Models for grinding HSS Lathe Tools

Holding the tool bits free hand and trying to keep them at the proper angle even with lines on the table doesn't work very well for me. So I use my machinist square as a guide. I found on my delta 1x42 belt sander that the edges of the table are basically square to the platen. I drew a sketch of a 3/8" x 2" HSS tool bit and then added the angles based on Mikey's instructions way back on page 4 of this thread. The side angle is 9.5 degrees and the front angle is 19.5 degrees. I rounded those to 10 and 20 degrees respectively. Easier to set the square. To grind the side I set my machinist square to 80 degrees. Here is the set up.

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For the front I set the square to 70 degrees.

IMG_3706.JPG

And for the rakes it is back to 80 degrees.

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This works really well for me. Thought that I would pass it on.
 
I'm too clumsy when grinding, so I cut some pieces of plywood at various sharpening angles and use them to press against the tool. The 30 degree one works great for threading tools!
 
You do what you gotta' do to make it work for you - glad you guys are sorting it out. I tend to just grind by eyeball. Having ground so many tools, I sort of know what each tool needs to look like. The shape of the tool is not too critical as long as it is strong enough and allows you to access the parts of the work you need to reach. What IS critical are the tool angles, the relief and rake angles. Those need to be understood and ground intentionally. That way, if the tool does not do what you want it to do then you can change it until it does.

To be honest, I have sorted out the tool angles I need for most materials I turn. However, if I run into a material that doesn't cut as I think it should with a current tool that I'm using I have no reservation about changing the angles on the tool. If I think I need more side rake or back rake or whatever, I just grind it and see what that does. To me, that's the fun of being able to grind tools and it has taught me a great deal about how a tool cuts and how a lathe works. I hope the information found in this thread allows you to do the same.
 
@mikey I finally got to grinding a RH square tool today, without reference to a model. I didn't bother buying keystock. After buying one of the lots of 100 3/8" HSS stock linked to earlier in the thread, I was too impatient to wait to try the real thing, and besides I had a project to do. I mostly freehanded the tool against the wheel with checks against a protractor. I don't have a radius gauge but I probably put about a 1/64 radius on it. I roughed the radius on a 120 wheel, and smoothed and honed it, and all edges, with diamond cards. Didn't get to the arkansas stone because I'm not sure where my honing oil is and my father taught me never to put metal to arkansas stone without oil. :rolleyes:

When I turned 6061 with the square tool, resonance was visible in the bright surface. Facing, while good, was not quite as beautiful, so I could stand to hone the side cutting edge some more.

(I've been playing with the idea of making parametric 3D models that people could set the angles they want and a scale factor, then 3d-print them out. I'd like to put that on thingiverse so that people can just adjust the angles in the thingiverse customizer, download, and print, but OpenSCAD doesn't have chamfer support I need to easily model the nose radius. So far it's just an idea.)
 
Congrats! Any chance we could see a pic of your tool and a pic of this resonance? A Square Tool should produce a near mirror finish on 6061 so I'm curious why you are not getting that.
 
What I assume was resonance is a regular ripple in a near mirror finish on the turned surface. The "near mirror" quality is obscured by oil and grease now, and the parts are installed. No pictures handy right now.

I clearly have some roughness in my side edge. I pivoted the tool counterclockwise a few degrees and facing quality improved enough to see the reflection of the tool, though not quite as clear as I got from turning. I can't see the roughness in the side cutting edge, but I can sense it with my fingernail (unless I'm making things up), so I think that going back to the coarse diamond stone and working the edge some more should be my next step. That, and maybe ordering a loupe.
 
mcdanlj said:
What I assume was resonance is a regular ripple in a near mirror finish on the turned surface. The "near mirror" quality is obscured by oil and grease now, and the parts are installed. No pictures handy right now.

I clearly have some roughness in my side edge. I pivoted the tool counterclockwise a few degrees and facing quality improved enough to see the reflection of the tool, though not quite as clear as I got from turning. I can't see the roughness in the side cutting edge, but I can sense it with my fingernail (unless I'm making things up), so I think that going back to the coarse diamond stone and working the edge some more should be my next step. That, and maybe ordering a loupe.
Click to expand...

Look at the edge under a light source. If you see light reflecting off the edge then it needs work - one of the faces is not flat. From your description, it sounds like the tool is cutting almost okay. Sharpen it and it should work well for you.

Nice that you ground a useful tool!
 
@mikey — Question about definitions: For the square tool, you say that all the angles are 15⁰. Except for the End Cutting Edge Angle, which you call out at 80⁰. However, all the references I find (including Machinery's Handbook) define the End Cutting Edge Angle relative to the tool axis. I think you are defining it relative to the Side Cutting Edge Angle instead? Then the End Cutting Edge Angle would be, by Machinery's Handbook's definitions, 25⁰, and you would hold the tip of the stock to the belt at a 25⁰ angle (on a 15⁰ table for the 15⁰ End Relief Angle). I'm referencing that not only because it's the major reference in the field, but also because the illustration of the angles there looks suspiciously similar to the illustration at the end of your "How to Grind a Turning Tool" document. The illustration is vague, not explicitly identifying the reference for the End Cutting Edge Angle, but the text seemed clear to me. Am I barking up the wrong tree here?

For my square tool, I had missed that one exception to your 15⁰ rule and used a 30⁰ End Cutting Edge Angle by MH's definition, for a 70⁰ included angle at the tip.

I don't want to get needlessly pedantic. However, I'm experimenting with my idea for a parametric 3D model, which requires precise definitions of each of the terms. ☺

At this point I figure that maybe specifying the included angle of the point would be clearer for the newcomer who is the target of the parametric model.
 
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OK, I have a model of the square tool — as cut against a wheel — mostly rendered:

https://github.com/johnsonm/lathe-tool-models/blob/master/square-tool.stl

I haven't figured out an accurate way to render the nose radius because I don't have a chamfer function in openscad that can follow an edge, especially a curved edge, but honestly if it's 3D-printed the plastic won't be a sharp edge anyway.

OpenSCAD source for this model is at https://github.com/johnsonm/lathe-tool-models/blob/master/square-tool.scad — and it's meant that you can plug in whatever angles you are interested in, and see what it would look like. I haven't uploaded it to thingiverse because it's not done and I'm not sure whether the customizer will work anyway because rendering the curves resulting from cutting it against the wheel is expensive.

I think it's not worth putting on thingiverse until I add the ability to model cutting it against a belt grinder and make it the default, which will make it much cheaper to render because it will avoid most of the rounded edges that are so expensive to render.
 
You are correct with regard to the end cutting edge angle. The books reference the tool axis so if you wish to define it that way in your model then that would be in line with the books and I would encourage you to do that.

Now, let me tell you why I define it the way I do. The many, many illustrations and discussions in the books have confused and confounded machinists and would-be machinists for decades. You can see the drawings and you can read the terminology used to describe each feature but translating that into actually grinding a tool from that information is not easy. You have multiple tool faces, each of which angles in two planes, and the angles change with the material the tool is used with. If you went to a trade school the instructor would tell about all of this but then he would show you how to actually grind the tool. The hobby guy doesn't have that advantage. He has the books. This has turned the majority of the hobby machinist population into a source of carbide insert tool buyers and users, largely because it is easier but also because that is the way the industry has gone so it has to be better, right?

What I've tried to do in everything I've written on this subject is to simplify it as much as I could. I tried to define the what/where/why/how/how much for everything so that the average guy could grind a tool of his own, by himself, simply by reading what I wrote. And I used Mikey-logic to do it.

For example, the side cutting edge angle and end cutting edge angle of the Square Tool is what it is because it is intended to allow you to access a shoulder. The shoulder itself is 90 degrees but an included angle of 90 degrees on a tool will rub so I made it less than 90 degrees so it wouldn't rub. I used the side cutting edge angle as a reference so I could tell the guys to make the included angle at the tip less than 90 degrees; the precise angle is not important. To grind that angle, the simplest way is to use the side cutting edge as a reference and draw a line that is less than 90 degrees to it and then grind to that line. Simple, easy to understand. I chose 80 degrees because it allows a 5 degree cushion between each edge that the eye can easily equalize in that corner when orienting the tool. Mikey-logic, you see?

I have taken other liberties beyond this angle thing. I have tried to explain what each angle does and how they can be modified to allow the tool to work better on a small lathe. That is a major departure from every reference text on tool grinding that I am aware of. I am probably the world's only proponent for small nose radii; every other resource and anecdotal opinion will tell you that bigger is better (but they don't tell you why).

I hope this clarifies this for you. Okay, okay, I went beyond clarity into diarrhea of the brain but it was important to explain that I had good reasons for going beyond the texts.

At least now we have guys who can actually grind real lathe tools - good enough for me.
 
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