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Rear parting toolpost

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appletree:

--- Quote from: vtsteam on January 16, 2022, 06:14:28 PM ---Here ya go:
(not John Stevenson)

--- End quote ---

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 don’t have much difficulty and my lathe does not have an extended T slotted cross slide.
Just being devil’s 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

vtsteam:
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.

appletree:
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

vtsteam:
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:

appletree:
Hi Vtsteam it’s good to talk, we can all learn something new, it’s 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 “don’t be afraid to ask the question, if you don’t understand, probably nobody else does, just too scared to ask “ so keep talking Phil

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