Creating a bolt hole circle on Sherline Mill

BiggerNoise

BiggerNoise

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My part is a 4" aluminum disc. I'm doing a six hole circle .500 in from the edge.

I don't have the rotary table just yet and the travel of the y axis keeps me from just using trigonometry.

For this part, I can probably get away with Dykem and a protractor, but I'd appreciate any tips for skinning this particular cat. FWIW, there's a center hole as well, so I can set this thing up to spin about its center.
 
Yes! that would work.

(but why for those of you who'd like a geometry refresher...)

The two radii and the chord are all the same length, so that's an equilateral triangle which has 60 degree angles.

PXL_20220424_202843272.MP.jpg

Thank you!
 
It sounds like you were able to use constructive geometry in this case. But if using dividers and eyeballing the holes isn't quite good enough, another way would be to make yourself a smaller-diameter bolt circle, also with a hole in the center. It really should be called an "index circle". Bolt them together and index off the holes in the smaller one. Using a dowel pin would be the way to go. It could be difficult to get the index holes + center hole precisely aligned with the X axis so you still would get to do some trig. A carrier plate that remains fixed to the table, also with a hole, would help there. The bolt circle(s) would rotate around that so you'd just need to get the X coordinates right, and that would be easy since you know the radii of the index and bolt circles.

For best results the center holes would need to be a close fit to a dowel pin, but that's easy enough to do with a reamer.

A bit of work for a one-off. However, after that you could use the carrier plate for other similar problems. If you can stand to have a set of index holes in your part you wouldn't even need to make the small "bolt circle".

I used a similar scheme to mill three slots in my home-made fixed rest (for the fingers to ride in). In that case it wasn't because I didn't have enough travel but because I've got a manual mill and cutting the angled slots was a nonstarter. Using the pin-indexed approach I could do them all along the X axis.
 
homebrewed said:
It sounds like you were able to use constructive geometry in this case. But if using dividers and eyeballing the holes isn't quite good enough, another way would be to make yourself a smaller-diameter bolt circle, also with a hole in the center. It really should be called an "index circle". Bolt them together and index off the holes in the smaller one. Using a dowel pin would be the way to go. It could be difficult to get the index holes + center hole precisely aligned with the X axis so you still would get to do some trig. A carrier plate that remains fixed to the table, also with a hole, would help there. The bolt circle(s) would rotate around that so you'd just need to get the X coordinates right, and that would be easy since you know the radii of the index and bolt circles.

For best results the center holes would need to be a close fit to a dowel pin, but that's easy enough to do with a reamer.

A bit of work for a one-off. However, after that you could use the carrier plate for other similar problems. If you can stand to have a set of index holes in your part you wouldn't even need to make the small "bolt circle".

I used a similar scheme to mill three slots in my home-made fixed rest (for the fingers to ride in). In that case it wasn't because I didn't have enough travel but because I've got a manual mill and cutting the angled slots was a nonstarter. Using the pin-indexed approach I could do them all along the X axis.
Click to expand...
That's a couple interesting approaches, thanks for that.

This part is low enough precision that dividers will suffice, but I'll hang on to these. I'm sure I'll eventually get to the point where eyeballs aren't quite enough. I have to make three more of these plates, I might try out the index circle just to see what sort of trouble I can get into.
 
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