Need advice on steel to use for Pen Mandrel

"Wheelz" wrote: I'm a woodturner and mostly turn pens. I use a pen mandrel with a 1/4" rod that bends just way too easy and results is out of round turnings. (clip) ^^^^^^^^^^^^^^^ I am also a wood turner, and a mechanical engineer. Your problem is not in the composition of the steel rod. Steel has a modulus of elasticity that hardly varies with the type or the heat treatment. Mostly what changes is the yield strength, and you are not reaching that level of stress with your pen mandrels. My guess is that you are applying too much compression with the tail stock, causing the mandrel to buckle. Just bring up the live center firmly enough to eliminate any play. Then, if you want a little more support, use the fingers of one hand (the one not holding the tool) behind the spinning wood. You should be okay.

Wheelz,

The DEFLECTION of your mandrel is quite independent of the type of STEEL it is made of.

However, heat treated steels can take larger deflections before the deformation becomes permanent.

My choice of steel would be SAE 4140 heat treated to Rc 44 to 46, then finish ground. If that is not possible heat treat it to Rc 38 t0 40 and finish turn using tungsten carbide tooling.

If all else fails make the mandrels from SAE 1144, also known as "stressproof". Not as good as the options above but far better than using cold drawn steel rod or soft (ie. non-heat treated) drill rod.

Just to re-iterate: This will not reduce the deflection but will reduce the likelihood of permanent deformation.

To reduce deflection (vibration) the mandrel would have to be made larger in dia., or shorter in length, or made from a "stiffer" material such as tungsten carbide. Unfortunately these are the only options available for increasing stiffness. But as stated earlier, using heat treated steel reduces the chances of permanently bending, and scrapping, the mandrel.

Trust this helps.

Wolfgang

Is it getting bent while roughing a square blank down to round?

Wes S

As everyone noted, different steel won't help the bending issue. What will help it is to rework or buildup a custom lathe setup so that the mandrel can be held under tension, not compression. Given the small size of the work I would think you could rework one of the cheap mini metal lathes quite readily for that purpose.

Pete C.

Your mandrel would bend quite a bit. But even a tiny center in the rod, and a live center in the tailstock would make a huge difference. ERS

"Wheelz" wrote in news:1152930540.982285.283110 @h48g2000cwc.googlegroups.com:

You could make one or have someone make one out of carbide. Just buy a carbide drill blank the appropriate size, braze a bit of steel bar to the end, chuck it in a lathe and face, center drill, turn, and thread the steel end.

You'll break it before it will bend.

Huh? How the heck will keeping the mandrel under tension cause it to bend? Perhaps you didn't understand what I said, I'm suggesting modifying a small metal lathe to use a pen mandrel that is kept under tension, not using a conventional tail stock. Something like a cone fitting at the end of the head stock spindle opposite the chuck to hold the mandrel rod centered in the bore. Let the chuck clamp on the mandrel for the drive and have modified tail stock that is hollow with a thrust bearing and centering cone on the far end. Mandrel under constant tension to reduce whip / bending.

Pete C.

The OP says "it bends" and "results in out of round turnings" so the trouble is NOT deflection. You can get perfectly round turnings with a mandrel that deflects. But once it bends, you're done.

It appears that the mandrel rod is held in tension so a stronger steel could be cranked to a higher tension, uses the wood as a column in compression. Adding a center point hole to the end of the rod for support with a dead center would do wonders.

I agree that 4140 at Rc 44 would be a good choice.

snipped-for-privacy@hotmail.com wrote:

RoyJ wrote: ...

...

A wooden pen shell isn't strong enough to significantly tension the mandrel. The wall of a finished shell usually is under a millimeter thick; some that a friend of mine made have about half-mm wall.

The OP hasn't said whether his/her setup uses a center, live or dead; it doesn't really look like it for pictured mandrel (

), but some previous posters have assumed the bending occurred with center support in use. Anyhow, I imagine that center support, if not already in use, could make a big difference.

-jiw

"James Waldby" wrote: (clip) The OP hasn't said whether his/her setup uses a center, live or dead; it doesn't really look like it for pictured mandrel (clip)^^^^^^^^^^^^^^^^^^ The link shows two mandrels without tailstock support, but they are also shown without headstock support. That is because those are catalog photos. In use, the mandrel with the appropriate Morse taper is inserted in the spindle, AND A LIVE CENTER IS BROUGHT UP TO THE OTHER END.

If this had been posted to rec.crafts.woodturning or to the pens newsgroup, everyone would have known this. So I repeat what I said earlier--the problem is undoubtedly caused by too much force on the live center. This causes the mandrel to bow, and will cause the middle of the cuts to be off axis after the bow is relaxed.

Greetings Pete, I responded to the wrong post. What I meant was that using the pen mandrel with the end unsupported would indeed cause it to flex. But with support from a live center in the wood lathe tailstock it would flex much less. Sorry about the confusing reply. ERS