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Material for a Chuck Backplate

Mild Steel, Grey Cast Iron or SG?

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Brian G17/06/2020 14:48:54
705 forum posts
28 photos

I need to make a 5" diameter backplate like that in the photo (but without the protruding cap head screws) but cannot decide what material to buy. Despite the inevitable mess I'm leaning toward cast iron in case the milled recesses around the rim would affect the stability of mild steel, but have no idea if grey or ductile cast iron would be best.

Thanks for any advice

Brian G


Steviegtr17/06/2020 14:55:38
1266 forum posts
119 photos

I am sure there was a thread on here recently if you do a search.


old mart17/06/2020 15:01:03
1829 forum posts
148 photos

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.

Journeyman17/06/2020 15:05:57
805 forum posts
141 photos

I can see no reason why a slice off a 5"dia EN1 steel bar should not suffice. The milled flats seem only to serve as a space for the cap screws to go so could be left off just making the backplate a little thicker which would then provide plenty of room for the counterbore. If the cap screw heads in the picture are getting in the way why not replace them with countersink ones?


SillyOldDuffer17/06/2020 15:53:02
5932 forum posts
1282 photos

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.


Andrew Tinsley17/06/2020 16:58:37
1150 forum posts

+1 for a slice of EN1. Much easier and cleaner to work with than cast iron. ARC's back plates are steel and they are just fine.


Lee Rogers17/06/2020 17:38:29
64 forum posts

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 .

Mark Rand18/06/2020 00:04:20
900 forum posts
5 photos

Whatever you've got or whatever you can get will be adequate for the job. Cast iron, EN1, EN3, EN24 all work on backplates I've made and I'd be happy with brass as well if I had a suitable lump without another use lying about.

Possession is nine tenths of the rule! So use whatever you can get most readily in the size you need. Although the EN24 needs a bit more effort to machine...

Pete Rimmer18/06/2020 06:29:05
734 forum posts
50 photos

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.

Andrew Tinsley18/06/2020 09:31:10
1150 forum posts

The old wives tales are certainly still current. Do yourself a favour and get a slice of EN1,


old mart18/06/2020 15:32:25
1829 forum posts
148 photos

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.

Paul Ainsworth18/06/2020 16:07:36
97 forum posts
15 photos

Is it for a DB10? I bought a backplate for a C6 from ARC, only difference is 52mm nose hole. I sleeved and bored to 50mm to fit the db10s.

SillyOldDuffer18/06/2020 17:11:18
5932 forum posts
1282 photos
Posted by Andrew Tinsley on 18/06/2020 09:31:10:

The old wives tales are certainly still current. Do yourself a favour and get a slice of EN1,


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


Ding dong!


Pete Rimmer18/06/2020 19:16:53
734 forum posts
50 photos

All those high-speed steel CNC chucks don't seem to suffer from ringing.

Nigel Graham 223/06/2020 00:22:25
667 forum posts
15 photos

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.


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!

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