Lathe Rear Tool Post For Parting Off

If anyone is coming to grief with parting off on their hobby lathe, then it might be worth sharing my experience. When I had a Warco WM180 I had a huge amount of success making an inverted parting tool holder and running the lathe backwards.

Now that I’ve upgraded to a WM250V I thought I’d save myself some time and buy the ready made rear tool post from Warco.

(Unfortunately my WM180 inverted tool holder was too small. I didn’t fancy making it again, so the quickest route was to buy a ready made rear tool post from Warco)

Anyway, a video of it in action is here. I’m very happy with it.

Can I suggest that the video shows the tool holder in an odd position? The load on the tool bears on the loose strip and the three socket screws, when good practice might suggest that the solid metal is firmer. In other words, if you turn the tool upside down, you need to turn the tool holder too - surely? See the video at 15 seconds in. (And ditto the post clamping screws, but that is more complex to get right)

Not that I know anything about good practice - have you seen inside my garage ...?

Cheers, Tim

I built one from a Hemmingway kit; the original angled side allowed too much overhang at centre height so I turned it round & used a bought holder upside down... a much improved set up ... & much better rigidity. There are more pics of the machining in my album... Rear tool post.

Establishing centre height...

First cut after machining & assembly ...

1"dia MS washers...

George.

Edited By mechman48 on 20/10/2016 16:14:53

I'm not sure if it's right or not to hold it like that, the fact the strip is angled a bit looks a bit weird but so long as it's held firmly in it's seat it should be ok?

I maybe have a bit more courage or a bit madder or maybe my lathe is able to take it but i have used small part-off blades in the past and found that they are ok for small diameters and i use a front mounted tool.

I found they tended to bend inevitably as you extend them further and push them to part bigger diameters. I found my only option beyond that was to grind custom length parting tool from HSS and the tool width was more like 4.5mm to have the needed rigidity for 2" diameters. And this tool would do stainless okay.

PS. those washers look very nice, mechman! 

Michael W

Edited By Michael Walters on 20/10/2016 16:25:00

... PS. those washers look very nice, mechman!

Thanks MW ... iirc & fwiw they were machined off under power feed, approx' 400 rpm with the minimum feed rate as set on the machine... & plenty of neat cutting oil.

George.

The advantage of the rear tool upside down is that a much deeper tool can be used also the cutting forces tend to tension some lathe in a way that is beneficial. Dovetail pulls up for instance which tightens it but more often it's down to the saddle being lifted on lathes such as the 7's that have a flat rear rail.

Vertical prismatic V's self tighten under cutting loads and what happens problem wise relates to the bed width to some extent in relationship to the load. They are still pretty stiff even when worn and can also offer pretty large surface areas to take the loads. I suspect the over all proportioning is a black art really.

I've seen some very impressive demo's of 1" replaceable carbide tips tools parting off large diameters. Must try one some day, 5" dia aluminium and steel getting up to that size.

John

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Surely David you are running the lathe forward but reversing the cross slide's direction and not running the lathe backwards.

Martin

That’s a really good point with regards to turning the tool holder upside down – I’m not sure why I didn’t think of that. I think I’ll make the video unlisted until we get to the bottom of it here.

Martin – yes the lathe is running forwards (maybe it’s does not look it from camera strobing effects?).

David

why didnt I think of doing it that way?? Thanks

I must admit since trying an inserted parting tool, parting has become more successful, if not less exciting. Unlike HSS the tool 'honks' if fed in too slow, which puts you off and makes you want to slow down instead of speed up.

I had to part of several slices of 1 3/4" brass, not abig challenge, but it was amazing to see the chips pouring out of the cut like water at 1200rpm

Neil!

Please dont invert front parting tool if the chuck unsrews .I always part off from the back as the chuck screws on Click on my photos Nobbyrt on S7 fro mthe back

Like Hopper did with Brian's lathe you might benefit from shimming the rear underside saddle gib for a very precise fit. I think Hopper had to do a bit of light file work on the underside of the rail.

The Warco WM240 and some others do have a T slotted cross slide. Things bolted down need to be pretty flat though as distortion can tighten the dovetail.

John

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In an old machinist magazine there is an article about a Chap parting off 11" stock. He stated the biggest He did was 18". Would be hard to go without breathing that long while doing 18".

You could always try deep parting PB 4" diameter!

stated the biggest He did was 18".

Any idea of the width of his parting off tool?

Would it need to be very wide if you did it in stages? I admit my experience of parting off large diameters is nil, 18" won't fit on the Myford but even with something like 1" dia I start with only a short length, say 1/4" of the 1/16" wide tool blade sticking out of the holder and then withdraw the tool and then extend it, then the tool which is very strong vertically is constrained by the groove sides to stop it flexing sideways,  I would repeat the process depending on how large the bar is to ensure only a short length of blade is unsupported, or am I being too cautious?

John

Edited By Sandgrounder on 21/10/2016 19:43:15

I switched over to a rear mounted insert type tool a few years back and couldn't be happier !

I think that most of the problems people have with a front mount tool is that they mount it in the toolpost that is sitting on top of the compund slide and this is not ideal as the compund slide can flex especially when the parting tool has  a lot of overhang .

When machining / turning it is usually good practice to keep the tool overhang/ as close as possible to the tool post which is the exact opposite to what you need for a parting tool although only having enough of the parting tool sticking out to get to the centre of the piece being parted off is also good practice.

I found that my little  lathe from China  didn't have the most ridgid toolpost/ compound slide set up so fitted a rear mount post , this gives the machine  a little more flexibility when parting off as i can still use the front toolpost to face and champfer then use the rear post to part off and no tool changes !

That being said there is a downside ! that is there is one more thing i have to keep in mind and that is making sure the rear post doesn't get smashed by a spinning chuck when turning a piece and when I'm machining a large diameter i have to remove the rear post - easy enough as it is one bolt and the toolpost uses a tongue and groove set up for alignment with a permanently mounted base plate .

One day when i get time i will move the rear post into the front postion to see if there is any appreciable difference when parting off but it won't be staying there as a rear post can be set up for more than just parting off , it can be set up with a champfer  tool , grooving tool, facing and turning tools .

Lets just say you need to make 20  bolts  and your rear post is made indexable you could set up a turning tool upside down in the rear post along with a parting tool  then set up your front post with a threading tool which can also be used to do a light champfer .

So the sequence of operations go : 

Rear turning tool  to machine to thread dia then index to parting tool to cut the undercut under the shoulder .

Switch to front post to champfer and cut the thread .

Rear parting tool to part off  .

Restart sequence .

Ian.

Edited By XD 351 on 21/10/2016 21:21:40

Without drlling through and bolting a rear toolpost directly to a cross-slide without T-slots, are there preferred ways to mount them?

Ed.

Why should parting off from the rear and running the lathe in reverse be any better ? I do know that taking a facing cut from the rear with the lathe in reverse often produces a better finish on my lathe but I have no idea why.