# Concentricity Issues



## pgfitzgerald (Aug 8, 2013)

It's been a few years since I turned any pens. I've been back at it for a few weeks and I now remember some of why I ended up putting the hobby away for so long.

I have issues with concentricity. I've read as much as I can find on the subject and it appears as though my lathe is fine, though I'm not 100% sure.

At this point, my guess is it's the bushings. The only thing that is confusing me is that it seems like it's not just limited to one set.

I didn't really document any of my findings, so I'm happy to go recheck things. I certainly could have missed something. Any help would be appreciated.

Thanks!


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## ossaguy (Aug 9, 2013)

I had that same problem before I switched to turning between centers.I got some of the bushings from John at Penturners Products,a carbide tipped dead center and the shorter tool rest also from there and it works great.

I wish I'd done that from the start.


Steve


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## Tieflyer (Aug 9, 2013)

Do you use a mandrel?  I've bent one, made everything go wonky.  I started using a mandrel saver as well, takes the pressure off my mandrel and keeps me from bowing it when I tighten the tail stock


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## Dan Masshardt (Aug 9, 2013)

Also, I've found that if the ends of your blanks aren't truly square, you'll get runout.


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## kovalcik (Aug 9, 2013)

+1 on the mandrel saver. I use one and they work great.  I also use an adjustable mandrel that I can shorten up for shorter pen blanks.

Is the entire pen out of round or only in the middle or the ends? Is it worse on the headstock or tail stock end?  SInce it happens over different bushing sets I would guess a mandrel problem (bent, too much pressure on the end) or a misalignment with the lathe itself.


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## randyrls (Aug 9, 2013)

Paul;  First, the blank ends have to be square with the brass tube, not the blank.

One method I use is to turn the blank(s) just down to round.  Don't size them yet.  Change the lathe speed to low speed, turn it on and lay a turning tool shaft on the bushing.  If you feel any vibration, the outer dimension of the bushing isn't rotating concentrically.  Stop and check to see where that vibration is coming from before continuing to turn.

BE CAREFUL when doing this not to touch the blank.  I am not responsible if you hurt yourself.  Not everyone is as obsessively safety conscious as I try to be.

Added later;  Take the mandrel and unscrew it from the taper.  Lay it on a FLAT SURFACE and roll it.  Any bends will be apparent.  Take the  mandrel and put it in the headstock.  Bring the tail stock up to but not touching the mandrel.  Spin the lathe by hand and watch for any sideways or up-and-down movement of the mandrel.  The mandrel should spin but not move otherwise.


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## pgfitzgerald (Aug 9, 2013)

Thanks for the replies!



ossaguy said:


> I had that same problem before I switched to turning between centers.I got some of the bushings from John at Penturners Products,a carbide tipped dead center and the shorter tool rest also from there and it works great.
> 
> I wish I'd done that from the start.
> 
> Steve



Couple questions. 1) I'm assuming you do the upper and lower barrels separate. Is that correct? 2) Is carbide tipped a requirement or does that simply help it last longer?



Tieflyer said:


> Do you use a mandrel?  I've bent one, made everything go wonky.  I started using a mandrel saver as well, takes the pressure off my mandrel and keeps me from bowing it when I tighten the tail stock





kovalcik said:


> +1 on the mandrel saver. I use one and they work great.  I also use an adjustable mandrel that I can shorten up for shorter pen blanks.
> 
> Is the entire pen out of round or only in the middle or the ends? Is it worse on the headstock or tail stock end?  SInce it happens over different bushing sets I would guess a mandrel problem (bent, too much pressure on the end) or a misalignment with the lathe itself.





randyrls said:


> Added later;  Take the mandrel and unscrew it from the taper.  Lay it on a FLAT SURFACE and roll it.  Any bends will be apparent.  Take the  mandrel and put it in the headstock.  Bring the tail stock up to but not touching the mandrel.  Spin the lathe by hand and watch for any sideways or up-and-down movement of the mandrel.  The mandrel should spin but not move otherwise.



I do currently use a mandrel. It's an adjustable one with a headstock collet. I just took the shaft out and verified that it is perfectly straight by rolling it on my cast iron table saw top as well as against my lee valley machined aluminum straight edge.

I shot a short video that shows the tailstock end as I turn the spindle by hand. You can plainly see there is something wrong. Pen Mandrel Problem - YouTube

I'm no expert, but I'm guessing this is likely gunk in the headstock taper or on the mandrel taper. They feel clean, but I do see surface discoloration. (Damn you Memphis humidity!)

It's a bit difficult to tell where along the barrels the problem is occurring because it's not consistent from pen to pen.

I just turned a cigar pen and noticed a couple of things. In this case, it appears as though the turned barrels are just fine. They're the right diameter and everything aligns properly once pressed together. Before removing the barrels from the mandrel I was puzzled to see that the blanks and the bushing didn't align... like this (exaggerated of course):








Dan Masshardt said:


> Also, I've found that if the ends of your blanks aren't truly square, you'll get runout.





randyrls said:


> Paul;  First, the blank ends have to be square with the brass tube, not the blank.



I don't think that's my problem, but I will be careful with that in the future.



randyrls said:


> One method I use is to turn the blank(s) just down to round.  Don't size them yet.  Change the lathe speed to low speed, turn it on and lay a turning tool shaft on the bushing.  If you feel any vibration, the outer dimension of the bushing isn't rotating concentrically.  Stop and check to see where that vibration is coming from before continuing to turn.
> 
> BE CAREFUL when doing this not to touch the blank.  I am not responsible if you hurt yourself.  Not everyone is as obsessively safety conscious as I try to be.



I have certainly noticed this. I'm guessing it's either the inside and outside of the bushings not being concentric (see graphic above), the mandrel not spinning true, or a combination of both.

...

Again, thanks for all the responses! I hope what I have added in this response will help lead to a resolution.


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## ossaguy (Aug 10, 2013)

Hi Paul,

    On the double barreled pens,I do them one at a time,and the carbide dead center just lasts longer especially on the smaller tubes,it doesm't get a ridge worn in the taper like normal steel ones.

     I don't know if Penturners Products still offers the one-time-only set of bushings that allow you to slip on just about any std bushing on them to turn TBC,but that's what I use.They work perfectly.

      The 60 degree live center that John sells costs a little more than some of the others,but it really lasts good.The steel seems like it HHS,vs regular ol'steel.

      It just solved so many problems I had,some of what you have described.It made pen turning fun again!


Steve


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## Dan Masshardt (Aug 10, 2013)

Couple more things to pile on. 

I recently started making sure that my bushings were really clean (acetone bath) and was surprised how much gunk came off.  

Also, checking runout in various places along the line with a dial indicator would be a good thing. 

Harbor freight sells them cheap.  I got the tool post adapter from Rick Herrell.


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## Tim'sTurnings (Aug 10, 2013)

If anyone is going to get a dial indicator, Grizzly has a great deal on an indicator, magnetic base, adjustable arm supports and a molded case for only $17.95. Great deal!
Grizzly.com
Tim.


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## low_48 (Aug 10, 2013)

Not major runout on something hanging that far away from the head. Loosen the collet, lightly bring in the tailstock and tighten the collet. Go ahead and measure runout then. Shouldn't be more than a couple thousandths.


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## pgfitzgerald (Aug 17, 2013)

Okay folks, I finally found some time to go check...

Runout on the mandrel at the headstock is 0.0005".
Runout on the mandrel at the mid-point is 0.004".
Runout on the mandrel at the tailstock is 0.002".

So at this point, I'm thinking I just have crappy and/or dirty bushings. I'll try thoroughly cleaning the ones I have and I'll also see about getting some good ones from John at Penturners Products.

Thanks!


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## Dan Masshardt (Aug 17, 2013)

pgfitzgerald said:


> Okay folks, I finally found some time to go check...
> 
> Runout on the mandrel at the headstock is 0.0005".
> Runout on the mandrel at the mid-point is 0.004".
> ...



Or you're ends aren't as square as you think they are.  This is my problem when I have runout most of the time.


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## pgfitzgerald (Aug 17, 2013)

Dan Masshardt said:


> Or you're ends aren't as square as you think they are.  This is my problem when I have runout most of the time.



I'm currently using a PSI barrel trimmer with a carbide cutter.

What's the best way to go about checking whether the ends are truly square?


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## edstreet (Aug 17, 2013)

pgfitzgerald said:


> Okay folks, I finally found some time to go check...
> 
> Runout on the mandrel at the headstock is 0.0005".
> Runout on the mandrel at the mid-point is 0.004".
> ...




What is the runout on the head itself? with no tooling on it.


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## edstreet (Aug 17, 2013)

pgfitzgerald said:


> Dan Masshardt said:
> 
> 
> > Or you're ends aren't as square as you think they are.  This is my problem when I have runout most of the time.
> ...



By putting a chuck on the lathe, put the blank in the chuck and use a skew to trim the ends.


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## pgfitzgerald (Aug 17, 2013)

edstreet said:


> What is the runout on the head itself? with no tooling on it.



Almost nothing. Probably less than the width of the line printed on the face of my dial indicator.


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## edstreet (Aug 17, 2013)

pgfitzgerald said:


> edstreet said:
> 
> 
> > What is the runout on the head itself? with no tooling on it.
> ...



so the problem is in the mandrel head itself.  Have you tried to strip it down and not using the tailstock?  You could also flip it around and put it in a chuck and sand the taper.


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## Dan Masshardt (Aug 17, 2013)

edstreet said:


> By putting a chuck on the lathe, put the blank in the chuck and use a skew to trim the ends.



Does your method assume the tube is dead straight through the blank?


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## pgfitzgerald (Aug 17, 2013)

The issues I'm seeing are far larger than the maximum runout on the mandrel itself, so it makes sense to me that the problem lies elsewhere. Since I can visually see issues with the bushings as shown in the graphic I made, I'm inclined to think that is the most likely culprit. 

However, there may be an issue with the squareness of the ends of my blank as pointed out by Dan. I don't have a chuck; Are there any other methods by which I can check the squareness of my trimmed blanks?


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## edstreet (Aug 17, 2013)

pgfitzgerald said:


> The issues I'm seeing are far larger than the maximum runout on the mandrel itself, so it makes sense to me that the problem lies elsewhere. Since I can visually see issues with the bushings as shown in the graphic I made, I'm inclined to think that is the most likely culprit.
> 
> However, there may be an issue with the squareness of the ends of my blank as pointed out by Dan. I don't have a chuck; Are there any other methods by which I can check the squareness of my trimmed blanks?



Runout builds up the more with tooling.  Also how flexible the mandrel shaft is makes a huge difference.  Then you have the gap between the bushing hole and the mandrel shaft diameter.  Sadly most of the mandrel bushings has some epic amounts of slop in there.  Slop factor + runout can be a very terrible thing.  Another factor is factory bushings has minimal 'grip' into the blank and that to causes a larger error factor.

Assuming the tube is square to start with, that is 90 degree shoulder from the wall edge to the opening, you can actually do the job by hand.  Not only can you see the tube end but know how much and in what area needs work.  Lapping on a flat surface with sandpaper you can net some very spot on accuracy.

There is one thread in which we did several measurements and posted results and the topic was on TBC vs mandrel.
http://www.penturners.org/forum/f14/tbc-mandrel-112218/ Super epic goodness in that thread.


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## pgfitzgerald (Aug 22, 2013)

Got my bushings and dead center from John Goodin and all I can say is wow!

I took one of the the turned barrels from the last pen I had on my mandrel and "cheap" bushings and put it between John's bushings.

What a difference! I took one quick pass to even things up and it's so much better than before.

I'm a TBC convert now!


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## healeydays (Sep 27, 2013)

I have been having a bit of problems with concentricity issues and it's definitely the bushings.  I only TBC and being a frugal New England Yankee I never tossed the old bushings.  Well, they are now in the trash and I now have new ones from JohnnyCNC at Penturners Products and it is night and day.

Mike B


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