Proposed New Bushing/Mandrel Design

[JimGo]

Punkinn posted a complaint that we hear often about the fact that there are at least two different mandrel sizes. Some people in her thread, as well as in previous threads, have suggested that Bereas's B mandrel is a better choice where you can use it, because it is more stable than the standard mandrel. That got me thinking about alternative bushing/mandrel sizes, and the problems many of us have with misaligned bushings, bent mandrels, etc. I came up with the idea below for a new approach to bushing and mandrel design, and I'd love to hear what you think of it. Basically, I want to get rid of the mandrel completely, and instead use interlocking bushings. The threaded bushings should help keep the bushings appropriately aligned (i.e. the edges perpendicular to the rotational axis), and the thicker diameter should help reduce "whip" and other issues. These will obviously cost more than standard bushings because of the additional machining, but even at 3x the standard cost of bushings (i.e. about $15 rather than $5), it would still be worth it in the time it would save me from having to fix my "out of round" problem (don't flame me Lou, I know they aren't REALLY out of round).

What do y'all think of this design? Is it likely to be more or less accurate than the current bushing and mandrel system?

Any feedback is appreciated.

Oh, and a note to Berea, PSI, CSUSA, et al...please steal this idea! I don't think there's enough money in bushings to worry about trying to protect my IP in this case. Although, a note of recognition would be nice, as long as they work better than the current system!

(by the way, the last sentence of the last paragraph was a joke, in case it didn't come through that way!)

 

[C. Scott]

JimGo,

That looks like a neat idea. However, for it to work the ends of the blanks must be perfectly flush with and perpendicular to the tubes. Also, would the threads will get progressively tighter as you turn the piece?

I have always thought that a super hard mandrel would be the answer. The best thing I have done so far to make make pens more accurate has been to turn one barrel at a time.

That being said, I hope some real experts will give their assessements. I'm like you, I'll buy it if it substantially helps me make a better pen.

 

[Paul in OKC]

I think the disadvantage is going to be so many threaded connections in one setting. Threads are not always the easiest thing to keep aligned, especially if you hae more than one 'joint' to line up. I have toyed with the idea of making sort of 'plug' bushings, one for each end of the tube. One end would fit either a drill chuck or the morse taper. The other for the live center. This would only let you turn one half at a time, though, unless the middle part was double ended. Only problem is the number of sizes needed.

 

[JimGo]

Good point Paul. WHat if they weren't threaded (except for the left-hand most one in the drawing, so it attaches to the MT)? For example, they could have a slip-fit into each other. Regardless of whether threaded or slip-fit, if the connecting pieces (e.g. threads) were long enough, you could even make spacers for them to allow people to extend the body/cap as we do now.

Chris, we already need the ends of the blanks to be perfectly flush with and perpendicular to the tubes. By extending the piece that sticks into the tube, my goal was to reduce the amount of "wobble" that could exist. But this is a good point. My plan was that the threads would end at the standard length of each tube. So, you wouldn't be able to go smaller than the standard tube, but if implemented as discussed in the previous paragraph, you could extend the length of the cap or body.

 

[Ron in Drums PA]

Jim
This is a good idea, but I don't think it would be cost effective with the increased amount of machining required to keep every component perfectly aligned.

 

[Ron Mc]

Jim,
The nice thing about your design is that the different manufacturers could sell spacers that slide over the bushings that would be the right OD for different kits. The unit would have to be adjustable to different lengths but if done right it could be a base unit of bushings for let's say a Baron. Then when you want to make a Jr. Statesman 1st series you could just purchase 1 spacer for the lower end.
Interesting.[]

 

[Mikey]

I would just be happy with a larger, thicker mandrel that doesn't flex. Of course, you would need bigger bushing IDs, but dang, that would be worth the $ extra.

 

[JimGo]

Hey Mr. Titanium (a.k.a. Bruce Boone), would a Ti mandrel help reduce the flex?

 

[ed4copies]

There is a historical analogy. The mini european (with stepped tube) had no mandrel, may still be available. I found it to be a PIA, stopped making the kit cause there was always at least one part out of round.

See if others had better luck!!

 

[Paul in OKC]

Maybe a combination of our ideas, if this makes sense. Only thread the end that screws into the morse taper, make the middle section double ended. Make each part that slides into the tube ab out 1"-1 1/4" long and two sets at the mandrel diameters, one at the .246 and the other at the B size. This would let you use what ever bushings needed for which ever mandrel size, without the mandrel, and with good support.
Make any sense?
I will try to make a sketch later and post.

 

[JimGo]

I thought about making the middle section double-ended. That would certainly result in fewer threaded components. However, some like to make only 1/2 of the pen at a time, so this design allows for it (although you'd mess up the threads in the cap-end, but that's probably a relatively minor loss).

My idea though was to have a set of these for each pen, rather than just having something that is a substitute for a mandrel (i.e., eliminate the need for separate bushings).

Thanks for the feedback!

 

[Scott]

Hi Jim,

I think you could make a set of these that would work exactly as you hope they would, especially given Paul's sage advice! But, to make them economically and market them at a reasonable price for penturners, might be too much to ask. Like Paul said, screw threads are not always the most tightly accurate things on the planet. A straight rod with a slip fit would only be as accurate as the allowances built into the slip fit. Of course, the same thing could be said for the slip fit of a bushing onto a conventional mandrel. But if you're multiplying that microscopic instability times five joints, it could become a problem.

I think for the time and money, we would be better off with a series of mandrels that are as thick and stiff as the pen kit will allow. If you make a number of any style of pen, it would be worth it to get the appropriately sized mandrel and bushings. The expensive part of this is the part that holds the mandrel, but with so many of us using collet chucks to hold mandrels, that is not as big a deal as it could be. The reason I would stick with a one-piece mandrel is that there is a structural continuity there that would tend to hold things in place even when there are forces that would throw things out of line, such as the faces of blanks being uneven.

This doesn't mean that you shouldn't move forward on this, because if you can find a joining system for the various parts that would also provide a rigid structure, this would be a great system!

Good Luck!

Scott.

 

[Fangar]

Tolerances would kill me. Can you say wobble? I have a hard enough time milling a flat, consistent notch in a piece of rod

[]

Fangar

 

[TomServo]

Scott: if you're going to make a mandrel for every pen tube size, you'd be better off to just make 'double' pin chucks, then you eliminate the ends of the barrels pushing the rod out of round. Plus there's no nut to tighten at all (a time saver swallowed up by keeping track of the pins..). It would be easy, and one of these days I really will order some drill rod to try it, and get some piano wire to use for pins. Why not machine the headstock end of the mandrel to fit a standard adaptor? I don't think you'd lose a significant amount of strength as long as you kept the smaller section as short as possible.

edit: also, the difference between kits is more often the final size of the kit, rather than the barrel size - most Berea kits use either the 7mm or 10mm tubes. OD is what varies more than ID... (which makes you wonder why they didn't use a bigger B mandrel in the first place!)

 

[RussFairfield]

There was a time when there were a few pen kits but there were no mandrels. It wasn't that long ago. We turned the pens barrels between centers. We had to use micrometers to measure the diameters within 1/1000" for a proper fit, and fight with the centers at each end; but all things were concentric, straight and perfect.

Then someone invented the 2-piece mandrel. It was 2 bushings with a fixed pilot that went inside of the tube. One was held in the headstock, and the other supported by the tail-center. We made the first ones ourselves out of wood and plastic, and then they were made from steel. They were almost as accurate, and easier to use, than turning between centers. The wood in the lathe was also short and there was no vibration. The last of these that I remember on the market were in the Packard Catalog, and they disappeared a couple years ago for lack of interest.

Then somebody got the bright idea - Let's stack everything up on a single long mandrel and turn everything at once because it is faster. Straightness of the rod, trimming of the blanks, diameter tolerances, and the accuracy of the bushings all became critical factors for an accurate pen. And, we have been been complaining about mandrels, vibration, and eccentric pen barrels ever since.

I would suggest that all of our mandrel problems would be solved if we did it the old fashioned way - one barrel at a time. Does it really take that much longer to make a better pen?

We might also want to know why there are 2 sizes of mandrels from Berea. In the beginning there was only the Cross-style pen, the one we call the Slim-Line; and the mandrels were sized to fit the tubes for them. The fact that not all tubes are created equal, and that some are measured by OD and others by ID, and some are measured in metric while others are in decimal inches, made for some variation in the tubes from the different manufacturers.

Then came the Parker-style ball point pen, the one we now call the Americana or flat-top. It was a larger pen, and Berea decided to make a larger mandrel that was sized to the ID and length of the brass tubes used in this new pen. Hut, who was reselling the Berea kits also went with the larger mandrel. Woodcraft, who was selling the same pen, decided to stay with the Cross mandrel and made their own stepped bushings. PennState and Craft Supplies, who were using different suppliers in Taiwan, and slightly different sized brass tubes, also stayed with their smaller mandrel and made stepped bushings available.

As the styles of pens proliferated, each supplier went with what they had for mandrel diameters, and started making bushings instead of new mandrels, because it was easier and cheaper that way.

And, that's how we got to where we are.

With the price of the kits reaching $30 for plain gold plating to over $100 for the up-scale kits, we deserve more than a turning system that is based on a SlimLine pen, and wasn't really the best for even that pen. Personally, I think it is insanity to turn both barrels at the same time for a pen the size of an ElGrande on a 1/4" mandrel. I think it is time for our manufacturers to re-think their whole philosophy of mandrels and bushings. Maybe it is time for someone else to get into the mandrel business.

 

[JimGo]

Thanks again for all the feedback. Russ, I've been leaning more and more toward doing exactly what you described. As I worked on my last set of pens, I got to thinking that any bow in the mandrel or misalignment in the MT is going to be more evident the farther away from the headstock I move (simple geometry), thus sticking with a single half of the pen should really reduce any problems I'm experiencing. My only problem with this will be the CB-less slimlines and other similar styles, where the pen needs to flow together. I could mic it, but I don't think I have the skills to get a fit within 1/1000".

Hmmm...maybe it's time to invest in one of those adjustable mandrels, or a Beall Collet Chuck.

 

[RussFairfield]

Jim, go with the Bealle collet chuck. It is worth the extra money. The adjustable mandrels that I have seen weren't precision enough for there to be no run-out on the mandrel rod at chuck. The collets in the Bealle chuck are.

 

[RussFairfield]

Jim, every individual piece will have tolerances for diameter, concentricity, and squareness. NMothing is perfect. All of these tolerances are additive. The best case is when they are randomly distributed and they cancel each other out. The worst case is when the all accumulate in the same direction and they are added together; The more pieces there are, the greater the error and the possibility for the error.

The solution is to have the least number of pieces and joints possible. You need to be thinking towards having fewer tolerances to accumulate than are present in the mandrel rods and bushings.

 

[Joe Melton]

Don't blame it all on the mandrel setup, guys. I recently made a bunch of Sierra pens, and the out-of-roundness of the finished blanks was terrible. This didn't really matter that much, since the wood is not flush with the Sierra fittings (it is below it). However, it tweaked my interest.
I measured the thickness of the walls of the bushings, the part that fits inside the tubes, and found it varied from about .062" to about .070". In other words, the o.d. of the bushings is not very concentric with the i.d. Because the bushings are getting worn, I didn't check the thickness of the other end, which gets damaged by the tools and sandpaper, but suspect it was just as bad when the bushings were new. Whether it is or not, though, the walls of the blank are going to vary in thickness since the other end of the bushings is off.
I've not measured bushings for other kits, but perhaps others would like to check this out.
You'd think the bushings would be made on an o.d. grinder, so concentricity would be guaranteed pretty close. I don't know how these bushings are made, except quickly. You can't spend a lot of operator time on these and make money at $3 a set, even with Chinese labor costs.
Joe

 

[btboone]

Sorry Jim, I tuned in late to this thread. Titanium is pretty flexible, so something like drill rod steel might be better and certainly much less expensive. All the parts screwing together will definitely add up to a lot of wobbling. I would say that if you wanted all those parts to screw together, they should have a non locking taper to handle most of the alignment, and the threads just handle the axial part. It looks like some of the parts could be kept really thick except right at the bushing sides. The real way to connect the parts, although not very machineable, is with castellated parts (like a chess piece) that have tapers in the fingers that stick up. This aligns them to perfect concentricity, but drives them as well. Bicycle cranks have gone to this technique.

I agree with Russ on this one. Have a couple mandrels, each as thick as possible made from drill rod. The stuff costs something like $3 for 3 feet of it from places like MSC or Mc Master Carr and it's deadly accurate in any size you want.