# The Universal Threading Chart For The Ml7 (and Other Lathes)



## Tersti (Jul 23, 2016)

Hello hobby machinists!

I've been making changewheel charts, skip the next two paragraphs if that's all you're interested in.

I want to cut a 2BA thread on my ML7 because I'm finding the allen bolt which is used to engage and disengage the back-gear is turning out to be a nightmare to reach. The only way I can get to it with the allen keys I have means taking off the top of the near-chuck spindle bearing. I have thought about butchering my allen key to make it stubby enough to fit and I might still do that if nothing else works out but figured if I only need one hex-head bolt and I have stock for some other projects coming anyway I might as well give it a go if I come by an afternoon free. (If not I know I can just buy the bolts, annoying when you only want one though.)

I've been having a look around and couldn't find anything much good in the way of changewheel charts for 2BA on an ML7, plus I have a really strange set of changewheels. I've been plugging away at the numbers and worked out a method for building the gear trains on pen and paper including a neat trick using prime factors to narrow down the search space, but since I need to cut an approximation to 2BA this is still a real pain. So I finally caved in and delegated the job to my PC.

My program computes every possible gear train ratio with the gears the user specifies and number of studs to mount them on (not counting any needed idler wheels) - remembers the gear trains required used to produce each ratio - calculates the thread pitch produced by each ratio - filters based on those pitches - sorts the pitches - then prints the pitches out with their gear trains to a file. I've included a few runs of the program which are applicable to my ML7 and set of gears: 20, 20, 25, 30, 35, 38, 40, 45, 50, 55, 60, 65, 70, 75.

The program is written in Python (3) which means you can open and tweak the parameters if you have a different set of changewheels or a different leadscrew. (And obviously a Python interpreter and the good sense to verify for yourself that the program isn't malware.)

The charts have no awareness of what's a good approximation to a useful thread or what's a garbage thread, so everything's included. This is useful because it tells you approximations, so you CAN cut metric threads without a 127 tooth gear, or at least threads that are close enough. To cut a 1.00mm pitch thread I would choose the 1-stud metric chart and scroll down to where the 0.9s meet the 1.0s and pick one of the gear trains either side of 1.0. I was really surprised to find that my 38-tooth gear cuts a less accurate metric thread than a (20x65 / 55x75) combination (0.061% error!). If none of the options on the 1-stud chart look good then you can always go to the 2-stud chart and see if there's something better there. With 3 studs the program might start to, err, strain, but it should come up with some really bizarre but accurate approximates where the correct changewheels aren't available.

For instance on my ML7 with my gears I can cut a metric 1.00mm pitch thread with 0.012% error if I use the gear train (30x35x45 / 40x50x75).

For all my searching I couldn't find a resource like this anywhere else online, I'm sure I can't be the first person to do this so have I re-invented the wheel? Where do you other members normally go looking for resources like rare pitch changewheel charts? Any opinions on error margins like 0.061%? I would have thought a 2% error was acceptable for most hobbyist work.

Any suggestions on what charts I could now derive from the universal chart? I'm aware of Whitworth, UNC, UNF, BA - any other charts in need of some more choices on any particular lathes out there?


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## 4GSR (Jul 23, 2016)

Your charts are really not telling us anything.  Can you go back and put labels to each column?  How about combining to one sheet? Put it in Excel format. I suspect that's what you have it in to begin with.  To me, it's just a bunch of numbers.  I know what you are accomplishing here but many seeing this will probably be clueless.  And thanks for sharing! Ken


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## Tersti (Jul 23, 2016)

4gsr said:


> Your charts are really not telling us anything.  Can you go back and put labels to each column?  How about combining to one sheet? Put it in Excel format. I suspect that's what you have it in to begin with.  To me, it's just a bunch of numbers.  I know what you are accomplishing here but many seeing this will probably be clueless.  And thanks for sharing! Ken


Hi Ken, that's a good shout, I think looking at too many unlabelled CSV files in the academic world has rubbed off on me. I don't have the sheets in Excel format, the Python program included just writes the data out to the text files in a way that's just about human searchable and readable. I don't know of a good way to import the files into Excel since I never really use it, and since one of those charts is 26008 lines you'll have to find someone more patient if you want to copy it out by hand.

Anyway I've just touched up the output files to say what each column is. The first column is always either the threads per inch or metric thread pitch (clue in the filename). The second column is always the set of driving gears separated by commas. The third column is always the set of driven gears separated by commas. I'll follow through with an example:

Today I cut an M8 thread, I'd figured out I could build a gear train on the gears: 20x30 / 40x38 which cuts a 1.2533mm pitch gear and set the lathe up for that. That's putting two gears on one stud and using the mandrel and leadscrew for the other two gears, so one "stud". If you scroll down through my_universal_chart_metric_1stud.txt you'll indeed see the entry "1.25329   20,30   38,40". Interestingly the chart includes the line: "1.25076   25,65   55,75" which cuts a closer approximation so I tore the gears for the old approximation off and put the 25 on the mandrel driving the 75 on one stud turning with the 65 on the same stud with the 65 driving the 55 on the leadscrew. I've fit a nut on the thread I cut with it and I honestly can barely tell the difference between the quality of fit from the "approximate" metric thread and the "perfect" imperial threads cut on the same lathe.


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## 4GSR (Jul 23, 2016)

Thank you.
Ken


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## MozamPete (Jul 24, 2016)

Have you looked at ML7Gear written by Duncan Munro?  I think it would do what you are after and you can customize the available gear set to the ones you have at hand.

http://metal.duncanamps.com/software.php


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## Charles Spencer (Jul 25, 2016)

I used Excel to convert the files.  I did it for one of them.  If this is useful I'll do it for the others.


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## Tersti (Jul 26, 2016)

MozamPete said:


> Have you looked at ML7Gear written by Duncan Munro?  I think it would do what you are after and you can customize the available gear set to the ones you have at hand.
> 
> http://metal.duncanamps.com/software.php
> 
> ...



Ahaaa that's what I was looking for. Shame it only works on Windows. Looks like it has some awareness about how to fit gears too, that's pretty cool. Thanks.


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## Catcam (Apr 4, 2017)

Tersti said:


> Hi Ken, that's a good shout, I think looking at too many unlabelled CSV files in the academic world has rubbed off on me. I don't have the sheets in Excel format, the Python program included just writes the data out to the text files in a way that's just about human searchable and readable. I don't know of a good way to import the files into Excel since I never really use it, and since one of those charts is 26008 lines you'll have to find someone more patient if you want to copy it out by hand.
> 
> Anyway I've just touched up the output files to say what each column is. The first column is always either the threads per inch or metric thread pitch (clue in the filename). The second column is always the set of driving gears separated by commas. The third column is always the set of driven gears separated by commas. I'll follow through with an example:
> 
> Today I cut an M8 thread, I'd figured out I could build a gear train on the gears: 20x30 / 40x38 which cuts a 1.2533mm pitch gear and set the lathe up for that. That's putting two gears on one stud and using the mandrel and leadscrew for the other two gears, so one "stud". If you scroll down through my_universal_chart_metric_1stud.txt you'll indeed see the entry "1.25329   20,30   38,40". Interestingly the chart includes the line: "1.25076   25,65   55,75" which cuts a closer approximation so I tore the gears for the old approximation off and put the 25 on the mandrel driving the 75 on one stud turning with the 65 on the same stud with the 65 driving the 55 on the leadscrew. I've fit a nut on the thread I cut with it and I honestly can barely tell the difference between the quality of fit from the "approximate" metric thread and the "perfect" imperial threads cut on the same lathe.



Hi there are direct import facilities in Excel to put the data into Excel.
the file attached is done for your first file

I noticed something funny in the upload of this file. it would not accept the current Excel format as it did not recognized the format when trying to select a file for uploading. It did however recognize the older format for the 97-2003 excel spread sheets - now attached


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