Machining cast iron in a lathe or a mill.
Machining cast iron in a lathe or a mill can be a bit of a problem for anyone new to this metal.
In the process of casting, when it is at a very high temperature in a liquid state, as the liquid is poured into a cool mold, then the sudden change in temperature of the contact surface causes the liquid iron to solidify very quickly, while the remainder of the iron cools more slowly.
What this does is to form a very hard skin on the surface of your casting, maybe only a few thou’ thick, and it is this surface that causes most problems to engineers.
It is mainly due to the mold not being hot enough at the start of the pouring stage of metal casting or the sand has a little too much water in its make-up, or again, the casting mold has too little ventilation to let any flashed steam out.
High-speed steel cutting tools tend to get blunted very quickly owing to the hardness of this skin. In fact, carbide tips can many-a-time, simply shatter, so what most engineers do is to start off with a well-used tool and force it under the skin and literally rip it off. Once you are through the tough, hardened skin of the cast iron, then using lighter cuts with a sharper tool will keep its edge longer.
For machining cast iron in a lathe or a mill, carbide-tipped tools tend to work better, but need that first layer taking off with a high-speed steel tool first as these can be re-sharpened easily, but this depends mainly on the hardness of the cast-iron surface.
Cutting Speed for Cast Iron
Another problem associated with machining cast iron on a lathe or mill is the speed of the cut.
Cast iron machining in a lathe or mill needs a very slow speed to cut it properly – too high and there will be sparks everywhere and your tools will need re-sharpening very quickly. As a general rule of thumb, to get a smoother finish on cast iron, you tend to need a finer tip on your cutting tool with very slow transition along the work.
Cast Iron and Dust Control
We all think of cast iron as being a solid lump of metal, but what it is, in fact, is a bonding of very fine particles of iron, and for this reason there can be iron dust literally floating around in the air while you are machining cast iron.
Holding a magnet near the cutting tip has been tried as a way of capturing this iron dust, but is very limited in that respect as much as the particles and dust are thrown out in all directions and are seldom close enough to the magnet’s attractive forces, which obviously has to be out of the way of the cutting tip and workpiece.
Another option which helps is to have a vacuum cleaner sucking the dust from the cutting tip. Again, this has to be mounted close to the tip, but not in the way of the work.
An adequate method of protecting yourself from this dust is needed, with a dust mask being the basic method through to a dust extractor near the cutting point being the another option, otherwise you will find yourself covered in black particles after only a few minutes of machining this metal, and that is not counting the dust you will have breathed into your lungs without any protection.
Fluid like water or cooling fluid used in a lathe is a further option, where these particles are captured by the flow of water, and there is no need for any type of lubrication to help the cutting as graphite is usually included when cast iron is produced.
All lathe beds and milling ways should have some protection from this dust and the easiest option is to have some newspaper sheet placed over them. Do not use a cloth as if these get caught up in the moving parts, they are a likely distraction from your work and can lead to physical damage done to yourself if you make a grab at it to remove it.
Generally, if you happen to have a casting that ruins your tooling, as sometimes these castings can have very large parts of ultra-hard sections, you can ask your supplier for a replacement casting and they usually oblige.
All they do is drop the hard one back in the pot and recast it again, but hopefully in a better manner next time.
So, all in all, it boils down to low-speed cutting, along with using dust protection and getting that first bite under the very hard skin to produce good parts from iron castings.
If you feel you need to have a good surface finish on your cast iron parts, as usually they tend to be a bit rough after machining, then using a very fine emery cloth to smooth it down is the best option.
Happy metalworking from the Steamshed gang.