# Metric Threading with QCGB not using 37/47 gears



## bhusted (Nov 4, 2021)

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?


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## BROCKWOOD (Nov 4, 2021)

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?


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## bhusted (Nov 4, 2021)

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.


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## benmychree (Nov 4, 2021)

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.


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## Flyinfool (Nov 4, 2021)

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.


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## DAT510 (Nov 4, 2021)

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.


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## Canuck75 (Dec 12, 2021)

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


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## GrayTech (Dec 12, 2021)

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.


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## KyleG (Dec 21, 2021)

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.


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## mickri (Dec 21, 2021)

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.


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## bhusted (Jan 4, 2022)

Thanks everyone for your responses.  Somehow notifications on this thread didn't get to me so I'm sorry for the delay.  

@Flyinfool I understand that none of the conversions for an English lathe to cut metric threads are going to be perfect other than the 100/127.  I don't often need metric pitches, but want to have something that works for many common pitches in my arsenal when needed.  As you said with the short length of thread on a nut (probably all I'll ever need) the accumulated error is negligible.

@Canuck75 With your 63/80 setup, what settings do you use on the QCGB to set metric pitches?  Do you have to install other gears to open up more options?  Maybe I'm missing something because I don't see many common metric pitches in the converted chart using this ratio.

@KyleG I can cut English pitches just fine.  There should be a 2:1 ratio from the spindle to the gearbox input.  Mine came equipped with 24/48.  You can see from Canuck75's picture that he has the same 2:1 ratio using 28/56.  I'm working from a spreadsheet that I made to try and help myself to understand the challenge.  I cannot guarantee it is free of mistakes, which is why I asked for some advice here.  I may be totally wrong or missing something.  

@mickri I'll look again for threads in the Craftsman/Atlas forum on this.  I have one of these machines and many people use 56/44 to transpose for metric pitches.  I've never used the Craftsman to cut metric threads either.  I did look at Halligan's video on this and it seems that he was doing what I was thinking of by trying to effectively transform the 8 tpi leadscrew into a 6mm pitch


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## mickri (Jan 5, 2022)

The 60 gear in your gear train is an idler gear.  It does not have any affect on the ratio.  What you need to do is to replace the 60 with a combo gear to change the speed of the lead screw to a metric pitch.  On my craftsman lathe a 44/52 combo changes the lead screw from 8 tpi to 7.5 mm.  If you have a craftsman 10 or 12 lathe I would use it to cut metric threads.  Very easy to switch between imperial and metric threads on a craftsman lathe.  All you have to do is move the sliding gear.  In the out position you cut imperial threads.  In the in position you cut these metric threads.   7.5, 6, 5, 4, 3, 2.5, 2, 1.5, 1.25, 1, .75 and .50.  If you should need to cut any other thread, imperial or metric, the manual for the qcgb does all of the math for you.  It lists the gears needed to cut several thousand different threads.  To use the manual for metric threads you need to convert the metric to tpi.  For example.  One of my projects needs a 1.80 mm thread.  25.4/1.80 = 14.1111 tpi.  The qcgb manual lists the gears for a 14.1137 tpi which is close enough for a 1/2" long thread.


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## KyleG (Jan 6, 2022)

@bhusted Yep, I goofed. I edited my original post so as not to confuse posterity.


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