Kit Pen Alignment Question

August 4, 2015

I've made a couple of dozen kit pens now, and with the last two I've noticed a problem I can't figure out. They're Designer Twist kits from Penn State Industries, and the upper barrel isn't square to the center band. In other words when it's all assembled the upper barrel is tight to the center band on one side, but there's a small gap on the other side.

My first thought was that I hadn't gotten the blanks properly squared off before turning. However the other three ends (both ends of the lower barrel and the clip end of the upper barrel) all seem to mate up perfectly with the hardware on both pens. Since both ends of all four blanks were squared the same way it seems odd that the problem would only show up on the center band end of the upper barrel, and on two pens to boot.

My second thought is that the twist mechanism isn't perfectly aligned in the lower barrel, causing there to be an ever-so-slight angle between the two barrels. But given how far the mechansim inserts into the lower barrel and the relatively tight tolerances between the mechanism and the lower barrel I'm having a hard time imagining how it could be "cocked".

Has anyone run into a similar problem? Any ideas how I might avoid it with the next set of pens?

August 5, 2015

I made quite a few of the Woodcraft version of that pen, and am I correct in assuming that you glue the center band on a tenon thats been turned on the upper barrel? If this is the case, then the problem likely is related to the manner in which the tenon has been cut. Since the center band's fit on the upper barrel solely depends on the tenon, I doubt the problem is related to barrel trimming or the fit of the twist mechanism. Just make sure you are getting a nice square shoulder on the tenon. I like to turn down the bulk of the tenon with a parting tool, then use the point of a skew chisel to just very lightly clean up the shoulder to make sure its perfectly square to the tenon. One trick I have learned is just very slightly undercut the shoulder of the tenon so you ensure that the center band will fit tightly against the shoulder of the tenon.

Make sure you are watching the tailstock pressure on the mandrel as well. If you are bowing the mandrel at all, it can cause all sorts of funky problems, and you may not get a square shoulder for the center band to butt up against. Hope this helps!

-John

August 5, 2015

John, I should have specified that I use the "no tenon" version of the kit, the center band just slips over the twist mechanism. You might be on to something with the mandrel though, I've noticed on a couple of my pens that the outer surface of the barrel isn't completely concentric with the tube. The only way I can imagine that happening is if I'm bowing the mandrel a little.

August 5, 2015

Ohhhhhhh, yea, I forgot they came out with a no-tenon version. I've never used it before, so I'm afraid I'm not familiar with the mechanics of that particular kit. I would imagine the problem is related to the mandrel possibly bowed then, especially if you are getting oval shaped, or non-concentric barrels.

-John

August 5, 2015

The out of round issue is normally the cause for this and it can come from a couple of problems.

If your headstock and tail stock don't quite align, this should not cause a problem with mandrel, because it will still run true reference a center point, but it will not be parrallel to the bed of the lathe. This isn't a problem unless the tail stock is loose, and is off center a little different every time you lock it down.

When it comes to bushings and mandrels, there can be two causes.

One. As mentioned above, if you tighten the tail stock to much you can cause the mandrel to bow. This causes the center of the mandrel to arc out slightly giving an off center turn. This can also permanently bend the mandrel.

Two. The bushings are not a great fit. I have found this with most of the stock bushings. The portion that fits into the tube can be just a hair small so that there is a minute amount of play. While this seems negligable, it isn't and it will cause one side of the pen to be thiner than the other. Also, if the center hole is not a good fit to the mandrel, leaving a little play, then you will get the same result.

Also keep in mind that between the metal bushings and the metal mandrel, you will get wear causing a loose fit and resulting in the out of issue. The only fix is to replace them. Some people considet this to be a cost of doing business, and replace the bushings every one or two pens.

These problems, are why I don't use bushings or mandrels for my finished turning anymore.

One solution is to buy custom made between center bushings (BCB). These bushings are generally of a higher quality, and are far more acurate than the stock bushings when it comes to fitting in the tubes, eliminating that problem. They do not use a mandrel, so that problem is eliminated. The ends of these bushings are machined with a perfect 60degree cone in th center. You will need a 60 degree dead center for your headstock and a 60 degree live center for your tail stock. The high tollerances will fix the above mentioned problems. The downside, is that they are expensive, and unless you are careful not to shave the bushings when turning your blanks, they will also wear out and eventially need to be replaced.

Because of the cost, I don't use the BCB. My solution is the two centers mentioned above and a vernier calliper.

I use a mandrel and bushings just to rough turn my blanks down to within 2mm of the bushings. I do this, because the mandrell and bushings will hold the blank tighter, to withstand the torque of rough turning off the corners.

I then take off the mandrel and put the dead center in the headstock. I then remove the bushings, and put just the blank between centers. Bring the tail stock up gently, and tighten just enough to hold the blank. There is enoug friction for the dead center to act as a drive center. I measure the component pieces that but up to the blank, using the vernier. Then I turn the blank down to size using gentle cuts and measuring the blank with the vernier until I have a perfect fit. Gentle cuts are a must because if you are to agressive, the blank will spin on the dead center. If this happens, you just need to snug up the tailstock a bit.

As a word of caution, if you are to agressive tightening the tailstock, you can crack your blank. However, I have been using this method for a couple of years now and I have only cracked a couple, back when I first started.

Hope thus helps.

August 5, 2015

Fantastic information Darrin, thank you! I've seen several makers talk about using calipers to measure the diameter rather than bushings and it makes a lot of sense, especially if I can eliminate the oval/off-center problem at the same time. Off to look for 60 degree centers...