2 years, 5 months ago-
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Hello Members
I am looking at cutting a stone for the single stone competition. I note that the Width for that stone [Novice] is 10 mm +/- 2 mm & the Gridle Thickness is 0.4mm +/- 0.2mm. What instruments are used to aid in achieving the correct dimensions?
In engineering I am used to the 10 by principle. If the dimension is 10mm, I would need an instrument that would read to 0.1mm.
Kind Regards
Ian ORRELL
Measuring width is quite easy using any caliper which reads to 0.1mm. At the machine, I use a carbon fiber dial caliper, which can’t damage the stone even when I’m clumsy.
Girdle thickness is harder. It can be measured directly with an instrument like the Bausch & Lomb Lenscope with a reticule, but that’s relatively expensive and awkward to use at the machine. The best way I know is a comparison with a thin wire or other object near the desired width. I’ve found that a jeweler’s saw blade works well for most girdles. Saw blades are in various thicknesses, so you’ll have to use a micrometer to measure them. You can also use a length of wire – there’s a useful chart at AWG to mm
I’ve read that competition cutters some years ago used the back edge of a broken hack saw blade as a reference to determine girdle thickness.
For more direct measurement of girdle I use the appropriate bit from a pin vise drill set. Since you can mount it in the vise it’s easier to hold still.
Good idea, Joseph. In the US, wire gauge sizes start at about 0.34mm and increase in quite small increments. Metric drills below 1mm are not common, but they are available. I just found on Amazon, listed as “Zona 37-151 High Speed Steel Twist Drill Set, Metric” a set for $16.30 which has 0.3mm-1.0mm in 0.05mm increments, and up to 1.6mm in 0.1mm increments. So they are available.
I read about the use of brass feeler gauges and ordered a set from Amazon for $6.89. They are listed as “Stanley Proto J000BR 6 Blade Non-Magnetic Feeler Gauge Set”. I saw where someone cut them back into a blunted point for ease of use. I haven’t done that yet, but will soon.
I use a set of wire spark plug gauges that I got from Ace Hardware or a local Auto Parts store. These are usually in thousands but you can go on line or with an app make the conversion to millimeters. The wire gauges won’t be exactly hundredths of a millimeter but you can get an good approximation. When I try to measure the girdle with my feeler gauges, my hands are not steady enough to be exact, but I can get real close.
Tom Mitchell
Hello Members
Thank you for your ideas on what use for measuring the girdle thickness.
Much appreciated.
Kind Regards
Ian ORRELL
I second the use of ‘spark gap’ feeler gauges, readily available from your local auto parts supplier. I sprung for the better of the sets which gives me around 20 different calibrated flat pieces of metal that I could use to measure just about any girdle on any stone.
The end is rounded off, so I used a nice flat lap to grind off the rounded edge so the end corner of the flat piece of metal can be placed right on the girdle, magnified and verified very precise. The gauges can also be use to make sure your micrometer is at least pretty close (at least on the < 1 mm scale).Carl
When I did my stone for the competition a few years back I just got some scrap pieces of paper, measured the width, and cut a straight piece with some scissors. Then when cutting the girdle I just would place the piece over it and eyeballed the facet till it roughly was the same width.
It can be measured fairly accurately with a caliper however I like to use feeler gauges. You can by a set an any auto parts store for under 10.00 and always be positive about girdle thickness. I hope this helps and good luck with the 2018 entry. Large facets with exacting meetpoints can be lost during polishing. be careful with your polishing to maintain those meetpoints. pay attention to where they say to take your measurements from. And always get your girdle with enough width for mounting the stone. Mounting puts pressure on those edges and your girdle width must be strong enough to not crack under pressure.
When I was first learning, I was taught to use a wire but I never liked that idea because a wire is round. It lays against the girdle facet and then you have to sort of eyeball the width of the wire and project that dimension onto the girdle and make a judgement call. I bought a brass feeler guage set and combine whatever sizes are needed to make the .3 mm or whatever. It is not perfect because they are odd sizes and don’t add up to the .3 mm exactly but I felt it gave me a better reading than the round wire.
There’s a set of small plastic cards that you can do this with, called the Melee Meter. One card in the set is a girdle measurer. I resell these card sets from the vendor at $70. Stuller also sells them, if you have an account – they buy in bulk and can sell them a bit cheaper than I can.
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