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Cutting a Large diameter using mini-mill

Discussion in 'A BEGINNER'S FORUM (Learn How To Machine Here!)' started by prasad, Feb 25, 2017.

  1. prasad

    prasad United States Active User Active Member

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    Guys

    I need advice. I want to cut 14 inch diameter curvature on 2 inch wide aluminum pieces as below. I made a sketch as attached below.

    untitled1.JPG
    My Grizzly G4000 lathe is a 9x19 and can not be used to turn 14 inch diameter. That leaves me only my mini-mill to do this machining. It is not a CNC and I calculated the x-y movements for the cutter in small steps. The process is moving the milling cutter manually in X-Y steps as if it is a CNC process. Steps are roughly 0.0315" in X axis. On Y axis the cutter is moved in much smaller steps to follow the radius desired. I tried milling it using a 3/4 inch end mill. The result is as attached below.

    20170220_132104.jpg
    The above image is what I made. You may notice curved surface has small steps but that is OK. It is the process involved that makes it tedious. I wish there is an easier way. I do not own a rotary stage and do not plan on buying one either.

    Any ideas?
    Thanks
    Prasad
    Eastern PA
     
    Last edited: Feb 26, 2017
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  2. JimDawson

    JimDawson Global Moderator Staff Member Director

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    I have to say that you did an amazing job on those.:encourage: That is not easy to do by hand.

    Rotary table is one option, another option would be to make a fixture that could rotate around a pin (bolt) with a 7 inch radius. Put a long handle on it and rotate into the cutter. I have done this before, works pretty well.
     
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  3. prasad

    prasad United States Active User Active Member

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    Jim, Thanks for the encouragement. Yes it was really hard to keep track of XY movements. I seem to lose the order of movements and go wrong after a few steps. Then I started using a printed sheet and a pencil to mark off each completed step.

    I do not have a rotary table. I do not want to buy one at this stage. I had thought of building some sort of rotating fixture but later thought of trying it the way I did. I held the piece in a milling vise and I used to thick blocks that extended above the vise jaws. In spite of the two thick blocks I am getting too much of chatter. If I run the mill at low speeds I can see heavy vibration of the piece being machined. So I am not sure how a rotating jig would perform. I am looking at reducing the depth of cut/penetration next and try doing it in two passes. So far it was done in just one pass.

    Thanks again
    Prasad
    Eastern PA
     
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  4. Cadillac STS

    Cadillac STS United States Active User H-M Supporter-Premium

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    Is the mini mill large enough to swing a 7 inch boring bar? If so could mount the metal in a vise 7 inches out on the table, swing a boring bar and come down from the top on it. Just need the bar and a fixture to hold it in a collet.
     
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  5. coherent

    coherent United States Active Member Active Member

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    That's a tough task with a small machine and no cnc. I'm sure a fixture that could hold the plate and swivel would work, but it would have to be fairly stout with solid attachment to the lathe either in lieu of or to the toolpost and a way to feed/adjust the feed around the swivel slowly . Cadillac's idea would work also. The g4000 only has 8 3/4" of swing over the bed though and 1 3/4" doesn't leave much room for work holding/fixturing.

    The only other way I can think of on the mill is like you did it, but would it be possible to rig some sort of simple "pantograph" to use as a simple guide to follow the curve on paper? Sounds basic but it would allow you to visually make your cuts without all the X/Y computations & time. I've seen some old pantograph mills, but don't know much about them.
    I applaud what you accomplished, I wouldn't have the patience. It just goes to show that if there's a will... there's a way!

    Of course you can always swing your mill head and mount a monster fly cutter and wear lots of protective gear...
     
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  6. prasad

    prasad United States Active User Active Member

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    My mill is a mini-mill. It is quite weak and flimsy. It's column lacks rigidity and flexes too much. I doubt if it can handle the forces if I fit a boring bar as small as a inch. Forget 7 inch.
     
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  7. willthedancer

    willthedancer United States Active Member Active Member

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    That incremental method is what I would have used. Nice job.

    Sent from my Moto G Play using Tapatalk
     
  8. gjmontll

    gjmontll United States Active User Active Member

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    A few thoughts on your project... I'll try to avoid repeating the other good ideas...

    1. I just measured my X2 mini-mill's spindle center to column clearance. It's too small, only 6.625", so forget about boring or fly cutting.

    2. And I suspect any boring or fly cutting at such a large radius on these interrupted cuts will absolutely break the gear teeth. (Based on what I've heard. I did the LMS belt-drive upgrade before it became a problem.)

    3. What are these pieces used for? There may be other ways to accomplish their functionality. What is the required tolerance on the arc?

    4. Is the full arc needed? If not, perhaps 3 precisely-placed pins that define the arc would work.

    4. Router bits will work on aluminum. I've used them in my mini-mill. In your case, maybe actually use a router, guided by a template jig, making a series of passes until reaching the final depth.

    Good Luck,
    Greg
     
  9. john.oliver35

    john.oliver35 United States H-M Supporter - Premium Content H-M Supporter-Premium

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    What if you modified Cadillac's idea and made a bar to hold a Dremel with a grinding wheel 7" off-center from the minimill's bore? Then you could manually swing the 'grinding arm' and adjust the X-axis to pull the part in to the depth you need.
     
  10. Tony Wells

    Tony Wells United States Vice President Staff Member Administrator

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    You didn't specify the tolerance of the radius, or the positional tolerance,or even surface finish requirements, so my idea may not work, but there is a way to do this within a reasonably close tolerance provided you can get an accurate template or pattern. Can you turn something as large as 14" on your lathe? Or have a friend who can?

    It would require a bit of tooling to be built, but it might be worth it to you, especially if you are doing more than a couple of these. Of course, toothpicking it, as you have done, is one way, but it is a test of patience as you have learned.
     
  11. 12bolts

    12bolts Global Moderator Staff Member Active Member

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    +1 on Jim's suggestion
    fixture.jpg
    You could probably get away with making it from heavy plywood. Secure the base board to your mill, with the pivot point 7" from your cutter and sweep the pivoting arm back and forth manually.

    Cheers Phil
     
  12. willthedancer

    willthedancer United States Active Member Active Member

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    Seen a fixture like this made of iron with multiple fulcrum holes as an attachment for a shaper.....

    Sent from my Moto G Play using Tapatalk
     
  13. coherent

    coherent United States Active Member Active Member

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    That would work pretty well. I've used a jig with a router to cut a convex in hardwood. Great results.
    I too use carbide router bits ocassionaly in my mill. They work surprisingly well on aluminum. Especially the roundover bits that cost a fortune if you buy round over end mills. On the other hand I've used end mills in a router with so so results. RPMs too fast for the cutter profile I'd guess.
     
  14. brino

    brino Active User H-M Supporter-Premium

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    Also a little "out of the box", but.......If you could find a ~14" carbide-tipped blade and use a big table saw, I could picture a hinged jig to lower it onto the blade (or raise the blade into it?), move over one kerf width and repeat. I doubt you could find a dado blade that diameter.

    Oh hey, here's a 14" blade that's made for metal.....reasonable price too....
    http://www.princessauto.com/en/detail/14-in-dry-cut-metal-saw-blade/A-p8141756e

    -brino

    EDIT: just remember to respect the max RPM on the blade!
     
    Last edited: Mar 2, 2017
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  15. GarageGuy

    GarageGuy United States Active User Active Member

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    You could use a smaller diameter cutter and speed up the spindle. Since you're taking light cuts anyway, you could use a 1/4" or 3/8" mill instead of 3/4". In aluminum, I've had good luck with 2 flute end mills, but that also depends on the alloy. If it is gummy aluminum, slower speed with lots of WD-40 as a cutting fluid.

    That is a relatively quick and easy solution. If you used a radial arm saw, or compound mitre saw and carefully mounted the work piece to the table, any carbide tipped wood cutting blade will do a nice job cutting aluminum as long as you take it slow and easy. Clamp or hold your work very securely so it can't get away from you. Slow and safe.

    GG
     
  16. prasad

    prasad United States Active User Active Member

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    Thanks all for your advice and encouragement. I went ahead and completed the radius milling. The pieces fit quite good. The assembly is almost ready. I had to make a dedicated drill and tap jig for drilling a large 14 inch diameter cast aluminum ring using a hand held drill as was simply too big for any of my machines. It too worked great. I will share pictures in a couple of days.
     
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  17. brino

    brino Active User H-M Supporter-Premium

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    Congratulations on the success!
    Did you continue with your original method or use one of the proposed solutions?

    Great, looking forward to it.

    -brino
     
  18. gjmontll

    gjmontll United States Active User Active Member

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    Great to hear that you worked it out. If I'd known that it was a drilling jig when I replied 3 weeks ago, I'd have shown the jig I made last year to drill radial holes through the wooden/steel rims of the cannon model I've been working on for 2 years. (and chronicled here in my "Greg's French 75 Mm Field Artillery Model" thread. This was about a 7" diameter, my Mini Mill was big enough - but not in your case! I'll be interested to see your setup. Greg
    2016-01-23 16.26.37.jpg
     
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