Lathe slightly off, help.

[Aces-High]

I'm am noticing that my turned blanks are slightly off. Thought it might be the mandrel, bought a new one with same outcome. Any ideas?

 

[More4dan]

Turn between centers, it will be more accurate and I find it easier and quicker. Your bushings may be worn if it's not the mandrill.


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[WriteON]

Headstock/Tail Stock centered? Not sure if that has anything to do with it...just a thought. Did the problem just start?

 

[TonyL]

Anything placed in between the headstock spindle and the tail stock ram can cause and compound misalignment. Assuming you head and tail stock components are ok, I would:
Check that your HS is secure (in case you have one that rotates).
Your TS is seated and locks properly - you shouldn't be able to move it when locked.
Place two centers that you know to be "true" in your HS and TS and then slide the TS to the HS, lock the TS. When locked, the points of the centers should meet point-to-point. Make your TS locked and your TS center quill is locked (smaller lever near the ram)

If your lathe and centers are "true". It can be anything - bent or loose mandrel, loose fitting bushing, dust or glue inside the tube, dull tools causing you to exert too much shear force against the mandrel (causing it to bend).

I also want to add that I am not a machinist or experienced woodturner; I am sure you will get better advice from those that know much more than I do...and there are plenty. Having said that..do a search on the IAP using key words: "Mandrel Bender". You will see the "heck" I went through and the wise advise that I was given. Needless to say, I don't use a mandrel any more. Some folks can use one without a problem; I cannot. Much success!

 

[vmsherp]

A simple cause of this is often ca or other finishes adhered to the bushings. Glue inside the tube can cause the same effect. Either will cause you to wind up with a nice round pen barrel with a tube that's not centered perfectly inside it. Clean everything carefully and see if that's all it is before your next pen.


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[Tim'sTurnings]

I agree with what vmsherp suggested. I also agree with the comment that your tools may be dull which forces you to press the chisel too hard on the pen blank, or the glue on the bushings sounds reasonable to me. Also make sure you do not over tighten the nut or whatever you use on the mandrel. I use a mandrel all the time and I have never had the problem that you describe. Good luck.

 

[More4dan]

The tailstock and headstock that are not aligned will create drilling problems but you will still get "round" turnings. Metal turners will offset the tail stock on purpose to machine a taper but it's still concentric. Either your bushing to mandrill has too much clearance space that varies with tool pressure or as others have said something (glue) has the blank off center from the turning axis. See if you notice back and forth movement of your blank when you chuck it up and push and pull by the bushings. You could also check with a dial gauge to see if the bushings aren't centered as the lathe turns.

Sometimes sanding can be the issue when the wood is softer on one side verses the other. Using a block when rough sanding can help.

I use a tapered reamer to clean the tube ID before turning and definitely before assembly. They can be had from Harbor Freight for about $3.

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[Dehn0045]

I had similar issues early on. My tailstock has a little bit of slop, and my alignment might not be perfect, but not enough to cause the errors I was seeing (at least I thought). I switched to turning on a mandrel or using TBC bushing until I am fairly close to size then switch to TBC and measure with digital calipers to get correct final size. My results have improved dramatically, which suggests that the tailstock slop and alignment were not the primary problem. Another thing that I learned is how badly sized bushings can be -- even the specified sizes from the vendors sometimes don't match the actual pen parts (I'm talking as much as 0.015").

Here is a good reference for TBC: http://content.penturners.org/library/techniques/turn_between_centers.pdf

 

[Woodchipper]

Interesting, makes we want to check the TS and HS on my lathe. Could there be a problem with the lathe itself not level? Curious...from a new guy to pen turning.

 

[Paul in OKC]

HS/TS alignment have little to nothing to do with the issue you describe. It can be density of grain, bent mandrel, loose bushings....If you are using the stock wood turning live center, it may not be 60* and the point may be hitting the bottom of the center in the end of the mandrel. Had that happen when I first started turning. You can file the point off a bit, or get even a cheap 60* live center.