Difference between ARMS and Picatinny rails? 3

[Helepolis]

Hi all,

as the title states, what is the difference between the rails?
I found the correct dimensions for the Picatinny rail but nothing on the ARMS.

Thanks!

 

[JNieman]

MIL-STD-1913 rails are a standardized and clearly defined set of requirements constraining the allowable geometry for mounting hardware on the rail.

A.R.M.S. is a company that has made different flavors of dovetail rail, including the design which became the MIL-STD-1913 rail.

If you mean any other rail, like Weaver.. it's less standard and companies would make their own Weaver-mod rail. Companies are not very likely to publish full specs on non-standard products they market.

Is there a specific ARMS product you're curious about, or what.

 

[Helepolis]

The thing is that i've been assigned to design an adapter that is universal for both the rails, but the request was vague as it sounds, "make it work".
So if i got you right, the two rails are basically the same, or more accurately the Picatinny is a standardized to mil-spec ARMS rail.

 

[JNieman]

That's easy enough.

Buy the part you're supposed to make it mate to. Make it mate to it. Allow for reasonable variance.

 

[Helepolis]

That's easy enough.

Buy the part you're supposed to make it mate to. Make it mate to it. Allow for reasonable variance.

That is the last option that i would use.

I prefer to try and fined the technical drawings/official dimensions of the product.

 

[JNieman]

If you say so.

It's better to know the 'nominals' for a more accurate target, I agree, but with a part in-hand you also get to do testing of your product right away, and then use it for marketing/demonstration purposes if needed later. Potayto, potahto, c'est la vie, etc etc

Good luck

 

[IRstuff]

There's plenty of information on the web:

http://armsmounts.com/

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[Nescius]

From the A.R.M.S. website:

"A.R.M.S.®, Inc. is the original designer and manufacturer of the dovetail dimensions for Military and Civilian use since 1980. Those original A.R.M.S.® dovetail dimensions were eventually adopted in 1995 as the Mil-Std 1913 Rail."

I had a thread about the 1913 drawing and how it's not very good:

http://www.eng-tips.com/viewthread.cfm?qid=406027

Pylfrm presented a reasonable interpretation at the end of that thread. It bugs me deeply that the standard drawing is poor. I work with it semi-regularly. Many rails on the market would not meet any reasonable interpretation of the drawing, but stuff still works...usually. Lots of rails are extruded with no secondary machining, but extrusion alone can't reliably conform to the "drawing" in my experience. Life goes on.

 

[3DDave]

Looking at MIL-STD-1913 I am horrified at the dimensioning and tolerancing, which fails to conform to the requirements of the 'Y14.5 standard and fails to completely describe the allowable acceptable variations one might encounter.

This is one of those cases where contacting the original designer of the rails should be the best bet, but since they would have supplied the drawings to create the MIL-STD, they would also be the ones who didn't understand** how to document the requirements in a usable fashion. The diagram was revised in 1999 and they made it worse.

http://quicksearch.dla.mil/qsDocDetails.aspx?ident_number=115317

should be the MIL-STD-1913 page.

**Or they do understand and want their competitors to be at a distinct disadvantage due to the incomplete spec, including you.

 

[IRstuff]

"**Or they do understand and want their competitors to be at a distinct disadvantage due to the incomplete spec, including you."

Unlikely, given that EVERYTHING that you want to attach to the rails also need to be standardized, such as scopes, lasers, grenade launchers, etc., so they all need to be able to reconcile tolerances and dimensions to meet their requirements, and the requirement typically calls out only MIL-STD-1913:

3.2.14 Mounting Interface
The XXX mLRF shall be capable of being mounted and aligned to a weapon platform via the MIL-STD 1913 rail interface system (Picatinny Rail). The XXX mLRF will be mountable and fully functional on either the top or right side rail without modification to cables or connectors.


However, anything that attaches to the rail has an extra degree of freedom, since they typically clamp to the rail with an adjustable mechanism.

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[JNieman]

Right. There's typically a screw driven clamp on one side, opposite a fixed 'jaw'. It's a pretty resilient system. Others have a fixed jaw opposite a cam operated (usually) by a throw lever. It hasn't been this popular because it was finicky. It's pretty idiot-proof.

 

[3DDave]

There's a difference between a resilient system and having to create resilient work-arounds to a bad description. There is not enough information to determine acceptable variations. It's not the first time suppliers ignore the allowed tolerance and tighten things up to make it work. I'm just surprised at the garbage description.

 

[SwinnyGG]

That drawing doesn't look THAT bad to me- I'm curious how you would modify it.

Mil-spec small parts drawings like this are always bare bones. It defines enough dimensions to build a part that is functional- it isn't, and isn't intended to be, a drawing you can hand to a fabricator so they can build a part without any additional information.

 

[IRstuff]

Interestingly, the original drawing in the 1995 release looks to me to be a better quality drawing than the one from 1999

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[SwinnyGG]

From what I can tell the only difference really is the addition of the limit on root radius at the bottom of the profile.

The second one looks to me like a different draftsman just opened it in his own autocad seat with different leader/arrow settings and didn't fix them for legibility before saving the drawing.

 

[3DDave]

jgKRI - what exactly is datum C? If it's targets how is a (m) modifier appropriate? If it is to be a fixed gage, then what controls the sloped areas? Why would divergence from the MMC condition of C allow more tolerance to the position of the vertical rectangular support and the overall width of the mating section?

The update mis-typed the datum reference from [C(m)] as [-C-(m)] for the .835 -.005 dimension.

 

[SwinnyGG]

Before you dive into trying to interpret the not-so-great tolerancing scheme, ask yourself:

What is the purpose of this drawing?

How do accessories actually interface with this shape?

Everyone seems to be focused intently on how the angled sections of the shape are dimensioned and related- what seems to be missing is the simple fact that in the real world, the angles of those faces being incredibly precise doesn't really matter that much.

I would never call this a great drawing, but from what I can tell if I needed to design something to mate with this shape, there's enough information there for me to do it.

 

[IRstuff]

As I indicated earlier, the clamps that attach the accessories provide a degree of freedom that allows it tolerate the sloppiness of the drawing.

The intent is not a precision mount, per se, but a reasonably precisely repeatable mounting.

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[JNieman]

Repeatable being quite subjective when you have things being mounted from a simple white light device to optics meant to provide reasonably accurate point of aim out to 1km.

But yea, I think that drawing is why some folk think "Good enough for gov't work" doesn't mean "Rigorously evaluated to comply with standards that ensure a reasonably low risk of mission failure, loss of life, or loss of vehicle" lol

 

[SwinnyGG]

Anyone who shoots long range will tell you that there isn't a mount out there, anywhere, that will allow you to remove and re-install your optics and maintain perfect aimpoint at 1000m. If there were, optics wouldn't need to be adjustable.

The point of the 1913 rail isn't to provide sub-micron positioning of accessories- its to allow a zillion different things to be mounted in whatever configuration is required, quickly and without much fuss. It accomplishes that pretty effectively even though it's governed by a drawing that isn't first class.