View Full Version : help! thread cutting on lathe

04-30-2006, 07:32 PM
I recently acquired a box of the old Lyman Ideal loading tools. They are all based on 30 threads per inch and I don't have this setting on my lathe. It jumps from 28tpi tp to 32tpi. The closest I can get is by changing two gears to convert to metric threading. .8mm is close to 30 tpi but not exact. I get this pitch with a 46 tooth gear next to the spindle, 80 tooth idler and 60 tooth gear on the lead screw. quick change settings are c-6.
There must be a setting which will allow me to cut 30 tpi exactly but I can't figure it out.
Would appreciate any help from any one.

04-30-2006, 07:48 PM
Ummm, What are you trying to do, they are already threaded, correct?

04-30-2006, 07:51 PM
I am trying to make some additional parts that are missing.

04-30-2006, 07:56 PM
Knowing a little more about your lathe might help. Like make and lead screw pitch.

04-30-2006, 08:03 PM
jclayton, you may have to put on your calculating cap and figure out what gear is necessary to make your wishes come true, and then purchase or make said gear. Not as big a deal as it sounds.

04-30-2006, 08:13 PM
My lathe is a 10 year old Chinese 12" by 36" geared head machine. The lead screw is 8tpi. I should try the metric conversion with a c-5 setting. This would give me a .875mm pitch--maybe 30tpi? But I would still like your help.

04-30-2006, 08:27 PM
Marv Klotz has a program titled "change" at his web site:


It runs from a command box (windows98, ME, 2000 or XP) or DOS. With it, you can plug in your gear tooth counts and see if any missing threads are actually available. You first have to plug in the gears and examine the ratios called out for several threads support by the qc to see what it is doing. Den

04-30-2006, 08:58 PM
Years ago when I was making my living as a gunsmith, I found replacement barrels for a Model 12 Winchester required the barrel tenon to be 30 tpi, Because original barrels were expensive and very hard to find, many customers would have me fit another barrel from other makes of shotguns. This was to salvage the guns that were cut down or had damaged barrels. As best I remember Mossberg were the easiest to do but Remington were also used. To get the required 30 tpi I used the power feed, not the thread cutting gears, and reversed the lathe to keep it in time. The closest feed rate I had was .035 per revolution but it was very workable for the inch or less needed for the barrel ring. The true feed would be .033 per rev. A little practice may allow this method to work for you.

04-30-2006, 09:04 PM
Ok, had a few minutes to play so here's the output from Marv's program for the gears you seem to have:

(other combos discarded)

40:60-40:80 0.333333 0.000E+000 % **
40:80-40:60 0.333333 0.000E+000 %
40:60-40:80 0.333333 0.000E+000 %
40:80-40:60 0.333333 0.000E+000 %

What it means is that with your qc set to 10TPI (B4) and IF you can physically get the gears into any of these arrangements and IF I didn't screw up the calculations.

For example, 60/40 * 80/40 * 10TPI = 30 TPI (using the B4 setting)

added - Actually, a better solution is simply the 20TPI (C4) setting and the 60:40 (thru the 80 idler if you have to).

Good luck ! Den

04-30-2006, 09:23 PM
On South Bend lathes with the standard QC gearbox, you have to change the stud gear (normally 20t or 40t on my lathe) to something else to get the lathe to cut 30tpi. I can't remember offhad what the replacement gear is, but it might not be the same for your lathe, anyway. The important thing is the general principle of what you need to do: get another stud gear that will give you the right gearing to get 30tpi when the QC gearbox is set on "something." I'm too tired to figure it out, but it shouldn't be too hard if you're fully awake. I think on my lathe, if I got a 30t stud gear and set the QC gearbox to cut 40tpi, I"d get 30tpi...but you ought to work it out for yourself.

The South Bend "heavy 10" lathe has an extended range gearbox that has a setting for 30 tpi so this isn't a problem...but for most other lathes, it is.

Shuswap Pat
04-30-2006, 09:42 PM
If all else fails, and you can't get the required gears, set up a ratio with two sprockets. I wanted to cut metric threads, on my imperial lathe, and only had 30, 60 & 127 tooth gears( sorry - Q/C gearbox for imperials). I still couldn't get all the right combinations, so I used RC35 sprockets and chain to get the required ratios. 30/36 teeth - set the Q/C to 36, and it will cut 30 tpi.

04-30-2006, 10:58 PM
Thanks people.What a great place the internet is.
80 years old and still a bumbling amateur machinist, but it sure is fun.

05-01-2006, 04:54 AM
If Evan finds out what you called him he will be mad. ;)
"80 years old and still a bumbling amateur machinist, but it sure is fun."


05-01-2006, 09:56 AM
If your screw gear (the one on the end of the lead screw, next to the QC gear box) has 56 teeth as on my 9" SB, you can get exactly 30 tpi by substituting a 60-tooth screw gear and setting the QC on 28 tpi. K. Steiner