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View Full Version : Gas block I.D. vs race O.D.


owlcreekok
January 06, 2009, 08:16
I have a new (undrilled, unmolested) metric gas block. It measures .690" I.D.

I am turning the race for it on a new chrome moly barrel.

What should my target race O.D. be ?

I would prefer BTDT responders but any input welcomed.

Gracias.

shlomo
January 06, 2009, 08:36
Be vewwwy, vewwwy careful, depending on how (and how well) you measured the ID of the gas block. If it were me, I'd chuck up a stub of something, and turn a test cylinder to the target OD, and see how it fits.

I've had a couple times when I've measured the hole ID with a dial caliper, turned the shaft to the target OD, and found it to be a thou or two loose, Lautrec.

Just my amateur $.02.

owlcreekok
January 06, 2009, 09:38
Yep yep. I have the journal really pretty at 0.6975. Polished and all. I turned it to .6985" at the last, a real light finishing cut, slow feed and a little faster spindle. Measured, (Mitutoyo digital verynear caliper) then used crocus. All I have is 120 grit and 320 and I did a quicky. The polish took a uniform .001". Hence the 0.6975".

Kinda "skeered" now, so I got off the journal and am chasing the muzzle end down ickle at a time until I get schooled or more nerve or both.

I have a 1" outside mic. I also have some gnarly inside calipers. I suppose I could set the internal tweezers to the gas block ID then check with the mic.

I need to get a decent set of outside and inside calipers, don't I ?

No, I didn' buy that toolpost. Spent that money on a .223 finish reamer and GO/NOGO .223 gages.

If it were me, I'd chuck up a stub of something, and turn a test cylinder to the target OD, and see how it fits.


Damn but I would love to do that. I am really loathe to un do the set up I got. (I need to take some pics too) I think I shall finish up the barrel from forward of the block race to muzzle, do a couple other little nit picky things than take it off the setup. Prolly part off the excess barrel and do exactly what you suggest, shlomo. Turn me one of them anti-ohfukc gages.

:shades:

shlomo
January 06, 2009, 10:02
I didn't realize that you already had the barrel set up. Still, shouldn't be too tough to put it back and indicate it to the journal you have roughed in. The journal is relatively non-critical for alignment and concentricity, unlike the operations you do on the ends with the holes in 'em. And, just between us girls, the bore ain't likely to be concentric in the journal anyways. Those bores wander relative to outside profile more than many of us like to think about.

For guys like us (hobby mosheenists) it's probably worth it to take it out, cut a test cylinder to fit, measure it with the mic, and then reset the barrel to finish turning the journal. Better to spend an extry hour or two than trash the project.

If it was me, I'd make the journal a .001 interference fit. If your tool is sharp and free-cutting, hitting that is pretty easy.

Be easier with a QC, though.:devil: :rofl:

owlcreekok
January 06, 2009, 10:15
Originally posted by shlomo
If it was me, I'd make the journal a .001 interference fit. If your tool is sharp and free-cutting, hitting that is pretty easy.

How about expounding just exactly WITF "interference fit" means. Pweeze.


Originally posted by shlomoBe easier with a QC, though.:devil: :rofl:

shlomo
January 06, 2009, 10:52
Originally posted by owlcreekok


How about expounding just exactly WITF "interference fit" means. Pweeze.



It just means making the journal .001 larger than the hole. It'll make the gas block a light drive-fit, or a "wring-fit".

You can do this by carefully making your dummy cylinder a firm slip fit, measuring with the mic, and then making the journal of the barrel .001 larger.

Rule from HTRAL: Never put a shaft in a bore without some kind of lubricant on it.

I know whatcher thinkin', so don't say it.

Reckon I deserved the bird. 'S okay. You deserved my barb as well.

Frickin' lantern posts.:redface:

owlcreekok
January 06, 2009, 11:26
Wait till Slow gets on here later. That drunk bugger was telling me last night to make the journal one or two thou' S M A L L E R than the block. He was reciting all he did when he fit his .458 SOCOM barrel to the Izzy block he used.

I might be confused, but at least the "interference fit" definition you gave makes sense.

Yeah, yeah, I know perzackly how to hate on lantern posts. If I know fuggal else, I know that.

PS- I put $30 in the pot for you. Just in case you hafta raise hell to get upgraded. :tongue:

Temp
January 06, 2009, 11:43
Leaving a .001" press fit on something in the .69" diameter range will definitely do the trick. Just make sure you have it positioned where you want it before you thump it in place,..because you're not going to be able to turn it.

If it was me,..I'd turn the first .100" of the journal .001" under the diameter of the ID of the gas block so that it would start on there good and straight, also.

I do that on a regular basis when I'm reaming press fit holes for dowel pins to be pressed into.

First, I ream the hole .001" under the dowel size, then go back with a .001" oversize reamer and drop it down into the hole a little bit so the dowel get's a straight shot into the hole.

Temp
January 06, 2009, 11:48
Also,..ditto on the careful measuring of the gas block hole.

When you're dealing with press fits, .001" here and there makes all the difference.

A .001" press fit is gonna go on real snug. A .002" press fit is gonna be a booger.

shlomo
January 06, 2009, 11:51
There ya have it from somebody who REALLY knows his onions, instead of a pretender/swarf-manufacturer like me.

I like the idea of making a section of it on OD or a thou under, to get it started straight. Next time I hafta do somethng like this (soon), I'm gonna give that a try. :biggrin:

Thanks for the chip-in. Still think it's too much x2. :whiskey: But I ain't raisin' no hades nohow about activating. Nature will take its course in due course.

owlcreekok
January 06, 2009, 11:58
Thanks, Temp. :beer: I ought ta frame that post and laminate it, frame and all.

You prolly will do the same with the letter I am sending you, slow mufuggah I am.

"Swarf Manufacturer" :rofl:

I should be so versed me dam self. :sad:

:bow:

Temp
January 06, 2009, 12:00
Originally posted by owlcreekok
You prolly will do the same with the letter I am sending you, slow mufuggah I am.



It ain't somethin' I'm worryin' about.

shlomo
January 06, 2009, 12:36
Owl--

While we're talkin', and it's on my mind--

I'm in the middle of making a jig for welding bolt handles, and I wonder if you know about so-called "gun taps." Great for thru holes, but not so much for blind ones. Shoots a string of chips ahead of the tap, cuts like butter, and no need to do the "half-turn, then back up to break the chip, another half-turn, back up..." routine. Just turn it right on down in a continuous run. :D

Took a pic of the string of chips, as I had the camera handy for a change. I'll load and post if yer interested.

Trust me, if you've never used 'em, they're light-years ahead of the traditional style. Kinda like QC posts.:shades: Just sayin'.

owlcreekok
January 06, 2009, 12:40
Post it up. I need a few more "gun taps" anyway.

I got a slew of pics of this barrel-.223 FAL build. I just gonna wait and post them all at once when done.

shlomo
January 06, 2009, 13:26
Originally posted by owlcreekok
Post it up. I need a few more "gun taps" anyway.

I got a slew of pics of this barrel-.223 FAL build. I just gonna wait and post them all at once when done.

:rofl: They call 'em gun taps because they "shoot" a string of chips ahead of the hole, thus:
http://img210.imageshack.us/img210/4258/002yz7.th.jpg (http://img210.imageshack.us/my.php?image=002yz7.jpg)

Can't wait to see the photo record of the PudelGewehr. :bow:

owlcreekok
January 06, 2009, 13:43
"Pudelgewehr"

BWAHAHAHAHAHAHAHAAAHAAHAAAHAHAHAHAHAHA

I got a choice between Izzy LB HG's or FNC HG's.

I also got a big ol honkin M60 flash hider too.

Slow threatened me with bodily harm if I use the M60 FH :eek:

shlomo
January 06, 2009, 14:00
Originally posted by owlcreekok
"Pudelgewehr"

BWAHAHAHAHAHAHAHAAAHAAHAAAHAHAHAHAHAHA

I got a choice between Izzy LB HG's or FNC HG's.

I also got a big ol honkin M60 flash hider too.

Slow threatened me with bodily harm if I use the M60 FH :eek:

I vote for the FNCs. Not a lot of that out there.

Ya oughta consider makin' a F/H of yer own design. After all, cylinders and tubes are what a lathe excels at.