Blanks are odd shaped after turning. How do I fix this?

[JamesB]

Lately I've noticed that while turning the blanks they will be flush with the bushings on one side be larger on the other side. Sometimes it's not all that noticeable on the finished pen but today I made a Mesa (Sierra) with an acrylic blank and I was floored by the result. It was so obviously bad!!
I rolled the mandrel shaft on the scroll saw table and it looks flat, the lathe centers are just a hair off but I haven't had the issue always.
Could I be pressing too hard with the tool laterally and flexing the shaft? Would the bushing cause it? I've only made 3 pens with that set of bushings so they shouldn't be worn yet.
Thanks for your help

 

[ctubbs]

Number one, take a brush (I use a 12 gauge shotgun cleaning brush) and brush the inside of the head and tail stock tapers. You may have a small piece of trash in there. It is also possible you are placing too much pressure on your tools causing the mandrel to flex. Be sure to keep your tools sharp. I find that to be my number one cause of applying too much pressure on the tools. If you have a dial indicator, use it to locate where the deflection is occurring. This will give you a starting point.
Charles

 

[its_virgil]

You say "lately I;ve noticed..." then later you say you have only made 3 pens with this set of bushings. Has the out of round problem happened with other pens of just with the sierra bushing? Those two comments are a little confusing to me.

There are several things that can cause what you describe. Can you isolate the out of round problem to a single set of bushings? Does it happen with several sets of different kit ushings? Are you tightening the live center or the brass nut to tight? Is youir mandrel a MT2 or other MT mandrel? If so, the taper in the headsotck can get dirty causing the mandrel to missalign.

Dull tools require somewhat more pressure than sharp tools. The pressure can cause flexing which will produce the our of roundness you describe.

If the problem just started with this new set of bushing then the bushings may be the problem. The mandrel hole and the outer circumference of the bushing can be non concentric and out of round pens will result.D

Do you use a 60 degree live center in your tailstock?

I could go on but I think you get my point. You must isolate the cause (which can be a chore in itself) then fix it.

I'll stop for now and let the TBC guys give you their advice.:biggrin:
Do a good turn daily!
Don

 

[glen r]

There are a few things that you could be doing wrong starting with putting too much pressure on the chisel (unlikely as this would be corrected when sanding) tightening the mandrel nut too much, tightening the tailstock too tight or having a crappy set of bushings. Bad bushings are not that unusual unless you have a set that are made for turning between centers like those that Johnny CNC makes. Your lathe centers being just a little off would compound any of the conditions that I mentioned. Hope this helps.

 

[randyrls]

Lately I've noticed that while turning the blanks they will be flush with the bushings on one side be larger on the other side.

I rolled the mandrel shaft on the scroll saw table and it looks flat, the lathe centers are just a hair off but I haven't had the issue always.

Would the bushing cause it? I've only made 3 pens with that set of bushings so they shouldn't be worn yet.

James; Check out this link to a previous post of mine. You need to break down the situation to the cause of the out-of-round condition.

A dial indicator is handy, but running the lathe at low speed and a metal ruler on edge and held by both ends can be used. Any up and down movement of the ruler is a sign of a problem. Go one step at a time and start with the lathe itself, then add the mandrel, then bushings and bare tubes, then blanks. At some point you will notice the up and down vibration. You say the mandrel isn't bent, but this is a way to check one thing at a time and fix the problem.

http://www.penturners.org/forum/f30/mandrel-bent-questions-94151/#post1367207

 

[StephenM]

That would happen to me from time to time (but not always) and since pen turning time to me is more precious than trouble-shooting time, I just stop the lathe, turn the blanks 90 degrees or so on the bushings and see if I can feel a difference. If I can, I just turn them to the bushing again and they're good to go. In my case, it's probably pushing too hard but now I just rotate everything as a matter of course and don't have problems anymore.

 

[JamesB]

Thanks for the posts. Rotating the blanks on the bushing is something I would not have thought of. I used the wall street II bushings for the Mesa so I think I'll double check it and then try and turn a dowel down and see what happens.

 

[leehljp]

Rotating the blanks is like putting a bandaid on the problem, not curing it. Sooner or later, the culprit needs to be found and fixed.

Don asked if you were using a 60° live center. That would be my first thought also - that you are not using the 60° live center.

Mandrels do cause this problem from many perspectives:
Too tight of tail stock,
Mandrel flex,
Mandrel bent slightly,
Too tight mandrel nut,
Blank ends not square.

There is a way to eliminate the above mentioned list - TBC. Less is more.

 

[JamesB]

I agree with you Lee about the bandaid. What is TBC?
I'm using the live center that came with my Shop Fox mini lathe and the MT1 mandrel from PSI. It's probably a combination of things, this whole process was bad. I didn't drill it straight and had barely enough material on one side to even cover the bushing, but I figured since it was going to my everyday pen I could make it work. Then I tried to rush it, then tried to disassemble it and totally wrecked it.

 

[leehljp]

I agree with you Lee about the bandaid. What is TBC?
I'm using the live center that came with my Shop Fox mini lathe and the MT1 mandrel from PSI. It's probably a combination of things, this whole process was bad. I didn't drill it straight and had barely enough material on one side to even cover the bushing, but I figured since it was going to my everyday pen I could make it work. Then I tried to rush it, then tried to disassemble it and totally wrecked it.

TBC - Turning Between Centers. Use a Dead drive center for the head stock and a live center for the tail stock.

You basically told me your problem: You use the live center that came with your lathe, and it was made for wood, not metal as in the mandrel. The mandrel has a little dimple in the end (60° dimple) and it requires a 60° live center. The wood live center is not 60° and it wallows around in the dimple just enough to cause your problem.

IF you continue to use the mandrel, you will need a 60° live center. They are available from several places.

Grizzly, WoodCraft, PSI, JohnnyCNC, LittleMachineshop.com, Amazon and others have them. IS the ShopFox a 2MT (Morse Taper)? I am fairly sure yours is - anyway, you want to make sure that you get a 2MT 60° live center. But if yours is a 1MT, you will need a 1MT 60° live center.


Back to TBC. Turning between centers is a method that is much simpler than using a mandrel. Much less parts. It consists of a Dead (non-live) center in the head stock and the live center on Tail stock. Put the bushings into the squared blank and put that between the two centers. Tighten and turn. Once the blank is close to size, remove the bushings and put the blank back on without the bushings. Measure with calipers for sizing. Finish. Measure finish with calipers too. Perfect finish sizing! No mandrel flex from chisel pressure; No mandrel flex from tail stock being too tight; no bent mandrel problems. No bushings stuck (glued) to the blank. No chipped blanks from separating the bushings after finishing with CA.

Some people don't like this as they prefer to finish both blanks together, which can only be done on a mandrel. That is fine. 90% of the people that use TBC after having used mandrels stay with TBC. It just eliminates lots of problems that are associated with mandrels.

 

[GoatRider]

Another thing that can go wrong is you don't have your blanks squared up. When you then tight down the mandrel, the mandrel bends a little.

I use a pen mill to make sure my blanks are square.

 

[rsulli16]

i had a little of that too. what i do now is during sanding i will lossen the knurled knob and rotate the blanks on the bushings a qarter to a half turn. do this a couple of times while i change grits, seems to have aken care of it.
another 2 cents worth.
Sulli

 

[leehljp]

i had a little of that too. what i do now is during sanding i will lossen the knurled knob and rotate the blanks on the bushings a qarter to a half turn. do this a couple of times while i change grits, seems to have aken care of it.
another 2 cents worth.
Sulli

Again, "rotating" is like putting a band-aid on the problem. The problem needs to be cured.

50% of the time with "new to pen turning users" it is using a wood live center on a metal mandrel. Mandrels require a 60° live centers, or the "for wood" live center will wallow around in the mandrel dimple. It sure goes smoother when the proper tools and equipment are used. Much less frustration and people will notice the increase in quality also.