Rear tool post???

[steam4ian]

G'day all.
I have read of people using a rear tool post for parting and claiming it gives better performance.
My question: Which way does the lathe turn to use it?
If it turns forward, as for normal cutting, then the back of the saddle is lifting from the bed. OTOH to turn in reverse means the chuck could unscrew on some lathes.
I would appreciate hearing what others are doing regarding parting and the value and facility of a rear mounted tool post.

Regards,
Ian

[Prowe]

Ian,

A rear tool post is used with the lathe rotating in the normal direction and having made an example of the G. Thomas design and fitted it to my Super 7 some twenty years ago I would not be without it. I rarely consider whether the spindle speed is correct – I just wind in the parting off blade albeit with care and hey presto the component falls away.
My rear tool post sits more or less permanently on the cross slide and it is very rare that I ever remove it, machining a large casting that is bolted to the boring table would be about the only reason.

Perhaps I have over simplified the operation but I can thoroughly recommend rear parting as a process, it works for me every time. I would also recommend that you read any of G.H.T’s articles on rear parting if you can get hold of them, he explains the process far better than a mere mortal like me.

Regards,

Phil

[jackrae]

I understand your confusion
The important point is that the tool is mounted upside down.
By this means it is effectively cutting on the upstroke, so holding the slide down onto the bed
Happy parting
Jack

[districtgrandmaste]

Like others I nearly always part off using my Rear tool post on my Myford Super 7.

Two thinga are important - a sharp tool and tailstock support!

[houstonceng]

jackrae said:

I understand your confusion
The important point is that the tool is mounted upside down.
By this means it is effectively cutting on the upstroke, so holding the slide down onto the bed
Happy parting
Jack

steam4ian is correct. Rather than holding the saddle down onto the bed, the back of the saddle will attempt to rise when using a rear TP with the parting tool downside-up.

If the "holding-down mechanism" under the rear sheer of the bed isn't in tip-top condition and adjustment, you'll have as much problem parting with the rear TP as you might with the front one.

[Tim Bayliss]

Hi there Ian,
The historic reason for using a rear parting tool post was that in the early days the lathe headstocks were not very rigid and used bronze bushes for bearings. When parting from the front this tended to 'lift' the headstock and cause vibration, etc. due to the weakness of the headstock/bush arrangement. The issue of the cross slide lifting when using a rear toolpost was secondary and the gibs could always be nipped up a bit anyway.
I used this arrangement for years on a ratty old ML4 with a really worn cross slide and had no trouble, but trying to part from the front was a nightmare due to the weak headstock.
I now use a Chester Craftsman which is very rigid but still have a rear parting toolpost more or less permanently fitted, I find it quick and convenient.
Best regards,
Tim

[jackrae]

I must admit I gave up using my rear tool post when I bought one of the tipped parting tools and life has been bliss ever since - though they do waste quite a bit of metal due to the tip width.
jack

[Tel]

I thought rear tool posts were mandatory - should be a hefty fine for not having one. Apart from the obvious reason of improved parting there are all sorts of nifty little winkles you can do with 'em. F'rinstance, if you need to part off aa number of identical short pieces just set a facing tool in the front, parting tool in the rear - set the relationship between the two so you can face and part with a simple wind in, wind out motion. With a little thought you can save a LOT of tool changes with one.

[abby]

Top tip Tel!

[SteveW]

Guys,

I occurs to add that the geometry of the rear tool post under heavy loading tends to tip the tool tip away from the material effectively reducing the cut.

A conventional tool holder (from the front) will tip into the job and increase the loading even under light cuts. The resulting positive feedback generally leads to all the trauma associated with parting off.

I made mine out of a lump of 1-3/4" square mild steel with a big slot to hold my Greenwood parting tool upside down at centre height with a couple of Allen screws to keep it there. The hold-down is a single 10mm coach screw with a head whittled down to fit my 'T' slots.

[ace]

I have parted off some small 1" round stock with my rear tool post and it sounds like frying bacon, hissing and crackling as it curls away from the cut. However I never manage to achieve this lovely cutting action every time, sometimes it chatters, sometimes it jams up but every now and then it works great.

ACE

I think it has a lot to do with the material you use, I have a small stock of metal thats totally c**p when used in the lathe, it tears and leaves a poor finish, it was purchased from the hardware shop.