Cast iron - 160mm dia

Cast iron is more rigid than mild steel. And it machines very nicely when cutting the thread in it, if a screwed spindle type. But for practical purposes, mild steel will do the job if need be.

That's odd, because cast iron seems to have a rather lower Young's modulus than low carbon steel.

Andrew

That's where theory deviates from practice. Maybe it's because Young's Modulus measures deformation under tension whereas cast iron's greatest strength is compressive? Or its superior resonance damping properties? Either way, I've never had much luck bending good grey cast iron. It usually seems to want to break before it will bend. Never managed to break a chuck backplate though. (One of the few things I have not managed to break over the years )

Edited By Hopper on 05/07/2019 12:15:07

Cast iron reduces resonance in machinery so I'v been told, my mild steel back plate , about twice as heavy as one used on a lathe such as a Box Ford, or Myford.

Ian S C

I have one 9" faceplate fixed to a steel 1 3/4" x 8 backplate of about 3 1/2" diameter. The backplate was bought cheap on ebay, I have no idea what it was originally intended for. It is fixed by six double diameter studs, 6/8mm diameter, the 8mm in the plate. Unusually, the studs are titanium, ten a penny in an aviation museum.

The ci with the inclusion was sold to me as meehanite continuously cast by a reputable company, I would have no idea if it was something else.

While Young's modulus is normally measured in tension the concept is equally applicable to compression. I'd agree that cast iron is better in compression than tension. But that doesn't imply that cast iron is necessarily better than other materials in compression. The fact that cast iron breaks without bending much simply means that it is brittle, not that it is necessarily stiffer than other materials.

I'd also agree that cast iron is good at damping vibration. But that is to do with the internal structure of the material, and in particular the flakes of free carbon. These absorb energy as the material lattice vibrates and thus reduce the amplitude of the vibration. Values in the link below imply that larger flakes are significantly better than small flakes at energy absorbtion:

**LINK**

Andrew

Comparing the material properties may be interesting but I suspect that the real reason is that c.i. can be easily cast with a nice boss on the back.

Rod

For volume production, to reduce machining time and the quantity of swarf requiring disposal, you need the raw material to be near net shape..

A backplate would be easier to cast than forge, and the moulds would be cheaper than the dies for a forging, reducing capital cost, probably with a smaller labour content as well.

Howard

That only applies to those with threaded spindles, those of us without them have to turn the cast boss away so a slice of bar is the better option

.

Contact details please

MichaelG.

We don't sell the titanium, if I'd found steel ones, they would have been equally useful at the time. They, and 8/10mm ones were originally used in Westland Lynx gearboxes, but were replaced by steel which is superior. Having a large stock of them saves me having to make them myself.

Sorry ... I was taking your statement literally

Hi

My favourite location for cast iron is ebay. Look for large postal weights these can be bought very cheaply and machine great.

Knightrideruk

Out of the darkness comes light.............

Steel makes perfectly adequate chuck backplates, and it is stronger than cast iron. Cast iron is cheap which is why it continues to be so prevalent. Easy to cast into a rough shape then machine to a finished article. Cast steel is not nearly so easy to make.

Digressing briefly away from the subject of the thread, I have always wondered if perhaps cast iron would be a suitable material for the crankshafts in small ic engines, I mean those of only small capacities say up to 7 - 10 cc. I feel that it would be much easier to machine than steel but I am curious as to whether it would survive the stresses in the engine?

Dave W

More like 10p each when I bought a few studs (that look like rivets) at the 'Jet Age' museum in Gloucester. Ostensibly they were from the TSR2 project.

Ian P

Cast-iron is strong in compression but relatively weak in tension, and it's also rather brittle. I think the alternating stresses in a crank-shaft would soon break it. Before steel I believe crank-shafts and other reciprocating parts were made of Wrought Iron for the same reason. Does anyone know what Stephenson used?

Several 19th century cast-iron bridges failed, perhaps most famously the Tay Bridge, which tragedy led to North British Railway Engine No 224 being nicknamed 'The Diver'.

The most common cause of collapse of these structures seems to have been failure of cast-iron members under tension, though quality problems like blowholes featured too. At best not a good material when bending and pulling are involved.

Dave

Thanks Dave, I thought it would be too easy to use cast iron, looks like I need to persevere with steel, I find that rigidity of the lathe is the problem when turning crankshafts with any degree of overhang with the tool.

Dave W

Can't help with that one - all the crankshafts I've made ( only 3 ) have been made up.

Would Brass or Aluminium be worth trying? Whilst not as strong as steel, they might be 'good enough' for a small engine.

Or perhaps there's a technique for turning steel crankshafts an expert could explain for us? I just realised it's yet another metalworking job I don't know anything about!

Dave

Thanks Dave, I think I will go for a made up assembly instead of turning from solid, not sure about brass or aluminium although should be easy to turn even with a large overhang of the tool. I think webs and shaft made separately, in steel, and then pressed together with interference fit or maybe loctite.

Dave W