Thread pitch math? (AKA...can I cut this Metric thread on my SAE only lathe? - Page 2
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  1. #21
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    Quote Originally Posted by iwananew10K View Post
    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.
    Banjo swap?

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    as far as I can tell the factory metric conversion consisted of a whole new banjo with the 127/100 (or 110,120??) plus a handful of smaller gears.

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    Quote Originally Posted by michiganbuck View Post
    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 ...
    Is m6 thread ALLWAYS 1MM PITCH? Edwin Dirnbeck

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    Quote Originally Posted by iwananew10K View Post
    as far as I can tell the factory metric conversion consisted of a whole new banjo with the 127/100 (or 110,120??) plus a handful of smaller gears.
    Please excuse my ignorance, I've never rebuilt a lathe....just ran them forever. A "Banjo" is a piece, part or assembly in the power feed tranny?? Or?

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    Quote Originally Posted by edwin dirnbeck View Post
    Is m6 thread ALLWAYS 1MM PITCH? Edwin Dirnbeck

    It can be a .75 pitch (finer threads.)

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    banjo is the arm that the end gearing mounts to.

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    OR.. it can also be something sometimes mistaken for a musical instrument (thats the one with strings on it)....

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    Quote Originally Posted by cyanidekid View Post
    OR.. it can also be something sometimes mistaken for a musical instrument (thats the one with strings on it)....
    Well at least I know the difference between a violin and a fiddle. Violin has "strings" where as a fiddle has "strangs." Thanks!

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    Quote Originally Posted by iwananew10K View Post
    as far as I can tell the factory metric conversion consisted of a whole new banjo with the 127/100 (or 110,120??) plus a handful of smaller gears.
    Somebody know of a how what when and where on this? I really need to set up my Clausing 6913 to cut metric threads if I can. Somebody know of a how-to article or at least a parts list somewhere?

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    Quote Originally Posted by EPAIII View Post
    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.

    Some good information there EPAIII.

    An interesting note on inch to millimeter conversions.Attachment 247557
    Attached Thumbnails Attached Thumbnails 1-inch-25.4mm.jpg  

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    Quote Originally Posted by edwin dirnbeck View Post
    Is m6 thread ALLWAYS 1MM PITCH? Edwin Dirnbeck
    M6 is always 1mm pitch. If you want a different pitch then this will always be stated.

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    if you post a pic of your gear train, with tooth counts, I might be able to suggest some options...along with what metric pitches you want.

    You do have an 8tpi leadscrew?

    Here is what I think the factory set up would have been like..

    image.jpg

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    I programed hundreds of different metric and inch threads on an old masak nc lathe. It became very simple. The nc control didn't care if you were cutting metric or inch threads. You just input the distance between threads(pitch). If you are making a RUFF thread on an engine lathe and the lathe doesn't have the exsact thread ,just calculate the pitch you want and see how close your lathe will come to this.YOU MIGHT GET LUCKY.Edwin Dirnbeck

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    When you say or write "M6" you are implying that you mean the standard 6mm metric thread which has a 1mm pitch.

    BUT if you say or write M6-0.85 then that would be a non-standard metric thread.



    Quote Originally Posted by edwin dirnbeck View Post
    Is m6 thread ALLWAYS 1MM PITCH? Edwin Dirnbeck

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    I do not know which country the author is referring to as "this country" nor do I know the date of that text. I do know that the inch has been redefined as EXACTLY 25.4 mm. "Exactly" means that all additional places that you may want to express on that number would be zeros. 25.4 to a million decimal places; to a trillion decimal places, to a googleplex (that is a real, defined number) of decimal places. Never any other digit except zero. This is the DEFINITION of the inch: it is defined in terms of the metric meter (0.0254 meters) This should be the accepted value by all countries in the world. But who knows, perhaps some still have well preserved barley corn.

    There were older definitions and conversion values but they were very close to 25.4. And they are now obsolete.



    Quote Originally Posted by LexD View Post
    Some good information there EPAIII.

    An interesting note on inch to millimeter conversions.Attachment 247557

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    If he is doing gun work I am not so sure this is a good idea. When you start by cutting one pitch in the lathe and then switching to a die that assuredly cuts the correct, but different, pitch something has to give. It has been stated that the die will cut the correct pitch, but this is not necessarily true. The final pitch will be somewhere between the lathe pitch and the die pitch, but where may vary depending on factors not discussed here.

    But if we assume that the die does follow the correct pitch, then the lathe cut threads will represent some metal that was removed that those correct pitch threads would have left on the thread. It does not matter which flank of the thread this missing metal is on, at some point of engagement with the (correct pitch?) mating thread there will be a smaller amount of pressure or even NO pressure between the two threads. If you are mounting something, like a scope or a butt plate, it may make little or no difference. But if you are fitting a barrel to the receiver, then, no matter what torque value you use, it will be effectively held by fewer threads than the designer intended. And that could be a dangerous condition.

    I would not use this method if the thread is in a critical location.



    Quote Originally Posted by Hazzert View Post
    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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    Quote Originally Posted by MonCeret Gunsmit View Post
    Somebody know of a how what when and where on this? I really need to set up my Clausing 6913 to cut metric threads if I can. Somebody know of a how-to article or at least a parts list somewhere?
    Shoot me an email through the forum later today, or even a PM. We made some Transposition Gears for SouthBend lathes a while back and may have one last compound gear on the shelf. ( Same Diametral Pitch and Pressure Angle as the Clausing ) I am not positive about this, but can check later. A bit busy today, hence the need to remind me.

    Related, but possibly pertinent if you really wish to have the ability is that we've been getting asked to make another run so even if we do not have one in stock right now, we would likely be making some in the next month or two.

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    Quote Originally Posted by EPAIII View Post
    I do not know which country the author is referring to as "this country" nor do I know the date of that text. I do know that the inch has been redefined as EXACTLY 25.4 mm. "Exactly" means that all additional places that you may want to express on that number would be zeros. 25.4 to a million decimal places; to a trillion decimal places, to a googleplex (that is a real, defined number) of decimal places. Never any other digit except zero. This is the DEFINITION of the inch: it is defined in terms of the metric meter (0.0254 meters) This should be the accepted value by all countries in the world. But who knows, perhaps some still have well preserved barley corn.

    There were older definitions and conversion values but they were very close to 25.4. And they are now obsolete.
    Way off-topic but United States still uses two different definitions for inch.
    the "metric inch" and US Survey inches&feet. Survey inch is some 2 parts per million different..

    "When international measure was introduced in the English-speaking countries, the basic geodetic datum in North America was the North American Datum of 1927 (NAD27), which had been constructed by triangulation based on the definition of the foot in the Mendenhall Order of 1893, that is 1 foot =
    1200/3937 meters: this definition was retained for data derived from NAD27, but renamed the US survey foot to distinguish it from the international foot.[2] For most applications, the difference between the two definitions is insignificant – one international foot is exactly 0.999998 of a US survey foot, for a difference of about
    1/8 inch (3 mm) per mile – but it affects the definition of the State Plane Coordinate Systems (SPCSs), which can stretch over hundreds of miles.[10]

    The NAD27 was replaced in the 1980s by the North American Datum of 1983 (NAD83), which is defined in meters. The SPCSs were also updated, but the National Geodetic Survey left the decision of which (if any) definition of the foot to use to the individual states. All SPCSs are defined in meters, but seven states also have SPCSs defined in US survey feet and an eighth state in international feet: the other 42 states use only meter-based SPCSs.[10]

    State legislation is also important for determining the conversion factor to be used for everyday land surveying and real estate transactions, although the difference (2 ppm) is of no practical significance given the precision of normal surveying measurements over short distances (usually much less than a mile). Twenty-four states have legislated that surveying measures should be based on the US survey foot, eight have legislated that they be made on the basis of the international foot, and eighteen have not specified the conversion factor from metric units"
    United States customary units - Wikipedia


    LexD's piece of old text is probably from between 1944 and 1959 as it mentions British standard from 1944 but talks also about old US inch that was used until 1959 "metrification"

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    Fwiw ...
    The US threads use a different way on measuring threads - vs metric - but any thread on a firearm classed as say A 2A/2B or maybe 3A/3B, if it measures and cuts withing the specs is by definition *right*.

    "Close enough" is mostly "good enough" except for those who are making lab stuff for optronics, and similar.
    The firearm, missile, or nasa stuff wants the thread to be "in spec" and "close enough-inside spec" is good enough by definition.

    For those making high-end threads, the limits go way up, and so do the costs.

    A really good leadscrew thread can easily cost a VMCs or a good german sport cars price tag.
    If You don´t think so, You are just not looking at the right place and accurate enough.
    Think of modern telescopes and KTM tracking mounts.

    Does anyone know what the matched, double, ballscrews on a Mori DCG VMC cost ballpark ?
    Able to interpolate to better than 0.7 microns TIR round *at the workpiece* on cuts in a bore ?

    I suggest without any hard data that the screws are probably 10-20k$ each.

    My point:
    Don´t say "perfect" or "best-possible" on a professional machinists forum where lots of people can make "perfect" threads 10x better (or much better) than the common US or EU thread classifications.
    And charge accordingly.

    According to Moore & Wright gage blocks are lapped to 0.01 microns, not sure if it is flatness or surface or local feature error.

    Certainly, today, the "best" threads can be made to similar surfaces and accuracies approaching similar levels, to some extent.
    Whatever best means.


    Seriously, if anyone has any costs or data on accuracies or specs on the DCG/Mori screws accuracies and costs I would be very grateful.

  22. #40
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    Quote Originally Posted by hanermo View Post
    Fwiw ...





    My point:
    Don´t say "perfect" or "best-possible" on a professional machinists forum where lots of people can make "perfect" threads 10x better (or much better) than the common US or EU thread classifications.
    And charge accordingly.
    I get what you're saying. However I think most of us assume, when one makes a post or uses a chosen verbiage that an "average scope" of the average machinist in the average machine shop is assumed unless other wise specified. I worked for several years in the Jig & Fixture shop @ K25 Oak Ridge where previously the intricate parts of the bombs were machined. In that shop we machined most things to a 50 mil. If a guy posts "what do I need to do to hold tight tolerances with a harbor freight lathe"....I don't crow about ""true" tight tolerances." I presume he is asking how to do the best possible machine work with...what he has to work with. My opinion. Thanks.


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