Cutting 27tpi On Grizzly G4003g

[epanzella]

Working on various engines I often have to make adaptors. It seems like most of the adaptors I wanna make require a 27tpi thread at one end or the other. Of course the threading chart on my G4003G goes from 26 to 28 tpi, skipping 27. Has anyone come up with a gear setup for the G4003G that will do 27tpi?

 

[4GSR]

It's been known to thread 28 pitch in place of 27 pitch when it was needed.

The lead error is only .001323" per revolution. Only about .007937" in six threads.

At least get the thread cut to almost full profile and run a die nut over the thread to straighten it out to 27 t.p.i.

Change your stud gear going into the QCGB by two teeth and that should get you 27 pitch if set in either 26 or 28 setting.

 

[kd4gij]

If that lathe will do 1.0mm thread it is close to 27tpi that it will fit

 

[Wreck™Wreck]

27 TPI is what 1/16 and 1/8 pipe threads are for some reason, I have no idea why, much like 1" through 2" pipe threads are 12 1/2 TPI also for reasons unknown to me. I can only think that someone in the distant past wanted to make things difficult or there is some obvious relationship between the diameter and lead that I do not understand.

 

[4GSR]

............ much like 1" through 2" pipe threads are 12 1/2 TPI also for reasons unknown to me.....

1" through 2" are 11-1/2 T.P.I. not 12-1/2 T.P.I.

 

[Wreck™Wreck]

1" through 2" are 11-1/2 T.P.I. not 12-1/2 T.P.I.

Correct, Typo, one key off, the question remains why 1/2 and not 10 or 12 or even eleven, 11 1/2 must have some relationship that is not readily apparent.

 

[epanzella]

I think they did it to drive us crazy. I'm currently working thru different combinations of the change gear that came with my lathe. Nothing yet.

 

[Andre]

A 27 TPI thread is not too unusual to cut, however a lot of lathes can't cut it. Including my South Bend.

In South Bend manuals there is usually a page titled: "Cutting 27 TPI threads" or something to that extent. You need to purchase a specific gear to cut it, unless you have one of SB's more toolroom oriented models, which usually have that setting in the gearbox.

 

[dave2176]

If my math is correct here, (somebody feel free to jump in) 27 tpi is .03703" pitch.
45 gear on F and 60 gear on G with QCGB set at C3 it will produce a metric thread pitch of .9473mm. So .9473 times .0393 thousandths in a millimeter equals a pitch of .03722" which is less than .0002" off per thread. Should be close enough.

The bad thing is you are cutting metric so not so easy as with full thread dial use.

Dave

 

[rick9345]

27 TPI is what 1/16 and 1/8 pipe threads are for some reason, I have no idea why, much like 1" through 2" pipe threads are 12 1/2 TPI also for reasons unknown to me. I can only think that someone in the distant past wanted to make things difficult or there is some obvious relationship between the diameter and lead that I do not understand.

"Lost in the mist of time"