Angle of HSS turning tool to work

mikey said:
To be honest, I haven't seen a really good book on this subject but there are tons of videos on YouTube. Most of the videos discuss tools suitable for use in a QCTP. The thing that sets a more modern design apart from tools intended for a lantern tool post are the back rake angles - they are either smaller or non-existent in tools made for a lantern tool post because the tool post holds the bit at an upward angle already so you don't need to grind the angle into the tool.

Here is a thread I posted on grinding these tools. At the end of the original post are some links that may provide more info you might find useful. http://www.hobby-machinist.com/threads/how-to-grind-a-hss-turning-tool.52581/

When you use a HSS tool in a QCTP, the angle of the tool can be whatever you want. The questions I think you have are "what angle should you use, when should you use it and why?" I suspect every one of us has asked ourselves these questions at some point so you're not alone. The answers aren't that complicated so I'll try to explain them.

If you use an inserted tip carbide tool then the tool shank is intended to be held perpendicular to the work. All the clearance angles are designed into the tip geometry of the tool and insert and all you need to do is set the tip of the tool to center height, get it perpendicular and you're set. Brazed carbide tools are, in general, used the same way. HSS tools are shaped and have varying geometries so we need to understand things a bit better to get the best use out of the tool.

A HSS turning tool cuts most efficiently when the lead angle of the tool is appropriate for the operation; to really understand this tool angle thing, you have to understand what lead angle is and what it does.

fig161_02.jpg


The lead angle (LA) of a turning tool is determined by the side cutting edge angle of the tool and the way the tool is angled to the work. When the tool shank is perpendicular to the work then the LA is defined by the side cutting edge angle, as in the third example above. We can change this LA by turning our QCTP. So, what is the big deal about LA and why does it matter?

Basically, the LA of the tool alters cutting forces and affects finishes. The lower the LA, the lower the cutting forces are and the rougher the finish is; the greater the LA, the higher the cutting forces are and the better the finish is. This cutting force thing has a lot to do with the amount of surface area that is in contact with the work. When the LA is high, more of the side cutting edge is in contact with the work; when the LA is lower, less cutting edge is in contact. If you use a high LA and then try to take a heavy cut, cutting forces increase and you get chatter; backing off on the depth of cut reduces cutting loads, the chatter goes away and your finish improves. Conversely, using a lower LA brings less surface area into contact so you can take deeper cuts without chattering, albeit with a less refined finish. So, what does this have to do with tool angles and tool shapes?

The angle of the tool in use is determined by the operation. If you are roughing, angle the tool so that the LA is lower (like the middle pic above). If you are finishing, increase the LA (like the pic on the right, above). When the LA is low, you can take a heavier cut; when the LA is high, you have to take a lighter cut. The lower LA is used for roughing and the higher LA is used for finishing. For facing, the LA is negative such that you are cutting with the forward part of the side cutting edge, up near the tip. Facing depths of cut can be light or heavy, depending on your needs, because only a small area is in contact and the potential for chatter is lower.

Tool shapes are not as critical when we use a QCTP but they do have an influence on how much or little you angle your tool. You will find that the LA you end up using for a given operation will put the side cutting edge at an angle very close to what the older shapes did when their shanks were perpendicular to the work.

Bear in mind that we're just talking about tool shapes and lead angles here. Cutting forces are definitely influenced by these things but the relief and rake angles have a much greater impact. That is a discussion for another time.
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The main thing about lead angle is that if you are roughing with zero lead angle, the tool tends to creep in, and likely cut undersize if you are roughing close to size; the lead angle puts pressure outwards on the tool, so that the tendency to creep in is eliminated.
 
I have a question about back rake angle, what is a back rake angle?
why it 's not needed with a QCTP?
if it is not often ground in a turning tool then why do we even mention it?

Please remember I have seen pictures of back rake angle (when I googled it), it sounds like it is used for woodturning when a sharper angle is needed but the point of the question is, nowadays most people who grind their HSS cutting tools for turning and facing,do not incorporate a back rake angle in their cut but it's always mentioned .
 
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Remember the quality of HSS tools is much better then it was when most of the old machining books were printed. I have been machining for over 60 years and can remember some of the poor tools of the day. In those days some of the angles made more sense then then they do today. I have even used dowel pins to make cutting tools in a pinch for cutting alum and other soft materials. Most tools today use top rake but they call it chip relief. I hand grind all my tools in my home shop even the radius tools used to cut the balls in my Avatar. I don't think many machinist of today
grind tools old school. There is nothing wrong with that but in the old days you could not buy ready made tools like you can today.
Jimsehr
 
"In those days some of the angles made more sense then then they do today", This is funny, recently i've bought lot of HSS tools and first thing i thought those angles make no sense, how ever ground them was more concern about the chip braker then the cutting angle.DSC_0020.JPG
 
I started machining over 60 years ago and I thought some of the books on grinding tools were out dated then. And if I took as much time grinding a tool as the guy in the video did I would have been fired in no time. I worked in a production shop where time is money. Also guys in home shops are not making 1000 pcs at a time so if the cutting edge won't last for a lone time it is no big deal. If the tool works for what the home shop needs cut it is fine. I have tried some of the new cheap Chinese insert sets and they work great.
Jimsehr
 
Jimsehr said:
I started machining over 60 years ago and I thought some of the books on grinding tools were out dated then. And if I took as much time grinding a tool as the guy in the video did I would have been fired in no time. I worked in a production shop where time is money. Also guys in home shops are not making 1000 pcs at a time so if the cutting edge won't last for a lone time it is no big deal. If the tool works for what the home shop needs cut it is fine. I have tried some of the new cheap Chinese insert sets and they work great.
Jimsehr
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Cheap Chinese carbide insert work great for a short time, they seem to chip, break easy, the high quality german, dark grey, are my favorite, cut great and take lots of abuse and don't chip just cut and cut.
 
Back rake makes HSS tools cut more freely, rolling the chip back away from the tip of the tool; the Armstrong and similar too holders hold the tool on a 15 deg. back rake, but that much is not necessary; I think that the idea was that in sharpening the top of the tool that eventually it ends up more nearly parallel with the tool holder, as more is ground off the nose of the tool. It gets to a point where you sharpen the cutting flank, the front and top of the tool, you keep up the same routine, and the tool can remain the same shape without cutting down its thickness behind the top rake grind, reducing its strength or making it necessary to cut the end off and start over with a whole new grind as you would with a quick change (parallel) tool holder. Bottom line, you get longer tool life with the 15 deg. holders.
I do not agree that we had inferior HSS tools 50 years ago; the same premium brands and types are still here and available, although some shops may not have used them back then, as obviously they are more expensive.
 
So it sounds like it is a personal preference to grind a back rake in the tool and not necessary :confused:.
 
Personally, I grind HSS tools for the lathe for use in QC tool holders with no back rake, just side rake, and avoid sharpening on top as much as possible. I do not much use HSS tools for most work, but instead use triangular insert positive rake tools for most work except for the roughest and interrupted cuts, for which I use negative holders and inserts until things are pretty much rounded up, then switch to positive rake; this is for my 19" Regal LeBlond; for my 9" Monarch, I use HSS tools in a QC tool holder, ground as above.
I was "raised" in school shop with Armstrong type holders and learned to grind them mostly using the full 15 deg. back rake, only with ground in chip breakers, ground on the corner of the wheel; these curled the chips and made them less dangerous to the operator than a simple top rake which had to have sufficient and appropriate feed to accomplish the same effect, they were simpler for the learner to control chips, but are easier to damage with their more acute cutting edge. When I got the old Monarch I thought I'd just use to tool holders that came with it; I'd forgotten what a pain they are to use! I quickly bought a small starter set of QC holders.
What I think I meant was that tools with back rake cut more freely, but with QC tool holders that hold them parallel, you have to cut way into the tool bit back from the cutting point and with further sharpening, the bit is nearly ground half way through its height, weakening it, and ultimately making necessary cutting to tool off and starting the grind all over again.
 
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