Dwell Time When Sizing -- Any Actual Basis in Fact??

[jds holler]

This is a subject I've seen mentioned a few times here, and I'd like to know if anyone with metallurgical knowledge could verify or refute the process.

The "process", basically being the pause of a few seconds or more, leaving the case inside the sizer die to achieve a more thorough shrink of the case. I guess the theory in question would be that the dimensional change of the brass isn't instantaneous, but might happen to a higher degree with the passing of time.

I don't want to offend anyone by bringing the process into question, but would like to find out if it's a bunch of hooey.-- or not. jd

 

[Dusty Stevens]

If you ever get some cases with very inconsistent bump give it a try.

 

[BoydAllen]

My goodness, one would think that anyone with a press, fired brass, and a suitable FL die, oh and yes, case lube could have confirmed this quite easily, assuming that he had the proper tool to measure from datum to head. Yes, it is absolutely true. I did the test a loooong time ago. I would think that the effect would be less extreme with new brass than with old, work hardened brass, but of course we would have to test to really know, given the minimal value of a "you would think".

 

[LHSmith]

I have better control on bumping cases that resist initial sizing by adding a thou or so with shims -or - with a JLC die adjuster. I know some claim it helps to increase dwell time but it's too hit or miss for my liking.

 

[ridgeway]

Test it

 

[Keith Glasscock]

I know what I do in the reloading room is based on the testing I have done.

I guess the question is whether it is worth testing. The answer is and emphatic yes.

 

[tmwinds]

If the brass is not bumping consistently a proper annealing of the brass will help more than anything I've tried. I do delay slightly at the top of the stroke before lowering the ram. I set my dies to bump the shoulder .002 and I can hit this target everytime if the brass is freshly annealed.

I know fguffey says that you can not bump a shoulder but I have tools that will, a FL die and press.

 

[Bart B.]

With the ram topped out at the end of its stroke sizing a fired case with several thousandths shellholder clearance to the die, watch the gap between shellholder and die close down. When the space between them no longer gets smaller, its as far into the die as the case will be.

Take 10 fired cases, resize 5 lowering the ram as soon as it tops out. Resize the other 5 keeping the ram topped out for 10 seconds before lowering it. Compare case headspace average across both sets of cases.

 

[chkunz]

With the ram topped out at the end of its stroke sizing a fired case with several thousandths shellholder clearance to the die, watch the gap between shellholder and die close down. When the space between them no longer gets smaller, its as far into the die as the case will be.

Take 10 fired cases, resize 5 lowering the ram as soon as it tops out. Resize the other 5 keeping the ram topped out for 10 seconds before lowering it. Compare case headspace average across both sets of cases.

Then report your results here for all to see. I will if you will.

 

[ClassCat]

This is a subject I've seen mentioned a few times here, and I'd like to know if anyone with metallurgical knowledge could verify or refute the process.

The "process", basically being the pause of a few seconds or more, leaving the case inside the sizer die to achieve a more thorough shrink of the case. I guess the theory in question would be that the dimensional change of the brass isn't instantaneous, but might happen to a higher degree with the passing of time.

I don't want to offend anyone by bringing the process into question, but would like to find out if it's a bunch of hooey.-- or not. jd

Don Getner was a Senior Research Metallurgist Specialist for both University of Pennsylvania and Drexel and an avid bench rest shooter. His research on this was inconclusive based on variance of Brass lots (consistency) He was also a GREAT Bullet maker

 

[Prose]

I just did a dwell test two nights ago between once fired and 5x fired lapua 260 brass. All annealed every firing. No dwell, slight dwell, and 2-3 second dwell all produced brass with the exact same measurements. Reason for doing the test was out of the same curiosity of the OP. Was kinda surprised with the results considering how often it is brought up. Test sample for once fired was 25 as well for the 5x fired. All brass was same lot.

I will continue to periodically measure to ensure the desired results, test be damned.

 

[riflewoman]

The reason the tests have either been inconclusive or showed no benefit is the structure of metals. Metals are crystalline in nature and tests show that creep in metals takes a long time unless you are at a temperature above a third to the melting point of the metal in a absolute temperature. Lead creeps because it’s melting point is 1100 degrees Rankine (absolute in Fahrenheit). And room temp is 530 R. Brass melting temp is 2170 R. So we don’t meet the criteria.

What you see in sizing variances is the variances in brass hardness, thickness and variances in the yield points of the brass locally. This is where a refined grain structure makes it more predictable.

But what do I know, I’m not a denizen of a “hide” somewhere...

 

[jds holler]

Thanks all for replies. I was kind of skeptical about this -- maybe still am. I like the Bart B. test mentioned, and will try it.

With the ram topped out at the end of its stroke sizing a fired case with several thousandths shellholder clearance to the die, watch the gap between shellholder and die close down. When the space between them no longer gets smaller, its as far into the die as the case will be.

For the particular loads I'm doing right now, I'm using brass of very dubious background, and getting some inconsistency in headspace. Nothing serious, but noticeable with some brass. Maybe a little dwell time will bring more consistency.

I'd still love for a genuine metallurgist to step forward and say either "Hell Yeah!" -- or "It's all BS!" jd

 

[riflewoman]

I was a structures design engineer for a major aerospace Company in the Seattle area for more years than I like. Does that count?

 

[dmoran]

I incorporate dwell time into my press operations, primarily for case neck tension consistency more so then for shoulder bump, but also for seating equality with bullet seating. My own results are what I care about, which hold dwell time worthy of both consistency and equality gains.
Donovan

 

[Murray Brook]

I always found that as my brass aged I would need to re-adjust my sizing die to achieve my .002-.003 shoulder bump.Then I read that pausing at the top of the sizing stroke would help this. I took 5 of my 25/06 cases that were fired 6 times and were having this shoulder bump problem .Sizing them with no pause gave me a reading on my Hornady comprator of 2.031. Taking the same cases and pausing appx 3 seconds at the top of each stroke with a 180 degree rotation gave me a reading of 2.029. Works for me.

 

[Damon555]

When I started reloading....pre-internet.....I thought I had the process figured out. My loads shot very well as I gained experience over the years. Then the WWW enabled everyone to communicate varying techniques easily. It was amazing the ridiculous things that people were doing to try to improve their loads.....Of course I had to try many of them just out of curiosity.....After no statistically significant improvement I never bothered with them again.....This process appears to fall into the insignificant improvement category.

Us handloaders are an interesting lot.....for some reason snake oil seems to be an integral part of our repertoire.

 

[jds holler]

I was a structures design engineer for a major aerospace Company in the Seattle area for more years than I like. Does that count?

I would imagine that your credentials would include at least a four year degree which I know includes a fair amount of metallurgy and materials properties -- perhaps much more. Soooo, what say you concerning the elasticity vs time with our brass cases?? jd

 

[Damon555]

The reason the tests have either been inconclusive or showed no benefit is the structure of metals. Metals are crystalline in nature and tests show that creep in metals takes a long time unless you are at a temperature above a third to the melting point of the metal in a absolute temperature. Lead creeps because it’s melting point is 1100 degrees Rankine (absolute in Fahrenheit). And room temp is 530 R. Brass melting temp is 2170 R. So we don’t meet the criteria.

What you see in sizing variances is the variances in brass hardness, thickness and variances in the yield points of the brass locally. This is where a refined grain structure makes it more predictable.

But what do I know, I’m not a denizen of a “hide” somewhere...

Please don't confuse us with the facts! When people actually think that a .09% improvement is statistically significant there's no convincing them otherwise.

 

[riflewoman]

As I alluded to in my post up thread, there are variances induced with the variances in actual yield point of brass, the actual strain induced in the case, thickness and grain structure of the cases themselves. Think about this: the ammo company presses operate at about 50 strokes per minute. That means the cases are formed in stages but the total time in a die is about half a second. From straight cylinder to tapered and necked case is only 4 or 5 stages. If there was any significance to the “dwell” theory, then the presses would have to run much slower.