Turban Präzipress

[JimSC]

I imagine the very looseness of the ram is centering things up.

That is how it works. The "looseness" in the shellholders on the tight tolerance presses is why they do not damage the brass when sizing. On the coaxes the float in the dies accomplishes the same thing. Same with the float in the turrets on most turret presses

Don't get me wrong the Prazi is a gorgeous piece of machinery and I would enjoy having one on my bench but that new barrel, jug of powder, or case of bullets always seems to take priority

 

[BoydAllen]

That appears to be the calculated SD. It was calculated, not measured directly.

All SDs are calculated. If a chronograph shows SDs it is because it has been programmed to calculate it. Perhaps it would be good to read, in detail, what SD represents and how they are calculated. I think that many who refer to them do not know these things.

 

[WSnyder]

So we have reloading components being measured to seven places?

PT Barnum is once again proven right.

Those are SD numbers, calculated not measured. He's not claiming he "measured" anything to seven places.

 

[BoydAllen]

Those are SD numbers, calculated not measured. He's not claiming he "measured" anything to seven places.

Perhaps someone can check my memory on this. I seem to remember that the results of calculations should only show the degree of precision of the least precise measurement.

 

[jimmymac]

If you measured to X digits then the calculated result needs to be X digits.

 

[WSnyder]

Perhaps someone can check my memory on this. I seem to remember that the results of calculations should only show the degree of precision of the least precise measurement.

My guess is that he's simply plugging the measured numbers into a spreadsheet that spits out the SD's and right or wrong going with it for simplicity sake.

How many significant figures for standard deviation? | semaths.com

Answer: Since the standard deviation can only have one significant figure (unless the first digit is a 1), the standard deviation for the slope in this case is 0.005.

semaths.com

 

[RegionRat]

Yes, in the real world we report the two true digits plus the one uncertain if we use the SD for more calculations, for example taking a QC sample of a lot and estimating the ES of that lot. That just helps prevent rounding errors when you use that number in more calculations.

When we just report an SD of something as a description of how scattered the data is around an average, one or two digits is "good enough in court"...

There are two basic kinds of SD numbers in our game here. One is where you actually measure every piece in your population, the other is where you take just a sample of that population.

The only reason to watch out for how we throw these numbers around, is that if you take a sample of something for an acceptance criteria and use it to judge the stuff based on an extrapolation of the SD and use it against some tolerance.

To be fair to folks, you give them the benefit of those certain digits plus the one uncertain, and then round that product off to the certain ones. Folks don't argue it until there is lots of money on the line and it is the difference between a deal or a law suit.

 

[JimSC]

Perhaps someone can check my memory on this. I seem to remember that the results of calculations should only show the degree of precision of the least precise measurement.

which explains my point about the float, slop, play or whatever term you care for in the shell holder and why all those measurements in that review are meaningless

 

[BoydAllen]

which explains my point about the float, slop, play or whatever term you care for in the shell holder and why all those measurements in that review are meaningless

I don't think that they are meaningless just because he wrote down too many places after the decimal place. My point is that according to the results, the tightest fit did not produce the least runout. Look at the first and last columns of the table and then look at which presses did the best in each category for runout.

 

[RegionRat]

Jim, Boyd, I have no horse in the race... but it sounds like you guys mostly agree.

The shell holder becomes a place where there is a built in degree of alignment freedom in what is holding the case head.

That shell holder "floats" the brass to the die with values that are much larger than the ones being shown on the press ram designs, so there are clearly more things at work when we take into account what it takes to produce well prepped brass than how tight (or loose) a press is made.

 

[Ultimag]

I'll stick with my dillion 550c &my area 419 zero press they everything the parazzi does and they are made in the usa

 

[BoydAllen]

Jim, Boyd, I have no horse in the race... but it sounds like you guys mostly agree.

The shell holder becomes a place where there is a built in degree of alignment freedom in what is holding the case head.

That shell holder "floats" the brass to the die with values that are much larger than the ones being shown on the press ram designs, so there are clearly more things at work when we take into account what it takes to produce well prepped brass than how tight (or loose) a press is made.

A while back a friend did some testing. Using a case and FL die in his Rock Chucker, he raised the case until it just contacted the die, at the base opening. (I think that he had the bushing out of the die.) then lowered the ram and pushed on the case to see if there was any movement. He did this to the rear and to both sides, with the shell holder opening to the front. What he found was that he could move the case a little left and right, that he could not to the rear. At my suggestion he had already reworked that shell holder, reducing the diameter of the flange that fits in the ram slot and the necked down portion, both to allow more freedom of movement, so when he discovered that he may have not had room for alignment to the rear, he chucked up the ram and opened up the top of the shell holder cut so that it looked like an old fashioned key hole and that gave him what he needed. Evidently the restriction had been in the necked down part of the shell holder.
In any case, after the mod, his case runout was very slightly improved. Looking at some of the older press designs, the shell holders were retained by a couple of set screws. I think that it would be interesting to come up with a ram design such that a shell holder could be adjusted to center. Related to this, I have replaced all of the spring clips in all of my presses rams with O rings.

 

[JimSC]

Related to this, I have replaced all of the spring clips in all of my presses rams with O rings.

I did this myself a while back.

Sorry if I offended anyone and do not want to beat a dead horse. That being said full length sizing is a basically a cold extrusion process. When the case is inserted into a FL die it is being compressed both on the axial and laterally. There is no movement possible. That is why lube is necessary and if they are not lubed the cases will bind to the die.

Let's assume that theoretically you have a press where there is no movement possible in either the case or the die while that case is being forced into the die. With zero float if the case is misaligned by .001 in X direction then that case will be bent .001 in X direction as it is being compressed. With float in the shell holder or the die the case is free to move over .001 and center itself within the die with no damage. I wish I was a graphics guru, that would be very obvious with some diagrams and or a video

As I said earlier I may buy myself either a Zero press or a Prazi myself one day. Not because I think it is necessary to make better ammo but simply because I admire bneautifully made gear. One of neighbors owns a vintage 67 Rolls, does it get him to the store any better than my wife's Honda ? Not really but it sure is a work of art to ride in. That is how I look at both the Prazi and the Zero Press.

 

[BoydAllen]

One fly in the ointment that has nothing to do with the press is the neck bushing, if your die has one. For most calibers the neck hits the bushing before the body of the case contacts the die. There is a new innovation that covers this from Short Action Customs. I have not tried one yet, but you might venture a look if you are interested in that sort of a thing.

 

[JimSC]

One fly in the ointment that has nothing to do with the press is the neck bushing,

That is one reason I switched from Redding S Style bushing dies to one piece dies. My reasoning is that one piece dies are cut with a single through reamer same as the chamber is cut. Then I expand with a expander mandrel. I still have some minor ( .003 - .005) concentricity issues with brass that has been fired through a AR. In theory the one piece FL die and mandrel method should cure that

 

[SG4247]

Just one observation:

I used the RCBS max press for years. The Redding, Forster and other popular dies I have were all
loose in the threads of the press, so an o-ring was added to all dies to “take up” the excess clearance in the threads, and allow the die to float. This worked pretty well, but I noticed the die cocks slightly or shifts when sizing some brass cases that are tight in my die.

In the Prazi press, the die threads are an exact fit. My dies thread into the press with very, very little clearance. I still used the o-ring, but it doesnt do much for float, because the die is tight in the press threads.

The data I have shows the Prazi makes better cases than the RCBS press for runout using the same die. I am wondering if that is mostly due to the die being constrained to be perfectly perpendicular (square) with the shell holder surface - due to the better machine work done on the Prazi.

I use the same shell holder in both presses.

 

[JimSC]

I use the same shell holder in both presses.

is the slop in the shell holder greater than the slop in the dies with the RCBS ?

Like has been said, the precision of any system is only as good as the component with the least precise tolerance

 

[SG4247]

The shell holder is tight in both presses.

My observation above was to highlight where the precision is possibly needed, to make a better, straighter, re-sized case. Just trying to figure out what really matters in the manufacture of a good vs not-so-good press.

 

[Dusty Stevens]

If you want good ammo out of the super high tolerance presses something better move. In this case the shell holder needs to be extra sloppy. Unless the shellholder and die is machined into those uber tight tolerances you are magnifying any tolerances you have in the die or the shellholder (which are awful btw)

 

[SG4247]

Huh, I am inclined to agree, that a loose tolerance low cost RCBS quality type press better be sloppy on the shell holder or die threads or both, in order to allow floating alignment between the brass case and the die.

Seems a little counter intuitive at first, but that is why the o-rings help significantly on my RCBS Max press. It is sloppy as hell.

Interesting.