the insert stays put because its in a taper and the forces acting on it push it further into that taper. the holder and the inserts also have matching V gloves top and bottom giving sideways integrity.

I use the insert type almost to the total exclusion of the plain type, works every time for me, even when used for quiye large diameters. If set correctly they are fine, but being an insert you have to make them work hard.

Edited By KWIL on 03/08/2023 16:30:46

I have both types of tool but must admit not had much success with the insert type, probably because you need to run at speed to be successful and having had some fearful lockups when using speed I am afraid to use its full potential. I now use the HSS blade type and find it very successful, you do need to make sure that there is the minimum amount of slack in your setup, not always easy on model makers lathes but from your posts it appears that you have probably got a commercial lathe. Dave W

Success with parting off came when I purchased one like your first pic and mounted it on a rear tool post never looked back since to the point Ive got the insert type but never use it

H

As Harry Wilkes mentiond, a rear toolpost mounted directly to the cross slide has great advantages. They are much stiffer and the swarf removal is helped by gravity. The top half to the one I made on a mill can be turned round for the 26 and 32mm blades which we use in 1.6, 2 and 3mm tips. Being able to lock the saddle front and rear also helps.

Edited By old mart on 03/08/2023 18:33:30

I use the insert type 95% of the time – they're good, but resist the temptation to use cheapo inserts from ebay (other sources of dodgy inserts are available). I bought some and they mostly worked fine, but sometimes would shatter for no apparent reason. They'd probably still be an OK deal compared with more durable but expensive, quality ones, but the break up often left a fragment of carbide embedded in the cut, ready to destroy the next insert or HSS blade you tried.

I am afraid I am a bit behind the times when it comes to this new-fangled tooling and a little too long in the tooth to change now. I use the good old-fashioned J&S type parting tool holders with the Eclipse type blade mounted upside down in a very rigid rear tool-post. On the odd occasion when I have 'pinged' the blade it's a two-minute job on the grinder to put the edge back on. With lots of suds pumped on it all works for me.

I've used these things for about 20 years now:-

https://www.rdgtools.co.uk/acatalog/PARTING-SYSTEMS-FOR-SMALL-LATHES-1728.html

…in my Warco WM250V and Myford Speed 10 before that.

Every so often I decide they get so worn and chewed up that I buy a new one. I recently started using my third.

I won't say they always provide the user with the slickest and smoothest partoff experience, especially on grumbly steels – but they do actually work, often well, and they're cheap.

Edited By Mick B1 on 04/08/2023 11:23:36

I agree absolutely. I always used to have trouble parting-off, even with a rear toolpost. Since I changed to the insert type a good few years ago I've had much better results.

Running in reverse is completely safe if you do not have a screw on spindle. You do need to still get the inverted tool tip on the centre line and if the saddle can be locked it will be a bonus in stiffness. That tool you illustrated would be much too much bother to get thinned down to a usable 3mm or less width for parting. If you can use a blade for carbide inserts or a tapered section HSS tool in a holder it would be better. I would recommend getting 2 or 3mm wide tooling for most sizes of lathes for best results.

That tool could be used for grooving or be made into a general turning tool. Possibly right and left handed at the same time provided you could get it shimmed to centre line both ways round.

Edited By old mart on 04/08/2023 17:27:38

Edited By old mart on 04/08/2023 17:29:07

Edited By old mart on 04/08/2023 17:31:13

I find blades in a rear-tool post work best for me, but it's not really the cutter that makes the difference. Which tool is the wrong question, should be 'What's the best way to part-off?'

First secret is rigidity. Parting-off applies more force to the cutter, job, and lathe than most operations. They are all likely to bend, causing chatter and dig-ins. When a dig-in occurs, the forces involved make it likely be violent.

Second is the need for a steady feed at the right rate. Steady is vital, and most humans are wobbly. The tool has to cut continuously, not too deep or too shallow.

Third is making sure the cutter is aligned correctly, going straight in, not applying side-pressure because it's at a slight angle.

Fourth is making sure swarf doesn't collect in the slot and jamb the cutter. Cutting fluid in applied in quantity helps.

Example likely to fail:

• A small lathe, these are light and bendy
• protruding job, sticking out of the chuck, also bendy
• a long slim cutter in a towering tool post perched on the top-slide, perched on a cross-slide, perched on the saddle, gibs loose or unlocked. The whole arrangement is bendy, plus all the slides can twist
• Operator mounts the tool at a slight angle, causing heavy side pressure. Maybe gets the height wrong too – parting off is fussy!
• Operator moves the tool by hand. He is unsteady, and him varying the feed-rate causes the cutter to bounce. It often collides violently with the job rather than peeling metal off at a sensible rate
• Operator is low to lubricate and stop swarf jambing the cutter

Counter example likely to succeed:

• Heavy lathe – big lathes are far more rigid than small ones
• Job protrusion minimised
• Short deep cutter able to resist pressure, protruding just enough to make the cut.
• Cutter mounted on a hefty rear tool-post bolted directly to the saddle.
• Ideally run lathe in reverse. This pushes the rear toolpost and saddle down, rather than lifting them.
• Operator selects correct RPM and feed-rate and engages automatic feed. Automatic feed maintains a constant cutting force, and eliminates human error.
• Operator pays close attention to clearing swarf

Only real men can part-off on a mini-lathe. They're light and don't have a rear tool-post or automatic feed. The tools are small and bendy.

Myford size machines are much less challenging, They have more weight and probably have a rear tool-post and automatic drive. Parting off from the front is do-able, but more likely to go wrong. A rear tool-post is generally reliable.

Parting off on bigger than Myford is almost trivial. Lots more weight, and even the front tool-post is impressively rigid. Automatic drive etc available. With a rear tool-post, parting off is trivial.

My lathe is a WM280, rather heavier than a Super 7, but not massive.

I can part-off by hand using the front tool-post if I'm really careful, but failure is always an option Front-tool post with automatic feed is almost 100% reliable – mostly works, occasional grief. Rear tool-post is 100% reliable unless I do something stupid.

Dave