Bent Mandrel?

[Chrisjan]

I quickly turned a pen this afternoon from hard acacia (or in Afrikaans Doringhout), but was shocked when I took it of the mandrel. The damn bottom barrel was turned off center. Not Out of Round though; I checked it on a hole stencil -its round alright but the brass tube is to the one side making the wood thinner on the side. Went back out to my lathe to check the mandrel; it doesnt look bent.

I dont have a clock gauge to check it, any ideas on how to check or how to fix...? Or should I just get a new one? Its a MT2:question:

 

[SDB777]

Ah, you'll soon be turning between centers......this was the reason I switched. A little much pressure on those cheap pieces of steel and bingo...she's bent.


Did you at least 'save' the piece or is it too far out to bother with assembly?







Scott

 

[SCR0LL3R]

If your mandrel mounts in your morse taper, clean both inside your lathe's taper and the tapered part of the mandrel. Also if it is held by a collet chuck check that that's all clean as well. I had a pen come out quite out of center like that and my mandrel seemed like it was slightly bent when I checked it. By cleaning the morse tapers and checking the collet chuck, my mandrel was once again near perfect.

 

[Chrisjan]

@Scott - Assembled it but it doesn't look nice at all - I thought you wont really notice i, but you do. I'll tap them out with the punches to reuse the parts again... As for the wood, will just turn it off once the mandrel is straight again

@SCROLL3R - I did clean the tapers both sides again and will do the collet and rod too before I do another test piece first

 

[GoatRider]

Another thing that can cause this is the end of the blanks not being square. When you tighten down the mandrel, the gap closes, bending the mandrel. It might not hurt the mandrel, it probably doesn't go past elastic deformation. But that's what pen mills are for.

 

[terryf]

Doringhout is very hard - are you using HSS or carbide tools? If its the 8 in 1 cheapies then its probably due to blunt tools and too much pressure.

I told you the other day that I only turn one barrel at a time on a mandrel for this very reason.

Take the rod out of the mandrel completely and roll it on a flat surface - you should be able to see if its bent or not.

Like Scott suggests, get a 60 degree live and dead centre and turn between them. Problem solved!

 

[ed4copies]

Check the bushings---is the hole in the middle, or is IT off center?

I have found many are not drilled dead center.

 

[Constant Laubscher]

It think the hole in the bushing bigger than the mandrel or as Ed said the hole is not in the center.

 

[Jim Burr]

All the more reason to lose the mandrel and go BTC. Some of the "hard as a rock" woods will do this every time. I think a new mandrel rod is about 12 bucks from CSUSA...I haven't bought one in years, but I seem to think it's close. That would have paid for a set BTC bushings.

 

[Chrisjan]

I dont think it wiil be the bushings - was a 7mm tube. I mostly use only one tool when cutting most blanks - a Proper HSS fingernail gauge and I regulary sharpen it, but it might've been a bit dull I'll admit... BTC are gonna take longer right and I still need to 'see' both parts turning to know what the damn thing will look like... Has anybody had success with the mandrel saver before?

 

[okiebugg]

pressure

Many times I have experienced similar problems....back in the day.

Most of the time, I find that if a blank is loose while turning, you tighten the nut on the mandrel. This puts pressure on the length of the mandrel and causes it to temporarily bow while turning and causing OOR problems. I always check, even to this day, the round status of every tube/blank I turn about halfway through the process to make sure I havent overtightened the nut. This over tightening can also put so much pressure on the ends of your blanks to the point that I always 'kiss' the ends to the end mill after turning. Works for me.