Changing tool bit geometry to suit the tool holder

[Tony Wells]

Hey David.....aren't you becoming a bit verbose?

 

[Conrad Hoffman]

Just made this mistake recently. I ground a cutoff tool, but forgot it was going in an angled holder. Took me a minute to figure out why it wasn't cutting! You can definitely improve threading tools, but don't forget that the trailing edge is already operating with negative rake. Unless you feed with the compound, don't make it worse by putting more positive rake on the leading edge. IMO, the safest bet is to grind the top equal to the thread helix angle so both sides cut about the same. In tough materials it can help a lot.

 

[Conrad Hoffman]

To clarify on the threading tool, you need the 60 degree included angle for the basic thread geometry if the top of the tool is flat and on center height. If you start messing with the top of the tool, the 60 degree angle will need to change slightly so the finished part still has a 60 degree thread. Fortunately you can make small changes to the top of the tool without worrying about it. If you use x-ray vision to look through the tool into the thread, you'll see that the left side of the thread presents itself to the tool with positive rake- a good thing. The right side of the thread presents itself to the tool with negative rake and thus doesn't cut as well. Feeding the tool with the compound at 30 degrees (29.5 or so is best practice) very little is asked of the right (trailing) side of the tool so all works well. Still, if the top surface of the tool is angled slightly to match the helix angle of the thread, both sides operate with the same amount of rake. If feeding with the cross slide, this improves the thread quality somewhat. IMO, threading tools need to be razor sharp and I've got a couple pages on my site that you might find interesting; tool sharpening and metric threading. Even if you're not doing metric threading, there's some useful general info a bit further in the page.

 

[Tony Wells]

Probably a square turret post that comes standard on many lathes. Want one?

 

[Tony Wells]

We don't do flame wars here. That's one of the things that are different here. Respect, I believe it's called. You're just as entitled to your opinion and choice as anyone else here.

I'm pretty much a carbide advocate myself. BUT, HSS surely has a place in many, if not all shops. There are numerous advantages in a production environment, obviously, but even in a hobby shop, carbide can make things easier and faster. You just have to know when to use it, and when to break out the grinder to make a HSS tool. In a commercial shop, time is money, so carbide will win out nearly every time. Every time on a CNC. On a manual machine, there are some things that really are best handled by a hand grind. Our rework guys used HSS tools quite a bit to chase after the NC machines for some profiles that you just couldn't cut manually otherwise.

But this is a home shop forum, and our discussions center on what is good for the hobbyist. Carbide isn't for everything or everyone, but it surely should be considered, and no one should be ridiculed for their choice.

 

[Conrad Hoffman]

IMHO, the trick with carbide in the home shop is that it needs to be far sharper than it usually is. Tune up cheap brazed tooling with a diamond wheel and it can do a nice job. I always keep a couple brazed tools loaded in holders for basic facing and turning because I rarely have to sharpen them. That alone saves time. I also seem to modify hard socket head cap screws constantly and the carbide works well for that. You can get inserts that are sharp and excellent, but I'm too cheap! OTOH, I was just making some custom cutters in O-1 this weekend and couldn't get a good finish until I ground the right tool in HSS.

 

[cyrusb]

No doubt carbide works great, but there are times when carbide just won't do, interupted cuts, special forms etc. And as a "hobbyist", for me the cost is not justified. If you are having problems with hss finishes get out your India stones and polish the tool surfaces. A polished tool creates a polished part. Something I learned years ago with my T rest and gravers. When done right no secondary finishing is nessasary. My .02

 

[Tony Wells]

Small radii generally I put on with a stone.

 

[Conrad Hoffman]

I use the same trick with my grinder. You can also use a piece of fine wet/dry paper on a sheet of glass, but it's not as good as a stone because you get a slightly rounded edge.
Tool Sharpening!