You are pushing the edge of the envelope! Both the diameter of 75mm and the material being stainless are against you.

But all you can do is try. Definitely use an inverted parting tool and flood with plenty of oil of some sort from an oil can etc. A carbide insert parting tool or an HSS blade with T-section might be good to have too. Keep the speed low. For stainless at that diameter definitely backgear territory with HSS tooling. Maybe backgear medium speed, which is about 50rpm to start with.

40mm or so of stick-out on the parting tool blade is a LOT. I would maybe start with it sticking out the more usual 15-25mm and go as deep as that allows before adjusting the blade to stick out further.

I would also use the old dodge of make a groove of a reasonable depth, then move the carriage say 10 thou in one direction and take another cut so widen the groove. Keep doing that as you progress so there is plenty of clearance on the parting blade for most of its length. Also, it might pay to set the parting blade exactly square to the lathe axis using a dial indicator before commencing. And lock the carriage in position when cutting.

Being stainless – albeit the easier to machine 303 grade – don’t let the tool rub or it will work harden the job very quickly. So you have to keep the feed up to it, which may take some ticker.

Good luck!

(And do let us know how it goes.)

Which raises the question: How long is the parent material? Lot can be done with the fixed steady too if it is a longer piece. Or if shorter, might be easier to just face it down until you get your 12mm?

If you can, I would cut and face. Depending on the length of your bar it would seem much easier and safer than parting off.

JA

A parting tool won’t go beyond it’s reach, which is generally quite small.  This carbide insert blade, photo pinched from ArcEuro of one in it’s holder, suggests it can do something over a 40mm deep cut, making Ø75mm possible at the extreme end:

Alas, how far the blade will reach, probably isn’t the limiting factor.  That’s the rigidity of the whole set-up: blade, holder, toolpost, crosslide, and the lathe itself!  Big lathes are better than small ones for parting-off because they have more beef.  In this context a Myford S7 is a small lathe, best considered ‘bendy’.

Substituting a Gibraltar rear tool-post will help, even more so if the lathe can run safely in reverse with the cutter upside down, not an option with screw-on chucks.

I’d saw it and then face-off.

Clever chaps have noticed the rod might be quite short:  is that so?  I’d still cut it in my band-saw but holding it in a clamp fixture I made for cutting short lengths.   The clamp is an example of a workshop problem solved by making a jig or tool that enables an otherwise awkward cut to be made.

Dave

Dave a rear toolpost with the tool inverted doe snot need the spindle reversing so no problem with a screw on chuck.

Hacksaw will get it cut without parting, just do a bit at a time while working on something else. A piece of angle iron in the bandsaw and the work clamped to that will support it and is what I often do for short lengths, saw cuts through teh vertical leg of the angle.

If using the type of parting tool Dave shows then start with a small amount of stick out and cut to that depth say 20mm then extend it in 10mm increments. Works OK for me, this is 70mm iron

If the lathe is weak or under powered then using a narrower parting tool will put less load on it, this is a bit of 115mm thick wall steel tube being parted with a 1.5mm imported parting tool and holder

On 18 July 2024 at 14:08 JasonB Said:

Dave a rear toolpost with the tool inverted doe snot need the spindle reversing so no problem with a screw on chuck.

…

 

“Doe snot” surely wins typo of the year.  Available from all purveyors of fine tools I’m sure!

True a lathe does not have to be run in reverse to part off with a rear toolpost, but there is advantage of doing so.

My previous post misled, and hope this diagram makes what I meant to say clear:

A lathe normally turns the chuck anti-clockwise, in which case a rear parting tool should be inverted – top of diagram.   The advantage of turning the tool over and turning the chuck clockwise (in reverse), is the cutting forces push the tool-post and saddle down on to the bed ways.   The disadvantage of parting with the cutter inverted and the chuck turning normally is that the forces tend to reduce rigidity by lifting the saddle.

Here comes a controversy!  There are tabs fitted to the back of the saddle to stop lifting.  But I don’t believe they have a precision function.   They are there to stop the saddle levering off catastrophically if something goes horribly wrong.   Although the bed is provided with a machined edge so that tab clearance remains the same over the saddle’s full range of travel, I’ve never seen a lathe bed that was precision ground in this area to be a slide.  Absence of precision and a way of lubricating the tabs suggests strongly to me that they are not meant to be adjusted to slide tight under the bed.   If I’m right, ‘improving’ this part a lathe is a waste of time!

Anyone know for sure?

Dave

Paul ,if you glue a piece of timber to the end of the bar with gorilla glue,the foaming type and clamp overnight,it will allow you to clamp it in the bandsaw.I have sawn off 1/2 thick by 5″ for gears using this method,just saw the wood as close to the bar as possible and the glue and whats left of the timber can be remave with a wood chisel.This idea I coapied from Brian Wood and have used it even for turning with no difficulty,I have not tried parting off though.

Frank

To add to Hopper’s post, the advantage of a rear toolpost, with inverted tool, and conventional spindle rotation is that cutting forces tend to pull the tool ‘out of cut’, and dig-ins are reduced. Myfords aren’t as rigid as we might like.

I have seen it recommended that, for parting, the tool should be on or below centre height. If anyone thinks that it’s safer to set a tool with its tip below centre height, because the machine’s inevitable flexibility may allow cutting forces to push a struggling tool out of cut, they are wrong – even if it’s strongly counter-intuitive. It’s safer to set tools a very small amount above centre height, so that there is an horizontal component of the force acting on the tool towards the operator, ensuring that feedscrew backlash doesn’t allow the tool to be pulled deeper into cut. (Below centre height reverses the direction of the horizontal component).

Hoppers point about the load going into the head and not the caps seems a good reason reason. Would not apply to a machine with taper bearings set into a solid head.

Also I suppose any uplift will tend to tighten the cross slide dovetails.

Rear post with a parting tool  is fairly common on a lathe set up for production work with a lever action cross slide  so the front side can be used with a conventionally mounted tool. Though probably better suite dto smaller components that 3″ stainless

This is my rear toolpost for parting off. Sollid as a rock with the cutting blade inverted it alows me to part off pieces which were imposible before.

David

On 18 July 2024 at 18:16 JasonB Said:

If you are going to leave the tool upright you may as well leave it at the front.

…
Me I’ll leave them at the  front hanging off the Quick Change toolpost and whack it in under power feed

Successful parting-off, I think, depends mostly on rigidity.   Therefore anything done to increase rigidity reduces the possibility that a part-off will fail.  Jason owns a 220kg WM280 lathe, twice the weight of a 90kg Myford (both weights approximate).

I have a WM280 too, and can confirm mine also parts-off OK from the front tool-post.  But there are some ifs and buts:

• Unwise to part-off without locking slides that don’t need to move
• Depends on the operator feeding the tool in smoothly, applying constant pressure throughout.  Being a clumsy oaf I find this hard to do.  Skill is certainly a important: pull back or press harder when chatter starts? Chatter is much more likely when I part-off by hand, I believe because I can’t spin the handle at a constant rate.  The same machine and set-up parts-off reliably when I step back and let the powered cross-slide do the job, chatter almost unknown.
• Important to minimise tool and job overhang

Compared with a rear-mounted Gibraltar tool-post, any front tool-post is a wobbly affair, and all things being equal the extra overhang added by a QCTP will make them even less rigid.   For anyone not familiar a Gibraltar tool-post is a sturdy block of metal that resists bending.  Photo pinched from Warco, theirs is the simplest possible design, George Thomas and others have a few bells and whistles:

I consider it ‘best practice’ to minimise rigidity before parting-off, especially on any lathe smaller than say 150kg.   The most rigid configuration is a Gibraltar rear tool-post, ideally with the lathe running in reverse.   Noting that Jason is at least twenty times more productive than me, perhaps I should save time by using the front-tool post:  if it does the job, I shouldn’t fuss!

Much depends on the lathe and exact set-up.  I think this explains why some have severe difficulty, whilst others sail through.  As a generalisation, parting-off is low-risk on a big machine, and it’s the owners of light lathes without power feed who struggle.  The difference is due to big machines being more rigid than small ones relative to the much same bending forces both experience whilst parting-off.

Dave

I repeat myself with this argument

Rear tool post vs front tool post

Alan