Ordinary cast iron will be easiest to get hold of, but if you can get the ductile iron, it is much less messy when you machine it. Steel would be third choice. Check whether that bore is tapered.

Cast-iron deadens vibration while steel tends to ring. Otherwise I doubt it makes much difference. Made mine out of cast-iron and have moaned about the mess ever since!

Any decent cast-iron should do, but beware of old weights, street furniture and ornaments and similar scrap. Nasty cast iron is very nasty indeed, full of inclusions, hard spots, blow-holes and cracks.

Dave

Steel will bruise or deform easily, particularly troublesome when at the trial fit stage. I'd go for cast iron and if the bore is > 1inch try an old weight lifting weight .

Steel is perfectly fine for making a backplate. The ringing is a common-repeated non-issue. When I skim my cast iron clutch pressure plate it rings like crazy but t doesn't happen in the bike. Same goes for backplates.

Low-cost chucks are cast iron because the bodies are cast to their gross shape which minimises machining and material costs. Same goes for backplates – anyone anywhere could cast them. If you took the time and cost factor out of the equation they would be made from cast steel or machined from solid.

I bought a steel backplate in 1 3/4 X 8 on ebay, it was only 4" diameter, and it is now on the back of a 9" faceplate which had the Myford thread, but not enough meat to modify to the larger thread. It is held on with 6 titanium stepped studs, from a Westland Lynx gearbox. The 8mm ends screw directly into the faceplate and the 6mm ends go through the steel backplate and use aircraft locking nuts with integral washers. The biggest advantage of steel is the lower ammount of mess made when it is machined.

This old wife was quoting Rollason's 'Metallurgy for Engineers':

Ding dong!

Dave

I am sure Rollason was correct but he no doubt was writing for the professional designers of massive machine-tools and marine diesels as well as of those working closer to our sizes of components.

One of the bits of lab equipment I operated at work was a vertical cylinder about 14 ins diameter with a wall a good inch thick, topped by a massive plate lid needing an overhead hoist, and floored a few inches below by a deep-skirted, cast-aluminium piston. The lid was mild steel but the cylinder, including its downwards extension forming supports etc., was cast-iron for that very reason – damping. Just to make sure, the whole caboodle was also bolted to a massive concrete plinth.

A chuck back-plate on a moderately-sized lathe is frankly pretty unlikely to resonate noticeably. Yes, if you dangled it from a bit of string it might make a nice if perhaps high-pitched dinner-gong, but in service it is heavily clamped between the mandrel and chuck and would probably only ring if at all at very particular speeds or under certain cutting conditions. More likely any ringing you hear is from the work-piece.

Nevertheless even quite small machine-tools do use cast-iron for its stability and self-damping as well as the other reasons Rollason gives; but some modern ones are built around welded steel fabrications filled with special forms of concrete. I recall reading this of the products of a major German builder of very large machining-centres; but I think the Peatol lathe successfully uses much the same principle.

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Having typed that about gongs, I recall once being a passenger on a freight ferry from England to Sweden. The steward's dinner-gong was a steel pipe blanking-plate suspended from a piece of wood!