Tubes and Mandrels

[Angusga]

I like to use the mandrel saver in conjunction with the threaded shafts as I can tighten the blanks onto the mandrel with the lock nut and then use lighter pressure on my tailstock without having the blanks catching as easily. I'm ordering a mandrel today that adjusts length through a collet at the headstock. I'm going to make a mandrel saver that will thread onto the shaft with a 60 deg. end to receive a tailstock live center to help with concentricity while also bolstering the mandrel at the tailstock end. I may also make a mandrel saver that can attach to my One Way live center with a better fit over the threads on a mandrel. The fit of the 7mm pen tubes on these mandrels is less than ideal causing vibration and chatter especially on two piece blanks. I wish the mandrels were slightly bigger in their diameter. This is only problematic with the smallest 7mm pen tubes when turning a two piece blank. With a one piece blank I use shop made between center bushings.
I've made end and center bushing sets to use between centers with two piece blanks but get quite a bit of vibration with them and have to use excessive tailstock pressure. It does work but requires more sanding to remove the chatter and bring the blanks to concentricity than I'd like. It would be nice if the tool manufacturers addressed these issues more precisely.
What have some of you folks done to increase accuracy and concentricity?

 

[KenB259]

I use the between center mandrel system and finish the last .020 -.030 between centers with no mandrel.

 

[wood-of-1kind]

I turn between centres and stopped using mandrels altogether. I do use bushings between centres that have a 60 degrees
opening for both dead/live centres and this method has improved the concentric issues that I used to have. When I used
to use mandrels to make 7mm pens(tubes), I would turn only one(1) blank at a time and never two(2) on the mandrel to minimize the flex and increase the accuracy.

 

[egnald]

I have a collet based mandrel "driver" on the headstock side and a collet based mandrel "saver" on the tailstock side. The collets clamp down when I tighten the tailstock providing a "perfectly" centered mandrel on both ends (well as "perfectly" as the tolerances in everything permit. Unfortunately the company that made mine stopped a few years ago along with a bunch of other companies that made similar, collet based systems. As of right now, it's hard to find anything even remotely similar.

I think if I didn't already have a "precision, collet based system" I would probably buy a PSI Morse Taper Maxi-Mandrel or maybe a Morse Taper Keyless Pen Mandrel System for the headstock side since they use a collet for holding (and centering) the mandrel. For the tailstock I would go with a PSI Live Tailstock Chuck Adapter and a PSI Collet Chucking System, which comes with a 1/4-inch ER32 Collet. This would put a collet based live center on the tailstock side. For the mandrel itself, I use a Letter "C" or Letter "D" drill rod, typically M2 tool steel because it is less expensive and is very hard. I get them at McMaster-Carr. The diameters are very tight tolerance, 0.242 +0.0 /- 0.0002 for "C" and 0.246 +0.0 / -0.0002 for "D".

I almost always use the "D" as it provides the best fit for most of my bushings, but sometimes I will get a set of bushings that are a little too tight for "D", then I drop back to "C". Almost always this has been with standard, PSI "Slimline" tubes and bushings. (Originally the mandrel sizes were established by Berea and were set at 0.247 +/- 0.002 more like a "D" drill rod, but PSI specifies theirs at 0.243 +/- 0.001 - more like a "C" drill rod).

It would involve a lot, but it would provide tight tolerance, easily adjustable mandrel lengths (no spacers required), and collet based clamping and centering.

Dave