# Single Point Turning a Radius by the Numbers



## rake60 (Nov 21, 2012)

When I was working in an industrial job shop, the most dreaded job was cutting the rope groove in a a wire rope sheave. 
It took a good eye and steady control to manually form a perfect groove that fit the gage.
An imperfect groove could scrap the part.

*I HATED THEM!!!!!*

That was something I was never very good at so I found a way to cheat that also works quite well for turning a radius on a lathe.

First you would leave a step of material in the work piece to allow stock for the radius.
If you need a 1/8" radius leave a block of material that is 1/8" above and 1/8' away from where the radius will finish.
I'd usually just roughly hack in a bunch of rough steps with the turning tool from there to make it 
rough 45 degree angle. Nothing critical at this point.

Then find a nice, free bolt circle calculator like _*THIS ONE*_
For the bolt circle, enter double the radius you are wanting to cut. If you're looking for a 1/8" radius enter .250"
At "Number Of Holes" enter 45. You will only be using the first 12 of those coordinates so don't let it numbers overload worry you.
Then enter "Angle of the first hole (Deg)" to 90

Touch off the part on the OD and set your indicator or digital on ZERO for the X axis.
Then move to the start position of the radius and set another indicator or digital to the positive value of the radius. 
If the radius is 1/8" set it at .125

Now make the negative X values from the caculaor positive values.
Then  just dial your tool to the coordinates on the calculator results.

I know it all sounds a little complicated but it isn't once you try it a few times.

In my professional career, I have free hand machined nearly perfect 8" radius's using this method.
I used 360 holes in the bolt circle programs to do that, but for a small radius 45 holes is enough.

Rick


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## Herbiev (Nov 22, 2012)

Thanks Rick. Very handy.


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## modeng2000 (Nov 22, 2012)

A book was published by Guy Lautard in 1990 called Tables & Instructions for Ball and Radius Generation

John


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