Author Topic: Gemmaster - isolating protractor indicator to install stop light  (Read 2164 times)


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I have recently purchased a GM4 and am seeking some advice. It has been suggested that I have a 12v light installed to indicate when the stop point is reached. Does anyone have any experience with isolating the protractor indicator from the rest of the machine body so this light can be installed?
Thanks in advance.


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Re: Gemmaster - isolating protractor indicator to install stop light
« Reply #1 on: January 16, 2023, 11:20:55 PM »
Welcome to the forum emmemmjae.
Do you mean Gemmasta GF4 ? I have the red light on my machine, it was pretty useless. I remember rewiring the circuit as a depth of cut indicator from Tom Herbst books, wasn't impressed with that either. Long time ago. Then I discovered encoders  ;D



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Re: Gemmaster - isolating protractor indicator to install stop light
« Reply #2 on: January 18, 2023, 01:36:45 PM »
The light is pretty useless, A Beale Woolley depth of cut indicator is a bit better.

I made a few of them to put on the clubs gemmasta machines, they work OK, certainly better than nothing but are prone to issues with dirt on the hard stop mucking up the operation, easily fixed with cotton bud and a bit of metho.

Now to your Question about how you do it, this is from memory, I haven't done it in a few years. You need to electrically isolate the handpiece and swivel from the rest of the machine using thin plastic, the thin plastic I used was recycled from bubble packs of screws or something.

Just check with an Ohm meter many of the Gemmastas already had the handpiece isolated electrically, so one lead of the meter on the handpiece other on the body of the machine anywhere. If it is a short circuit you will need to remove the big brass nut from the swivel and dismantle it to see where you need insulation. It's easy enough to do and you wont muck up any serious adjustments.

Sorry I can't be more specific, I don't have a Gemmasta any more and I don't go to club either. Give me a shout if you need an analog meter to do this job, I have a couple of spares that i would be happy to rehome. Other than that a cheap analog ohmmeter will do the job.
« Last Edit: January 18, 2023, 01:44:11 PM by Rusted »

Faceting Frank

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Re: Gemmaster - isolating protractor indicator to install stop light
« Reply #3 on: January 19, 2023, 03:15:21 PM »
Hi emmemmjae.
Never used a GM4. But looking at a photo of the quill, the distance from the pivot point to the stone is about five times greater than from the pivot point to the protractor indicator. So, installing a light to work with a stop on the protractor indicator would be a waste of time. The reason being that even if you managed to get the light stop to consistently work at an accuracy of 0.01mm, the 5-1 disadvantage would mean the facets are being cut at a depth accuracy of 0.05mm.
Your, going to have to reduce that 5-1 disadvantage by increasing the distance the light stop is away from the pivot point.


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Re: Gemmaster - isolating protractor indicator to install stop light
« Reply #4 on: January 21, 2023, 03:16:20 AM »
Hello emmemmjae,

To answer your question and as Rusted said, the isolation is done by a small plastic part fitted on the axis in the faceting head. The axis of the protractor is electrically linked to the machine frame. The faceting head is fitted on this axis (over the plastic insulator) and electrically linked to the other pole of the battery.
When the faceting head reaches the stop it closes the circuit

Images :

Best Regards,
« Last Edit: January 21, 2023, 03:20:37 AM by Lanzeyrion »


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