[chmedly]

Seeing as the smaller the area involved, the less hold down force is applied... I'm looking for opinions about where the tipping point is. If my table is x" by x", is it worth it to build a DIY vacuum table or should I just use fixtures and/or plastic brads? This is for primarily woodworking with plywood products. Anything from 1/4" to 3/4" material. But some of it will be very fine work where the accuracy in the z-axis is critical so pulling bow out of the material is important.

[awerby]

On small things, a vacuum table isn't going to be much use, since (as you already know) they work better the more surface area there is to grab onto. You would be better off with a vacuum pod, which concentrates the vacuum into a small area. But don't expect it to take any warpage out of 3/4" material. That's something you can hold onto other ways. Where vacuum really helps is with much thinner stock, which is hard to hold onto well enough to keep it from pulling up into the cutter.

[ger21]

Seeing as the smaller the area involved, the less hold down force is applied... I'm looking for opinions about where the tipping point is.

The "tipping" point can vary greatly.
It's affected by material, sharpness and geometry of tool, feedrate, and depth of cut. And vacuum pump.

But some of it will be very fine work where the accuracy in the z-axis is critical so pulling bow out of the material is important.

If the top face of your material is convex, the vacuum can pull it down. But if it's concave, with the corners lifting, you'll have trouble getting the vacuum to pull it down, and it a lot of cases, it just won't work

[chmedly]

The "tipping" point can vary greatly.

Do you think a 32"x24" table size, used with 3/8" baltic birch and 1/8" endmills, is too small for a DIY vacuum table (lighthouse motor type of thing)?

If the top face of your material is convex, the vacuum can pull it down. But if it's concave, with the corners lifting, you'll have trouble getting the vacuum to pull it down, and it a lot of cases, it just won't work

I've worked with a 50"x100" table with a "real" vacuum pump so I have some ideas about what to expect. Generally the material would be placed with the top face convex as you have suggested.

[ger21]

Do you think a 32"x24" table size, used with 3/8" baltic birch and 1/8" endmills, is too small for a DIY vacuum table (lighthouse motor type of thing)?

No, but it depends on the size of your finished parts, and how much vacuum you actually have. But 1/8" tools don't apply a lot of force when cutting.

[wizard]

This is one of those questions best answered with "it depends"!!! Probably not the answer you want but an honest one.

When using any sort of vacuum retention or compression system the best you can get is 14PSI or so per square inch. On a 10 inch square surface you might be lucky if you get 1400 pounds of force. I say lucky for a number of reasons, first most of the commonly used vacuum pumps will not pump down to complete vacuum. Second the design of the chuck in most cases implies a lot of idle area, that is area the part rest against that doesn't really factor into the amount of force being applied. Then you have the issues of leaks.

Seeing as the smaller the area involved, the less hold down force is applied... I'm looking for opinions about where the tipping point is. If my table is x" by x", is it worth it to build a DIY vacuum table or should I just use fixtures and/or plastic brads? This is for primarily woodworking with plywood products. Anything from 1/4" to 3/4" material. But some of it will be very fine work where the accuracy in the z-axis is critical so pulling bow out of the material is important.

Pulling the warpage out of plywood is tricky. You might get a piece to clamp down but that induced stress will be constantly working against your vacuum chuck. This will reduce your confidence the the piece will stick.

A couple of other points. if you build a vacuum chuck don't rely upon it to prevent parts from sliding. Instead use fences or other solutions to keep the part in place. Fences can have a big impact as the vacuum doesn't have to resist the mechanical forces pushing the plywood.

[mactec54]

Seeing as the smaller the area involved, the less hold down force is applied... I'm looking for opinions about where the tipping point is. If my table is x" by x", is it worth it to build a DIY vacuum table or should I just use fixtures and/or plastic brads? This is for primarily woodworking with plywood products. Anything from 1/4" to 3/4" material. But some of it will be very fine work where the accuracy in the z-axis is critical so pulling bow out of the material is important.

Size of your table has nothing to do with it, it will depend on your vacuum pump you have as to how big your table will be

There is no such thing as too small, you make a pod to suit the job you are doing, below are some pods for you to look at, they come in all shapes and sizes, some are better than others, but this will give you some design idea's to use for your table

With pods you don't have to worry about your material being warped or twisted

You can also make them as thin as you need to suit your machine

Vacuumpods.com "Buy a Better Pod for Less..." Pads
CNC Vacuum Pods and Cups | Vacuum Pods by Tooltec - Tooling for the woodworking industry
VACUUM CLAMPING
Better Vacuum Cups, Inc. - Home of the Green Vacuum Cups
NEMI
http://advancedcncinnovations.com/

[JerryBurks]

If you are thinking of a porous spoilboard table to hold down larger sheets with a high power regen blower, I suspect you need several square feet to hold that down properly or maybe less when using only 1/8" bits as Ger21 mentioned. I would guess you may be able to flatten 1/4" sheets but probably not 3/4" . If you have a full sheet size machine that is of course all no problem.

For anything smaller a high vacuum/low volume pump with pods if the ticket. I use them all the time and clamp or use tabs only for odd shaped and extremely small parts. A double piston pump can get 25" or better of vacuum with a well sealing pod/gaskets and will hold (from my experience) with at least 5-10 square inch surface when cutting with an 1/8" bit (80-120 ipm). I need maybe 50-100 square inch surface to cut with 1/4" bits (200-300ipm). If you have fences and don't rely on the gasket friction you may even be able to cut smaller parts. But don't expect to flatten any sheet with such pods.

[ger21]

Whether or not you can flatten a sheet depends on how stiff it is, how much vacuum you have, and how porous it is.
And with pods, you need to be able to push it down flat and tight to all the pods at the same time. Sometimes it works, sometimes it doesn't.