Pen12a

The most often heard complaint from pencrafters, new and experienced, is the problem of eccentricity between the outside of the turned pen and the inside of the brass tube. Too often, the result is that nothing fits very well. This section will discuss some of the problems and some of the things that we can do to work around them.

How significant are a couple thousandths of an inch? Lay a piece of typing or copy paper down on a smooth tabletop. Run your finger lightly across the table and up over the edge of the sheet of paper. Rub it lightly back and forth across the edge of the paper. It feels like a big ridge doesn't it? This paper is only 0.003" thickness, 0.004" at the most. Small variations do make a difference. Add a couple more thousandths and it becomes a visible difference. Remember that 1/64" is only 0.016", and that is a very visible dimension.

Penmaking is all about tolerances and the fits between the various parts of the things we use to make them. A few of the parts whose tolerances could add up to make a significant difference are:

-- Runout in the lathe spindle

-- Bore of the Morse Taper in the spindle

-- Spindle and the tailstock alignment with the lathe bed

-- Morse Taper in the Spindle

-- Morse Taper on what we are putting into the spindle

-- How the mandrel rod is attached

-- Roundness of the rod

-- Diameter of the rod

-- Screw thread, if one is used

-- Straightness of the rod

-- Fit of the bushings on the rod

Every one of these required a "fit" and a manufacturing "tolerance". It is a miracle of our modern manufacturing methods that things are not worse than they are.

The Lathe

I am not going to discuss lathes as individuals or brand names. Considering their reasonable price, most of them give us excellent quality. There is always the occasional lemon, where all of the machining tolerances have added in the same direction. It will be almost impossible to turn a pen or anything else on such a machine.

The Mandrel

Most pens are turned on some type of a mandrel that is held between the headstock and the tail-center. There are several ways to do this. My experiences with the mandrel rods has generally been good, but there are some differences in how they are adapted to the lathe spindle.

I have never understood where the 7mm designation came from. 7mm is equal to 0.276". All of the pen tubes that I have are 0.2665" outside diameter (OD), with a 0.249" inside diameter (ID).

I often hear that the "sloppy" fit of the brass tubes and the bushings on the mandrel rods is a source of the eccentricity problems we have with our pen barrels. All of the tubes and rods I have measured tell me something different. The smallest rod that I have measured was 0.245" OD, and the largest is 0.247" OD. There is a little more variation in the bushings and they run from 0.249" to 0.251" ID.

Comparing all of these numbers tells me that the worst case would be that the pen blank was eccentric by no more than 0.002", and the bushing by 0.003". This isn't enough to be of any concern , and we need to be looking elsewhere for the source of any problems that we might have. Someone else may have a different set of dimensions from other sources.

These are the typical ways that we can hold the mandrel rods for turning our pens. The bottom is the rod and MT2 adapter, The rod screws into the adapter. This one is from Hut Products.

Above that is a MT1 adapter for the same rod.

The rod is attached to the middle adapter with a set screw.

And, the top is the Jacobs Chuck

The Threaded Adapter

This adapter has several points where tolerances and fit are important:

-- The taper must match that of the lathe spindle.

-- The threaded hole in its center should be in the center of the adapter, and the threads must be a good fit with those on the end of the rod.

-- The threads on the end of the rod must be concentric with the OD of the rod and straight.

Fortunately, all of these things are fairly easy to machine accurately. It follows that these are among the most accurate adapters available.

The Socket and Set-Screw Adapter

While it is easy to drill a hole that is concentric with the center of the adapter, these holes have to be drilled with a clearance for the rod. The reality is that the rod diamater is 0.245" and the 1/4" drill bit that was used will cut a hole that is 0.252" ID. The rod is held in place with a single set-screw and all of the difference is pushed to one side, making the mandrel rod have an 0.003" runout at the spindle. This off-set also translates to a poor alignment of the rod because of the shallow depth of the hole. Next to the Jacobs Chuck, this has always been the least accurate adapter I have used. It is no longer available, and it has been replaced with one that uses a collet to hold the mandrel rod.

The Jacobs Chuck

A chuck can be the most accurate or the least accurate way to hold a mandrel rod. There are a lot of parts inside of these chucks, and sometimes the tolerances will be in the wrong direction. This is not the place to save money if we are going to use the Jacobs Chuck for turning pens. The one shown in the photo is the "Golden Goose" brand that is available from Craft Supplies, and it is typical of the chucks that are being sold in the less than $40 price range. Its only problem is an angular error, with the centerline of the chuck being at a slight angle to that of the headstock. This will give a a considerable wobble to the far end of a straight rod. This type error will be typical of a Jacobs Chuck in this price range. It is plenty accurate for general use, but not for pen turning.