Trimming offcase head or Datum ?

[rsbhunter]

I have been using a WFT case trimmer, and I am happy with the ease and speed. I had been using a Lyman hand turn bench mounted trimmer, which is alot more labor intensive. Which one to use? Maybe a no brainer, but it makes me wonder which keeps case length more consistent . I anneal each time, use a Foster FL sizing die honed to my neck diameter, so my bump is really consistent. Just my OCD kicking in..I know that all top shelf trimmers , Girard, etc trim off of datum....Thanks, rsbhunter

 

[SeabeeKen]

How consistent were your cases using the WFT?

 

[rsbhunter]

Usually they stay within +/- .001-.002. That's probably good enough for the shooting I do, but my OCD kicks in and........even when I pull the fired brass to measure base to datum for fired length, I can get .001-.002 difference.... And using a vernier caliper that is only +/- .001 accurate, I guess it's not worth worrying about? rsbhunter

 

[Hoser]

Headspace gauges go off the case head, so I will stick with my Henderson.

But headspace gauges have almost zero to do with trim length.

 

[rsbhunter]

I know that the case length is ok, oal, as I own the reamer and have the print. I was just curious if there was a general consensus on the "better" method...Thanks for the reply's, rsbhunter

 

[SeabeeKen]

Seems to me that trimming based on the shoulder would be off by the variation in shoulder springback, and being off by about 1-2 thou.

 

[rsbhunter]

Possible, but as stated , I anneal every time, and check cases about every 2-3 to verify the base to datum point (set back) is accurate to .001 at most. They are sized within 5 minutes after annealing. Hopefully this keeps the sizing consistency accurate... rsbhunter

 

[jepp2]

Lube application consistency and dwell time in the die are potentially larger contributors to the amount of bump by your die (ignoring die adjustment of course).

Case length variations of the amount you are seeing (0.001-0.002") is way down the list of things you should worry about. And keep in mind, indexing off the shoulder means your case neck lengths are very consistent. A larger factor in accuracy in my opinion, than the case measurement from the case head to the case mouth.

 

[rsbhunter]

Jepp2, that is the info I was searching for....my case lube is the Lanolin/99% isopropyl mix, in a zip lock, rolled around and taken out to let the alky flash off. I'm getting good results on targets, but as always, chasing the possibilities.... Thanks for the info...rsbhunter

 

[D-4297]

Been using a Trim-Pro II with the three-way Cutter head for several years now . Cases come out to .000 - +.001 and very seldom more than +.002 ....Maybe I learned something after thirty-five years in the Tool Room .

 

[Mulligan]

Lube application consistency and dwell time in the die are potentially larger contributors to the amount of bump by your die (ignoring die adjustment of course).

Case length variations of the amount you are seeing (0.001-0.002") is way down the list of things you should worry about. And keep in mind, indexing off the shoulder means your case neck lengths are very consistent. A larger factor in accuracy in my opinion, than the case measurement from the case head to the case mouth.

^^^^ this is spot on^^^^
CW

 

[Ray123]

I wouldn't worry about .002" ...if it were me. The most accurate method of trimming case overall length would be off the case head or rim off the cartridge cause that's where you measure from. With the primer removed so it's presence doesn't interfere with your measurement. The case datum for headspace will vary slightly no matter how careful you are in sizing...and has nothing to do with case overall length...Unless you use a trimmer that indexes off the case headspace datum line...which will have slight variations built in...and a slightly less accurate way to trim case overall length by its very nature...but likely to too be much faster...and it's good enough for a more critical measurement...your rifles headspace!
So either method works and I wouldn't even be concerned about. 002" in total variation of cartridge overall length. I trim, located off the case head, in a bored to dia Hardinge deadlength collet, in a collet fixture mounted a Bridgeport mill...very fast, very accurate. But highly impractical for most, unless you already own these expensive tools for other purposes.

 

[rsbhunter]

Thanks to all for the replies, figured I was chasing shadows....my dad was a machinist at Pratt & Whitney for years, long ago. I could read a micrometer and dial calipers when I was 12.....remember the talk about being within 1/10000....makes .001-.0015 seem like a long road trip !!!! Than again , rsbhunter

 

[mikecr]

indexing off the shoulder means your case neck lengths are very consistent.

Except it's a lot harder to qualify your shoulder datum, given that it's angled, and varying.
And any error there is big to results.

Case OAL is easiest to measure accurately.
Headspace is more difficult, and adding one error (by some amount depending on angle & tool).
Then if trimming off shoulder, you're adding potential angle error AGAIN.
Well, maybe it will add/subtract,, maybe cancel.

This is similar to the base to ogive (BTO) people go through the trouble to measure -while failing to qualify ogive radius, which affects their nose datum. With this, they're acting on bad information.

 

[jepp2]

Case OAL is easiest to measure accurately.

That is how I measure. But you raise some points I hadn't explained. I use WFT trimmers. so the chamber matches the shoulder angle. I set my headspace the same, so my case head to datum is the same for rifles of the same cartridge. Means I don't have to reset my die or case trimmer to get identical results.
But all your points are certainly valid. With almost all measures based on a comparator with no standard basis, everything is just relative.

 

[rsbhunter]

That is exactly what I believe....everything we do (to a large degree) in case prep, is a degree of accuracy. Some measurements are easily quantifiable, others are taken from a datum point. With a datum point only being a reference point, it's the closest most of us have to measure with a repeatable result. I don't shoot on a national or even state level competition, just club matches, so I temper my reloading capabilities with the level of accuracy I'm at. I will continue to reach for perfection, but accept that I am doing this for fun...rsbhunter

 

[Ringostar]

Ok, here’s a question. If your cases are more consistent after firing, would you consider trimming before sizing?

 

[rsbhunter]

I was under the impression that trimming was done after resizing.....the case length changes during the FL sizing operation..... But, I could be wrong...rsbhunter

 

[Ringostar]

I was under the impression that trimming was done after resizing.....the case length changes during the FL sizing operation..... But, I could be wrong...rsbhunter

Oh, I do understand trimming after sizing. I was just throwing something out there, if someone is getting inconsistent sizing. Then using a trimmer that indexes off inconsistent shoulders. Would you consider trimming first, and plan for case growth from sizing.

 

[rsbhunter]

The problem with trimming before sizing presents problems in both methods of trimming...whit the bench lathe and the WFT is the neck is to large. The mandrel is loose, and the neck to large to fit in the WFT. I'll continue to size, then trim...I'll just watch the length on every firing...I appreciate all the info and advice from all....rsbhunter