Cast Iron vs Mild Steel

Hello,

I often get a bit confused to what material is suitable for a project and I wonder if I can get peoples thoughts on the pros and cons of using Mild Steel or Cast Iron for models & home workshop tools. Including (but not limited to) machinability, finish, rust, strength, hardness, surface to surface friction, price etc

Many thanks

Thomas

Where to begin Thomas?

Most of the cast iron I use comes in the form of 'castings' (just saying that sounds a bit daft) but the casting process means that specific shapes can be made fairly easily - with the machining bringing any required surface down to size. So for instance, the wheels on my locos are cast, as it would be more work to fabricate them. I have recently acquired a set of Stent T&C castings for this reason. I mention the Stent because some people have produced basically the same design from 'stock' material. There are different types of CI but generally it machines very well (once under the outer skin) and can form a good bearing surface because of the graphite it contains. It can however be prone to cracking as it is not nearly as strong as steel.

Mild steel is much stronger and will come in stock sizes. Generally larger assembles will need to be fabricated by welding, brazing or screw fixings. Mild steel is relatively 'soft' but can be surface hardened. It generally does not make a good bearing surface but these days most will use some form of other bearing within any steel structure.

Both will rust but in my experience CI seems to absorb a degree of oil (?) over time and my older CI machines seem less liable to rusting than the steel parts in the same environment - but this is purely anecdotal.

Generally, for my tools and models I use mild steel and only purchase castings for specific applications. You didn't mention cost in your question (edit - sorry you did e.g. 'Price'  ) but this is an important consideration. For instance, the Stent castings will normally cost a lot more than the stock material required to build the same grinder.

This is a large subject and I would advise you have a surf of the Internet for any specific areas that might be relevant to your needs.

Regards,

IanT

Edited By IanT on 04/02/2014 17:04:28

Edited By IanT on 04/02/2014 17:04:55

Edited By IanT on 04/02/2014 17:05:24

Thank you once again Ian.

I was more referring to the Cast Iron blocks and bar you can buy from people like College Engineering rather than actual castings with specific shape.

The low friction quality of cast iron intrigues me, especially as I am about to embark on a radius/ball cutter of the Steve Bedair type. I instinctively thought mild would be better as it was stronger and feels heavier. Perhaps cast is better for it instead?

One thing I noticed at MEX was the cast iron bar looked rustier than the mild steel stuff.

I used steel for my Bedair tool, there is not going to be a lot of movement so steel on steel with a bit of oil will be fine. The base plate and rotating block could be done in iron but the tool holder would be better in steel as its thinner and the small thread for the tip retaining screw could strip over time.

The steel you see is not rusty as its got a layer of oil on it, the rust on any CI bar should only be a fraction deep and is of no consequence and no indication of the lovely material below.

I mostly use the CI bar where its wear and self lubricating qualities are needed such as piston rings, cylinder liners, eccentric straps. Its also better for carving steam cylinders out of if you don't want to go down the casting route.

J

Edited By JasonB on 04/02/2014 17:40:11

Continuous cast iron as supplied by CES is a joy to machine. It is a very forgiving material that can be worked with great accuracy and is less likely to take a 'set' than steel 9although more likely to crack if shapes are poorly designed). Beware that bores which seem to be a close fit can open up - this is caused by retention of a layer of fine dust (I think) a fraction of a thou thick.

The only downside is that the fine dust has a similar effect on your nasal packages to snuff, so consider wearing a dust mask if you machine a lot.

Neil