bent , bowed mandrels....

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opfoto

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Dec 28, 2004
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I have 4 mandrels that are in need of straightening...This has become a major headache for me. :mad:
I mark a line along the shaft every inch or so then I put them in the adjustable collet and check with the dial indicator for wobble. I start to straighten with about an inch into collet working my way from long to short with a rubber mallet. Maybe I should use a sledge instead. After completing that I recheck the wobble all the way out and it seems worse than when I started. What am I doing wrong....(TBC is not an option right now)...?
 
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ldb2000

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There is no surefire method of straightening a bent mandrel . You might be able to reduce the amount of bow in the mandrel rod but you stand only a slim chance to ever get it really straight again . Switching to TBC will cost about the same as a new mandrel rod . All you need is a Dead center which costs about $10 .
 

opfoto

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Please do not take this as a sign of disrespect....esp to those that TBC and JohnnyCNC.

I do appreciate TBC as another option as was suggested and I will pursue that option in the very near future but for now I was hoping for a few suggestions as to how to fix my particular problem. I understand that if I TBC then I would no longer have this as a problem.
 

JerrySambrook

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Marc,
It is probably cheaper on your brain if you just buy some replacement mandrells. Unless you like tinkering to straighten something like that out, it can be a very futile "sport"

Other than that, turning between centers is a great way to go.
I do not totally agree that all you need is the dead center for the headstock, because my little bit of experience has shown to me that most bushings meant for the mandrell are not good enough to mount between centers, due to the fit between the bushing and the tubes.

Jerry
 

randyrls

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I have 4 mandrels that are in need of straightening...This has become a major headache for me. :mad:


Marc; It is difficult to straighten a mandrel once it is bent. Woodcraft sells bare mandrels for about $2.99 so that is your best option. You may want to reverse your process and start at the short end and work to the long. When you do this, you do not want the tail stock in contact with the end of the mandrel and you want the lathe off and unplugged.

I would roll the mandrel over a flat surface like a table saw or drill press. I consider the mandrel to a consumable; good for many pens, but when it is bent, I remove it from the MT taper and replace it.

I believe that you have one of the adjustable mandrels that can extend to different lengths. If so, does a new mandrel show as straight? There are many sources of run-out. You mentioned you have a DI. I would start by checking the inside of the MT taper on the lathe spindle nose, then check the inside of the MT mandrel adapter with no mandrel in place, then check the mandrel at the closest point to the headstock.

I had problems with bent mandrels at one point. I discovered that I was not getting the blank end and tube exactly 90 degrees to each other. I was "pulling" the mandrel crooked when I tightened up the brass mandrel nut.

Possible causes why a mandrel bends:

  • Too much pressure from the tail stock hand wheel.
  • Brass mandrel nut too tight.
  • Blank end and tube not at 90 degrees to each other.
  • Faulty bushings.
  • Head stock and tail stock not aligned center to center.
 

Canedriver

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The problem your going to run into is even if you get the mandrel perfectly straight it WILL bow or warp in the future. Pretty much EXACTLY where it did it before. Once bent no matter how much you straighten it, it is now weakest at the bend point so slightest pressure will bend it again in the same place.

I just ordered the TBC stuff from johnnycnc because of this.

If you have ever "pulled" or "popped" a dent out on a car you will see what I mean by this. Smallest pressure will create a dent in the same spot.

I actually bought a couple spare mandrels and caught my nephew "playing the drums" on my work bench with them :mad:
 

AceMrFixIt

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I bought a mandrel saver for my lathe. It operates like TBC without the hardware. Of course you will need a straight mandrel to start with. It will take the pressure off the mandrel and put it on the bushings and tubes. I noticed less wobble when I used it the first time. Give it a look.
 

bitshird

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Adamsville, TN, USA.
Marc, unless you have some precision V Blocks, a surface plate preferably cast Iron (granite would be too easy to chip) a decent Dial test Indicator, I would just go ahead and buy a new mandrel shaft, Or buy a 3 ft piece of .25 drill rod a 1/4 28 die and make your own,you could sand the mandrel down the few thousandths to allow it to fit bushings or go buy an OD grinder.
But all of these options other than making you own are going to cost far more than buying a few mandrels from Woodnwhimsies for 6.95 each.
I think I have discovered one reason mandrels get bowed/bent in the beginning is folks are using the live centers that come with the lathe, of course another thing is what is you conception of tolerance for straight, dead zero isn't going to happen even with a brand new mandrel!
I think I'd go the 6,95 route and get a couple and also if you don't have one get a real 60 degree live center, Woodnwhimsies also have them reasonably priced, and you'll be one step closer to turning between centers.
 

Mike Rinelli

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Mar 24, 2009
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Waukesha, WI
Hi Marc,

I had this problem too, no matter how careful I was, eventually, my mandrels began warping.

I solved the problem with the following items:

LCDOWEL Collet Chucking System with 5 Collets
http://www.pennstateind.com/store/LCDOWEL.html

and,

PKMSTS2 #2MT Pen Mandrel Saver
http://www.pennstateind.com/store/PKMSTS2.html

The Collet system is pricey, but I use it for a ton of turning, it screws on to the headstock, so it runs very true.

Haven't had any problems since.

-Mike
 

Daniel

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Reno, NV, USA.
Please do not take this as a sign of disrespect....esp to those that TBC and JohnnyCNC.

I do appreciate TBC as another option as was suggested and I will pursue that option in the very near future but for now I was hoping for a few suggestions as to how to fix my particular problem. I understand that if I TBC then I would no longer have this as a problem.

Marc, You ask for an answer that does not exist, It was stated but not emphasized in he very first response to your question.
"There is no surefire method of straightening a bent mandrel . You might be able to reduce the amount of bow in the mandrel rod but you stand only a slim chance to ever get it really straight again"

Now if you still want to overcome and cash in on that very slim chance, here is what I know about bending metal.

I will start by assuming that your mandrel is bent in the first place and that this is the reason you need to straighten it. When the mandrel bent the metal on the outside of the bend stretched in addition the metal on the inside of the bend may have smashed. The change to the end inside of the bend woudl not be as severe as the outside and may not have happened at all. when you try to straighten the metal it does not squish that stretched metal back into place (it will to some degree if you heat the rod first) and what you are actually doing is stretching the meatal that was to the inside of the bend. now you have metal on one side that has been stretched and metal on the inside that has been squished and then stretched. if the inside of the bend does not stretch then it will cause the stretched metal on the other side to bubble out. the distortion to the metal is not the same on each side and actually causes the rod to be off set along it's length. the metal is actually weaker on one side of the mandrel now than on the other. as well the strength of the mandrel as a whole has been weakened in that spot and will be more prone to bending again. it will in fact eventually weaken until it breaks if it is bent over and over. if the mandrel bends again it stretches the outside even more and will continue to do so every time it is bent again. this means that the more times you put it in your chuck and smack it with a mallet, you are just stretching it more and more. the actually way to straighten a rod is to bend it one way bend it to an equal degree the other and then press it into a straight line. Reheat it so that the metal can re align and then you have a nice straight but much weaker piece of metal. At the very best even if you can straighten the mandrel My bet is it will be to weak to turn pens on anyway.
 
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Marc,
I use a mandrel to turn my pens It would be good to find out why they are bending I have been using the same mandrel that I got from woodcraft a year ago. I tighten the tailstock until it touches the mandrel then go just a hair tighter. I also use the factory tailstock insert that came with my $200 lathe.
 
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