Metric Threading with QCGB not using 37/47 gears

[bhusted]

I need to setup for cutting a metric thread and almost pulled the trigger on buying the 37 and 47 tooth gears that people using for metric transposing when you can't fit a 100 and 127 on a smaller machine like my 10" Logan. I started looking for more information about how to set the QCGB using the 37/47 combo and got stuck because most of the resources I could find were for lathes without the QCGB. I did some math and found that maybe the 37/47 combo isn't the best way to go for me. has anyone else run into this?

The lathe has a 24t stud gear and a 48t screw gear with a 60t gear to bridge the gap as it is normally. This gives a 2:1 reduction from the spindle to the gearbox and allows for anything from 8TPI to 224TPI. My thinking is to speed up the gear box so that the 8TPI setting effectively becomes a 6mm pitch. I found a few references on PM that seemed to show South Bend did this for their metric setup. In order to speed up the QCGB the only things I would have to change are the stud gear and the screw gear to get a ratio of 0.944882. This can be approximated by a combo of 17/18 or some multiple and accurate to 0.05%. Using this ratio would provide metric pitches of 6, 4, 3, 2, 1.5, 1, 0.75, 0.5, and 0.25.

As anyone else with a QCGB machine tried setting theirs up this way? I'm thinking of using a 51/54 ratio. Maybe there is something I'm missing?

 

[BROCKWOOD]

I can only wait for the math magicians to chime in. Perhaps your 60t idler can be reduced to allow fitment of the 100 / 127 arrangement?

 

[bhusted]

My concern isn't that I can't fit the 100/127 because the 37/47 ratio is close enough for anything I would need to do. My question is really that the 37/47 ratio allows for only a few metric pitches and I would need to install even more gears on the banjo to utilize the pitches I mentioned above. If I can get all of the pitches I would need by only purchasing 2 gears instead of a whole set of change gears to use on my machine that has a QCGB, why buy the unnecessary gears? I just feel like I must be missing something.

 

[benmychree]

On my lathe, only eleven additional gears are necessary to cut every metric thread to be imagined, besides the 120/127 transposing gears; to cut the pitches that you list, only 3 change gears are necessary besides the transposing gears.

 

[Flyinfool]

A lot also depends on the length of the thread. If it a short part like a nut you will never notice the thread being off a couple percent (as long as it is not a critical or high load application) If you are threading a long shaft then the accumulated error could make a difference.

 

[DAT510]

Sorry I don't know the specs for the Logan Lead Screws (though I suspect its 8 tpi), QCGB Ratios or how the gear train is setup. But, I did create this spread sheet for my Jet 1024 lathe when I needed to cut some metric threads that weren't listed on the Charts. The cells are live so if you update the ratios, gears etc to the specs of your logan, it should give you the info your looking for. It is currently set up for the 100/127 transposing gear, but all you need to do is change the 37/47 or other ratios you're considering and you can see what calculates out.

Hope this helps.

 

[Canuck75]

bhusted,-
I'd go for a 80/63 transposing gear set. Nice size and almost no error. Made my own for my SB plus the complimentary stud gears.

Canuck75

 

[GrayTech]

There's an android app called pocket lathe gear calculator. I think you can use it with gearbox lathes if you know the internal gear ratios. May be useful.

 

[KyleG]

How are you coming along with this? I'm working the same problem myself. It looks like I can catch the "fractional" pitches (.5, 1, 1.25, 1.5, 1.75, etc) or the "decimal" pitches (.4, .45, .5, .7, 1.5, etc) but not both.

It's interesting that you mentioned you have a 48T gear on the gearbox input. Is it cutting pitches properly? My Logan 825 has a 24T on the reversing gear, and a 60T on the gearbox, and has been working for me. (Photo attached)

EDIT: This is wrong. I mixed a photo I pulled from the internet with a similar one I took of my lathe. I am set up with a 24/48 pair for inch threading just like bhusted. Apologies for any confusion this caused.

 

[mickri]

Halligan142 has a video on how to calculate what gears are needed to cut metric threads on a South Bend lathe with a qcgb. He walks you through all of the math. You could use your gears instead of the South Bend gears to get the answers for your lathe. Also there are several threads in the atlas/craftsman forum on how to do this on a 12x36 lathe with a qcgb. The threads are probably 4 or 5 years old. Do a search and you will find your answers.