Parting off tool

[Tractor manParticipant @tractorman]

Hi all.

The turret lathe i bought came with a short tool post which turns out to be a parting off tool post.

Never seen this type before which presents the tool behind and below the work thus providing quite a rigid set up.

Lathe has to run in reverse and it irks really well.

[Tractor manParticipant @tractorman]

[Chris Evans 6Participant @chrisevans6]

For parting nothing beats a rear mounted toolpost

[not done it yetParticipant @notdoneityet]

Some like to part off that way. I don't have too much trouble in the normal rotation (and the Raglan could only be used in reverse if a collet and drawbar were in use). I think it is more to do with how rigid the set up is, particularly the cross slide position on the saddle.

[peak4Participant @peak4]

This might seem a really silly question, but why does the lathe have to run in reverse?

Being entirely self taught, I don't have the benefit of wide and varied training, so I've never seen one of these before.

If the tool was sharpened appropriately, wouldn't it work fine in the normal direction, pulling the tool towards you; that way the swarf, particularly fine brass chips, would naturally fall away from the tool.

Also the load would be towards the iron casting, with the screws just holding the tool in place, as normal, rather than trying to prevent it being pushed downwards past them.

[Tractor manParticipant @tractorman]

George Thomas wrote a good piece on parting off and it’s all to do with the geometry of the tool position relative to the diameter of the workpiece. Conventional tool placement allows the tool to dig in deeply but placing it in a different alinement means if it catches it cannot dig in but is pushed away from the work,not deeper into it.
I can’t explain it as well as he did.
This tool holder and position is by far the most rigid I have seen as the tool is not able to flex and is being pushed into the bed of the lathe. I can’t comment on the possibility of digging in but I will do some experimenting. Mick

[MWParticipant @mw27036]

Posted by peak4 on 10/04/2017 21:11:38:

This might seem a really silly question, but why does the lathe have to run in reverse?

Being entirely self taught, I don't have the benefit of wide and varied training, so I've never seen one of these before.

 

No it's okay,

The lathe has to go in reverse because the tool would be presented on the "wrong" side of the lathe. The conventional way has the work piece rotating in an anti clockwise movement down, and into the cutting face of the tool.

Working from behind the lathe, a rear tool post cannot hold a conventionally ground tool because the workpiece would be rotating, still in an anticlockwise movement lets presume, up and into the underside of the tool, not cutting in the way you want it and most certainly rubbing rather than cutting.

It is because of this that the tool has to be placed upside down so that the right cutting face is presented, with the clearance angles all there, to the work piece.

A lot of cheap lathes in the past did not have a reverse function, so the lathe would only spin in an anticlockwise way, so came the rear toolpost, along with it's upside down tools,

this also meant that if the user was canny, they could have two toolposts in play without needing to stop to switch tools for parting off.

The theoretical reason why it is also good for parting off is because the tool is less likely to slip under the work piece, be drawn in and snap either the tool or the work piece, normally the tool. So a rear tool post would encourage the tool to go "up" and away from the work piece rather than the opposite. This tends to happen if the tool is too low on the centre line.

(Very nice tool post/ parting off tool, Tractorman!, Is that two, separate, dovetailed slides on the same cross slide?!)

Michael W

Edited By Michael-w on 11/04/2017 07:12:02

[mechman48Participant @mechman48]

I built a rear tool post from a Hemingway kit & it does very well, especially on steel, I won't go into all the geometry as Geo Thomas, & others have explained it very well. I haven't used it for some time as with the small stuff I'm doing at the mo' the conventional front position works well. look in my album 'Rear tool post' for pics…

​Looks nice but too much overhang for centre alignment…

So modified for a more rigid set up… lovvly jubbly…

George.

[Tractor manParticipant @tractorman]

Hi Michael W yes it is. It’s a capstan Lathe so can be set up with a front and rear tool post.

It’s the first time I’ve seen such a low tool post but it makes sense and works great as there is no overhang as there would be with a standard height post.
I’ve only just got the lathe so I’m still learning about its use. Mick

[Alan JacksonParticipant @alanjackson47790]

I made a similar vertical parting tool and it worked very well. It was described in Thread: Parting Off MEW225.

Alan

[steamdaveParticipant @steamdave]

Looks like it works similar to a tangential tool.

Dave
The Emerald Isle

[peak4Participant @peak4]

Thanks Michael, I do appreciate that, I actually use an inverted tool in the rear toolpost of my Myford and also it's Warco clone.

One advantage of that is that the whole slide is lifted against the dovetails, rather than being pressed away from them, which surely helps improves rigidity as well.

I was clearly half asleep when I posted, and was thinking more of using the tool set lower in the toolpost as a shallow grooving tool, rather than for parting off.; then extrapolated to parting without thinking it through.

[Neil WyattModerator @neilwyatt]

Rear toolposts are all smoke and mirrors if the lathe has to run in reverse.

The lathe just becomes mirror image of a normal toolpost with the lathe running forwards. All the forces are exactly the same and the geometry is identical.

Just imagine standing behind the lathe using a rear toolpost and running eth lathe backwards. All that's different is that the headstock is on your right instead of your left.

Now a rear toolpost with an inverted tool DOES have a completely different geometry.

The toolpost above works well becuase it is well designed – rigid with minimal overhang – not because it is fitted at the back.

Neil

[pgk pgkParticipant @pgkpgk17461]

Posted by Neil Wyatt on 11/04/2017 11:52:23:

((Rear toolposts are all smoke and mirrors if the lathe has to run in reverse.))

I'm glad you wrote that 'cos i was wondering what the heck i wasn't understanding, having come to the same conclusion

((The lathe just becomes mirror image of a normal toolpost with the lathe running forwards. All the forces are exactly the same and the geometry is identical.

Just imagine standing behind the lathe using a rear toolpost and running eth lathe backwards. All that's different is that the headstock is on your right instead of your left.))

But it's much harder to reach the handwheels

((Now a rear toolpost with an inverted tool DOES have a completely different geometry.))

presumably not to a front inverted tool run in reverse..

((The toolpost above works well becuase it is well designed – rigid with minimal overhang – not because it is fitted at the back

Neil.))

pgk

[MWParticipant @mw27036]

Sorry guys, i'm not sure why I said reverse, I think it was something to do with the tool.

But really, forwards, always forwards with a rear toolpost. Anti clockwise direction.

The bar/object spins round past the halfway mark of an anticlockwise rotation and comes up and into, the upside-down cutting face of the tool, in order to cut.

Michael W

 

Edited By Michael-w on 11/04/2017 13:12:47

[SillyOldDufferModerator @sillyoldduffer]

Posted by Neil Wyatt on 11/04/2017 11:52:23:

Rear toolposts are all smoke and mirrors if the lathe has to run in reverse.

The lathe just becomes mirror image of a normal toolpost with the lathe running forwards. All the forces are exactly the same and the geometry is identical.

…

Neil

Down here in the West Country we talk just like Vicky in Little Britain. 'Yeah, but no, but yeah but no but yeah…'

I use a rear tool-post with a carbide insert parting tool and run the lathe in reverse. I do so because the geometry of the post and my tool-holder won't hold the upside down tool at centre height. Zut alors!

For me success in parting-off came from both rigidity and keeping swarf out of the slot. On my lathe the problem parting off using the front too-lpost is that the thing towers above the saddle and has many wobbly joints and adjustments.

The leverage involved in this lanky set-up means that a parting-off tool cannot be really rigid.

So, my experience is that a rear tool-post is a much more better arrangement even if the lathe is running in reverse and the parting tool is the right-way up. It's simply more rigid than the alternative.

In practice reverse cutting with a rear tool-post works well, though I accept that cutting forwards against an upside down tool should be even better. Although my 'to-do' list includes making a suitable rear tool-post, I'm not rushing because what I have is easy and produces acceptable results.

I've experimented with upside-down cutting and an HSS tool that does fit my rear tool post. I wasn't able to convince myself that gravity helps clear swarf out of deep slots. Whichever way up the tool is orientated, swarf jamming and/or welding to the tip is my big problem, even with generous lubrication by hand. Am I doing it wrong (again!) ?

Dave

[Neil WyattModerator @neilwyatt]

I parted a bit of 1 1/4" diameter steel a few days ago, using a carbide tool (on the front).

Problem was I just couldn't feed it fast enough, I had to slow down to 450rpm to stop it howling!

Neil

[Neil WyattModerator @neilwyatt]

More thoughts on RTPs + parting in general

Two types:

Inverted ones that run with lathe in normal rotation, benefit from pulling cross-slide up on to the vees and eliminating top slide

Non-inverted ones used with lathe in reverse that are very rigid.

The benefit of carbide tools? the 'scoop+break' shape generates relative short swarf that is also slightly v-shape in cross-section and so is less likely to jam in the cut. GHT's top-grooved tool worked in the same way.

Overall?

One of my observations is that people who part off under power-cross feed rarely report problems! Could it be that, as well as being a spur to eliminating obvious problems (like poorly set or blunt tools, or obviously felxible setups), task-specific tool-posts and specialised parting tools give the operator greater confidence, leading to a less cautious approach to the work and therefore improved results?

Neil

[Tim StevensParticipant @timstevens64731]

It is also the case (I submit) that a dedicated toolpost which can be left in place and which does not interfere with the standard arrangement is a significant benefit in time for many operators. But surely the main need is to have a toolpost which relies on firm support from wide metal surfaces (and not on a thin lip at each end of a thinnish cross slide) – with the added benefit of the slack being taken up by gravity rather than working against it?

Cheers, Tim

[JourneymanParticipant @journeyman]

Posted by SillyOldDuffer on 11/04/2017 14:17:48:

The leverage involved in this lanky set-up means that a parting-off tool cannot be really rigid. ….

Dave, you need to get rid of the wobbly bits! Get one of these:

Very rigid and can be used anywhere on the cross-slide of my WM250. Can even be used as a rear toolpost with the normal topslide fitted as well. Details **here**

John

[SillyOldDufferModerator @sillyoldduffer]

Posted by Journeyman on 14/04/2017 10:21:52:

Posted by SillyOldDuffer on 11/04/2017 14:17:48:

The leverage involved in this lanky set-up means that a parting-off tool cannot be really rigid. ….

Dave, you need to get rid of the wobbly bits! Get one of these:

Very rigid and can be used anywhere on the cross-slide of my WM250. Can even be used as a rear toolpost with the normal topslide fitted as well. Details **here**

John

Ooo, that's nice. And the swarf guards too.

Ta,

Dave

[SillyOldDufferModerator @sillyoldduffer]

Posted by Neil Wyatt on 14/04/2017 07:16:13:

…

Could it be that, as well as being a spur to eliminating obvious problems (like poorly set or blunt tools, or obviously felxible setups), task-specific tool-posts and specialised parting tools give the operator greater confidence, leading to a less cautious approach to the work and therefore improved results?

Neil

There's a lot in that I think. Watching a carbide parting off tool at speed really opened my eyes – I didn't think it was possible. Now I do it all the time and it gets easier the less I worry.

A big problem for beginners, especially us self-taught types, is getting a feel for how hard to drive our new toys. There have been many posts about stripped gears in small milling machines not to mention those reporting burned out lathe motors and fried electronics. Perhaps this is down to over-confident he-men thrashing their hobby kit? My novice errors were at the other extreme: tentative, erratic light cuts causing rubbing, chatter, disappointing metal removal rates and many dig-ins whilst not parting. For a while I convinced myself that my new mini-lathe couldn't cut steel at all; actually driven with appropriate confidence it was rather good at it.

Internet videos help but for quick learning there's no substitute for a tutor who knows the ropes.

I'd be encouraged if MEW issued beginner certificates:

• Bronze – Often mangles metal and/or breaks tools and machines.
• Silver – Eventually produces work more-or-less to size but can't guarantee a good finish.
• Gold – Works accurately and quickly with a predictably good finish. Isn't embarrassed by showing the results!

I'm about half-way to getting my Silver…

Dave

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