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I'm trying to transfer a caliper diameter to the end face of a cylinder on my wood lathe. How do I determine that the caliper is centered in order for that diameter to be transferred?

For example I have a cylinder like this:

enter image description here

and calipers like this:

enter image description here

Say, hypothetically the cylinder has a diameter of 10 units. I want to mark the bounds of a smaller, 5 unit circle using the caliper. How would I do that in a way that's precisely centered?

If the question is unclear, please let me know and I can include more info

  • Well you generally mark circles with a compass ;-) But because this is spindle turning you wouldn't normally mark a diameter, instead the standard practice is to turn and check, turn and check until the cylinder matches the gap in the cailpers (or anything that can hold a fixed gap and can be slipped onto the turned cylinder from the side, even an open-ended spanner can work). – Graphus Jun 2 '18 at 19:41
  • @Graphus thank you so much, i really appreciate this. Yes, I understand checking the diameter periodically as Ihollow the end of the cylinder. I use the parting tool. Is there any way that I can mark the diameter witt the placement of the calliper. It would need to be centered, as with a compass. This is tricky to do since one (a human) has to rely on estimating... Any thoughts – max pleaner Jun 2 '18 at 19:59
  • Oh you're hollowing the end, sorry that wasn't clear. For reasonably accurate marking out you need a centre finder to locate the dowel centre (picture shows a commercial one, there are many different DIY versions of this tool that aren't difficult to make to the required accuracy), then a compass for marking the circle you want. If you don't own a compass already if necessary you can make one quite easily, but even the most basic children's compass will work fine and those can be very inexpensive. – Graphus Jun 3 '18 at 12:28
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Spin the piece on the lathe and put a pencil as close to the center as you can. This should draw a small circle. Set a compass to the desired radius (half the diameter). With the piece stationary put the compass's pin in the center of the circle and draw your desired circle.

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