fixture question

[fourperf]

good morning guys. I have a fixture question. I have an odd shaped part with no holes in it that I need to profile then flip over and chamfer the other side. How do you hold down a part to machine that has no holes ( a square or round part for that matter also). I have found myself sometimes adding holes to the part just for the sake of holding it down but in this case I dont want to do that.

Thanks

Mark

[Geof]

I have done something similar to this by having two sets of clamps and two programs. The only difference between the programs is that one jumps over one set of clamps and the other jumps over the others.

Clamp the part down with one set, and run the correct program.

Install the second set and remove the first set; run the correct program.

It may be necessary to tweak the programs a little bit at the places where they merge. You sometimes get a witness mark where the X,Y motion stops for the Z to lift over the clamp which can be minimized or avoided by having the Z ramp up or the X,Y pull away on a little tangent arc. Then the other program has a ramp down or in inward tangent arc to start. If you are doing a finishing operation such as tumbling or bead blasting it is probably not worth worrying about.

[fourperf]

thanks geof.

mark

[Donkey Hotey]

Hey Geof, it seems that you are emphasizing running the correct program. Have you done this and run the incorrect program?

Mark:

Have you considered carpet tape? Spray the table down with brake cleaner and get it super clean, then stick the part down. It depends on having a lot of surface area but it works for large, flat parts.

[Geof]

Originally Posted by Donkey Hotey Hey Geof, it seems that you are emphasizing running the correct program. Have you done this and run the incorrect program?

I do not dignify cheeky questions with any response.

I do know however that clamps tend to be stronger than cutting tools.

[PinMan]

Not sure if thgis will be any help for your application, but in the past, I have come into jobs where clamping and unclamping can be a huge time killer.

Another option you have is making a 'squeeze' or 'split' fixture. Basically you make your fixture with your odd shaped pocket adding a certain amount to the length of your pocket. Somewhere around .125 is sufficiant. Cut your fixture in half right through the middle of the pocket. Square up your sides where you cut it, then drill holes under your pocket in your fixture. Running dowels and 1 threaded rod or bolt in the middle, you can use a cam lever to tighten the part in the fixture.

It takes a little longer to make the actual fixture, but you cut out a good bit of time in the long run when you are interchanging or flipping parts.

You can then add option an stop with a note to flip part and a subroutine to do the other side, eliminating the need for another separate program.

Hope I didnt confuse you lol.

[fourperf]

Thanks for the ideas guys.

have you done carpet tape greg? you probably would have to take tiny cuts.

mark

[Geof]

Originally Posted by PinMan .....You can then add option an stop with a note to flip part and a subroutine to do the other side, eliminating the need for another separate program. Hope I didnt confuse you lol.

Be very careful with optional stops for flipping. If you forget to push the optional stop key you may have problems. This is why I use two programs, I get forgetful in my old age.

[AMCTony]

I have used the carpet tape thing for one offs but I like super glue better. It is more rigid and gave better results. With this method glue the part to cut to a base piece of metal like an aluminum plate. Removing the part involves heating the plate to around 300 degrees or so so using this method with plastic parts is not recoomended. Excess glue on the parts comes off pretty easy with a razor blade and is very hard and brittle after it is heated. i used this method to make some speciallity radar components. The tolerance was pretty tight and the part was 6061 about 7 x 10 inches x 0.150 thick. The entire center was cut out so only a rectangular ring remained that was about 0.375 wide. It had tones of thru holes, chamfers and recesses on it and the glue method was the only thing that we could get to work without going fixture crazy. In fact the whole setup cost less than $20. 1 aluminum plate for a base and a bottle of super glue.

[slatern44]

If you only need to make a couple of parts Black Max from Loctite works great. Just make sure you use the 7452 primer with it so it will cure in about 10 seconds or you will be standing there holding down on you part for awhile. The stuff has great side load holding power but hardly any up and down strength. Then just make a metal shim about 0.040 thick and tap it under your part with a hammer and it will pop off. Acetone will take off the left over black max.

[Donkey Hotey]

I haven't personally used it but a bunch of guys do it at my day job. It's the only way to hold thin parts without damaging them. They mostly use it for fly-cutting thin materials. Machining is machining though. It should work as long as you have lots of surface area and take light cuts.

Of course, it's always safer to use the clamps and just be sure to use the correctprogram.