Expander/turning mandrels?

[russell m]

I am useing sinclairs mandrels & sizer & neck turners. My turning mandrels are .0012 & .0014 smaller than my expander mandrel. Is this ok. I have been told that the diff needs to be more like .0008 to .0010 diff.
Its time for me to hit the rack. I will ck back tomorrow. Thanks everyone.
russell m

 

[kansasvet]

The springback of the brass from the expander mandrel should be just right to fit the turning mandrel perfectly. In a perfect world.
I use a K&M, in 20 cal. the expander is .204 and the turning mandrel is .203
22 cal. the expander is .224 and the turning mandrel is .2225
6mm cal. the expander is .243 and the turning mandrel is .2415
6.5mm cal. the expander is .264 and the turning mandrel is .2625
30 cal. the expander is .308 and the turning mandrel is .307

I do not have the instruments to measure less than .0005 so they may actually be slightly different than my measurements.

I would think it difficult to determine the amount of springback for brass of various makes with different levels of annealing.

 

[BJS6]

I use the Sinclair turning gear as well.

I experimented with several mandrels and found that, for turning with a small bettery screwdriver as the method for spinning the cases, an expander 0.0008 larger than the turning mandrel was ideal for accuracy of the cut. Tighter made the job a chore and the additional twisting in the hands effected the accuracy of the cut.

You may find that the 1.4 thou bigger expander just a little loose, size and turn a few cases and see how it feels and check the accuracy of the turned necks. The 1.2 might work OK.

In theory slightly loose shouldn't matter since the cutting action will hold the case neck on the mandrel but that will only apply to a point.

Yoiu may even find that if the cases feel a little loose when just expanded they will be OK after sitting a day, the brass seemed to spring back enough over a little time to alter the fit just a smidge.

Bryce