Uneven turns (bent mandrel?)

[usfwood]

Greetings again.
I have a question, and at first glance the answer might seem obvious, but there may be another answer. I have turned several of the slimlines. I typically use the skew to plane down the blank to the bushings. Often, not everytime, the blank will be even with the bushing on one side and proud of the bushing on the other. Why would the blank not turn evenly. I have a fairly light touch with the skew as I get close, and do not feel I am putting enough pressure to cause the mandrel to deform.

The mandrel rod does not seem bent when I roll it on my table saw surface. And I do not believe that I am over cranking the tail stock in to the end of the mandrel. And since I know this question might come up, I am using the stock live center with the Rikon lathe. Oh yeah, I have the Rikon minilathe.

Thanks,
Darin

PS. They were Woodcraft pen kits and bushings.

 

[leatherjunkie]

I had this problem and still do sometimes.
reading here has helped me to fix it.

check the tightness of the nut on the mandrell.
if its too tight it will cause this problem.

what i do is turn to about 1/8" of the bushings then stop.
i then loosen the nut and rotate the pen blank about 1/2 turn on the mandrell, then snug up the nut.
after that i will go to skew and use light cuts to finish turning to the bushings before sanding.

this has helped my pen blanks to straighten out.
and i just got a beall collet chuck system with a 60 deg live center. sure makes a big difference to me.

hope this helps you out

 

[usfwood]

Thanks. I could try the rotation trick.
I have looked at the Beeal sp? chuck, and they sure look sweet. They are just a lot out of my price range. I was thinking of getting a length of 0.250 drill rod and making my own replacement rod. The drill rod is ~$2.60 from MSC for a 36" peice.

Has anyone else tried this? The drill rod os O1 oil hardening if I wanted to try to harden it after running the center drill and die.

D

 

[jtate]

Try putting your tool rest edge parrell to the mandrel and almost touching it. Turn the headstock by hand and the bend will probably become apparent.

I've been doing what Leon suggests on nearly everything lately - just in case there's a slight bend of which I am unaware. I only turn it a quarter turn though. Just 'cause I read quarter turn somewhere ion this site.

Additionally, I've been removing both everything from the mandrell and replacing it on the mandrel in the reversed order, just before I take my last cuts. That is, if the point of the pen was toward the tail stock, I reassemble everything on the mandrel so that the point is toward the headstock.

These two things have made a difference for me. Maybe it's just hocus-pocus but I ain't arguing with it.

Also, the place on the mandrell that will be least susceptible to this is the point closest to the headstock so that should be where the part of the pen is where you most don't want this to be issue - usually that's the nib end of things.

 

[jtate]

Also, I have better luck when I run the lathe really fast while cutting (and really slow while sanding). Others may have a differnet approach.

What sort of speed are you using?

 

[RussFairfield]

The mandrel is not always at fault. The differences in grain orientation on opposite sides of the blank can cause out-of-roundness from the cutting tool and sanding.

 

[Randy_]

Whether or not it is the cause of this particular problem...and it certainly could be...you need to get yourself a 60° live center. The stock live center is almost certainly a 30° center designed to be used with wood and not metal.

 

[Tangboy5000]

As you can see there are many reasons for this problem. When it was happening to me I asked around and had someone watch me turn a pen. I was just pushing too hard and warping the mandrel.

 

[ctEaglesc]

I will guess in most cases the Russes answer is the cause.I have found that the case from the amount of glue ups I make.I like to use "end grain" versus long grain of the same woods for slight color differences in my blanks.
Many times it is a combination of factors.But end grain is harder and doesn't sand the same as lng grain.Especially if the endgrain has been stabilized with CA.
I went and got a 60* live center.I don't think they are the panacea they are made out to be.
The mating of a live center is similar to a minature Morse taper.They mate in a mandrel slope to slope as it were.
The angle has nothing to do with how it fits as long as it does not bottom out in the depression.The shoulder of the tip of the live center "rests" on the end of the mandrel.As long as the tail stock and head stock are in alignment the shouldn't be any movement laterally.
JUst my observation.
I have read instances where the introduction of the heavy duty 60* centers did not solve any problems at all.
To me it is just a live center and since I have 3 lathes a live center is just that something to keep the other end of the mandrel steady.

 

[Randy_]

Originally posted by ctEaglesc
<br />.....The mating of a live center is similar to a minature Morse taper.They mate in a mandrel slope to slope as it were.
<b>The angle has nothing to do with how it fits </b>as long as it does not bottom out in the depression.The shoulder of the tip of the live center "rests" on the end of the mandrel.As long as the tail stock and head stock are in alignment the shouldn't be any movement laterally......

This subject is difficult to describe with written text....pictures would be a big help. If I understand correctly what eagle is saying...and that is not a given...his statement is misleading at best and just flat wrong at worst! The angle "DOES" matter. A quick check of a HS geometry book will show that a 30° live center tip will never rest, shoulder to shoulder, in a mandrel with a 60° centering hole, no matter the size of either......it just ain't possible. The tip of the LC will always bottom out in the hole in the mandrel unless there is a clearance hole at the bottom of the centering hole. If there is a clearance hole, then the contact will be a ring configuration; but will not be a smooth shoulder to shoulder fit like a Morse taper. That being said, when both the mandrel and the LC are new one should still be able to turn round pen barrels. The problem is that over time, the incorrect contact between the tip and the mandrel will cause the tip to wear....a well documented occurrence by the many posts here at IAP. Tip wear could cause the LC to be off center in the end of the mandrel which could cause out of round pen barrels. How much of a problem this is, I don't really know and there certainly is no argument that there are many other potential causes of out of round blanks as mentioned by others in this thread. It is also possible that other causes are more likely to be the source of your problem.

Nonetheless, as I stated earlier, whether or not it solves your particular problem, you do need to use a 60° LC for best results when turning lens on metal mandrels.

P.S. For any who are interested, you can display the degree symbol by holding the ALT key and typing "248" and then releasing the ALT key.

 

[ctEaglesc]

As far as your last statement is concerned based on the few pens you have turned(or at least shown) I would have to disbelieve you. Based on the amount of pen I have turned and shown without one I Stand by my original statement it is not necessary.
If you can do it you show it if you can't you just write about it.

 

[Randy_]

Originally posted by ctEaglesc
<br /><b>As far as your last statement </b>is concerned based on the few pens you have turned(or at least shown) I would have to disbelieve you. Based on the amount of pen I have turned and shown without one I Stand by my original statement it is not necessary.
If you can do it you show it if you can't you just write about it.

I take it that you don't disagree with my first statement....the one where I said you were wrong about the angle thing??

 

[ctEaglesc]

Like I said those than cannot <b>do</b> just write.
My first 200 pens were done with a standard 30* live center.
I saw no difference in an expensive 60* live center.
The pens I make out of my blanks reguire a little more precision than drilling a hole in a solid piece of wood.
A long time ago I stated I looked at members albums to view the credibility of their statements along woth other information.Posts etc.I have seen your album.You write a good game in teal but that's about it.

 

[usfwood]

Wow, such controversy, over the angle of the center drill in the end of the shaft. First let me say that I can agree with both of you. The center drills usually do have a pilot that provides clearance at the tip, and would give the ring contact. I have seen wear on the live center of the lathe at woodcraft due to the mandrel. And I do believe that this could throw out the alignment/runout of the mandrel where it contacts the live center. But I believe also that if the live center does not bind or slip when started, the live center will not wear. It all depends on the friction of the bearings, starting inertia, etc. For that reason, I can see why some people would not run into problems using the stock live center.

Regardless of the above, I found the runout was being caused by the collet. The axis of the collet with relation to the morse taper axis is way out. This appears to have been causing a bow in the mandrel rod when the collet was tightened before the live center was engaged. I will try to clean/polish the inside of the cup where the collet is compressed to see if the alignment can be improved. The collet and collar do not seem to be precision machined. If this does not work, my drill press chuck has a #2 morse taper, and will run much more true than the pen mandrel collet/holder assembly.

I will let you all know the results.

By the way, the bushings and pen sleeves are not the same fit on the mandrel shaft and there seems to be a few thousandths of play between the busings and the rod. I do not know if this is enough slop to cause a problem or not.

Thanks to all for the replys, I think I will also try the blank/ bushing shuffle on my next pen. It makes good sense.

 

[loglugger]

May take on this is that all mandrels are not made the same, some have a recess the point of a 30- degree live center will go into and other don’t witch will cause the center to wear and not remain centered all the time. If you use a 30-degree center very long it will wear a ring on the taper and will get off centered more. I used a 30-degree center for the first 100 or so and them started to have trouble because of wear, got a 60-degree and the trouble went away. I put the mandrel in the lathe, put the blanks and bushings on lose, bring the tailstock up close to the mandrel and turn the lathe by hand to make sure the mandrel runs true, then tighten the nut that hold the blanks tight, then turn the lathe by hand to make sure it still runs true. Glue in a tube or on a bushing, a burr on the end of a tube or the blanks not true will cause the mandrel to wobble. If everything is true then the tighter you tighten the nut the stronger and straighter the mandrel will be.
Eagle, You won’t find any that I have shown ether, If I could make this camera work I might think about it. If you will go to the Freedom pen project you will know that I at least made 6 or 7 hundred in the last 3 years but you won’t be able see any of them.
Bob