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Rear mounted tool post

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Eric Cox01/10/2010 13:50:12
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557 forum posts
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What is the advantage of a rear mounted tool post for parting off as opposed to using the standard tool post
mick01/10/2010 14:13:10
421 forum posts
49 photos
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.
Richard Parsons01/10/2010 14:29:35
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645 forum posts
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The disadvantage is your tool is upside down so you do not get coolant directly onto its cutting edge. 
KWIL01/10/2010 14:42:35
3681 forum posts
70 photos
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.
Michael Cox 101/10/2010 14:58:06
555 forum posts
27 photos
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
Eric Cox01/10/2010 15:26:48
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557 forum posts
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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 ?
Nicholas Farr01/10/2010 17:16:06
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3988 forum posts
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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

mick01/10/2010 17:26:28
421 forum posts
49 photos
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.
wheeltapper01/10/2010 17:54:47
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424 forum posts
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Don't try that if you have a screw on chuck.
could be Very nasty.
 
Roy
peter walton01/10/2010 18:37:16
84 forum posts
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
wheeltapper01/10/2010 18:47:14
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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
Nicholas Farr01/10/2010 18:59:25
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3988 forum posts
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Hi Peter, now that's a saleable reason and one I can understand clearly.
Regards Nick.
chris stephens02/10/2010 00:31:07
1049 forum posts
1 photos
Oh dear not this again, I thought we de-bunked this myth months ago.
chriStephens 
Andrew Johnston02/10/2010 10:37:33
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7061 forum posts
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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
KWIL02/10/2010 11:36:53
3681 forum posts
70 photos
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.
chris stephens02/10/2010 12:50:41
1049 forum posts
1 photos
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 
 
Gordon W02/10/2010 13:31:25
2011 forum posts
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.
Andrew Johnston03/10/2010 10:35:30
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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
AndyB03/10/2010 11:19:03
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167 forum posts
7 photos
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

john swift 103/10/2010 11:58:38
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318 forum posts
183 photos
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

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