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InsaneEPP
10-13-2004, 03:21 PM
I'm in the planning stage of a CNC router. If you are using a vacuum clamping system, your workpiece is directly on top of the vacuum surface, correct? If you need to cut a piece completely free of the table, how do you do it? I have read that you can cut almost through and then sand through the rest, but what if you are cutting aluminum? Seems like vacuum clamping is not an end all and that t-slots may still be needed. How are others holding down their work pieces?

Also, would it be advantageous at all to have a surface that was replaceable?

Thanks much!

Andreas

buscht
10-13-2004, 04:16 PM
Andreas, vacuum clamping will not work in all instances, but is much better than mechanical clamping on large parts.

Keep in mind that vacuum clamping relies on atmospheric pressure to work. This is about 12.7 lbs per sq inch. If your part is 1" x 1" then a vacuum clamped part only has 12.7 lbs holding it in place. This probably won't work. But if your part is 12" x 12" then you have 144 sq inches x 12.7 lbs = 1828 lbs hold down pressure. Very impressive!

Most vacuum clamping setups use a spoil board between the tabletop and your workpiece. This is usally a piece of MDF.

Draw your part in CAD. Use this picture to make some grooves offset from the outside of the part by 3/4". Drill some holes through the MDF. Clamp the MDF to your table and you now have a proper spoilboard setup.

There are many other ways to do this and I could make a picture if you need it.

Back to your aluminum question. If the parts are big enough or the raw stock is big enough, you can use the vacuum to hold the part and waste material. Or you do as you mentioned, don't quite cut all the way through. In aluminum, leave .003". Then use a tabbing routing where you cut this away, leaving small tabs that hold the part in place. Release the vacuum and cut the part out with an Exacto knife.

Alot of professional tables has Tee slots and vacuum, so you are covered no matter how you choose to cut your parts.

BigDaddyG
10-13-2004, 04:22 PM
InsaneEPP,
If I knew more about your final product it would help greatly, but that said. A "sacrificial plate" does not alway work well with vacuum clamping, but I have seen it done. As for cutting Aluminum, on my Haas, even with conventional clamping, I use over size stock (if my finish is to be 1/2", I machine out of 5/8" stock, turn over and flycut the size to the finish dim). This process is much quicker because I spend no time on set-ups, just machine my part, outside edges etc, then when I fly cut, the part is square, etc..
Hope this helps,
Glen

InsaneEPP
10-13-2004, 04:32 PM
buscht, I sort of have an idea of what you're saying, but a picture would help oh so much. Does the vacuum get pulled through the holes you drill in the MDF? I assume the 3/4" offset is inside of the part? Will the part seal well to just the MDF or do you need gasketing material?

Rekd
10-13-2004, 04:41 PM
Andreas,

I do covers for microwave housings all the time on vacuum. I run down to .002 or less from the fixture so as to not break the seal, (alum parts, alum fixture), then just tear them apart after I take them off the machine. It's like alum foil.

The fixture has to be real flat, with a fairly accurate machine. Also, I run strictly HAAS', and thermal expansion must be monitored throughout the day: as the spindle/ambiant temp goes up, the tool/spindle/head assembly expands, moving the tool down and will (at least on my machines) move as much as .002 or more depending on the temp swing..

buscht
10-13-2004, 05:49 PM
Here's a simple picture.
The outer black line represents your raw stock.
The smaller rectangle is the finished part. The large circle represents a hole in the part.
I drew a .5" offset around the part to represent the outside edge of a router bit.

The (red)purple lines represent the dado grooves in the MDF spoil board. Remember, you need maximum surface area for better holding. So the more grooves the better. Except the air pressure can squish and break a part without adequate back up.

Notice that the vaccuum grooves are on the waste material and the good part. But in no instance do they cross the router path or go into the area of the hole. Any cross over will cause a vacuum leak and the part will release.

I am showing holes in the MDF board in the dado grooves. These are through holes which allow the vacuum to pass through the router table up to the grooves which hold the part in place.

If the back of your parts are flat and smooth, you probably don't need any gasketing. Just press the part down with your hand until the vacuum gains enough pressure to hold the part in place.

The MDF board can be held down by the same vacuum to the table, or bolt it in place in a spot where you won't cut it with the router.

There is a little bit of work to make these spoil boards so they might not be the best for setups of one piece, but if you want to make dozens of parts, they are the way to go.

ger21
10-13-2004, 07:41 PM
Remember that vacuum can bleed throught the edges of MDF, so sealing the edges might be necessary

InsaneEPP
10-13-2004, 08:42 PM
These are all good replies. I thank all of you, especially buscht for putting a picture up for me. Apparently there are at least a few ways of doing this. Can I ask what is the typical spacing for the holes in the table? Are there usually many holes at perhaps every square inch, or less at 4 per square foot? Seems like you'd want many holes if you were vacuuming directly to the piece, but perhaps less if you were going the templated route. I would think it would be slightly easier to plug less holes.

buscht
10-14-2004, 09:00 AM
Andreas, What I drew was a picture of a custom made spoil board for a specific application.
Alot of people use a grid system. Grooves are 1/4" wide x 3/16" deep and spaced ona square grid, roughly 2" apart.
You would use a 1/4" dia gasket material for the seal and to seperate vacuum chambers. The gasket looks like a rope made of rubber.
Anyways, you set this up so that you never route into the spoil board more than 1/16". This insures that you will never mess up your vacuum grooves.
Run your program without a part on the table. This will cause a1/16" cut into the spoilboard. Use these marks as a guide on where to place the gasket material.

The hole through the board or table are simply a way to get the vacuum to the part.

A common misconception that more holes give you a better vacuum. More holes will allow the vacuum to come up to full pressure quicker, but not any stronger. Remember, its sq inches that count. Your vacuum grooves create the sq inches.

You need more holes in the table to make your setup more flexible. You stand a better chance of hitting a hole.

Most people keep all the holes plugged at all times with either a screw, cork, or test tube stopper. If you only open up the holes you need and recover them when you are done, its not too bad.

This setup is proven in industrial setups every day and is one of my favorites.

I agree with sealing the edge of the MDF.

There is another type of vacuum that is used for flat panel furniture manufacturing. It uses a very powerful low pressure high volume vacuum that actually will suck through the porosity of the MDF spoilboard. No grooves or anything is needed.

The problem with this is that it's very expensive $10,000+ and you need large smooth parts to get it to work well. 2ft x 2ft works pretty good.

dmgdesigns
10-15-2004, 09:08 AM
Great question Andreas. I was about to post the same question.

I am in getting close to completion myself. I am waiting for parts to be machined. I will have a 66" X 39" table with about 55" X 32" travel. Most of my products will be made from 1/4 to 1" MDF and Oak sheet material. These products will ranging in size from 2"X3" to 9"X15" of varying designs.

My thought is to make a 1 1/2" thick MDF table top to cover the AL top I have now. Mill a 1/4" deep inset the full travel of my machine. Then mill 1/4" wide by 1/2" deep groves below this inset. These groves lead to a manifold for the vacuum line. The 1/4" thick MDF interchangeable top would fit in the inset. It would have 1/4" holes drilled along the groves except for the tool path of the specific part being milled. The area of the groves will be approximately 173 sqin in volume. Check out pictures.

I will have a gasket between the Vacuum table and the removable tops. Would it be necessary to have a gasket between the top and sheet goods? I am thinking yes, maybe rolled gasket material glued to the top. Might be a little expensive though. Also what kind of pump would you suggest? I am thinking of something like a vacuum from a dust collection system but not sure. How big a pump would you suggest?

Thanks for your help.

ger21
10-15-2004, 09:36 AM
My thought is to make a 1 1/2" thick MDF table top to cover the AL top I have now. Mill a 1/4" deep inset the full travel of my machine. Then mill 1/4" wide by 1/2" deep groves below this inset. These groves lead to a manifold for the vacuum line. The 1/4" thick MDF interchangeable top would fit in the inset. It would have 1/4" holes drilled along the groves except for the tool path of the specific part being milled. The area of the groves will be approximately 173 sqin in volume. Check out pictures.

I will have a gasket between the Vacuum table and the removable tops. Would it be necessary to have a gasket between the top and sheet goods? I am thinking yes, maybe rolled gasket material glued to the top. Might be a little expensive though. Also what kind of pump would you suggest? I am thinking of something like a vacuum from a dust collection system but not sure. How big a pump would you suggest?

Thanks for your help.

If your going to be making basically removable vacuum jigs, don't use 1/4". Use 3/4", and don't drill all the 1/4" holes. Ideally you want to route a groove around the perimeter of your part, and then some additional grooves under the part to give you more vacuum area. You only need one hole to your grooves below to let the vacuum pull. Use a gasket material in the perimeter groove. A good inexpensive source is here: http://www.allstaradhesives.com/

Or, they sell a 1/32" self adhesive gasket material for quick jigs.

Are you saying to basically hook up a dust collector for a vacuum? I'm not sure if it will pull hard enough. I've heard a lot of people using shop vacs, but you have to provide an air bypass to prevent from burning up the motor.

There is a lot of information in the forums on ShopBot's website about vacuum holddowns. You might also take a look at this page.

http://skalarcnc.netfirms.com/pods.html

buscht
10-15-2004, 09:44 AM
Mark, I'm not sure who your question is addressed to, but I'll stick my nose in.

There are only two reasons that you need a big vacuum pump.

1. Air leakage
2. To shorten the time it takes to obtain full pressure.

If you have a good seal and can wait a minute or two for pressure to build, a small pump is all you need.

I can guarantee that you will have leakage though, if nothing else, right through the MDF pores.

Check out this site for relatively cheap vacuum pumps http://www.joewoodworker.com/veneering/makingbags.htm

Now, on to your design.

Its absolutely wonderful, in my opinion. But you are making some mistakes in applying the vacuum.

1. No real problem with your 1-1/2" MDF top idea. I like the pocket idea for dropping in the spoil boards. A few suggestions.

a. Cross hatch your grooves, the more surface area, the better. The little squares will still support the 1/4" MDF ok.

b. You don't need to go that thick on the MDF 3/4" would be adequate, especially if you seal it.

c. Consider dropping the pocket idea. Its alot of work to mill out, will collect dust big time, and be hard to clean. Two dowels along the back, and one along the side will allow you locate the 1/4" spoil boards just fine and the vacuum will hold everything in place.

d. Make a groove around the outside to hold your gasket material. Have the gasket material stick up about 1/16" to 1/8". The groove allows the gasket material to compress flat and it won't cause a bulge in the 1/4" mdf

e. Use your machine to surface the 1-1/2" MDF perfectly flat to your router.

2. The biggest design problem is with your 1/4" MDF spoil boards. A bunch of holes will not give you enough surface area to hold any part. You will need to create grooves in the top of the MDF, and just have through holes to get the vacuum to pass through sheet. I know this from years of experience. Do a calculation; A 1/4" hole has only a .049 sq in surface area, 100 of these holes is 4.9 sq inches. That's only 62 lbs.

3. You may or may not need gasketing on top of the 1/4" spoilboards. Its all about how smooth your raw parts are. Oak is usually grainy and hard to get a good seal. If your parts are warped, you either need to push them down or use gasketing. I have had luck with using masking tape around the perimeter to get a seal. It just figured in the cost of the parts.

No matter what you need to do here, its not a big deal. You have a good design that will work. Experimenting with the 1/4" spoilboards isn't too expensive or time consuming.

A couple of other points. I don't like using a dust collection vacuum as it stays on all the time. I'm afraid of it overheating, plus its noisy. The vaccuum bag pressing pumps have automatic shutoffs, that once the proper pressure is acheived the pump shuts off.

A cheap gasket material can be found at most hardware stores. Its some kind of window/door seal. It looks like foam rubber with adhesive backing. about 1/4" wide x 1/8" high.

Good luck.
Trent

buscht
10-15-2004, 09:53 AM
Mark, Gerry must have posted while I was typing.

Most people do use 3/4" MDF spoilboards and there are good reasons for it.

wood is porous, the vacuum will seep right through the board. Thicker is better.

A thicker board is more forgiving of mistakes. Alot of times these boards are placed right on the aluminum router table. Its easy to make a Z height mistake and route right down through the board into the table. 3/4" give you more margin for error.

3/4" boards take more abuse in the factory setting.

All that being said. We run 1/4" spoilboard all the time and absolutely love them.
They are lighter, cheaper, easy to move around. Tempered hardboard in 1/4" will make a wonderful seal all on its own.

Trent

strat
10-19-2004, 02:36 AM
take a look at this one i just used the mdf and sealed with a paint called plasti dip the neoprene foam i got from grizzly and found the vacpump at a surplus place works pretty good for the bigger stuff but need to make a smaller one for smaller parts
http://cnczone.com/forums/attachment.php?attachmentid=2135&stc=1&thumb=1

Claude Boudreau
02-20-2008, 06:26 PM
If you have a one square foot of material on your table and vacuum on the other side of it, full atmospheric pressure will push it down. But if you only have 1/4 inch holes let say every square inch, the hold down pressure would be the sum of the surface of the holes times atmospheric pressure per square inch?Iam planning to build a vacuum table and wonder if the hold down pressure is gonna be enough.

Vac-Clamp
02-20-2008, 08:28 PM
Hi Claude.
A few things to keep in mind when vacuum clamping. Vacuum generators and pumps rarely provide anything close to absolute vacuum. (absolute vacuum is about -14.7 psi). A good generator/pump will produce about -12 psi with reasonable evacuation properties. You also need to keep in mind that there will be leaks between the workpiece and the gasket/seal material. all of this means that you will probably be left with about -11 psi with everything in your favour.
Assuming that you are working with a rigid workpiece and a compliant seal you will end up with a "chamber" which, in your example, is 144 square inches but not very tall. It is the volume of this "chamber" that you will be evacuating to provide the clamping force.
As the level of vacuum increases your workpiece will pull down to the face of your vacuum table. If you had a series of secondary seals around the 1/4" holes then the total force would be the sum total of the area of the holes. However it is very likely that you would not have a perfect seal around the holes and air would still be able to flow, albeit very slowly.
If you build a vacuum table look to incorporate some type of grip points. These grip points will hold your workpiece still and steady while you are working.
Errol
v-clamp.com (http://vac-clamp.com)