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Author Topic: Concave cutter variable oscillator.  (Read 720 times)

Faceting Frank

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Concave cutter variable oscillator.
« on: April 14, 2021, 07:49:30 PM »
Just finished the concave facets for the “Happy Birthday Concave” for this years, AFG annual comp. It has 24 concave facets on the pavilion all going to a point, they are the only facets on the pavilion and as you will have already worked out very thin. Never cut concave facets this thin before and what I’m finding is at the end of the oscillating stroke at the polishing stage, any variations in the cutters surface is, showing up on the facets.
Don’t have time to muck about with trying to fix this, because time is running out getting my stones finished for the comp. The only way I can think of fixing this is to vary the length of the stroke, so its not starting and finishing in the same place all the time.
Anyone else noticed this in concave cutting?

FlashGP

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Re: Concave cutter variable oscillator.
« Reply #1 on: April 15, 2021, 07:39:59 AM »
Hi Frank

I finished my entry last weekend, it looks great.  I didn’t notice the issue you mentioned, but have designed my oscillator to have an adjustable stroke length.  I can set the stroke at 30, 40 or 50 mm.  I run it at a 40mm stroke length.

My cutter and polishing mandrel set are all machined to within 0.02mm diameter of each other.

I found that the ability to position the cutter with 0.01 mm increments was invaluable.  The cutter needs to be centered on the reference facets.

The design looks great when finished.
Yours Sincerely
Flash (Gordon)

Faceting Frank

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Re: Concave cutter variable oscillator.
« Reply #2 on: April 15, 2021, 06:28:07 PM »
Hi Gordon,
I make my own mandrels, this particular mandrel is 12mm dia. Copper coated with solder and turned in a lathe. I think the tin content is 40%, it would be a lot softer than a BATT lap but polishes really well. Couldn’t see anything on the mandrel in the area that was causing it to cut slightly deeper. Haven’t attempted to cut any of my mandrels to the tolerances your talking about.
So was thinking of making an adjustable pushrod for my geared oscillating motor, so I could alter the stroke if this problem occurred again.

FlashGP

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Re: Concave cutter variable oscillator.
« Reply #3 on: April 19, 2021, 08:00:46 AM »
My push rod is a male tie rod end screwed into a female tie rod end, both from Banggood and is adjustable so I can stop the forward throw before I run out of bearings.  I simply adjust the position of the mast so the stone is centred on the mandrel.

My stroke adjustment is via a series of taps on the rotating arm driven by the oscillator motor.  If I had a lathe, I would use a wheel with a series of tapped holes spiralling towards the centre, it would look cooler and give finer adjustments to the stroke length
Yours Sincerely
Flash (Gordon)

Faceting Frank

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Re: Concave cutter variable oscillator.
« Reply #4 on: August 27, 2021, 10:35:42 AM »
Got this year’s Australian Facetors Guild competition results from their website. Surprised at coming second with stone 0.10.6, “Happy Birthday Concave”, scoring 99.52. I wasn’t happy with the polish I was getting from the solder coated copper mandrel I was using. It did produce as good a polish as a BATT lap would produce. But I think being 60% lead, which is very soft, the surface was becoming uneven as I was using it and it was creating an uneven surface on the facet. But the judge didn’t seem to mind which was good. So will skip making a variable stroke pushrod.
Going to make an aluminium mould to coat a copper mandrel with an old BATT lap metal. So, when I get round to making it, I will do a post on it.

Faceting Frank

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Re: Concave cutter variable oscillator.
« Reply #5 on: August 30, 2021, 08:11:01 AM »
That must be you who won Gordon?

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