Level cutting surface

[drodda]

I removed my spoil board down to just the 3/4" plywwod on the bottom that is bolted to the angle crossmembers.

I glued 3 sheets of 5' x 10' 3/4" MDF to the Plywood with titebond 2 Extend and clamped it and left it for two days.

Next I removed all the clamps and trued up the leading edges of the stack to give me some 0,0 lines. I surfaced the table next with a 1/2" bit. This all looked great when done. It was nice to have a fresh table top without all the scars. I went with the thickness to try and get rid of the table movement that I was getting.

Today I went out and cut my first part with the new table and the marks in the table were very inconsistant. So I cut a angled groove across the table and found it to be as much a .08" off in areas around the table. Does everyone have this same problem with the table changing or is it just a black hole located in the center of my shop? This is very frustrationg as I trued the table to do some inlay work and luckily I found the problem before I actually lost quality wood to the scrap pile.

What can I do to limit or reduce this problem? Is it that after the glue and removing the top skin of the table it swells or is there something else that I am not seeing. I am getting very frustrated with this as it seems to happen all the time in my shop which is heated and located in the dry winter air of Iowa.

Any advice would be greatly appreciated?

[Brady Watson]

Dave,
You may have compounded the problem with so many layers...Instead of one layer swelling/contracting, now you have several at the same time. Since you've got all of that meat there, you will want to surface in the morning before you do a critical part.

When it comes to parts that absolutely have to be perfect in the Z, I scale things back and avoid trying to do an entire 4X8 area. I'll do just a strip of parts. That way I know I can control/isolate Z fluctuations.

The other thing that pops into my head as I read your post is, how is the material to be cut being held down? What is the material being held down? It is not uncommon for the material to be ALL OVER THE PLACE for flatness...So you'll want to take a digital caliper and sample the material in random places, noting the deviation. Avoid using drywall screws, as it actually lifts the material & dimples the spoilboard. (Kreg screws are the best) If using vacuum, make sure your plenum is not bowing. At times I work with Celtec PVC in 1" thicknesses...it has varied in thickness as much as .11".

Rest assured you are not alone with this issue. Even the 'big boys' running the big iron have this problem, which is why some have an aluminum base plate instead of plywood. They also compensate for expansion as the room/machine heat up, changing their table base zero values to a higher value in the late AM/early PM for changes in height.

-B

[drodda]

Thanks Brady,

The only problem is that I am building large card tables and table toppers. This prevents me from doing samll parts at a time since it is a very large piece of plywood that is getting cut into table shapes and then other pieces are getting inlaid into the hardwood that is being glued to the plywood. Then the Bot planes it all to a even surface. At least that is how it is supposed to work.

Where would you get a 120" x 60" piece of 1" aluminum to use as a base for the spoil board?

[Brady Watson]

Dave,
1/2" aluminum is adequate. You should be able to find this through your local metal supplier. You can epoxy your MDF right to the AL. Inspect the AL sheet before you mount it to make sure it is flat. You will also want to invest in a real machinist's level. I think grizzly sells decent ones for not a lot of $. Get that table dead level.

Since you already have the layers of MDF on the table, I would be interesting in knowing how you make out doing the following:

1) Surface entire top flat again using a large surfacing cutter (1.25" or larger). Are there any ridges? If so, adjust Z axis. A properly adjusted Z will show ZERO ridges (you should not be able to catch your fingernail on stepover marks AT ALL)

2) Do a C2 & zero that surfacing bit (after cutting SB flat) at 6,6. Tell the machine MZ, .122 (or the EXACT thickness of your ZZero plate). Then slide plate under bit. It should be a snug fit. If not, adjust for it in my_variables.sbc and redo the routine.

3) Move tool to several locations on the table, leaving the Z @ your plate thickness. (You may find it helpful to ZZ the Z after it is at the perfect plate height) Use the plate as a feeler and feel for tight/not fitting or loose areas. Write those areas down (record X,Y coordinates reading off of SB3 DRO)

4) Analyze the data. Where is the greatest deviation in Z occuring? Do you find that the plate is tight @ Y30 and not so at Y2 or Y58 ? If this is the case, then the center of the X car is sagging. This is not so much an issue on a 4' tool, but the 5' tools can sag a little. Contact me offlist for a remedy...after you have flattened the spoilboard again & taken Z samples.

-B

[paco]

My two 1 inch MDF support board and spoil board assembly is cupping upward to as much as 0.03" within 3 hours with sheet goods (whatever it is) with one Fein T3 vacuum... and cupped back down within about the same amount of time.

Talking about frustration?!

The support board part is bolted every 12 inches or so. The spoil board is screwed every 8 inches or so and sealed at perimeter. They are laid down on 5 3/16" X 2" X 2" and 2 3/16" X 2" X 3" angles.

Frustrated I am a bit...!

I'm about to beef up the support angle with replacements 1/4" X 2" X 3" all around... but I'm not 100% sure if it'll help at all!?!?!?!?!

I've considered AL support board... but Ooouuch on the bill!... and will it help... at all???

I thought about sealing the whole thing with what ever would keep it from breathing... but will I be willing to do this every time I need to surface?!?!?!??

So far, I don't clearly understand what going on and that what has prevent me from trying "too much".

I don't have any of that problem if I don't use the vacuum but I just love to see my goods being held under pressure!

Dave, I'm with you... but I don't have any real trick to give you. Sorry.

I do am partly relief that I'm not alone though!

[drodda]

Brady,

I have battled this for over a year now and have always resurfaced before doing critical work. I just resurfaced my table tonight. I do get small lines with a 1.25" bit during the resurface. I have lived with these from day one since shopbot has not found it in there hearts to put a very simple Z adjustment plate on the setup. This might cost them all of $5.00 in materials. I have bought all the parts to fine tune the Z but have not found a break in production to make this modification. Now that I have walked away from the day job I tend to try to keep the bot moving as my family likes to eat. I own a survey level and this is what I leveled my bot with during assembly. I will check the table tomorrow but for now I am cutting my boards to get this table out. I will check into the grizzly machinest level though.

Will the new bot carrige help with this at all on the sagging or is it the same problem with it too.

Thanks Paco sometimes a pat on the back and to know your not the only one is help in itself.

I will contact you offline later Brady as I have a couple ideas to run by you anyhow. You know the secret kind!:-)

[Brady Watson]

Dave,
The *new* PRS gantry is much stiffer than previous versions...There are a few tricks you can do on the gantry to make things better.

If I surface the spoilboard before doing a job, the cut depths are very consistent...but the majority of the materials that I cut are all over the place.

Yes the 1/2" AL is very expensive at 1st glace...but I think that it will help tame that table and keep it flat. It really ties all of the metal Xmembers together getting them on an even plane & keeping them there. Before you go the AL route, you should stiffen up that gantry.

-B

[richards]

Dave,
Here's a link (http://www.talkshopbot.com/forum/show.cgi?tpc=7&post=40868#POST40868) to another post on the forum that shows how I square up the Z-axis.

I've used that dial indicator several time to align the Z-axis with excellent results each time. However, with the extreme range of temperatures that I face in the shop, the table surface can easily shift throughout the day. (Right now at 7:00 a.m., it's below zero F. in the shop. Hopefully later on in the day I'll get the temperature up to at least the mid 40's. But, not everything heats and cools at the same rate, so shifting goes on constantly.)

[conceptmachine]

Has anyone tried using bosch or 8020 extrusions for a table base?it might be less expesive than a solid sheet of aluminum i havnt priced any so i cant compare prices.
--- shawn

[cnc_works]

Just as an addendum to Brady's suggestion of the aluminum, I was fortunate enough to find an inexpensive 4' X 12' sheet of 1/2" aluminum when I was setting up my PRT here in Portland. I cut it down (anyone need a 4' X 4' piece?) and bolted it to the cross members, then bolted a sheet of MDF to that, then screwed on a second sheet of MDF on which I can replace as needed. My table seems to stay very flat, only needing surfacing because of cutting scars.

I'm not sure, but it just occurred to me that my hold down setup could contribute to the stability. Instead of vacuum (I do very little sheet goods) I have installed a 2" square pattern of T-nuts in the bottom MDF sheet (only about 1100 of them) and cut matching 3/8" holes in the top sheet. I wonder if all that surface area of exposed MDF might help to hold an even expansion and contraction over the entire sheet. Anyway, works for me.

Donn

[drodda]

Update:

Perhaps it was that once I removed the skin off the top layer of MDF it went wild. Today I started the day off with a table resurfacing just to check the table and it was very close if not perfect from the surfacing Yesterday. So I proceeded to finish my project. I would still like to make these things more acurate for the table staying level but am out of panic mode for the time being.

Mike,
While it is -1 outside here right now it is a toasty 70 degrees all the time inside the shop. So I don't think the temperature changing is the problem with my table fluctuations. Sometimes I even turn it down to keep the sweat down.

I am still going to get my Z adjusted better but can live with it again for a week or two as I need to get the next project started.

Once again stanks for all the help,

Dave

[charles_o]

I originally built this t-slot table
8542so that I could quickly change jigs and fixtures. I since have added a six zone vacuum plenum made from mdf and sealed with a sanding sealer. The t-slot stays true but the vacuum plenum does vary. I deal with flatness issues by flatening the bleeder board and since I can cut off all zones but those needed, I can flatten a small bleeder if I don't need the full 4x8 table.

8543

[andyb]

Brady,
Thanks for the tip on using pocket hole screws instead of drywall screws. I just finished cutting out MDF cabinet doors and the pocket hole screw helped me get the MDF flat. I've always had a problem with the material wanting to raise up around where the drywall screws were, but the pocket hole screws actually pulled the material down flat to the table.. The dimples are gone too. I'm working on a vacuum table but until I have time to get it setup I'll keep using the pocket hole screws. Thanks again for the tip.

Andy B.

[Brady Watson]

Andy,
No problem. Those are the best screws I have found for our application. Unlike the Kreg brand, http://www.mcfeelys.com offers washer head square drive coarse thread screws from 1" to 3". You want the coarse thread because it really digs in better than the fine thread. When you go to McFeelys, type 'pocket hole' into the search box. They are about the same price as the Kreg ones, but you can buy in bulk...and as you probably already know, they are worth more than the price of the screw...I've never broken one of these & my material never lifts off of the spoilboard when I tighten them.

-B