Author Topic: Milling a circle  (Read 717 times)

Offline Paulc

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Milling a circle
« on: October 17, 2019, 09:12:55 PM »
If one doesn't have a rotary table, how would you mill the  circumference of a circular piece with a small stem (think banjo shape)?

Offline Jo

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Re: Milling a circle
« Reply #1 on: October 17, 2019, 09:24:31 PM »
Does it have a hole in the middle? If so you can use that as a pivot point to do the milling. If not you can clamp/mount it on a jig that can provide the pivot underneath.

Jo
Enjoyment is more important than achievement.

Online crueby

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Re: Milling a circle
« Reply #2 on: October 17, 2019, 09:32:25 PM »
Does it have a hole in the middle? If so you can use that as a pivot point to do the milling. If not you can clamp/mount it on a jig that can provide the pivot underneath.

Jo
Yes  - have seen that done, with a long handle attached to let you control the rate the part rotates at (you don't want it to get spun by the cutter). Stop blocks could be used to limit the distance (angle) it can rotate if you are doing a portion of the circle.

Offline Paulc

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Re: Milling a circle
« Reply #3 on: October 17, 2019, 09:45:54 PM »
Yes, it will have a hole in the center, so that sounds like the way to go.
Thanks!

Offline Hugh Currin

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Re: Milling a circle
« Reply #4 on: October 18, 2019, 01:27:23 AM »
It just scares me to have a piece able to rotate when cutting. This is likely the easiest but it scares me. I've had enough pieces take off on me.

Depending on size you could use a boring head with the boring bar set to cut the OD. Also could put an axle though the center then clamp in a vice, take a cut, rotate part, clamp, cut, and repeat.

Maybe I don't understand the part, but for a circular cut can you do it with a lathe?

Thanks.
Hugh

Offline Doc

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Re: Milling a circle
« Reply #5 on: October 18, 2019, 02:16:26 AM »
Keep the end mill on the smallish side and do not climb cut and you should be fine.

Offline Dave Otto

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Re: Milling a circle
« Reply #6 on: October 18, 2019, 02:20:23 AM »
Like Guy Lautard says in his book The Machinist Bedside Reader “Don’t feed it the hungry way “

Dave

Offline Chipswitheverything

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Re: Milling a circle
« Reply #7 on: October 18, 2019, 01:18:49 PM »
Paulc, it's a bit difficult to advise on this query without some idea of the size, thickness and even material of the component, also the size of the hole in the centre about which it might be pivoted.
 I'd tend to go with Hugh's comment, that this could be a route to an unpleasant accident if things don't go well.  I understand that you may not want to buy a rotary table, but a RT gives that control and safety factor.
 Years ago ( maybe now sometimes ) model loco builders often machined the rounded ends of coupling rods by swinging them around a pin on a vertical slide in the lathe, but it needed a headed pin ( washer and locking bolt ) that stopped the component from pulling out and perhaps gave a bit of friction as well, and the radius being machined was quite small and the rod to hang on to was pretty long, giving reasonable control.  If there is a doubt about the job, could you saw out and file the thing?   Dave

Online crueby

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Re: Milling a circle
« Reply #8 on: October 18, 2019, 01:41:11 PM »
As Dave said, a lot depends on the size and precision required. Lots of times I'll just rough saw a curve and use a belt/disc sander with a table to finish off the curve.

Offline Mcgyver

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Re: Milling a circle
« Reply #9 on: October 18, 2019, 03:40:43 PM »
I've never encountered a part like that where the curve wasn't just for aesthetics.  As such, if so...hand tools will provide perfectly acceptable accuracy.  Crude methods do not have to mean a crude result.

about 25 years ago I built a UPT (a great project used over and over again).  At the time I didn't have an RT so made the drill head yoke by chain drilling and filling.  Eminently doable, mostly whats needed is patience.  lay it out, file carefully and finish with abrasives.

Here's the semi-amusing part....I went to take a picture of the drill head as an example of this and I thought "I know exactly where that piece I chain drilled it of out is"...so I photographed it as well.   I mean, I can barely remember what I did last week, but I can put my finger on a scrap of metal used 25 years ago?  I scare myself.














Offline Paulc

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Re: Milling a circle
« Reply #10 on: October 18, 2019, 09:02:13 PM »
Thanks to all. The part is .25 aluminum and various parts with this configuration can be seen here
https://modelengineeringwebsite.com/USS_Monitor_engine.html

I'm making what I call a kinematic model version, not steam powered but with an external drive to make all the bits move.

Offline Paulc

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Re: Milling a circle
« Reply #11 on: October 19, 2019, 09:47:41 PM »
Finally broke down and ordered a rotary table.

Now just have to figure out how to use it!:rolleyes:

Online crueby

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Re: Milling a circle
« Reply #12 on: October 19, 2019, 10:09:34 PM »
Finally broke down and ordered a rotary table.

Now just have to figure out how to use it!:rolleyes:
You won't regret it, very useful!

Offline Chipswitheverything

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Re: Milling a circle
« Reply #13 on: October 22, 2019, 03:30:16 PM »
Nice pictures of the Mini drill atop the UPT.  I'd say it reminds me of mine, except that you have the most beautiful paint finish on the filled casing, looks lovely, and also the nifty screw depth stop in addition to the stop collar on the spindle.

  Paulc, getting the RT sooner rather than later is a good move, it opens up quite a lot of helpful machining possibilities.   Dave