Press & Die Thread Pitch

[xcalibor67]

After reading some concentricity issues on reloading, made me wonder why the original developers chose the "Standard" thread pitch used for presses and dies. After running scenarios through my head, I figured the thread pitch was chose due to speed vs concentricity, what people would live with. Is it right in thinking that a much finer thread would hold the die more concentric in the press as well as giving it a more precise adjustment? Or does it merely depend on the amount of slop between the die threads and the press threads, as we know that when you tighten the locknut on the die, it can "Cant" the die a thousandth or so to one side, thus affecting the concentricity between the press ram and the die.. Don't ask me why I thought of this, lol its just one of those days...

 

[lightman]

You have put more thought into this than I ever have. Finer threads make sense, don't really know why they chose the course threads.

 

[Comrade Mike]

I think the answer is simply no threads like with the Forster coax

 

[steveno]

good luck trying to find somebody that will make you a press and dies with a finer thread.

 

[rcmodel]

I have no idea.
But I think your compromise between accuracy & speed of changing dies is the most likely.

The old Lyman 310 / Tru-Line Jr. Dies were 5/8" x 30 TPI.

And it took forever to screw then in & out.

rc

 

[mdi]

I would tend to think it's in the manufacturing tolerance more than the TPI. Threads are also determined by 50% and 75% of nominal diameter; 7/8-14, .8281" and .8125". But, fine threads have a smaller tolerance, tighter fit between matching threads. I don't know why 7/8-14 was chosen over 7/8-20. In theory, your ideas are prolly correct, but in real life the 7/8-14 die threads have worked quit well for several decades and those wanting better can go with "in-line" dies and use an arbor press...

 

[mahansm]

The thread pitch doesn't make that much difference. It's the tolerances and clearances of the threads. That's up to the manufacturers of the dies and equipment. Sources for error include press bore off center, press bore canted, threads canted relative to press bore, die bore not concentric/aligned with die threads, lock ring face not perpendicular to lock ring axis, press top surface not perpendicular to ram travel, etc..
A good quality manufacturer will set tolerances in the drawings to what he considers an acceptably low number and then do quality control to ensure that those tolerances are held.
Machining gets much more expensive as tolerances shrink and this is reflected in the final price of the equipment.

 

[WestKentucky]

I would think that a tapered setup like spark plugs use would likely be ideal, but I don't really know why that thought comes to mind. What has you so interested to think so deeply into the engagement of the press and ram

 

[Otto]

Higher thread counts would increase the time it takes to install and remove a die...14 TPI provides enough tolerance to let the die self-align.

 

[Jesse Heywood]

Another point is fine threads don't last as long as coarse in cast metal. Without the strength from grain structure the fine threads fracture easier.

 

[rcmodel]

An excellent point, and one I didn't think off!!

There's your real reason right there I betcha!

rc

 

[xcalibor67]

Well, I think what got me on the subject was digging out my ole No.3 22 hornet. After I purchased a redding S bushing neck sizer, I could produce pretty good rounds. I understand the hornet necks are super thin, and naturally a bit finicky, and any concentricity issues become more profound. I think with me doing some machining in the years past got me to wondering why they used the thread pitch. I did figure it was speed VS alignment most cost affective.. You mention the finer threads would not hold up as good in cast, but most presses have steal inserts now. Personally I do not load for quantity, as I load for myself only and don't mind spending a few hours with a cup of coffee and my press, so finer threads would A-ok with me. Some good input though.

 

[Poper]

You mention the finer threads would not hold up as good in cast, but most presses have steal inserts now.

It wasn't always that way. By the time manufacturer's began this as a standard practice, there were lots and lots of dies and accessories already out there in the standard 14-28 TPI pitch. Besides, it works quite well for the great majority of the ammo loaded. Add to that point that few people can shoot as well as the potential of their rifle and/or ammo, and that makes the extra cost for the extra precision something few people are willing to pay for.

Just my 2 cents.

 

[rcmodel]

Try adjusting the die with a case in it.
Wiggle the die to center it in the threads.
Raise & lower the handle and jiggle the die while adjusting.
Then toggle the press down tight and lock the lock ring and lock the set screw.

The die will then be square with the press the next time you screw it in tight.

( Unless you are using Lee o-ring lock rings.)

rc

 

[243winxb]

Square dies in press.

The RCBS news letter said to have the shell holder in contact with the bottom of the FL die to square it. Then set the lock ring. But if partial full length resizing, this may not work. This is where the Redding competition shell holder set would be useful.

 

[rcmodel]

That is true.

But you still need to rattle the die around in the treads and then square it against the shell holder before locking the lock ring.

That was what I was trying to say above.

But I didn't do a very good job of it.

rc

 

[gamestalker]

I wrap Teflon tape around the threads, problem solved.

GS

 

[9mmepiphany]

I think the answer is simply no threads like with the Forster coax

I think this ^^^ , in Post #3, really is the best solution

 

[rcmodel]

But first, you have to identify a problem with standard 7/8"x14 die threads.

Which in 50+ years of reloading, I haven't been able to do yet.

Rc

 

[BCRider]

The Forster still uses threads. But instead of using threads in the body of the press it's relying on the quality of the lock ring of the die.

And after considering this I'm thinking that this could be a weak point if the thread in the lock ring are not axial to the faces to a tight degree. And because it's not locking thread against thread it becomes more important to use a good and correct style ring. So the Lee "rubber band" lockrings are right out. Ditto the set screw locking RCBS rings. That leaves us with the pinch lock style rings sold by Hornady and a few others for the best application in the Forster press.

The Lee rings with the rubber bands can't be relied on to not creep in use without having something to lock to. The set screw RCBS rings are no good because the screw will pull the threads into a centered lock on the one side while the side with the set screw can wobble around within the thread fit tolerance with no idea of where within that range it'll lock firm. So we're left with the pinch lock Hornady or similar rings that become a variable diameter thread that pulls in to close up the thread tolerances.

But even that style of locking ring is only as good as the axial to face tolerance. If it's out a little then the die will sit at an angle in the Forster style mount.

Going back to the regular threaded setup as seen in the Lee, RCBS and other brands the coarse thread is not a limit on a good axial alignment. The angles in the threading will self align the die to the press mount when a properly made and true lock ring pinches the threads to lock the die in place. At that point the threads move against each other and self center themselves unless there's a lot of damage or a very poor surface finish to the threads. In this style mount the only real issue is how good an axial tolerance was used for cutting the threading for lining up with the bore of the ram in the base of the frame.

A while back I actually scrapped a Lee Classic SS press for this issue. I did a test and found that the die mount threading did not line up with the ram by a surprisingly large angle. At that point I would not have given it away because I don't hate anyone that much. The only decent thing to do was toss it into the scrap metal box for recycling.

 

[beag_nut]

The thread pitch doesn't make that much difference. It's the tolerances and clearances of the threads. That's up to the manufacturers of the dies and equipment. Sources for error include press bore off center, press bore canted, threads canted relative to press bore, die bore not concentric/aligned with die threads, lock ring face not perpendicular to lock ring axis, press top surface not perpendicular to ram travel, etc..
A good quality manufacturer will set tolerances in the drawings to what he considers an acceptably low number and then do quality control to ensure that those tolerances are held.
Machining gets much more expensive as tolerances shrink and this is reflected in the final price of the equipment.

Exactly what mahansm said, and nothing else.

 

[9mmepiphany]

The Forster still uses threads. But instead of using threads in the body of the press it's relying on the quality of the lock ring of the die.

I thought the Co-Ax used the dual floating guide rods and floating jaws of the shell holder to assure alignment

 

[fguffey]

If a reloader thought the die was not perpendicular to the shell holder it would only require raising the ram and measuring the gap if the die was canted.

A reloader could use the ram to seat the die in the threads, after seating the die with the ram the reloader could secure the die to the press with the lock ring.

F. Guffey

 

[xcalibor67]

With the PA die im using, the die end does not even protrude below the press, its recessed about 1/8 up in the press. Seems some of you hit on the same idea I was thinking about squaring the die to the ram. Im going to stack washers on the shell holder until they fit snug at full cam, then I will measure total thickness and have my bud to make a solid bushing on his CNC to use for squaring the die body to the ram/shell holder. Maybe it will help, maybe not, but I do know I can reach up and grab a loose die and physically cant it to one side and snug the lock nut by hand and it will stay in that position. My m8s say im just to damn finicky lol. But of course I was the original lil boy who took apart a clock just to see how and why.. P.S this is a seating die.

 

[TexasDon]

A note on slop in die threads:

I have a very old RCBS 357 steel sizer/decap die that has a 1/4-28 bolt through it that holds the decapping pin assy.

It has soooo much thread slack that you can push the decap pin to one side and tighten the lock nut in that position. ( I've broke a few pins that way )

Like RC said, you have to have a case in the die on the shellholder and wiggle then tighten up everything for best results.

The new dies are made a lot better and I don't need to do that anymore.
But I still check.

TxD