Cutting Metric Threads Through A Qcgb On A Craftsman 12x36

mickri

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Some of my projects will involve cutting metric threads and I wanted to figure out how to cut metric threads on my 12x36 lathe.
I know that there are numerous threads and videos about how to cut metric threads on an inch lathe all over the internet. I think that I have read most of them. There are mind numbing discussions about this gear or that gear and combinations thereof and error rates taken out 5 & 6 decimal places. But virtually all of the threads and youtube videos seem to assume that the reader already knows and understands how the gears work. Well for a newby like myself you are just stuck out in left field trying to figure this puzzle out. Especially how it works with a qcgb.
The first speck of light at the end of the tunnel came when I ran across a video on cutting metric threads through the qcgb on a South Bend model A 9 lathe.
What this guy did is gear his lathe so that his carriage moved 6mm instead of 1 inch in 8 revolutions of the spindle. I may be using some incorrect terminology here. To do this he geared his lathe so that the lead screw spun 1.89 times faster than normal using 54 & 80 gears instead of the normal 20 & 56 gears on his South Bend. With this change his lathe when set to cut 8 TPI now cut 6mm. 12 TPI cut 4mm, 16 TPI cut 3mm. 24 TPI cut 2mm, 32 TPI cut 1.5mm, 48 TPI cut 1mm, 64 TPI cut .75mm, 80 TPI cut .60mm, 96 TPI cut .50mm, 160 TPI cut .30mm and 192 TPI cut .25mm. I thought to myself why wouldn't this work on my 12x36.
I was now on a quest to figure out this intellectual challenge and was getting nowhere fast. Until I ran across a thread on this forum about slowing down the speed of the lead screw on a craftsman 12 with a qcgb. http://www.hobby-machinist.com/thre...on-a-qcg-late-model-12-craftsman-lathe.39019/ I want to give a huge thank you to Robert D for his explanation of how changing the gears effects the speed of the lead screw.
With this new found knowledge I set to trying different gearing until I found a combination of gears that would spin the lead screw approximately 1,89 times faster than normal. The closest that I have found so far is replacing the normal 40 gear with a combination 56/44 gear run off the inner 32 gear and replacing the normal 48 gear with a 40 gear. With this gearing the lead screw moves 5.988 mm in 8 revolutions of the spindle which is an error of only .002% if I got all of the math correct.
So am I totally lost here or did I figure this out correctly?
Chuck
 
I think that you are probably right but a member of the Yahoo Atlas_Craftsman group came up with a method for cutting at least some common metric thread pitches on an Atlas QCGB equipped lathe without affecting most of the Imperial pitches. It works on all 10" and 12" machines with QCGB. The expense consists of one 42T and one 54T gear, and giving up being able to cut 4.0 through 7.5 TPI without getting your hands greasy. All of the thread pitches from 8 TPI through 96 TPI are still available. I had intended to do up a formal threading chart for it but haven't had time.

You will find a detailed explanation of the method (with photos) in DOWNLOADS under Atlas/Craftsman/AA\A/C Lathes\A/C Lathe Charts, Tables, Metric & Misc Docs. File name is A30secondmetricconversion2rev2.PDF. It is currently at the top of the list in Downloads as I just uploaded it but of course it won't stay there as more files are uploaded.

Cutting metric threads isn't on my bucket list but I bought a pair of gears from Clausing JIC.
 
I had seen what must of been an earlier version with no pictures of what you uploaded. What I had seen only mentioned being able to cut the 1.00mm, 1.25mm & 1.50mm threads and I wanted to be able to cut some of the finer metric threads so I kept searching. After reading what you uploaded I will go that route. I will try to do the math to see what other metric threads can be cut and do a threading chart. It may take me a couple of days to get it done. I'll post the results. Now I have to decide do I try to make the gears or just buy them.
Thanks for you help.
Chuck
 
Right. Carvel originally posted this idea early this year. More recently, he (or maybe it was I) decided that the information should be expanded and placed in Files for downloading. He asked me to proof-read his edited copy and if I had any, make suggestions for changes or additions. It occurred to me while doing that that there were other metric threads that it might be close enough to cut, and I came up with a few of the more obvious.
 
In addition to what Robert posted they also have a section in the MOLO on doing metric threads with a QCGB. I also have what I believe is out of an earlier MOLO that has a lot of other feed setups. Atlas10QCgearsetups.pdf Maybe Robert can tell me which one it came out of as I would like to find it in a printed copy.

Also when doing metric threads you need to be careful when releasing the half nuts. Some places say to never release them until the thread cutting is complete. Another site I read and what I did was when you release them you turn off the lathe at the same time. You then pull out of the work, next you reverse the lathe and when the thread dial is at the place you released from you engage the half nuts and let the carriage move past the end of your threads. You then turn off the lathe and place the motor in forward, set the cross feed for the next cut and do the process again until completed. I did this to cut metric threads on a ER 32 collet chuck I made.

IMG_1571.JPG IMG_1574.JPG
 

Attachments

  • Atlas10QCgearsetups.pdf
    4.8 MB · Views: 66
Rob,

Your PDF is a copy out of a version of the MOLO (because the right half of the last page shows the tab divider for 8 - LATHE ATTACHMENTS) that I didn't know existed. Because the page numbers start with "1" instead of around "95", it is from one of the versions that all say COPYRIGHT 1937 on the copyright page on the back side of the title page. Because it covers the Atlas 1500/1700 the MOLO was printed between 1947 and 1952, the years that those models were in production. There may be a few of them around but I routinely ask eBay sellers which lathe make, model and size is mentioned on the first page of Part 7 - THREADING, and so far haven't turned up anything except either the Atlas F-Series 10 inch or the Master Craftsman 12 inch. But I accidentally bought one about a year ago (think it was something else) that turned out to cover the Atlas 6 inch so have been looking for one covering the Craftsman 6 inch. I'll add Atlas QCGB to the list.

The exact pages that you have a PDF of were also printed as a standalone and sold with the QCGB conversion kits. My friend in South Africa has the only original I know of. But one could turn up one day. He scanned it and sent it to me. I converted it to TIF and did some cleanup. And you can have a PDF copy of that if you wish. It's in DOWNLOADS in the Atlas Lathe Manuals category, I think. If I didn't upload it, let me know.

One last comment before I quit for dinner. If you look on page 4 (printed page #, not file page #) of your PDF, you will see that you could have cut the metric threads (nice job, BTW) with less trouble if your machine has a FWD-OFF-REV switch. For the ID threads, the process is the same except that instead of backing the cutter "Out" after each pass, you back it "In".
 
Rob,
Thanks for the input. One of my projects is to make an ER chuck which requires metric threads. Hence my research into metric threading. I saw the info on metric threading in the MOLO and it seemed like you had to change gears for every thread. That is probably more precise but over the short, 1 inch or less thread lengths that I anticipate I think that I will be fine with the settings posted above. Had some time this afternoon and did a metric thread chart. I did the math for every TPI on the qcgb. Most of the threads were not usable so I did not include them in the chart I made. My chart is a simple excel spreadsheet. A copy is attached.
 

Attachments

  • Metric threading chart.xls
    7 KB · Views: 59
Thanks. That saves me some time.
 
Robert, Thanks, I was able to find the PDF of the QCGB and it is a lot nicer copy than the one I had. I think I will print that one out as I like to have a printed copy by the lathe.

Also I do have a reverse switch but when you are threading to a shoulder you need to be able to release the half nuts. You just need to make sure that the threading dial does not do a full turn but that is not a problem. My explanation might not have been so good.
 
Rob,

I don't understand why you have to release the half nuts just because you are threading to a shoulder. Cut a thread relief to give you a little lee-way. And use a much lower spindle speed or surface speed than you would use for turning the same material. And don't run your belts loose.
 
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