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Cutting Metric Theads on a PM1236 lathe

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  • #31
    Harold, all you will need to change between metric and imperial threading is to move the spacer on the lower gear (and adjust for proper mesh etc).

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    • #32
      Harold. You can use your theadigdial but make sure you have the right gear on it don't ask how I know.
      There should be gears supplyed with the lathe you should find them in the toolbox. And you should find wich gear you have to use from the chart below the threadigchart on the gearcover..
      Best reg
      CS

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      • #33
        CS, I know that a selection of threading dial pinions is supplied for machines with a metric lead screw but are they usable on an imperial lead screw cutting metric threads? The pinions that came with my CQ6230A-1 lathe are 14,15 and 16 teeth.
        Last edited by The Artful Bodger; 08-04-2013, 04:04 PM.

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        • #34
          Originally posted by crancshafter View Post
          Harold. You can use your theadigdial but make sure you have the right gear on it don't ask how I know.
          There should be gears supplyed with the lathe you should find them in the toolbox. And you should find wich gear you have to use from the chart below the threadigchart on the gearcover..
          Best reg
          CS
          Good Morning,

          While photographing the area requested by a different poster, I noticed that the Thread Dial Chart (displayed on the lathe) specifically indicates TPI with NO MENTION of metric. This would strongly suggest that the Threading Dial was not intended to be used. I had previously surmised that if a Thread Dial was used, a different gear would be needed on the chaser but how is it to be used? When cutting TPI, the chart indicates that certain numbers may be used to start a thread and in some instances only a single number must be used (or possibly an unmarked space(s) between two numbers).

          How have you used your dial?

          EDIT: while I was typing, Artful was typing and sending. He beat me by a few seconds.

          Harold
          Last edited by hwingo; 08-04-2013, 04:12 PM.
          For those having fought for it, Freedom has a flavor the protected will never know.
          Freedom is only one generation away from extinction.

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          • #35
            Harold, the threading indicator is not difficult to use and of course you can use it for imperial threads on your machine.

            The indicator has a number of marks, lets say six and it might have intermediate marks as well.

            To use, close the half nuts as a mark ('1' for example) passes the index. The carriage will start to move and you make the first cut and disengage the half nuts then retract the tool and wind the carriage back to your starting point. Re-engage the half nuts when number '1' passes the index again.

            Depending on the pitch of the thread you are cutting and the pitch of the lead screw you may be able to use more than one of those marks and that is what the threading indicator table shows. If the table shows a '/' that means you can use any mark for that pitch. But only for imperial threads on your machine.

            There is quite an advantage in being able to use the threading dial on a single phase lathe as it is not necessary to stop and reverse the motor. Be aware that if yours is single phase and you 'engage' reverse with the carriage handle while the motor is still turning in the forward direction the motor will continue to run forwards!
            Last edited by The Artful Bodger; 08-04-2013, 04:28 PM.

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            • #36
              Originally posted by The Artful Bodger View Post
              Harold, the threading indicator is not difficult to use and of course you can use it for imperial threads on your machine.

              The indicator has a number of marks, lets say six and it might have intermediate marks as well.

              To use, close the half nuts as a mark ('1' for example) passes the index. The carriage will start to move and you make the first cut and disengage the half nuts then retract the tool and wind the carriage back to your starting point. Re-engage the half nuts when number '1' passes the index again.

              Depending on the pitch of the thread you are cutting and the pitch of the lead screw you may be able to use more than one of those marks and that is what the threading indicator table shows. If the table shows a '/' that means you can use any mark for that pitch. But only for imperial threads on your machine.

              There is quite an advantage in being able to use the threading dial on a single phase lathe as it is not necessary to stop and reverse the motor. Be aware that if yours is single phase and you 'engage' reverse with the carriage handle while the motor is still turning in the forward direction the motor will continue to run forwards!
              Good Morning!

              Yep, I know what you are saying. I use my threading indicator very frequently and have grown quite fond of it. That which I was addressing is, CS stated that he used his dial with *metric* threads on his imperial lathe. Since I have never cut metric on an imperial type lathe, how was he using his threading dial? Even if he changed the gear in the threading indicator, why would he need a dial or how would he know when to engage the dial? It was my impression that when making the second, third, fourth, etc. pass, the machine is turned off, the half nut remains engaged, the cross feed is back out enough to clear the threads, the lathe is switched to reverse, the lathe is turned back on, and you switch again and send the carriage on the next pass.

              Harold
              For those having fought for it, Freedom has a flavor the protected will never know.
              Freedom is only one generation away from extinction.

              Comment


              • #37
                Harold, as I understand it one cannot use a threading indicator when cutting metric threads using an imperial lead screw and vice versa. It is all to do with how the threads are defined.

                I believe your procedure when keeping the half nuts engaged is correct.

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                • #38
                  I am going to stick my neck out here and say I do not agree with some of the restrictions posted by others.

                  I have a similar Chinese lathe with the normal 120/127 conversion gear at the back. Only my back gears are LOT more messy that the previous photo!!! Mainly carbon dust mixed with oil I think, from the cast iron gears.

                  The 127 tooth gear compensates for the imperial leadscrew - no big deal. When you engage it the lathe becomes 'metric'.

                  Now, the lock nut. It can be a right pain waiting for a single phase motor to wind down before it can be reversed. I very often don't both waiting. I disengage the clutch on the motor to stop the spinning, disengage the locknut on the lead screw, extract the tool from the thread and wind the carriage away back to the start. Re-engage the lock nut and give the leadscrew a bit of a nudge with the motor clutch to get rid of the slack. Position the tool tip to the first turn of the thread. (It's still 'out' from the previous cut of course.)

                  Ah - but it may not be aligned with the previous cut! True, true, but you will have the right pitch.

                  All I do now is tweak the little slide on the carriage to line up the tool tip with the existing cut thread. Then I back the tool away from the chuck by hand for two turns of the chuck, move the tool tip in to the right depth, and start another pass.

                  Beats hell out of rewinding for 30 mm of a 0.8 mm pitch in hard metal.

                  Cheers

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                  • #39
                    Originally posted by rcaffin View Post
                    Beats hell out of rewinding for 30 mm of a 0.8 mm pitch in hard metal.

                    Cheers
                    Why does the hardness of the metal have anything to do with it ?

                    Is it a different operation for brass ?

                    What about licorice in case I want to make anything for a Bridgeport ?
                    .

                    Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.



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                    • #40
                      Originally posted by John Stevenson View Post
                      Don't worry Harold, very subtle English joke.
                      Aren't all English jokes subtle?

                      Originally posted by hwingo View Post
                      ...I used to work with some English folks in Saudi Arabia and their sense of humor was quite foreign to me...
                      Proof?

                      Originally posted by Richard P Wilson View Post
                      Yes, so subtle even some of us English folks don't get it...
                      Building a stronger case...

                      Originally posted by John Stevenson View Post
                      ...In fact I do all my threading, imperial and metric this way and the threading dial has been swinging free for about 12 years.
                      That's how I've always done my threading.
                      Keith
                      __________________________
                      Just one project too many--that's what finally got him...

                      Comment


                      • #41
                        Originally posted by rcaffin View Post
                        I am going to stick my neck out here and say I do not agree with some of the restrictions posted by others.

                        I have a similar Chinese lathe with the normal 120/127 conversion gear at the back. Only my back gears are LOT more messy that the previous photo!!! Mainly carbon dust mixed with oil I think, from the cast iron gears.

                        The 127 tooth gear compensates for the imperial leadscrew - no big deal. When you engage it the lathe becomes 'metric'.

                        Now, the lock nut. It can be a right pain waiting for a single phase motor to wind down before it can be reversed. I very often don't both waiting. I disengage the clutch on the motor to stop the spinning, disengage the locknut on the lead screw, extract the tool from the thread and wind the carriage away back to the start. Re-engage the lock nut and give the leadscrew a bit of a nudge with the motor clutch to get rid of the slack. Position the tool tip to the first turn of the thread. (It's still 'out' from the previous cut of course.)

                        Ah - but it may not be aligned with the previous cut! True, true, but you will have the right pitch.

                        All I do now is tweak the little slide on the carriage to line up the tool tip with the existing cut thread. Then I back the tool away from the chuck by hand for two turns of the chuck, move the tool tip in to the right depth, and start another pass.

                        Beats hell out of rewinding for 30 mm of a 0.8 mm pitch in hard metal.

                        Cheers
                        Your post would make a little more sense if you would edit the term locknut to the correct term halfnuts.
                        James

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                        • #42
                          Some basics of threading indicator dials:

                          A threading dial or indicator is designed to synchronize the thread being cut with the thread on the lead screw.

                          For cutting English/inch threads, with an English/inch lead screw this synchronization is fairly simple because the English/inch threads were WISELY specified in TPI, using whole or round numbers. The most common lead screws on these lathes are 4, 8, and 10 TPI. 8 TPI is the easiest case so I will use it for example.

                          Any thread that is an integer multiple (1X, 2X,3X, etc.) of the lead screw's pitch will synchronize at any position on the lead screw and can be cut without any thought to synchronization. Just engage the lead screw and cut. No problem.

                          Now, most English/inch thread dials are based on a four inch synchronizing distance. Why? Well consider, a lead screw with a whole number of threads per inch (TPI) and a thread being cut with a whole number of TPI WILL synchronize after one inch of travel on each. Thus, an 8 TPI lead screw and even a really odd number of say, 17 TPI being cut will be geared to both travel one inch for those numbers of revolutions. And since both of these thread pitches are whole numbers, the threads will be in step or synchronized at one inch intervals. So, any whole number of TPI will synchronize after a distance of one inch. Some, like the multiples of the lead screw TPI, may synchronize sooner, but all of them will synchronize at one inch intervals.

                          So why don't the dials use a one inch distance? Well, some English/inch threads are specified with 1/2s, like 4.5 TPI or 11.5 TPI. A thread with a 1/2 in it's TPI spec. will be exactly 1/2 thread off at a distance of one inch. But it will synchronize after a two inch distance. So a threading dial based on a two inch distance will handle all integer and half increment threads.

                          But still, some threads may have a 1/4 in their spec. These threads will require a full four inch synchronizing distance. This is why most dials for English/inch threads are based on a four inch synchronizing distance. The four numbered marks on their dials are at 90 degree angles and are marked 1, 2, 3, and 4 or that number of inches. The marks between the numbered ones are for half inches and quarter inches. A simplified dial could have only one mark and all English/inch threads would synchronize at that single point. But time is money and the additional marks allow a machinist to engage sooner when the thread being cut allows more synchronizing positions. That is what the rules on where to engage are all about: saving time.

                          Metric threading is more complicated. Unlike the wise men that invented the English/inch system of threads, the metric guys dropped the ball. They decided to base the metric threads on the lead in mm instead of threads per mm or per cm. By using round numbers (like 1.5mm, 1mm, 0.9mm. 0.7mm, 0.5mm, etc.) the math becomes more problematic. For instance, assuming a 4mm lead screw, a 1.5mm thread will require a 12mm synchronizing distance. And a 0.7mm thread will require a synchronizing distance of 28mm. That is 7 full turns of the lead screw.

                          Due to the way that metric threads are specified, metric threading dials are a lot more complicated. Many have their own change gears for different threads.

                          Since there is an exact, rational (whole number) ratio between the inch and the mm (10/254), it is possible to synchronize metric threads that are being cut on an English/inch lead screw (and vice-verse). However, the problem is that the synchronizing distance is a lot longer than 4 inches. Thus, the threading dial required would need a large gear that engages the lead screw or a large internal gear ratio inside it. This could be done. As an example, a 0.45mm thread is equal to 56 4/9 TPI. This is an exact conversion. So, due to the ninths fraction, we would need a nine inch synchronizing distance. 9 X 8 = 72 revolutions of the lead screw. Not too bad. Another example would be a 0.7mm thread. This is exactly equal to 20 8/25 TPI. With the 25s fraction, you would need a 25 inch synchronizing distance. 25 x 8 = 200 revolutions. This could take several minutes of waiting for the dial to indicate synchronization. And if you miss it, you must wait again. This is why using a dial to synchronize metric threads with an English/inch lead screw is not a practical thing. Leaving the half nuts engaged and backing up the work is the better solution.

                          Also, larger dial diameters would be needed to prevent false lineups.
                          Paul A.
                          SE Texas

                          And if you look REAL close at an analog signal,
                          You will find that it has discrete steps.

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                          • #43
                            It is possible to cut metric threads, using the thread dial, and dis-engage the half nuts. But there's a bit of a trick to it.

                            Set up for your first pass, and engage the half nuts when a number passes the thread dial index, say "1". Remember this number, it's important.
                            I'm going to use "1" in my description below, but it can be any number, as long as you always use that number.

                            Make your first pass, and when you get to the end of your thread wind out the tool and dis-engage the half nuts. You MUST then stop the lathe
                            before the thread dial makes a full revolution. Do not move the carriage by hand.

                            Reverse the lathe and re-engage the half nuts when "1" passes the index mark, let the lathe take the carriage back to the start point. You can
                            dis-engage the half nuts at this point, but again, stop the lathe before the thread dial makes a full revolution.

                            Set up for your next pass and repeat the above steps, making sure to engage the half nuts on "1" again.

                            By engaging the half nuts on the same number you are engaging the lead screw in the same place on the lead screw, it's as though you never
                            dis-engaged.

                            I'm not sure what would happen if you allowed the thread dial to make more that 1 revolution after dis-engaging, I haven't tried it. I suspect
                            that instruction was so you didn't pick up the lead screw in the wrong place.

                            Anyway, the above has always served me well. YMMV

                            Best regards
                            Mark

                            Comment


                            • #44
                              Originally posted by Paul Alciatore View Post
                              For cutting English/inch threads, with an English/inch lead screw this synchronization is fairly simple because the English/inch threads were WISELY specified in TPI, using whole or round numbers. The most common lead screws on these lathes are 4, 8, and 10 TPI. 8 TPI is the easiest case so I will use it for example.

                              Any thread that is an integer multiple (1X, 2X,3X, etc.) of the lead screw's pitch will synchronize at any position on the lead screw and can be cut without any thought to synchronization. Just engage the lead screw and cut. No problem.

                              This is also true of course for a metric lathe. For example the very common metric CQ6230a (metric version of the PM1236) has a 3mm pitch lead screw and can cut metric pitches .5,.6,.75,1,1.5 and 3mm by engaging the half nuts at any point.
                              Last edited by The Artful Bodger; 09-02-2013, 08:55 PM.

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                              • #45
                                If the thread dial goes around one or more times you just have to make it go back around the other way the same number of times.

                                You can move the carriage back manually while the half nuts are disengaged as long as you watch the thread dial and make sure it goes back around to the original spot on the same revolution before reengaging the half nuts.

                                It might be advisable to let the dial go back a little too far and then close the half nuts at the proper time after the lathe has been restarted forward as this would eliminate any problem with backlash in the thread dial or half nuts.
                                Originally posted by MarkA View Post
                                It is possible to cut metric threads, using the thread dial, and dis-engage the half nuts. But there's a bit of a trick to it.

                                Set up for your first pass, and engage the half nuts when a number passes the thread dial index, say "1". Remember this number, it's important.
                                I'm going to use "1" in my description below, but it can be any number, as long as you always use that number.

                                Make your first pass, and when you get to the end of your thread wind out the tool and dis-engage the half nuts. You MUST then stop the lathe
                                before the thread dial makes a full revolution. Do not move the carriage by hand.

                                Reverse the lathe and re-engage the half nuts when "1" passes the index mark, let the lathe take the carriage back to the start point. You can
                                dis-engage the half nuts at this point, but again, stop the lathe before the thread dial makes a full revolution.

                                Set up for your next pass and repeat the above steps, making sure to engage the half nuts on "1" again.

                                By engaging the half nuts on the same number you are engaging the lead screw in the same place on the lead screw, it's as though you never
                                dis-engaged.

                                I'm not sure what would happen if you allowed the thread dial to make more that 1 revolution after dis-engaging, I haven't tried it. I suspect
                                that instruction was so you didn't pick up the lead screw in the wrong place.

                                Anyway, the above has always served me well. YMMV

                                Best regards
                                Mark
                                Don Young

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