Tight Gun= *SLOPPY* tolerance

[Hohn]

I see a lot of people falling into the misconception that their super tight pistol represents "tight tolerances."

It's actually rather the opposite. A tolerance is the allowed variation. If you have very tight control of the variation, then you can precisely guarantee a given amount of clearance or flatness.

My job requires engineering parts with micron-level tolerancing. The fuel pumps I work with have a clearance of the piston in the bore of about 3 microns. That's about 3% of the *width* of a human hair.

Yet, you can always insert the piston in its bore. It has no play, no slop, yes-- but it also has no hangups, no catches or "tight spots" that need to break in.

When a gun maker ships you something with catches and tight spots that have to "wear in" it means that they couldn't hold the tolerances tight enough to produce the desired small clearance. So they produced something that has no clearance (i.e., it hangs up) and insist that wear produce the clearance their manufacturing process couldn't.



I'm not trying to throw anyone under any particular bus, but wanted to make very clear that tight *tolerances* and tight *gun* are not only NOT the same thing, but they are diametric opposites.

 

[Ricnzak]

Are the parts you work with manufactured with special machinery to get those results? I would assume there are machines and practices better than others. I have no experience in metal manufacturing is why I ask. Very interesting stuff to me.

 

[Hohn]

Absolutely they are. They are all ground, rather than cut. You simply cannot cut drill or ream a hole to tolerances that tight.

Anything where utmost precision is required will be ground (abraded away, like sandpaper) and not cut.

Imagine you are trying to get a piece of wood to fit somewhere by sanding the high spots until it does.

Now imagine you're doing it with 1200 grit sandpaper instead of 80 grit. Takes forever, right? Yes. But precise? Heck yes.

 

[SmileDocHill]

Yeesh! Another thread preaching about tolerance.

 

[BE Mike]

Can't we all just get along?

 

[gmcttr]

...Now imagine you're doing it with 1200 grit sandpaper instead of 80 grit. Takes forever, right? Yes. But precise? Heck yes.

I used to work with 1/2 micron diamond dust before finishing up with the much finer plastic film abrasive polishing laps. Of course I didn't have to get two parts to mate up, just had to have all the points line up and a high polish on faceted gemstones.

 

[Vigilant]

Then talk to us about castings, and how MIM is better than real steel for parts?

 

[Hohn]

There are some steels whose incredible properties can only be achieved by powder metallurgy.

Castings can be as strong as billet if the castings are HIPd afterward to assure no porosity. Same for PM.

MIM vs "real steel" is the wrong question. It's WHICH MIM against WHICH STEEL that matters. There are soft "Real steels" that far inferior to MIM. And there are MIM grades far inferior to "real steel"-- you simply cannot make a blanket statement about either.

Crucible Tool Steel and Specialty Alloy General Information

 

[Vigilant]

There are some steels whose incredible properties can only be achieved by powder metallurgy.

Castings can be as strong as billet if the castings are HIPd afterward to assure no porosity. Same for PM.

MIM vs "real steel" is the wrong question. It's WHICH MIM against WHICH STEEL that matters. There are soft "Real steels" that far inferior to MIM. And there are MIM grades far inferior to "real steel"-- you simply cannot make a blanket statement about either.

Crucible Tool Steel and Specialty Alloy General Information

Gun parts.

 

[S8MS-01904]

Absolutely they are. They are all ground, rather than cut. You simply cannot cut drill or ream a hole to tolerances that tight.

I beg to differ. I'm holding 2-4 microns or less of roundness on valve bores in cored aluminum. So reaming is possible in a high production environment.

 

[WebHobbit]

OP - I am not doubting your expertise BUT...with firearms I would rather have TOO TIGHT than too loose....after all we can take away metal but we can't add to it! Great example is overly large revolver cylinder charge holes - too big mean you need a NEW CYLINDER.....too small they just need some reamer work.

 

[churchmouse]

There are some steels whose incredible properties can only be achieved by powder metallurgy.

Castings can be as strong as billet if the castings are HIPd afterward to assure no porosity. Same for PM.

MIM vs "real steel" is the wrong question. It's WHICH MIM against WHICH STEEL that matters. There are soft "Real steels" that far inferior to MIM. And there are MIM grades far inferior to "real steel"-- you simply cannot make a blanket statement about either.

Crucible Tool Steel and Specialty Alloy General Information

I understand exactly what you are saying and yes.

 

[Hohn]

I beg to differ. I'm holding 2-4 microns or less of roundness on valve bores in cored aluminum. So reaming is possible in a high production environment.

That's impressive.

But it's also different than holding 2-4 of cylindricity over a 30mm length, and *definitely* different than holding 2-4 clearance when you have an entire 2nd surface and its own cylindricity to account for.

It's the stack up that gets you.

 

[Hohn]

OP - I am not doubting your expertise BUT...with firearms I would rather have TOO TIGHT than too loose....after all we can take away metal but we can't add to it! Great example is overly large revolver cylinder charge holes - too big mean you need a NEW CYLINDER.....too small they just need some reamer work.

Absolutely true!

I think I need to correct something I mistakenly implied in my OP. I'm not saying a tight gun is wrong or that it can't make for an excellent pistol. Indeed, they are--after break in-- fantastic and will provide outstanding performance, accuracy, and reliability.

And I should probably ask the mouse to edit my Title, but "sloppy" was a "sloppy" choice of words-- we're still talking about relatively precise fits.

If nothing else, it might be entirely defensible to make a "tight" gun just on cost alone-- the cost of holding the tolerances required to avoid the tightness can be exorbitant. (priced a Cabot recently?).

So I'm not saying a tight gun is inferior or even a poor engineering. I'm just saying it's not "tight tolerance" because truly tight tolerancing eliminates the need for any interference fitting or wearing in.