Rear mounted tool post

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Rear mounted tool post

This topic has 23 replies, 14 voices, and was last updated 3 October 2010 at 22:57 by John Olsen.

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1 October 2010 at 13:50 #56331
Eric Cox
Participant
@ericcox50497
What is the advantage of a rear mounted tool post for parting off as opposed to using the standard tool post
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1 October 2010 at 13:50 #5208
Eric Cox
Participant
@ericcox50497
1 October 2010 at 14:13 #56332
mick
Participant
@mick65121
Put very simply, the cutting action with a rear tool post pushes the cutter away from the work, which means it won’t have a tendenacy to dig in, which is what happens with a tool post mounted parting blade.
1 October 2010 at 14:29 #56333
Richard Parsons
Participant
@richardparsons61721
The disadvantage is your tool is upside down so you do not get coolant directly onto its cutting edge.
1 October 2010 at 14:42 #56335
KWIL
Participant
@kwil
I cannot agree completely, if the action on the cutter is to push it away when on the rear post upside down it can only because your leadscrew nut is shot or else the bearings in the headstock need attention. This all arose in the old days of white metalled headstock bearings, which wore on the top, so reversing the action merely forced the spindle downwards on to the least worn part. In addition, since the rear toolpost tends to lift the tool, it drags the v slides tighter which makes moving the topslide more difficult.

The only advantage of a rear tool post is that the chips drop off and do not jam in the gap, this is the real problem of front parting.
1 October 2010 at 14:58 #56336
Michael Cox 1
Participant
@michaelcox1
With a front mounted parting tool, if the tool digs in then the tool moves downwards. In so doing it is forced between the workpiece and the cross slide. These are two fairly immovable objects so the tool is forced to dig further into the workpiece. With a rear mounted toolpost the tool will swing upward, away from the workpiece and the tool tends to self release from the dig in.

I do not agree with Richards comment regarding coolant. If this is applied to the groove it gets carried straight round to the cutting tool. In fact the deeper the groove becomes the more coolant that is carried round.

Mike
1 October 2010 at 15:26 #56338
Eric Cox
Participant
@ericcox50497
Personally I can’t see the difference, one is just an inverted image of the other. How many of you have experience the tool digging in and alleviated the problem by using the rear mounted tool post.or is it just apocryphal ?
1 October 2010 at 17:16 #56344
Nicholas Farr
Participant
@nicholasfarr14254
Hi, I havn’t used a rear mounted parting tool. I have used a reasonable rigid parting tool in the convetual manner on a large industrial lathe with gallons of coolant, which was in very good condition and no matter how you set the cutter you always get that day it wants to dig in. I have come to the conclusion that the digging in seems to be down to the material that you are parting off. My observations and theory is the material drags the tool and crosslide in towards the clearance side of the crosslide screw. I therfore can’t see where the differance would be as far as digging in is concered. Of course, I may be wrong.

Regards Nick.

Edited By Nicholas Farr on 01/10/2010 17:17:38
1 October 2010 at 17:26 #56346
mick
Participant
@mick65121
Oh dear, I did say put very simply! Draw yourself a sketch of a rear mounted tool and a conventualy mounted tool, then draw an arrow in the direction of rotation and yu”.ll see what I was getting at. Push off, in this context, has nothing at all to do with lead screws and shot bearings. If you haven’t got a rear tool post and your conventunal tool post will allow a tool to be mounted up side down at centre height, run the lathe in reverse and you’ll notice the improvement. Turning in this way will also cut down on vibration, thats why most auto lathes were designed to cut left hand. Happy turning.
1 October 2010 at 17:54 #56348
wheeltapper
Participant
@wheeltapper
Don’t try that if you have a screw on chuck.

could be Very nasty.

Roy
1 October 2010 at 18:37 #56351
peter walton 1
Participant
@peterwalton1
I always thought the main reason for the rear toolpost was you could leave a parting tool in situe and use as and when required.

A lot easier when doing the odd small run of components.

Peter
1 October 2010 at 18:47 #56353
wheeltapper
Participant
@wheeltapper
Posted by peter walton on 01/10/2010 18:37:16:

I always thought the main reason for the rear toolpost was you could leave a parting tool in situe and use as and when required.

A lot easier when doing the odd small run of components.

Peter

that’s why I’ve got one.

only trouble is, my Chester Comet has a short cross slide and the rear tool tends to get in the way .

I’ll have to redesign the post so I can rotate the tool out of the way and return to the same position easily.

Roy
1 October 2010 at 18:59 #56354
Nicholas Farr
Participant
@nicholasfarr14254
Hi Peter, now that’s a saleable reason and one I can understand clearly.

Regards Nick.
2 October 2010 at 00:31 #56365
chris stephens
Participant
@chrisstephens63393
Oh dear not this again, I thought we de-bunked this myth months ago.

chriStephens
2 October 2010 at 10:37 #56389
Anonymous
Chris,

Pray enlighten this ignoramus (me) as to the myths and debunking thereof.

My gut feel is that there is no inherent difference in parting off with a front or rear toolpost. The differences that might exist are due to the way the rest of the lathe handles the cutting forces.

Regards,

Andrew
2 October 2010 at 11:36 #56390
KWIL
Participant
@kwil
From the front every time, but the cutter MUST be on centre height, if above it rubs, if below the work will tend to ride over, hence the myth of digging in.
2 October 2010 at 12:50 #56395
chris stephens
Participant
@chrisstephens63393
Hi Andrew, I am not going to speak to you if you continue to put yourself down!

You have it aright with second para. The myth is that there is some kind of magic difference between front and rear mounted parting tools. There are some reasons why a few people might find the rear mounted better, one might be that if it is permanently mounted it might be better set square to the lathe axis, whereas a parting tool just plonked in the QC toolholder might not be. Any improvement in parting in this circumstance is not a sign of inherent improved ability, just the benefits of a permanent fixture, or operator error.

As to the debunking, you will have to go back through posts sometime earlier in the year.

Note for those who lack experience;

Myth and debunking not withstanding, if it works for you go for it. The best teacher is experience and if you have found something that works, for your set of circumstances

and is not wildly dangerous, then go ahead and use it.

christephens
2 October 2010 at 13:31 #56399
Gordon W
Participant
@gordonw
I asked questions re parting off a year or two ago, here are MY findings, as a newcomer to model engineering. All forces should go thru’ the strongest parts, ie. down thru’ the beds, not lifted up into a small area of gib strip. I bought an expensive tipped parting tool, the tip fell out when unpacking, and was never seen again. Bought 2 new tips, and still had problems. Stripped the lathe down, polished gib etc. and readjusted everything. Tightened the top slide solid, hardly ever use it anyway. Now can part off with ease, for most work I’ve gone back to HSS in a simple holder, set at c/h. For deep cuts, ie. over about 10 mm, withdraw tool and move back, to make width greater than tool, this allows for not being square to axis. If chatter or noises start, slow spindle speed. These are just my findings, not expert advice.
3 October 2010 at 10:35 #56428
Anonymous
Chris, oooops, sorry about that. Note to self: must ask for some anti-self-deprecation pills next time I go to the quack.

I read all the previous postings on parting off last night. It has certainly cleared up a few things in my mind.

I have had the odd dig-in when parting off using HSS tools. Usually when I’ve been in a hurry, so most likely due to operator error. Since buying a ‘cheap’ industrial insert style parting off system, I have had no problems (fingers crossed and touch wood). I generally part off at a few hundred rpm with a feedrate of between 2 and 4 thou per rev, depending upon the material. I use power crossfeed for parting off. I wonder if hand feeding may be the cause of some problems, as it is difficult to maintain a small but consistent feedrate?

I part off brass, bronze and cast iron dry. There is no problem with jamming as the swarf is granular. For steels (plain and stainless) I use coolant. The design of the insert seems to produce steel swarf in the form of a tight coil, like a clock spring, and a U cross section, so it is narrower than the groove.

Mind you I do still keep a hand close to the power feed knock off when parting off, just in case.

Regards,

Andrew
3 October 2010 at 11:19 #56431
AndyB
Participant
@andyb47186
Hi all,

I used to have problems parting off from the front and then got one of RDG’s rear parting tool posts for Myfords for £30 (I couldn’t make one for that!).

I had to grind a flea’s nadger off the HSS blade to suit my Drummond and what a difference!

I have a feeling that part of my problem was the squareness of my tools to the work and another is that I can maintain a more consistent feed turning the cross slide wheel anti-clockwise; I am right-handed.

Certainly consistent feed rate is vital and I can part off shims at 20 thou in mild steel!

I do use a slightly more than drip feed of coolant from a washing up liquid bottle.

I have to say that Drummond beds are very stiff.

I would not go back.

Andy

Edited By Andy Belcher on 03/10/2010 11:20:34

Edited By Andy Belcher on 03/10/2010 11:22:03
3 October 2010 at 11:58 #56432
john swift 1
Participant
@johnswift1
Hi

Your reference to the inconsistent feed rate is a factor

In the early 70’s I could break parting tools on the school lathes with ease

my dads reply was it’s only a toy lathe , they are not rigid enough ,

followed by – don’t be timid you have to go for it and be consistant with the feed

several years later I was co-opted into making reapairs to the controls on a EMI-MEC Sprint plug board capstan lathe.

watching the war issue BSA auto’s , DANOBAT 40’s and 50’s , and EMI-MEC’s running

the first thing that I noticed was the parting tools were thinner than than I remember on the school lathes

also depending on the machine mounted on the rear or vertical slide

and very close to the collet chuck

Interestingly one machine was being used to cut up some 1/2″ x 1/4 “

brass bar into thousands of pieces about 3/4″ long to make adaptors for some switch gear

two lengths of bar being feed through a 1/2″ square collet to a stop and the parted

the machine run for days just needing to be supplied with new bars !

starting virtually from square one at the start of the year with a CL300M lathe that needed some attention to the speed control etc

my first attempt at parting took me back to my school days

the toolpost deflected forward and the workpiece trying to climb onto the tool

I’ve now solved my problem

It just took new taper bearings , new gibs and a lot of work ensuring nothing move when it should not do so

John

Edited By john swift 1 on 03/10/2010 12:00:33
3 October 2010 at 13:28 #56441
chris stephens
Participant
@chrisstephens63393
Hi Guys,

There is one good reason why Andy’s rear tool post would work better and that is that it is directly mounted to the cross slide. Without the extra set of dovetails in the top slide, to add slop, the toolpost must be more rigid and therefore a more consistent feed can be achieved. This does not make the rear toolpost better of itself, but it does overcome certain inadequacies in a poor lathe.

Certainly on the SMEE stand at Ally Pally last January I found parting off a little trying on a Mini-Lathe, you could see the top slide moving down quite markedly as the tool tried to dig in, a rear tool post might have helped, but on the other hand a few hours spent “finesse-ing” the gibs would have cured the problem too, and this work would have helped normal turning as well!

chriStephens
3 October 2010 at 15:48 #56444
AndyB
Participant
@andyb47186
Doh!

Thanks Chris, of course it is!
3 October 2010 at 22:57 #56459
John Olsen
Participant
@johnolsen79199
A couple of points from above that I would like to emphasise.

First the narrow parting blade. I only had a Unimat 3 for many years, and had a lot of success with a parting off blade about 1mm wide. This was ground from a piece of 1/4 inch HSS and could go in just over a quarter of an inch. Very successful, lasted many years, and made me wonder what the fuss was about. Of couse the Unimat is only a tiny machine, you would not be wanting to ask it to part off two inch diameter 4140 bar. Mind you, within their reasonable size range, these small machines are actually quite rigid.

Then with the Myford (ML7) …I applied the lesson and used the narrowest parting blades I could find, with pretty good success until the last few months. Then I discovered that the headstock bearing had developed a bit of play. I had not had any trouble turning things accurately to diameter, but I noticed that I was having trouble parting off, and realised that the job seemed to be trying to ride up on the tool. So checked the bearings, took some shims out, and it is all behaving much better now. So rigidity is important, reasonably enough.

I have used a rear tool post in the Myford, it didn’t seem to generally be any better than the quick change holder in the front. However the rear tool post I have has a disadvantage, the tool is set on a rake which is nice since it avoids having to grind the top to get rake. But there is no other centre height adjustment on the holder, so to get the correct centre height you have to accept a given overhang. I think this is a bit too long on mine, so will reconstruct the head to combine an adjustable centre height with the built in rake feature. I will probably make another head with zero rake for brass etc.

I have a tipped parting tool for the Myford, however the tip is wider than I consider acceptable. I think about 3/32 or 2.5mm is about the maximum for an ML7. A Super 7, with its better bearings, should probably be able to cope with more.

regards

John

Edited By John Olsen on 03/10/2010 22:59:26
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