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Why is HSS tool set above center

Gard

Aluminum
Joined
Mar 18, 2016
My 56th rev of the SB book "How to run a lathe" states to set the HSS tool bit 5 degrees above center for normal straight turning of mild steel. Grind the bit with sufficient front clearance. My question is who came up with this and why 5 degrees? I suspect there is some science and logic behind it but I have not been able to find reference to this anywhere else in my very limited library or a google search. Clearly this is not done with modern carbide tooling. However, is it still considered good practice for HSS? I have posted photo on the hobby-machinist forum also. I think fig 56 suggests the front angle relative to vertical should be the "3 to 15 degrees" plus the 5 degrees (from above center)?
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TO make sure it is not below center.
If it is below center, when you lower feed or stop feed it will move up and make a groove in the part
When it is above center, the force of the cut pull the tool down into the larger diamter of the part, and when you stop the lowering of the cutting force allows the tool to move up to a smaller diameter, thus not groove
 
In books from the late 1800's far more extreme angles above center are recommended. The justification being that this causes some of the cutting force to load the crossfeed screw.

Why that was an important consideration with old lathes I do not know. Maybe they used a lot more rake and had problems with the tool getting sucked into the work.

But it does prevent cutting accurate diameters with a DRO or the crossfeed dial and also requires constant fiddling with height which are pretty significant disadvantages.
 
Excellent points. I have seen those effects before but had not considered the effect of cutting height. Seems obvious in hindsight.
 
Prior to about 1900, High Carbon cutting tools would have been used since HSS wasn't invented yet. But for whatever reason, those rocker tool posts and tool holders seemed to be used everywhere for a very long time. Even the worst off shore qctp today would be multiple times more rigid. So there would have been large amounts of flex with the old school rockers. My best guess is that above center recommendation could also have been an effort to compensate for the large amounts of unavoidable deflection. I've never set my tool height anywhere even close to being that high. But I'm also using either a 4 way tp or a qctp.

I logically can't see how that above center recommendation would help load the cross slide screw any better. Even a light .003" finishing cut with the tool exactly on center is going to exert more than enough to overcome any friction within the slide and fully seat it's thread flanks back against the nut threads.

I don't necessarily agree with only following what the books say. There imo starting recommendations just like cutting speeds and feeds. Experimenting a bit with tool height on various materials will show you what works the best for the lathe, tool type and material better than what the books suggest. From my perspective, gaining experience is mostly about learning what doesn't work.
 
What I learned, with both heavy cuts and slender, flexible material, there is a tendency for the tool to "climb" the work when the tip is at or below center, due to all the dynamics involved. Setting the tool slightly above center, combined with adequate front tool clearance to eliminate rubbing, reduces the tendency for grooving.
 
Carbon steel tools were sometimes used in a 'shaving position' basically on top of the round profile,similar to a woodlathe ..........this was claimed to leave a better finish on the soft steel used in the day
 
I've got a suspicion that the advice wasn't originally based on science so much as on results based on experience, and I expect that there have been as many scientific explanations over the years as there have been explainers.

I'm put in mind of the reasons why there is a left-hand thread on a left-hand (foot?) bicycle pedal. It would have been used originally because if you used a right-hand thread your pedal unscrewed itself and dropped off in the road. Cycling publications have been full of scientific explanations and the resulting arguments for well over a century and will continue into the foreseeable future.

George
 
5Dgr above centerline ???
That on its own is bull
Through every point at the circumference of a circle you can place a centerline
So also 5Dgr above centerline :willy_nilly: :willy_nilly::willy_nilly:
And once set the tool does not mind if it came from a position at 1 -2 5-6-8-10 or 12 o`clock
So with this the guideways are involved They play a part
Sloppy guideways may need this kind of fooling around
That is where quality of the machine plays a role


Peter
 
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5Dgr above centerline ???
That on its own is bull
Through every point at the circumference of a circle you can place a centerline
So also 5Dgr above centerline :willy_nilly: :willy_nilly::willy_nilly:
And once set the tool does not mind if it came from a position at 1 -2 5-6-8-10 or 12 o`clock
Peter
I am not sure 5 degrees above center is the best description by itself but the page from the book in my first post has a sketch that I think shows what the 5 degrees means. A line from the tip of the tool thru the center of rotation will be 5 degrees above the horizontal. This amounts to approximately 3/64 inch of height above center per inch of part diameter.

South bend came up with 5 degrees, a similar "textbook of turning" by Hercus has 2 degrees, not sure when it was published. It also mentions setting up the tool so any sideways flex in the holder is away from the part. Apparently older books recommended larger numbers:
In books from the late 1800's far more extreme angles above center are recommended. The justification being that this causes some of the cutting force to load the crossfeed screw.

I have a soft copy of "On the art of cutting metals" by F.W. Taylor published at 1906 ASME. Covers 26 years of experimental work to find the optimum cutting tool, feed and speed to remove the most amount of material in the least time with a baseline 20 minutes of tool life. I am not sure it qualifies as scientific but sure looks like good engineering to me. A quick read did not show tool height being a major variable studied. They developed lots of tables and equations that summarized the experiments and a slide rule tool they sold. Over 30,000 to 50,000 experiments they created 800,000 lb of chips in 26 years. File is too large to download but I could send it to anyone interested.
I am guessing there are other papers and textbooks that followed in the early to mid 1900's. Taylor mentions some other researchers that measured tool forces but he thought tool life was better.
 
Gard...that sound logical that through testing the experts found that above center proved better. The percet better amount might be based on tooling and the material.

Carboloy school gave us a time for carbide life, I thinkit was 32 minuets per insert that we should shoot for. likely based on years of testing.
 
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I am not sure 5 degrees above center is the best description by itself but the page from the book in my first post has a sketch that I think shows what the 5 degrees means. A line from the tip of the tool thru the center of rotation will be 5 degrees above the horizontal.




I
Yes That is my point
What has the horizon got to do with this
They assume your crosslide is horizontal
Plenty of lathes around with the slide not horizontal
But what if I lock the crosslide when turning ??

It is all a fooling around for compensating a bad lathe like a SB
Peter
 
Peter,
I am not sure if you are asking a question or just making a comment about SB. Assuming the former, here is fig 15 and 16 from the Herco book that shows how changing the height affects the front clearance and back rake angles. I think this chart is true regardless of what kind of lathe or tool holder you have. If anyone has a SB lathe installed and leveled per the instructions, the cross slide should be level within a small fraction of 1 degree.

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Fwiw I got curious and did a Google search about lathe tool height setting. At the industrial level for manual lathes and recent information, there doesn't seem there's that much to find. Oddly Shereline lathes mentions for shafts above 3/8" diameter to set the tool "several thousandths below center". With small low powered lathes, maybe that helps to simulate a more positive rake the material sees? I'd guess that's only for OD turning, and facing tools would obviously be set on center. I still think it's better to experiment a bit for the lathe, tooling and material and what seems to work the best than pick a single recommendation and just blindly follow it.

Somewhat related to our cutting tools I guess. But in the open pit mining industry, large 10"+ diameter multi tooth carbide tipped rotary blast hole drills are real expensive, multiple thousands of $ for each one. When they try a different manufacturers drill bit, they always experiment with rotation speeds and feed pressures to figure out the best tool life for that particular brand of drill bit. The average rock type, it's hardness and drilling conditions (dry or wet holes) makes a big difference to the overall tool life span. What the manufacturer recommends is just considered as a guide line for where to start. Even after that, the number of holes each bit produces is logged and anything out of the ordinary is going to get questioned. It doesn't seem to get mentioned that much on PM, but some with there own machine shop businesses have mentioned they experiment with feeds and speeds for there own particular cutting tools and material requirements as well.
 
Why would it be different with "modern carbide tooling".
I am no expert on carbide but I think its edge is generally more blunt, "the angle of keenness" (from the SB page in my first post), between the front relief and top rake. This is recommended as 60 degrees for HSS cutting steel. Some examples I have seen of carbide are more like 75 to 80 degrees. If there is insufficient front relief, the tool will rub on the part if it is set too high. I am sure some carbide can be set a little above center but not as high as HSS. I suspect the sharper edge of HSS can result in lower cutting force and required HP at least in some cases. Do any carbide manufacturers have recommendations on tool height other than on center?

I am always experimenting with HSS height as well as angles to see what works best. Most of my work is one off prototype and repair, I think if I was doing higher production it might be easier to really dial in the settings. Then I probably would be looking for a newer lathe and insert tooling. I think all the replies to this post have been interesting and helpful.
 
I set carbide on center for od. On id I was told years ago to set slightly above center, if set on center when the bar deflects down it will cut oversize. A couple finish passes the same depth probably makes it moot.

Dave
 
Yes That is my point
What has the horizon got to do with this
They assume your crosslide is horizontal
Plenty of lathes around with the slide not horizontal
But what if I lock the crosslide when turning ??

It is all a fooling around for compensating a bad lathe like a SB
Peter
Agree, it's fooling around to get a light weight machine to do what you need it to do and likely unnecessary for stiffer machines. However, I will argue that the discussion is still valuable no matter what machine or cutting process under consideration. Understanding of how and why things work the way they do can be applied to situations far beyond a SB lathe.

That's is why I cringe when I see "that's how I was taught" versus understanding. True understanding can be applied to new problems. "how I was taught" is a great starting point but limited to solving the same problem, not new ones.
 
To fully understand what is meant one must indicate that the crossslide is considered horizontal
Then those workarounds also apply in the same way to those lathes with a (worn;)) slant bed

Peter
 








 
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