Parting-off

[Paul LParticipant @paull58212]

I have a stainless 303 bar Ø75 that i need to part off (12mm long) . I have a myford S7 and a decent parting off tool but is this a viable process? is there a limit on what Diameter can be parted off?

If it viable any tips would be appreciated.

Paul

 

 

[HopperParticipant @hopper]

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.)

 

[peak4Participant @peak4]

I’d be inclined to add tailstock centre support, to save the workpiece being pulled out of the chuck.
If the final part will have a hole in the middle, no problem, but if not, maybe leave a thick stub when facing the end; centre drill that, and machine off later.
I have in the past, superglued addition material to the end of a workpiece to allow for a running centre.
Just be wary about the heat breaking the glue’s adhesion.

Bill

[HopperParticipant @hopper]

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?

[David George 1Participant @davidgeorge1]

What lathe are you using as on a large lathe like a harrison I used at work somtimes you could part off much biger than that but we would usually cut it off on a cut saw and then face it off.

David

 

[JAParticipant @ja]

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

[Paul LParticipant @paull58212]

Thank you both for your help.

The bar is only 50mm long.If it had been longer I would have cut a 12mm bit off with the band saw but 50mm was the smallest amount I could buy and that cost £45.

It needs a Ø28mm hole in the centre so I assume it be better off boring that out first and then parting off.

Any other ideas of how to cut a 12mm slice off without parting welcomed.

Paul

 

[SillyOldDufferModerator @sillyoldduffer]

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

[HopperParticipant @hopper]

Yes boring the 28mm hole first would DEFINITELY make life easier. Reduces the amount of stickout of the parting blade by 14mm too.

You could still support the job with a tailstock centre by turning up a stepped plug that fits snugly in the hole and has a centre drill hole in the end. Make the plug depth less than the 12mm where your parting off will be. Usually best to remove the tailstock centre for the last little bit so the offcut piece does not jam sideways when it parts company with the parent metal.

Depending on your bandsaw, you still might be able to hold a 5Omm length in the vice on it, if you put a piece of scrap etc of the same 75mm width in the other end of the jaws to hold them parallel. That would be easier than parting it off. Again, boring the 28mm hole first would be helpful and reduce cutting time. Run your bandsaw slower than for mild steel, maybe about half the speed. Cut the slice plenty thicker than finish size and face it in the lathe afterwards to get it all nice and flat and square.

[JasonBModerator @jasonb]

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

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