Go/no go thread gages

By pipe hand (*)

Date 09-22-2008 07:03

depending on what type pf threading is being done dia is probably to small before threading
SC

By flamin

Date 09-22-2008 12:06 Edited 09-22-2008 13:22

Go/NoGo gages measure Pitch Diameter. They aren't really intended to measure the Major or Minor diameters. Pitch diameter is the area in the thread where the cross section of the thread, and the groove of the thread are equal. Most specs allow for 3 turns max with the NoGo, if the hole is deeper than 3x the diameter. With shallow holes, this usually wouldn't be allowed because it significantly compromises the strength of the thread. Also, the right gage needs to be used for the class of fit requred, 2b or 3b for internal, 2a or 3a for external. Class 2 fits are generally more common than class 3, which have tigher tolerances and better fit up than class 2.

Jason

By Mikeqc1 (****)

Date 09-25-2008 11:27

Go thread plug gages are made to check Pitch DIA , Major Dia , thread angle and lead at the maximum metal condition, the no\go gage is made to check pitch DIA only in the direction of minimum metal condition of the threads.

MDK

By bozaktwo1 (***)

Date 09-22-2008 16:41

If your no-go plug gage threads into the hole, then the threads were cut too loosely. This can result from a couple different conditions, but usually it happens when there isn't sufficient material to cut the threads on. A 1-14 will be fairly sloppy to begin with, but the no-go gage should not thread in.

By Bill M

Date 09-23-2008 15:28

1"-14 sounds like an odd thread size call-out. Must have been fun finding that tap. What is your tap drill size? Are you reeming to tap size? How deep are you tapping? Are you machine tapping these or are you doing it by hand?

By michael kniolek (***)

Date 09-23-2008 19:58

1-14 is "special" off print from the 60s The No go plug indicates pitch DIA (agreed?)
the drill size i think was .9375 (sound right?)although i measure the holes to be .95 plus after tapping.
I got a whole **** load of these from a new vendor i may need to take a road trip to see whats up, the fit of what they gave us compared to the older ones is bad real bad (steam nozzle) so the gages used on the old ones make sense go threads nice, the no go stops just after the treads engage.
these threads other than this are all busted and chipped junk.
So do you all agree that the no go tests pitch dia?
Bill all good questions i will be sure to ask.
MDK

By flamin

Date 09-23-2008 21:24

1"-14 in not a "special" size. It's just a standard UNF thread. The recommended tap drill size for a 1b or 2b class is 15/16" (.9375), as you mentioned, although it can range from .923" up to .942". BUT, the hole being at .950, even though it is oversized could still gage properly. Chances are that when the part was tapped it wasn't done properly. The wrong tap, a dull tap, wrong feeds and speeds, or wrong cutting oil could easily cause those things to happen. I would suspect any of the latter before the I would suspect the minor diameter hole size.

Jason

By Bill M

Date 09-24-2008 17:55

My Machinery's Handbook lists 1"-12 as the standard UNF

1"-8 is a UNC
1"-12 is a UNF
1"-14 is a UNSf (formerly NF)
1"-18 is the UNS
1"-20 is a UNEF
1"-27 is a UNS
1"-28 is a UN
1"-32 is a UN

Twist drills are roughing tools that may be expected to drill slightly oversize. Try drilling undersize, and reeming the hole up to the correct tap drill size. De-burr /chamfer the hole a bit, then use a fresh tap and then inspect with your go / no-go gage.