Herbert Lathe converter plate

Hi There Gents.
I have done a lot to make a converter plate for my old Herbert 4 Preoptive take a Forkardt Chuck which is in excellent condition.
I have made a plate in half inch mild steel, - as a trial to see if I can make one using more appropriate materials, like cast iron later. I have fitted it to the flange on the Herbert and have been able to turn it down to an accurate diameter. The flange has a recess to locate anything bolted to it and this isn proving to be the problem, as if I take it off now, I don't know if I can bolt it on again centrally as I have it mounted on countersunk bolts and these will not be available if I turn it over to get at the back. The Old Herbert Inspection Number is: 94-2949.B.
Do you think an inch thick plate turned to match the Herbert flange and then turned in situ to match the Forkardt with a small flange that fits into the hole in it's backplate, drawn on by the mounting bolts threaded into the plate, will be OK? Is there any approved design for these things. I am thinking of taking the final design to a proper machine shop to get the recess turned out, but I need to know what the DIN number for that Herbert flange is I have seen lots of converter plates for sale but they always quote the specification of the mounting and apart from the diameter which I have measured at 19.03 mm as far as I can see and the holes in it form an equalateral triangle 7.88 cm radius. I don't have the definitive size. I cannot take these measurements to an engineer and risk saying, please make me a plte like this.
Costing only 20 pounds I have discovered a cast iron disk of over an inch thick and the right sort of diameter. It is a spare dumbell in a sports shop and so I am going to look round the car boot sales for a cheaper one.
Does anyone know what the spec. of thisHerbert mounting flange actually is, please?
Thanks a lot George.
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Lots of worrying about specs when you've pretty much got the thing done! I've got a much smaller lathe that uses a similar mounting method, 4 studs with nuts on the back of the chuck to mount on a flange. I wanted a larger chuck, so I got a cast iron round, took dimensions off the nose of the spindle flange and turned a recess, I rough-bored the center hole first by bolting it onto the midget faceplate that came with the lathe. Once the recess was bored, I turned things around, chalked up the spindle side of the mounting plate-to-be and gave it a rap with a rawhide mallet. This leaves chalk everywhere but where the holes are. Drill and tap for studs. Doesn't matter if they're off a bit, the recess on the plate, if properly cut, will center the works. Now bolt the thing onto the spindle, face the chuck side off, true the rim to size and fit that side to the recess in the chuck. Repeat with the chalk to mark the chuck mounting holes and drill those holes oversized a few thous. Again, if you turn the spigot on the mounting plate properly, the chuck will be centered. Mount the chuck and you're done! If the chuck mounting bolts are going to foul on the spindle flange, you're going to have to go with a thicker plate and use counterbored capscrews. This way you've got the mounting plate fit to YOUR lathe, no one else's and it's pretty much machined in situ.
Stan
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On 18 Oct, 23:55, snipped-for-privacy@prolynx.com wrote:

Yes Stan, that just about covers it, the thing that worries me is the accuracy of the press fit flange. To get it to fit as tightly at the original chuck, which is a ground fit and has three push off release screws to take it off after it has been drawn on by the three mounting bolts the fit has to be extremely accurate. I am not sure I can work to that accuracy when I have to turn it round after mounting it on the flange to face the edge etc. Don't forget that, - at the moment I do not have a working chuck at all. If I clamp it on using just hex bolts onto the flange do you think I can turn out the recess to the accuracy required for correct registration. As I have mentioned I am a beginner really, but I do need the lathe for repair jobs on all my old machines. I shall have to do all this again as I need quite deep threaded holes for the 16 mm mounting cap screws, the half inch plate will not be thick enough for this.
Thanks George.
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