Thank you for that, Michael.

I have wondered about this too, having seen warnings about cast-iron swarf particles on the lathe.

The cited document is fascinating – I had not realised cast-iron is so complicated! I have even book-marked it in my growing links folder labelled "Engineering".

Any intrinsic abrasive might be the silicon compounds and carbides, but the silicon is not accused of forming silica or silicates in the iron. I would think the greater hazard for the machine-tools is tiny grains of casting sand still trapped in cavities or corners even after fettling and cleaning.

Sand is mainly silica – silicon dioxide – occurring in nature as granules of quartz weathered from igneous rocks rich in that mineral, and from eroded flint / chert (silica agglomerations in sedimentary rocks); and nearly as hard as diamond. Glass is the same stuff, being basically fused sand.

Cast-iron is a family of alloys – over 30 available commercially – and they have rather different properties.

The simple view is cast-iron a mixture of soft Iron and Graphite which as an excellent lubricant. As it can be used to make cheap bearings, one might think it's not abrasive. Wrong!

Looking closer, Cast Iron is mainly a mixture of Cementite, Austenite, Ferrite, and between zero and a lot of Graphite.

• Ferrite and Austenite are both soft
• Graphite is an excellent lubricant but the amount present depends on the alloy
• Cementite is Iron Carbide. As might be expected of a Carbide, it's hard, brittle and very abrasive.

Machining cast-iron breaks the structure into fine particles, many of which are abrasive. The abrasive problem is multiplied because castings often arrive with an extra-hard skin due to chilling.

Mixed with graphite the carbide particles form a grinding paste which gets into the ways and bearings and causes severe wear. In the good old days second-hand lathes used to mass produce cast-iron components were a bad buy. Much more rapidly worn out than the same machine worked hard cutting Brass, Steel, or Aluminium; they lasted a few years rather than decades.

Hence the apparent paradox. An old lathe might happily run its spindle in a plain cast-iron bearing for years, whilst the much better lathe that cut the bearing had a short life and is long gone. In the first lathe, the abrasive particles were cleaned off and the high graphite content dominates. The Carbide matrix only causes trouble if it disintegrates, which it will if the bearing is overloaded. Conversely, the lathe that cut the bearings broke the hard matrix and was sprayed with abrasive Carbide.

Silicon is also hard. it's added to promote Graphitisation, to improve strength, and to resist corrosion. If a Silicon containing cast-iron is machined, the Silicon isn't as abrasive as Carbide and there's less of it. Henchman rather than Chief Villain!

Most ordinary cast-iron contains lots of graphite and, although a professional would choose more carefully, they can be used to make effective light duty bearings. Other cast-irons are unsuitable because they don't contain enough Graphite, perhaps none. Best not to use unknown scrap cast-iron to make important bearings.

Difficult to clean up properly after cutting cast-iron – it goes everywhere. Old fashioned factories didn't bother – even when labour was dirt cheap, it was cheaper to run lathes flat out until they couldn't hold the generous tolerances of the time and then scrap them.

Apart from the mess I don't think machining cast-iron in a home workshop is an abrasive problem unless a massive amount is done with no attempt to catch bits and clean up.

Dave

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Edited By SillyOldDuffer on 27/05/2022 11:11:25

I understand the comment above about protecting beds, slides etc from the dust, but I'd suggest using paper, rather than rags.
At least when paper gets caught in something rotating, it will tear, rather than suck in your fingers.
There's a reason why some of the commercial; blue rolls are designed to rip easily.

You can hold the paper down with tape, thin elastic bands, or small magnets.
Depending on the application thin foil ready meal containers might have a use as well; I use one for catching parted off pieces.

I also keep an old copy of the Yellow Pages; the paper works well under a vice, workpiece, etc. to stop it sliding on the mill table, or it provides a clean work surface for re-building things such as brake cylinders. When it gets dirty, just rear off the top sheet.

Bill.

The outer skin of a casting is often HARD (Cast iron camshafts often have chills in the mould, to chill and garden the nose of the cam )

We received a batch of cylinder block castings which ruined every cutter in the transfer line. (Once loaded, it was impossible to stop and extract them! The problem was that the foundry had knocked out the boxes near to where the cladding on the building had been removed, allowing snow to blow in onto the red hot castings, and chill them!

Even with dust extraction, a surprising amount of dust escapes.. Using greased plates, and weighing the dust collected, indicated that over the area of the shop machining that particular cylinder block, there was one, very finely divided, floating about in the air above!

So in the air above the machines, the powdered hard skin of cast iron can be very abrasive and damaging to the machine on which the work is being done..

When machining cast iron, I always put a powerful magnet under where the swarf will fall, and cover it with paper. Carefully removing the paper brings away a lot of the dust (Not all unfortunately ) Away from the magnetic field the swarf can be tipped into a suitable receptacle for disposal.

Makes cleaning up so much easier!

Howard

I've turned and milled a lot of it, all the bevel gears, pinions and the liners for both my traction engines. It is lovely to machine, soft and consistent with no hard spots or inclusions.

Having said that most of the iron castings for my traction engines have also been soft with no hard spots. I think my supplier uses a small, traditional, foundry.

Andrew

If you can catch the dust with a vacuum cleaner or magnets while machining and cover up the parts if possible where the dust drops all the better. I hate cast iron, but SG or ductile cast iron is lovely stuff to machine if you can get any and the swarf is much like steel.

Edited By old mart on 27/05/2022 21:23:19

I have worked for 6 years to learn how to make my own iron castings, and while I am no iron "expert" by any stretch, I have learned a few things about iron, and how to get consistent and machinable gray iron castings.

I use scrap iron motor end bells, and they are consistently good quality material that are not too difficult to break up into smaller pieces.

My first iron castings were about 1" thick, and they machined easily without any additive.

I then tried some thin castings, perhaps 1/2" and thinner, and succeeded in making tool steel, which would dull any sawblade, and was totally unmachinable.

I discovered ferosilicon, and so now I use a slight amount of it in each melt, and this prevents the "chills" in my thin pieces. The "chills" are hard spots in the casting that are created when the metal solidifies before the graphite flakes can be established in the metal.

My first iron casting used Petrobond, which is an oil-based sand, and while iron can be cast in Petrobond, my sand erroded badly at iron temperatures, and so I had some sand inclusions, which luckily I was able to repair.

I discovered "resin-bound" sand, and it is a 3-part material, with resin, hardener, and catalyst.

Resin-bound sand is designed to work well with iron and steel castings, and I have never had any further mold problems using resin-bound sand; ie: no voids, no inclusions, no defects of any kind.

The continuous cast gray iron bars machine beautifully, but in a blind test, I don't think I would be able to distinguish my castings from gray bar, as far as machining or quality.

One more trick that seems to help get consistent iron castings is to let the casting remain in the sand mold overnight, to cool slowly. Not absolutely necessary, but it seems to help make a very machinable iron casting.

You definitely don't want to pull a hot iron casting out of that sand and quench it in water; that would be asking for trouble.

Edit:  Any more than a slight amount of ferrosilicon (75% grade) will cause excessive shrinkage and will cause hot tears in the castings.

Edit02:  Avoiding slag inclusions in castings is a matter of using the proper gate and runner configuration.

The runners should deadend into a spin trap that is open to the top of the mold, and the gates should be on the top of the runner, so that the gates skim the slag off as the iron enters the mold.

Voids are often caused by poor sprue and runner layouts and sizes.  I try to actually rest the lip of the crucible on the top of the mold, to prevent any waterfall effect.  The sprue should fill quickly, and remain full throughout the entire pour, so as not to aspirate air.  The runners terminating in a spin traps give the initial flow of metal and any entrained air/sand/slag a place to escape, without entering the mold cavity.

The velocity of the molten metal should be controlled by the gates, not the sprue.  The mold cavity should be filled as fast as possible without causing turbulence in the metal.  High velocity in the metal stream causes many if not perhaps most problems in iron castings.

Edited By PatJ on 27/05/2022 23:04:40

The swarf from ductile (SG) iron is not horrible black powder like ordinary cast iron, but shiny like steel chips. The other advantage of ductile is that it isn't brittle like ordinary stuff. You are lucky to have the choice.

Much useful information from Pat J, thank you. Noel.

As Howard Lewis has suggested above, a strong magnet is a worthwhile item when machining CI.
But I'd suggest you put the STRONG magnet into a polythene bag and 'stick' it near the cutting tool. When there's a good collection of CI dust on the outside of the bag, just extract the magnet from the bag over a waste bin and reuse. Small neodymium magnets are ideal and cheap enough online.

(One other issue to watch – don't wash your hands after machining CI in the kitchen sink or SWMBO will be irate next morning at all the rust marks which will have appeared. Removal thereof will require painting with Jenolite or similar).

The other thing with cast iron is to wear a face mask.

The alternative is pretty unpleasant when you blow your nose over the next 24 hours…

Neil