Looking to build more stable table

[EricSchimel]

Hey all! Right now on my 4x8 PRS I've got a pretty standard table setup:

Bottom layer is plywood, middle layer is sealed MDF with plenum routed into it and the top layer is LDF (Trupan). All three layers are glued together.

This setup has worked for several years got all of the plywood I've been cutting. Recently I've picked up some regular production work that's all in plastic. One job in particular requires very accurate pocket depths.

What I've been finding is that the Trupan moves daily, I'm not all that surprised by this.

I'm exploring ideas for a new table. I've priced out aluminum extrusion which I'm sure would work, but it's a LOT of money.

I've also looked at making my bottom layer and my plenum out of Extira. From everything I've read it seems to be really stable. My thinking right now is that I do that, and instead of glueing down my LDF I leave it loose and only put it on when needed. (I could keep it from sliding with some pins)

For the rest of my plastic cutting I have jigs that I could place (and vacuum down to) the plenum directly.

So the question(s) are: Has anyone else fooled around with building a really stable table? What materials have you tried?

[coryatjohn]

The cheapest and most stable material would be plate steel. Steel is literally an order of magnitude cheaper than aluminum. It's also stronger, stiffer, and has a lower coefficient of expansion. The negative is that a 4x8 sheet of 1/4" plate would be so freakin heavy that you'd need a forklift to move it around.

[MaxFrenzy]

Hey Eric,

So, the plywood base and LDF are going to be more susceptible to changes in humidity over the sealed plenum, which you've noticed. However, a lot of the success for these accurate depths are going to be dependent on making sure your vacuum is holding your plastic securely/evenly throughout the area you want to pocket. With plastics not being porous, I've always had good success holding them down with the bleeder & blower style vac tables. My question is if you feel like you have good hold down, and you zero to the top of your material and run a pocket, what sort of deviation are you getting when you measure depth of one area to another? Have you tried testing your z-zero on several spots of the piece before cutting? One thing to consider is that the thickness of the material may not be consistent throughout. Spindle tram is another, but I'm assuming you've covered that. I've had production runs like this before where the depth was important. Ideally, you'd be able to skip re-zeroing in between cuts since the work piece thickness SHOULD be identical, but in many cases isn't. In one instance, I found that re-zeroing to the top in between pieces gave me better consistency. It's something you could try. Overall though, in the best situations, I could keep around 5 to 10 thou difference between two pockets that were a foot or two apart on certain pieces but other times, it could be 40 or 50.

[EricSchimel]

So I've been far down the road on this...

Firstly here's a shot of the jig:

PXL_20201005_175937025.jpg

Basically it's a jig that's made out of HDPE. I've run gasket around where the part goes and I've got a high pressure pump that sucks the rough cut plastic down to the jig. That part works awesome.

To make the jig I had to perfectly flatten my table. I even ran a dial indicator all over the table to make sure it was as dead flat as I could get it.

Then I probed the sheet of HDPE and it wasn't totally flat, close, but not perfect. So then I shaved just a bit off the top of the HDPE. I then cut in my gasket channels and installed the pins.

I have a probing setup that I run to put the jig down on the table and locate it in XYZ (And rotation). I also dial indicate it to make sure it's flat.

This all works, however every day/half a day it all goes to hell and I need to resurface the table.

Essentially I have this whole jig sitting on top of what's effectively a sponge.

If I can put the jig directly on the plenum I think I'll be much better off. the thing I want to be sure of is whatever I made the base layer/plenum out of is as stable as possible.

Hopefully that puts a little more clarity on it...

[coryatjohn]

I did a job that required cutting thousands of plastic parts (acrylic) to very close tolerances. One thing I noticed is that the wear on the end mill was the biggest contributor of error in my machine. Something to consider.

[EricSchimel]

Tool length is measured at the start of each job, sometimes twice daily so that's not an issue here. Side wear of tools has already been accounted for and solved on this job.

The issue is the the table moves throughout the day, sometimes it's higher in on spot, lower in the other, etc.

A few hours later you can probe it again and there's a chance things will be different.

I'm really looking to stabilize the table so I'm not constantly chasing down Z tolerance issues.

[dlcw]

Eric, another thing to think about is setting Z-zero to your table bed instead of the material. This way, if/when, the material varies in thickness, it won't be an issue as all depths are cut based on the table, not the material. This is how eCabinets works. This way you take out the variable of material thickness. It seems to work well with mortise and tenon joints for my cabinets. Just my opinion....

[EricSchimel]

Yep, I already do that. I have an inductive zero plate bolted to the metal frame of the ShopBot. I also have a Z prox switch at the top. All zero measures are done off of that plate and referenced to the Z prox switch at the top for maximum accuracy. The cap on the Z prox switch is made out of hard plastic so that regardless of the geometry of the bit I get an accurate reading.

While nothing is perfect, the big fluctuating variable is the table.

[waynelocke]

I am getting ready to use 3/4" phenolic for my plenum. You might look into that. I think that it is pretty stable.

[coryatjohn]

Question: Is your vacuum pump under your table? If so, that could explain the changes you're seeing. Vacuum pumps throw off a lot of heat and if that heat is hitting the underside of your table, especially on one side or end, it's going to twist the entire machine.