Vacuum Hold Down Calculations

[kivimagi]

I'm interested in adding a vaccuum hold down to my shopbot table. I'll be using a similar grid as the shopbot 4 zone part file, but modified to hold my pieces, which are small in height, but long in width.

I want to hold down a 1/4" x 5" x 96" piece of oak, so I can cut parts out of it with an upcut spiral bit.

I'm curious how I could do the calculation to determine if the area underneath my piece, coupled with the vacuum source I have will be enough to withstand the cutting forces.

Thanks

[Brady Watson]

Ryan,
Generally speaking, 29.9 inches of mercury on your vacuum is equal to 1 atmosphere or about 14 pounds of force per square inch.

I don't believe that you need any sophisticated setup to hold down the parts you described. I often make an accessory vacuum table for each part that I do (provided that I am doing volume quantities) I make them out of whatever I have at the shop, usually 1/4" luan ($10/sheet) and some 2X lumber to create a plenum (sandwich 2X lumber with luan). I use silicone to seal the box. On one end I machine a hole for my Rigid shop vac hose to plug in, and on the top I machine a grid about 1/4" to 1/2" smaller in all directions than the part going on top. I then place 3M adhesive-backed neoprene foam to the extents of the part. If there is going to be excessive side force on the piece, I put wooden stops so that it doesn't shift around.

I have used this design to do several different products. It worked very well for reproduction 3" X 96" 3D trim molding. Be sure that the inside of the vac box has supports and that there are holes in it for the vac to go through. If you don't properly support the top luan skin, it will flex under suction.

Hope that helps...don't underestimate the power of a shopvac...You don't need a vacuum pump $$$ to make it work well. The shopvac is perfect because it has the CFM to overcome leaks and it gets the vacuum plenum evacuated and sucking almost instantaneously.

-Brady

[greg_russell]

Brady,

When you are machining the grid, how wide and deep are you machining the grooves? Given that you are using 1/4 material I presume that the groves are pretty shallow?

Are you using a standard 3M foam, like what you would use to seal a door, or is it something else?

Thanks,

Greg

[ron brown]

"1/4" x 5" x 96" piece of oak, so I can cut parts out of it with an upcut spiral bit."

This could be opening a can of worms. Brady is right on the ability to use low-power vacuum and hold parts well. "Up-Spiral" bits tend to lift parts. It is difficult to keep vacuum on parts as pieces are cut out as the vacuum will be lost as channels are opened.

If the parts are all identical I might try using a vacuum mask to hold each part individually and use a stratigy having a gentle release cut. I would be trying down-spiral or straight cut bits to avoid pulling up the small parts. It may be the pieces are just too small to hold with low vacuum and/or too "leaky" to hold with high vacuum/low volume pumps.

The material shown in this thread http://www.talkshopbot.com/forum/mes...3971#POST18579 from "All Star" might work well in this application.

Ron

[jthelen]

http://www.allstaradhesives.com/
This link is in the other chain too.

Go to this web site and look at the gallery tab. The parts cut on these spoil boards are pretty small. The trick is to keep the vacuum sealed under the parts. Look at his gallery pictures and you will see what I mean. I have used this product and it works great with my Fein shop vac as a vacuum pump. John at All star is very knowledgeable and helped me a lot when I designed my vacuum system. Give him a call and tell him what you are trying to do, he will get you setup.

[kivimagi]

Ron,

I'm currently using a down-spiral cutter, and I'm able to cut the parts without a vacuum, but the edge quality is poor. I'm using an "onion-skin" method to keep the parts together, and all the chips end up wedge in at the bottom, as is expected.

I'm hoping to clean up this edge with a vacuum hold down, and an up-spiral bit. Maybe there is a better method. Tabs, Compression bits??

Regards,

[paco]

Upcut are, I believe, the best cutter for hard wood... though straight are quite acceptable too... tabs are essential from my experience; as for leaving a skin at the bottom, you'll need it quite thick... compression bits are for laminated material you want to be clean on both edge... interesting but expen$ive tool bit!! I don't know about your strategie but you might consider a quite non-agressive one; low feed, mid RPM, 1/2 diameter tool bit stepdown (multipass)...

[Brady Watson]

Ryan & Greg,
I often drill right through the 1/4" luan and just make a swiss cheese type grid of holes that fit inside of the part extents.

I would try just using a straight bit and see if you gain anything over the downcut.

-Brady

[davidallen]

I've used compression bits with good success. you want to cut into the spoilboard enough to limit the lift of the up cut portion but still give a good edge. most bits have the bottom 1/4" as up cut and the rest as down cut. so going 1/8 to 3/16 into the spoilboard should do the trick.

another option I've tried is to leave an "onion skin" then run the pieces through a drum sander to remove the skin. you'll lose some thickness to the piece but it'll be finish sanded and separated.

the All Star jig tape looks interesting. if anyone tries it let us know how it holds up to sawdust and chips.