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  • Internal Thread Cutting

    When cutting external threads you set the compound to 29.5 degrees, and advance into the cut with the compound. This causes the cutting to occur on one edge of the thread, and the other edge is cleaned up with the 0.5 degree difference between 29.5 and 30 degrees.

    What is the preferred method of cutting internal threads? One way would be to reverse the lathe's direction of rotation and cut the far wall of the tube, leaving leave the compound at 29.5 degrees, keep the same cutting direction as you use on an external thread (from the end of the tube inwards). This would require a right hand threading tool.

    A second way would be to set the compound to 60.5 degrees beyond the 90 degree point, and again advance the compound to deepen the cut.

    I suppose the simplest is to use the cross slide to deepen the cut, however this cuts both side of the tread at the same time, perhaps leading to poorer threads.

    What method is preferred?

  • #2
    when you cut a right hand external thread the compound is angled to the front right 29/29.5/30 deg as you like and the thread is cut toward the headstock. The compound is used to advance the depth of cut and the crossfeed is used to back the cutter out of the thread and is usually set at "0" on the dial and returned there to start the next pass.

    For an internal thread you can set the angle of the compound to the front left or to the rear right. The compound is still used to advance the cutter into the thread for each pass.

    I prefer to set the compound to the front left as long as I have clearance to do so. Sometimes the compound will have to be set to the right rear to clear the work or the lathe.
    It's only ink and paper

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    • #3
      If I'm cutting a matching bolt and nut I leave the setup as is and reverse the spindle for the nut. Otherwise I don't have a preference.

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      • #4
        And when setting the compound to 29.5 degrees, I was taught to take the last couple of thousandths with the cross slide to clean up the thread profile on both sides of the vee.

        Keith

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        • #5
          If I do any internal threading I just turn the tool upside down and thread the back of the bore instead of the front.

          The lathe turns the normal way,the cross slide or saddle moves in the same direction towards the back of the lathe as doing external threads and the chips fall away from the tool to the bottom of the bore.
          It makes it easier to set the tool at tip at 90 degrees and you can see what's happening easier.

          Allan

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          • #6
            Originally posted by Carld
            .......

            For an internal thread you can set the angle of the compound to the front left or to the rear right. The compound is still used to advance the cutter into the thread for each pass.

            I prefer to set the compound to the front left as long as I have clearance to do so. Sometimes the compound will have to be set to the right rear to clear the work or the lathe.
            But remember that the tool would need to be sharpened either left or right hand depending on which angle you choose.
            Paul A.

            Make it fit.
            You can't win and there is a penalty for trying!

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            • #7
              When doing internal threads I just advance with the cross slide. With a well sharpened tool it works quite well enough. Just remember that the deeper the tool cuts the more it cuts so reduce the advance correspondingly.

              Free software for calculating bolt circles and similar: Click Here

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              • #8
                I normally would just advance the cross slide (or is retract????????) as it gives me more posite control as to the end point of the threads if there is a relief groove at the end of the threads. Another reason also for cxutting on the backside with the spindle in reverse. With a properly ground tool bit, good still boring bar and sufficent cutting lubricant this should pose no real problem. Now these are internal threads. Not the high buck 6" scale

                Forty plus years and I still have ten toes, ten fingers and both eyes. I must be doing something right.

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                • #9
                  Originally posted by Allan Waterfall
                  If I do any internal threading I just turn the tool upside down and thread the back of the bore instead of the front.

                  The lathe turns the normal way,the cross slide or saddle moves in the same direction towards the back of the lathe as doing external threads and the chips fall away from the tool to the bottom of the bore.
                  It makes it easier to set the tool at tip at 90 degrees and you can see what's happening easier.

                  Allan
                  Won't this try and "lift" the saddle up
                  The shortest distance between two points is a circle of infinite diameter.

                  Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

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                  • #10
                    Originally posted by Spin Doctor
                    Another reason also for cxutting on the backside with the spindle in reverse. With a properly ground tool bit, good still boring bar and sufficent cutting lubricant this should pose no real problem. Now these are internal threads.
                    You wouldn't happen to have a picture of the cutter used for that? I'm grinding one (several attempts so far) to do the same.

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                    • #11
                      Originally posted by Paul Alciatore
                      But remember that the tool would need to be sharpened either left or right hand depending on which angle you choose.
                      Ok, what am I missing? I thought threading tools are 60deg and symetrical?
                      What would the difference be in right or left hand????

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                      • #12
                        Originally posted by dp
                        You wouldn't happen to have a picture of the cutter used for that? I'm grinding one (several attempts so far) to do the same.
                        Alas no. The tool was HSS ground for a 12mm Acme. This form has an included angle of 30D. The tool was held in a 1-1/2" bar made from 4140 TG&P. The tool was ground on a surface grinder using a special grinding fixture made for making up tools with accurate angles on them. The fixture has a 7D side relief built into it. Basically you set the desired angle with either the protractor head of a combination square or bevel protractor. The tool nose was measured on a optical comparator. Actually the first operation was to waste out as much as possible using a 60D threading tool as the total surface area with the Acme is quite large. The nuts are made out of Ampco 18 and used on an industrial washer used for on a machining line for automotive engine blocks. There are two acme screws at each end which are used to lower and raise the whole side of the washer. In order to make them I had to first make a master gage that matched the thread on the machine
                        Forty plus years and I still have ten toes, ten fingers and both eyes. I must be doing something right.

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                        • #13
                          Originally posted by Spin Doctor
                          Alas no. The tool was HSS ground for a 12mm Acme. This form has an included angle of 30D. The tool was held in a 1-1/2" bar made from 4140 TG&P. The tool was ground on a surface grinder using a special grinding fixture made for making up tools with accurate angles on them. The fixture has a 7D side relief built into it.
                          Heck fire - I should be able to do that with my grinder I converted from the radial arm saw

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                          • #14
                            Ok, what am I missing? I thought threading tools are 60deg and symetrical?
                            What would the difference be in right or left hand????
                            The clearance angle of the leading edge of the tool must be sufficient to both allow for the helix angle of the thread and provide cutting clearance. The helix angle will vary even with the same TPI depending on the diameter of the work. The trailing edge of the tool only needs a cutting clearance angle which will normally be provided by the helix angle of the thread.
                            Free software for calculating bolt circles and similar: Click Here

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                            • #15
                              Originally posted by Sparky_NY
                              Ok, what am I missing? I thought threading tools are 60deg and symetrical?
                              What would the difference be in right or left hand????
                              The top rake on the tool is usually ground to slope directly away from (perpendicular to) the cutting edge. This actually causes negative rake at the other side of a 60 degree tool. This negative rake is OK to shave off a few tenths on every cut, but may not perform well if used for the heavier cut of the primary cutting edge.

                              You can grind the rake to slope from the point if you want the tool to cut equally in either direction but I do not usually do this. To get the same effective rake angle on the cutting edges, you would have to grind double the angle as measured from the tip back. (Rake measured perpendicular to cutting edge = 7 degrees: rake measured along line from tip and bisecting the cutting edges = 14 degrees.) This rake will reduce the height of the back parts of the cutting edges as compared to the tip, due to the clearance angle under the cutting edges. This, would reduce the angle of the thread by several degrees from the initial value of 60 so that would have to be compensated if you are going to make a 60 degree thread. If you feed with the cross feed instead of the compound at 29.5 degrees, this error will be doubled as it will be on both edges instead of just one.

                              By grinding the rake perpendicular to the cutting edge, you maintain the angle of the primary cutting edge by keeping all parts of it at the same height. The other edge of the tool will have a change in it’s angle, but it will cut in increments along a line determined by the 29.5 degree feed angle (compound setting) and that side of the thread will only be off by 0.5 degrees (30 - 29.5 = 0.5) or less. So you will get a thread form that is between 59.5 and 60 degrees, which is close enough for most work. This is a very good reason for feeding with the compound instead of the cross slide if you need a high quality thread.

                              Some commercial threading inserts (most?) will have the rake angle formed by a raised edge or groove behind the cutting edge. This allows the complete length of both cutting edges to be kept in the same horizontal plane as the tip and will prevent any of the angular problems I mentioned above. Normally, HSS tools sharpened in the shop will not have this kind of feature as the top surface will usually be a simple flat that slopes back from the edge (or tip).

                              All of the above applies equally to both internal and external threads.
                              Paul A.

                              Make it fit.
                              You can't win and there is a penalty for trying!

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