Cutting Metric Threads Through A Qcgb On A Craftsman 12x36

Robert,
After your comment about me being the first to ever report a dual 48 gear I had to go out into the garage and take a closer look. Sprayed some PB Blaster on the 48's and the 40. What I thought were the rounded over ends of a bushing turned out to be a washer that was stuck to the gear from old, dried grease. The PB Blaster dissolved the grease and the washer fell off into my hand. A couple of lite taps with the handle of a screwdriver freed everything up on both the 48's and the 40.

I am still curious about not having a spacer on the 40 sliding gear to keep it in place.
 
Installed the gears today. I can now cut the common metric threads while still retaining the imperial threads from 8 tpi up. I also have realized that no spacer is needed on the 40 sliding gear because the thin washer on the combo gear keeps the 40 in place. I was surprised at how tight the bushing fits into the gears.

IMG_3598.JPG
 
mickri,

If you will read the operating instructions on the QCGB, you will see why the 40T gear is called the "Sliding Gear". You've already figured out why there is no need for a spacer. If you did the conversion following the instructions in "A 30 second metric conversion Version 2 Rev2", you'll already know that installing the 52/44 tooth gear pair only cost you the ability to cut 4.0 to 7.5 TPI threads. You use the sliding gear to switch between Imperial and Metric. You should go to Downloads also and get QCGB MM-Inch Thread Charts Atlas And Craftsman.pdf.
 
I downloaded all of the documents you mentioned and more months ago. One document that I have downloaded and may be one of the ones you mentioned has every possible tpi that can be cut through a qcgb by changing the gears. It was from that document that I found the gears that I would need to cut a 1.80mm thread. I converted 1.80mm to tpi which is 14.11 tpi and scanned the document to see if any combination of gears would cut 14.11 tpi through a qcgb. I found a listing for gears to cut 14.1136 tpi and those are the gears that I referenced in a prior post. I know that with the change I can no longer cut threads from 4 to 7.5 tpi and that all threads from 8 tpi and up can still be cut.
 
OK. Sounds like you mostly have the situation under control. But one of the two files that I mentioned was not uploaded until this afternoon.
 
Using the 51/54 or 63/60 gear combinations are only an approximation, the correct gears are 127/120 this will give a true metric pitch, the problem is that a 127 t gear is quite large and will not fit on many smaller lathes, but should fit on most 12" and even some 10" lathes.
 
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