Metric Thread Cutting Without Conversion Wheel

[Nigel Graham 2Participant @nigelgraham2]

On an Imperial lathe.

This won’t work for all threads, of course, and not over long thread distances.

Assuming you don’t need a long length of thread that can lead to significant accumulated errors, and can finish the work with a die, compare threads to find the nearest pitch or TPI. (The respective diameters do not matter.)

 

As this example:

Recently I cut an M8 thread on components turned on a Myford ML7 with gearbox, all set for Imperial threads.

Careful comparison of the Zeus Book tables revealed its 1.25mm pitch is very close to 20tpi.

Set the lathe to 20tpi, use a 60º tip (as for 1/4″ UNC), cut a few “thou” shallow, finish with a carefully-adjusted die.

 

“Why not” , you may ask, “just use an M8 die straight off in a tailstock die-holder?”

Simple: the commercially-made die-holder set I have does not fit tightly enough in the tailstock, it is a clumsy contraption to use, and trying to cut anything fatter than perhaps 4BA on sight with it is an utter pain. Also, more importantly, I needed the thread to be concentric with the part’s plain shank, turned with tailstock support.

 

NB: It won’t work so well for other pitches. Let’s examine the first few of the most common M-coarse threads more likely screw-cut rather than direct die use. The integer part of most of the TPI values is used among the inch-size threads but for this purpose should only be used over short lengths like stud and pin ends, and from half- to two-thirds depth as a “roughing cut” for the metric die. For closer analysis make comparative plots.

M6: 1mm pitch = 0.039″ = 25tpi almost.

M10: 1.5 = 0.0591    17tpi.

M12: 1.75 = 0.0689   14.5tpi

M16: 2.0 = 0.0787     12.7

M20: 2.5 = 0.0984     10.16

M24: 3.0 = 0.1181     8.47

Coming too close to the leadscrew pitch for comfort.

[halfnutParticipant @halfnut]

Needs must. Good idea.

If you have a set of standard Myford imperial change gears, you can cut all metric threads using the chart below.

Accuracy, as shown, is generally in the range of + or – 1 in 1000 to 8000. That is one thou per inch to 8 inches. More  precise than your leadscrew.

Ignore the final column headed No. Of Translator Used. It refers to the mathematical calculations, not to any gear. Just follow the gear train in the main body of the chart.

[Thor 🇳🇴Participant @thor]

Hi Nigel,

I have on a few occasions done the opposite, turning imperial threads on a metric lathe and finishing with a die. It works fairly well for some imperial threads.

Thor

[tminusa@yahoo.co.ukParticipant @tminusayahoo-co-uk]

L C Mason gives some tips for producing near metric threads on page 58 of his book “Using the Small Lathe”

[Nigel Graham 2Participant @nigelgraham2]

The standard change wheels for the ML7 will cut common metric threads, but in this case, having fitted a gearbox, I had to improvise a bit because I don’t have a suitable “top-end” driving wheel for the output of the reverse tumbler.

At least, so I thought but it’s possible one of the others in my standard set will produce closer approximations.

This is an early pattern gearbox, so I don’t know if later ones give a more comprehensive range.

.

That driving gear is meant to have only 12 teeth for the full range of the gearbox to work, but that’s a horribly frail looking thing so I normally use a 24T wheel and set the gearbox to twice the wanted TPI. The 12T wheel is still necessary for 32 and 40 TPI.

For ordinary turning (I can never remember if “sliding” or “surfacing”), even with the 24T wheel the gearbox still gives a very fine longitudinal feed.

[BazyleParticipant @bazyle]

halfnut – your chart hasn’t embedded. probably only accepts JPEG pictures. It may be showing on your computer as it is a local link.

[Bazyle]

On Bazyle Said:

halfnut – your chart hasn’t embedded. probably only accepts JPEG pictures. It may be showing on your computer as it is a local link.

It was a photo taken on my phone so should be Jpg. I will try again.

[John HaineParticipant @johnhaine32865]

Try this:

https://ridethegeartrain.com/index.php?fnname=chooseLathe%3EbeforeSubmit_notIndep_handleHidVar&language=English&adflag_f=no&previouslathe=Imperial_no_gearbox&gearboxfitted_f=no&lathetype=imperial&threadrequested=mm&threadtype=metric&threadsize=1.5&compoundincluded=1&autocompound_f=yes&errormax=0.8&maxresults=2000&changegearstring=16%2C20%2C24%2C32%2C36%2C40%2C56%2C56%2C60%2C+100&compoundgearstring=127%2F100&changeleadscrew_f=no&leadscrew=8&powerfeed_f=no&carriagefeedratio=0.34&crossfeedratio=0.1&changeablestud_f=yes&studgear=40&changeablelsg_f=yes&leadscrewgear=56&range=single&lolim=4&hilim=8&interval=1&prefintro=yes&prefquickstart=yes&preftutorials=yes&prefourlinks=yes&prefinputtext=yes&prefspecstext=yes&prefgeartables=yes&prefgearboxtables=yes&lsg=56&newgear_f=no&customdone_f=no&numn=1&numl=1&primaryratio=1&stdstudgear=40&stdleadscrewgear=56&stdleadscrew=8&stdcarriagefeedratio=0.34&stdcrossfeedratio=0.1&stdchangeablestud_f=yes&stdchangeablelsg_f=yes&stdgearboxfitted_f=no&stdcompincluded=1&stdauto_f=yes&stdchangegearstring=16%2C20%2C24%2C32%2C36%2C40%2C56%2C56%2C60%2C+100&stdcompoundgearstring=127%2F100&zout_p=yes&zin_p=yes&deglab_p=no&ptab_p=no&pcurve_p=yes&psymb_p=no&pline_p=yes&pcircle_p=yes&fullsearch_f=no&restricted_f=no&dp=18&diststudlsg=5.1&diststudcomp1=6.3&distcomp2lsg=7&distcomp2comp3=5.1&distcomp4lsg=5.1&feedonly_f=no&mode=afterDataInput&pagename=chooseLathe&helppage=chooseLathe&gearboxbrand=Myford_7_imperial_new_model&returntext=Return+to+the+Data+Input+Menu

 

[Peter_HParticipant @peter_h]

If you DO feel like cutting gears (I was daft enough to) then the key gear is 127T being a multiple of 25.4 mm/inch. I mad a vertical bearing mounted shaft which clamped over the toolpost. With the blank free to rotate and be moved back and forth by the crosslide feed, the gear was cut by a hob driven on a bar in the lathe chuck. It was easier to get rotating if I gashed the blank with a slitting saw and dividing head but not always needed. I needed a 6 slot spline in the hub, which I did with a keyslot broach and a mandrel to index each slot by 60 degrees relative to the one just cut. That way I built up a set, albeit only from ally, for the Chipmaster and wrote a program to give me all the setting for a specific metric thread. I’m not sure if I found the root message on this thread, but if the poster is using a Myford, then the 20DP gears are (or were?) easily available from RDG and ebay.

 

Peter

 

[Nigel Graham 2Participant @nigelgraham2]

Peter –

The original point was indeed overcoming a lack of suitable gearing to cut a particular metric thread; but whilst I am slowly plucking up the courage to try gear-cutting I’d not want to risk using my efforts on the lathe’s screw-cutting mechanism itself!

More likely I’ll investigate similarly close approximations using the Myford’s own standard change-wheels. Without the gear-box these will give a number of metric pitches anyway – but the gearbox removes much of that facility.

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