Raising up a PRS Alpha Table

[bstern]

I have a PRS Alpha and am having Z Flex problems when cutting 3/4 Melamine in one pass.
I was told by shopbot that since I am only going to cut small panels that raising the table would fix the problem.

I need to raise the table about 5". (huge difference in Z flex). The Table is 96 X 48.

As this is critical, I don't mind spending a few extra dollars to make it right. I plan on using a pvc sheet for my vacuum plenum for stability.

Should I use full sheets of ply? Should they be Glued, Screwed or both?

Any help for the best design would be helpful.

Bob

[dubliner]

Just curious if youre using a router or spindle & what bit & speeds youre using too? Is the deflection in the x direction, y or both? As for a raised table I would make a removable box so that you could revert it when necessary, if that would be never, then six more sheets of 3/4 material would definitely make it a heavy vibrationless machine.

[bstern]

HSD 4hp, 7 ips.
Flex is in both X&Y and seems to be caused by the long lever when Z is fully extended.

You can see the the flex with minimum pressure put on by hand. pull the z up 4 inches and its solid as a rock.

All (work related) stuff is done on 3/4 to 1 1/2 sheet goods so raising will not be much of a issue.

[dubliner]

Thanks Bob, reason I asked is I have the same setup but havent started cutting yet & will store this in the memory banks, would slowing down IPS or multiple pass be impractical?

[bstern]

At this time I climb cut to an onion skin and then to the final pass in conventional. This works OK on rectangles but slows things when I could cut in a single pass on large parts.

Cutting small circles for rafix fittings I have to adjust the diameter of the hole. Works fine normally but if I need to rerun the sheet if say by final cut outs were not deep enough it cut further on the circles making them too large for the connectors.

I want to run at 8ips accurately. I guess this is kind of pushing the machine to its max. I think giving up some Z room for accuracy will be a good trade off for me.

[scottcox]

Bob,

You could build a torsion box from 2x6s and using your shopbot, surface one side of the box frame, then add your bottom ply layer with pre-drill bolt holes. Flip, surface the other side and add your ply / pvc spoiler. Have bolt holes in your table to match up and voila!

That's the two cent plan.

[mthomas]

I have used 2lb density EPS (Extruded Polystyrene aka styrofoam) sheets from a local EPS firm to temporarily, and sometimes almost permanently, raise my cutting surface. If there is a supplier (or even better, like in my case, and actual producer) you can get the sheets for a ridiculously low price and can request any thickness you want in 1/8" increments. I used to layer 1/4" backer or or melamine board on them to protect them, but they are surprisingly durable. I can't speak for your situation, but I can get a 48x48x12 solid block of virgin EPS for under $75 dollars picked up right from the dock. Even though it is a pretty good non-slip surface for the spoilboard, I usually sink (very lightly) a few coarse thread screws through the spoilboard into the EPS block just to make sure the board doesnt shift when cutting.

If you're worried about the EPS boards crushing under the weight, I can assure you that a 4 or 5 inch sheet of 2lb density EPS will not "squish" with 2 full sheets of mdf stacked on it. The weight is so distributed that the EPS weathers it just fine.

All in all, when you factor in the light weight, extremely low cost, and the fact that they are pretty darn flat right off the production line, you may want to consider them as a possible solution.

[henrik_o]

Aha, now I finally find this thread!

Yes, we will also make an adjustable height table for our bot. I got the 14" Z PRS, so this is a must.

I haven't figured out how to do it, but I am exploring using recycled dentist's chair bases. Since there's zero market for used dentist's chairs that are older than ~5 years they can be had for practically nothing. I have acquired a couple of units and will experiment with them when I get the time. I can say that they are extremely rigid, the only question is if two will be enough or if I need four. It needs to be rigid, but beyond that the less the better since every added unit means one more calibration to get level.

[bill.young]

Hey guys,

To add to Michael's foam idea, a stressed skin panel already has material laminated on both faces of the foam which will give the foam some protection and also give a surface to attach to. The faces are usually OSB but if there's a place that manufactures them in your area (there's one near me and I'm in the middle of nowhere!), you might be able to talk them into laminating a different face material onto one for you like plywood, if you don't like the OSB.

I've used a stressed skin panel as my table for years and have been very happy with it...I think they would work well as a table-raiser.

Bill