Hss Tool Bit Grinding: Thoughts, Observations, Questions.

Arrak Thumrs

Arrak Thumrs

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I have ground lathe tools for specialty things in the past. i.e. various boring bits, strange shaped tools for esoteric situations, etc. That said, my go-to turning tools have been HSS inserts from A.R. Warner for a long time now. It felt time to get more of an education in tool angles by grinding my own. One thing that I've long thought about was the difference in adding positive back rake. The HSS insert tool holders are all neutral rake.

A few observations so far. A larger nose radius seems to be needed on my hand ground, 5 degree positive rake tools. For the size of work and DOC typical in my shop, the neutral rake inserts work best with a 1/64" nose radius. I was getting a poor finish with my new tools until I increased the nose radius to more like 1/32". I guess there is some connection to the back rake angle?? I don't know, but it seems to be the most likely option. Curious.

What angles do people like for clearance and back rake? It is a little confusing looking at the usual sources on tool bit angles. They are nearly all referenced to lantern tool posts and holders that already include an angle. I arbitrarily chose 5 degrees for everything. 5 degree side clearance on both sides. 5 degree positive back rake.

I don't see any reason to make excessive clearance angles. 5 degrees seems more than enough, yet I see on tool grinding examples some much larger angles at times. I don't get why.

Back rake. How much can I put on? 10 degrees would seem to be an awful lot, but is it? Nearly all of my machining is steel (free-machining on up to 4140 & tool steels).

Thanks for your thoughts! :)
 
7-7-7 or so is my general angle for everything. The nose radius on my main RH tool is around 3/32. Massive, and takes some extra power to cut but it leaves a nice finish.
 
If I recall, around 10-15* is the break even point where you maximize power and still maintain tool edge life. I don't really know what I grind mine at though. I just use the tool if it cuts well and alter it if it doesn't cut well. As far as nose radius goes, I think mine are around 1/64 or so. The larger the radius, the nicer the surface finish, but as Andre said requires more power and thus slightly less depths of cut.


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"I don't see any reason to make excessive clearance angles. 5 degrees seems more than enough, yet I see on tool grinding examples some much larger angles at times. I don't get why."

Sometimes pictures (drawings) exaggerate the angles to make them visible. I ,too, like the old 7-7-7 rule. But I'm an old codger too. Just do what works for you. I have both small low powered lathes and a big gear headed powerful beast. The little ones were first and I learned a lot about tool geometry from these micro machines. Positive rakes are a must. The big honker uses negative rake tools well and makes BIG chips. The finishes with both big and small machines are great but it takes some playing to learn what they want to work well. Play with they and enjoy the lessons they will teach you.
 
Do you hone your tools on a stone? I find that a little bit of that improves finish a lot. Also when putting a radius on the nose be sure to extend it all the way down the tool. If you just round off the tip while leaving the lower part sharp you reduce the clearance at the nose.
 
I am probably the worst person to answer this one, but I'll be honest. Disclaimer: No science or measuring involved here so don't do what I do.

I work mostly in aluminum. I use 1/4" HSS for most finish work and 3/8" Tri-cabide insert tools for rough work because they're a workhorse. I like to use my left, right and center HSS cutters with a 10* clearance angle, 10* sides and flat top for L & R. I like the L & R to have a full radius that I use for a scraper on a backside finish cut. I grind them to a sharp point and do a quick "knock the point off" (10*) wrist action maneuver. I own a set of radius gauges, but never bothered to measure. I sharpen my HSS tools with my 240 grit belt sander(!) and do a quick hone with a flat stone.

It may not be the quickest, most efficient or politically correct -but if you've ever seen my work, I usually get very good finishes with this simple, non-scientific, or industry stds. method. Obviously, I could take the time to make it more professional, but why bother if my simple method works for my hobby guy projects.

Now, bring on the haters.
 
Thanks for the replies, everyone! :) Good to know I've started from a reasonable place. I'll experiment with increasing the angles a little since I seem to be a couple degrees less than many posters.

Fabrickator, I don't see any reason for you to be "hated?" As respected a source as George H. Thomas champions the use of his "linisher" (British term) for tool grinding. I use my belt sander to touch up my parting tool regularly. Why not?

John, I rough grind on an 8" grinder with ? wheels (blue, coarse grit) to get rid of the bulk. Then I grind on a 7" 38A80 (I think) wheel to finished size, shape and angles. Then I use a Norton "sharpening stone." I wish it was labelled... Even the original box does not explain exactly what it is. My best guess is a 320-ish grit, hard Arkansas stone. It is orange and holds shape well. Oil is needed during use or it clogs and burnishes rather than cuts. The stone is pretty fine for using right off the grinding wheel. I'm thinking a more coarse, intermediate slip stone would be nice to add before final sharpening on the one I have now.

No mention yet of "side rake." It goes on the top face of the tool bit. There is "back rake" which is often mentioned. That is the slope back toward the shank of the tool bit. There can also be "side rake" which is the slope down toward the trailing side of the cutting edge. Back rake and side rake together create a compound angle. I forgot all about side rake when grinding my tools. It doesn't seem to have as prominent a place in discussions I read of tool angles. 'Guess I'll experiment and see what it might or might not accomplish for me.
 
The simplest answer is "it depends". Depends on available HP, machine rigidity, material, DOC, feed speed, desired tip life, desired finish (which is also a function of feed and DOC). The easiest answer is to just play around with different grinds. To be honest, most grinds, especially if you hone/ stone the edges, will cut most materials just fine. I think the optimisation is required for difficult materials (like copper) and small lathes, which need all the help they can get.
 
in my limited experience side rake helps with chip formation, as well as increasing the shear effect of the cutting edge which reduces cutting forces but can cause problems in gummy or grabby material. Chip formation is also a function of speed, DOC and feed too, so it's not the only answer.
 
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