The following is distilled from a couple of posts I made to the Facetor’s Digest in September 1999.

I have followed the thread on the Beale/Woolley Depth of Cut Indicator with interest. As I reported a few weeks ago I’ve fitted one to my Ultra Tec and have enjoyed the benefits which it brings to my enjoyment of faceting. As a result of some correspondence and experimentation in the UK., I became curious as to whether the B/W indicator is actually measuring the contact resistance or whether they are measuring the ‘digital flutter’ which is produced because laps are not exactly true.With my version of the B/W indicator attached, I placed an oscilloscope across the stop contacts on my Ultra Tec (Figure 1). The ‘scope showed that the contacts were alternately ‘making and breaking’ as the imperfections in the lap/spindle caused the quill to move up and down, hence opening and closing the stop. The output, as the facet approached its correct depth of cut, was analogous to a square wave with a variable mark/space ratio.

Figure 1. The position of the two attached wires on the back of the Ultra Tec head

I believe that my meter is integrating the output from the contacts and producing a variable current reading because of its inherent inertia and inability to accurately follow a rapidly varying signal. The meter does not measure a changing contact resistance. The response time of an oscilloscope is so much faster that it can follow these variations.If you cut at say 400RPM the lap is rotating at 6.67 revs per second. Say there are 3 imperfections on the lap. The frequency of the pseudo square wave produced will be 20 hertz. My digital ohmmeter samples twice per second – 2 hertz – hence the problem a digital meter has in following the change in signal generated from the B/W indicator. An analogue meter has inherent mechanical inertia and again won’t follow such a signal.I hope this observation will be of material assistance to others, however it does not detract in any way from the enormous assistance that the B/W Depth of Cut Indicator (Figure 2) can provide.

Figure 2. My depth of cut indicator

As a result of the follow up posts to my findings when monitoring my B/W Indicator with an oscilloscope I have made further tests.First of all, I ought to detail my set in greater detail as Tony Michael asked for clarification as to the oscilloscope settings.I am using an Ultra Tec which is about 25 years old – but is in good shape and recently re-calibrated (by myself). The run out of the master platen was certified as 0.0002 TIR when new. This is too accurate for me to re-measure with my kit.The oscilloscope is a twin beam ‘scope with a timebase from 0.2 Sec/div to 0.2uSec/div. The Y scale can be set between 20V/div and 0.5mV/div and is set to DC coupling.The circuit diagram of B/W Indicator I built (Figure 3) has a 50uA meter internal resistance 4k3 ohm, with a 1k0 ohm shunt. In series is a 10k5 ohm current limiting resistor. The battery is a 3v lithium rated at 2Ah.

Figure 3. The circuit diagram of my Beale Wooley Deepth of Cut Indicator

The ‘scope was placed across (in parallel with) the contacts on the facet head. Note this will give a zero volt reading when the contact is closed and a 3v reading when open.

Now when the lap is rotating the voltage measured by the ‘scope varies from 0v to 3V. The offset in relation to ground was not discernable.

I have done some further tests with a stationary lap. The lap was newly faced in the lathe and is 10mm thick.

By setting the Y amplification to 10mV/Div, I was able to see the contact resistance change as the contacts were opening & closing however this signal only changed from 0 to ~25mV before jumping to 3V. Now I have measured the contact resistance at this closure and it is approx. 400 ohm. It is worth mentioning at this point that the residual ambient electrical noise is ~5mV ( from fluorescent lamps, domestic appliances etc ).

As a result of the above, I would surmise that when someone is using a stationary lap to set up a machine he is measuring at a different order of magnitude from that used when cutting. When cutting (with my setup) the integration of the pseudo square wave over time swamps out the contact resistance changes. I assume that this is because my spindle / cutting lap run out is, by far, the greater source of errors.

I have tried using the machine with the newly faced aluminium lap rotating and also without the lap; running the stone against the integral master platen. The master platen was cleaned specially for this test. (Because I had the stone against the master platen I used a piece of soft glass on the dop – hardness about 51/2.) Both the aluminium lap and the master platen show the same effect. I have also checked the effect of vibration introduced by the motor and spinning lap. I placed the dopped stone on a block adjacent to the lap. No change of reading was observed.

However I noticed on a couple of occasions that the readings with the stone on the spinning aluminium lap suddenly started to ‘flutter’ more than it had earlier. I have started to wonder whether this is as a result of dust settling on the lap or glass particles abraded from the stone by the aluminium. Cleaning appeared to alleviate the problem.

I believe there are two conditions which are in existence. With a stationary lap the resistance of the contact can be measured, this enables the machine to be set up accurately. When the lap is rotating the effect being measured is much greater. One is measuring the “digital flutter” of imperfections in the lap and lap/spindle set up. In this case the inertia of the measuring instruments come into play. I could NOT achieve greater cutting accuracy by measuring the change in contact resistance.

I hope I have managed to explain myself clearly, I now think I understand both effects much better.

Jon Rolfe suggested that one can convert the signal into an audio output. This of course does not help those of us who have hearing problems whether deafness or tinitus. Perhaps this is why the indicator has made life easier for me. Carl Downey suggested using a bi-colour LED, I would prefer a needle as some people may have difficulty in discriminating between the colours produced.

However at the end of the day what matters is ‘Does it work for you?’ We don’t need a full knowledge for the theory and science behind the method. If it improves your enjoyment of faceting then great.