Cutting a Large diameter using mini-mill

[prasad]

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.


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.


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

 

[JimDawson]

I have to say that you did an amazing job on those. 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.

 

[prasad]

I have to say that you did an amazing job on those. 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.

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

 

[Cadillac STS]

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.

 

[coherent]

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...

 

[prasad]

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.

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.

 

[willthedancer]

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

Sent from my Moto G Play using Tapatalk

 

[gjmontll]

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

 

[john.oliver35]

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.

 

[Tony Wells]

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.