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Why is HSS tool set above center

It seems to me after many decades without training that one needs to be more careful with tooling the lighter the lathe or mill or whatever is. I have a SB9A that has quite a bit of wear and with some careful planing and sharp tools I have gotten consistently predictable results. That being said, I have used much heavier lathes in beautiful condition with not so good tooling and also had good results. For those of us who can’t afford or don’t have room for a tank, practice, experiment, and be careful for excellence.
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

Yes let's assume the cross slide is horizontal. I guess 50 or 100 years ago there was a state of the art factory with a row of brand new SB lathes turning out barrels or shafts all day long. The lathes had been properly installed and leveled. The SB factory Rep said you will get best production if you use these cutting angles with the tool above center for normal straight turning. The amount above center is more for larger diameter shafts. Tool is on center for taper turning and threading. If the Rep found an old worn out lathe he would probably recommend sending it back to SB for reconditioning.

Today we have more horsepower, heavier more rigid machines, different tool holders with carbide insert tooling with CNC control. Someplace along the line setting the bit above center has fallen out of favor.
Grinding a toolbit without back rake is faster and easier than grinding in back rake, and the thickness of the toolbit isn't reduced by repeated re-sharpening. If such a toolbit is set so that the top surface is parallel to the cross-slide travel, and the tool is set "on center", its effective rake will be zero.

If, on the other hand, that no-back-rake toolbit is set so that its top surface is parallel to the cross-slide travel but above center, the cutting-edge-to-workpiece geometry is that of an on-center toolbit having back rake.
I am no expert on carbide but I think its edge is generally more blunt,
Those of us dealing in making carbide tools with 400x, and 1000x magnification since the late 1960s may disagree on "more blunt".
There are many reasons to go above centerline.
Your top goes negative but you decrease the heel.
So chip flow pressures up top goes up but less heel gives me more time in a run.
Try grinding 30 degrees on that front edge.
"Blunt" It is common to put a .001 or even .005 radius on the edge of carbide tools. This not so common in HSS.
This changes a whole bunch of things. HSS "hones" itself very fast. Carbide does not.
The important look here is not the new tool but a tool into 5 to ten percent of it's life.
Many coatings used on carbide need a hone due to build up on the tip.
It is complicated and hey how about K or T lands.
Makes no sense that adding 20 or 30 degrees negative at the critical point of cutting makes the tool hold size and last 5 times longer.
One would think this bad. Then one gets into the whys.
I am no expert on carbide and chip formation. Armature hour here but but a whole lot of tool testing done.
We all have a HSS grind or carbide insert grade that has seemed to work like magic. Few ask the why and how.
Do I need lower heel to help dampen that cut (above center).
Am I too low and get accelerated flank wear? Did I push to much up top and push too much heat into the tool face?

Here comes a question to all.
I look at an operation and think maybe some changes may help a lot. (or not)
But the customer is buying this now and wants a low price.
Do I argue with them and try to sell them something with a lower usage or just give them equivalent? Lower usage means smaller orders to me.
They do not want to spend the time doing tool testing which is a pain in the ass for sure.
But, tis the job of every tool salesperson to slit his or her own throat. That is how you make happy customers that will give you more down the line.
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There are many reasons to go above centerline.
I did not realize going above center was recommended for carbide for strait OD turning.
Try grinding 30 degrees on that front edge.
Are you suggesting modifying or regrinding carbide inserts, I have tried this a few times with some sucess.
HSS "hones" itself very fast. Carbide does not.
The important look here is not the new tool but a tool into 5 to ten percent of it's life.
Many coatings used on carbide need a hone due to build up on the tip.
Not sure what you mean by hone, is this wearing and dulling of the tool cutting edge?
Here comes a question to all.
But, tis the job of every tool salesperson to slit his or her own throat. That is how you make happy customers that will give you more down the line.
I agree with this, if I like the way a tool works I will buy it again and more likely try other things from that vendor.
Is there any more modern references you can recommend or is it a lot of tribal knowledge?
I have set my TB at center off my tail point or what ever was convient, but if the old lathe companies say a little above from extensive testing then that is likely best for some unknown reason, likely better for HSS and carbid.
Perhaps one coud save 20 bucks a year or avoid some chatter doing it that way.

Carboloy had a super strong lathe at the 8 Mile Road shop . I kept a lathe chip for years in my tool box to show what a supe big and thick chip looks like. They were doing tests all the time to evaluate every thing about turning and likely had a ton of lathe data.
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Not sure what you mean by hone, is this wearing and dulling of the tool cutting edge?
A hone is a small .0005 to .005 rounded edge intentionally put on the tool cutting edge.
Many reasons for this what seems like a dulled edge.
So now the biggest contact point and cutting edge is not the tool top plane but something below it.
We do this with with brushes, vibes, slurries in the making.
You can call it intentional dulling I guess.

And then the engineered or "shaped" hones.
Light hone in some areas and heavy in other. This done by very special machines.

Some here : https://conicity.com/introduction
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To add to Bob's great post #27
A hone can avoid the sharp edge crash that can happen at the startiing of cuttiing, often called break in time..once past the break-in a cutter can have a long tool life time..Yes it can be put on by the manufacturer of by the tool guy or jobsetter at the using machine. For some tools a certain wire brusing makes the break-in to the cutter edge..perhaps 2 seconds on the wire brush.
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And my thanks about the additional information about carbide and that link. That's something new to me for sure.

I did get curious and checked a few of my older machinist reference books. One that's fairly basic was written by Fred Colvin in 1941 titled Running an Engine Lathe does mention very briefly 4 way tool posts, but overall it seems the rocker tool posts were much more common in commercial shops at that time. Qctp's don't even show up. He does say for work over 1" in diameter, the tool should be set slightly above center, but no numbers for how much above were given. Below 1" diameter, he recommends the tool should be set on center. It was also pointed out the lack of rigidity most tool holding methods had at the time, and especially those rocker tool posts. Braised carbide only got a short sentence or two mentioning it's much higher turning speed capabilities and it being very expensive at that time. None of this helps much of course.

Since 1941 would have been well into the massive build up during WW II, I think this book would have been aimed at all the mostly untrained people being hired to operate machine tools at that time. Fred from what I can gather, was a well known authority about machine tool practices, and I have another about tool & gauge work he co-authored back in 1907. That one was much more detailed and quite technical, so I assume he knew what he was writing about.

https://en.wikipedia.org/wiki/Fred_H._Colvin for any that might be interested.
Yeah I guess it is MB. I still like Montana better. Nicer scenery, cleaner, better roads and even less people. 😀

And not to throw this thread OT, but a total tooling and machine tool desert where I am. And the main tool supplier here in Canaduh would be KBC Tools. There prices bear zero resemblance to your American KBC even though there the exact same company. Even if I try buying from any tool supplier out of the U.S. I'm looking at adding right around 50% on top of the U.S. prices for the money exchange difference, freight, brokerage fees and the government's 14% taxes on top just getting it through our customs. I'm unsure if anyone in the U.S. can access KBC's Canadian website, so for an example and since I have one I got out of the U.S. Today's price at KBC Canada for a Narex VHU 36 sized B & F head is $6191.40 + that 14% tax on top. And that's just for a bare head, no tool shanks, boring tools, storage box etc like I bought.

Yeah it's too bad I've so far never run across just how expensive carbide was from it's invention date in the 1920's and on up. And I doubt that early carbide had anything like the same durability and quality we now have. Afaik the early carbide and probably even into or past the 1960's still couldn't withstand much or any interrupted cuts at all. But maybe Bob has run across old prices for carbide tipped lathe tools at some time?
I'd agree, plus the carbide grain structure seems to be much better or lets say more predictable as the production processes got better. That and better binding agents and probably better controlled results as we learned and understood more. But I don't really know much about carbide, so my best guesses really don't count for much.