Lathe cutting tool height

I've just picked up a copy of the well-known - and respected - book "How to Run a Lathe", published by South Bend Lathes, this the 34th edition, of 1938. Like several other, older books I have and have seen, it advises: 'The cutting edge of the cutter bit should be about 5 [degrees] above centre, or 3/64 in. per inch in diameter of the work... for ordinary straight turning.' It then advises that, for taper turning, threading, and cutting 'tenacious metals', the cutter should be exactly at centre height - as we'd all expect. The accompanying illustration is of what looks like a small HSS bit in a substantial toolholder (standard industrial practice) - but not a spring tool holder.

As far as I'm aware, the 'above centre height' advice is now obsolete (except, perhaps, for some boring operations), but I don't understand why it was ever advised for 'ordinary' turning. Any ideas?

I have looked at the advice given in "How to Run a Lathe" but have always been told to place the cutting edge at the lathe centre and that is what I have done. When using a long boring bar may be slightly above. 3/64" above per inch diameter seems a lot to me. It will be interesting to see what the experts say.

Thor

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I have always assumed it to be in the hope of compensating for the inevitable slop and spring in the path between the headstock spindle and the cutting-tool.

That said: 3/64” per inch does seem unrealistically high

MichaelG.

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Edit: __ for those less fortunate than Kiwi Bloke

http://www.vintagemachinery.org/pubs/1617/3789.pdf

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Edited By Michael Gilligan on 20/04/2023 12:51:50

My contemporary Camm handbook recommends grinding 8 to 10 degree clearance angles, which are compatible with South Bend's 5 degree above centre recommendation.

I guess modern lowering tool height is due to the improved rigidity of lathes and cutting tools after the invention of HSS. Carbon tool-steel cutters were still common during the 1930s when the South Bank book was printed, and they're not as strong as HSS. With a relatively weedy cutter, starting above the centre-line reduces the chance of a dig-in, plus depth of cut is limited by the clearance rake of the cutter. Both reduce stress on the cutter. Remembering that much production lathe work was done by unskilled operators, starting deliberately high reduces the chance of carelessly setting the tool below centre.

HSS being markedly stronger than ordinary tool-steel makes much deeper cuts possible provided the lathe has sufficient rigidity and the motor has enough poke. Fast metal removal is vital in a production environment.

Despite the strength of HSS it's still unwise to start cutting below centre-line because as soon as the cutting edge makes contact it's committed to a deep cut and likely to go pear shaped by bending under the pressure.

Dave

As SOD points out, if the clearance angle is greater than the nominal 5 deg then it will still cut.

For boring, above centre height reduces apparent top rake, so a little can help get the tool down the hole, but dont overdo it.

My own view is that tools should be set to centre height to +0/-0.25mm tolerance.

Martin

I have not looked but assume if you looked at lathe tool manufacturers ie Sandvik etc you will find some answers. Obviously cutting edge technology plays a significant part in the multitude of forces but what you are trying to achieve is the lowest force to shear the material. That is why there are so many different carbide tip geometries to suit different materials and their conditions. Obviously this will also apply to HSS or any other material you may want to use, but with differences in angles and material removed. In industrial production applications tooling is developed over time to produce the lowest cost per part and can vary from machine to machine. So see what works for your application and don’t rely too much on others.

I don't recall the over-height being so much because of the affect on tool angles but the reason is to provide a component of force outwards or back onto the feedscrew/nut which takes up backlash, reduces chatter and hopefully snatch or dig-in. The book is aimed at beginners, trainees etc using old worn lathes so relevant and helpful. Not necessary for a big engineering lathe with backlash adjustment and solid structure.

I suspect SOD is mixing up strength and stiffness. Does HSS have a higher Young modulus than carbon steel? The big advantage of HSS is that it retains it's hardness at high temperature, hence you can use higher turning speeds.

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[my emboldening] That’s an interesting interpretation ^^^

MichaelG.

The advice in my copy of "Machine Tool Operation" by Henry Burghardt, 1936 edition, (first published 1919 ) basically echos the advice it the South Bend book. Here is the relevant pages giving the rational. It starts about half way down the first page.

This book came from my first high school in 1959 when it ceased being a "technical school".

Edited By John Purdy on 20/04/2023 18:06:23

Edited By John Purdy on 20/04/2023 18:18:51

Edited By John Purdy on 20/04/2023 18:19:34

Most current advice seems to be on the centre line, although the exceptions are slightly above for boring, which allows for some tool deflection and when using inserted parting off tools for solid work, the tip should be 0.1mm, 0.004" high.

Done my time back in the early 70s and we were always told to set on centre height or a couple of thou below, boring tools were only raised so that the bottom of the bar would enter the hole without rubbing, parting off was always done at centre height.

As an Apprentice in the late 50s at R-R with ex Sentinel instructors, we were taught to set the cutting edge on the centre height. Never above it, because the tool will rub rather thancut.

Fo a tool set above centre line to cut properly, it will need to defelect downwards, so that the cutting edge DOES lie on the centreline.

The olnly exception to thnis is the shear tool, and possibly slim boring bars.

Howard.

Out of curiosity I've had a look along my bookshelf and find that seven relevant books from 1917 to the 1980s don't mention tool height at all. Of the others, Know your Lathe (Denfords, 1953) is in agreement with the South Bend Book and states that "For ordinary straight turning the correct height of the tool cutting edge is about 5 degrees above centre, or 3/64 inch in diameter of the work… Always place the cutting edge of the tool bit exactly on centre… for every type of taper turning and boring, as well as for cutting screw threads and turning brass, copper and similar tenacious metals."

The Practical Metalworker (ed. Bernard E Jones, undated but probably 1920s) shows similar thinking: "... for other operations [than taper tuning and thread cutting], presuming the clearance to be sufficient, advantage is often felt in placing the point some three or four degrees above the centre, as this affords smoother cutting, and sometimes acts beneficially in preventing chatter. [My emphasis.] Clearance is also affected by the work diameter... the chance of rubbing on a small rod being obviously much less than on the periphery of a large circle."

A quick sketch shows that the quoted value of 3/64" per inch of diameter translates to about 5½°, and that 3 to 4 degrees translates to about 1/32". Those degrees are measured round the lathe centre of course, and are not related to the clearance angle of the cutting tool.

Engineering Workshop Practice (ed. A. W. Judge, 1947 edition) says "In setting tools for turning it is important that the points be set exactly on the centre of the work. If the point be set high… the front of the tool will rub below the point. This is more pronounced on small-diameter material*, and in order to make the tool cut, more clearance would require to be ground, thus weakening the point. With the tool point on the centre, the tool functions most efficiently, although it is sometimes found necessary to slightly lower this in order to eliminate chatter when taking a heavy cut. Should the cutting point be lowered too much… it ceases to cut and merely planishes or scrapes the surface of the work." [*This is the exact opposite of The Practical Metalworker, and can be shown to be wrong.]

The Amateur's Lathe (Sparey, 1984 edition) doesn't mention tool height directly except in relation to taper turning and screw-cutting, though he does suggest that the parting-off tool should be set slightly below centre, not above "as is sometimes advised".

The Myford Series 7 Manual (Bradley, 1982) says that "For all ordinary turning and boring, the tool should be set with the cutting edge at exactly centre height".

At the CEGB apprentice workshops in Penarth (now a housing estate) in 1969, we were taught to put the tool at centre height. My conclusion from all this? You pays your expert and you takes your choice!

George

Edited By Georgineer on 21/04/2023 22:17:30

Thanks everyone for your input. It's strange, isn't it? The problem with so much teaching and knowledge acquisition is that the 'whats' are taught, in preference to the 'whys' - because it's easier, and the teacher can be a regurgitator, with little understanding.

The 'anti-dig-in' idea sounds reasonable, but does it withstand close scrutiny? A tool on centre height still experiences an outward force. I suppose 'dig in' susceptibility depends on the geometry of the deflection of the tool bit, holder, tool-post, slides, etc. With a weedy Myford (for example), a dig-in is an unstable condition, the bit 'nodding' further into the work. It's not pulled in, it's pulled down; the whole tool support assembly effectively pivots somewhere near the topslide base, and this pushes the tool inwards (Long ago, I applied forces and measured deflections, then calculated effective centre of rotation of tool tip). In this case I don't think that an above centre-height setting would help. But - this may have been the original idea, because I'm sure I have an ancient book (somewhere...) that believed that the forces on a lathe tool pulled the cutter towards the centre. Similar thinking may be the reason why some old lathes (and the current Cowells - it's a living fossil...) have the carriage gibs at the front. I wonder whether the high setting results in a little burnishing of the cut surface, which was thought to be better cutting?

By the 1930s, South Bend lathes were pretty hefty, and well designed, albeit with plain spindle bearings, so lack of rigidity should have been less of a problem.

However, none of this musing really explains why the texts advocating a high setting for parallel turning set this aside and allow geometrical considerations to demand on-centre-height setting for tapers. Inconsistent information worries me...

(edits for two-fist typing - and before coffee, too)

Edited By Kiwi Bloke on 21/04/2023 22:47:15

My inclination is that the tool should be set at the height which the manufacturer advises; and this may not be the same for a ground HSS tool compared with carbide inserts.

It also begs the question as to which surface is doing the cutting.

It looks to me that the tool it's self was causing this, the older forged tool needed to wear to be put on centre, the later Armstrong pattern holder used an entirely different straight blank

If one look at the extremes, it seems if you set it too high, as the work turns towards you, the tooltip must dig in as the work forces itself through it going down on the tip and if setup strong enough a deep cut went through, if not something has to give as it bites in.

On centre you have the choice of fine cute not digging in.

Below centre it will rub on top of the tip rather a drag I think.

Has anybody set it on centre, then attached a test dial indicator to the tip and measure the deflection - to - tool pressure aplied as cutting takes place ^ extreme case like parting-?

.....To see how the depth of cut you want to use/apply match up to the deflection in your case/lathe/tooltip shape and choice of center height/above centre height, so depending on your lathe setup rigidity, may you have to set it slightly different for different depth of cuts-?

Edited By Chris Mate on 22/04/2023 05:29:30

Interesting to see this similar discussion from 2007 : **LINK**

https://www.practicalmachinist.com/forum/threads/turning-above-centre-height.94384/

MichaelG.

After 55 years of being told to set tools on centre height I think that will be a hard habit to break. I wonder if centre height which is a definite position is easier for the novice to get right than getting into trouble with the tip set too high.

Mike