Vacuum Table Issues

[ChrisR]

Background info: We got our Shopbot about a month ago, had the frame assembled in a week (missing/wrong part issues). We then put down our base table layer (3/4 finish ply) followed by screwing the vacuum plenum MDF where the instructions told us to, followed by cutting our vacuum grids and holes and planed it flat. We then used flooring glue to mount the MDF spoil board (which was also planed flat) and ran the plumbing for the vacuum.

Main problem: When we saw an older model demonstrated, it had an incredible amount of suction where the owner placed a 12x12 sheet of ply and we couldn't lift it. On ours, while we can't shift the sheets in the x and y direction, we CAN left the board (4x8 full sheet) off the table. While we thought this would be ok as it would allow us to remove finished pieces easier its proving problematic as the half inch birch finish ply we are using isn't perfectly flat and standing off the table.

There's been numerous discussions as the the possible cause... 1. the plumbing is constricting the air flow 2. Too much glue was used 3. We didn't use the right MDF or should have used a ULDF. Short of planing off the whole spoil boardI'm at a loss.

I'm currently stuck between the boss, warehouse manager, and machine maintenance guy (first two have metal millwork and CNC experience) and I was kinda put into this because I am the most knowledgeable with computers and design work where I am.

Any thoughts and help would be greatly appreciated.

[srwtlc]

Since you screwed the plenum layer to the base (should have glued it instead), did you first seal the back face and edges? You could be loosing a lot of vac from between the two if you didn't seal it first. You'll also get more movement over humidity changes that will cause even more surface variation from screwing this layer down. Did you seal the plenum also to keep bleed between zones? Sounds like you have a fair amount of vac loss going on. What are you using for a vac source.

[Kyle Stapleton]

What are you using for a vacuum?

[Kyle Stapleton]

You beat me

[jTr]

Chris,

Short-term first-aid without further details on your vac source:

1> Be certain to put convex face up (Of course, you knew that I'll bet!)

2> Use 1.5" wide masking tape around edges - Fire up your vac, climb on that sheet to flatten, then apply tape as an edge seal. You'd be surprised how much this helps, and the masking tape wont damage your bit. As cutting takes place, the theory is the tensions will be released by the kerfs and the sheet will relax allowing the smaller pieces to lay flatter and get a better grip on the table. In a pinch, this may get you through the job at hand.

By the way, your description of no slide but easy lift of the ply is exactly how my system performs. Just have to avoid the junk plys - any less than $50 a sheet typically spells disaster.

Qualifier:Baltic Birch has never needed any of the above remedies on my table (4 lighthouse motors for vac source), so it seems likely you have a significant leak somewhere.

Good luck,

jeff

[jTr]

Oh - and don't lift finished pieces off the table until part cutting is complete. As each piece is removed, the vacuum seal is being opened, weakening the hold on remaining pieces being cut. (It's also not recommended from a safety standpoint).

jeff

[ChrisR]

Sorry for the late reply, was super busy at the job last half of last week and when I go home, everything work related gets put out of my head. I'll reply in order,
@ Scott, We applied 3x coats of poly after we screwed the board down (as the instruction manual told us to) but next time we're definitely looking into using a gasket where ever possible. As for the humidity, they set it up in the warehouse and the weather in Myrtle Beach is all but dry. Hopefully when they actually build the room for this and possibly another one, it will be climate controlled. .

@ Scott and Kyle, it's the 16.9 regenerative vacuum I've seen in other peoples posts when I searched about this.

@ Jeff Thanks for the idea about using the tape as an edge seal. When we went to the local technical school to look at one before we bought it, the instructor had siliconed the whole edge of this boards. As for the quality of what we're doing, we're getting it from their local 84 lumber and was just told it was C2 Birch ply? (I'm not much of a woodworking guy.... yet, I just happened to be a fast learner on computers) Thanks for assuring me this is the way it's supposed to be with the lift as well. We're doing monogrammed letters with it and have to tab everything.

Now is the fun part where we're learning how long a bit lasts. We're using Centurion compression bits .25 inch, doing .5 inch full depth passes at 1.25 ips and 12000 rpm. All seems to be running good, nice clean edges There's a decent amount of length in each of these pieces as well as lots of small corners. So far we're getting about 5 sheets per bit before we have to change.