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[4]

How do I grind the bit for my circle cutter?

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Ken from ontario

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#1
I made an adjustable circle cutter last year similar to many you see on the market , never cut anything with it until yesterday, the cutting bit for it is a 1/4"x 1/4" HSS blank , I searched the net but couldn't find any reference to this type of cutter, only thing I found was Lee Valley's bit:
99k0901d1 (1).gif
I tried that profile with an end cut angle of 10°, no relief angle, side cut, just the end cut ,well it did cut but burnt where there was friction , I have never done any cutting tool grinding but I'm thinking my next profile should be like "V" shape,similar to the tip of a nail,with two (or 4 sides).: e5703a1d-b46e-489a-9f1a-4e37ca1def15.jpg
So my question is, what's the most logical profile for this application. should I keep it like the first picture but with a 15° -20 ° end cut?
 

francist

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#2
I have used such a cutter once, 40 years ago in a Rockwell radial arm drill, and with dramatic results. An extra-large escutcheon plate was needed to conceal the carnage to the custom made mahogany door... :eek:

You may find some better information by searching "fly cutter wood" as that is typically what they are sold as. At least the ones I've seen in hardware stores around here.
Capture fly cutter.JPG
As for tool shape, I think I would be tempted to think of it like a scraper chisel for a wood lathe. Flat top, relief below (or behind in this case) the cutting edge, and clearance on either side. Somebody else may have a better recommendation though, as I said my only experience with one was not particularly positive.

-frank
 

Dredb

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#3
Tank cutter. Regard the cutter as a parting tool, flat top with front and side relief, no top rake, it will cause it to dig in. The cutter you show would be OK for plastic water tanks.
 

T Bredehoft

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#4
Watch your surface speed. Heat will burn the wood and could dull your tool
 

Dabbler

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#5
Look on youtube for 'trepan tool' or 'trepanning cuttter'. It is different from a parting tool in the types of rake you grind into it. PM me if you have trouble finding a resource.
 

Ken from ontario

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Thank you all, I expanded my search a bit more under ,parting, grooving, trepanning,, found a few more ideas. Tomorrow I'll try what I think might work .
Thanks again for the suggestions.
 

mikey

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#8
Hey Ken,
I own one of these contraptions. Used it on wood back when all I had was a drill press. Even then I thought it was the most inefficient tool design I had ever seen. Unbalanced, lacks rigidity and the tool geometry was really poor - burned the wood. I presume you will only use this on wood; I wouldn't even attempt to cut metal with it.

With that said, if you want to try grinding a tool for it then I would like to suggest you grind it with the shape of a left hand lathe turning tool bit. This shape would shear instead of plow through the material. Since wood is not hard (assuming you will only use it on wood) then I would use side and end relief angles of about 15-20 degrees to reduce cutting forces and provide enough relief to hopefully minimize burning. I would keep side rake conservative at about 10 degrees to improve edge strength and keep back rake low at about 6-8 degrees to focus the cutting forces at the side cutting edge. It would be nice to use more rake to clear chips but you'll be cutting inside a groove and chip clearance will be what it will be.

There are many other options for large hole cutting in wood. I like using a plunge router with a circle cutting jig - clean, efficient and much safer. Good luck.
 

Ken from ontario

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#9
Hi Mike, thanks for chiming in, you are right on about the design, the only reason I made this cutter was just to have fun making something, it was one the first tool I made while practicing but overall, it is a terrible design.

Last week my son asked me to cut a 6" hole in a 1/4" plywood to be used as an outlet for his portable A/C, of course I had better options but wanted to try this contraption I had made and also was curious how a bit should be ground for this tool.
The profile you mentioned is more likely the one that would cut without burning the wood too much ,thank you for precise explanation , all I need to do is to re-grind the cutting bit and see what it can do.
:encourage::encourage::encourage:
Ken.
 

RJSakowski

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#10
I grind my cutter as shown in your OP. I will grind relief on the outside to prevent dragging on the outside of the cut and relief on the bottom as well. I also put some positive rake on the leading face. Here is a drawing of a typical grind.
Circle Cutter Bit.JPG

I predrill the pilot hole and use a dowel pin instead of the pilot drill. I wouldn't attempt to use the circle cutter in a hand drill as it is difficult to control the feed rate and to keep the circle cutter perpendicular to the work surface. A drill press minimally or better a mill/drill. For large circles, slow the rpm's down

In the past, I used to cut hundreds of 4" diameter holes in .063" aluminum sheet as well as holes in acrylic and steel. When cutting sheet stock, I will start the hole on one side and flip the sheet to finish the cut. This results in a virtually burr free hole.
 

Ken from ontario

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#11
Thank you RJ, the grind on the bit you posted looks like the trepanning cutter I found picture of online. wasn't sure it would cut the 1/4" thin plywood efficiently ( thought it might dig into the plywood too much) so I passed on it , I could grind one end of the bit that way to compare with the one Mike mentioned.
Thanks for your suggestion and the pics.
 
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