I use both HSS and insert tooling. Nothing special, tool geometries as for low carbon steel, speeds similar, or slightly slower, and feedrates the same. Cut dry, cast iron swarf plus coolant equals a horrid black sludge. To some extent it depends upon the source of the cast iron. if it's continuously cast then it is soft and easily machined with any tooling. If it's a casting then inserts might be better choice as some castings have hard spots or surfaces depending upon the foundry technique.

Andrew

Agree with Andrew, though I prefer carbide just in case there are any hard spots in the skin. Cast-iron varies enormously depending on who made it, why, and how it was treated in the foundry.

Engineering cast-iron is much better than old sash-weights and the like, but casting often causes a very hard skin to form. Although thin, it can blunt HSS, which is why I mildly prefer carbide just in case. Underneath cast-iron is soft and generally machines well. Angle grinders get the skin off too,

However!!! Main thing to expect of cast-iron is mess. Filthy stuff and it goes everywhere, including back into the house. Best kept out of slides and bearings etc because cast-iron particles are abrasive. Mixing with oil produces a kind of combination grinding paste / paint. Dry dust leaves black marks on whatever it touches. So I cover the vicinity with paper sheet and clean up often.

Dave

Never leave cast iron dust/swarf on a dry bed of a lathe or milling table in a damp (water) atmosphere. It will go solid and cannot be removed. The answer is to clean everything (I use the same trick as David but with kitchen paper) and then oil all metal surfaces.

JA

I recently did the chuck register on a RDG supplied 100 mm Boxford machined casting which was no problem at all.

Raw castings sometimes have random hard spots in odd places, the ones I've had shriek as the tool skids over them, then it's carbide time.

If you have a lot of turning to do on another job, then graphite dust up the nose requires a mask. Some people use the shop vac while turning to deal with that

I don't understand why that should be? Cast iron, particularly the continuously extruded stuff, is fairly soft. Cast iron is also one of the few materials that is happy working against itself. So why would cast iron swarf be abrasive? Of course if there's sand or slag in the mix from a casting that would be abrasive, but not the cast iron.

Andrew

If you have enough hands a cunningly positioned hoover pipe an collect a lot of the swarf before it lands

If you have got a chilled casting that will take the edge off HSS tools in an instant, Carbide will cut it OK but you still have those very hard bits of swarf flying about. So if it can abrade HSS it could probably affect the machine's surfaces, not that I bother covering anything I just get on with it.

Fair point, although I understand that in cast iron that cools slowly there is more graphite than iron carbide. I've probably machined more cast iron than anything else, at least by weight. I'm reasonably organised in cleaning the lathe and haven't noticed any wear. But my lathe does have hard plastic wipers on the saddle and it has an induction hardened bed.

Andrew

Graphite is wonderful to machine. I used to make spark erosion electrodes from graphite blocks milling turning grinding and benching on a shadowgraph to shape. Lovely stuff and machine wrecking if graphite gets into bearings and electrics. The boss had a go at machining an electrode at night, and firstly left open electric cabinet and opened a filter cabinet with fans on and the whole factory was covered in a later of powder graphite, took a week to clean up.

David

Bo'sun:

I don't machine cast-iron very often but when I do, I use moderate to low speeds. That helps control the mess!

Cast-iron is often used for machine parts because it has a better vibration-damping quality than mild-steel.

Yes you could use mild-steel for a back-plate, as far as I can see, and I dare say some do. After all it is not very large and is heavily clamped so not very likely to ring.

If you are prepared to machine a casting but would prefer steel to iron then you may as well buy a sawn slice of free-cutting mild steel. As you say, buying it ready-machined would put the price up but more significantly a back-plate is normally finish-turned to match the lathe and chuck anyway.

It was reckoned that in the shops where cylinder blocks were machined that the haze contained a complete block casting.

I had greased plates, a foot square, situated in the shop where we machined V8 blocks. Having washed off the dust from the plate, weighed in the cleanliness cage, and calculated the floor area of the shop, there was, indeed, a block floating around above us, in very finely divided form.

Although I didn't work in the machining area, any time spent there during the summer, meant that my brow and head were a nice shade of yellow buy the time that I got home.

Howard