Work Holding and Cutting Thin Aluminum on a Mill
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    Default Work Holding and Cutting Thin Aluminum on a Mill

    Hi all; I've moved on from my tiny import CNC mill I converted myself to a real (though still a budget model) 3 axis CNC machine with 24" x 15" table travel. I'm a total amateur at CNC machining and have slowly been learning all the ins and outs as I go. I think I've gotten to be reasonably good at the few operations I need to make my parts but I recognize that I have a very long ways to go. For reference, I sell small parts to people making radio-controlled four to eight rotor helicopters (quadcopters).

    With my previous machine, I was only able to make one or two parts at a time and it took forever to machine much at all. This new machine is both much larger and faster so I want to leverage that all I can. The problem is, I'm not sure the best way to do that and I'm hoping you guys can point me in the right direction.

    I've attached an image that represents the type of parts I make currently - basically 2d profiles around the whole perimeter with some drilling and a little bit of pocketing to create the retaining channels. The material is either .25" or .09" - I'm looking for advice on how to machine these style of parts as quickly as possible - basically set up the machine to create a ton of them and come back in a few hours to find them all done.

    I've tried doing a variety of things on my smaller machine and some things work well but none of them seem to scale up effectively. To date I've done this:

    1) Drilled a series of mounting holes in a piece of raw stock. These holes corresponded to the design so it worked out nicely. I then created a fixture plate out of 1/2" aluminum and used shoulder bolts to pin the material to the fixture plate. Finally, I aligned and touched off the plate and let the machine rip. The problem is the shoulder bolts are quite small (4-40) as the parts are tiny and the bolts work themselves loose while machining the part. This has broken bits, lost steps, etc. All bad. Obviously I could use a thread locker but this would dramatically slow down set up. It seems there should be a better way.

    2) Simply cut the part and let it pop itself loose when done, about 1 in 3 or 4 times the "popping loose" nicks the part bad enough that I can't use it. I know this is TERRIBLE but it is fast and simple and seems to work with these tiny parts. The issue I have with this approach is that as the material gets thinner and larger (to cut multiple parts at once), it gets hard to clamp it properly so that it won't bow up in the middle. These are low precision parts (0.01" is fine) so a tiny bit of bow is fine, but it causes other problems like the bit lifting the material and such and that is where I run into problems.

    So what's the trick for parts like this? I've heard carpet tape works well and I can do #2, just with added tape. I've tried that a bit and the results are interesting but you still need to clamp the material as the tape alone isn't enough to keep the material from sliding while being cut. Since the tape is a little spongy, things have a tendency to bow up on you even more. Also, once the parts are machined, it's VERY hard to remove them from the tape itself. Things really get stuck together. Is there a trick to the tape?

    Should I go a totally different direction on this? Thanks much in advance!

    -Mike
    Attached Thumbnails Attached Thumbnails v3-1.jpg   v3-2.jpg   v3-3.jpg   v3-4.jpg  
    Last edited by webgeek; 09-07-2010 at 05:16 PM. Reason: Added image more images

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    One solution is small tabs around the outside contour. In this approach, you first mill all of the features other than the outside contour. When milling the outside contour, two or three small tabs, a few thousands thick, are left to hold the part. All of the other material is milled away. This leaves you a second operation to complete the part - but it is not a difficult task. Some CAM systems have a feature for programming tabs.

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    Hi Bruce, thanks for the quick reply. I should have added option 3 in my list. I've used holding tabs in the past and then manually cleaned them up. This worked pretty well but was very tedious and time consuming. Is there a good way to quickly remove the tabs? I'd heard someone use a router table + flush trim bit and then a spindle sander. That would be better than the nasty filing I did. Is there another way to do that better? Thanks!

    Mike

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    .....here I go again.......

    If you have many to do, how about a drill bushing fixture to drill four holes in the blank, (yes manual but very fast. Blank clamps in fixture which slides around on the drill press table, could stack several in fixture.) and then bolting the blank to a stand-off fixture to elevate the part above the mill table, no critical locating here. Then run the exterior routine on all the parts in a run. Note: very thin parts might require a shaped platten between part and nuts. If practical, the blanks could be stacked here too.

    2nd op, clamping each part in a fixture that does provide precision location, clamping (pneumatically?), outside the interior cuts and running the interior program, which removes the areas containing the original four bolt holes.

    Yes, that means two seperate loadings and routines, but would be rock solid and it might be faster than filing off tabs and hand finishing, especially with enough parts to make the fixtures pay off.

    Bob, who has never run a CNC machine of any kind.....

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    I use my table saw on aluminum all the time. If your parts have a straight edge (your drawing appears to have such) then put the tabs on this edge and use a table saw to clean the tabs. Shears will easily rough cut the tabs. With a fixture, you could do 5-10 a minute. If you do a lot of table saw cutting, it is worthwhile to get a purpose built metal cutting blade. I use a Morse blade for aluminum.

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    Are there enough of these to justify dedicated fixtures?

    Machine all of your holes, internal profiles, and channels.

    Double-stick tape should work well at this point.

    Now ...

    Make a fixture with posts matching the profile of your center cut-out but slightly under-size, with a tapped hole in each post.

    You will need a post for each part in your sheet.

    Slip your sheets onto your fixture and clamp them down.

    The taller the posts, the more sheets you can stack.

    Machine all the outside profiles.

    Sort of the reverse of Bob's suggestion.
    Last edited by KilrB; 09-07-2010 at 03:36 PM. Reason: Not wordy enough.

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    Could you find an insert with a nylon thread in it so your shoulder bolts don't back off so you could use your existing fixtures?
    Michael

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    Webgeek, if you decide to try the tabs approach, SheetCAM from SheetCam homepage handles tabs nicely, I think they just added a feature that automatically creates the tabs for you (in older versions you had to add them manually). They also have "nesting" features to help you maximize the number of parts you get out of one sheet, and it will "fill" a sheet from a single part drawing.

    Actually looking at your parts again, it looks like they have holes on the corners. I'd make a fixture out of aluminum plate that has tapped holes at these corners for clamping as many parts as you can fit on your table (and reach). Do a slight counterbore at each tapped hole so you can drill into each hole a little without hurting the threads.

    Then as a first op, clamp the raw plate on top of the fixture, either with standard table clamps or if you want to get fancy, build these first op clamps into the fixture. Then run a program that drills all the holes in the parts.

    Then screw the raw plate down to the fixture using the hole locations at the corner of each part, remove the first op clamps and run the second op program to cut the profiles and pockets.

    When you make the fixture, put some locating pins or blocks on it that just fit your table slots, this makes it easy to quickly square it to your table, and since it will get machined for the first time with these pins locating it, it will be very true to your machine. I also drill a tapered hole at (0,0) on my fixtures to make it easy to re-zero the machine to the fixture.

    I also place my fixtures on the machine table at in the exact location every time (measured from the end of the table), this way any out of square of the slots doesn't affect the accuracy.

    Paul T.
    Power Technology

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    Vacuum table....
    Dave P.

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    Candle wax.....

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    i would clamp your sheet stock down in a way that allows all the center stuff to be milled. once the windows are milled away, exposing some much larger threaded holes, bolt down a fixture plate conforming entirely inside the profile, and mill the profile.

    this way you can use just a one or a couple nice big hold down bolts instead of many tiny screws.

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    I haven't done that much workholding with tape and glue, but here's some info I had on file, from snippets from (I think) this forum, but I've mislaid the posting handles of those who wrote it (apologies on not being able to give credit where it's due – I did a search but didn't turn anything up)

    <<Double sided Tape:

    Golf club repair shops stock good quality double sided tape which works well, maybe better than carpet tape. Other leads: Permacell, 3M (various grades including 9500PC, #410)
    The two usual ways of breaking the grip are heat (eg hot air gun) or solvent (acetone,
    Brake Clean, alcohol, laquer thinner – try till you find something which works well)

    If milling thin parts on double sided tape, use 3M or Anchor brand, and (if using coolant) run a bead of holtmelt around the edges to delay coolant reaching the tape.
    Works ok for parts with large bonding surface areas. It doesn’t adhere well enough for small parts. Kind of resilient and parts may creep. For lathe work, apply additional clamping force from the tail stock (using a false or pad running centre if necessary). On large pieces, use strips of tape with gaps between so that solvent (mineral spirits) can get in to release.

    Mitee-grip wax paper, or nylon mesh fabric, is reputed to grip VERY well, and is immune to many coolants (probably not alcohol, though?) Need access to boiling water. (search this forum for more info)

    “Super glue” : The problem with this is getting the part de-bonded after you have finished machining without distorting the part. Solvents, such as nitromethane (or maybe acetone, work, kind of, but if you have a good thin bond line it may require some coaxing with a razor to get the part free. Super glues don’t like shear so you have to keep shearing forces light.

    There is also an interesting heat activated tape that hot stampers use for holding dies on the press. Heat activates it but it is not a thermoplastic, ie it stays bonded when hot, a sharp rap in shear makes the plate pop off. I don't know what it is called but a hot stamp foil supplier would have it>>

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    Wow, thanks for the huge list of information everyone! I'm still trying to absorb everything and understand it. I've also posted a few more pictures to show some other parts I have to make.

    Quote Originally Posted by Robert Campbell Jr.
    Yes, that means two seperate loadings and routines, but would be rock solid and it might be faster than filing off tabs and hand finishing, especially with enough parts to make the fixtures pay off.
    This is kinda what I do now, but I use the interior holes of the parts and shoulder bolts. The 4-40 bolts work lose so I'd need to use something larger. This technique doesn't work for all my parts though, just some. I still like the idea for some other things I cut so I'm filing this away as a good idea for future parts. You also mention "pneumatically" clamping - can you explain that? I assume it's referencing a vacuum table but I'm not sure. Thanks!

    Quote Originally Posted by Bruce Griffing
    I use my table saw on aluminum all the time.
    Wow, these parts are far too small to use a table saw - the largest part is only like 3" square. I'm too chicken to get my fingers anywhere near the blade - particularly since a nasty kickback while I was being safe badly broke the tip of my finger.

    Quote Originally Posted by KilrB
    Sort of the reverse of Bob's suggestion.
    What you described is very close to what I do now except with the addition of the inner "towers" which would actually solve a problem I've run into before. I love the idea of a little cap on the towers to "clamp" down the parts - this makes good sense too. Thanks!

    Quote Originally Posted by mthomure
    Could you find an insert with a nylon thread in it so your shoulder bolts don't back off so you could use your existing fixtures?
    I'm not familiar with nylon threaded inserts? I've seen screws with little lock patches on em, is it something like that? Thanks!

    Quote Originally Posted by PaulT
    SheetCAM from SheetCam homepage handles tabs nicely, I think they just added a feature that automatically creates the tabs for you (in older versions you had to add them manually). They also have "nesting" features to help you maximize the number of parts you get out of one sheet, and it will "fill" a sheet from a single part drawing.
    I just checked out SheetCAM, I think I like VisualMill better but I LOVE the nesting feature. If anything sells me on SheetCAM, it's that killer feature

    Quote Originally Posted by Dave P
    Vacuum table....
    I'm under the impression these parts are too small to grip properly without a huge amount of vacuum.

    Quote Originally Posted by Limy Sami
    Candle wax.....
    That's a new one I've not heard of before. I assume it would work just like glue and the like. I imagine it's hard to get it thin and even though?

    Quote Originally Posted by dsergison
    i would clamp your sheet stock down in a way that allows all the center stuff to be milled. once the windows are milled away, exposing some much larger threaded holes, bolt down a fixture plate conforming entirely inside the profile, and mill the profile. this way you can use just a one or a couple nice big hold down bolts instead of many tiny screws.
    Sadly, this would only work for a few of the parts but it's still a great idea and I agree with it for those parts. Thanks!

    Quote Originally Posted by Troup
    I haven't done that much workholding with tape and glue, but here's some info I had on file, from snippets from (I think) this forum, but I've mislaid the posting handles of those who wrote it (apologies on not being able to give credit where it's due – I did a search but didn't turn anything up)
    Wow, lots of great info and ideas in that post. Thanks!

    Thanks for all the suggestions and advice everyone. I'll be giving this all a shot and see what works out best in the end. I really appreciate it! Thanks!

    -Mike

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    Mike,

    I had to make some parts very similar to yours, I was machining them from 0.062" copper and aluminum.

    I wanted to easily be able to do low volume production (like run 10 or 20 parts in a shot). I tried everything - including the suggestions above.

    Vacuum won't work for parts with lots of holes and open features like you have.

    Fixturing with screws doesn't really work because the thin sections deform when you try to cut them since they aren't necessarily held firmly unless you use TONS of clamps which takes forever and is a PITA to set up.

    Tape works but it's gummy and gets everywhere.

    Tabs was a pain - very hard to get them removed and leave a good looking part. And for max rigidity you need a lot of tabs which means even more secondary finishing.

    Anyway, I talked to someone who does this stuff all day and he gave me the perfect solution.

    I bought a hot plate at Walmart for $20. I cut rectangles of aluminum on my table saw (with a carbide tipped blade) about 1" larger than my parts to act as holders. I cut stock for my parts. I put the holders on the hot plate and let them get hot - took a glue stick that you buy anywhere for cheap money, smeared it around on the plate, then stuck the stock on it and set it aside for a few minutes to cool. Machine the part, then back on the hot plate to melt the glue... pull the part off and drop it in a 5 gallon bucket filled with acetone, cover and let sit for a few hours. Pull it out and the glue is gone - any remnants of glue are hard and flaky and a nylon nail brush removed them in about 5 seconds.

    The nice part of doing it this way is there is very little hands-on time.. takes 10 seconds to smear the glue, 10 seconds to pull the part off, and 5 seconds to brush off any remaining hot glue after the acetone bath. You can cut 10 or 20 or 100 "fixture" plates if you like and load up your machine with as many of them as the table will hold, then walk away and let it do it's thing. And while waiting for the hot plate to heat up the plates takes 10 minutes or so, you can be doing other stuff while that is happenning, and doing other stuff while the acetone is cleaning up your parts.

    You should see the results I got - razor sharp lines and no burrs on 0.062" thick copper (which is a notorious bitch to machine) using a 1/16" end mill. The work is supported 100% all over, so no matter how intricate your cuts are, you get great workpiece support. I was cutting parts that looked like little mazes and they came out perfect every time. Best of all, since you have great workpiece support, you don't have to back off your feed and speed to get a nice finish - you can let 'er rip.

    and glue sticks are cheeeap. I was buying them in bulk from ULine.

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    Smile OT uncalled for comment

    Quote Originally Posted by Limy Sami View Post
    Candle wax.....
    I'm reminded of the song "Dinah, Dinah, show us your leg..."
    replacing the words "But Dinah uses axle grease..." with
    "But Limy uses candlewax...."

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    I like the glue stick idea. I've been using might grip. It's a sheet adhesive from the mighty bite folks. You stick your fixture plate on a hotplate, slap on a sheet of adhesive and your aluminum stock. Throw some weight on your hot plate and heat it up. mill your parts and reverse the procedure.

    The acetone and hot glue is a cool trick though.

    My wife sews a bunch of paragliding gear with computerized pattern tackers. She glues all the parts with a very special hot melt adhesive to hold them in place. Wickedly cool, and great fun when the competition buys glue sticks at Walmart to try to copy her work. So far I know 2 guys who have replaced their machines from urethan hot melt glue. Funny thing is, I never though of soaking them in acetone to clean the hook, shuttle and race. Maybe I can buy them cheap....

    Thanks for the idea!

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    For your sheet stock parts, shear or saw blanks, and an additional heavy blank that will be used as an overall clamp. Stack and clamp to fixture plate using soft strap clamps and locating against three pins. Drill and mill holes and cutouts. As was previously suggested, strategicly placed tapped holes allow clamping through part, allowing strap clamps and pins to be removed. Mill external contour. Parts cut from plate similar, though you can't stack as many. Only downside is having to reclamp halfway through. Small price to pay in exchange for completing a number of parts in one cycle.

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    Cant at least some of these parts be lasercut ????
    Or redesign them so all can be lasercut
    IMHO you cannot beat lasercutting with milling

    Peter from Holland

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    Webgeek, It's not hard to get a thin layer of wax, warm fixture plate until wax flows freely, drop on part, place a heavy, cold, and flat chill block on top, rinse and repeat.

    When removing part, heat agin for next one, dip completed in solvent to remove wax, jobs a goodun.

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    About kickback on a table saw. Kickback occurs when a workpiece is not stable on the table. Normally, this is due to one of two things. The workpiece is not flat and rocks on the table during the cut or the workpiece twists in the plane of the table during the cut. In either case, the blade grabs and can throw the workpiece. A simple plywood fixture, very easy to construct, would allow you to cut parts less than 1" in size. I have cut parts this size many times on the table saw without any problems.


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