... what I always refer to as the frog is actually technically called the "Common Crossing"? And this is measured as a 1:4, 1:5 etc?
Yes and yes (at least in the steam era)
[quote="P-J.S., post: 70372, member: 729"Would these always be a 1:5, or 1:4 etc or would they be made up to suite a specific location? Using templot you can set them such as 1:4.75, 1:5.25, but all the Jigs seem to be in whole number ratios?[/quote]
Crossing angles of 1:N etc. would be common to all railway companies... angles of 1:N.5 would be known probably within most railway companies... angles of 1:N.25 and 1:N.75 were used by some pre-grouping companies (GWR and LNWR at least) although not for all values of N. The range of "N" might not be the same for all railway companies.
When looking at turnout data for pre-group companies, the crossing angle can be expressed as "N x" where N is measured in feet and x is measured in inches (I have only seen values of 3, 6 and 9 for x). So 1:6 3 (or .3 or /3) reads as 1 foot separation (of Vee legs) at 6 feet and 3 inches from sharp point of Vee (which is a short distance in front of the blunt point of the vee).
Manufacture of filing jigs from the S7 Group is a process which involves milling, case hardening and grinding so filing jigs are made only for simple crossing angles like 1:5, 1:7 etc.. Manufacture of S7 Group crossing assembly jigs is done by CNC milling of paxolin hence the S7 Group is able to offer such jigs with 1:N.5 angles.
There are photos of S&C work set out in the Swindon PW shops which indicate that bespoke crossing angles were possible... I have seen no evidence that the GWR ever used bespoke switches.
regards, Graham