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Old Yesterday, 07:38 PM   #1 (permalink)
 
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Default The Price of Speed

Here's an annoying thing. I had read the recommendation to set the lathe on its highest speed when using a friction polish, and I did this for the first time. The piece cracked; I assume because it got too hot.

I've actually only used my lathe (Delta MIDI Lathe) on its lowest speed, and had wondered about the higher ones. It certainly behaved like a new beast, going that fast, and I was optimistic at first that it was going to open the door to better quality, but not so far....



Thoughts about lathe speed? And is cracking like this a known problem?

Also, any idea what kind of wood this is? It was a scrap that's been in my shop for years. My guess is some kind of oak, as it's heavy.
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Old Yesterday, 08:06 PM   #2 (permalink)
 
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Cracking when applying a friction polish at high speed is unusual. I used friction finish when I started turning pens but I don’t use it on pens any more. I do use it on some other small turnings that won’t be handled as much. I always apply it with the lathe on the highest speed. The whole idea of a friction finish is you need to create friction (heat) for it properly cure.
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Old Yesterday, 08:51 PM   #3 (permalink)
 
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Since it was a piece of scrap wood, I would not be too concerned. It does look to be an open grain, so Oak is a good possibility.

Chuck up another and have a go at it! Heck, I'd have 5-8 scrap spindle pieces on the ready and just get some practice in. Try different speeds, try different tools. At the end you will have used up some firewood and gained a lot of experience.

As to the cracking, there are many variables. Don't worry too much, as an older piece in your shop, there are many possibilities and I doubt you will ever identify the key.

A few years ago I got a bowl from a Salvation Army store. It needed to be re-"turned" (re-turned on the lathe, not taken back!!!) It was dry as a bone, decent shape, but with a few dings and scratches it looked like a good possibility. I rechucked it, got out a sharp gouge, and it blew up at the slightest touch. Not even close to being saved. Why? No idea, but it cost me .50 to try.
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Old Yesterday, 09:48 PM   #4 (permalink)
 
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Looks like oak to me also. If it was scrap, it may have been too dry for the heat the lathe created. I think I would have put some oil on it first.
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Old Yesterday, 10:52 PM   #5 (permalink)
 
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Yes, I've done that before on old wood. Slathered it with the polish, which is walnut oil and shellac, and let it sit for a few hours.



I wonder if I could save this tube by turning all the wood off it, and then try again....
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Old Yesterday, 11:36 PM   #6 (permalink)
 
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here is a video that may interest you and others that use friction polish. Maybe something in there to learn from. This guy did the testing and sometimes that is what it takes to get the answers we seek.

how to apply friction polish on a lathe - Bing video
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Old Today, 12:43 AM   #7 (permalink)
 
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The word "dry" for wood is typically used to describe wood that has reached equilibrium moisture content ( it does not mean 0% water). Change ambient conditions and the equilibrium moisture content will change. Wood swells and shrinks as moisture content changes, mostly across grain but some with the grain as well. Glue a brass tube in the middle and cracks are almost inevitable, even with an abundance of caution. Different woods behave differently, crosscuts and dense hard woods being the most finicky. Excess heat is one of the most commonly cited risk factors. Stabilizing is a proven countermeasure, but can add substantial cost. In this case I am thinking that the wood was the primary issue. CA or Wipe On Poly finishes require less heat, and therefore should be less prone to initial cracking, either might be a good option for you. That said, friction polish has been used by many penturners with lots of success, and I'm sure most apply at high speed with pretty high temps...

Forgot to say - definitely save the brass by turning the cracked wood off and try again
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Old Today, 08:25 AM   #8 (permalink)
 
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I agree with the rest, issue with the blank. Friction polish can only get so hot before you're forced to stop from burning fingers. Oh, I have the same lathe as you, and run it all full speed all the time (granted, I do drill and square on another lathe) for pen-sized spindles.
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Old Today, 08:50 AM   #9 (permalink)
 
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Quote:
Originally Posted by mark james View Post

A few years ago I got a bowl from a Salvation Army store. It needed to be re-"turned" (re-turned on the lathe, not taken back!!!) It was dry as a bone, decent shape, but with a few dings and scratches it looked like a good possibility. I rechucked it, got out a sharp gouge, and it blew up at the slightest touch. Not even close to being saved. Why? No idea, but it cost me .50 to try.
I would wager it was oscillations that caused the catch. Already finished to reasonably thin dimensions the bowl has some flex and will vibrate like a bell as soon as the gouge starts cutting. You would have been better off using an 80 grit gouge on it.
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Old Today, 09:29 AM   #10 (permalink)
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After clicking on the photo and seeing it up close, it looks like you may have added too much pressure in tightening - see the right end. It seems to have the faintest of curve/pressure on the end. I can tell that you are sanding the bushings as much as the pen blank. You are coming off the bushing and sanding the blank down more than the bushing. It is thin there. Combined with it being Oak, too much pressure and extra heat probably did it. The fact that it did not fly apart says that it probably is a stress crack from the end pressure rather than from the speed alone. Heat probably played a part but it combined with the end pressure

Ambiguous and relative terms cause problems:
This is actually a personal (ambiguous/relative) thing that everyone has to deal with, and many do it effectively without noticing. Pressure, heat, tight/loose/snug, light/heavy are all ambiguous terms that depend on the individual to interpret. Equivalent light touches with carbide scraper from a 250 lb person would be considered heavy/aggressive by a 120 lb person. Mildly warm sanded blank by me would be considered fairly hot to touch to my wife. "Mild" sanding pressure to me is "heavy" sanding pressure to my wife. "Tightening" is one area that I have seen to have a wide area of personal interpretation for the same term. Barely snug to a 250 lb would be very (or overly) tight to a 120 lb. ON your blank, on the right end at the top of the blank, there seems to be a minuscule amount of crushing of the wood at the point of where the crack starts. That indicates too much pressure for the thinness of the wood at that point.

I do remember 2 or 3 lifetime bowl turners coming here (IAP) 12 - 14 years ago and admonishing pen turners for turning at 2000 or 3000 RPM. They were used to turning 8 to 14 inch bowls at 300 - 800 RPM at the most. 2000-3000 seemed excessive to them as they had been turning for eons at the lower speeds - not taking the time to think that 3000 rpm on a 3/4 in blank was slower cutting than 800 on a 12 inch bowl edge.

Getting to the root cause is something you will have to ultimately figure out for yourself. These guys have given several areas to look for finding the cause. I consider things like this to be part of the journey in making great pens. BTW, that oak blank looks great! It has character!
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