Author Topic: Rear parting toolpost  (Read 1593 times)

Offline appletree

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Rear parting toolpost
« on: January 16, 2022, 10:57:15 AM »
Rear parting toolpost, Iím sure this must have been done to death. I personally have no problem parting from the front on my as new Raglan 5 inch, I also donít have a T slotted cross slide, ( I ordered one from John ward 3 years ago but it never came), sadly I understand John has passed. Back to the question, I have read that under dig in conditions the tool is pushed away on rear parting and dragged in on front parting. I canít see this as the mechanics in my opinion are the same in both cases. I can see that rear parting is more rigid unless you can remove the top slide and mount the parting tool holder directly on the cross slide, additionally the chips will tend to fall away from the cut at the rear but for me that is about it. Looking for a definitive explanation Phil

Offline chipenter

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Re: Rear parting toolpost
« Reply #1 on: January 16, 2022, 01:37:52 PM »
A rear parting tool is upside down so the force is opposite to a front parting blade .
Jeff

Offline appletree

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Re: Rear parting toolpost
« Reply #2 on: January 16, 2022, 02:16:52 PM »
Clearly the force is in the opposite direction, but surely the force is the same. Looking for the actual science behind the 2 options Phil

Offline BillTodd

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Re: Rear parting toolpost
« Reply #3 on: January 16, 2022, 02:40:52 PM »
It's about the direction of the flex. A normal mounted tool is forced down the effect of the leverage from the bottom of the toolpost upward is to press the tool further into the work-piece exacerbating the bending force.

A rear mounted toolpost reversed the cutting force benring the mechanism away from the job,  so reducing the cutting force and thus regulating the cut .

This is the same mechanism that is designed into spring tools,  i.e. the cutting force is used to bend the tool away from the work.

Bill
Bill

Offline appletree

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Re: Rear parting toolpost
« Reply #4 on: January 16, 2022, 03:46:51 PM »
Sorry to be unable to see this, as far as  I can see the work pushing down on the tool which is on centre height will move the tool tip down and away from the work (reduce the depth of cut) this I understand is how the sprung tool works, the rear tool post is the same in my opinion except the word down is replaced with up. Really want to understand this. Thanks for the replies Phil

Offline awemawson

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Re: Rear parting toolpost
« Reply #5 on: January 16, 2022, 03:48:34 PM »
It's all about negative feedback (rear tool) and positive feedback (front tool). The rear tool lifts above the centre line disengaging the cut.

The other way of achieving negative mechanical feedback is to use a sprung tool holder, where the parting tool deflects below the centre line if there is a tendency to dig in

Andrew Mawson
East Sussex

Offline Muzzerboy

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Re: Rear parting toolpost
« Reply #6 on: January 16, 2022, 03:48:39 PM »
It's far better to part off with the machine in reverse if you need to use a rear parting tool. Obviously that requires a spindle nose that isn't simply threaded, for fear of the chuck coming loose. I'd suggest that if you DO need to part off with an upside down tool, you should set up the saddle gibs very carefully. How many people can honestly say they have adjusted the rear gibs recently - or ever? The tool forces act to lift the saddle, so any backlash will result in the tool lifting ie diving under centre and even a well adjusted gib leaves some backlash.

I know people talk about the tool springing or moving as if it's a benefit but the best situation is one where the tool has a well controlled, consistent position and not prone to moving when the tool force exceeds the weight of the saddle. With the spindle turning in reverse, there is no need to worry about backlash, as the saddle is pressed against the flats of the bed.

Offline vtsteam

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Re: Rear parting toolpost
« Reply #7 on: January 16, 2022, 04:13:14 PM »
Hi appletree. I think in your mental picture of the front tool, the tool bit is the only thing that is bending. In that case, yes, a tool set exactly on center height will bend away from the point of contact. And if you have an absolutely rigid toolholder, toolpost, carriage and lathe ways with no slop, well then no problem with a front toolpost.

But if you have a smaller hobbyist lathe, then the above conditions are unlikely to be  perfect. So you will get dig ins, even when the tool is exactly on center.

Why?  you are imagining a case where the center of the radius of flex is horizontal and parallel with the tool bit. But with the rest of the system flexible, that radius will effectively be much lower. In that case the tool will tend to dig in.

When it digs in the tool may break, or in a less severe case, you get chatter. The max amplitude of chatter, a harmonic vibration, is much greater then the normal tool cutting force, so once it starts It is hard to stop, and it has the power to flex many of the lathe components, including the carriage attachment to the ways.

A rear toolpost also has flexible connections to the ways, BUT, with the center of that bend radius low and the direction of rotation now upwards instead of downwards, the tendency is to push the tool away from the work.

And this set of behaviors is all borne out by the fact that big solid lathes tend to part material easier than small hobbyist lathes. They are simply more rigid. And they act as you have been imagining. That's why a rear tool bit is more often recommended to cure parting problems with small lathes. But there is one other advantage for either -- a rear tool holder can carry one tool while the front toolholder can carry another. Thus two different operations can carried out without changing tools.
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline BillTodd

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Re: Rear parting toolpost
« Reply #8 on: January 16, 2022, 04:20:01 PM »
Sorry to be unable to see this, as far as  I can see the work pushing down on the tool which is on centre height will move the tool tip down and away from the work (reduce the depth of cut) this I understand is how the sprung tool works, the rear tool post is the same in my opinion except the word down is replaced with up. Really want to understand this. Thanks for the replies Phil

No the down force at the tip is resisted from way behind the centre line forcsng the tip into the work.

Spring tool are purposely more flexible than the lathe toolpost so it flexes above the centre line which does reduce tool contact.
Bill

Offline vtsteam

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Re: Rear parting toolpost
« Reply #9 on: January 16, 2022, 04:44:46 PM »
Below the centerline.
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline appletree

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Re: Rear parting toolpost
« Reply #10 on: January 16, 2022, 05:00:30 PM »
Thanks for the replies, I am afraid that i think if you were to draw the front and rear mounted parting setups out cold on paper they are the same. I fully appreciate, particularly on a lightly constructed lathe the rear position has practical advantages. I always spend time to tool grinding, setting up and use power cross travel, and have no problems, the one time I tried a carbide inserted tool it ripped the tip out and wrecked the holder.
Tool rigidity and lack of ďplayĒ in any type of lathe work is worth striving to achieve. Given that my Raglan 5 inch was not designed to accept a qctp (along with many other lathes) I am now tending to use the standard single toolpost supplied with the lathe, this avoids overhang past the end of the topslide, concentrates the forces where they were designed to be, reducing leverage and bending forces. I figure I am not in mass production nor trying to make money, I can afford a bit of time for tool setting.

Offline awemawson

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Re: Rear parting toolpost
« Reply #11 on: January 16, 2022, 05:29:45 PM »
Appletree, if my memory serves, and it may not !, the late and much lamented John Stevenson from you home town of Nottingham (well  Long Eaton) wrote quite an extensive treatise on this subject on this very forum some years ago.Worth searching for iirc
Andrew Mawson
East Sussex

Offline appletree

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Re: Rear parting toolpost
« Reply #12 on: January 16, 2022, 06:06:53 PM »
Started looking for Johnís post, drawing a blank so far, will resume tomorrow, thanks for the lead phil

Offline vtsteam

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Re: Rear parting toolpost
« Reply #13 on: January 16, 2022, 06:14:28 PM »
Here ya go:
(not John Stevenson)
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline chipenter

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Re: Rear parting toolpost
« Reply #14 on: January 17, 2022, 01:50:41 AM »
On the clubs Colchester Master parting off was impossible due to the were on the compound , the front dovetail was worn under the tool post , with a dig in you could see the tool post move forward and snap the the blade , dismantling and blueing showed that it had worn on the front end by 5 or 6 thaw twisting the compound under pressure , two days of scaping has improved the rigidity enabeling parting off .
Jeff

Offline appletree

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Re: Rear parting toolpost
« Reply #15 on: January 17, 2022, 04:25:22 AM »
Here ya go:
(not John Stevenson)

Hi Vstream thanks for the image, I guess that answers the question, I was just interested, I think even so a lot of the issues for most people will be the lack of rigidity working from the front. I shall continue using my front mounted tool post as I donít have much difficulty and my lathe does not have an extended T slotted cross slide.
Just being devilís advocate, when parting from the front and looking at the drawing, increasing the tool extension towards infinity would reduce the bending radius towards 0 deg, just saw crazy thought. Phil

Offline vtsteam

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Re: Rear parting toolpost
« Reply #16 on: January 17, 2022, 02:57:18 PM »
Phil, I only provided the answer and dwg because you asked for an explanation of the "why" of a rear toolpost, not as a recommendation of what to do with your lathe.

Although, parting isn't always easy for even a rigid lathe system -- particularly for thin or springy stock, and very particularly for turning crankshaft journals for a model. If you've ever tried to do this from the solid, you know what I'm talking about. And here's another reason:

A lathe tool which is flexible, even when mounted on a heavy rigid base, still has an effective center of bend radius below the center of the turned piece. See the first dwg below. Thus it may chatter or dig in. This is because the top surface of the tool bit is the point of contact on the workpiece, while the support resisting the cutting force is at the tool's lower surface.

Interestingly, the deeper a tool is the worse this situation, and parting tools are often very deep to resist bending. As a result they don't bend, which is good up to a point, on a heavy stiff machine. But on a more flexible small lathe they do dig in and either break or spring the carriage and connections -- possibly causing damage. They are the worst possible shape for that kind of lathe.

A crookneck bit (seldom seen any more) raises the the bend radius above the workpiece center. It hinges above. Therefore the tool bends away from the cut rather than digging in. It was used for older and less stiff lathes, and often had to be made up by blacksmithing.

It just occurred to me that one way to get around the problem of a conventional lathe bit being pointed on the upper surface, yet resting on its lower surface, would be to braze a short length of tool to another, as shown in the second drawing below. This would put the cutting support in line with the cutting edge, and the tool would tend to bend away from the cut rather than dig in. It also would likely be more rigid than a crookneck cutter, being shorter and more in line, and a lot easier to come by. This could definitely be shop made with a torch and solder, and doesn't require a blacksmith.
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline appletree

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Re: Rear parting toolpost
« Reply #17 on: January 17, 2022, 03:18:12 PM »
Hi vtsteam l only gave the answer I did as a roundup, I never was asking for advice just knowledge. Thanks to for your further comments in your last post. I have had  little John and raglan lathes 45 years. My current lathe is effectively 1 owner from new 1960 stood for 30 years, it is as new with no play anywhere, big and strong enough to ge useful I also have a Wilson 8 inch. I use both carbide and HSS tooling, there i a place for both, I have lots of good old style tooling, sprung looks, Jones and shipman tool holders etc Phil

Offline vtsteam

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Re: Rear parting toolpost
« Reply #18 on: January 17, 2022, 03:41:50 PM »
Great, Phil, you've mentioned that you don't have problems parting with the Raglan a few times so far. I never doubted it or the quality of your other tooling and prior lathes. Others reading this tread with lesser lathes or a curiosity about rear toolposts may still be interested in the theory and possible solutions, which was the reason I've continued talking about it.  :beer:
I love it when a Plan B comes together!
Steve
https://www.youtube.com/watch?v=4sDubB0-REg

Offline appletree

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Re: Rear parting toolpost
« Reply #19 on: January 17, 2022, 04:11:24 PM »
Hi Vtsteam itís good to talk, we can all learn something new, itís good for people new to the hobby too, many who have not got an engineering background. I used to say to my kids when they were at school ďdonít be afraid to ask the question, if you donít understand, probably nobody else does, just too scared to ask ď so keep talking Phil

Offline awemawson

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Re: Rear parting toolpost
« Reply #20 on: January 17, 2022, 04:15:57 PM »
I fitted a rear tool post on my Colchester Master 2500 for parting off many years ago. The lathe is rigid and pretty solid, but I confess that since I acquired an inserted tip carbide Sandvik parting tool set up I never use the rear tool post  :palm:
Andrew Mawson
East Sussex

Offline PekkaNF

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Re: Rear parting toolpost
« Reply #21 on: January 20, 2022, 07:57:25 AM »
For a light weight chinese lathe parting off was never that great. I have heard about the tool flex.

1. I replaced the top slide with a foot for oversized QCTP

2. I made a parting off tool with a jack screw support. bit like crank shaft turning tool.

These helped a lot, but I noticed that the head stock mounting to bed casting is not too rigid. Maybe I should access that next....and tailstock could have a longer stroke as well...

Offline appletree

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Re: Rear parting toolpost
« Reply #22 on: January 20, 2022, 04:17:42 PM »
Good thinking out of the box

Offline PekkaNF

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Re: Rear parting toolpost
« Reply #23 on: January 21, 2022, 03:16:47 AM »
Yeah, started thinking how much support I need to resist bending and turning. Actually pretty much on cantilever beam. Why not to put support under the tip, a little doodling on the paper, then 1:1 scale drawing on paper and see how to clear the rotating part in most of the use cases. Last step was the height control. I realized that I don't need stop on QC tool holder in the usual manner, because there must be adjustment on the jack screw: Threaded the holder body and put there nut and standard hex socket screw.

Then I needed to figure how to hold the insert blade securely in the holder. The blade had rounded edges, one dove tail curret had just the perfect rounding (and bevel) to mach. Hold down is simple, Just a cross bar and set screw top of it to press it down. I considered a key or pin to draw it inside, but all fits were hard push fits, I haven't detected there flex.

After the initial idea it was one evening of drawing and another in making. I didn't finish it at all, because I was convinced that it was a prototype, but to my surprise it worked! I also reasoned that the blade could be deeper to allow cutting of larger diameter, but the lathe does not seem to rigid enough for any larger on steel and that was the blade I had in hand.

Offline jackary

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Re: Rear parting toolpost
« Reply #24 on: March 02, 2022, 06:14:20 AM »
I made this vertical parting tool to try to avoid the lack of rigidity and flex of either a front or rear parting tool due to the cantilevered projection of the cutting tool. It works well. A front mounted version could be made similar to your supported tool.



Alan