View Full Version : change gears and feed speed for threading

12-04-2012, 11:23 PM

I've got a grizzly G4000 9x19 lathe and I need to cut some 30 tpi threads to match an existing part. 30tpi is not one of the threads listed in the thread plate on the machine. the G4000 has change gears as well as a 9 position selectable feed rate lever. They are used in combination to cut the proper tpi. From my searching on the internet I can only find calculators that provide the correct change gears to achieve the right tpi. But what setting do I put the feed rate lever on? Or am I stuck with only the tpi's listed on the feed plate. I'm sure there's something I am missing. Thanks for any help!


J Tiers
12-05-2012, 12:02 AM
It appears from the on-line manual that you get several gears with the unit, plus the metric conversion gearset. The rest of the adjustments come from the "feed lever", which really is a partial quickchange gearbox. The choice of the "A" and "B" gears selects what series of threads the gearbox settings give.

You get a 30 and 60, plus a 28, 36, 42, 45 and 80. I'm not clear on that because there seems to be another 30 required as well, for the 16, 18 etc thread series, and I'm not sure where it comes from.

You would need to add a gear combination that would give a 5/6 ratio and use the 36 tpi setting, or a 3/4 ratio used with the 40 tpi, etc.

The 40 tpi setting uses the 30 tooth gear in position "A", which is the ratio setting position, and the 60 tooth as "B". But ANY gear will do in "B".

It looks like you can set up for 40 tpi, then add the 80 and 60 linked together with the 80 driven, and the 60 driving the next gear.

That would work, BUT there are two issues.

One, there may be no place for them, and/or no way to link the 60 and 80.

Two, the 80 may already be used in the gear train, with no good way to get the 60 tooth linked up with it and engaged with more gears down the train.

12-05-2012, 12:30 AM
Thanks for the reply! I'm not sure that the 80/60 gears will be able to link, as you said, but I will look at it. Could you quickly explain how you determine the proper gears by ratio, or direct me to some literature so I could better understand the subject? Thanks again!


12-05-2012, 12:34 AM
Here is the lathe:


Here is the manual:


For threading, start at page 40.

12-05-2012, 08:29 AM

Observe from the threading chart (page 42) that the tpi increases with the number of teeth on gear 'b'. (Which drives the input shaft of the gearbox.)

It appears that you should have a 45T change gear, so setting up for 20tpi but replacing the 30T at 'b' with the 45T will give you 20 * 45/30 = 30tpi.



12-05-2012, 10:24 AM
Barrington beat me to it. He is it right with 30A 45B Lever 4.

Here are some other combos to get 30 tpi:
A B Lever
36 60 2
36 45 7
42 45 9

There's probably more.


12-05-2012, 01:39 PM
Thank you, thank you! Great explaination. I do indeed have the 45 tooth gear. I should be in business!

12-05-2012, 04:18 PM
Just to expand the explanation - for 'tpi' the middle row of numbers is key.


For each corresponding lever position the final tpi is just that number multiplied by 'b'/'a'



J Tiers
12-06-2012, 12:19 AM
hah..... very good...... I DID miss that the 45 is 1.5x 30........