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Home > Shop Tips > Grinding HSS Lathe Tools By Hand: Part 3 – Grinding a Threading Tool

Grinding HSS Lathe Tools By Hand: Part 3 – Grinding a Threading Tool

In this final video of Barry’s 3-part series on grinding HSS lathe tools Barry walks us through the process of grinding a threading tool.

If you missed part 1 or part 2, they can be found here:

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Hi, Barry Young here with part 3 of how to grind tool bits. Today we are going to grind a threading tool.  This is used for external threading on round pieces in the lathe.

This is a rather large one. It’s 1″ square. We use much bigger ones in industry and much smaller ones at home in our home workshops.

Today I’m going to grind this piece of mild steel to make it look like a threading tool. I use mild steel because it grinds quicker here and I won’t waste your time. So, off we go.

You must have a center gage, like this. Some people call it a “fish tail”. Center gage is the correct term. It’s not a “thread gage”. Now, in this fish tail, right on the end is a groove, a v-groove. We are going to make this tool match, fit in that v-groove. When we are done the tool won’t go like this, or like this, it needs to come straight out of that V.

Ok. I usually just put the tool bit right down onto the tool rest and rotate it 30 degrees, and plunge. Now, I’m grinding this much quicker than you will to try and make the video short. And that’s why my tool bit is changing colors. When I grind a tool bit for use I avoid changing colors.

Alright, now I’m going to just flip it over and do the other side exactly the same way. Grind grind grind. Well, we’re getting close. Now I’ve got it roughed out. Let’s check it with the center gage and see which direction we have to go here. Well? What do you think? Let me take a look here. Looks like we’ve got way to blunt a tip. So, I’m going to find out where I was, and then I’m going to crank it over some more to make it actually sharper.

Ok, let it cool down here a minute. Let’s check it. Is that a little better? Looks like there’s a … now its a little too sharp. It’s got some slop in there. So I’m going to have to grind the tip away. I’m going to find out  where I was and just rotate towards the tip.

Same thing on the other side. And lets check again. Now we’re fitting. Pretty darn well. Ah, it’s still a little bit too sharp though. Grind away a little bit more of the tip. Notice I’m leaving it on the wheel long enough to where it makes one solid cut. That’s good enough for this demonstration, but in the home workshop you can go until that’s perfect.

The last face I need to grind is the top rake. So, turn 90 degrees to the wheel, and notice that the shank end is farther than then end being ground. What that will do is give me an angle like this, or a “top rake”. And what I’m doing now is grinding until I see sparks coming off the tip.

Now if you’re doing soft materials you’ll make this even more pronounced. Like for plastics you can go as much as this much, 10, 15, 20 degrees even. Teflon, probably 30 degrees. That’ll make it extremely sharp. Now I have a tool that’s approximately the correct shape and I’ve got the top rake in. The problem is that because I’ve flipped this tool bit over I don’t have the compound angle that I was looking for on each one of these faces. To get that, now I can’t be on the tool rest. I lift the tool, line it up, you can feel it when it aligns to where it was, and then lower the shank until you get sparks off the top. Like this. As soon as sparks come off the top you know you’re there. Do the other side the same way.

And once we finish these two compound angle cuts you’ll find yourself to be the happy owner of a new threading tool.

Now, there’s some things people do wrong with this tool. Let’s talk about that. One is too much slop, which means you need to grind away the tip, as I did earlier.

Another thing they do is what you can see here – faceting. Lets make that one solid grind top to bottom.

And another thing, they don’t remove these burrs on the end of the tool. So, I’m going to get rid of that burr and I do that just by working it back and forth edge to edge, one edge and then the other.

Now we have a tool that will cut threads. That’s how you do that. Thank you very much!



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About Tyler

Tyler is a hobby machinist and 3D printing aficionado. He teaches computer programming and web development at Highline College near Seattle. Tyler founded Projects In Metal in 2008 because he was frustrated by the lack of free plans available for hobby machinists.

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