Question for machinist

[John L]

I have 5/16" square lathe bit or cutter in my fly cutter for cutting nail nicks set up in the arbor of a small Atlas horizontal mill. It cuts a nice nick except I break bits occassionally. I would really like to extend the cutter even more for a longer nick, but am worried about breaking even more cutters. What type steel would be best for this cutter knowing that there are so many steels machined into lathe bits. I bought a huge variety of old bits and one lasted just about forever until I screwed up and broke it....wondering what type it could have been? Any ideas here are appreciated.

 

[Nathan the Machinist]

The cobalt and super high temp high hardness HSS are more brittle and less impact resistant. Plain old M2 (the cheap stuff) is probably a good choice. Grind it to provide enough relief behind the edge, but no more than you need - which can be tricky to do well. And a zero (or near zero) rake face is more durable.

Rather than stick it out more, why not get a larger fly cutter? They're not expensive. More than about 1" sticking out is asking for trouble.

Perhaps David Wesner can chime in. He is older than the hills and probably remembers fly cutters. I use inserted face mills. If you talk to him, ask him if he ever heard of them...

 

[John L]

Thanks Nathan...my Atlas uses an arbor similar to the arbor used for cutting wheels or slot cutters except this arbor has a 5/16" square hole perpendiculasr to the arbor and you stick out the cutter at this point. That is why the only way to achieve a longer nick is to stick it out farther. I will check M2 bits. Thanks.

 

[drewfacehotm]

you can try a braze on tool, basically a mild steel square shank with carbide brazed on the end of it. they are pretty cheap and cut pretty good, you can increase you rpms making it less likey to break.

 

[David Wesner]

The cobalt and super high temp high hardness HSS are more brittle and less impact resistant. Plain old M2 (the cheap stuff) is probably a good choice. Grind it to provide enough relief behind the edge, but no more than you need - which can be tricky to do well. And a zero (or near zero) rake face is more durable.

Rather than stick it out more, why not get a larger fly cutter? They're not expensive. More than about 1" sticking out is asking for trouble.

Perhaps David Wesner can chime in. He is older than the hills and probably remembers fly cutters. I use inserted face mills. If you talk to him, ask him if he ever heard of them...

Hardy-Har-Har First time we meet you get a shoulder punch for that one ................ if these old bones can catch you

John, Nathan gave you the goods. He always has great advice/help :thumbup:

Whenever I'm doing any milling one of my first considerations is rigidity. Be it in the spindle diameter, the length of spindle I have hanging out, my set-up, and also tool deflection. Tool deflection in the case of your single cutting edge flycutter is going to be your enemy. Extending it will probably increase the problem you're having. I'm not saying you won't get away with it. I do it in some cases too Try backing up your blade with a good solid angle plate or, if you're using a vise, add another piece of steel behind the blade getting the nick, to stop any potential "bad vibes"

When grinding a tool for a flycutter,(any cutting tool for that matter) when it comes to tool life, you'll find there is a fine line between between too much relief on the back side and/or not enough. In this case you need some material to support the shock the point will encounter when engaging the material being cut ...... but....... you also want a free cutting tool (that fine line I mentioned).

Something that might help increase tool life for you in this case would be to put a small radius on the very tip of your tool bit. Sharp points will fail 10 X as fast as one with a small Rad. Sometimes a Rad. as small as .010-.015 can increase tool life drastically. Do it by hand, the last operation, after generating your preferred geometry on the tool, using a fine stone.

If a new, larger, flycutter isn't in your budget, you can always make one. I have several I've made over the years that I still use to this day. Probably one of the more simple tools to make in the shop. Like Nathan mentioned though, new ones are cheap ......... less than $20. That might be the best route.