Models for grinding HSS Lathe Tools

[tmenyc]

Thanks, Mikey.

 

[mikey]

Tim, I took a closer look at your tool. It looks like you're using a wheel to grind with, which is fine. When you regrind it, try to get the top and bottom of the tool face even on the wheel. There will be a dip in the center of the face due to the radius of the wheel but try to make even contact at the top and bottom of each face. Same thing when you hone it; try to get evenly honed areas at the top and bottom of each face. Whenever two faces intersect, you should not see any light reflecting off that edge when the tool is sharp. On a knife tool like this, keep the nose radius really small but round and even from the top to the bottom of the tool. The overall shape of the tool is good, though, so hang in there.

 

[tmenyc]

Shall do!

 

[mcdanlj]

keep the nose radius really small but round and even from the top to the bottom of the tool

I decided to try to add the nose radius to my 3D model to show what it should look like. "How hard could it be? Just cut off the end of the nose and merge in a cylinder with the nose radius and bob's your uncle!"

When I did that, the cylinder intended to represent the nose radius was very much in the wrong place. Off by several degrees, and off by more with more aggressive relief angles. I realized that the angles are compound and, for example, if you set the table angle to 15⁰ for side relief angle, and then hold the tool to the belt at a 15⁰ angle on the top for a 15⁰ side cutting edge angle, you'll get a 15⁰ side cutting edge angle but have a bit more than 15⁰ side relief angle.

This isn't enough to matter when you are grinding these tools! At the angles @mikey talks about, you'll generally be within about a degree, and remember that @mikey's recommendations are based on these simple table angles and tool angles, not on trig functions and compound miter math, so do it the easy way to replicate his starting points, and then when you are adjusting based on your machine, your stock, your desired finish, etc, you'll be adjusting by at most a few degrees at a time, and the change you are actually making to the underlying angles is all you care about, so it still doesn't matter.

It does matter when you're trying to make a precise computer model (as I am), and it does matter if your goal is to replicate precise angles from published works, if and only if the angles in use are sufficiently large. And, it turns out, one of the specific clear points from @mikey's research was that hobbyists might prefer larger angles than are typically published for commercial work. But if you are trying to precisely replicate some industry-specified model that has, say, a 25⁰ side cutting edge angle and a 20⁰ side relief model, you'll have to set your table at 18⁰ rather than 20⁰ in order to get 20⁰ of side relief, because asin(sin(25)*cos(20)) ~= 18. The same kind of adjustment would apply to large end cutting edge angle and end relief, of course.

This is not generally important, and if laying out piles of right triangles in three dimensions and muttering SOH-CAH-TOA over and over doesn't sound fun, you should ignore the whole thing, On the other hand, if you enjoy piling up trig functions through three dimensions and keeping track of multiple reference frames while doing so, let me know and maybe you can find the bug in my model that is breaking my ability to consistently 3d-model the nose radius.

 

[mikey]

And I thought tip geometry was complicated! I had the easy part, figuring out how the geometry works and how to grind it to do what I wanted it to do. If I had to sort it out with Trig I would have gone to inserted carbide tools and stayed with them! Kudos to you for doing this, @mcdanlj!!

 

[TonyBen]

I'm almost done grinding my first tool on a 1/2" HSS steel blank. I used Adam's video and I'm making a square RH turning tool since the majority of what I'll be doing is trimming barrel shoulders and making square cuts. I just need to do the top angles, round the nose and hone the surfaces. I'll post pics later.

I went out and bought a Porter Cable bench sander with the vertical sanding belt and the round sanding wheel. I've been using the round sanding wheel and the angle guide that came with the sander. I also bought an angle finder and found the angle guide on the tool to be 1˚ off.

I'll have to set up the bench grinder to make the last two top cuts.

I've got two 3/8" blanks I can work with also.

Tony.

 

[mikey]

Please keep us posted on how your tool turns out and how it works for you, Tony.

 

[TonyBen]

Here's my first attempt. Will try to finish the top cuts today. The bench sander came with 80 grit. I bought a finer belt for finishing later.

Tony

 

[mikey]

Looks good so far, Tony. At some point, see if you can find some ceramic belts for that machine. They grind much faster and cooler than aluminum oxide. Also pick up some stick wax to lube the cut. This extends the life of the belt, reduces friction and has many other uses in the shop.

 

[TonyBen]

All done. I did add a nose radius using some 240 grit sandpaper on a flat granite stone after I snapped these images last night. I had to use my 6" bench grinder to do the top cut as I couldn't get the compound angle right on the disk sander

I did a facing cut on front of my 3-jaw backplate last night with this tool to try and bring the runout down and it went from 0.013" to 0.010" on a 1" steel shaft.

I hate facing cast iron as theres just so much fine dust.

Looks like I'll be installing the 4-jaw and turning an aluminum bar with this tool (I don't have a large steel bar right now), then I'll clamp the 3-jaw to the bar and face the back of the 3-jaw backplate where it would contact the spindle shoulder and try again.

This tool should work real well for shoulder turning. Now I just have to figure out what to do with the other side. I might make a left hand knife tool since there are times that I have to turn down a boss for a gas cylinder or an operating rod guide and I have to chuck up the barrel the opposite direction and cut from the other side.

I have two more 3/8" blanks and I will make a threading tool as well as a boring bar. I do need to make custom muzzle guides for odd size flash suppressors and muzzle brakes that keep me from using my cleaning rods from the muzzle end. I'll have to buy some delrin to work with which I've never cut before on a lathe (or a mill for that matter).

Tony.