Thread pitch math? (AKA...can I cut this Metric thread on my SAE only lathe?
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    Default Thread pitch math? (AKA...can I cut this Metric thread on my SAE only lathe?

    How do you do the math to determine metric thread pitch in .XXX" feed per revolution, So a guy can see if he can cut that metric thread on his SAE only lathe?

    Thanks!

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    For example.

    .7mm pitch .7/25.4= 0.02756 inches.


    Nothing will match perfectly on your lathe. That's why they have metric change gears with a 127/100 tooth combo gear.

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    A m3 thread is .5 mm or .01968"

    m6 thread is 1mm or .03937"

    *A metric thread die will get the correct mm thread.

    oh I see Erich answered ...

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    Here find information for a Logan lathe
    I have not read it it so don't know if correct ,I suspect it is correct.

    Metric Threading on Logan Lathes

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    Quote Originally Posted by jancollc View Post
    I know how to covert mm to inches....I was overthinking it via the word "pitch" thrown in....

    Thanks guys!

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    Still a good question, and a good title other guys can search..Had you mentioned the lathe perhaps a PM guy would have the gear.

    Yes if a mentionable lathe name...

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    The answer to the conversion of metric lead to inch lead has been given. I find it easier to convert metric lead to inch pitch (threads per inch), which can quickly be compared to the available pitches on the lathe's thread chart. That conversion is done by dividing 25.4 by the metric lead.

    For example, 25.4/.8=31.75, which means that a .8 mm lead can be substituted with a 32 TPI pitch in some cases.

    Similarly, 25.4/1.25=20.32, so a non-critical short M8 thread could be made with 20 TPI if you have a terrible need and can't do it right.

    Larry

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    Or, you can just find the closest match on your inch thread pitch gauge. Like Larry said, ornamental or lightly loaded short threads will often work fine even if you only get them close. Of course, this only works if you have the metric part in hand. Also, remember that when 'short' is used in this context, it refers to thread engagement length, not part length. In other words, you may have a male part with several inches of thread, but thread engagement is equal to the width of the nut, etc.

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    Depending on the thread; roughing it in with the closest TPI like Larry's example and finishing it with a die might be the best compromise between quick and proper barring the correct gearing for metric threads.

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    IMHO Gun work requires the best quality thread ...there should be no compromise.. if you can not make the thread you should not do gun work.

    Customer should be told "I can't do it right." but I will fudge something up and likely you won't know the difference.

    I have some very nice custom guns and so would not wish to have "can't do it right" threads...

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    I occasionally have the same situation in reverse; my lathe leadscrew is metric and I require an "inch" thread.

    Rather than going to the trouble to fit the "inch" leadscrew that I have, I use this program;

    https://www.varmintal.com/gearsvb61.zip

    The errors in the pitch are usually minuscule; probably better than the leadscrew error.

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    Ordinance threads. If the machine or the gunsmith can/wont make it to spec... Machinist hand book explains the limits for each thread class.

    https://www.natool.com/sites/default...atp101-102.pdf

    We need a 13 TPI and this machine only has 12.. but if we reduce the wire size to .xxx below spec we can/might make it fit into the nut.. nobody will know the difference..

    No wonder Gordon picks on USA quality.
    Last edited by michiganbuck; 01-18-2019 at 09:19 PM.

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    It does not matter if the lathe is mention-able here or not. A lathe has a lead screw and that screw will be either English or metric. English threads are measured in Threads per Inch while metric threads are measured in MilliMeters per Thread. You may notice that these fractions ("per" indicates division or a fraction) have an inverse nature and that complicates the math just a little bit. In essence, these are just two different ways of expressing the thread pitch.

    Since the inch is now DEFINED as EXACTLY 25.4 mm or 0.0000254 meters, that number constitutes an exact conversion from one system to the other. To convert between inches and mm you simply multiply or divide by 25.4.

    When we use gears, as in setting a lathe up for threading, we must use whole numbers of teeth. You can not make a gear with 25.4 teeth. So we must multiply that exact conversion number by a factor that produces a whole number (of teeth). The first, and most obvious such number would be 10 and 25.4 x 10 = 254. But that is a rather large number of teeth for a gear so we would like a smaller number. And that is possible because 254 is an even number: it is divisible by 2. 254 / 2 = 127. Stated another way, 25.4 x 5 = 127. And 127 is a prime number so it can not be evenly divided by any other number. If we want an exact conversion between inches and mms, we must have a gear that has 127 teeth or some exact multiple of that number. Because the other change gears on a lathe can have many different values, we have some freedom in choosing the companion gear to the 127 tooth one. What we need is a number that will work well with the other gear ratios that are available in a standard set of lathe change gears or in a quick change gear box. This is going to be a number with some small numbers in it's list of prime factors. Some tooth counts that are commonly selected include 120 (2 x 2 x 2 x 3 x 5), 100 (2 x 2 x 5 x 5), and 50 (2 x 5 x 5). By introducing these two gears into the lathe's gear train, what was an round number distance in inches becomes a round number distance in mm.

    For many purposes, the exact conversion that is possible with the 127 tooth gear is not really needed. There are several approximations that can be had with some other combinations:

    37 and 47 teeth provide a ratio of 47/37= 1.27027..... An error of about 0.021%. This is a very popular conversion.

    80 and 63 teeth provide a ratio of 80/63 = 1.26984..... An error of about 0.012%, an even more accurate conversion at the expense of using larger gears.

    Either of these two approximations is usually considered good enough for most lathe work. They may even be better then the accuracy of the lead screw which is the basis of the accuracy of any screw threads made on that lathe. There is little sense in having a conversion that is a lot better than the lead screw's accuracy. You may notice that the two errors above are in opposite directions. It is even possible to improve the accuracy of a screw that is cut in the opposite system as your lead screw by choosing the gear ratio that partially compensates for the lead screw's error.

    In addition there are other ratios that can be set up with standard, manual change gears, that will be close approximations for many metric threads. The accuracy of the approximation will vary from thread to thread and there is no guarantee that you can cut any particular metric thread with a given lathe and gear set. It is hit and miss with these individual approximations.

    So, the short answer is, YES you can cut threads of the other system of measure. A lathe with an inch lead screw can cut metric threads and a lathe with a metric lead screw can cut English (inch) threads.

    The thing to look out for when choosing the gears that you will use is the exact gear arrangement that is needed for each thread that you wish to add to your lathe's list. A quick change gear box will be a limiting factor because there are only a few possible combinations of gear ratios (threads or feeds) that these boxes are set up to produce. You can add additional gears in the gear chain that leads to the quick change box and they can, in theory, allow any thread to be cut. But each additional gear is an expense and is another gear that must be manually changed when those threads are to be cut. A lathe with a manually set up chain of gears offers a lot more flexibility. Manual change gear sets usually contain gears in a sequence "by fours" or "by fives". that means that the tooth counts on the gears of the set are even multiples of the numbers 4 through 13 or more. This produces a bunch of prime numbers in the various gears and those primes are what is needed to set up as many threads as possible. An example of a set of change gears "by fours" would be:

    4 x 4 = 16
    5 x 4 = 20
    6 x 4 = 24
    7 x 4 = 28
    8 x 4 = 32
    9 x 4 = 36
    10 x 4 = 40
    11 x 4 = 44
    12 x 4 = 48
    13 x 4 = 52
    14 x 4 = 56
    15 x 4 = 60

    Gears with other tooth counts are needed for some threads. For instance, a 27 TPI thread usually requires an additional gear with a multiple of 9 teeth. And threads with a prime number, like 29 TPI would require a gear with either that number of a multiple of that number of teeth (29, 58, 87, etc. teeth). Since 29 is not a prime number that appears anywhere in the list of gears, by fours, above, that number must be introduced with a separate gear.

    Another thing to look out for is will the gears fit on your lathe. There will be some gear ratios that seem ON PAPER to work for a particular thread. But when you go to the lathe you find that you can not physically assemble them in the needed sequence. I have a two arm bracket (banjo) for my lathe that allows additional gears to be mounted, but even with this some sequences are not possible.

    Most sets of manual change gears contain one pair of gears with the same tooth count. These are used to cut threads with the same pitch as the lead screw. Example: two 40 tooth gears.

    Manual change gears also make cutting odd threads a lot easier. For instance it is possible to find ratios for things like worms for a worm gear. Worms will have a pitch that involves PI as a factor and are thus irrational. These will always be approximations as you can not get an irrational number with gears that have integral numbers of teeth. Depending on the lathe, some of all of the gears between the spindle and the quick change gear box can also be changed manually. This provides additional flexibility in setting up threads.

    It has been said that the lathe is one of the most versatile tools in a metal shop. This is quite true.

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    Your best bet is to buy the change gear and learn how to use it...and check threads with a thread pitch gauge.
    You can keep a slug/rod of aluminum just for checking a thread pitch before running the part..The diameter doesn't matter for a pitch hecking rod so you can use theon over and over many times,.

    Fooling around to get one or two metrics just close/or near niss wont be much help..


    IMHO
    If turning or boring any gun threads, you should have a metric and imperial thread gauge sets, a set of wires, a micrometer, the wires formula, a machinist’s hand book or a good threads chart.
    The in-feed amount is very tricky because any part run-out will make that not a reliable number. Plus, the size of your nose flat can throw the in-feed number off.
    I have wire measured a thread then found a higher-grade nut that fits very well and used that for a go gauge. but that is only an OK gauge. it is just OK because for a customers job it should be wire checked.
    But a nut or even a real gage is not good if you don’t hone or file the kick-up that can build up at the OD from the cutting action.

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    Thanks for the additional info everyone. GREAT info here. I agree with the my customer deserving better than a "kinda close enough" thread pitch. My initial plans was to machine the tenon of the barrel to half or 3/4 final depth with he next closest TPI on my SAE lathe, then finish the thread with a (hopefully) and adjustable die, or a rethreading die.

    I have a Clausing 6913...If I can find change gears, I'll get them and figure it out to be able the correctly cut the occasional metric pitch that comes through the shop.

    Thanks again.

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    You guys are going to laugh at this, but there is a method to cut accurate metric pitches without the proper change gears. It takes a skilled person and isn't practical except in the most dire circumstances.

    Say you want to cut a .8MM pitch. Set the lathe up to cut 32 threads per inch. Think of the leadscrew as the side opposite and the taper attachment as the hypotenuse. .8MM is equal to 31.75 (31.749999999999999999999999999998) threads per inch. If the taper attachment is set at 5.071° and the workpiece is offset to be parallel (offset tailstock, offsettable center, etc) , the 32 tpi of the lathe will cut .8MM pitch.

    Edit: This is one time you can use the thread dial on metric threads cut with an imperial lead screw.
    Last edited by David_M; 01-20-2019 at 10:12 PM.

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    That's cute.

    Also not germane for most things, but there's that arrangement of putting an additional slide under the compound, then setting up a bar like a taper attachment with a bell crank to advance or retard the tool from the nearest standard pitch to make the one you need -Imperial to Metric or vice versa. There might be a name for it but I don't know it. I've only seen it set up as a demo.

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    Quote Originally Posted by David_M View Post
    You guys are going to laugh at this, but there is a method to cut accurate metric pitches without the proper change gears. It takes a skilled person and isn't practical except in the most dire circumstances.

    Say you want to cut a .8MM pitch. Set the lathe up to cut 32 threads per inch. Think of the leadscrew as the side opposite and the taper attachment as the hypotenuse. .8MM is equal to 31.75 (31.749999999999999999999999999998) threads per inch. If the taper attachment is set at 5.071° and the workpiece is offset to be parallel (offset tailstock, offsettable center, etc) , the 32 tpi of the lathe will cut .8MM pitch.
    That is an old mold maker's trick to compensate for shrinkage. There is another way if you can spend the time. Set the compound parallel to the bed and calculate the difference per turn between the closest English thread and the metric one. Divide that by 4 and hand turn the work a quarter turn at a time, set in the correction on the compound, and repeat. A lot of work but can make a good thread.

    Bill

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    ok, pretty sure the Clausing uses 16DP x 14.5PA gears??

    if so, that's a good thing as that's common and reasonably affordable.

    if my google-fu is on, the Clausing gear train is like a SB
    "single tumbler" machine so a metric conversion set requires a banjo swap?

    there *are* ways to get conversions so close that you can't even really call it a fudge with a gear or a few without the banjo swap.

    so, there's that.


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