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View Full Version : Cutting a 27TPI thread on an Atlas 10F



sean69
03-10-2019, 09:29 PM
Hi all;

I have a 27TPI thread to cut on an Atlas 10F ... I have a quick change gear box installed on the lathe, but it has no setting for 27 TPI

I have a quick change gear set and a bracket (I have always called it a banjo...) BUT am missing some of the gears required to get the correct ratio (2x32 tooth gears) that and the bracket position A (the one to be used of course) has been welded several times & I can see a crack behind one of the welds - I don't have much faith in it holding together.

There are already about 6-8 gears on the bracket for the quick change box to be able to engage the lead screw forward/reverse control and engage the spindle. (of course none of them were 32 teeth!!)


Does anyone know if I can swap out the gears in place now (to engage the quick change box) with different sizes to be able to change the ratio and get that 27TPI?

-thanks
-sean

Stu
03-10-2019, 10:08 PM
Try this.

https://www.hobby-machinist.com/threads/change-gear-table.71526/

Page 32

lakeside53
03-10-2019, 10:20 PM
27 is missing on a lot of lathes.. 'cos it's hard to get gears to do it. Prime numbers and all that stuff

Read this : https://www.practicalmachinist.com/vb/general/how-cut-27-tpi-threads-lathe-343738/

RichR
03-11-2019, 12:13 AM
Try this.

https://www.hobby-machinist.com/threads/change-gear-table.71526/

Page 32

You need to create an account to access that. I found another source here:
http://www.machinistweb.com/forum/showthread.php?t=2711
While you still need to create an account there, they don't ask for a bunch of personal information like hobby-machinist does.
Just a user name, a password, and an email address.

old mart
03-11-2019, 07:46 AM
I agree with mf205i, if they are pipe threads, 1/8 npt is 27tpi, and 1/2 bsp is 28tpi, blame the Americans, they had to change for the sake of change.

MattiJ
03-11-2019, 07:55 AM
Got metric gears?
Some oddball combination might be enough close

My imperial Kerry would do 0.9375mm pitch that would be 27.09 TPI

rjs44032
03-11-2019, 09:36 AM
I've done this on my Sheldon. I will need to know the configuration of your end gearing and QCGB. Then I can tell you what gears to use.

Best Regards,
Bob

lynnl
03-11-2019, 01:44 PM
If you are cutting pipe threads, cut the threads at 28tpi. Do the math and you will see that the pitch error is .0013 per inch and thatís only a couple of tenths pitch error over the length of a typical pipe thread.
Mike

Wait a minute.. not to suggest 28tpi won't work, but 27tpi = ..037037 pitch; 28tpi = .035714 pitch.
The difference between the two is .001323. That's the error over the distance of one thread, not per inch. ...isn't it?
So over, say 1/2" or 14 threads engagement, the error is 14 X .001323 = .018519, by the time you reach the last thread.

sean69
03-11-2019, 01:56 PM
Try this.

https://www.hobby-machinist.com/threads/change-gear-table.71526/

Page 32

Thanks!

That is precisely what I am looking for - the lathe is a different model with a different QCGB but the setup may be the same :) I can start looking for a manual that matches my model.

thanks!!
-sean

sean69
03-11-2019, 01:59 PM
Wait a minute.. not to suggest 28tpi won't work, but 27tpi = ..037037 pitch; 28tpi = .035714 pitch.
The difference between the two is .001323. That's the error over the distance of one thread, not per inch. ...isn't it?
So over, say 1/2" or 14 threads engagement, the error is 14 X .001323 = .018519, by the time you reach the last thread.

I may be able to fudge it like that as the number of threads that are engaged is 4 -5 ... HOWEVER the part is a retaining screw for an 1873 Springfield Trapdoor sear bar. kind of the 'most critical' pieces in the action (lock) and quite precise fit. I'd like to try to get it 'right' before trying something else (and not damaging the lock plate threads in case someone comes along with an original screw - the metal is VERY soft)

sean69
03-11-2019, 02:10 PM
Thanks guys - you all helped out a little.... found the appropriate supplement here:

http://vintagemachinery.org/pubs/51/16337.pdf

it lists the setup for the change gears and my QCGB together :)