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  1. #1

    Low profile work holding

    Some of you might remember that i picked up some outside work a little while back and bought a pncnc 440 to do it .

    I've been asked to do a second job for the same guy and I'm not so lucky for it to be a single operation with a single tool like the first job .

    What do you guys feel is the minimum amount of material to grab onto .
    For referance the part is roughly 4 inches square and about .200 thick aluminum .

    I was thinking about sinking some mitee bite knife edge clamps into a fixture plate and only gripping on the top edge if the clamp @ about .030 so I can use 1/4 stock and have just enough material to be able to face the part and profile the outside edges in one shot .
    And have a bare minimum to face off in the second op

    My goal isn't accuracy, just quick with a nice clean finish on the outside edges .

  2. #2
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    Re: Low profile work holding

    I assume that you are considering Mitee-Bite TalonGrip clamps. I have successfully used their clamps for many projects. However, they suggest a grip of 0.060 rather than the 0.030 that you mention. I would not be comfortable with such a small grip. Consider also that most extruded aluminum has a small corner radius rather than a square corner. The rounded edge will further reduce your grip.

    Additional thoughts:
    You mention that the parts are about 4x4 inches. Could you process them in perhaps triples and separate later? The gap between parts would provide more options for clamping.

    Is it possible to cut partially though the stock (perhaps 0.15 deep) on the first operation and finish the cut in the second operation when the part is flipped? This would allow for more grip allowance.

    I don't know your location but is 8mm stock available? That would provide ample clamping room.

    With 0.25 by 4 stock clamping tightly at the bottom edge will cause the stock to bow upwards. If you facemill the top surface the result will be thinner in the middle and must be considered in the final dimensions.

  3. #3
    Community Moderator Jim Dawson's Avatar
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    Re: Low profile work holding

    If you have a lot of these to do, it might be worth building a vacuum fixture, with interchangeable fixture plates. We made one for handling small thin parts like this, if this is something of interest to you I can provide pictures. Uses a cheap vacuum pump from Harbor Freight.

    If there are any holes through the part that can be used as screw holes, then screw it down to an aluminum sub-plate.

    Given the final thickness, I might start out with 5/16 or 3/8 plate. With 3/8 plate you should be able to do all the work in a vice with step jaws.
    Jim Dawson
    Sandy, Oregon, USA

  4. #4
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    I use Sterling Toggle Clamps for thin parts. These are designed to hold lithograph plates in printing presses. Drill a 3/8 diameter hole drop in the clamp and tighten.

    Gary

    Quote Originally Posted by Portlyinohio View Post
    Some of you might remember that i picked up some outside work a little while back and bought a pncnc 440 to do it .

    I've been asked to do a second job for the same guy and I'm not so lucky for it to be a single operation with a single tool like the first job .

    What do you guys feel is the minimum amount of material to grab onto .
    For referance the part is roughly 4 inches square and about .200 thick aluminum .

    I was thinking about sinking some mitee bite knife edge clamps into a fixture plate and only gripping on the top edge if the clamp @ about .030 so I can use 1/4 stock and have just enough material to be able to face the part and profile the outside edges in one shot .
    And have a bare minimum to face off in the second op

    My goal isn't accuracy, just quick with a nice clean finish on the outside edges .

  5. #5

    Re: Low profile work holding

    There isn't enough of them to make it worthwhile to set up a vacuum pump or get to crazy with fixturing .
    And a bit of bow in the middle isn't a big deal tolerances are quite large .
    Parts looking good is all he is really after .

    I didn't think .030 was going to be quite enough to hold them , more or less was hoping I was wrong about it .
    This guy really hooked me up with the first job and I want to do this one as cheaply as possible as a thank you .
    I can just go with 5/16 stock easily enough and deal with a few extra minutes run time .

  6. #6
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    Re: Low profile work holding

    Quote Originally Posted by Portlyinohio View Post
    There isn't enough of them to make it worthwhile to set up a vacuum pump or get to crazy with fixturing .
    And a bit of bow in the middle isn't a big deal tolerances are quite large .
    Parts looking good is all he is really after .

    I didn't think .030 was going to be quite enough to hold them , more or less was hoping I was wrong about it .
    This guy really hooked me up with the first job and I want to do this one as cheaply as possible as a thank you .
    I can just go with 5/16 stock easily enough and deal with a few extra minutes run time .

    You don't need a crazy fixture, just put a cheap piece of aluminum down as a fixture plate. Drill and tap it to match 4 holes in the corners of your stock. Now set up a quick and dirty repeatable drill and counter bore for your stock (counter bore so you can face the stock with it bolted down) and size them big enough to make 2 or 3 parts at a time. Use tabs on the parts on the last .030 or less. When the first side is done cut them loose from the tabs and deck off the tabs when you face the other side to finish. You should save enough time making 2-3 at once to make the other steps more than worth it. The thinner stock should be no problem like this.

    Option B might be to just do the superglue and tape thing. At 4 inches and 1/4" aluminum that should be a pretty ideal type of setup for the tape and glue trick.

  7. #7

    Re: Low profile work holding

    I know the super glue and tape thing works , seen it on YouTube plenty of times , I'm just not brave enough to try it .

    But doing two at once and leaving tabs isn't a bad idea , wish I'd of thought of that myself .
    A plus is it might run long enough to let me fiddle around with other work in between . Or at least let me keep the mess cleaned up as I go along .

    If this continues I'm gonna have to give some serious thought towards some enclosures

  8. #8

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    Re: Low profile work holding

    I can vouch for the superglue and tape thing. Tried it a while back just for fun and I was shocked at how well it worked. The only warning I would throw out there is when facing for the second op I used my usual superfly cutter. I reduced the depth of cut since there wasn't much to take off anyway and slowed down my feed a bit but I think I was still taking .002 chips. Worked fine until I got to the far side of the part. The unsupported shelf of aluminum decided to bend rather than cut, cut up into the cutter and stalled the spindle... briefly. Then the tape let go and things went flying. From that point on, I used a 3/8 2 flute endmill for the second facing op and had no troubles at all. Bear in mind, I'm a machinist with all of 7 months experience so if I can make the glue work, anyone should be able to.

  9. #9
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    Quote Originally Posted by Portlyinohio View Post
    Some of you might remember that i picked up some outside work a little while back and bought a pncnc 440 to do it .

    I've been asked to do a second job for the same guy and I'm not so lucky for it to be a single operation with a single tool like the first job .

    What do you guys feel is the minimum amount of material to grab onto .
    For referance the part is roughly 4 inches square and about .200 thick aluminum .

    I was thinking about sinking some mitee bite knife edge clamps into a fixture plate and only gripping on the top edge if the clamp @ about .030 so I can use 1/4 stock and have just enough material to be able to face the part and profile the outside edges in one shot .
    And have a bare minimum to face off in the second op

    My goal isn't accuracy, just quick with a nice clean finish on the outside edges .
    Unless your doing a lot of them, just use
    375" stock in a vise with parallels. Plenty of safety room. Mill .01 past model bottom, flip over and face off carrier stock. Way easier than special work holding, and you'll deliver a nicer part with no "bow".

  10. #10
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    Re: Low profile work holding

    Yeah, I've held on 1/8" or so in vise, and done the "cut beyond the bottom, flip it over, clean it up" and it's great. Done it in 6061 aluminum, 1018 cold rolled, 303 stainless, 6-4 and grade 2 Titanium. The 440 can't push the things out of the vise if you have 1/8". (I typically use soft jaws when flipping because profiles are rounded, not straight.)

    I've used superglue and tape, but I have some planarity issues (2 thou or so end to end) and prefer soft jaws.

    Unless you make 10,000 or more of the part, the cost of your time and the machine will be significantly higher than the cost of the material, although I appreciate the desire to not waste material :-)

  11. #11
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    Re: Low profile work holding

    Superglue/tape: TRY IT!!!

    I use it all the time- as long as you've got enough surface area, it's great. I've gone as small as 3-4 sq inches on quarter inch parts- which works if you don't take full thick cuts. Use MDF as your spoil board, cut 0.05 deeper than the stock, and just plane it flat again (OK, that takes a planer).

    The trick is to use blue (or better, green) tape, and follow the Saunders protocol to the letter: tape down, burnish flat, degrease with acetone/MEK/IPA (any one works fine, just don't use MEK substitute), and glue up under pressure. You will be stunned by how hard it is to remove the work. If you can put the stock into a small partial pocket (2 sides is plenty) or use a sheet metal screw or two thru required holes, you can make more aggressive cuts in thicker material.

  12. #12

    Re: Low profile work holding

    I look and see all the successful superglue videos and I just can't make myself do it , and I know it'll work .
    Heck I've used it lots of times on the lathe ....

    I ran a couple test parts this afternoon ,with 5/16 stock I can do two parts at once and break them free by hand and face off the extra quick enough to not need to do anything else .
    1 extra tool change and a few extra minutes run time .
    And I think doing the first op in pairs instead of singles makes up for that extra time .

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