Picatinny Rails?

[Razor18]

Thanks, but my question was not about wiggle room, it was pure geometry of the MIL-STD as shown in every image on th net, but I probably couldn't explain it precise enough.

The way it's drawn it can't be machined, cause some of the dimensions seem to be "over-defined", meaning there are 3 specific dimensions where if 2 are made right, the third can't be the value specified.

If you look at the drawing I'm sure you can follow. The heart of the rail is in the middle of the rail profile, datum C, which is a rectangle .748 +/- 0.002 wide, and exactly 0.108 high. On both sides, the corners of this datum C are touching 45° lines, these lines intersect in a 0.835 +/- 0.005 width (rail width), without any chamfer.

The vertical sides of datum C and the 2x45° lines form two little triangles on both edges of the rail. The base of those triangles is the side of datum C, which must be 0.108. Perpendicular to this vertical base, the "height" of those triangles (on the image it's actually horizontal, and isn't drawn) from the middle of the base to opposite corner of triangle is obviously half of the difference between datum C width and rail width (0.835 - 0.748) / 2 = 0.0435.

Now in a symmethrical triangle, if the height (which is 0.0435 here) is not equal to half of the base length (0.108/2=0.054 here), then the angle of the triangle sides can not be 2x45°, it's geometrically impossible. So, back to the MIL-STD drawing, if you require datum C to have the drawn dimensions, AND require the side angles to be 2x45°, then rail width can not be 0.835. From those 3 conditions, only 2 can be met at a time.

So which one of the required dimensions must not be met so strict in your opinion to still have a match between rail and any proper mount?

 

[Sully2]

So what if the drawing isnt totally complete or for that matter 100% accurate? You trying to MAKE a rail from the drawing or just buzzing someone else' s *** about it.

 

[Razor18]

I don't do CAD drawings for fun, of course I want to make it on machine. But if you can't do it on paper first, based on the drwaing's dimensions, you can't do it on machine eighter. That's why drawing are for. Your question shows you didn't even try to understand the geometry problem, and ask me about wanting to buzz someone's? Have you tried making it maybe?

Regards

 

[Sully2]

I don't do CAD drawings for fun, of course I want to make it on machine. But if you can't do it on paper first, based on the drwaing's dimensions, you can't do it on machine eighter. That's why drawing are for. Your question shows you didn't even try to understand the geometry problem, and ask me about wanting to buzz someone's? Have you tried making it maybe?

Regards

I designed jet engine hardware for 37 years partner...and what you say isnt true. Look up the term "Kellering" and then try and decipher how you would draw it! And dont tell me jack squat about CAD drawings either....thats what I used from 1974 till 2004

 

[Razor18]

If this is the case, I honestly don't understand your first answer. Wouldn't take too much for you to start drawing datum C (0.748 x 0.108), put the 45° lines onto the corners of datum C. Then just measure if the width of the 2x45° edges can ever be 0.835. No, it will be 0.856. If you find, how to machine it for 0.835, I would really happy to know it, honestly. I want to make it, but can't, till this question is solved, or the rail won't fit. I really didn't try to ask here for nit-picking.

 

[DocB]

I appreciate you teaching me something new (about air guns)
Thanks for that.


Thanks, hope you have a great weekend.

Respect is earned, not something you are entitled too.

Happy shootin' gun)

Respect, like dignity and integrity are always implied until one is found unworthy, young sir. Perhaps you should be a bit more humble and accepting of advice and mentorship when it is freely offered. Arrogance goes hand in hand with the ignorance of youth whom believe that they are worthy because they have watched a video or two.

The gentleman whom you so disrespectfully offended is a long time member and mentor of many on this site, a respected machinist, designer, engineer, business owner whom has probably held more weapons in his life than most professional soldiers. He is one of the many on this site who have 'been there, done that', made the mistake, and then shared the lesson with us.

You will be surprised to learn that there ARE actual rocket scientists, ballistic engineers, master gunsmiths, world champion marksmen, optics engineers, and genuine experts. Perhaps a bit of humility on your part will earn you the respect that you are so quick to deny others of.

Please enjoy the site, learn from others here, and remember only liberals use insults when they are loosing arguments!

Another ole'timer...

DocB

 

[bruce_ventura]

...Respect is earned, not something you are entitled too...

I disagree. I think we earned it when we were born. We're all entitled to it.

And thank you DocB. Good to see you're still kicking. I'm still looking forward to sharing a beer when you get back. Keep the photos coming.

 

[westcliffe01]

Ok, I am not a professional cad jocky but I am an engineer and do about 90% of my own CAD work except for loading them in SAP.

I'm trying to understand your issue with drawing this section.

If we assume that the 0.748 dimension is the "gauge" distance for the dovetail and that the 0.108 gauge height is centered on the 90 degree corner, then working outwards the dimension to the theoretical corner intersection is 0.748 + 2x(0.108/2) = 0.856.

So the fact that the print shows a dimension (which appears to be corner to corner) of 0.835" suggests that the rail does not have a sharp corner but in fact has a small flat which is (0.0105^2*2)^0.5*2 = 0.0297" wide (vertically).

If you think about it, it is never good engineering practice to put a sharp corner on something which is going to fit into an outer part. That is because no-one can make a tool that cuts a true square corner (at least not for very long). So cutting tools have a radius on the edge and the part that fits into the feature needs a flat or chamfer to clear the radius so that it can be properly seated on the 2 45 degree faces as intended.

I was going to write you a description of how to draw it, but I don't think you need it now ? All clear ?

I will add that a good print would show a detail of that corner feature so you could see the 0.030" flat, but this was probably done a long time ago and they were probably initially cut by artisans who understood how things needed to be made to fit together and not by operators hired off the street who were shown where to push the button.

I will tell you that the hardest thing one ever has to accomplish as an engineer is to get artisans to trust your work. The problem being that the work they get from about 90% of the engineers is incapable of fitting together and the shop gets blamed for delays at the end when it does not go together... The day that guys from the shop floor walk in your door with suggestions about how to make your work better (instead of *** is this sh&$%t ?) you have "arrived".

If this is the case, I honestly don't understand your first answer. Wouldn't take too much for you to start drawing datum C (0.748 x 0.108), put the 45° lines onto the corners of datum C. Then just measure if the width of the 2x45° edges can ever be 0.835. No, it will be 0.856. If you find, how to machine it for 0.835, I would really happy to know it, honestly. I want to make it, but can't, till this question is solved, or the rail won't fit. I really didn't try to ask here for nit-picking.

 

[Sully2]

I disagree. I think we earned it when we were born. We're all entitled to it.

And thank you DocB. Good to see you're still kicking. I'm still looking forward to sharing a beer when you get back. Keep the photos coming.

Ive got to disagree. Being born is a biological incident...nothing more. Becoming respected is via your actions over time and your interface with others on this planet

 

[bruce_ventura]

I completely respect your opinion, which I would do even if we were total strangers.

 

[Hunter02]

Alright. So I regret what I said to sideflipcar and apologize. I wish I had never said it. I didn't realize the thread was older than dirt. And I should have responded to his comment how I did. His comment about my dad (that I never had) ****ed me off as well. I did feel his original comment was disrespectful and I overreacted.

There are young people out there that do know a dcnt amount about shooting, and I commonly see them shot down, soley because they soley because they are young.

I am on here to learn and help people out that have questions.

I have no hard feelings, and will take it a a learning experience.
Sorry to those of you I offeneded. Hopefully we can accept each other differences and call it a day.

Happy shootin

 

[DocB]

I disagree. I think we earned it when we were born. We're all entitled to it.

And thank you DocB. Good to see you're still kicking. I'm still looking forward to sharing a beer when you get back. Keep the photos coming.

Hey Bruce!

I'm telling ya, a nice cold Sam Adams or two would be great right now! lol.. That's for sure.

Yep, still over here kicking rocks and dirt clods around, my incoming count is up to 894! lol.. I just wish these guys would behave.

Thanks for doing what you do sir, I'm sure that I've met a few of your 'students' over here where I am, they're 'fair' shots! lol!!!

There's a reason that Dignity, Honor, Respect, and Integrity are the values instilled in all recruits, they are the bedrock of men becoming MEN.

Watch out for all those whacky liberals out there on the 'left coast' Bruce! We'll get together and share a few, talk about horses and shooting someday soon!

DocB

 

[Razor18]

.

If we assume that the 0.748 dimension is the "gauge" distance for the dovetail and that the 0.108 gauge height is centered on the 90 degree corner, then working outwards the dimension to the theoretical corner intersection is 0.748 + 2x(0.108/2) = 0.856.

So the fact that the print shows a dimension (which appears to be corner to corner) of 0.835" suggests that the rail does not have a sharp corner but in fact has a small flat which is (0.0105^2*2)^0.5*2 = 0.0297" wide (vertically).

Dear westcliife01,

THANK YOU, this is exactly the kind of answer I was looking for from Someone who tries to understand the question instead of answering some nonsense, like "the drawing is not perfect, so what?" (let alone from someone who's been engineer, claiming to have "been there done that" on jet engines for decades...). Drawings are invented to show how to do something, and if something isn't on the drawing, how could anyone expect it to be done right?

Yes, I got the same answer from another credible source too: the edge-to-edge dimension of 0.835 is in fact the distance of the "chamfering" after cutting the 2x45° edges, and not the edges themselves, as drawn. With this additional information the drawing is perfectly enough to mill a rail, but on the drawing there is no sign of any chamfering, so again, the drawing is simply wrong, and no rail can be done based on that without this additional information, which is missing from the drawing. Would like to watch the jet engineer try to mill it by the original drawing to make him understand.

Thanks, I appreciate your approach to the problem.

 

[Sully2]

Dear westcliife01,

THANK YOU, this is exactly the kind of answer I was looking for from Someone who tries to understand the question instead of answering some nonsense, like "the drawing is not perfect, so what?" (let alone from someone who's been engineer, claiming to have "been there done that" on jet engines for decades...). Drawings are invented to show how to do something, and if something isn't on the drawing, how could anyone expect it to be done right?

Yes, I got the same answer from another credible source too: the edge-to-edge dimension of 0.835 is in fact the distance of the "chamfering" after cutting the 2x45° edges, and not the edges themselves, as drawn. With this additional information the drawing is perfectly enough to mill a rail, but on the drawing there is no sign of any chamfering, so again, the drawing is simply wrong, and no rail can be done based on that without this additional information, which is missing from the drawing. Would like to watch the jet engineer try to mill it by the original drawing to make him understand.

Thanks, I appreciate your approach to the problem.

If you are referring to ME...I never said I was a jet engine ENGINEER now did I? I said I DESIGNED engine hardware ( basically for the military group of engines) for my whole career.. And if you were SMART ENOUGH to get a copy of spec SI-212,010 or PITF3 CL-A you would see that INTERPRETATION OF DWG states that not every little thing has to be specifically dimensioned because many are FALL OUT edges , surfaces. The resultant of other actually dimensioned surfaces being machined and whats left is whats left.