Hello Zal.
The trouble with using a Model ‘A’ headstock, despite later ones having a D1-3 Camlock, is that the centre hight is only 4 1/2 inches. But because it is a genuine toolroom class lathe built to the correct proportions, the width of the bed across the very large slideways is more than twice the centre hight at 9 1/2 inches, so it will cope with a centre hight of around 6 1/2 – 7 inches to swing your 12 inch chuck. Most workshop type lathes of around 6 1/2 inch centre hight have beds that are only around 7 1/2 – 8 inches wide with much smaller slideways, so you were right to choose the Model ‘A’ bed.
A simpler but just as effective as a Camlock type spindle nose is a design used on some German machines such as Wieller and also Prazimat’s DLZ, and even on a few cheap Chinese bench lathes such as the Sieg C6 and Chester’s belt change DB 10 G.
This looks at first like a thick plane flange type fitting but behind the flange is a thin knurled collar. The nuts behind the collar that secure the chuck are just loosened, no need to remove them, and then the collar is rotated a few degrees to align the larger holes machined in it which allows the chuck to be withdrawn complete with the mounting nuts.
The backplates look a bit like a Camlock type. They have at least 3 large diameter pins with a smaller thread and nut on the end. This system is safe to start and run in reverse and to use a spindle brake in both directions, and is almost as quick to remove and replace as a camlock but would be simpler and quicker to make, as well as the backplates.
The short locating register or spiggot on the front of the spindle nose does not have to be tapered. it could be a parrallel type. After all, threaded spindle noses use a parrallel register and a shoulder to very accurately locate chucks and other fittings, the thread does nothing more than pull the fitting on and hold it there. Many lathes currently on the market that use a plain flange type fitting also use a short parrallel register.
Hope this helps.
Edited By Lathejack on 12/05/2011 13:08:29