Hss Tool Bit Grinding: Thoughts, Observations, Questions.

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Arrak Thumrs said:
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! :)
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I have two sets of cutters,for brass and aluminum and another for everything else. Brass and aluminum are 5/16 square and have no side rake. Steel has about 8° or so of side rake. They also are 3/8" square,makes it easier to pick the correct cutter.Front rake and back rake about 10° or so for all cutters. For the best finish on long turnings the tool has a radius across the entire tool width. Other radius on cutters vary from 1/32" / 1/8" , depends on the stock and how close to a square shoulder needed.
 
The more rake you have, the more the cutter will shear. The more shear you have, the straighter the cutting edge needs to be. So the more rake you have, the larger the radius needs to be.
I just adjust the radius on the fly. Since the radius can be touched up with the tool still in the holder, I just look at where the cutting is taking place, and reshape the radius if needed. No idea what size radius I use, I just grind it til it looks right and cuts right.

This is the tool I use for most aluminum, it is resting on a 3" dia aluminum tube. It has somewhere between 10 and 12 degrees of side rake, about 20 degrees of side clearance, about 12 degrees of front clearance. The tool is positioned to give 0 or slightly positive lead angle, which results in a positive back rake on the cutting edge. Steel cutting tools look similar, but use an effective o or negative back rake, and only about 5 to 7 degrees of side rake and clearance.
The tool will easily make really deep roughing cuts, and the chips look exactly like the fuzz you get from a vertical shear tool when making light cuts. I used to use roughing and finishing tools, until I started grinding them like this - now I just use one tool.

The end of the tube shown was hanging unsupported 10" from the chuck. This is the test bar I use for leveling the lathe.
IMAG0503.jpg
I tried to get a pic highlighting any roughness in the cut. You can just make out the micro-grooving, it looks mirror smooth unless you look really hard for the cutter marks.

I never hone my tools - it is a waste of time. I grind on with an 8" 120 grit Norton wheel, and go right from the grinder to the lathe. You can see the radius looks really huge, for some reason the tip looks blunted in the image, but it is a nice smooth radius and still sharp enough to draw blood.

Hope this helps
-Josh
 
I have been a machinist for more then 50 years and my thinking is for light duty lathes use lots of very high positive rake on the top of tools high speed or carbide. I have used as much as 45 degrees top side rake. I made a quick video showing me cutting a .250 depth of cut on aluminum on a Logan 11 inch lathe. The lathe does not have power feed so I hand fed it. I got some comments about how light the cut was but no one showed me another light duty lathe doing that much depth of cut on their lathe. I went out to the shop after I got the poo poo comments and tried it on 1 inch dia c1215 and got the same results. I have owned lathes with 20 horsepower and that is another story.
jimsehr
 
Jimsehr said:
I have been a machinist for more then 50 years and my thinking is for light duty lathes use lots of very high positive rake on the top of tools high speed or carbide. I have used as much as 45 degrees top side rake. I made a quick video showing me cutting a .250 depth of cut on aluminum on a Logan 11 inch lathe. The lathe does not have power feed so I hand fed it. I got some comments about how light the cut was but no one showed me another light duty lathe doing that much depth of cut on their lathe. I went out to the shop after I got the poo poo comments and tried it on 1 inch dia c1215 and got the same results. I have owned lathes with 20 horsepower and that is another story.
jimsehr
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I totally agree. I "can" do .25" doc, but I seem to have the sweet spot at about .2" doc on my 11" Sieg SC8 for roughing aluminum. I run it as fast as reasonable so I have some inertia to overcome the lack of power. It really comes down to feed rate, if you can get the cutter to advance at just the right rate, it just peels the chip off the face like peeling an orange. 45 degrees sounds amazing. I will experiment some more with this. I have been hesitant to go any steeper because I worry about the carriage getting sucked into the cut. The carriage is pretty wimpy on the Sieg lathes, it would not take much to pull the lash out of the lead screw and crash the work.

One of the things I love about this forum, is that you don't get ragged on by a bunch of gurus for doing something they know is impossible in their view. I mean, we all know it takes a 2 ton Monarch to make a .25" doc, and small (read import) machines are not capable of cutting under a thou...
 
joshua43214 said:
I totally agree. I "can" do .25" doc, but I seem to have the sweet spot at about .2" doc on my 11" Sieg SC8 for roughing aluminum. I run it as fast as reasonable so I have some inertia to overcome the lack of power. It really comes down to feed rate, if you can get the cutter to advance at just the right rate, it just peels the chip off the face like peeling an orange. 45 degrees sounds amazing. I will experiment some more with this. I have been hesitant to go any steeper because I worry about the carriage getting sucked into the cut. The carriage is pretty wimpy on the Sieg lathes, it would not take much to pull the lash out of the lead screw and crash the work.

One of the things I love about this forum, is that you don't get ragged on by a bunch of gurus for doing something they know is impossible in their view. I mean, we all know it takes a 2 ton Monarch to make a .25" doc, and small (read import) machines are not capable of cutting under a thou...
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If anyone wants to see my carbide high positive cutting tool working it is on you tube .
enter hi positive tool
jimsehr
 
joshua43214 said:
One of the things I love about this forum, is that you don't get ragged on by a bunch of gurus for doing something they know is impossible in their view. I mean, we all know it takes a 2 ton Monarch to make a .25" doc, and small (read import) machines are not capable of cutting under a thou...
Click to expand...

Your C8 is a copy of an Emco Super11 that can easily take a 0.25" cut in aluminum or steel. As you and Jimsehr are saying, its the tool and not just the lathe.
 
mikey said:
Your C8 is a copy of an Emco Super11
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At the risk of nit picking it is the other way around. Most Chinese sourced machines are a cot of the sieg or a rebadged sieg.


Sent from my SM-N920C using Tapatalk
 
I typed EMco, not Enco. The Sieg C8 is a near carbon copy of the Emco Super 11 lathe, which is a precision Austrian-made lathe introduced about 20 years ago. I'm pretty sure the C8 did not precede it.
 
mikey said:
I typed EMco, not Enco. The Sieg C8 is a near carbon copy of the Emco Super 11 lathe, which is a precision Austrian-made lathe introduced about 20 years ago. I'm pretty sure the C8 did not precede it.
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ya, part of what motivated me to buy the SC8 was it looked like the Emco. The Sieg is a decent lathe, but I would not recommend any one buy it. It is really just a scaled up 7x mini-lathe. I have had almost nothing but problems with it, and now I am getting the ignore from both DroPros and Sieg. The head stock is out of square and it cuts a pretty severe convex, and one of the gears in the carriage has about 0.004" run out making it almost un-usable for fine adjustment. So I have to hope I can tweak the head stock, or I will have to scrape a new lathe into shape. And I will have to make some new gears. I am also pretty sure the carriage and cross slide are out of square by about 0.003" over 5" - pretty crappy by any standard.
I was told by DroPros I should be happy to have such a great lathe and such a low price...

I am currently looking for a replacement (waiting for a 10EE deal) so I can turn this one into a line boring machine.
I have no doubt the Emco can do real work, the SC8 can do some good work, but I have stalled the machine many times when pushing it.
 
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