Are TBC bushings needed? (AKA, where did I go wrong?)

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Fibonacci

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Feb 9, 2011
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So I finally got my proper live center and decided to try turning one of my CSUSA classica kits last night with some acrylester I picked up at the same time.

Given that I had determined the bushings I bought to be slightly eccentric, I tried to turn the kit with my calipers. I noticed as I went along that I had to keep tightening the tailstock to keep it from spinning. When I finally got to nominal, I pulled the blank off to clean the ribbons that had collected on the ends.

At this point, I discovered that the tubes were very flared and had rubbed a reasonable amount of copper off on the live and dead centers. Unfortunately, I don't think the kit is recoverable, and the blank had cracked on one end from the flaring.

So, where did I go wrong? The obvious problem is that I had too much pressure on the tube, but I only tightened it when the blank started spinning freely between the centers.

Are the bushings needed for the main shaping to make the load be purely compressive rather than partially compressive and partially radial? Any other suggestions for fixing this?

I didn't have any other problems with the material, so that is nice at least. I had not worked with inlace acrylester before. It polishes really well. I decided to finish it for practice.

EDIT: I want to correct an impression I seem to have given. I have NOT had issues with CSUSA bushings. I have had issues with cheaper ones I bought at a discount vendor.
 
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KenV

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I presume you are putting the tubes directly onto the live center and dead center and seeking to turn the whole thing just like you always did.

I use the bushings to provide end support and lateral support while roughing the blanks to close to final shape. -- The sixty degree angles and the interaction of the tube onto that cone is not enough to provide a lot of friction and will not handle agressive turning -- but will support light finish cuts, shear scrapes, and light abrasive use.

Heavy hand = spinning and pressure = flaring

I like TBC -- but use bushings most of the way down. (want a better friction fit with standard bushings - run a center bit in to provide a 60 degree recess and more surface contact).
 

Fibonacci

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That was the conclusion that I had come to. I just wanted some confirmation before cutting into my bushings or ordering new ones.

I know JohnnyCNC has TBC bushings that are supposed to be very accurate and already have the tapers.

Is there anyone else that has noticably better bushings than others? I used to get all of my bushings from one source that had them a bit cheaper than everyone else, but upon further inspection, most of them are not concentric. The hole for the mandrel is not quite in the center of them.

I guess I need to start over with bushings for now.

Or I could just use those bushings for the roughing and not worry about the fact that they are wrong and continue to use my calipers.
 

GoodTurns

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you will not go wrong with Johnny's bushings. If I am going to make more than a couple of a pen type, I get his and they are worth every penny.
 

Dave Turner

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Sylvania, Ohio
Up until very recently, I've done all my between center turning without any bushings. I do just like you did, I support the brass tube ends directly on the cones. I just crank in enough pressure on the tailstock to keep the blank from slipping on the cones. This does not require much pressure, even when roughing the blank from square to round. If the blank starts to slip, I'll give the tailstock wheel just a little nudge to tighten things up. I've never had the ends of the tube flare on me.

That being said, I recently have started using standard bushings (without any drilling modification) on the ends of the blank. I just run the blank and bushings directly between the dead and live cones. I do this to save me some time when roughing down the blank to near the final size. I don't have to stop and measure with calipers to see when I'm coming close. After I get close, I take the bushings off and turn directly on the brass tube until I get the size I want on caliper measurement. This eliminates any out-of-round that might crop up from poor bushings.
 

KenV

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Not knock intended on John G's bushings -- I have some and they are good value. There are a lot of bushings and I have the special ones for the high volume and higher value operations from John.

That said, the less controlled way (conventional mandrel busihings) works as well on centers as they work on a mandrels -- and are an easy way to start.

And bushings -- even John's -- are a guide with best match coming from the calipers matcing the fittings to the turnings.
 
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