7mm pen tube expanding after turning issue

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TN_Eric

TN_Eric

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East Tennessee
Greetings all

Recently, maybe the last 3 slimline pens I've made, I've been having an odd issue. At first I thought it was a singular event as after being turned, the pen parts would just "slip in" to the finished tubes. After the 2nd one, in a row I might add, same issue.

Prior to the 3rd one, I tested the tubes before inserting them into the blank, and after glue up and the "fit" as normal, as in needing to be pressed in to work. So I put it on the mandrel, turned it as normal. After turning and finishing, I went to assemble it and the same issue, the nib just "slid in" with out pressing, as did the transmission, and clip. It was like the tube "expanded" while it was being turned.

I did notice that I would have to tighten up the quill a few times while turning as the tool would catch the blank. Could the bushings be causing this? Time to replace them maybe? I have a bag full of 7mm tubes, and checked several of them and all seem good and match the tubes in the kits, so I can't say those were the issue.

Thanks.
 
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I would take a good caliper and test the dimensions of the bushings and tubes to see if you might have a bad set of tubes or bad/worn bushings. Your bushings should just slide into the tubes without any real force when the fit is right. Checking the measurements of the pen parts that will mate with the tubes is also a good way to isolate possible issues. Everything should be fairly close in dimensions for a snug fit. I would test before and after turning to see if anything changed, giving you a possible culprit for the issue.

It would take a good bit of force on the lathe to compress the tubes so I am less inclined to think anything you did in the turning process is causing this, but as a cautionary note, you don't need to heavily muscle down the tailstock nor the locking nut on your mandrel, just a snug fit to allow things to turn and be worked. Lessen your hand pressure on your tool when cutting the blank down - if you push down too hard, it will force you to overtighten your mandrel to keep the blank from spinning free of the mandrel. You shouldn't need heavy force on anything to be successful.
 
Well something is expanding the ends of the tubes. Are you using the right mandrel for slimlines. There is an "A" and "B" You want the "A" which is thinner. The bushings are just round washer type things or called spacers with no shoulder so can not see how you can expand the tube by over tightening the tailstock. The bushings do not go into the tubes. Mabe you can explain your set up process more to us. Because something does not sound right. Now I do not make slimline pens so maybe I am way off here but lets try. Maybe the mandrel shaft is bent ever so slightly and as you slide the tube over it expands the tube to conform to the rod. Lay the rod on a flat surface like the top of the tablesaw or whatever and roll it. It should roll easily and no wobble. If wobble then change mandrel. Do the tubes slide on the mandrel rod easily is the starting point.
 
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could you post photos of your tubes on the mandrel set up and ready to turn? It does sound like you've got an awful lot of pressure on the tubes and the ends are ballooning somewhat.

Do you do any other steps with the tubes? I know it's possible to sand away some of the interior of the tube to open up the fit of parts.
 
My first thought is that something you have or something you are doing has changed since you made your last successful pen. Are you using a different bag of tubes, different kits, mandrel, tools etc. Exclude nothing in examining this.
Second thought is to check the fit before and after each and every step of the process, just to see.
Third thought is to let someone else turn something in your shop. If that isn't possible mail some tubes to someone else to try. I've made hundreds of slimline from several different manufacturers and haven't run into this yet. But you never know.
 
I'll take some photos later tonight and post them.

As for the mandrel, I am using the PKMS2SET from Penn State, MT2

Just to clarify, this is on slimline kits only, using the 7mm bushings for the 7mm tubes.
 
How are you squaring the end?
I stopped using a pen mill because it was doing this. Slims are touchy. There is a very small difference in the tube and the parts. The tranny is usually the worst.
 
RDH79 said:
How are you squaring the end?
I stopped using a pen mill because it was doing this. Slims are touchy. There is a very small difference in the tube and the parts. The tranny is usually the worst.
Click to expand...
You make an outstanding point here and may really be onto something.

Even though the "standard" pen turning mandrel, or "A" mandrel specified by Berea (BereaHardWoods Co. Inc.) as
0.247" inches +0 / -0.002" inches, I use "D" Drill Rod (0.246" inches +/- 0.0005" inches) for my mandrel on almost everything except 7mm Slimline pens. Even though it is smaller than the "standard" mandrel that Berea specified, I've found that "D" Drill Rod can feel a little too tight for my 7mm brass tubes. For that reason, I keep a mandrel made with a "C" Drill Rod (0.242" inches +/-0.0005" inches) just for 7mm tubes.

I don't use a pen mill to square my blanks, but it wouldn't surprise me if a "standard" pen mill reamer wasn't on the larger side of the "standard" 7mm, "A" mandrel dimensions. If you are using one that is, it would most likely increase the ID of 7mm brass tubes slightly.

As someone already suggested, check the fit between each step to see if you can find out where the problem is. Even just slide a tubed blank on your mandrel, tighten it up, and then take it back off and check the fit (before turning), in case it is just the mandrel itself that is stretching out the tube.

Dave
 
I use a pen mill...very carefully and advance it a tiny bit at a time. I got to the point that I now measure the length of the tube with calipers. Why? I made a revolver pen that has the refill just inside the nib. Seems I took off a bit too much.
Watching this thread for the solution.
 
TN_Eric said:
Greetings all

Recently, maybe the last 3 slimline pens I've made, I've been having an odd issue. At first I thought it was a singular event as after being turned, the pen parts would just "slip in" to the finished tubes. After the 2nd one, in a row I might add, same issue.

Prior to the 3rd one, I tested the tubes before inserting them into the blank, and after glue up and the "fit" as normal, as in needing to be pressed in to work. So I put it on the mandrel, turned it as normal. After turning and finishing, I went to assemble it and the same issue, the nib just "slid in" with out pressing, as did the transmission, and clip. It was like the tube "expanded" while it was being turned.

I did notice that I would have to tighten up the quill a few times while turning as the tool would catch the blank. Could the bushings be causing this? Time to replace them maybe? I have a bag full of 7mm tubes, and checked several of them and all seem good and match the tubes in the kits, so I can't say those were the issue.

Thanks.
Click to expand...
Where did you get the pen kits from?
 
I may have found the issue. On the mandrel there is a rough part that I can't remove with out using a metal file and I'd rather not do that. I did order a new mandrel that will be here Friday. I'll see if that fixes it. In the image, a pen is pointing to the spot I can feel metal sticking out.
 

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That's why I went with the PSI Maxi-Mandrel that I bought from Exotic Blanks, because it uses a collet type design to hold the mandrel shaft. Then I replace the shaft with "D" Drill Rod from Grainger (0.246" inch diameter). That way I can easily replace the shaft if it gets buggered up somehow. For 7mm (Slimline) pens though, I usually swap it out for a "C" Drill Rod (0.242" inches) as they usually feel a little too tight on the "D". (I suppose I could probably just hold the Drill Rod in an ER32 collet, but I've always used the Maxi-Mandrel).

Since it doesn't have a threaded end to accept a brass nut, a mandrel saver tailstock is also needed. My favorite is the Hold Fast Mandrel Saver by TMIProducts that I bought from WoodTurningZ. It comes with a type of collet insert that tightens down on the mandrel shaft and centers it more precisely than what is achieved by the PSI style mandrel savers that just have a clearance hole through them.

So far this combination has worked very well for me.

- Dave
 
egnald said:
That's why I went with the PSI Maxi-Mandrel that I bought from Exotic Blanks, because it uses a collet type design to hold the mandrel shaft. Then I replace the shaft with "D" Drill Rod from Grainger (0.246" inch diameter). That way I can easily replace the shaft if it gets buggered up somehow. For 7mm (Slimline) pens though, I usually swap it out for a "C" Drill Rod (0.242" inches) as they usually feel a little too tight on the "D". (I suppose I could probably just hold the Drill Rod in an ER32 collet, but I've always used the Maxi-Mandrel).

Since it doesn't have a threaded end to accept a brass nut, a mandrel saver tailstock is also needed. My favorite is the Hold Fast Mandrel Saver by TMIProducts that I bought from WoodTurningZ. It comes with a type of collet insert that tightens down on the mandrel shaft and centers it more precisely than what is achieved by the PSI style mandrel savers that just have a clearance hole through them.

So far this combination has worked very well for me.

- Dave
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While not my thread thanks for the info Dave. That will come in handy later. I also think the idea of drill rod in a collet chuck might be a worthwhile constatation.
 
TN_Eric said:
I did notice that I would have to tighten up the quill a few times while turning as the tool would catch the blank. Could the bushings be causing this? Time to replace them maybe?
Click to expand...

Take the mandrel shaft off and put it on a hard, FLAT surface. Look for gaps between the surface and the shaft, then rotate the shaft and look for more gaps. It sounds like you have a bent rod, which is an all-too-common problem.

If you see light, the shaft is bent and needs to be replaced. When you over-tighten the quill, it will compress and bend the shaft.
 
TN_Eric said:
I may have found the issue. On the mandrel there is a rough part that I can't remove with out using a metal file and I'd rather not do that. I did order a new mandrel that will be here Friday. I'll see if that fixes it. In the image, a pen is pointing to the spot I can feel metal sticking out.
Click to expand...
so something has caused that burr. I would take a serious look at your bushings and whatever else sits on the mandrel to see if anything has a sharp edge. It's possible you may need to replace a bushing.

Also possible that you're tightening things down too tight on the mandrel and causing a bushing or something else to get just a little off center and bite into the mandrel. A little sandpaper will remove the burr no problem.

Yes, speaking from experience unfortunately.

I switched to a new mandrel and a mandrel saver last year and have never looked back. I keep pondering doing turn between centers but this seems to work well for me.
 
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