Ongoing Spoilboard and Z woes

[johnm]

Folks -

How often do you resurface your spoilboard, and for what reason?

We've had the 'bot about a year, and originally had two layers of 3/4" mdf... Doc Palumbo said that it would move around, and it did! So, off came the MDF and we then went with 3 layers of 1/2" ply, glued the entire sheets, including edges and screwed on 6" centers throughout on each layer, then added two layers of tempered 1/4" hardboard (masonite) glued without screws. While all of this was drying, we stacked the table with several sheets of plywood, dead elephants, dark matter, oceans, etc. to keep it evenly flat and weighted. When I initially surfaced the table, it took less than .010 to true it up. After I surfaced it, I applied two coats of shellac.

We often use an interim spoilboard under our cuts to minimize through cut damage to the table - its scrap 3mm hardboard. As a result, we don't have to resurface the main spoilboard that often. Crickets chirping, a happy camper... then.... nightfall...

About 3 months ago, we were doing something that was critical for Z work, so I resurfaced the table and it was out by .085 - I was very surprised (actually, I used a different word) So, got it flat, sealed it, went on our way...

Yesterday we were having trouble with the fit of some rabbets/dadoes and lo and behold... Rose figured that the table might be out. We did some checks, decided to resurface and this time we had to mill .095 to bring it true. What the ell is going on? Our metal table is a copy of Wayne Locke's (sp?) and the spoilboard is supported every 12 -18" or so, and the whole thing is sitting on a concrete slab. If any of y'all have seen .18 of some masonite, acting all nonchalant, yet out of place, I'd sure like to know about it.

While I guess humidity could be an issue, all of this has happened in a single season, and I find it hard to believe that 1 3/4" thick spoilboard out of plywood would move over 3/16".

Any ideas?

Thanks in advance...

John

[gerald_d]

John, you notched your steel cross-supports for an indexer, if I remember correctly? Reckon your steelwork is the culprit. The wood wouldn't be able to move if the steel were holding it firmly.

[paco]

I get my non treat MDF support/spoilboard to swell and shrink back within 24 hours (up to 0.03") with the vacuum... what word would you use about this one?!
I try to not bother about it but it does create problem with intricate and long pocketing toolpath and long 3D raster.
I haven't yet figure the right setup for me. I'm considering plastic/wood composite 3/4" sheet (about 75.00$ca) but cannot get Trupan here and can't suck through any thickness of MDF neither (vacuum bleed board).
I usually surface to get the top very flat (when it's gouged all around) or whenever it's not true.

[johnm]

Gerald -

Yes, I had wondered about table sag... But after a year? We are set up for an indexer with a well, but the corners all have gussets and the table is welded, not bolted together - a real trick to move it here, too...

I guess that the steel could be sagging - but Lord - at this point, given all of the table surfacing that's been done, the sag would have to be on the order of 1/2". Everything is running true and square otherwise - if the steel were sagging or deflecting even 1/8" the SB gantry would be running out of parallel, (paralell?... parrallelllllll?)and hanging up in travel, No?

Hmnnnnn....

John

[beacon14]

With all due respect John I think your plywood table construction could be part of the problem. 1/2" plywood can flex pretty easily, so glueing and screwing, even every 6 inches, probably resulted in a waffle pattern of tightly glued spots every 6 inches with the excess glue pooling up between the screws. Then even though you stacked lots of stuff on top of the masonite if you figure out the total weight of your stack divided by 4608 square inches you probably had less than 1/10 of a pound per square inch (based on 400 pounds total over 48" x 96"). Even with 4000 pounds you would have had less than 1 PSI. Also, gluing the masonite to one side of the plywood yielded an unbalanced construction - the masonite moves differently than the plywood, and without a similar layer on the bottom to balance the stresses movement is inevitable.

However, even with a more perfect table assembly, the fact is that both steel and wood move with changes in temperature and/or humidity, and move at different rates. I have come to the conclusion (my opinion, of course) that searching for a non-moving table is like searching for the Holy Grail. It just is not going to happen, short of maybe a solid block of granite resting on the concrete floor.

That said, your movement does seem excessive, more so than others with similar steel set-ups seem to be getting. Hence my suspicions about your wood construction. FWIW my table does not stay flat either, but rarely if ever moves more than .03"

[patricktoomey]

I got sick of my MDF plenum and base sheets swelling and contracting with changes in humidity so I'm switching to granite. It's supposed to get here tomorrow or the next day. I'm going to lay a 52 x 104 sheet of 1 1/4" granite on the steel table and then use 1/2" Corian cut with the waffle pattern for my plenum. I'll let you guys know how that works.

[gerald_d]

John, in this post (http://www.talkshopbot.com/forum/messages/27/15385.jpg) you can see what SB is offering as a cross-supports now - much stronger.
Our tables also have solid steel cross supports under about 1.5" of MDF. We can hear how the MDF and steel fight each other as the temperature changes. This fighting/slipping of the MDF on the steel loosens the bolts. Now the question is whether the table would stay flatter if the slip was allowed?.........

Patrick, how would you attach the corian, to the granite, to the steel? By the way, granite is 4x more flexible (young's modulus) than steel and slightly more flexible than glass. Maybe it is more stable with temperature changes?

[patricktoomey]

Gerald, I'm planning on beefing up the support steel with possibly more cross members to better support the granite. I'm going to have a pool table installer come in and help me shim an level it since he is experienced with getting slate to stay flat and dead level. Granite is very stable with temp changes and will not react to humidity. I'm thinking of bolting through the granite to the steel in one corner and then using an elastomeric construction adhesive at several other spots to allow for the different thermal expansion rates of the steel and the granite. I'm going to see if the pool table guy has any ideas there, I know he uses steel shims of different thicknesses to get the slates to stay flat. I'm going to bolt through the Corian to the Granite. I have calculated the maximum difference in the Corian to the Granite at 8 feet based on my temperature swings and find that they could theoretically move almost 1/4" relative to each other since the COE (coefficient of expansion) of Corian is much higher than the granite. I'll use a 1/4" round hole in the home corner of the table with a 1/4" bolt. Then on the far X side I'll make a 1/4" x 1/2" slot in the Corian so that the Corian can stretch in the X direction. In the corner opposite home in the Y direction I'll make a slot in the Corian that is 1/4" x 3/8". The corner diagonal from home will get a 1/2" x 1/2" hole. This should let the Corian expand and contract without trying to bend or get humps in it. My shop is now air conditioned so I actually should only get maybe .015" of expansion but I'll allow for maximum just to be safe.

Don't know if this will all work yet but it seems plausible. I'll let you know what happens.