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Clearance Angle on Tools for External Threading: Is It Really Needed?

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  • Clearance Angle on Tools for External Threading: Is It Really Needed?

    I was just reading through the other thread about rake angle on threading tools. The whole problem with the rake angle is when you grind it on a tool that has clearance angles on the two sides, the angle of the "Vee", as viewed from directly above, closes in and is less than the desired 60 degrees. So it is the side clearance that creates that problem. But do we really need side clearance on tools for external threading. Before I get jumped here, it is DEFINITELY needed for internal threading tools. For small diameter internal work, you may even need to grind secondary clearance facets to prevent the tool from rubbing. But on external threading tools, is it really needed?

    The clearance angle is needed for most cutting tools in order to allow them to advance into the material being cut. This drawing illustrates this principle:



    I was probably thinking about drills when I made this drawing, but with rotations it will fit any cutting tool. What it shows is that the clearance angle (on the bottom in the drawing) ALLOWS the tool to slide into the work on the ramp it has just cut. If that angle were zero then the tool could only skid on the surface of the work but it would not be able to penetrate. This would be very plain if you ground a drill with a zero clearance angle. It would rub and heat up, but it would make very little progress in drilling a hole and that progress would only be due to melted metal, not cut metal.

    So, for MOST cutting tools, side or end clearance is absolutely necessary. But consider a tap. A tap starts by cutting or grinding a thread form on the OD of the tap and then several flutes are cut or ground to produce cutting edges. These edges do not have any side clearance and indeed, side clearance would be bad as the tap could cut sideways. Now, a tap cuts mostly at the tapered tip where you may say that there are facets that create the clearance angles where that cutting is taking place. Perhaps, but I would argue that the tip is made this way due to ease of manufacture and the tip of the tap could just as well be ground as a cone with no clearance and it would cut just as well with perhaps just a bit more friction. Each cutting edge on such a conical tipped tap would simply strike the top edge of the hole and start or deepen it's own groove. It never cuts sideways so no clearance is ever needed.

    Now, consider an external threading tool. It does cut but the depth of the cut is constant. It does not need to be advanced into the work during the cut.



    The top illustration shows the need for front clearance when the tool is advanced into the work. The bottom one shows that the cut can continue with no front clearance if that advancement is ceased. Now you may say that the zero clearance tool in the bottom illustration may be rubbing on the work, but the curvature of the work prevents this. The tool goes straight down while the work itself curves away from the tool immediately below the cutting tip. So there is no rubbing.

    A threading cut is made with a constant in-feed: no further advancement into the work is made during the cut. The tool "penetrates" the work at the point of first contact and stays the same after that. OK, front clearance should not be necessary on a threading tool because a threading tool does not advance into the work during the cut.

    But what about side clearance? After all, the tool moves sideways like a tool that is turning a diameter. So is side clearance needed, at least on the leading edge? I see several factors here. First, if viewed from the point of view of the flank of the thread that is being cut, then the penetration here is also completely constant for the entire cut. This drawing attempts to illustrate this.



    Please pardon the odd shading: it is apparently an artifact of the JPEG conversion. What the drawing shows is there is no increase in the depth of the cut into the flank. So, there is no real need for a clearance angle if it is measured relative to the face of that that thread flank.

    But there is an additional complication in my argument. You may notice that the thread flank is itself at an angle due to the helix angle of the thread. Worse yet, this angle will change with the pitch and pitch diameter of the thread. I am talking now about the flank on the left or the leading flank. The flank on the right or trailing edge of the tool is angled away from the tool and will never interfere. I have two thoughts on this. First, the fact that the threads are on a cylinder instead of a flat surface means that they are curving away from the cutting tool immediately below the cutting edge. This will provide some clearance. Second thought is that the tool COULD be angled a bit vertically to match the helix angle. This angle will be slight and if there is a small error, the curvature thing I mentioned above should take care of any remaining error.

    If it is angled this way, the Vee shape of the nose of the tool will remain constant at different heights and there would be no effective clearance angle as measured relative to the flanks of the thread being cut. Such a zero clearance tool could be ground with a top rake which would not change the angle of the cut. If the angle is not exact, it would be best to make it smaller rather than larger than the calculated amount.



    I did not show any top rake in the drawing, but if it were added the presented angle of top of the tool would not change because the cross section is the same at any distance down from the top.

    In short, my conclusion is there is no real reason for clearance on a threading tool except for clearing the helix angle being cut. And there may be some advantage to avoiding clearance angles on a threading tool, including allowing top rake which would not change the profile of the thread.
    Last edited by Paul Alciatore; 12-26-2012, 04:55 PM.
    Paul A.

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

  • #2
    I agree you offer an interesting approach, but I disagree.

    If you make the statement that zero clearance means you can't start a cut by feeding in while shaping, the same should hold when external turning, when grooving or parting for example. The instantaneous tangent to the tool is parallel to the face of the no-clearance tool, so unless the tool is marginally below centre, you can't start feeding in without force.

    The same should hold for a threading tool, except that you aren't trying to feed in a width tool - to groove or part - you're trying to feed an almost sharp-pointed tool. The fact that your tool progresses down the length of the work is irrelevant - the instantaneous tangent still offers no clearance angle to the tool. The force you need to start a no-clearance pointed tool will be much less than the force you need to start a width tool. Indeed while I suggest that a no-clearance pointed too needs force to start it, the force tends to zero as the tool point radius tends to zero.

    So I'm postulating that a threading tool with no clearance angle on the tip or the flanks, and with any non-zero radius at its tip, must be used below centre to avoid forcing.
    Richard

    Comment


    • #3
      Originally posted by rohart View Post
      I agree you offer an interesting approach, but I disagree.

      If you make the statement that zero clearance means you can't start a cut by feeding in while shaping, the same should hold when external turning, when grooving or parting for example. The instantaneous tangent to the tool is parallel to the face of the no-clearance tool, so unless the tool is marginally below centre, you can't start feeding in without force.
      Totally true, I agree. A shaper is an interesting example. The tool in a shaper also starts in the air and meets the work head on at full cutting depth. It also should not need any clearance on either the nose or sides except to lessen the friction.

      The same should hold for a threading tool, except that you aren't trying to feed in a width tool - to groove or part - you're trying to feed an almost sharp-pointed tool.
      Yes, you are feeding it, but not IN. It starts in mid air already all the way in for that pass. There is no IN feed during the cut. And from the point of view of the flank you are cutting, there is no sideways feed either. It just follows the flank at the same depth for the whole cut.

      The fact that your tool progresses down the length of the work is irrelevant - the instantaneous tangent still offers no clearance angle to the tool. The force you need to start a no-clearance pointed tool will be much less than the force you need to start a width tool. Indeed while I suggest that a no-clearance pointed too needs force to start it, the force tends to zero as the tool point radius tends to zero.
      The force tends to zero as the depth of the cut is less. When the threading tool first contacts the work at the end(start) of the thread, it meets it head on, so to speak. It is already at full depth for that cut.

      So I'm postulating that a threading tool with no clearance angle on the tip or the flanks, and with any non-zero radius at its tip, must be used below centre to avoid forcing.
      I don't think you disproved anything.
      Last edited by Paul Alciatore; 12-26-2012, 06:27 PM.
      Paul A.

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

      Comment


      • #4
        Point taken, but I have further reasons for not following your approach. To start with, I would like to be slightly more precise. For a very small depth of cut, the clearance needs to be non-negative.

        Now, there are several reasons for wanting to feed in to a thread. Cutting from a shoulder, for exapmle. A lens cap, for example, hardly has room for a starter land.

        If you grind the tool to have zero clearance, so the tool is just two vertical flanks turned by the helix angle, then your geometry has to be really spot on.

        If you grind your flanks with enough clearance, you can forget all about the helix angle, and this means you can use square section tool stock in a normal tool holder. It also means you can use the same tool for LH and RH threads.
        Richard

        Comment


        • #5
          I think that any tool, not just a threading tool, that was constructed with no clearance angles would work acceptably initially but would become unusable very rapidly as even the most minute amounts of wear would increase cutting forces dramatically.

          The tap analogy can be misleading. Most taps made for use in power tapping applications have eccentric relief.

          http://www.ctemag.com/aa_pages/2009/0908_Tapping.html

          Dave

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          • #6
            The side clearance angle is needed to accommodate tool wear. If a tool doesn't have side clearance then as wear begins at the cutting edge the tool rubs immediately below, friction increases, heat goes up, wear rate increases, and tool life is very poor. In "Design and Use of Cutting Tools" by St. Clair this is the first topic covered. On page 12 he has this table of wear rate vs side relief:



            A threading tool follows the helix so it needs a little additional relief to accommodate this so most threading tools have about 10 degrees relief, i.e. a little more than the optimum side relief for best tool life.

            Comment


            • #7
              The wear thing is a valid point.

              I disagree about the tap analogy being misleading. But perhaps I should have said "manual tap". Nothing that I said was intended for CNC work.

              Paul A.


              Originally posted by becksmachine View Post
              I think that any tool, not just a threading tool, that was constructed with no clearance angles would work acceptably initially but would become unusable very rapidly as even the most minute amounts of wear would increase cutting forces dramatically.

              The tap analogy can be misleading. Most taps made for use in power tapping applications have eccentric relief.

              http://www.ctemag.com/aa_pages/2009/0908_Tapping.html

              Dave
              Paul A.

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

              Comment


              • #8
                Originally posted by GadgetBuilder View Post
                The side clearance angle is needed to accommodate tool wear. If a tool doesn't have side clearance then as wear begins at the cutting edge the tool rubs immediately below, friction increases, heat goes up, wear rate increases, and tool life is very poor. In "Design and Use of Cutting Tools" by St. Clair this is the first topic covered. On page 12 he has this table of wear rate vs side relief:



                A threading tool follows the helix so it needs a little additional relief to accommodate this so most threading tools have about 10 degrees relief, i.e. a little more than the optimum side relief for best tool life.
                And yet the source that Dave referred to did state that hand taps are made with zero side clearance. Most of the cutting that is done by ALL taps is at the tapered front end. Perhaps my theory about the clearance in that area was not correct and they are, indeed, made with clearance there to lengthen tool life. I guess that would apply to threading tools also, but just how severe is this "tool wear problem"? When manually chasing threads I usually touch up the tool before each job and it wouldn't be hard to touch it up after a bunch of parts. I probably would want to do so anyway, even with a tool with clearance.
                Paul A.

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

                Comment


                • #9
                  Clearance is needed, not just to accommodate wear at the cutting edge, but also because the workpiece materials that we normally machine are elastic. The cutting action causes deflection of the cutter and the work at and slightly above the cutting edge. As the work rotates under and away from the cutter, the material elastically rebounds. The effect is small but important with most metals, but becomes quite large with some 'plastics'. The effect of this with insufficient clearance is to cause rubbing, which requires additional power to drive the workpiece and manifests as heat and marking of the workpiece. In some plastics the effect is so strong that lack of clearance can cause local melting.
                  Bill

                  Comment


                  • #10
                    Any cutting tool would need side clearance greater than the feed rate or helix angle as long as the top flat is parallel to the axis of the work (shaft). If you angle the top flat of a cutter equal to the helix angle, which would also be the same as the feed rate angle, then your thread cutter profile may work. The actual problem will be friction on the sides of the cutter just below the cutting edge and that is demonstrated by the chart from Gadgetbuilder. At a relief angle of 8 deg the chart claims 100% cutter life for the number of cuts made.

                    Using all the above, if you keep the top flat of the cutter parallel to the axis of the work (shaft) and grind side clearance equal to the helix or angle of feed then you should also add 8 deg more to the leading side clearance to give the cutter it's maximum life cycle.

                    If you angle the top flat of the cutter equal to the angle of feed or helix then the chart still recommends adding 8 deg side clearance on the leading side.

                    To sum up, I don't see how a thread cutter or a cutter turning the OD of a shaft with the top flat parallel to the axis of the work can ever work without side clearance greater than the feed rate or helix.
                    It's only ink and paper

                    Comment


                    • #11
                      If it is angled this way, the Vee shape of the nose of the tool will remain constant at different heights and there would be no effective clearance angle as measured relative to the flanks of the thread being cut. Such a zero clearance tool could be ground with a top rake which would not change the angle of the cut.
                      If you grind a top rake in a threading tool then feed in directly surely that would means that some part of the cutting edge will be presented to the workpiece below centreline. That would change the cutting angle wouldn't it?

                      This begs another question I had not thought of until just now. If you're cutting a thread with a high helix angle, do you measure the angle of the vee along the axis of the part or across the axis of the helix? Seems to me that on a fine thread the two would be the same but on a high lead thread the vee as viewed across the helix angle would be narrower than viewed from 'above', how I normally look at a thread shape.

                      Hell, another thing just occurred to me - if you use a zero-rake tool when cutting a high helix/lead thread, you're automatically putting a top rake on the tool because of the high travel speed. Aren't you?

                      I have to go lie down now...
                      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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