Here in South Africa where I live it is very difficult, and extremely expensive, to obtain the correct sizes and accuracy wires to use in the 3-wire method for determining thread pitch diameters, (and thus confirming that your single point threading is cut to the correct depth). So, until recently, I have been doing what most machinists do - screwing a nut onto the threaded bolt, and calling it OK if the nut goes on easily, but tightly...
Some time ago I purchased a new micrometer that has a 0.001mm resolution "sight unseen" through the internet, and although I'm not very happy with the "feel" quality, it is giving me consistently the same (rounded) results as my trusty old Mitotoyo 0.01mm resolution micrometer - hence it is at least accurate to 2 decimals of a mm. Anyway, to make a long story short, this micrometer came boxed with a set of exchangeable "anvils", one set being 60º wedges, another a set of 60º cones, and a third set are wide, flat disks. Last weekend I opening the box to remove the micrometer in order to take some readings on a rod that I was turning down to make a special bolt, and the 60º wedge shaped anvils caught my eye just as I was thinking about the critically sized threading that I had to do... light-bulb moment... Why not use a flat disk anvil on the one jaw, and either the wedge or the cone in the other jaw of the micrometer - then I can calculate the thread depth by measuring the thickness of the (outside) major diameter of the "bolt" normally, and the diameter of the "bolt" with the wedge fitting into the thread on the one side, and subtracting the one measurement from the other should give me the thread depth...
Bit difficult to explain - let me show you some photos to make this clear...
The micrometer fitted with the flat disk and wedge anvils into it's two jaws:
View attachment 117086
This is a close up of the 60º cone and wedge anvils:
View attachment 117087
And here is a photo showing the flat disk straddling the outside of the threads on the one side of the bolt, and the 60º wedge fitting in between two threads, measuring to the minor diameter of the bolt (M16x2).
View attachment 117088
So, is this an accurate way to measure screw threads???
I wanted to confirm if this idea is actually worth something, or just a "flash in the pan", so I raided my supply of bolts and machine screws, and measured these bolts varying from 20mm to 4mm diameter.
View attachment 117089
Keep in mind that all the bolts were Metric, and all were bought from shops (and one could thus hope that they were manufactured to prescribed tolerances).
Results are rather encouraging, I think - I measured 38 bolts, and found that three of them had major diameters outside of the normal 6g tolerance, and these were excluded from the final assessment. As can be seen in the spreadsheet summary below, the average measured thread depth of all the "in spec" bolts came to 0.72mm, whilst the calculated thread depth was 0.71mm when using the formula: thread depth = dmax - d1max (also known as 5H/8)
, and 0.80mm when using the formula: thread depth = 0.6134P
(which is the most used formula). For interest sake, I also looked up the recommended total thread depth for two carbide inserts manufactured by Sandvic and Mitsubishi. Lastly, I calculated the % deviation of each individual measurement, expressed as a % deviation from the two calculated formulae, and highlighted the calculated depth closest to the actual measured depth.
View attachment 117090
I do accept that the wedge has a very narrow blunt edge - perhaps 0.25mm, and that there would be a smallest limiting size beyond which one would not get accurate readings - the above figures show that the % deviation increased substantially with the 4mm diameter machine screw readings, and this probably exceeds the minimum readable size. Also, there is no allowance for the changes in the helix angle resulting from the different pitch sizes, but I suspect this will be negligibly small.
I really would value any feedback on this: - Are any of my assumptions wrong? What am I missing? Why are the larger bolts closer to the 0.6134P values, and the smaller ones closer to the 5H/8 values? Why did I do this, do I need to see my shrink