class of fit for barrel thread tap

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SteveM
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Joined: Mon Jun 27, 2005 6:18 pm
Location: Wisconsin

class of fit for barrel thread tap

Post by SteveM »

I need to make a tap to chase the barrel threads of a S&W .357 mag frame.

The threads on the barrel are 0.667 on the OD and 36tpi. I measured with my thread pitch micrometer that is for 32 to 40 tpi and it shows 0.650 thread pitch diameter, and based on calculations I found online, that looks right (calculations have 0.649).

My question is: what diameter should I make the tap?

That boils down to what class of fit is required for an application like this.

Note that the frame is already threaded, it just needs to be chased. I just don't want to make the threads any larger than they are and make a sloppy fit.

Steve
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GlennW
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Location: Florida

Re: class of fit for barrel thread tap

Post by GlennW »

I would make the tap match the barrel threads since that is what needs to screw in there.
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!
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Harold_V
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Re: class of fit for barrel thread tap

Post by Harold_V »

Class of fit, even for interference fits (which I think is a class 5), should be determined by the pitch diameter, NEVER the major or minor diameter. Thus, the major diameter should be in keeping with the formula for that particular thread and fit. The size is bastard, or non-standard, so S&W may not make that information known and it may not be found in thread specifications, although the formulas found in the H28 manual would allow for making a determination of the proper sizes required.

If the task was mine, I'd make a tap with the major diameter a couple thou larger than the barrel thread diameter, guaranteeing that the major diameter of the barrel did not interfere with the minor diameter tapped in the frame. Pitch diameter of the tap? Do as Glenn suggested. If the barrel fit previously, and there's no deformation of the threads on the barrel, it should fit as it should.

Armed with that thought, I'd highly recommend you pay close attention to the threads on the barrel, to ensure that they have the correct form and are not sporting deformation.

H
Wise people talk because they have something to say. Fools talk because they have to say something.
earlgo
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Re: class of fit for barrel thread tap

Post by earlgo »

Many years ago I swapped out a barrel from a S&W M&P and found out the hard way that the frames are relatively soft. So beware when applying too much torque to the unsupported frame. I ended up casting a cerrosafe block that encompassed the entire barrel frame junction so it could be gripped in a vise.
cerrosafe block
cerrosafe block
(The mating half is missing so it must have been used for another project.)
Just a caveat if you need it.
--earlgo
Before you do anything, you must do something else first. - Washington's principle.
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