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Fly Cutter Tool Geometry

You tool bit will need to be sharpened to give it a proper cutting edge. You need to relief the bottom (where it would touch your work) so only the tip is cutting. Grind back angles in two plains (the bottom clearance), next you need to provide a bit of a chip break (the top rake).and you can grind in some horizontal clearance as needed - like if you were using a boring bar.

This image might help: FlyCutterSharpening.jpg

The first two images should be looked at reverse to how they are drawn to match your direction of rotation.


Well-Known Member
What's the proper tool geometry for a fly cutter?

This doesn't look right to me..... but?

View attachment 5395

I found this online http://tomstechniques.com/reference/

I'm still perplexed with the end face grinding?
Its upside down, the long-short side of the HSS should be on the top with the long on the high side. The extra material on the non cutting side helps to balance the tool. When i use my boring head i add a counter balance for the same reason. Balance is important on our small machines
Brent H's contribution doesn't look anything like Tom's Techinques offering. Still confused.

I'll pay for the HSS tool bit and postage if some one will volunteer to grind me a sample to use as reference. Once I see the angles I will be able to make my own.



Premium Member
Another approach could be to use one of those carbide braze on lathe tools and mount that in the fly cutter. KMS Tools and busy bee have them. Cheap. They look similar to Brent’s drawing. You want the kind With the cutting edge on the right side of the tool. So you can spin it clockwise. Tom is very keen on cutting his own cutting tools and he has a point. I’d rather just get on with it. These ones..

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Active Member
Premium Member
Pretty much any geometry that works for a lathe tool bit will work in a fly cutter. As long as it's rotated in the correct direction.


Ultra Member
Premium Member
My experience has been once you have a decent facing mill, the flycutter stays in the cupboard. The FM can take decent cuts but actually finishes better than a flycutter. It only makes sense, 3-4 times more cutters at work given the same feed, heavier tool, integrally balanced, typically more rigid because it fits the spindle taper socket, cutting geometry is always very close & consistent... Once upon a time one could say carbide inserts cost a lot more than HSS or carbide tipped cutters & you could utilize same lathe tools. But that cost gap has narrowed so as to be negligible. I have a 2" diameter 4-insert FM like Jangers middle picture, really happy with it. Inserts last a long time. I think its a good starter size because its in the range of what a light machine will allow taking heavier cuts. You can always just take stepped passes to traverse wider stock just like an end mill. But you can just as easily shave a thou off & get a wicked good finish to tight tolerances.