Small Point Re Parting Off

[Antman]

Hi,
From what I have read parting off can be a major headache. I was a bit nervous of trying at first but now I manage to part without too much drama. However one thing I want to ask: Say you want to make something with parallel faces, like a thick washer. One side faces easily. Do you just part off to length? I find that the face left by my parting is none too smooth.
Ant

 

[ShedBoy]

Can you post a picture of set up and finish, it may be something minor which will be obvious to others. I tend to grind my parting tool with a slight angle so the right is a tiny bit longer, this seems to part it off cleaner for me, I use this to part off to length with no problems with a good finish.

Brock

 

[gbritnell]

Hi Ant,
Because most cutoff tools have parallel sides the back side of the cut isn't always smooth. The swarf tends to drag in there and scuff up the finish. Here again that's dependent on the material and depth of cut. Usually to get a nice 'washer' one has to part off the piece and then make a small split chuck to hold it and face the cutoff side.
gbritnell

 

[mike os]

try a right hand parting tool, leaves teh back of the parted piece "good" & no clean up, might need to face the blank again tho.

 

[Blogwitch]

As Gbritnell said, plus, you really need to have a beefier parting tool because usually the taper is caused by the tool flexing sidewards at the tip.

With regards to your statement about parting off being a headache.

In all honesty, if the tool and machine are rigid enough to do the job, it is just like facing off, in fact I usually just power feed it off, and not just on non ferrous either, I do the same with stainless and normal steels, up to 3" thick. Don't forget, also the correct tool for the job makes a difference, I grind up tools from 0.010" wide for parting off thin brass tube (to prevent wastage) to anything up to 1/4" wide for heavy jobs, there is no such thing as a do it all parting tool. I have at least half a dozen that I use regularly.

It is mainly technique if you are hand feeding, and earlier this week, I started to train my grandson in the use of machinery for the engineering college course he is about to start.

He has never used my lathe at all, in fact only an hour or so of any lathe use before, when he did a short course at school. I decided that the main problem with all starters is parting off, so with ten minutes instruction on how to set it up, feel, sight and sound on how to do it, he parted me off 18 long lengths of thick walled steel tubing to make some lead hammer handles, with no problems. I then had him part me off a couple of short lengths of 1" steel, no problems at all.

In my view, most of the parting problems are in peoples minds, they read or hear that it is a problem, and become scared of it, when in fact it is just another machining operation out in the big wide world.

Just to prove it isn't BS, here is a vid I did when I got one of my latest acquisitions.

[ame]http://youtu.be/BG4qEw3eMcQ[/ame]

In fact it wasn't brass it was cutting, but one of the bronze family. You can tell by the type of swarf being produced.

Bogs

 

[precisionmetal]

Antman,

Your information is correct: parting is a common (maybe the most common) procedure leading to tool breakage in a lathe.

I've been running an engine lathe for over 40 years, and while I break very, very few tools, I'd have to say that I've had more mishaps during cutoff operations than all other turning operations combined.

If the partoff tool has parallel sides, my suggestion is to take a moment and indicate it in (set an indicator on the side of the cutting blade and run the cross slide in and out). Make sure it's dead straight. That will help a lot. As soon as you lose the dead sharp cutting edge on a parallel cutoff tool though, it WILL drag on the sides, and that's where the heat/problems come from.

A tapered blade (widest at the front) always makes things easier and is quicker to set up too (just an eyeball is all you need).

Plenty of lube or coolant, keep the cutting edge very sharp, and back out immediately if you see chips "pack up" in the groove. Those things will avert most disasters.

PM

 

[Ken I]

All the advice above is good.

When I want the back face finished or to size, I would use a RH part off tool (front relief so that the part comes off without a pip).
Also I part leaving on some material but stop short of parting off, leaving enough material so that I can take a finishing cut before parting through the last bit.

Obviously the blade must not flex and you must leave enough material to drive the finishing cut (surprisingly little).

Many variables and different tools for different situations (as Bogs said).

When in doubt or size / finish critical, follow George's suggestion and chuck it after parting.

Ken

 

[don-tucker]

That's exactly what I do Ken,
Don

 

[JohnS]

Hi Ant

As a newbie I too struggled with parting procedures on my mini-lathe despite trying all the recommended techniques. You start to blame yourself for not being able to carry out a basic operation which is straightforward to experienced engineers.

After a year I decided to upgrade to a larger lathe. This had larger and heavier chucks, 100mm as opposed to 80mm on the mini-lathe and of course a more powerful motor. An immediate and unexpected bonus arrived with this bigger and more powerful lathe. Parting off was suddenly no longer a problem and I assumed this was due to increased inertia.

I am not suggesting that you should consider upgrading your lathe but my experience may put your parting problems into perspective.

You may also find this article written by Bogs of interest http://start-model-engineering.co.uk/begin-with-bogs/parting-off-2/

John-Som

 

[modeng2000]

Just to add my thoughts to parting off, taking John Bogs advice I mounted my WM180 lathe on a piece of kitchen worktop and the difference in rigidity was amazing. I had assumed that the lathe bed was strong enough by its self but not so. I don't have to worry about parting off any more.

John

 

[Ken I]

Re John-Som's remarks - size is important - my mini lathe works fine with a 2.5mm part off but pretty much refuses to cooperate with a 3mm

Modeng2000's remarks - mounting the lathe rigidly (even to a robust wooden table) helps to suppress harmonics and improve rigidity.

2c

Ken

 

[Antman]

Hi Guys,
Thanks all for your replies. George I feel especially priveleged by your replies. And sorry I haven’t replied sooner … I’ve been thinking. It dawned on me … just make a tool. To which end I have started making a toolholder for my parting tool holder that works in my QCTP. That way I will easily adjust for height for tool shapes I can try using those wedge shaped HSS tool blanks. Mine are 0.6” high x 0.1” at the widest. I was stuck into using 1 parting tool and a bunch of shims … Duh. John, I think I have learned quite a bit since your post about half learnt techniques. I still get so rattled by it, I guess that’s why I ask so many Questions. Sorry for no pix I do really want to show off my slow progress in my workshop, but I don’t have a very effective camera right now. My lathe is fairly rigid a 9” Chinese basic machine. I do see it springing sometimes but I am getting to know how much of a cut it can take. I have all my machines on angle iron tables. The lathe’s table is heavy section angle, very rigid, bolted to the floor though a couple of planks and the lathe is bolted down onto jack screws.
Ant