Turning Tool And Facing Tool Questions

JR49 said:
Mikey, How about a Tutorial on the different degrees of the angles used , and for what purposes. I posted on the first page that I felt I was pretty good at tool grinding, BUT, after reading your posts, I'm not so sure anymore. I'm sure Carlo, and myself, and Joe, would like to learn more. Thanks, JR49
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I wrote those articles that Joe linked to some years ago to answer questions just like those we're discussing here. A guy on a forum was totally lost and couldn't wrap his head around how tool geometry worked so I wrote that just for him and then later polished it up for Rob, the owner of Machinistblog.

Its one thing to watch a video and see how a tool can be ground but it is quite another thing to understand how the angles actually work and then altering them to suit your purposes. Those articles will hopefully give you an idea of how to make your tools do what you want them to do. Have a look and we can discuss this more if you feel the need.

Oh, I should add that that article was for small lathe users because that is what the guy had - a Sherline lathe. The same principles apply to larger lathes as well. I can take a tool and grind it to reduce cutting forces on my 11" Emco lathe just as easily and it will cut far better than a tool with standard tip geometry.
 
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Guys, I've been thinking about our discussions and it seems to me that some of you are capable tool grinders. If that is the case then don't think you have to change what you're doing. On the other hand, for those of you who have smaller lathes that seem to struggle a bit then I suggest to you that you can grind a tool that will work far better than a standard geometry tool. The angles found in the typical angle table are suggestions based on what is known to work on larger lathes in production situations but if your lathe is not one of those larger lathes then using those angles may produce cutting forces that can cause problems for you. Every tool angle has a purpose and while not all of them need to be modified, when we do change them for the right reasons the difference in performance can be significant.

I know of one guy who does not use back rake on his tools; his videos show this. That's okay but you must also realize that back rake plays an important role in chip clearance, force focalization, cutting forces and cutting temperatures. Not incorporating back rake into your tool may be deleterious to your tools performance.

If you read the articles Joe linked to above you might have the impression that these angle changes only apply to little lathes and don't have a place on larger lathes but you would be wrong. Anytime we can make a tool cut more efficiently with less load and with greater precision then it is worthwhile considering it. The tools I use on my 11" lathe to cut 1144, 4140 and 303 all use larger relief angles, more back and side rake in order to reduce cutting temperatures and allow less cutting forces during the cut. These tools produce cleaner cuts with less work hardening and greater precision. It takes only a few minutes for me to modify a tool to do this and I can tell you that they work far, far better than standard geometry tooling.

I think we get locked into thinking that the table angles are locked in stone but they are simply baseline values. Where we take them from there is up to us and I encourage you to experiment with them. Once you see how much better a modified tool cuts you'll never go back to a standard tool.
 
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I would be very interested in hearing what everyone (but particularly Jim and Mikey) think about this statement; The sharper you make the cutting edge, by increasing rake and clearance angles, the better, more precise, smother, and with less cutting force, the tool will cut. HOWEVER, increasing the angles too much, will cause the tool to wear faster, so one must find a happy medium between excellent cutting features, and tool life, or time between sharpening.. What say you guys, JR49
 
Those are really nicely written articles, mikey. Well done!

-frank
 
JR49 said:
I would be very interested in hearing what everyone (but particularly Jim and Mikey) think about this statement; The sharper you make the cutting edge, by increasing rake and clearance angles, the better, more precise, smother, and with less cutting force, the tool will cut. HOWEVER, increasing the angles too much, will cause the tool to wear faster, so one must find a happy medium between excellent cutting features, and tool life, or time between sharpening.. What say you guys, JR49
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You are correct in concept, JR. The way I approach angle changes is to consider what it is I want to change and why I think I need to change it. You can reduce cutting forces by altering relief angles, side rake and back rake and you can also alter the degree to which you make those changes.

For example, say I am cutting 303 and I want to get the heat out of the cut as fast as I can. In that case I would increase my relief angle a few degrees to gain a sharper interface and increase side rake up to 4-5 degrees to really clear those chips fast. Since I do not necessarily wish to focus my cutting forces at the tip of the tool but prefer to take the load on the side cutting edge I would leave back rake at the standard setting. What this does is give me a sharper cutting edge with better chip clearance and I doubt you would see a breakdown of the cutting edge unless you have a very long production run. I may also increase the end cutting edge relief to improve shearing action at the cut, which can have an impact on work hardening also.

Or say I needed to cut some 1018 (I hate that stuff). I would increase side relief by about 4-5 degrees for better shearing action but keep side rake at baseline for strength. I would increase back rake 3-4 degrees to focus cutting forces near the tip and use a slightly larger nose radius to give me as clean a finish as possible. All of this allows the tool to cut as clean as possible in the lousy material with as good a finish as a tool can give.

I guess my point is that you should use the various angles to give you the advantage you need when and where you need it. You may need to touch up the tool more often but in my hobby shop that hasn't been an issue. I sharpen all my tools after each use or before critical cuts anyway.

EDIT: I meant to mention that increasing side and back rake will actually improve cutter life while reducing cutting forces and reducing cutting temperatures provided you don't take these angles to extremes. If you wish to have a sharper cutting edge without sacrificing edge strength, leave the relief angles alone and boost side rake - it gives you a sharper interface and better chip clearance without your edge dulling prematurely. Try it - it works.
 
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Hey JR49, I have a proposition for you. Why not grind what I call a "square tool" and tell us how you think it works in comparison to what you normally use. Pick a general shape that you prefer and grind it with 15 degrees of side and end relief, 15 degrees of side rake, 15 degrees of back rake and put a 1/32" nose radius on it. This tool will cut with very low cutting forces and will enable you to hold tight tolerances. Give it a try and report back.
 
Not to stick my nose in although I do love to learn. My entire strategy behind grinding tools is to have the cutting edge present itself with the least amount of force necessary. I try to use smaller nose radi in hopes the heat will stay in the work vs being absorbed into the tool and I try to adjust for proper chip clearance but I haven't quite figure that one out yet. So far the only time I've damaged a tool is when I'm not paying attention and touch off way to hard. Or I think I touch off and I don't and run the tool way to deep. My whole outlook has been to present the cutting edge too the tool with enough clearance to keep heat in the work and not the tool.


Regards-Carlo
 
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