Modifying tool rests?

[Geoff Whaling]

Have a look at this clip,

https://www.youtube.com/watch?t=14&v=U8tqr48Ir_I

Pretty nasty near miss from a tool rest failure. I bet the turner changed his underwear after that one.

Who do you go chasing when a modified tool rest fails?

[David C. Roseman]

I supposed if you felt the need to go chasing anyone, it would depend on how the rest had been modified, and just what the turner was doing when the accident happened.

The tool rest in the video was an original factory rest. No indication of how old, or how much it had previously been used or abused. Or even what part actually failed.

[Geoff Whaling]

With all the talk on modifying tool posts on another thread I thought this one was timely.

If you stop the clip and step through it frame by frame it appears that the tool post shaft has sheared at the boss at the top of the machining. I've heard of many fails like this and have seen the result of similar failures myself. It seems the machining of the shaft on these tool posts which are cast from material that appears quite grainy across the break, even to the naked eye, & that the machining creates a stress point at that shoulder.

I think this sort of fail is quite common as we regularly see posts asking about tool post repairs or about the fails.

It would be interesting to understand the machining and usage stresses and why they fail at that internal corner and not along the horizontal arm?

[Kyle Iwamoto]

There really is no reliable way to weld cast iron to a steel post. They are different materials and have different properties. If you turn at uber speed like he does and use a carbide scraper very aggressively, that is what can happen. Looks way cool shooting ribbons though, huh?

[hu lowery]

Have a look at this clip,

https://www.youtube.com/watch?t=14&v=U8tqr48Ir_I

Pretty nasty near miss from a tool rest failure. I bet the turner changed his underwear after that one.

Who do you go chasing when a modified tool rest fails?

When you modify something you own it, including taking responsibility for failures. A good bit is involved in machining a part for maximum strength. Obviously, radius the top of the shaft. A sharp corner there forms a stress riser and invites breakage. The final few passes should be calculated to make the shaft as smooth as possible with no torn grain or ridges showing. Some lubrication can help with this too. Reshaping the metal to transfer load as evenly as possible above the shaft may also spread the load and help avoid breakage when turning the shaft higher has significantly changed the design of the rest.

Regardless of what we do there are stress points at the top of the shaft and where it exits the banjo. Looks like the rest in the video failed where the shaft exited the banjo but I couldn't really see that well. Hard to say why it failed, or any metal part fails that has been in service awhile. "Abuse" including dropping the rest and accidentally hammering on it with tools when catches occur, many things that aren't deliberate and we don't think of as abuse may be a factor. Metal fatigue can be a factor. Probably usually a combination of factors.

Seems to me that the cast iron rests are subject to failure even straight from the factory. Might be the quality of some of the rests, might be metal fatigue from hours of use, maybe a combination of both. I don't care for cast iron rests of unknown properties although I own several that came with lathes. Cast iron can be very strong rivaling good quality forgings but I always suspect fairly low quality castings with "cheap" lathes. I suspect the majority of failures happen with rests that haven't been modified.

As wood turners I think we are more inclined than most to make or modify things to fit our uses or to use things for purposes or in ways that were never intended. When we get "creative" we assume the risks whether we are modifying something or repurposing it.

Hu