Purpose of spacer bushings on a pen mandrel

[Egk1313]

Could someone tell me, in different pen kits, what is the purpose of certain numbers of spacer bushings that are being required on a pen mandrel? Would it be an issue if I did not put the required amount on? Sometimes it just seems to make the mandrel way too long. Thanks in advance.

 

[jrista]

It is just a way of keeping the tool away from the ends of the mandrel, like the knurled nut. It is not really a requirement, and no number is technically correct.

 

[monophoto]

Bushings serve two purposes. First they are intended to be references for the required diameter of the turning. That is, in theory, if you turn down to the diameter of the bushing, then the turning should match the diameter of the metal components of the pen that mate with that turning. Obviously, you need to think several steps ahead to recognize that sanding further reduces the diameter of the turning, while applying finish increases the diameter. Ideally, there should be no perceptible difference between the diameter of the turning and the diameter of any adjacent metal components in the assembled pen. Also, over time, the process of sanding and applying finish will also modify the diameter of bushings. For that reason, it's a good idea to also do some careful measurements to assure that you are getting the desired diameters on your turnings. Likewise, careful pen makers who use bushings and mandrels also consider bushings to be expendable items that need to be replaced periodically.

As you say, the second purpose is to serve as a spacer. Basically spacers simply assure that there is physical space between the wood or plastic turning and the metal ends of the mandrel so that there is room for tooling without risk of the tool hitting the metal end of the mandrel.

 

[Woodchipper]

I made spacers from a HDPE scrap left at the recycling center. Square on one end, tapered on the other which is against the blank.

 

[egnald]

Greetings from Nebraska.

The most fundamental pen mandrels came with one fixed end having a Morse taper or a female thread sized to fit the headstock of the lathe. The opposite end had a small taper in the end to accept a 60-degree live center in the tailstock and it was threaded for about an inch or so for a knurled brass knob/nut. The length of the mandrel was not adjustable. Depending on the length of the tubes and turning bushings, spacers were needed to pad the length of components on the mandrel so that the brass knob/nut had something to press against to tighten everything on the mandrel.

A different design of mandrel essentially eliminated the need for the spacers by allowing the mandrel shaft to slide in and out of the Morse taper (with a collet type locking mechanism), thus making the length of the mandrel shaft adjustable so that the brass knob could tighten things up without the additional spacers.

A different design includes a "mandrel saver" that is essentially a tailstock live center with a through hole that can slide over the mandrel shaft. This has a similar effect to making the length of the mandrel shaft adjustable. It eliminates the need for threads and the brass knob all together as the "mandrel saver" presses against the parts to tighten them up on the mandrel.

I use both, the mandrel with the shaft that can be slid in and out of the Morse taper and a "mandrel saver". I use the Morse taper so that I can replace my mandrel shafts with drill rod. I use the "mandrel saver" to press against the blanks and turning bushings. I also use one spacer on each end. I do this so that I don't need to turn so close to the "mandrel saver" and Morse taper. I also think the little extra length helps prevent water infiltration into the "mandrel saver" and Morse taper mechanism when I wet sand.

Hope that answers you questions - Dave