Coincidentally got two new tools yesterday and learned a bit about expanding mandrels

[ntsqd]

The new tool that started this is a Mitutoyo digital micrometer with resolution to .00005" To me that last digit is the round-off direction, not an actual size measurement; if it is 0 then I round low, if it is a 5 then I round high. The other tool is a Porter Precision Products expander mandrel system. Porter sells the pins in two sizes for each caliber and I bought both sizes for each since I don't yet know what I'm going to want for neck tension. My thanks to whomever it was that posted a link to their page a while back, truly impressed with the product & wish that I had known about it before buying the Sinclair system.

That said, out of curiosity as to what diameter they actually are and to try out the new mic than anything else I mic'd a couple of the Sinclair TiN coated expander mandrels. I found them to vary in diameter by as much as .0005" I really wasn't expecting that. I guess the question is, is a .0005" out of round acceptable?

So then I mic'd some of the gauge pins that came with the Porter expander mandrel system and I got the same exact reading each place that I measured any of those pins. If the pin was marked ".223+" and I got .22305" in one location, I got ".22305" in all locations that I measured on that pin. I really would expect that given what gauge pins are normally used for.

 

[Mikecr]

With such a hyper-focus on this, you might consider that up/down sizing(beyond springback) does not set tension.
You are expanding only for a desired interference.
Ideally that interference would be no more than springback from cal, which is what seating bullets will expand to anyway. So this would be with a mandrel at or slightly below bullet diameter.

The difference, in this function, is that you can pre-seat using a hardened mandrel instead of soft bullets as pushed against their soft noses. Function #2 is to drive ~1/2 thickness variance outward, away from bullet seating interference, to lower loaded TIR.

Before you bust heavily on Sinclair's offering, consider that this IS a standard system in neck expansion system.
The die allows the mandrel to float. The mandrel has a rounded nose.
In contrast, pin gages are not expander mandrels, and collet hold of them is not floating.
I'm hoping that diversity in aftermarket mandrels(carbide, TiN, sizes, shapes) will continue to grow -for The Sinclair/PMA die atleast.

Neck tension is the force provided by springback against a given area of bullet bearing. Force x Area = PSI
This PSI of grip is adjusted by sizing LENGTH, and by attributes of the brass(thickness, hardness, alloy, diameter, etc).
Hopefully your mic can measure neck thickness, as it means more to tension than expander thickness.

 

[ntsqd]

The discussion of how to set neck tension is not why I posted my observations.

The pins supplied by Porter have had a lead-in angle smoothly ground into them. Something that a reasonably careful home hobbyist can do to. If you want to fine tune how far the neck is expanded you can't easily do it with the Sinclair et. al. system because the small increment in pin sizes does not (yet?) exist. This is easy to do with gauge pins, order the size that you want to try and carefully grind the lead-in angle on one end. Were I to guess I'd say that the Porter pins were ground with a Scotch-brite wheel or they were dressed with one after grounding with a very fine grinding wheel.

My Sinclair pins fit in my Sinclair mandrel die body tightly enough that they have no body to pin float of any significance. No idea how others make their parts.

A friend once commented to me that the nice about standards is that we have so many to chose from.

That said, I was and am not diving in that deep. I'm just reporting my observations. Mostly that I was surprised that the Sinclair pins that I checked were not as round as a gauge pin. Is this level of error a problem? It IS a pretty tiny deviance and I doubt that many would have ever noticed any effect from it.

 

[nksmfamjp]

I have a hard time worrying about errors under 0.001" unless they stack up and impact loaded round runout.

 

[ntsqd]

I too felt that the error size was so small as to be in the noise for most of us. I just surprised to find it at all and thought that there may be some folks out there to whom this might matter.

 

[WapitiBob]

The 5 tenths might be the coating job but there shouldn't be close to that much buildup. Out of round by 5 tenths is just sloppy work. Not sure how you grind something out of round like that.

 

[ntsqd]

Agree, the plating shouldn't be anywhere close to the same thickness as the deviance in OD.