A source of barrel eccentricity with a workaround

[Johntop]

My first post - be gentle and forgive its length.

I see several threads addressing my current pet peeve, eccentric barrels, but none that seem to resolve the problem I experience turning with a mandrel. I guess many folks don't consider it a problem since the pens I see our local "experts" produce are at least as out of round as my earlier attempts, but it still frustrates the heck out of me. I think I share a desire for perfection with a lot of you so I'm hoping for some advice.

I have a workaround that corrects a lot of the problem, but it seems pretty clumsy. The problem I see is that there is quite a bit of slop (say about .003") between the tube ID and the mandrel. I've looked at 4 different "A" mandrels and they are all within less than .001" of .245" Every tube I have put on has at least .002" slop (OK, tolerance). When I mount barrels on the mandrel they naturally seek some stable orientation and it’s obviously not perfectly concentric. Overtightening makes it worse, but there is no way to avoid the issue if the fit isn't perfect.

So when I turn the barrels the resulting product is round, but at least a couple thousandths out of concentric true. Plenty enough to see, much less feel.

So my first workaround (and proof this is an issue) is to reduce the slop by increasing the diameter of my mandrel. This is clumsy, but I simply built up a sheath of CA on a mandrel. I found it necessary to apply multiple thin coats to keep it true, but about 15 applications brought the mandrel diameter up by .0025". Had to then apply a coat of wax to slip the pieces parts on since bushings and tubes are now a tight fit. Tight enough that I also have to ream out the tube ends a little to take the flashing off in order to work them on.

The result is barrels that are almost perfectly concentric. Turning a slimline without a center bushing just for a test, the assembled barrels line up with no visible misalignment and almost not enough to feel as you rotate the mechanism. I'd say true and concentric to within significantly less than .001".

So my questions:
1) Any comments in general? I have not seen anything similar commented on and I'm wondering if I'm really off in the weeds. It sure looks like a lot of people accept a degree of misalignment that I find objectionable however, and I want to get rid of it.

2) Anyone out there who will turn me a set of .248" and .249" mandrels for a reasonable price (my local machine shop wants to take me for a ride for this kind of precision).

3) Is there a better way? I have not tried turning without a mandrel, but I don't want to turn to a bushing in the center for the pens I'm working on now so I don't see how to do without a mandrel in that case anyway.

Thanks in advance for any insight.

 

[Paul in OKC]

Topic comes up plenty, but at least you have it correct in that the barrels are out of concentric, not out of round as said most. I am not sure about what the mandrels are made from, ut I know you can get letter 'D' drill rod, which is .246 diameter. Other way might be to use .250 drill rod and do a lot of polishing. I am a machinist and usually very picky about things like this, but I have turned on a mandrel for 7 or 8 years now and just don't seem to have the same problem as most. 2 or 3 thousnadths of out of concentric is only .001 to.0015 per side.

 

[sbell111]

Honestly, I'm not sure how bad this problem really is. I haven't noticed that our pens have the problems that you describe.

For giggles, I just performed the 'remove the center ring and rotate the mechanism' test on the only pen available to me at my office; a four-year old comfort that was made by my wife. The barrels line up perfectly throughout the rotation.

I don't for a second believe that we make the best pens anywhere and certainly don't believe that my wife knew all the secrets back then (my pen was made within her first month or so of making pens). Therefore, I can only assume that the problem isn't widespread and is being caused by certain procedures that some people are doing (such as overtightening) or perhaps equipment that is out of spec.

 

[Johntop]

Thanks Paul, I'd thought about exactly the approach you suggested with the drill rod, but seems as iffy to me as what I have done already. Rather have one turned accurately.

So what I see when I rotate the barrels in a twist pen is the eccentric difference ranges between zero (when the barrels are aligned as they were on the mandrel) and about .002 out. I don't know about you, but that .002 is way more than enough to see, and bugs the heck out me to feel....

 

[ed4copies]

After experimenting with the "Mandrel-less" turning, I have confirmed that the bushings are frequently (more than 10% of the time) not drilled in the center. This will NOT apply to the 7mm, but it applies to nearly every component that is popular on this site.

I am slowly "weeding out" the bushings that "wobble" and my pens are getting better.

ALSO, you don't have as many "catches" as you near final dimension on pens with skins, etc around the tubes, then cast in polyresin. I now realize SOME of the catches were created by the blank "wobbling", so I caught the "proud" part.

We live and learn. Mandrel-less IS better. But you do throw away some bushings!!!

 

[ed4copies]

Another point, having turned over a thousand "slimline-based" pens. The 7mm tubes are NOT all the same. PSI is different from Berea or the Chinese. So, if you REALLY customize your mandrel to JUST fit, make sure you use the same manufacturer from then on. Also, if you use the CA method. WHEN it gets hot, it can become "sticky" again. So you better keep your heat down while turning, unless you want your pen permanently mounted to your mandrel.

<center>DAMHIKT</center>

 

[wdcav1952]

John,

You bring up some interesting issues. I do think the post would have been improved by leaving out the rather condescending remark in the following snippet:

I guess many folks don't consider it a problem since the pens I see our local "experts" produce are at least as out of round as my earlier attempts, but it still frustrates the heck out of me. I think I share a desire for perfection with a lot of you so I'm hoping for some advice.

I don't know anyone here who has self-described themselves as one of our local "experts".

Mitigate your condescension for others' efforts and you have a thought provoking post.

 

[Johntop]

Thanks for your insight ed4copies.

Yea, I discovered early on the tubes are not all the same diameter. In fact I initially thought picking the right (smaller) diameter would solve the problem. Helps, but still have issues. the variability is part of why I thought a couple different mandrel diameters would be valuable.

 

[Johntop]

Cav:

Appologies to the board. Didn't mean to be condescending at all. My intent was to express my frustration with the problem and the fact that folks at our local stores that I have been able to discuss it with in person have simply blown the issue off. Yet when I look at their pens they consistently display as much eccentricity as mine did (in fact they are often much worse). They just seem to consider it acceptable tolerence.

My own view is that this is something it would be worth our suppliers looking into. It would certainly differentiate for me the company that eliminated the play between the parts.

Thanks for your reply and coaching. I'll try be more mindful of how my words might look in the future.

 

[CrazyBear]

Cav

My reading of it was that john was talking about 'experts' local to him that he meets in person. I dont thnk he was referring to anyone on here

 

[Chuck Key]

You were on to the solution in your orginal post but when it came time to pony up you let it go. That kind of accuracy in pen kits, related supplies, machinery and tooling is going to cost you much more than we are paying for what is currently available. If you are serious about this there may be a few rides involved.

Chuckie

 

[wdcav1952]

Hugh, after re-reading John's post I agree with you. John, forgive my thin skin and keep on innovating!

 

[loglugger]

Check out jcollazo he has a better fitting mandrels.

 

[Aderhammer]

Have you heard about mandreless? Sounds like you have calipers anyways. Much higher precision.

 

[pssherman]

Another solution is a trick I use with sloppy bushings that do not fit well on the mandrel. To tighten them up, which could also be used to improve concentricity, use a spring-loaded center punch and put a series of dimples on the inside of the bushing at BOTH ends. They will be initially very tight on the mandrel and you may have to tap them on. The small points of contact will flatten out and produce a fit with no play. By adjusting the number and spacing of the dimples you should be able to correct some, or all, of the non-concentricity of the bushing.

Hope this helps.

 

[leehljp]

Originally posted by Johntop
My intent was to express my frustration with the problem and the fact that folks at our local stores that I have been able to discuss it with in person have simply blown the issue off. Yet when I look at their pens they consistently display as much eccentricity as mine did (in fact they are often much worse). They just seem to consider it acceptable tolerence.

Although I haven't met any pen turners in Japan, I do see a few once in a while when I am back home in the states, and am usually disappointed with what I see. Mine are not perfect, but the standard that some accept just makes me keep my mouth shut and pen in my pocket. I don't even cast my "fake" pearls before swine.

 

[PenPal]

Varying sizes of brass tubes,continue in to the fittings to press in , the drills we use are rarely the size marked.
I feel we are dealing with what is called Fair Average Quality often with no connection to the price per kit we actually pay.

Precision comes at a price.

Pricing your pen to the kit and timber or otherwise,to determine FAQ is proportional to the actual cost.

Normally I make a lot of pens per annum the majority as gifts,I do not skimp on quality timbers, the presentation is normally from my top pocket with the advice please use this pen.

On one occasion I made a pen for an Elderly Chinese man with a near million dollar lifetime collection of Jade. He accepted the pen, put it in the safe with the Jade. How priceless was that pen to him?

A long time known fact is the variation in sizes,there is no standard
in the world, nor is there a standard in most manufacture.We have seized on a pen to express ourselves and I for one am still exploring the kit I can afford to give away in large numbers to provide real pleasure to so many people.

I am so sympathetic to the expressions of that feeling we all have to do the best we can,extend that to the best use of what we have.FAQ.

The real enjoyment of the IAP is the whole spectrum of styles, makes, kits etc viva la difference.

Peter

 

[Johntop]

Thanks to all for their comments.

Bob, who is jcollazo?

Hank, your experience lines up exactly with mine: while my pens are far from perfect, perfection is my goal. The standard our local turners accept just seems way out of bounds. .001" sounds like a little number but if I can feel it, or worse see it, every time I use the pen I am continually reminded I didn't make it right no mater what fancy inlay or special material was used.

Steve, your experience gives me hope! Even some of my pens turned on "sloppy" mandrels have come out pretty good, but more have problems. I'm hoping some insight from this dialog will offer a clue about what to adjust/change. The approach I've been using seems awfully clumsy and I'd love a better way.

Chuckie, point taken. In fact I've been been discussing having a couple mandrels made from tungsten (twice the young's modulus of HSS steel so probably much less tendancy to whip, flex or bend) and ground to a range of exact diameters in .001" increments. That's my brute force (and expensive) approach, but I wanted to ask you guys how crazy I am before I go down that path. I know the tungsten bit is overkill, but any suppliers that want to offer "high precision" mandrels in various exact diameters would find me a ready customer. Anyone who can help me figure out why Steve's pens (and some of mine) come out right and others don't will find me most appreciative!

Paul, I like your trick, but I dont see how to use it to exactly center the tubes when they are a loose fit. That seems to be the primary source of the problem. Can you give a little more detail?

Cav, NP. I'm the new guy to this community, and I do appreciate the intent behind your input.

Thanks again to all.

John
281-222-2052

 

[Johntop]

So I just got a private note with some info I thought I'd share. I guess a lot of you on this board already know Joe (Jcollazo), but his A mandrels are listed as "about .249". That would mean at least .003 less slop than all the ones I have bought so far (from 4 different suppliers and typically between .245 and .246). I'm ordering one today and will let you guys know what my experience is.

Thanks Joe. BTW, if anyone has more ideas for better turning results with mandrels (other than the larger diameter approach) I'm still very much interested.

Thanks
John

 

[loglugger]

John, keep buying bushings untill you get a tight set along with Joes mantrel, then take care of then like they are gold.
Bob

 

[pssherman]

Originally posted by Johntop

Paul, I like your trick, but I dont see how to use it to exactly center the tubes when they are a loose fit. That seems to be the primary source of the problem. Can you give a little more detail?

John,
The trick I mentioned will correct loose bushings and may also work for eccentric bushings (if loose). If the tube is loose on the bushing, you can put a few dimples around the bushing step where the tube goes. That should tighten things up. If the tube is a 7mm tube, put a few very small dents in the edge of the tube, this should tighten/center the tube. When the barrel is finished, the dents can be easily removed with a round file.