Vacuum Fixture HELP!

[project5k]

I'm working on a product, that, if all goes well, i will need to be turning out multiple units as quickly as i can. I have a couple questions about fixturing, and how i can make my production process the easiest.

What i will be making will start out as a cookie, or slug, cut off of t-6061 alum, 2.5 inches in diameter, and about .750" thick.

My thinking was to take a couple of the larger "tooling plates" that i have that are all alum, and bore some countersinks into it, drill a hole through the center, and put a rubber "O" ring down inside, and through the hole in the middle of the depression, i would attach a suction, or vacuum pump of some kind, to hold the slug in place during machining.

I will need full access to the top and the upper .250 of the slug, so side clamps are out.

My thinking is that i would have multiple depressions, and thus could machine multiple parts in one pass, then, have a second set of depressions, that i could then flip the cookies over in, to machine the backsides.

My questions are as follows.

What should i use as a gasket material in the depressions, to make a seal between the cookies, and the fixture?

What can i use to make the vacuum? obviously a shop vac isnt really gonna work in this case, just not enough surface area to work on, so i'm thinking its going to have to me a fairly decent vac source.

and since i'm running full flood coolant, what do i do about any coolant that may get sucked into the vac ports, do i just have an accumulator of sorts, so that the fluid gets collected before the pump?

I was wondering if i could use something like the harbor freight vacuum pumps, but i didnt know if anyone had one, every tried what i'm doing, or had any idea if it would work...

i tried searching for some of this stuff, but everything i searched for came back, "no matches found"

I'm looking for any input, ideas, thoughts, and so forth, if anyone has done anything similar to what i'm talking aobut, and you have pictures, i sure would love to see them...

[Geof]

I doubt you will get enough holding power.

2.5" diameter is 4.91 square inches.

Full vacuum, which is not possible, is 14.7 pounds per square inch.

You will have less than 72 pounds holding your part down, it will certainly spin and very likely will lift out under the force of the cutter.

Side clamps are not out, you have 0.5" of space to clamp on the side; top clamps are out.

Do you have a decent sized vise? Make yourself some custom jaws with a 2.5" diameter hole about 0.45" deep and you will have plenty of holding power.

EDIT:

When you want to scale up for high volume start another thread and I will suggest how you can make a multiple holder using air clamping.

[JimPAC]

You'll spin your wheels trying to get a vacuum to work and it wont!
Geof has the solution"

[escott76]

Originally Posted by project5k I'm working on a product, that, if all goes well, i will need to be turning out multiple units as quickly as i can. I have a couple questions about fixturing, and how i can make my production process the easiest. What i will be making will start out as a cookie, or slug, cut off of t-6061 alum, 2.5 inches in diameter, and about .750" thick. My thinking was to take a couple of the larger "tooling plates" that i have that are all alum, and bore some countersinks into it, drill a hole through the center, and put a rubber "O" ring down inside, and through the hole in the middle of the depression, i would attach a suction, or vacuum pump of some kind, to hold the slug in place during machining. I will need full access to the top and the upper .250 of the slug, so side clamps are out. My thinking is that i would have multiple depressions, and thus could machine multiple parts in one pass, then, have a second set of depressions, that i could then flip the cookies over in, to machine the backsides. My questions are as follows. What should i use as a gasket material in the depressions, to make a seal between the cookies, and the fixture? What can i use to make the vacuum? obviously a shop vac isnt really gonna work in this case, just not enough surface area to work on, so i'm thinking its going to have to me a fairly decent vac source. and since i'm running full flood coolant, what do i do about any coolant that may get sucked into the vac ports, do i just have an accumulator of sorts, so that the fluid gets collected before the pump? I was wondering if i could use something like the harbor freight vacuum pumps, but i didnt know if anyone had one, every tried what i'm doing, or had any idea if it would work... i tried searching for some of this stuff, but everything i searched for came back, "no matches found" I'm looking for any input, ideas, thoughts, and so forth, if anyone has done anything similar to what i'm talking aobut, and you have pictures, i sure would love to see them...

As the other poster said, soft jaws. Make a copy of your vise jaws in aluminum. Cut them so you get a piece of the radius in each one. You don't need a complete circle in the jaws, just make sure they are closed on something when you cut the jaws. Even .25" of depth in the jaws will do a whole lot. At work I use soft jaws almost exclusively. It also makes setting your offset easy, as unless you unbolt them from the vise they will always be in the same place.
There are other kinds of fixtures you could make. There are low profile side clamps that use cam action. You'd want a pair of fixed blocks to center the blank against though.

[project5k]

ok, i had wondered if the vac would be strong enough, guess that confirms my fears that it wouldnt be...

So, if i'm gonna go the route of making half moon clamps, (vice jaws) then i could use compressed air and some short throw, medium to large diameter(I'm thinking 1 inch) air cyls and just do it that way.. i have 185psi compressed air on tap at all times...

another plus of going that route is i dont have to worrie about coolant inside of vacuum pumps and all that jaz.

[Geof]

Originally Posted by project5k ..So, if i'm gonna go the route of making half moon clamps, (vice jaws) then i could use compressed air and some short throw, medium to large diameter(I'm thinking 1 inch) air cyls and just do it that way.. i have 185psi compressed air on tap at all times...

185psi air, that is high!!! But not high enough.

A 1" diameter cylinder will give you about 145 pounds of force closing your air vise, not enough I am afraid. You would need something like a 3" or 4" diameter cylinder.

If you do not have a vise make yourself some halfmoon clamps that are closed using a couple of bolts, just like a split clamp except one half is machined into your plate.

[project5k]

hmm thats an interesting thought.. so tell me, do you really think i need over 2000 lbs of clamping force? a 4" cyl * 185 psi = 2323.6 lbs of force.. wow, sounds like i'm really gonna be crushing these things..

or is my math wrong? area of a circle is pi*r^2
3.14 * 2^2 = 12.56in^2
12.56 * Air pressure, 185 = 2323.6

now i know that there are some losses, and i know that my compressor comes on at 180 and turns off at 195 (according to multiple unrelated gauges)

but for your example on the 1" cyl, i get 3.14*1^2*185=580.9lbs of force.

where am i going wrong on the math?

[escott76]

Originally Posted by project5k hmm thats an interesting thought.. so tell me, do you really think i need over 2000 lbs of clamping force? a 4" cyl * 185 psi = 2323.6 lbs of force.. wow, sounds like i'm really gonna be crushing these things.. or is my math wrong? area of a circle is pi*r^2 3.14 * 2^2 = 12.56in^2 12.56 * Air pressure, 185 = 2323.6 now i know that there are some losses, and i know that my compressor comes on at 180 and turns off at 195 (according to multiple unrelated gauges) but for your example on the 1" cyl, i get 3.14*1^2*185=580.9lbs of force. where am i going wrong on the math?

R not D. .5x.5xpi

[project5k]

please explain further. a 4" diameter cyl would have a radius of 2", 2^2 or 2 squared is 4. 3.14 times 4 = 12.56

the formula is pi times r squared, right? thats the area of the circle, or piston, then multiply that times the air pressure to get the output force?

[HimyKabibble]

Originally Posted by project5k please explain further. a 4" diameter cyl would have a radius of 2", 2^2 or 2 squared is 4. 3.14 times 4 = 12.56 the formula is pi times r squared, right? thats the area of the circle, or piston, then multiply that times the air pressure to get the output force?

You can use a smaller cylinder if you provide a lever or cam to give mechanical advantage. A 1" cylinder with a 16:1 lever will give the same clamping force as a 4" cylinder with 1:1 lever, and you shouldn't need much travel on the workpiece end, so even a 1" travel cylinder should work. You could also use a hydraulic cylinder instead of a pneumetic one, and run MUCH higher pressures (like > 1000 PSI).

Regards,
Ray L.

[Geof]

I always like to err on the side of holding things too tight rather than too loose but notice I did give you a range; 3" for about 1300 lbs and 4" for about 2300 lbs.

The lever suggestion is also very viable and you can also get force multiplication by using a toggle clamp or a sliding wedge. These are more complicated to build but when your volumes increase it becomes worthwhile putting in the extra effort on the fixturing to reduce the reloading time.

An advantage with both the toggle clamp and wedge is that they can be self locking so if you lose air pressure you do not get parts flying around. In this case you need a double acting cylinder because force is necessary to unlock.

[escott76]

Originally Posted by project5k please explain further. a 4" diameter cyl would have a radius of 2", 2^2 or 2 squared is 4. 3.14 times 4 = 12.56 the formula is pi times r squared, right? thats the area of the circle, or piston, then multiply that times the air pressure to get the output force?

yes, but where you calced the area of a 1" cylinder you used 1" for the radius, and it's .5"
.5"*.5" = .25"
.25"*3.14*air PSI = 145