More vacuum questions?

[hiposner]

Sorry if this has been answered, I stayed up all night reading posts and I think I get the vacuum concept minus things! I like the t track in the plenum idea as we are a high school and every student wants to do something different and we always have small parts (thus I think the cams will work nice). If you put the t track in the plenum how do you do through cuts without damaging the plenum? Is there any reason not to just make a t-track board that would vacuum down over one or two zones? That way you could lift it on and off when necessary? Same idea for a screw down board? Am I missing something? Why put the track in the plenum? Also I bought 3/4 trupan for the plenum and spoil board, do I have to skin it first? I see mixed messages on this! Lastly, why glue the spoil board at all, won't it hold in place with the vacuum? Is there a simple method other than glue? I see some people said they taped it? How does that work?

[mikewallis]

Hillel, I'm at the same point you are so I can't give you an answer.
I'm probibly missing something as well but I got to ask the question, why go all the work of making a plenum, sealing it and adding t-track to risk cutting through it (The Plenum or T-track). Is there a step i'm missing.
I also am needing the best of both worlds, t-track and vacume. Vacume for the large sheets and hold downs for the smaller hardwood boards like Cherry. Does anyone have a setup that can accomidate both without risk to the plenum? From the looks of it the vacume will not be enough to hold the smaller hardwood boards in place so hold down system seems to be essential.

[dana_swift]

Howdy, I use vacuum, and T track both. They each have their purpose. Without knowing what your SB configuration is (its not in either of your profiles) I cant make much in the way of suggestions for your case.

I use more than one "table" each one has its application, one is MDF with t-track. That gets used for general purpose one-off projects.

I have dedicated fixtures and a low-flow hi-vac system based on an air-conditioner vacuum pump that gets used for frequently made parts. The secret to getting this to work is "allstar" gasketing tape. These fixtures are mostly made from HDPE sheets. I have made LOTS of them.

I am adding a hi-flow hi-vac system (not talked about often on the forum) it is based on a Becker pump and a plenum made from HDPE with a layer of 3/4" Trupan on top of that for a bleeder/spoil board. This fixture is for general purpose vacuum holddown. And "should" offer the best of both the hi-vac and hi-flow worlds. That remains to be seen.

Elsewhere in the forum I listed many of the hold down methods I have used and some experiences with them. That and others are good things to read up on. Brady Watson has a good section on hi-flow medium-vac systems in his column.

Clamps, screws, glue, tape, bolts, all still get used. I have to add one other clamp method I use and have not documented elsewhere.. "inertia".

Inertial clamping works well for some applications, especially creating the mounting holes in dedicated HDPE fixtures. I use 25lb bags of lead shot on top of the material being "held". That gives it extra "traction" with the spoilboard. Then I created dedicated bit definitions in Aspire (or PW) that have very low slew rates so the part wont shift. Something like a jog speed of 4ips, and a cut speed of 2ips. The inertial hold down is used for very short cuts, so the bit doesn't heat up from the low cut speed. The part being cut doesn't shift, but I have to use very high safe z heights to clear the bags of shot. It is very useful tho..

Once I have the bolt holes created, I switch to bolt hold-down.. and build up the fixture to use whatever hold-down I intend for it.

Hope that helps-

Good luck! Experience will reveal what works for you. Sounds like you are getting started.

D

[john_hartman]

Since non of the other seasoned bot users have responded I'll give you my take on the matter having owned a SB for about 6 months using a vacuum table.

"Is there any reason not to just make a t-track board that would vacuum down over one or two zones?" I like this idea a lot. I might have to use it if you don't mind..


For the Trupan you do not have to skin it. However I used Brady Watson's technique of milling the plenum into the back side of the spoilboard/ Trupan and flipping the smooth side over. Since the plenum is within the 3/4" trupan I have a little less material to suck through. Incedentally this method allowed me to use only one Fien III vacuum without any problem. Just try it to see.

I did not glue the spoil board down. I counter sunk 6 screws about 1/4" around the perimeter corners and centers then used HVAC foil tape to seal the exposed edges and hold it the to the sealed MDF sub-table. When the vacuum is not on you can push it down a little but as soon as the vacuum is on it sucks it flat.

I hope this helps a little.

[ron brown]

I read these earlier and thought 'how to respond'. Vacuum clamping is a great method of hold-down for certain purposes. For many other purposes it is frustrating or useless.

As I have stated in the past, I find little positive about MDF. In use, for high accuracy parts, it usually will need 'skimming' a few thousands off at start of work and every few hours after that.

And yes, one can use 'track' and a vacuum table. If I ever chose to do that I think I would make it where I could permanently mount the track and place 'ripped' sections of MDF held in position by nylon machine bolts.

Don't forget there are also gasket vacuum tables and pocketed vacuum jigs for finishing the backside of projects. Jigs will evolve in many ways you never expected them to once you start.

There will still be times the most efficient method of hold-down is plain old drywall screws.

Cutting method, bit selection, bit load, cutting speed and even cutting direction can make a lot of difference on if a holding method works or fails.

If anyone tells you there is only ONE way to hold a project, be very wary. There may be only ONE way that person feels it can be done. Also remember what works for a production job may not be suitable for a one-off project.

Good luck,

Ron

[beacon14]

Since my table is one of the ones that may have started some of the confusion, I'll jump in. One of the reasons I have T-track in my plenum is that I like to make things more complicated than they have to be. I'm only half-kidding, in that does take time and trouble to remove the spoilboard to access the T-track, and yes, the T-track makes an attractive target to wayward toolpaths and Shopbot gremlins, but it has it's advantages as well.

Because my spoilboard must be removable to access the T-tracks I can't glue it down, hence the tape. With the vacuum on, I run a strip of 2" Gorilla Tape (from Home Depot) around the perimeter, flush with the top of the spoilboard. It seals the edges of both the table and spoilboard and keeps dust from working under the spoilboard and from getting sucked into the ends of the T-tracks. Since I have to remove the tape and then re-apply it I only remove the spoilboard when necessary, typically 2 or 3 times a year.

For short-duration projects as you suggest I'll throw a piece of scrap melamine on the spoilboard, hold it down with the vacuum, and screw or nail into that. Quick, easy and effective. You could definitely have a T-track board that you could hold down the same way. But I also use the T-track to hold fixtures that need to stay in place for longer times (days) and I don't want to run the vacuum unnecessarily for long periods. I also use it to hold jigs which hang off the end of the table for machining workpiece ends, and there's not enough material on the table surface to hold the jig with the vacuum.

Having T-track in the plenum also forces you to use a multi-zone system and coordinate the zones with the T-track layout, so it's not for everyone. You really have to consider whether you can do it all with the vacuum and spoilboard. I like having the option. Another benefit is not losing the thickness of the spoilboard and scrap board when milling thicker projects - I did a 6" heart pine mantle that would not have fit under the gantry without removing the spoilboard.

Regarding holding solid wood like cherry, if the bottom surface is planed smooth I find I can hold most solid wood pieces with the vacuum by surrounding them with scraps of thin, non-porous sheet material and taping all the seams with blue tape. If you prevent air leakage and use the proper toolpathing strategies (light passes, sharp downshear bits, onion-skin passes, etc.) it should not be a problem.

Lastly, Trupan is not as durable as regular MDF, so you might want to save it for your spoilboards and use something denser for the plenum layer. Assuming you are routing grooves in the plenum the Trupan is not needed there.

[dana_swift]

School use has one additional requirement.. guaranteed hold down. For that vacuum may not be acceptable. When the bit gets a gouge bite on something it can make a quick flight across a room. In a school that would be entirely unacceptable. That will happen more often with novice users, especially students.

Think screws. Things are more likely to stay in place, and it can be checked by the person in charge before cutting commences.

Screws and clamps also require more thought be given to toolpaths and safe z clearance heights. That is a good lesson for students in my opinion. Learning how to put tabs on the perimeter of a part that gets cut free is also a learning experience.

Because most of my cutting is done with vacuum, I just assume a clearance height of 0.1 is plenty. But when I switch to "hard" hold-downs, I have to remember to do the extra thinking and planning. Sometimes I have forgotten that step and have some interesting scraps and broken bits to prove it.

In a school I expect the number of parts cut per day would most often be just a few. So the time to screw down the work is not a big factor. Student labor is effectively free. The situation is different for most of the forum users, so the advise switches to production-oriented answers.

For clamp hold down, the ones David Buchsbaum makes are very good, and don't cost much. Cost being another factor schools are really sensitive to. Especially recurring costs. Screws and spoilboard, or TTrack and Davids clamps are a good combo for going easy on the budget.

Also note: If you use David's clamps, you will need 3/8 T track.

Just some more thoughts I figured I would kick out here..

D

[3imoh]

David-

This sounds similar to a table design I have been designing on and off for a while...

It would be a zoned table that would actually pull vacuum through t-tracks. Instead of the standard aluminum tracks, I was going to machine the slots in some thick birch ply as the base layer. I would then inset aluminum bar on either side of the slot (pre drilling and counter sinking holes on the alu on the bot beforehand). This aluminum would be the bearing surface for whatever t bolts I used. I would setup the spacing of the slot so you could drop a bolt down into the slot and twist it to engage the track...IOW, you would not need to have access to the end of the t-track to use it.

I would then glue another layer of ply on top of that and machine slots that matched the t-track. On top of that I would glue MDF and machine the same slots. This could be surfaced down as needed to the ply. I have also considered doing all of this with PVC...it would be much more stable (and expensive).

Does anyone have any thoughts on pulling vacuum thru these tracks? I was originally thinking of having a single port in each zone, and having the vac pull horizontally along the tracks. My latest thought is to build a plenum below the t-tracks and have holes drilled thru the center of the track.

I know I would need to deal with a lot more debris and dust in the vac system. I was thinking of trying one of those small cyclones designed for shop vacs from oneida. I was also considering having a y valve that would pull all the vac thru a seperate filter that I would use just for cleaning the vac channels before using the vac for hold down.

I have 200amp 3 phase service at the barn that will eventually be my shop at home, so I am seriously considering a "real" vacuum blower. Though the tinkerer in me keeps getting pulled to a 9-15 type setup...

-howie

[beacon14]

Howie, not sure I totally understand how and why you want to pull vacuum through the T-tracks. The dust collector will not make a suitable motor for vac holddown as it won't generate much suction with the airflow restricted.

The spoilboard is a major key to successful results with a low-vac high-flow system. Open holes or slots like I think you are describing will allow too much air to flow through when cutting through the workpiece. The unbroken spoilboard allows airflow but also restricts the airflow so vacuum is preserved even as leaks develope by cutting through the workpiece.

[3imoh]

Hi David-

I wasn't planning on using a DC as the vac source, I was just thinking it might be good to put a small cyclone in front of a vacuum if I was going to have a system that might allow a lot of chips through it. Not sure how that would impact the vacuum, though.

My main motivation for this and what I think makes it unique is that I would like a ton of t-track in the table. It would be great to have t-tracks about every 6" or so (in y) running along the x direction.

When I started thinking about how that much extruded t-track would be breaking up the vacuum area (not to mention the cost to do a 5x10 table), I started thinking about making my own tracks using a combination of routed BB ply and aluminum bar as a base table where the vac could pull through. The top of the tracks would end up being 1.5 to 2in below the top of the table if I decided to top it with ply/MDF.

Each zone would have a perimeter channel cut for vac gasketing. I was planning on using a spoilboard or bradyvac style board over this for when I ran the vac system. I do all sorts of things with the bot, and I really want to have something adaptable to most any situation.

It is still on the drawing board and I probably won't mess with it much until I get the shop built where I have 3phase. I may end up building a small 4x4 version running off a fein to test the concept.

The basic idea is a zoned table with a bunch of deep channels in it. Wherever you wanted on any channel you could drop a long t-bolt in it, twist it (almost) 90 degrees and use it as a hold-down, if you wanted to use it as a vac table, you would put a spoilboard/bradyvac on top of it and cut away. If you really needed to, you could also drywall screw into the top layer of MDF in between the channels as this layer could be surfaced down to the t-tracks over time like a regular spoilboard.

Hope this makes more sense...I should try to draw this up at some point.

-Howie

[gundog]

Here is how I designed my vac table I got all of my inspiration from others on this site. I have 3 pieces of T track inside the 4x8 cutting area in the plenum material but not inside the vac zones instead it is between the zones. I also have the whole table perimeter surrounded with T track outside the 4x8 cutting area. I mounted 2x4’s on each side in the x direction planned to just under the plenum level between the rails and the plenum. The ends I used a piece of 2x4 and a piece of 4x4 bolted to the end leg brackets. I cut plastic and don’t use a spoil board because I do not do through cuts my parts are too small and the vac would not hold them.

If I did decide to use a spoil board I would not use a full sheet I would cut it to fit each zone that way the T track would be below the surface of the spoil board but you would still have access to the T track. I also think that would be better because if you wear one section out from cutting into it you would not have to replace the whole sheet. You could replace the worn section and zero your Z to your lowest spoil board section and surface that section to match the rest. By using this method you could still cut full sheets. I have made up some special clamp boards for the perimeter to hold things from moving latterly and the vac sucks the sheets flat. I made some cam type clamps from HDPE I clamp the boards on 2 sides then use the cam clamps on the other 2 sides to take any slack out the clamping with vacuum works great. It is nice to be able to turn off the vac and not worry about bumping the material and knocking things out of zero when changing bits, so far I am very happy with my table.

Mike

Here is a link to my table build.

http://www.talkshopbot.com/forum/messages/312/41126.html?1242436083

[hiposner]

thank you everybody for all your wisdom, it really is inspirational to be a part of such a great community! I hope my students get it some day! I am off to go backpacking. I am supposed to get my prs 48x96 in a week or so!

[beacon14]

Howie, that makes more sense. Sounds like a lot of work but if the system works it would be a great table.

[mikewallis]

As Hillel said, thank you all for your wisdom. Sorry for not replying earlier, I have a new born baby boy and no it's not the shopbot
.
I should have included my profile as mentioned. I have a 4x8" 2007 PRS standard with 4HP spindle. So far I've spent my extra time in the last few months getting familiar with the Bot and calibration (Shew!) As someone else stated "I'm having way to much fun".
I'm at the point to where I now need to focus in on my product line/production which starts with a hold down system. I have time as this is going to be added to our current services and time is not that critical.
Please bear with me as I'm new to CNC and vacuum systems and I'm still grasping the concepts of the vacuum hold down.
So if say a 12"x48"x1" piece of Cherry wood is planed but still has a little "teeder" on the table top will vacuum pull it down tight to the surface? I've never used vacuum so am not sure what to expect.
I like the idea of a bunch of t-track incorporated into the vacuum table but is it not necessary for solid planks using vacuum?
Another question: MDF seems to be the standard but I'm already seeing many draw backs to using it, mainly curling of the edges, why is a good quality ply wood not used in place of MFD.
Again I'm very "Green" so please bear with me.