308 can question

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Strike 6
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308 can question

Post by Strike 6 »

I am working with a machinist to build a form 1 can. When I met with him, he said he could not move forward without knowing the tolerances of the measurements on the plans I provided.

Below are two pages of the plans. Am I mistaken when I say the areas with the .001" or .002" with the Z next to them are the tolerances?

What am I missing?

He also recommends 17/4 steel for construction. I thought 316 stainless would be a better choice. Recommendations?

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57fairlane
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Re: 308 can question

Post by 57fairlane »

no, those markings are for concentricity and perpendicularity.

+/- .001" is pretty tight anyways. A one off deal probably isn't too big of a deal but on a production piece you would want .003" or so FWIW
Strike 6
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Re: 308 can question

Post by Strike 6 »

Ok, so the ID of the tube should be 1.625 +/- .002 or +/- .003?
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Enfield577
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Re: 308 can question

Post by Enfield577 »

Strike 6 wrote:Ok, so the ID of the tube should be 1.625 +/- .002 or +/- .003?
Think it through, with tolerancing you want to understand what you are trying to acheive. The baffles will fit inside the tube so if anything the tube wants to be large not small and the baffles want to be small not large

So for exammple on a 1" tube, tube tolerance wants to be say + 0.002 - 0.000 and the baffles want to be -0.002 + 0.000. See what I am trying to get at, you need a clearance so both parts work together.

If your guy knows what he is doing he should be ok as long as he making all the parts. He wants to get a 0.003 or so clearance on the baffles
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byersjus
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Re: 308 can question

Post by byersjus »

No criticism of Enfield, but asymmetric tolerances drive me nuts. I know the designer/draftsman wants to say "get as close to x without going over/under", but I think most machinists immediately calculate the "real" nominal and setup a symmetric tolerance. In the 1" tube ID example you provided I'd see 1.001" +/- 0.001".

Not saying the asymmetric is wrong, but I think designers and machinists see it differently (unless they are the same person). I would build the fit tolerance into the print. Ask yourself what is the maximum amount of gap you can withstand and then setup the tolerances so that if the baffle is at its lower limit and the tube ID at its upper you would still be under that max. If they were reversed (baffle upper limit, tube lower) they wouldn't interfere or even better there would still be a little gap to allow assembly.

My two cents.
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daviscustom
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Re: 308 can question

Post by daviscustom »

The most important tolerences will be on the fit of the baffles in the tube. That is a pretty deep boring operation inside that tube, so you have to decide if you want your baffles to slide in fairly easily...or if you are going to weld this together.....maybe lean towards a light press fit. If you don't plan to take it apart I would have him shoot for -.000/+.001 on the tube id, and +.000/-.001 on the od of your baffles....and make them the same dimension. It will take longer to do but....and he might whine about it....but he can do it if he knows what he is doing. He might need to hone the last gnat hair out of the tube id to make it more consistent...then he will need to make a tool to push the baffles in on the press....obviously with a small enough od that it won't get stuck in the tube.

Tolerencing on all the steps should be fine to be at +/- .005, except the last step next to the od of the baffle if you are using spacers you need to make sure it is not going to be too tight a fit in your spacer....don't need any extra hassles when putting this together....I'd make sure you have a couple thousanths clearance between the id of the tube and the od of the spacers....and the same couple thous. between the id of the spacer and the od of the first step on the baffle. Making certain the flat faces on the baffles and spacers are parallel to each other and perpendicular to the od is very important.

17-4 is perfect for the blast baffle and maybe the second one too just for insurance, but unless you plan on full auto or a lot of big mag dumps....that would be it, then use 316 for the tube and everything else.
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BeerWolf
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Re: 308 can question

Post by BeerWolf »

byersjus wrote:..... but I think most machinists immediately calculate the "real" nominal and setup a symmetric tolerance. In the 1" tube ID example you provided I'd see 1.001" +/- 0.001"...
That's what we use to do when we programmed the CNC's at Northrop, with a "not to exceed dim x" note added to the operator.
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