Let's Talk About Table Flatness

[Brady Watson]

So I thought I'd kick off some discussion about table flatness, consistency and versatility. I've learned a few things over the past month that I didn't know/encounter much before and thought I'd share some of them. Feel free to jump in and compare notes, ask questions etc. My company, among other things, is classified as a job shop. I machine just about any material in 2D and 3D, so versatility is important. By versatility I mean that sometimes I can get away with holding things down using some kind of clamps, screws or adhesive, while other times I need vacuum - and even less, but not out of the realm of possibility I have to bolt things down.

My shop is between two rivers and weather-wise I encounter everything from hot & humid to dry and sub-freezing cold. As you probably know, every material you put your hands on has a coefficient of expansion, and wood-based materials also change size by humidity. Material also moves around while it is being machined because it is being heated up by the action of cutting and the surrounding shop air which is also heating up. This is why I never stop a long toolpath and go to sleep because it WILL change dimension overnight as things cool down. What I didn't know was how much an ultralight MDF bleeder board could change in size during a long cycle time (13hrs).

My machine, 'Big Bertha', is a 5x16' machinable area PRT Alpha (7.2:1 all around). It started out as a custom tool with dual Zs, 5hp spindle on Z1 w/5x8' spoilboard and a Hypertherm plasma cutter on Z2 with 5x8' steel grid. I've since done away with the plasma and my current machinable area is 5x12 - if for no other reason than I can't get sheet goods larger than this. I've got a 2nd gantry parked down at the 16' end which I am going to bring back online. My current setup consists of 5x12' 3/4" MDO plywood as the support board, regular MDF for the vacuum grid (zoned out accordingly) and 5x12x 3/4" Trupan Ultralight MDF for the bleeder board. All layers are glued down including the bleeder (on top of grid squares AND zone perimeters)

In days gone by, I used my own vacuum plenum design (BradyVac removable sheets) made out of Trupan UL and for a while a Fein Turbo III (@7Hg") as the vac source. When used properly, it worked pretty well, and still (via screws) gave me access to the MDF table underneath for screws, tape etc. It was OK when the vacuum was on in terms of being relatively flat and I could machine it flat with the tool. When I moved things to the new shop, I did the dual vac setup, which allows me to select between a 5hp FPz (9.5 Hg") and 10hp Becker (25.5 Hg") depending on what I am doing. Hey, why run 10hp when you can run 5?

Ever since doing a fully dedicated vacuum system (which I really wrestled with - in terms of giving up some versatility) - I've had issues with the table not being as flat as I was accustomed to. My previous layup was MDO & Medex water resistant MDF - which I think they changed the formula on...and it was ROCK SOLID in terms of not moving around and staying flat. Now I'm finding myself flattening the bleeder more often than I think I should just to get things to be flat...for a little while (as in only hours). I recently did a 8'+ long filagree panel in 3D which took 13hrs to complete on the finishing pass alone. After doing a 3D roughing pass that left .03" allowance on it, I finish machined it with a 3/16" ball @ 8% because it had a lot of compound curves to reduce tool marks. About 1/2 way through the finishing pass, I was seeing that it was barely taking off that .03" of allowance - which meant that the entire table was sinking in...I found myself doing some black magic, stopping the tool every .05", nudging the Z down .003" each time until I was able to get that .03" back. When all was said and done, the table had sucked down a full .14" (YIKES!!!) across only 24". There's no way I could have put my name on that piece so it's hanging up on the wall of shame...

I wound up machining off what was left of the bleeder board and machining the grid flat. I glued down another piece of Trupan Ultralight and then machined it flat. I did the entire job over only to find to my extreme displeasure that it was doing the EXACT SAME THING - albeit a little less at .05" creep. I was able to fudge things a bit and it all worked out - but the 'builder's curse' still remains where I know all that is wrong with it even if the customer doesn't. (They literally high-fived each other in the office - which is next door to the Ferarri dealership...when they saw it - which erased some of my shame.)

So...I made some calls to some good friends who also do CNC routing professionally and we talked about the issues. My one buddy's shop is in Miami and he routinely cut 40-60 sheets at a time. He told me that it wasn't uncommon for him to replace his bleeder every 3 days (!!!) because the bleeder would either swell up, compress under the vacuum suction or dry out and change dimension in the winter - or a combination thereof. He was running a few Lighthouse vacs (9Hg")and was having this happen...it could only be worse with higher Hg". So what do the $500k+ CNC router machining centers use then? How do they get around this problem? Density...and more vacuum.

The big machines use several Becker vacuums (or similar) that pull 25 Hg"+ to get the CFM up AND they use regular old MDF. When they have all vac zones open, they pull 12 Hg" or so with nothing on top of the table - for a delta/useable vacuum range of about 13 Hg" out of their max 25 Hg". 13 Hg" is usually plenty to hold down sheet goods through a bleeder. THis makes sense that the higher density MDF will be more stable - and if I really take a look at the Trupan UL, it is about as free-flowing as anything out there. It would probably make a good air filter! So I think I'm all done with ULMDF - which is OK for guys running the Lighthouse/Fein setups. It just crushes down so easily under the suction of the Becker. It also must absorb a lot of moisture and in turn shrink from getting sucked out.

Just because I've been doing this for a while doesn't mean I don't get schooled from time to time :) The important part is learning and moving forward. One interesting thing is the little SB Desktop machine. I've had that for (I think) about 4 years now. It has the Bosch extrusion - same stuff used on the PRS table sides, as the 'support board' with a piece of 3/4" MDF bolted down every foot or so, with countersunk bolts. Now it's just a small machinable area (18x24") but that spoilboard is totally rock solid. When I flatten it, I only have to take off like .01-.015" and it's dead flat again with all kerf traces gone. It's simply amazing compared to what I go through with the big tool.

So I am strongly considering doing a similar setup on the 5x16. The MDO support board goes away & gets replaced with some aluminum bar that adapts the steel crossmembers to 1030 8020 aluminum extrusions - all the way across the 60" bed (20pcs). This alone will function as a torsion box, and I can get it as dead flat as possible using a dial indicator on the gantry. Parts to be machined can be clamped or bolted down using any number of clamps/fixures including gasketed high-Hg" vacuum pods.

On top of the extrusions, a regular MDF table, consisting of 1-man manageable sheets/tiles can be bolted down just like the little DT machine. Parts can be screwed down with a nice soft MDF board to protect the cutter. There's no more need to mess with big 5x12' sheets - they can be all put together and made into one big sheet as needed and replaced individually.

To keep the versatility of a universal vacuum system, the same system can be used. The MDF tiles can be removed and replaced with a BradyVac (BradyVac3 anyone?) I plan to use regular 3/4" MDF as the bleeder - machined BradyVac style about 1/4" deep for the grid. Then I'll glue on a 1/4" MDF backer with 2" hole for the vacuum port and flip the whole thing over. The same countersunk bolt scheme will be used (bolt holes coincident with 1" squares every 12" or so) along with o-rings under the fastener heads. The MDF backer can be sealed where it meets the extrusion and edges. Of course there will need to be a 3x3" piece of extrusion missing where the vacuum ports are...but no big deal.

See attached pics below...tell me what you think...what you'd change/add/do differently. It's not a cheap endeavor @ around $2k for the extrusions, but confidence in the machine is paramount and I don't think I'd rest easy until I've done my best to get positive control over the situation.

-B

309583095930960

[dlcw]

Brady,

I can say I don't have the weather/humidity extremes that south NJ has but I do have temp extremes. Hundreds during the summer and below zero for stretches at a time in winter. BUT, the humidity is pretty consistent. So with that said.....

I put a 3/4" baltic birch plenum on my machine when I first set it up. I then carved the air channels out of the plenum and sealed it with about 5 coats of shellac. Also, before I bolted it to the cross-arm supports, I sealed the underside with 5 coats of shellac. I then put the 3/4" LDF spoilboard on and glued it down in specific areas. I have 7 zones in my table and each zone is its own piece of LDF. Around the edges of each piece of LDF I glued on PVC edgebanding. Of course I skinned the back side before gluing it down. My system is hooked to Gary Campbell's Blackbox vacuum system with 4 lighthouse motors. I don't mount anything directly on my spoilboard. When cutting plywood I use a 1/8" piece of MDF, skinned both sides, and put my materials on top of that. This serves me two purposes. One, I don't cut into my spoilboard thus creating vacuum leaks. Second, when cutting is done, the sacrificial board and material are slide off the CNC to an outfeed table for part sorting. I then put a new 1/8" sacrifical board on the CNC, put the next piece of plywood on and start cutting while sorting parts on the outfeed table. I cut plywood in two passes, first pass is climb cut, second cut is conventional. If the parts are small I turn on the 2 pair of vacuum motors to hold things tight.

When cutting material that needs to be held using clamps, I mounted T-tracks to 1" MDO and mount the material to the MDO with clamps. The MDO is in turn held on the table using the vacuum system. With the MDO base being held by the vacuum, I've never had a problem with the MDO or the MDF spoilboard compressing. Granted I don't have enough vacuum to launch a jet fighter like you do... :)

I have made it a habit that before the start of each plywood cutting job, I skin the spoilboard to make sure it is dead flat, relative to the gantry. I also skin the spoilboard prior to doing a long carving job. I've done several large mantel faces with 15 to 20 hours of carving time and haven't had a problem. But I think a lot has to do with the fact that I don't have the humidity variations like you do, nor do I have the vacuum system you have. But at 2500 ft elevation, I couldn't pull the Hg's that you pull. Even from day to night and back to day, there is very little fluctuation in humidity.

Your table looks really high tech and quite honestly if you can compress that with your vacuum system you should market the vacuum system to the Navy to hold airplanes to the decks of aircraft carriers. I just wonder if your still going to have the spoilboard compression problem even with the great support underneath. Just my .02 cents.

[Brady Watson]

Don,
Thanks for the feedback. I didn't seal anything other than the edges of the plenum and bleeder this last round because time was ticking to meet the deadline. In terms of the current layup leaking...I can put a 4x8 sheet of acrylic on top of the 4x8 zones and pull 22 Hg" - a few more if I put some plastic sheeting around the perimeter edges because there is always some bleed thru at the edges of the active zones. So I don't think I have any major air leakage, but that does nothing in terms of moisture absorption.

I like the disposable 1/8" MDF trick. Not sure if I can even get 1/8" MDF from my supplier. I think 1/4 is the thinnest. I think I'd fill up the dumpster if I adopted that method though...I remember visiting a busy sign shop years ago and the operator was pretty well on top of things. He used a sheet of 1/2" Homosote of all things the same way you use the 1/8" material - except I don't think he flattened it at all. It pulls vacuum nicely and is cheap.

Dividing the machining surface into tiles or pods of sorts makes sense for a number of reasons. First, I can move them around without difficulty by myself & without fancy contraptions...unlike a 5x12 footer! Another reason it makes sense, at least in my mind's eye/theory is, the break up of the large surface with smaller sheets allows some room for expansion/contraction in the XY plane. Furthermore, it'll be much easier to really seal everything off on all sides on say a 2x4' panel opposed to a big sheet.

Recently I did a job where they spec'd out 5/8" MRDF - I never heard of the stuff before. I was able to get some - they called it 'double refined' MDF and say that it is easier to cut and paint with less fuzzies. I am wondering if it is similar to the Plum Creek stuff or other high quality 'cabinet grade' MDF. Anybody know more info on it? I may try making a sample pod out of that stuff to see how well it flows and resists crush down. When it was up on the table I could feel it pulling vac through it on top of the ULDF...so it should be good.

In pursuit of getting good, consistent material for a vacuum bleeder I called several manufacturers around Thanksgiving...Sierra Pine, Arauco, Roseburg - and explaining my application, issues and requirements were dicey at best. I might as well have mentioned that I was building a space ship and needed MDF to insulate the flux capacitor from the turbo encabulator (http://shortsleeveandtieclub.com/the-amazing-capabilities-of-the-turboencabulator/) - with so many CNC routers running vacuum in commercial applications, you'd think that at least somebody (MDF application engineer) would know at least a little something about our industry....Nope. So as with most things, it's doing my own R&D - even if it's only trial and error (which gets expensive sometimes!)

Just the other day I was wishing I had the T-track/extrusion installed. Customer needed these 1.75" thick x 6" wide poplar balusters first profile cut, then flipped up on end & sharpen the internal corners. I would have been nice to bolt down a fence and use cam clamps. I think I am going to pull the trigger.

Working on some quick disconnects for vacuum. @$60 ea, these (http://www.sharkbite.com/product/90-elbows/) are a lot more than I want to spend, so I am going to see if I can 'borrow' the design and 3D print something similar with a bolt on flange. That way I can just pull off the vacuum pod without too much fiddling...in theory anyway.

-B

[gundog]

I use vacuum to hold most of my parts cut from aluminum and plastic. I built my vacuum table from info I gained on here much of it from you Brady I never went to the boxes though. My vacuum motors are light house I run 2 of them most of the time I only run one motor to cut 1/2" Starboard I make tons of parts from that stuff I make the outside profiles to a skin of .050" so only the drilled or bored holes go through to leak air and a few profile slots.

The .250" aluminum has created more problems for me to cut due to the thickness not letting me cut to a skin because there is not enough material to effectively use a laminate trimmer to trim out the parts like I do with the plastic. I cut the aluminum parts and leave tabs the through cuts are making to much vacuum loss for my vacuum setup as built. I plan to add 2 more light house vacuum motors to the system. One thing I do now is I installed a series of aluminum dowel pins this helps a bunch now the vacuum only has to hold down and the dowel pins to stop any lateral movement. I turn off each zone after those parts are cut to keep as much vacuum as possible I program to do it that way by selecting which parts to cut in the order of my choosing. I may also look into a real vacuum pump.

I really like using vacuum my vacuum plenum has 5 zones and I use Valtera knife valves to control the vacuum zones. My spoilboard is Trupan glued down to the plenum in the high spots. I cut my plenum from counter top particle board I then sealed it with some shellac. The addition of the dowel pins has really helped with cutting the aluminum parts.

[Brady Watson]

Thanks Mike. Vacuum certainly isn't the end all be all of hold down. It's really a nice convenience for production, but there are times where I just can't use vacuum alone to hold things down if for no other reason than the parts don't have enough surface area to stay stuck - and as you point out, the kerf area can get quite large.

For times like these, I use my 'Vacuum Film' technique (http://www.talkshopbot.com/forum/showthread.php?11481-Vacuum-Film-Technique-A-Quick-Explanation) where I adhere the substrate to a sheet of .040 styrene or 3/16" coroplast. To mitigate some of the cleanup on some materials, like AL, I first put masking tape on the back of the substrate to be machined - then I put mounting tape (or fiberglass carpet tape) to the styrene/coroplast and laminate the two together (by hand). This gives you some buffer in Z, allowing you to cut all the way thru the AL etc and slightly into the plastic. You never lose an ounce of vacuum.

When all is said and done, you just peel the parts off. I have to do this method for adhesive-backed laminates (that must retain its own 3M adhesive/paper as part of the product) - there's just no other way. The two downsides are A) time/labor/materials laminating and B) getting residue off the profile/edges from the adhesive riding up the bit. Other than this, it works pretty well - especially if vacuum is marginal. Carb cleaner makes short work of cleanup on aluminum. You won't need tabs or have to worry about using a trimmer anymore...

--

I took some measurements out at the table rather than relying on the SB table drawing for crossmember locations. The PRT uses 2x2x0.25" angle (and 3x5x0.25) - Looking at it and imagining 431 lbs of AL extrusion on top...It would be bolted, but I am still concerned about deflection/sag at the center. I'm toying around with either sistering up another 2x2x0.25 angle to each existing one, or switching over to like 2x3x0.25 wall rectangle. Don't think I want to do center legs (like the BIG SB custom tools I've installed...PRS96-336 !!) - because that invoves making a strongback etc and it gets complicated. But it would be wicked stout!

-B

[909anchordown]

Hey all, I am delighted to see this discussion as I am experiencing table top stability issues with my 96x60 PRT Alpha that is otherwise still going strong. I bought it used and always have to work around the issue. One theory I have that may contribute to issues is this: The heat generated by the two lighthouse vac motors under the table cause the steel supports to warm up a little and expand causing the surface to bow downward in the middle of the table which is what I see though not 100% of the time. Another theory is getting material that is not flat enough and so when holding thicker and less flat sheet with the vacuum, the table top is being sucked to the sheet and they meet half way. Thoughts?
Marcel

[Burkhardt]

As for the vacuum quick disconnect of vac pods, I use a short piece of 3/8” O.D. hard tubing and slip it into 3/8” ID pressure hose that supplies the vacuum. Seals tight enough and costs next to nothing.

[Brady Watson]

Gert - I'd love to get away with 3/8" hose and couplings, but the plumbing has to be sized for the pump CFM. In my case about 200 CFM, where 2" is what the engineering charts point to for free air flow.

For small type 1 PVC pods...yeah - the 3/8" stuff is perfect for that.

Just got my hands on a Sharkbite XL 2" elbow...says it fits CPVC...NOT PVC Sch40 etc. So...admiring their handiwork/engineering. Will make something similar for 2" PVC. @ $60/ea, I'm sure I could improve on that. More later...

-B

[JMCS]

Just a thought,you have a lot of wood products right now, (base, plenum, spoil) that's a lot room for movement. Instead of using 8020, why not use a sheet of 3/4" 6061 Aluminum plate? With the 8020 you have a lot of smaller pieces that have to be aligned and secured to each other or they can move independent of each other. A sheet of 6061 weighs around 514 lbs. A bit heaver than 8020, yes, but a lot more stable. A few extra cross members will handle the weight. A heaver machine is a more stable machine. The expansion coefficient is minimal. You could machine your grid right in to it and use gasketing or use a structural bonder and attach another plenum board.
With regards to the MDF shrinking, could it be with a 13 hour run, the moisture content is going down in the board with that much air flow going through it? That time of the year, dry, dry,dry. With the big guys running heavy iron, there changing there spoil boards so frequently they don't have a problem with it shrinking.
Double refined MDF. Try Ranger Board by West Fraser. Awesome product, finishes out after machining like it has been sanded to 150. Also no extra edge sealing, the edges absorb the same as the flats.

[JMCS]

6061 3/4" Aluminum plate is $1905. Shipping is $267. Price is from Discount Steel .com Shipping from Minneapolis, MN to New York

[Burkhardt]

Gert - I'd love to get away with 3/8" hose and couplings, but the plumbing has to be sized for the pump CFM. In my case about 200 CFM, where 2" is what the engineering charts point to for free air flow. ...

-B

Wow, that is a lot of volume for pods and the skinny hose surely won't do. In my case I have only a high vacuum GAST pump that does maybe 4 cfm but I have used it successfully for custom shaped pods up to 15" x 20" or maybe double that size when 2 pumps are used.

I have modified my 36"x48" 80/20 table (consisting of 3-slot 4515 extrusions) that a few of the middle slots are closed (thin aluminum flashing taped on) and act as the vacuum conduit. I can punch a hole anywhere along the slot and only need a re-usable non-stick gasket (cut from foam-type drawer liner) to hold down a board for one-sided work, no real "pod" needed. I even hold down the indexer and tailstock or the machine vise that way when used. But that would not work as well for your high volume vac system.

When using any aluminum table on a steel frame, keep in mind the thermal expansion of aluminum is twice the expansion of steel and for an 8' long machine that can become substantial. It can be 10-15/1000" difference for a 20 deg F change which does not sound much but could bow the table when not allowed to move relative to each other (e.g. mounted on standoffs).

[Brady Watson]

Just a thought,you have a lot of wood products right now, (base, plenum, spoil) that's a lot room for movement. Instead of using 8020, why not use a sheet of 3/4" 6061 Aluminum plate? With the 8020 you have a lot of smaller pieces that have to be aligned and secured to each other or they can move independent of each other. A sheet of 6061 weighs around 514 lbs. A bit heaver than 8020, yes, but a lot more stable. A few extra cross members will handle the weight. A heaver machine is a more stable machine. The expansion coefficient is minimal. You could machine your grid right in to it and use gasketing or use a structural bonder and attach another plenum board.
With regards to the MDF shrinking, could it be with a 13 hour run, the moisture content is going down in the board with that much air flow going through it? That time of the year, dry, dry,dry. With the big guys running heavy iron, there changing there spoil boards so frequently they don't have a problem with it shrinking.
Double refined MDF. Try Ranger Board by West Fraser. Awesome product, finishes out after machining like it has been sanded to 150. Also no extra edge sealing, the edges absorb the same as the flats.

I was considering sheet AL (probably 1/2" is more than adequate, but $900/sheet @ 4x8' x 3 = $2700) and phenolic sheet as well; but to cover a 5x16' area they're both more expensive and have less utility than the 8020, because both lack t-slots. I'd like to bump up to the 1.5" stuff (1530/1545), but that brings the grand total closer to $4k for the job. Plus, a 1/2" or AL sheet is cumbersome to handle even with a front loader and cherry picker. It would also be dicey putting MDF on top for a bleeder...

Initial testing of a piece of 1030 shows that it's pretty darn stout between the crossmembers and I'll never put enough weight on it to have it deflect any measurable amount. I don't think expansion and contraction is going to be much of an issue. There's still no getting around doing a flattening routine to get things flat to the gantry/spindle from time to time. If I sense that things are too up or down, bolts holding the substrate to the 8020 can be loosened to allow it to relax and then tightened down again. As I said, the DT tool has proven to be excellent for flatness...granted this is a much larger area, but I think it's way better than relying on wood with temp/humidity swings.

I'm leaning towards replacing the 2x2x1/4" angle iron crossmembers with 2x3x1/4" wall steel tubing...It will provide twice the support and deflection resistance according to deflection calculations.

Yes, it's a lot of work getting them properly secured, aligned and flat - BUT - I only have to do it once. Plus, I have robots to drill holes for me exactly where I want them. There's only going to be about 860 of them by my calculation... :D

Thanks for the tip on the Ranger Board - I'll check it out.


Wow, that is a lot of volume for pods and the skinny hose surely won't do. In my case I have only a high vacuum GAST pump that does maybe 4 cfm but I have used it successfully for custom shaped pods up to 15" x 20" or maybe double that size when 2 pumps are used.

I have modified my 36"x48" 80/20 table (consisting of 3-slot 4515 extrusions) that a few of the middle slots are closed (thin aluminum flashing taped on) and act as the vacuum conduit. I can punch a hole anywhere along the slot and only need a re-usable non-stick gasket (cut from foam-type drawer liner) to hold down a board for one-sided work, no real "pod" needed. I even hold down the indexer and tailstock or the machine vise that way when used. But that would not work as well for your high volume vac system.

When using any aluminum table on a steel frame, keep in mind the thermal expansion of aluminum is twice the expansion of steel and for an 8' long machine that can become substantial. It can be 10-15/1000" difference for a 20 deg F change which does not sound much but could bow the table when not allowed to move relative to each other (e.g. mounted on standoffs).

Gert,
I also have a couple of little Gast pumps and use the smaller tubing with them. I learned early on to err on the side of bigger is better when it comes to vac tubing because I remember not being able to make a seal even with an 11-gal surge tank because the tubing was too small...'like sucking through a straw' - quite literally! It certainly did cross my mind to exploit the internal cavities of the extrusions for vac plumbing...but aside from needing multiple feeds to make up for the smaller equivalent tubing diameter, there was also the issue of sealing both ends of the extrusions air tight. That means drill/tap/gasket and bolt at least 40 caps...I think for me, on-board PVC pods etc with them bolted to the 8020 would be fine. In fact, it is very rare for me to use the small vacs on the big machine because the bigger vacs are available right there.

Most of the time I'll just make a 'direct air' pod, puck or fixture out of whatever scrap I have laying around. I made one last week, printed out an adapter and plugged in some 1.5" corrugated hose...The hose held up under 25 Hg" - so I am considering using some of it for the 'quick release' setup when I want to remove the vacuum grids and just machine on the 8020. I reverse engineered a push to connect fitting and scaled it up to fit a 2" PVC pipe...the problem is that it doesn't scale well - looks goofy(!) - and while it seemed like a good idea on paper, it's too wonky to use in real life. Then I was thinking about something more compact - why not a compression fitting?...then again, why not just plug a 2-2.5" ShopVac hose in or out of the zone and be done with it?! So I am thinking in this direction now. They have nice hose end fittings/cuffs for the corrugated pool/car wash hose, it won't crush down and leakage doesn't seem to be a problem. I'd just adapt from the existing PVC manifold or run to a flex hose and be able to plug/unplug as needed.

In terms of expansion/contraction - I'm going to see how it goes. My initial thought is to slot the thru holes to both deal with the possibility of piece to piece stack up errors and possibly give the material a place to go if it does expand. This is a tightrope between getting the proper torque to hold things down and allowing it to move. I can't say with any certainty that the MDO doesn't buckle/bow under weather changes - because I just don't have the instrumentation to pick that up. I will say, visually I can't tell - even with a straight edge and dial indicator. Last week for the heck of it, the shop was around 70F and I pre-loaded the dial indicator against the spoilboard. When I came in the next morning there was zero change on the indicator at 46F. Not very scientific, and kind of a novelty test, but I was expecting a change of at least .005".

Attached are pics of the reverse engineered push to connect and how HUGE it got (like 4.5" OD) - and a jig I slapped together last week out of 3/4 & 1/4" MDF - no gasket required. The hose feeds in from the bottom off the table. To secure the jig to the machine, I just put like 10 dollups of wood glue behind the jig to the spoilboard and weighted it down. It came off pretty easily when done...but did take a few pocks of material with it when I pried it up.

More later...

-B

[gundog]

For my clamping portion of my table I bought some extrusion from Hubbard CNC they sell on Ebay. If I had it to do again I would make my own clamping table from 1/2" x 4" 6061 bar stock by just cutting a rabbit out of the corner and placing them spaced so they make a T slot when installed. My extrusion fit 5/16" bolts if I were to make my own I would make it fit a 5/8" slot that way you can buy ready made clamping kits for milling machines.

[gundog]

This is how I would make my own clamping table this is an end view.
http://www.talkshopbot.com/forum/attachment.php?attachmentid=30988&stc=1

[Brady Watson]

Mike,
I certainly did consider doing exactly what you show. The problem is, 1/2" AL 6061 would actually cost a little less than the 8020 but still have to be machined, drilled and the steel drilled to accept all those pieces. It's definitely a great idea, but I think the 8020 is the way to go for my setup.

Saw the Hubbard stuff as well as the Techno stuff and other offerings on Ebay et al. All the 20mm stuff seemed too flimsy and I think they can only take a 5mm fastener - so that rules that out. I think 1/4-20 is the smallest I'd want to go.

I'm looking forward to popping my Kurt or Grizzly milling vices on the table. I'll have to make an adapter plate for quick registration, but it's great to be able to really crank down on the metal stuff when machining...Plus align it with an indicator and life is good. I had one on the DT machine when doing some cutting on an awkward to hold aluminum casting.

Got me thinking now Mike...I'm going to have to crunch numbers again and see if AL bar would really make sense for my work...

-B

[waynelocke]

I remade my table plenum last year with 4 2' X 4' X 3/4" pieces of HDPE. I used four pieces so I didn't have to deal with expansion over the whole 4 X 8. This replaced a mdf plenum and the original hassle of sealing it still leaves a bad taste and then even after sealing it it was still barely sufficient compared to the plastic. The top cost, I think about $200.

I know that some have criticized hope as plenum over the years but, although I don't remember the numbers, the expansion over a 20deg temperature change was not bad and was accounted for by drilling larger mounting hole towards the positive x and y directions. The plenum is much more efficient and to this point the table has stayed fairly flat. My shop is air conditioned and heated so there is rarely a large temperature change.

[Burkhardt]

I would add to the criticism of HDPE, even if it works in your air conditioned environment. Its rigidity is about 2% of steel and 6% that of aluminum so it needs a very good structural support and the thermal expansion is about 20 times that of steel.

Another thing comes to mind for any extruded material, be it plastic or aluminum that it has residual stresses from production and will deform from flat when machined, like skimming the surface or cutting slots or rabbets. There is an aluminum grade MIC6 that is stress relieved and will not deform much if machining is required.

[waynelocke]

I just replaced the mdf plenum and kept the two 3/4” birch ply sub base so that the hdpe structural concerns are not really an issue. The thermal expansion being 20 times that of steel doesn’t really matter. What matters is that the expansion is accounted for. The expansion on a 2 X 4 piece is still pretty small - at least by woodworking standards and this is dealt with in the mounting holes.

I use a Fein not a large blower like Brady and so I was not dealing with the same issues and trying to solve
the same problems. The suction improved immensely. After using this for a year, if I were going to redo it, I would do the same thing. The table is still pretty flat and when I need vacuum it’s better than the shellacked mdf. Best $200 to $300 I spent last year.

[waynelocke]

Any solution is a compromise , otherwise the choice would be easy and obvious. I do different things and have different requirements than Brady (and probably most SB'ers) and am not suggesting this as a solution for him or anyone else but it works well for me. I used an mdf plenum for 15 years and had plotted a replacement for years when I went with the hdpe. After a year's experience I cannot remember once lovingly longing for the mdf or encountering one time when I regretted not having an mdf plenum.

Given my stage in life, I measure return on investment in weeks and months - not years. Phenolic and 80/20 seemed like awesome choices when I was looking but I was not willing to spend $1000 plus. Having followed Brady's posts and having had a couple of conversations over the years with him, I have enormous respect and admiration for his work, his impressive knowledge and his sharing — I never skip a Brady post. His requirements are different from mine.

My only point is that using a "sealed" mdf plenum is akin to polishing a cow patty — it may look better but ...

[Brady Watson]

HDPE - been there done that. In my shop it moved all over the place with temp & humidity swings. I couldn't get the fastener tension right to keep it from curling. Too loose & it moved around...too tight and it buckled. I've also serviced machines where they had HDPE and it moved around enough where the spoilboard was like a rainbow - all bowed up, with chips and swarf between it and the support board at the frequency of where the screws were holding it down. Now this machine was in Miami - and temp/humidity swings are more extreme than most of our shops...plus how dilbert do you need to be to not notice your table getting all jacked up like that? <sigh>

In the past I've used composite decking (Depot Veranda because it was solid...don't think so anymore) and after running it through the planer to flatten off the texture, it was pretty good for making long pucks. Not sure I'd want to do an entire surface with it, but for hard wood vacuum fixturing it was pretty good.

If I do wind up making a plenum out of a non-wood material, first choice will be aluminum plate and 2nd choice phenolic. Even PVC/Sintra/Komatex moved around a little more than I wanted it to on the BT48.

I say if it works for you - great - keep doin' it. I don't want the added expense of heating/cooling my shop when I am not in there - so I have to bite the bullet and figure out what works best in my situation. We shall see my friends! :cool:

-B

[bleeth]

Got to throw in with the other side of the coin. A SB, which is a fabulous machine for the money, is not a super high resolution big iron machine. If you go into it with the appropriate expectations then you will be amazed. If you expect unreasonable perfection then you have only yourself to blame for your over inflated expectations and resulting disappointment.
I could make parts that competed with the big boys on quality any time, but they did it fast and easy and for us it took a lot of tweaking, patience and time. Brady has gotten more out of his mills than darn near anyone could. He is also the first to admit the immense amount of time and effort in mechanical education it has taken to do that. Nothing wrong with that at all. Fabulous in fact. But, that is also far beyond what most mill clients want to have to do. Note that when it comes to flatbed mills they all end up in the same place though, There is a spoil board thrown on top of the plenum. The spoil board changes shape during it's life. That affects part production. Learning how, when, and what to do about it is what separates the Pros from the Ams. No matter how flat, stiff, and non reactive your base below it is, the spoil board above and the live part you are cutting are still the final variables to deal with. To do that we don't need to be Scientists. Artisanship is plenty sufficient.

[Brady Watson]

Dave,
Where are these 'mills' you speak of? My mill weighs 3800# and is made from Mehanite cast iron. It's heavy enough to cut tool steel within .0005" reliably. My CNC router, with a machinable area over 53 times that of my mill, maybe weighs half that. I'd like to find the first dweeb that started the trend of calling machines even lighter than a ShopBot a 'mill' and slap them with a wet noodle. Many of these machines barely qualify as routers, let alone anything resembling a mill in the traditional sense. Some are calling some of these glorified plotters made out of plastic of all things 'mills' - please stop!!! JUST STOP!!! It's ignorance polluting & corrupting our language, and minimizing the efforts of legitimate millwrights and pattern makers - which were and are at the top of the machining pyramid.

Though my illusions and delusions over the past 17+ years of working with these 'light POS machines', I've managed to turn out countless iconic projects a lowly ShopBot owner/operator has no right doing. In fact, several jobs came my way via big iron turning it down because they were literally too slow doing 3D (by the customer's voluntary admission), didn't have provisions for holding the part down (using something other than universal vacuum), or were just plain out of their element in doing the job (scared)...

They say a poor craftsman blames his tools...I say a good craftsman invests in the best tools he can afford and maintains them like they are the lifeblood of the business. Furthermore, since these machines are electronic, they also need to keep up with the times, upgrading them to something newer than WinXP/2005 technology and mechanical components that improve the end result. I've been to too many shops where they beat the hell out of their ShopBot and never maintain it...and more often than not, 'upgrade' the machine with half-baked solutions that are worse than the OEM/stock configuration. Very few even clean their machines of dust/chips, let alone check, adjust or lubricate them. "That's just the way it is." - isn't spoke too often 'round these parts.

So to some, the idea of spending $2500 or so, plus all that labor to get the table surface the way that I want it to be seems crazy. I get that. But if it isn't reliable, or meeting my expectation of what it should be, I'm the kind of guy to put my best effort into it and make it right. No different than a guy with an early PRS 5x12 with 4 legs wanting to get the sag out of the table. If I machine parts and they aren't up to my standards, they don't leave the shop until they are done properly. All ego aside, my name means something. People know they aren't going to get junk when they deal with me - so it's my responsibility to always do my best - and that means, not turning a blind eye to deficiencies in my equipment, that can be corrected within my means. The foundation of the machine affects everything above it.


“...like a wise man who built his house on the rock. The rain came down, the streams rose, and the winds blew and beat against that house; yet it did not fall, because it had its foundation on the rock. But everyone who hears these words of mine and does not put them into practice is like a foolish man who built his house on sand. The rain came down, the streams rose, and the winds blew and beat against that house, and it fell with a great crash.”

Rock on...:p

-B

[bleeth]

Hundreds of things are properly called mills besides a metal working tool, starting with those places that grind grain into flour. Other than that we're pretty much peeing on the same tree as per usual! Getting what you do out of your tools is based on your extreme attention to detail, regular maintenance, and creative thinking. That isn't the norm out there. Like you, I've walked into shops where the owners had told me their machine was no good and in reality the issue was poor set-up and operation.
By the same token, the average user cranking out cabinet parts or v-carved craft signs isn't going to need the absolutely perfect bed either.
Sharing the knowledge for those few fanatics that do want or need to go the extra mile for absolute optimal results though, is always a good thing. Maybe a bit of it will seep down where it will help others as well! :cool:

[mporter88]

Hello.
May be my first post here. Lurker/learner forever though.
Have a question and think this maybe a reasonable place to ask it.
Any input appreciated.

I have a 24 x 48 Buddy standard with the HSD.
Love the machine.
Bot at home...Fadals at work.

Now taking a leap into a small personal production run of something I developed on the buddy.
Will purchase a longer stick so I can run 48 x 48
Will also build a 48 x 48 bradyvac with 2 Feins.

If one of you were going to do that..today..what would you do for a base to use all of that new powerstick?
I can give up some Z as I only ever cut panels....so i've thought about a box even.

I am simply routing 1/4inch (6.5mm) melamine board into parts with .125 carbide endmills...and want to get Z cut depth as consistent across the field as I can.

The product would benefit from equivalent depth in all pockets....so want to plan the new setup to FOCUS on that.

Would the Buddy support moving a slab of Aluminum?
Would it kill the steppers?

How about a glue up of Formica..flattened?
Would probably weigh almost as much.
Money is an object for me...but still curious as to best option.

Current base on the 24 x 48 setup (will be replaced) is 1 inch high quality subfloor ply.

Any input appreciated.

[bobmoore]

my vacuum system has been much more stable since I made my plenum out of corian. I have 2 5 horse becker pumps that can be run solo or n tandem with a combined volume of about 175 cfm at 25 inches of mercury. I don't recall the numbers but the corian had less expansion / contraction than aluminum. keep in mind that when you surface your spoilboard the vacuum is severely reduced (especially with a high vacuum pump) compared to when you put a sheet on to cut and cover all of that open spoilboard. We would get some bowing (I seem to remember about .018-.020 inches) even on the Komo vr 510s toward the center of the spoilboard. Bob

[khaos]

Great thread!

BLUF: I primarily do 3D work. I only share here for the lurkers and not to provide a solution for Brady's application.


I have to go through spoilboard at a decent clip. I have a 0.75" marine plywood base flattened and then I glue spoilboard on top. Short of going the route of extrusions Brady is leaning towards I can't see much improvement. My PRS 8'x4' lives in my non-heated/cooled shop literally 25' or less from the Nansmond River. Humidity varies from %20 to %80 within a few hours and approaches %100 often enough to mention. Heat swings normally about 20 degrees. So before I work I have to skim the table to remove the hills and dales less the shire attract hobbits. :/

I have an .75 extruded PVC 2'x4' vacum puck/pad that I screw down on the corners. This worked great for smaller and faster cuts (< 3 hours) but longer cuts > 12 hours would result in a wall of shame entry. From the results there is usually a very noticeable ridge in the work. :( I brush my racks and pinions regularly.

I made an 8'x8" puck for some custom molding and some flooring inlays out of a piece of TREX and that was a great piece of tooling those cuts were usually < 3 hours so it only bears mention because it was unaffected by humidity. The flooring inlays are sensitive to depth of cut as I use straight bits vs the vcarve method. I would skim a 64th or a 32nd off at the planer to finished the work up usually to compensate for some variance in the laminate thickness I cut on my bandsaw. :)

The best results for me personally have been using a high end subfloor that has a waxy feel throughout. I understand that my results are compounded by all the materials in the cut including and sometimes primarily the working material

I am not a production shop . And Vacum is the rarest form of hold down I use. Screws, bolts and job specific jigs are my lot.

[Red F]

http://www.talkshopbot.com/forum/showthread.php?19903-Buddy-BT48-Alpha-table-MKIII&highlight=buddy+table

Check out this thread. This is a great aluminum vacuum table built on a Buddy.

Truely any of AJCoholic's posts are great.

[ddrumguy]

happened to skim through this thread and thought i would share my setup and what a good friend of mine and i did to achieve around .003 on my table. I havent checked it in awhile but i typically need to account for that amount when cutting. My friend came to visit right after i got my prs alpha that i bought used and we tried cutting our first file and quickly found variations (big variations) in our first piece cutting in Sintra type material. 14 long hours later we had rebuilt the entire table and used a festool tracksaw track (about 8 ft long) to use as our straight edge.

first we chucked all the old table and started over with 2 - 3/4" cabinet grade sheets of birch and then topped it with an 3/4 mdf plenum and on top of that 3/4 trupan every sheet was glued with titebond. This was the easy part (if there was one). But the table was still off quite a bit. So we spent many many hours leveling, adjusting and i had also just added a set of legs from shopbot in the middle ( this is a 48x96) machine. The final honing in for use believe it or not was the flats that the v-ways are connected to. We found that some 3-5 thou shims at various points helped to get the gantry more consistent as we found that some high or low spots could be corrected by these shims. I bought a bunch of colored gel material at an auction once (had tons of it and have now thrown most in the dump) but kept some in case i ever needed it and this is what we used to shim those rails.

anyway as i skimmed through this thread i didnt see anyone talk about this so thought i would mention it. may help someone