|Y C Lui||27/05/2022 05:41:46|
|61 forum posts|
I have done some machining on grey cast iron recently and was amazed to find how easy it is but further reading on the subject makes me worry a bit.
Some on-line sources mentioned that the swarfs are highly abrasive and will do harms to bearing surfaces of machines. Others said that only the swarfs coming from the material on the surface is abrasive but not those from the core. Some even said that the graphite in the material is abrasive but my understanding has been that graphite is a lubricant .....
So it's all very confusing for me.
Can anyone shed some light on this ? Are there any difference between grey and ductile cast iron in this regard ?
Edited By Y C Lui on 27/05/2022 05:48:24
|Michael Gilligan||27/05/2022 07:27:07|
20182 forum posts
Light is shed, here: **LINK**
|Nigel Graham 2||27/05/2022 08:35:26|
|2133 forum posts|
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.
6393 forum posts
Yes it can be abrasive if it gets into the slideways of your mill or lathe. So best to clean it off well as soon as you finish machining cast iron. Covering the ways with a concertina cover or just paper or rags is a good thing to do too. Apart from the metalurgy etc etc etc, there are issues with the size and shape of the swarf chips that come off cast iron.
8691 forum posts
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.
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.
Edited By SillyOldDuffer on 27/05/2022 11:11:25
|Y C Lui||27/05/2022 11:46:15|
|61 forum posts|
Thanks, Dave. The explanation is exactly what I am looking for. Now I feel more comforable with machining cast iron.
1713 forum posts
I understand the comment above about protecting beds, slides etc from the dust, but I'd suggest using paper, rather than rags.
|Bob Lamb||27/05/2022 12:54:22|
133 forum posts
When I used the metal lathe over lockdown to finish a woodturning project I used clingfilm to stop the dust getting absolutely everywhere. It's safer than rags and less likely than paper to get knocked. It also moulds to the lathe so doesn't get in the way. It should work ok for cast iron dust unless it gets too hot? Bob
|Howard Lewis||27/05/2022 17:57:43|
|6104 forum posts|
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!
|Andy Carlson||27/05/2022 19:00:46|
|432 forum posts|
Out of interest... how does continuous cast iron round bar compare to a more traditional casting?
My gut feel is that it is less badly behaved - I think that sand is ruled out for example. Chilling - maybe better controlled? Carbides.. no idea.
|Andrew Johnston||27/05/2022 20:49:37|
6601 forum posts
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.
|not done it yet||27/05/2022 21:15:32|
|6809 forum posts|
Back a century and a half, plough parts (shares and mouldboards) were a problem. They either scoured well - and wore out quickly - or were hard-wearing but certain soil conditions meant the soil stuck to the plough parts like ‘something’ to a blanket, as they say.
Along came a a fellow who used ‘chilled iron’ to largely overcome the problem. A self-scouring but really hard material. Leave any metal filings, like this, between your lathe parts at your peril. Perfection with castings cannot be guaranteed 100%. Close, maybe, to 100% - but would you risk your machine? Particularly if the parts are not fully hardened.
Castings are often cast in sand. That is certainly abrasive (just think of ‘sandpaper for smoothing surfaces) and some may be adhered to the surface of the casting. Chilled castings are another issue, as detailed previously. Both those issues provide more than sufficient reason to clean away swarf carefully and completely. The graphite content will be soft - but cast iron is not all graphite!
|old mart||27/05/2022 21:22:54|
|3772 forum posts|
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
|Nigel Graham 2||27/05/2022 22:26:51|
|2133 forum posts|
What grade is used for car brake-discs?
I have cut up a ventilated disc for fire-bar material, but also have a couple of single-thickness discs reserved as cast-iron stock plate for more sophisticated use.
367 forum posts
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
|Y C Lui||28/05/2022 06:33:35|
|61 forum posts|
Yes, I can get ductile cast iron , EN-GJS-600-3 or 500-7. Do they product abrasive chips like grey cast iron ?
|old mart||29/05/2022 17:42:28|
|3772 forum posts|
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.
|not done it yet||29/05/2022 20:36:33|
|6809 forum posts|
Let’s remember that iron reacts with water and oxygen to form oxides. Hematite is used as an abrasive (for polishing - as jeweller’s rouge?).
If you wish to keep polishing your ways, go ahead and use it as a lubricant. I wouldn’t!
|noel shelley||29/05/2022 20:43:48|
|1344 forum posts|
Much useful information from Pat J, thank you. Noel.
|Mick B1||29/05/2022 20:50:28|
|2192 forum posts|
A powerful vacuum cleaner used assiduously after finishing will draw out most of the abrasive particles - so long as you've cut dry.
Back in the day the tendency was to use airlines that'd blow the stuff further in.
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