Balancing the design of a salvage

[Paul FallertParticipant @paulfallert28101]

In MEW #59, Ted Wale asked Scribe-a-Line about sizing an insert for a chuck backplate. The purpose of the insert was to afford a "second chance" at cutting the threads, which had gone askew on the first effort. Cross-section drawings were provided.

I have searched subsequent issues without success in finding a Scribe-a-Line response.

The question asked, I believe, about adequate metal to provide strength for safely supporting a lathe chuck. This question arises in other areas of model making, re-use and general repairs.

Can someone point to the answer?

[Paul FallertParticipant @paulfallert28101]

Iron Backplate Casting safety mod ?

[Neil WyattModerator @neilwyatt]

The cross section of the insert will be double that of the threads when the unthreaded portion is only half the thickness of the thread, so I would suggest that as long as it is thisk enough not to distort, it should be fine.

Neil

[Paul FallertParticipant @paulfallert28101]

Neil:

Thank you for considering this problem (I'm now assuming it was never answered in Scribe-a-Line). I am a retired accountant, not an engineer so I have a lot to learn.

I am also now in the process of machining a backplate, so this issue is front and center in my thinking (although so far I don't need to make an insert). The insert would be Plan B if the threading does not go as planned.

At Fig. 2, at the point where the inner right edge of the insert intersects with the backplate in the drawing, there is only 1/4" of metal (cast iron) in the (vertical) cross section. Is that considered a "shear or stress point"?

I was trying to compare it to a spigot. If this were a spigot, 2"OD and 1-1/2"ID made of cast iron, would there be a weakness at that same point that is overcome by the insert or by the fact that this is a tube? Tubal Cain and others suggest that cast iron is much better than steel for backplates, but cast iron is supposedly not as strong in tension as steel. The other concern raised by Ted Wales was to not press the insert it into the backplate due to the possibility of cracking the cast iron.

Paul

[Paul FallertParticipant @paulfallert28101]

To help readers, I have created an album which contains Ted Wale's Fig. 2

[Neil WyattModerator @neilwyatt]

My main worry would be the sharp corner acting as a stress raiser, if it could be radiused it would be stronger.

That is a week point though, avoid an insert like that if you can, or don't take it so deep.

Cast iron is better because it is unlikely to pick up on the threads and jam. Choose something like meehanite or a spheroidal graphite casting which is less brittle than common cast iron.

I think a second opinion would be useful here – anyone?

Neil

[vintagengineerParticipant @vintagengineer]

i would use a different thread tpi or pitch for the insert just to stop one thread following the other reducing the wall thickness.

[Clive HartlandParticipant @clivehartland94829]

Place/make a clamp to put around the collar, it will allow reversing of the rotation safely. It will also sstop any cracking.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Paul Fallert on 06/04/2017 21:36:29:

To help readers, I have created an album which contains Ted Wale's Fig. 2

.

Sorry, but I really don't like the look of that … It's a classic "stress-raiser"

It would be much safer if the insert [of suitable material] was half as long again.

MichaelG.

[Ian S CParticipant @iansc]

Michael, I agree that the insert should be longer, 1 1/4" to 1 1/2", , the internal corner should have quite a good radius, about 1/8" would do, and the OD of the insert should be relieved to clear the radius. A shaft lock adhesive and this could be reinforced with three grub screws in the positions indicated in the above diagrams.

Ian S C

[Paul FallertParticipant @paulfallert28101]

While looking for something else, I found that Peter Peters responded in MEW #70 (12-2000) with an extensive analysis and calculations. Anyone else who has an interest in making this kind of insert should find that helpful. It was not published in Scribe-a-Line, but in a separate article placed a page before Scribe-a-Line.

My shortened summary of Mr. Peters article:

1) Pins and screws are not advisable as they weaken the assembly (see Mr Peters reasoning)

2) Insert should be thinner to retain as much as possible of the original casting, and Mr. Peters offers design guidance in a simple formula

3) Insert should be longer (as per Michael and Ian) or if not possible, then shorter to avoid a stress riser.

MEW has a wealth of collected knowledge both published and on this Forum. Thank you for your contributions.

Paul

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