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It is only to be expected that the calliper IC will draw current spikes that translate to voltage noise. In a static state, CMOS logic draws only nano-amps. The operating current is due to charging and discharging internal capacitances on each clock edge. There needs to be adequate decoupling to limit the voltage excursion this causes. A battery may just be good enough, but a suitable capacitor should be better.

In view of the very small margin below the battery voltage for the chip to work correctly, I again suggest investigating a slightly higher external supply, but still with adequate local decoupling at the battery terminals, or even better as near the power connections on the chip as possible.

Do we know the clock frequency used? An electrolytic may not be best – a suitable ceramic is generally better at higher frequencies. The leads should be short to avoid adding stray inductance in series with the capacitor. A correctly designed surface mount capacitor fitment would be best, but it may be a bit late for that!

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Presumably the integrated circuit in a digital calliper uses the same type of CMOS as watches. Here, in the interests of best battery life, the FET device parameters are chosen to minimise the totem-pole currents (as well as otherwise operating reliably). The absence of external connection, and the fairly complete Faraday cage of the case means that watches are generally immune to ordinary levels of interference.

The same technology is not so sensible for digital callipers, particularly when an external connection is able to import RFI. It occurs that a possible investigation might be to increase the voltage of external supply slightly from 1.5v.

Initially, the supply current should be graphed against increasing voltage from 1.5v upwards to, say, 2.0v , or until the current indicates chip dissipation of a few milli-watts. For example, at 2.0v, 500 micro amps would result in 1 milli-watt, and hopefully such a small dissipation and over-voltage would do no harm. An increased supply voltage should provide higher noise immunity. Decoupling at the calliper would clearly still be essential.

Anyone looking for a project?

Perhaps I should have spelled it out, but the object of using the truncated M12 thread is to achieve a reasonable thickness of material between the threads. For 11mm, it is about 0.7mm, which is quite adequate to take the 3/8 Whitworth tapping forces. It is also fully strong enough in service, after all a wire thread insert works.

Manufacture is easy as outlined provided you can screw-cut in a lathe.

In MEW 291, I was interested in Brian Wood's account of coping with the curse of the wrong size draw bar thread. Having previously standardised on the once predominant 3/8 Whitworth threaded arbour, I too looked for an easy way to cope with the larger threads. There is no special merit in an M12 drawbar - 3/8 Whitworth can over tighten MT3 more than sufficiently! At one time, there were 14mm spark thread repair inserts that used only the outer half of a 5/8 ANF thread, and the same principle can be used here.

The modification is outlined for a M 12 threaded arbor, but will work similarly for ½ inch. First screw in an M 12 bolt to measure the length of thread. Next, bore out the inner portion of the M12 thread by drilling with 11 mm. A HSS bit usually works even for a hardened thread, or use a carbide milling cutter. Now screw cut an adapter that fits this truncated thread as follows.

Making the Insert

Grind up a lathe tool with a 60 degree V thread profile, then carefully form a flat on the end slightly less wide than one half the thread pitch - say 0.75 mm for the 1.75 mm of the M12 thread - it need not be exact. With a suitable steel bar in the chuck, reduce the diameter to about 11.9 mm over the required length with an additional short section at the beginning to gauge thread depthing, turned to be a close fit to the root of the truncated M12 thread (ie circa 11 mm).

Do not angle the top slide. Screw cut until there is a slight witness on the 11 mm gauging section. At this point there will still be a flat on the crests. Now, while keeping the same diameter setting, take cuts advancing the top slide (deburring as necessary), until the arbour can be screwed on, but without slack.

Finish the adapter by making the internal 3/8 Whitworth thread (ensure deep enough), and facing off the 11 mm stub. Screw on the arbor with thread lock, and once set (apply hot air gun to speed up), part off if feasible safely, or go part way through and hacksaw.

One made much earlier.

Allen,

The problem is obviously not the usual shear pin since the quadrant gear train still rotates up to the gearbox input. Firstly, best not run the machine under power anymore, since this could cause more damage if there are teeth off gears. However, looking at my moth-eaten parts list, there is a pin as the output coupling to the power feed shaft (this assumes you were not using the lead screw when the incident happened). It is not clear if it is designed as a shear pin. If you disconnect the power feed shaft (it has a slot driven by the pin), it should be possible to check it. Undo the RH bearing block and slide shaft and lead screw assembly to the right (no need to remove completely).

You do not report any gearbox noises, so it is hopeful that damage may be (relatively) minor. Unfortunately, it looks like you need to fully dismantle the gearbox to fix. There is also an oldham type coupling for some of the gears, failure of which would prevent those particular gears from working. Start by disconnecting the quadrant train to the gearbox, then turn input by hand, trying all gears. The gearbox can be removed for easier dismantling. Reassembly is a fiddle to get the selectors correctly engaged.

You are a bit unlucky to have this problem. The end stop release works reliably on my Bantam. However, I recall an article in MEW years ago where the author had trouble with the release. I do not remember the details, but the calibration of the springs that control the release force was incorrect. Perhaps someone can point you to checking if this could be the problem.

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