Parting On a Hobby Lathe

Hello Everyone

I’ve been trying to get the hang of parting using my Warco WM180 lathe. I have been following a procedure much like the below picture and when the diameter of the part is small everything works very well. The cut is smooth, the lathe is quiet, and the tool advances nicely. However, once I increase the diameter of the part (to say 1.5 inches) things become very different. The noise (chatter?) and vibration increases dramatically, and to the level that I think If I persisted it sounds like something very bad would happen.

Today I’ve been reading a post on a different forum that suggested inverting the parting tool and reversing the lathe. However, as you can see from my second diagram I can’t get this to work using my existing tooling. This is because I can’t change the height of the tool post, and so the parting tool cutting edge won’t be central and won’t make contact with the part before the blunt edge of the parting tool meets the closest edge of the part.

My question is have I misunderstood things or do I just need to get a different parting tool ? (and if so does anybody know where I can source it?)

David.

Most people that use the inverted method have a separate tool post behind the spindle so the lathe runs in the same direction.

Search "rear toolpost" its been done to death on just about every forum.

Have a look at the Eccentric Engineering advert on the right of this page, they do a holder than may fit your existing toolpost

J

Edited By JasonB on 02/01/2015 18:31:02

From a newbie. I did my first ever parting yesterday following 'toms techniques' and I'm glad to say I now have a few holeless washers as souveniers. The major points he makes is a rigid toolpost i.e QCTP type and bags of cutting fluid as well of course as a perfectly aligned sharp tool.

So forgive me if this is a baby teaching egg sucking to grandma but it may be worth looking at those things before looking elsewhere

I can get a 1/16" wide parting tool with 1/2" overhang to part off 2" diameter EN1A. Similarly a 3/16" tool actually works better, because you can get more cutting oil into the cut. but anything about about 90rpm causes chatter and any less than 70rpm and the motor stalls, so it's walking a fine line, but the critical elements are end of tool sharpened on a diamond wheel and feed enough to create a constant supply of neatly rolled up chips that bubble up out of the cut. It's painfully slow though, the tool feed rate is about one and a half thou as each chuck jaw goes past, so 4.5 thou thick chips.

i think most people parting larger diameters fail because they (1) don't drop the speed enough hoping for a 'flywheel effect' to keep the work from jamming, and (2) don't apply a steady enough feed. At around 75 rpm over-aggressive feed will stall the work but probably NOT smash the tool, so be confident.

The main conclusion from my experiments is that I probably could do with a bigger pulley ratio for more torque to get a steadier cut, but sadly the motor pulley is already the minimum 19mm I only have room to fit a mandrel pulley about 30% larger than the existing.

Neil

I'm no expert in the 'black art' of parting off, when I first tried it on my Myford with on of those parting blades in a holder my results were rubbish so I gave up, shortly after looking through the box's of tooling that came with the lathe when I purchased it I found two Sandvik parting blades and one holder so gave it a go and it cut through like butter so now I got brave and fitted the rear toolholder with a HSS tool I ground myself happy to say that too works OK but nothing like the Sandvik.

.

Harry,

Are those the ones with a little depression in the top? [which causes the swarf to "roll" so that it becomes narrower than the groove being cut]

Very clever design, and very effective.

MichaelG.

The most common problem with parting off is being wimpy on the feedrate. You need to keep the tool cutting or it will chatter. On my centre lathe I use a 3mm insert system in a 4 way toolpost. It seems to help locking the top slide and the saddle. Flood coolant is advantageous, except for brass and cast iron. I normally run at about half turning speed, so for 2" EN1A the spindle speed would be 260rpm. I use an absolute minimum feedrate of 4 thou per rev and, more controversially, I use power cross feed. I've never bothered investigating rear toolposts, as I don't have a problem with the front one. Here's a picture of parting off 3" OD cast iron; I can't remember if I was running at 180 or 260rpm:

On the repetition lathe, which has the tooling behind the work so normal operation is with the spindle running clockwise, I use HSS parting tools. Depending on the material, spindle speed is 500 or 1000rpm. There is no power cross feed so I just pull on the lever hard to get a good rate of feed. Seems to work fine. The key is to keep the cut going.

Regards,

Andrew

Experimented with a rear tool post on a previous lathe but found there was no real advantage.(holder now doubles as a larger paperweight until I can think of how to up-cycle it) On my large gearhead lathe I part of at around 200 - 300 rpm for material between 3" to 1" with either a 2mm or 3mm tool and depending on what I am parting, either an HSS or carbide insert tool, the carbides have an inverted vee already in the top if the insert. I've found that grinding a similar groove in the HSS improves cutting rate considerably. I'm with Andrew in that I always part off with power cross feed, I find this is the only way to keep the tool cutting consistently and without chatter. I don't have flood coolant so use either a drip feed of high pressure cutting fluid or an air powered mist coolant similar to the Hench fogbuster but homebrewed which is very effective. With Brass and CI, I shut off the coolant and just allow the air to clear the chips. Parrafin for Ally and soluble oil for pretty much all grades of steel.

As Andrew has shown above, I also use a rotating centre when parting, slacked off slighlty as the cut reaches the end. This is something I was always advised against doing for a variety of reasons however, I've found parting is less successful when not using a centre

Hi David and Guys
I always part off using a rear parting tool on my S7 Mk one
Nobby

David,

The above type of rear parting tool holder works well. The one in the pic. is from RDG (no connection),around £60, but there are many types out there. The blade takes GTN3 inserts. These curl the chip in from both sides so making it narrower than the groove being cut and helps prevent jams. Parting becomes a routine operation with this kind of setup. (And you'll notice a big saving on washing powder)

Use plenty cutting oil. Keep both sides of the cut wet. Provided you have plenty oil in the cut, I have found no difficulty in stopping the cut by backing off very slightly, if there is a need to brush away swarf that might build up and get drawn back into the cut. Spindle speeds up to about 250 rpms work ok, with a steady continuous feed into the cut. I prefer around 100rpm. As you come nearer the centre ease off the feed pressure BUT do keep it cutting, not rubbing.

Also, be positive when beginning the cut. It can sometimes take a good pressure to get the cutter to "bite".

Take any slop there might be out of gib strips and if you can, lock the apron down on the bed while parting off.

Make very sure the cutting tip is sharp and has no tiny chips out of it.

J

I have a Sieg C3, similar to the Warco WM180 I think. It used to suffer from chatter, when turning, parting off just about all the time. Parting off almost anything was a nightmare. I put this down to several things but mainly the overhang of the tool and toolholder.

So I made a post and clamp on style QCTH. As the Sieg C3 is a modern machine it has variable speed, and a bolt on chuck rather than the rather archaic screw on jobbie of most old machines. From the start the use of an upside down parting off tool was planned.

There are some photos in my album

As a result parting off is now a piece of cake, chatter generally is almost a thing from the past.

Best of luck!!

cheers

Bill

Michael

Yes those are the one's as you say very effective.

H

Would the second illustration above not try to undo the chuck with disastrous results?

Not on the OP's WM180 as its a flange mount chuck

Hi Ian,
It would if it was a screw on chuck. I was just about to say such as a Myford but it then occurred to me that one of the "must have " accessories on a Myford is a Dewhurst reversing switch (It seems it must be Dewhurst for some strange reason.) It would seem pointless to have reverse with a screw on chuck. Maybe some newer Myfords have a different chuck fixing. (It is about 50 years since I last used a Myford (ML7) )

Les.

.

Just a thought, Les ... Would reverse be 'safe' on the Myford when using their special 2MT collets ?

Can't quite get my head around the forces involved

MichaelG.

I use rear toolpost and run at normal turning speed, or one speed lower. IE 400rpm for a 1" diameter steel bar.

Keep the feed up aggressively and dont let the tool rub.

I splash a bit of cutting oil on with a brush.

Works for me. (1937 Drummond M-Type)

There is an art to sharpening parting tools. I grind mine flat on top and never with a dip or curve because then when you grind the front in future to sharpen, the cutting edge will be narrower than the unground section of tool bit. Jammo whammo.

Edited By Hopper on 03/01/2015 09:33:10

Ralph

Parting of a component in a lathe is always difficult regardless of the machine. Anyone who has not jammed a parting off tool has not made much.

One of the big problems that amature machinests have been programed that tungsten tips are the answer. This may be true for the larger more rigid lathes but for the small hobby lathe parting off can be a problem. I have always had problems parting bits all through my trade years.

The best parting tool I have ever used the blade was mounted at 45 Deg to horizontal and it never jammed. I don't know if you can get them now so I made my own. The reasoning is that if the blade is over loaded it moves away from the job.

I will attach some pic's

Larry

Edited By JasonB on 03/01/2015 13:20:53

As usual lots of good advice, and it seems like I have a few options. I can’t see any immediate way that I could fit the rear tool post (bearing in mind my beginner status), so that leaves the inverted parting tool holder from eccentric engineering. This look ideal, but the supplier is in Australia. Does anyone know of anything similar that is a UK based\supplier ?

Of course, I’ll also spend some more time trying to improve my skill levels!

David