[GLCarlson]

PathPilot assumes a standard form threadmill cutter (one with the pointy end of the triangle cut off, the normal thread form). If one does a lot of threadmilling, a form cutter is the right tool. If it's an occasional job, a set of $75+ form tools is a bit expensive for the small shop, or the right one may not be immediately available. And while a smaller threadmill can cut a larger thread in the same tpi, in practice one winds up with an assortment of threadmills of various pitches and sizes.

Single Form thread mills can mill any pitch including metric sizes, are used for cutting internal or external, right or left hand 60 degree threads. The tip of the included angle is ground to a point. These are also called sharp V double angle milling cutters. Available in HSS or carbide, one or two of these can accommodate a very wide range of threads. For the occasional job, a single form threadmill is a very convenient tool.

Unfortunately, when used with the standard PathPilot conversational threadmill program, the sharp V cutter will produce a thread that has incorrect depth. Presented below is a simple way to convince the conversational program in Pathpilot to accept a sharp V cutter.

To cut an external thread in PathPilot conversational with a 60 degree sharp V cutter:
Select the thread from the menu or enter book data.
Subtract minor diameter from major, multiply difference by 1.4
Subtract that result from the major diameter and enter as the new minor diameter. PP will calculate the new thread depth (set passes =1)
Set desired # of passes, origin, thread length, etc as needed and post code.

To cut an internal thread in PathPilot conversational with a 60 degree sharp V cutter:
Select the thread from the menu or enter book data.
Subtract minor diameter from major, multiply difference by 1.2
Add that result to the major diameter and enter as the new major diameter
Set passes, origin, thread length, etc as needed and post code

With number of passes is set to 1, the sharp V DOC will be shown. In practice, this approach produces a loose running fit; if one wants a tight fit, a bit of tweaking to the appropriate diameter will be necessary.

In more detail, the essence of this approach is to let PathPilot do most of the work, while spoofing it to allow the V cutter instead of the expected form cutter. The trick is to adjust the minor diameter of an external thread (and major diameter for an internal thread). To do this easily, first set up a standard thread from the handbook values or by selecting a thread from the conversational drop-down. Then, adjust the appropriate diameter. Make sure your V cutter has the correct length offset and actual diameter in the offsets table.

External thread example. Using the drop-down menu, PathPilot defines the 1/2-13 thread as:

major diameter 0.4985
minor diameter 0.4041 (handbook maximum minor diameter is 0.4069)
and if a 1 pass cut is taken, Pathpilot shows a cut depth of 0.0472.

To get the depth required for a sharp V mill, divide the handbook depth of thread (or the one calculated by Pathpilot) by 5, then multiply by 7. Or multiply the standard depth by 1.4. Either approach corrects the depth of the standard form of the truncated triangle to the full triangular depth of thread. In this example, 0.0661.

If one attempts to enter that larger cut depth, say 0.0661 for the depth for a sharp V cutter without adjusting the minor diameter, PathPilot balks. One may not enter that depth, it's simply refused.

However. If one adjusts the minor diameter to 0.3663, which is the Pathpilot major diameter minus 2 times the sharp V depth (or subtracts two times the difference between the sharp V depth (0.0661) and the calculated depth of thread for a proper form tool (0.0472, which Pathpilot has already provided with the 1 pass setting) from the minor diameter, PathPilot will now accept a depth of cut that works for the sharp V cutter (0.066). GWizard has sharp V and standard thread depth already calculated, if a simple lookup is preferred.

Once the minor diameter is adjusted all the other parameters are still there and still valid. One may change number of passes, thread length, etc. Pathpilot will now post a conversational threadmilling program that will use the sharp V cutter in the tool table, using the correct tool diameter and offset.

The same technique works for internal threads as well. However, internal threads require adjustment of the minimum major diameter by a factor of 1.2, not 1.4. To see how the geometry works for each case, look at any labelled thread diagram (eg, G wizard, Machinery's Handbook, etc)

For example, to cut a 1-8 internal thread

major diameter (minimum) is 0.9980 (handbook: 1.000)
minor diameter is 0.8446 (handbook: 0.865 to 0.890)
depth of cut is 0.1534/2, 0.0767

The major diameter of an internal thread should be increased by adding the adjusted double depth of cut (0.1534 * 1.2) to the minor diameter, to get a sharp V major diameter of 1.0287.

To see metric threads in Pathpilot conversational, switch to G21 (metric units) via the MDI.

Any thread can be cut by entering handbook values if a thread isn’t in the supplied list. PP will do most of the math.

This method has been checked on a few external and internal threads, both metric and UN.

Hope this is useful.

(NB- I've checked the math several times, but inevitably errors occur. And I apologize for the rather basic approach, which I'm sure is unneeded by the majority of members here- but may help the occasional individual who's just getting comfortable with threadmilling.)

[andyf97]

Great info there, many thanks for posting it.

My latgest issue with thread mills is notbfinding long enough ones. I need forvexample to put 12mm of m9x0.5 thread into a brass barrel but cant find any thread mill that long.

[GLCarlson]

Great info there, many thanks for posting it.

My latgest issue with thread mills is notbfinding long enough ones. I need forvexample to put 12mm of m9x0.5 thread into a brass barrel but cant find any thread mill that long.

Scientific Cutting Tools has a cutter that might reach. 0.182 diameter, sharp V, 0.650 long cutting depth. SPTM 182L. The 290L is good for 14-40 tpi and corresponding metric range. They look delicate, but so far have held up.

In the US, Zoro has them at a rational price, 60 bucks or so if I remember right (Grainger sells for about 2x that). I realize that may not be much help given your location, but they are available.

[adamvs]

Well for what it's worth, I noticed my Scientific Cutting Tools single thread mills are a little undersized, e.g., the ,75 cutter is .746, so that may have an effect on the outcome. I always mike mine a few times to get the diameter locked in. Also there are thread crests to take into account. It always takes me a test cut or two to get it right, and sometimes CAD the thread with the crest and internal radii to try to get it the first time. A bit of a PITA, but data worth keeping.

[GLCarlson]

Well for what it's worth, I noticed my Scientific Cutting Tools single thread mills are a little undersized, e.g., the ,75 cutter is .746, so that may have an effect on the outcome. I always mike mine a few times to get the diameter locked in. Also there are thread crests to take into account. It always takes me a test cut or two to get it right, and sometimes CAD the thread with the crest and internal radii to try to get it the first time. A bit of a PITA, but data worth keeping.

I've got a couple. One is dead on, one is under a couple thou. Cheaper than a drawer full of form cutters, some fiddling required.