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  • #46
    Originally posted by Johannes Frank View Post
    RC Rider The action is a Borden Rimrock they all have 18 tpi. in one of the images in the first post Im holding a thread gauge against the cut threads from the lathe. I have other barrels from 3 different gunsmith all are 18tpi. I will check for the run out. I have counted the threads on the lead screw over 2 inches and they come out 20 tpi over 2 inchClick image for larger version

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    The calipers showing 2.000" are not positioned on the same spot on the thread fillet. 0,8mm is the difference from metric to inch and photo seems to show this difference
    Helder Ferreira
    Setubal, Portugal

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    • #47
      When cutting threads, can you disengage the halfnuts, and then pick up thread a gain using the dial ? On Metric or on Standard threads ?

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      • #48
        Originally posted by 754 View Post
        When cutting threads, can you disengage the half nut, and then pick up thread a gain using the dial ? On Metric or on Standard threads ?
        Almost impossible to disengage and pick up again,you can go in on many places on one the lead screw, very shore between the spots. I always keep the half nut closed and revers the machine.

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        • #49
          If the thread you're cutting is a multiple pitch of the leadscrew, you can disengage the half nut anywhere as it will always be in sync. (Pardon my English, it's not my native language)
          Helder Ferreira
          Setubal, Portugal

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          • #50
            If it is a metric leadscrew, you should not be able to disengage, then fiind the thread again on 18 tpi..
            but you should be able to on metric threads.
            Last edited by 754; 09-29-2021, 01:30 PM.

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            • #51
              Do you think it'd be possible to get a measurement of the entire length of the lead screw, accurate enough, when compared to the total thread count over that length, to reveal how much, if any, that it's off?
              Depending on how long the lead screw is, that should result in a more easily detectable differential.
              Lynn (Huntsville, AL)

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              • #52
                Originally posted by Johannes Frank View Post

                Yes I know this and have tried, the manual say 65 t but that did not work. it is for a lathe with a 8tpi lead screw mine is 10tpi. Tey sema to have one manual for different lathes ?
                The thing is that whilst THAT set of pitches have 60 specified on the machine and 65 in the manual, other ratios on machine plate are identical to the manual. If the reason for the different ratios were a different pitch screw, then all of them would be different not just the one.

                There is an error, either on the machine plate, or in the manual. It might not be the cause of your problem but an error exists.
                Peter - novice home machinist, modern motorcycle enthusiast.

                Denford Viceroy 280 Synchro (11 x 24)
                Herbert 0V adapted to R8 by 'Sir John'.
                Monarch 10EE 1942

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                • #53
                  Originally posted by Johannes Frank View Post

                  Almost impossible to disengage and pick up again,you can go in on many places on one the lead screw, very shore between the spots. I always keep the half nut closed and revers the machine.
                  That is starting to sound more and more like they used a metric lead screw that for many uses would be close enough. If it was a true imperial based gearing there should be at least two places on the index dial where you could drop the half nuts and get a match. Only on threads with an odd number pitch on an imperial box is there a need to drop into the same index number for each pass. And for 18TPI it is divisible by 2 but not by 4. So there should be two drop in points possible on the threading index dial.

                  Reading Paul's concern over any wear or roughness in the threads affecting the two pin measurement idea I have to agree that the better test for this is to rig up a dial gauge so it can measure the travel and then using some manner of locating the chuck to manually turn the chuck through exactly 18 revolutions and see what the dial gauge shows for carriage travel. I'd mentioned using a level on one jaw. But another method would be to take out the slack and then while turning the chuck by hand to insert a length of something between bed and one jaw to stop the movement. Zero out the dial gauge or note the reading. Then rotate the chuck 18 turns and again use the same spacer to stop the same jaw at the end of the 18 turns.

                  Sadly this won't tell you where the issue actually resides. But at least it will prove for sure that there is an issue. After that you're back to the search on finding which part is the actual issue. But all this might be for nothing since if it does turn out to be metric based then even with some other changes you might simply not be able to get close enough to a true 18TPI for this more demanding purpose with this machine. Or rather that you might need to come up with a two part gear with the proper ratio to get closer to the 18TPI.
                  Chilliwack BC, Canada

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                  • #54
                    I am thinking its a simple as this.. try at least 2 metric pitches.. try to disengage and then re engage.. if it does .. Metric screw....no measuring needed.

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                    • #55
                      Originally posted by Peter. View Post

                      The thing is that whilst THAT set of pitches have 60 specified on the machine and 65 in the manual, other ratios on machine plate are identical to the manual. If the reason for the different ratios were a different pitch screw, then all of them would be different not just the one.

                      There is an error, either on the machine plate, or in the manual. It might not be the cause of your problem but an error exists.
                      I agree with that reasoning and it is in line with my suspicion that the problem lies with the machine plate, either the data is wrong or it is being read wrong.

                      Some basic research is required. My suggestions...

                      Measure the leadscrew pitch over a longer distance, say 6".

                      Determine what the ratios A,B and C are. For, example put a mark on the 'L' gear and turn the leadscrew (I think there is a convenient hex on the far end?) and count how many turns on the 'L' gear.



                      Find tooth count of the spindle gear, I think it will be 40 but that needs to be confirmed.

                      Armed with this information we can use a (on-line?) calculator to check the machine plate and make whatever amendments are required.

                      John

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                      • #56
                        Originally posted by The Artful Bodger View Post
                        Have we got an explanation yet?
                        I do not have a solution on this, I have tried many many times. The factory have no solution jet. I did cut today a 1,5 mm Threads in 30 mm stainless steel, I cut it again because found that the spider I made had a little loose threads that turn easy. So I tried again and the 1,5 mm threads are almost ok if I follow thru on the big wheel on the carriage. I will count the tooth on the spindle gear. I have now cut 9tbi 18tpi 16tpi and 1,5 mm threads, They all have in common the same characteristic, it is the thread gauge will rock on the threads, they are all a little to long,just. That has to be related to the lead screw? Does the rack and pinion have anything to to regarding threading? Another thing, The half nut is bending the lead screw when engaged, the top of the nut touch first and bends the led screw down.
                        Last edited by Johannes Frank; 09-29-2021, 07:40 PM.

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                        • #57
                          Assuming the leadscrew is a 2.5 mm and ten turns of the screw gives you an error
                          of 25.4 – 25= .4 mm or .0157” ( .0314 for 2 inches)
                          Assuming the leadscrew is a 2.6mm and ten turns of the screw gives you an error
                          of 25.4 – 26= (-) .6 mm or .0237” ( .0474 for 2 inches)
                          Since your machined Imperial threads are too long, it means the leadscrew is resulting
                          ( turning) the linear distance too much which would be the case for the 2.6mm leadscrew.

                          So the rotation has to be reduced in the drive train from 10 turns (26mm) to 25.4. ( 26/25.4= 1.024)
                          That is a reduction of .024 % in rotation , so a 50 tooth DRIVEN gear should be 51 tooth
                          OR a DRIVER gear should be 49 instead of 50 ( 50 x .023= 1.2 Teeth) to get you closer in accuracy
                          Nothing perfect, but better by 80 % from my numbers

                          Rich



                          Green Bay, WI

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                          • #58
                            Hi Johannes,

                            Threads can be loose if they are cut too deep.

                            I do not think the rack and pinion should pay any part in threading on a lathe that has half nuts. The half nut should not bend the led screw but some lathes have only one half nut and engaging can push the lead screw down a bit but usually there is something to support the lead screw against this happening. Dirt in the lead screw or half nut would make this worse.



                            The lathe probably uses the rack and pinion for longitudinal feeding so best make sure that is not engaged when using the half nut (most lathes have a system that prevents this).

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                            • #59
                              Actually, since all gears have whole numbers of teeth, there will be a place where you can re-engage the half nuts when cutting 18 TPI or any English pitch with a metric lead screw. The problem is that you must know that exact point and it will require a large number of revolutions of the spindle and the screw so it is usually faster to just keep the half nuts engaged and back up. Even if you need to back up by hand it will be faster.

                              Another thing; the 127 to 100 or 127 to some other round number is the only English - metric conversion ratio that is exact. This is because the inch is defined as 25.400000000000..... mm, EXACTLY. And if you first multiply by 10 to get 10.0000 inches that is 254 mm, again EXACTLY. 254 is not a prime number so, while it can be used as a tooth count on a conversion gear, you can also divide that by 2 which gives you 127 (for 127 mm). The other gear in the conversion process must only represent a whole number of inches so numbers like 100, 120, 80, 50 (10, 12, 8, and 5 inches respectively) and others have been used. All the other commonly used English-metric gear combinations are APPROXIMATIONS of the exact ratio and will have cumulative errors if you are seeking to find a point where the half nuts can be closed in sync with an original position.

                              So if you are using the 127 to 100 conversion gearing, then you will have an exact synchronization point within 127 revolutions of the spindle or lead screw. Often, there will be other sync points at lower counts, but 127 will always be such a sync point.

                              HOWEVER, if you are using one of the other, APPROXIMATE conversion ratios, then those nasty, cumulative errors can and probably will make the first real sync point occur only after a lot more revolutions.



                              Originally posted by 754 View Post
                              If it is a metric leadscrew, you should not be able to disengage, then fiind the thread again on 18 tpi..
                              but you should be able to on metric threads.
                              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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                              • #60
                                The thread charts on the front of this lathe DO show both English and metric threads. And the specifications shown by the sellers DO state that the lead screw is an inch (English) one.

                                But neither the photo posted here nor any of the images of this lathe I can find on the internet show any way to select between those two types of threads (English and metric). If the lead screw is indeed a 10 TPI pitch, then there must be some way to convert the rotation of that ENGLISH screw to a metric rate.

                                So, I must ask the OP, is there any selector knob or lever on the side of back or top or inside of the machine where you do that selection?

                                If there is no such selector, then there must be some kind of compromise gearing inside of the head stock that produces fair to good approximations of both English and metric threads; NEITHER of which is actually correct. And do not tell me that no manufacturer would do that. The Chinese factories have been making quill down-feeds on the inexpensive milling machines that are marked with inch divisions (0.001") while actually traveling just 25.0 mm for 1000 of them instead of the correct 25.4 mm. They have been doing this for years and show no signs of stopping.

                                I would perform some very careful measurements of all of the thread pitches on this machine, both English and metric. I am betting BOTH are inaccurate.

                                Caveat emptor!
                                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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