Hercules Graziano change gears.

The following discusses the change gears on the Sag14.
Specifically my sag14...see thread here.
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Mine is a Hercules clone.
Use this information at your own risk, this only applies to my clone.
The Hercules has only two ranges in the QC box...later Graz seem to have 4 and may need other gear sets.

In order to switch from metric to inch follow the directions on the plaque. In addition to setting the correct control positions the change gears need to be changed to match the plaque as well. Find the manual on line somewhere.
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The problem arises when, like me, you don’t have the original change gear set.
A perennial problem with buying a lathe.

Mine came with the metric set.
Since the Graziano is European its native is metric.
So, the metric is simple.
20T on the spindle -> a 40T idler -> 80T drive into the QC box.
As shown on the QC plaque, although it omits the idler.
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To drive the QC at inch rates a different gear train is needed.
The plaque shows...
31T spindle ->
58 driven/ 47 driver ->
64T QC box drive.
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These gears measure as mod 1.0,you are on your own for pressure angle.
My recollection is that metric is almost always 20*.

Now.
The ratio of the inch change gears is...
(31/58)*(47/64)=0.39251
As any nerd can tell you that’s not quite right.
That gives us a ratio of 2.548 not the defined 2.54.
Not to worry though that gives us an error of 0.3% or 3parts per thousand.
Over an inch that becomes 0.003 error...so don’t use the screw cutting for depth measurement, and it may throw off your clocking if you are doing something weird and long. Cutting a 10” lead screw would throw you off by 0.030.
I’m assuming there isn’t a 3:1000 gearset in the QC box some where to correct for the error.
When I buy/build or steal a gear set I’ll mearure it and update this post.

On topic questions and comments only please.
Give it a few days for pictures.

My SAG 14 cuts both metric and imperial on the standard gearbox. It's "natural" threading is imperial, due to the 3/8 leadscrew pitch. The standard banjo gears are 20:80 with any idler between.I thought all SAG's were like this.
I have done the maths on my gear- ratios and the metric pitch error is in the region of 3 in 1 million.Better than the leadscrew pitch error in manufacture when new, never mind for a 40 plus year old machine.
My box has a 4 ratio ABCD box followed by a 10 ratio 1-10 box, then a W / m selector.I thought all SAG 14's were like this. In this second box the drivers and driven gears are reversed for cutting metric threads and driven through a compound train of 4 gears (plus an idler.)This is all selected at the same time when m is chosen. The system is extremely cleverly thought out and arranged, with different pairs of gears having a different module in order to have the same centre distance. The manual shows this well, although not the module differences. I believe the SAG 17 is the exact same arrangement, but am ready to be corrected!

I hope this info helps: Not many Graziano's about here in the UK..