Announcement

Collapse
No announcement yet.

Metric threads possible on imperial sized brass rod? Hints please.

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Metric threads possible on imperial sized brass rod? Hints please.

    Although metric is our common means of working here I have found that the only brass rod available to me is of an imperial size. I was told by the supplier that it should be no problem to cut a 6mm thread using a die and die holder on the 1/4" brass rod and have therefore assumed that an 8mm thread could be cut on the 5/16" brass rod which was also supplied.

    Has anyone tried doing this? Is it possible to cut a metric thread on common imperial sized rods without turning the end of the rod first? I do not have access to a metalturning lathe but when threading mild steel or stainless steel have commenced the thread cutting with the die only after creating a taper on the bench grinder.

  • #2
    Welcome, it's a fairly straightforward job, file a taper on the end, about 1/4" will do and present a die to the end of the rod, its easier to stick the rod in the lathe and back the die up with the tailstock barrel, pull the chuck rondo while adding pressure with the tailstock, if you want really detailed just ask, if not don't be offended as it's impossible to gauge how much experience you have from the OP, M6 will cut on a 1/4 well, I cut brass dry but some use lube, you find a new die is best with brass, and keep it for brass, same with files, new ones brass, after wear steel, brass likes a sharp tool.
    It's an easy job, when you've done one and a bit mysterious when you haven't, like the rest of life.
    Hope you get somthing from this
    Mark
    Ignore the lathe bit I misread, just file a taper on the end and run it with a die, steady pressure, try to keep the die level or a drunken thread will result.
    Last edited by boslab; 11-18-2016, 12:41 PM.

    Comment


    • #3
      According to my imperial-metric conversion program, 1/4" rod (0.2500 inches diameter) is close to the nominal thread diameter for 6x1.0mm (.2361 inches). 5/16 rod (0.3125 inch diameter) is close to the metric conversion for 8x1.25mm nominal thread diameter of 0.3150 inches. The 6mm will be a little tight and may be harder to thread, the 5/16 a tad loose. In all cases, cutting a taper on the end at about the minor diameter of the thread to be cut, or a little more, is needed to start the tap. EDIT: dammit, I meant die, not tap.
      Last edited by Video Man; 11-18-2016, 06:24 PM.

      Comment


      • #4
        A M8 die will thread onto 5/16" rod a whole lot more readily than a M6 die will thread 1/4". The size for 8mm is near perfect.
        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

        Comment


        • #5
          I have to admit that I've never tried forcing a die with brass. On steel it seldom provides nice clean results though.

          If you do have problems I'd say it'll be with the 1/4" rod where the die has to chew off a touch over 0.015". And that's enough that it could easily prove to be too much for the die. Consider that the face of the die does not have correctly configured cutting edges. O sure, it's sharp like a razor. But once you're outside of the diameter of the teeth that edge has no front clearance angle. So it rubs and chews if it cuts at all. That drag means that the teeth are loaded up a lot against the rear face of the teeth. And that is what makes the threads a mess when I've goofed or thought I might get away with cutting away the crest stock with the die.

          The 5/16 is a lot closer to 8mm than 6mm is to 1/4. So in that case it's likely you would be able to get by without turning the stubs down first. On the 1/4" stock I tend to think you'll need to modify the die to cut that excess cleanly or you'll need to machine the stubs for threading down first.

          If you modify the dies so the first portion of the face just outside of the threading teeth did have some back rake then it would likely cut and thread just fine.

          Keep in mind too that if you do try grinding a die to have this front clearance rake that the angle needs to be slightly more than the pitch angle. After all you need a few degrees more than the pitch angle or it's still going to rub.

          Comment


          • #6
            He wouldn't have to turn 5/16" down because it's already smaller at 7.94mm.
            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

            Comment


            • #7
              Thanks boslab, Video Man, Peter and BCRider for your very rapid and helpful replies. You have set me on the right path to achieve what is required. I will reply in a little more detail to some specific comments. I am new to the forum so if there are better ways to reply I would welcome advice. Thanks to you I see more clearly now that the issue rests with the 1/4" rod being threaded with a 6mm die and that the 5/16" rod is actually minutely undersized. I am grateful for the clarification and appropriate solutions considering my equipment. I'll have a go at modifying my 8mm die so it leads in successfully as BC has explained. Thanks for such excellent assistance.

              Comment


              • #8
                I appreciate your very clear and helpful directions boslab. Your attention to detail in this process is extremely valuable.

                Comment


                • #9
                  Originally posted by Video Man View Post
                  According to my imperial-metric conversion program, 1/4" rod (0.2500 inches diameter) is close to the nominal thread diameter for 6x1.0mm (.2361 inches). 5/16 rod (0.3125 inch diameter) is close to the metric conversion for 8x1.25mm nominal thread diameter of 0.3150 inches. The 6mm will be a little tight and may be harder to thread, the 5/16 a tad loose. In all cases, cutting a taper on the end at about the minor diameter of the thread to be cut, or a little more, is needed to start the tap.
                  Using the Vernier caliper I see the 1/4" rod is 6.31mm or .249" and that the 5/16" rod is 7.94/7.95 or .312/.313". Your comment about the minor diameter had me looking at charts I hadn't referred to before and I see that I need to taper the 1/4" (6.31mm) down to 4.8mm and the 5/16" down to 6.5mm. Thanks for making that point Video Man.

                  Comment


                  • #10
                    Ummm... you mean the 6mm die, right?

                    Sorry about the confusion with the 8mm vs 5/16". I was jumping back and forth between the conversion utility and typing the reply. Got the size for the 8mm vs 5/16" bass ackwards. As noted it'll simply cut a smaller "crested" thread. The issue will be if there is enough engagement to suit your needs.

                    If you were doing enough of these and they had to be a decent fit it might be worth considering rolling the 8mm threads onto the 5/16" rod. That would deform the metal upwards and give you a full and proper size thread. But rolling threads is more of a production deal and needs a snazzy machine. But if you're doing enough of them and you need the full size thread it might be an option. Or if not in your own shop job it out to a local shop that has a rolling machine?

                    Comment


                    • #11
                      Originally posted by Peter. View Post
                      A M8 die will thread onto 5/16" rod a whole lot more readily than a M6 die will thread 1/4". The size for 8mm is near perfect.
                      .
                      I was delighted to hear that news Peter. I think when I thought I was in for an issue with the 1/4" that I presumed the 5/16" would be equally problematic. Thanks.

                      Comment


                      • #12
                        Originally posted by BCRider View Post
                        I have to admit that I've never tried forcing a die with brass. On steel it seldom provides nice clean results though.

                        If you do have problems I'd say it'll be with the 1/4" rod where the die has to chew off a touch over 0.015". And that's enough that it could easily prove to be too much for the die. Consider that the face of the die does not have correctly configured cutting edges. O sure, it's sharp like a razor. But once you're outside of the diameter of the teeth that edge has no front clearance angle. So it rubs and chews if it cuts at all. That drag means that the teeth are loaded up a lot against the rear face of the teeth. And that is what makes the threads a mess when I've goofed or thought I might get away with cutting away the crest stock with the die.

                        The 5/16 is a lot closer to 8mm than 6mm is to 1/4. So in that case it's likely you would be able to get by without turning the stubs down first. On the 1/4" stock I tend to think you'll need to modify the die to cut that excess cleanly or you'll need to machine the stubs for threading down first.

                        If you modify the dies so the first portion of the face just outside of the threading teeth did have some back rake then it would likely cut and thread just fine.

                        Keep in mind too that if you do try grinding a die to have this front clearance rake that the angle needs to be slightly more than the pitch angle. After all you need a few degrees more than the pitch angle or it's still going to rub.
                        I appreciate your very clear explanation and valuable ideas BCRider. As you say chewing off 0.015" (0.381mm!) is a fair amount to be removing I would have thought.
                        So I am intending to modify a die as you suggested. As I understand it the thread angle is 60 degrees so the lead-in on the die will need to be shallower than half the thread angle which is 30 degrees so I will aim to grind (using a dremel or die grinder - I hope) a slow angle of about 20 degrees or leave 160 degrees of metal on the leading edge of the HSS die to ensure the die accommodates the 1/4" rod. I look forward to trying this out. I might have to get a small aluminium oxide grinding wheel to fit in the dremel for the HSS die modification perhaps?

                        Comment


                        • #13
                          Originally posted by BCRider View Post
                          Ummm... you mean the 6mm die, right?

                          Sorry about the confusion with the 8mm vs 5/16". I was jumping back and forth between the conversion utility and typing the reply. Got the size for the 8mm vs 5/16" bass ackwards. As noted it'll simply cut a smaller "crested" thread. The issue will be if there is enough engagement to suit your needs.

                          If you were doing enough of these and they had to be a decent fit it might be worth considering rolling the 8mm threads onto the 5/16" rod. That would deform the metal upwards and give you a full and proper size thread. But rolling threads is more of a production deal and needs a snazzy machine. But if you're doing enough of them and you need the full size thread it might be an option. Or if not in your own shop job it out to a local shop that has a rolling machine?
                          Yes BCRider, I am sorry about that - I meant to say I would modify my 6mm die as you suggested . I will need to check my posts more carefully. Thank you for picking up that error. It won't be a problem with a smaller "crested thread as it is for a museum artifact mount fitting and has little weight involved.
                          Interesting rolling of threads came up with another matter a few days ago where a local firm said they would probably roll the thread for a batch of fitting on their sophisticated machines. Thanks for the useful advice -with another project that may indeed be the way to go if more thread depth is required.

                          Comment


                          • #14
                            If you have a split die for the 6mm thread do a pass with the die opened a little oversize as a first pass, followed by a pass at nominal size. might make the going a little easier.

                            Sent from my Nexus 6 using Tapatalk

                            Comment


                            • #15
                              ....o I am intending to modify a die as you suggested. As I understand it the thread angle is 60 degrees so the lead-in on the die will need to be shallower than half the thread angle which is 30 degrees so I will aim to grind (using a dremel or die grinder - I hope) a slow angle of about 20 degrees or leave 160 degrees of metal on the leading edge of the HSS die to ensure the die accommodates the 1/4" rod. I look forward to trying this out. I might have to get a small aluminium oxide grinding wheel to fit in the dremel for the HSS die modification perhaps?
                              Sorry, I missed this bit first time around.

                              You might be looking at the wrong angles here. The pitch lead angle isn't the same as the thread cross section angle. The pitch lead angle is set by the circumference of the thread and amount of advance per thread. On a 1/4-20 thread the circumference is pi x 0.25 = 0.785. And the advance for one rotation is one thread or 0.05. The pitch lead angle ends up being the arctangent of .05/.785 which is 3.64°. So all you need to do is on the webs just outside of the actual cutting teeth is to grind an angle of around 5° so these webs form a cutting edge to remove the excess brass and effectively size the brass down to the size needed by the thread cutting teeth.

                              The split die idea is another option. To deal with the larger amount to be removed you might need to relieve the outside diameter with some rounded ground out spots to allow the die to flex far enough. That also means it would not fit in a regular die holder though. But it's an option. I've never had any luck opening the dies up by much over 2 or 3 thou. And in this case we are looking at more up around 10 or 12 thou. That's a LOT for a small die.
                              Last edited by BCRider; 11-19-2016, 04:16 PM.

                              Comment

                              Working...
                              X