Machining Cast Iron?

Good morning All,

Simple question for you knowledgeable folks. I'm about to machine the register spigot on a Cast Iron back plate for an ER32 Collet Chuck. I haven't machined Cast Iron before, so what tool material and geometry would you recommend?

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

Thank you Andrew,

The backplate is fully machined (apart from the spigot), so I suspect no chilled margins.

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

I put a magnet in a plastic bag under the chuck to collect dust and swarf and you just remove the magnet from the bag over a bin and the swarf falls off.

David

Edited By David George 1 on 30/10/2020 10:34:40

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

Best to wear a mask when machining cast iron, all those fine particles will damage your health if inhaled.

Emgee

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

+1 for a magnet to collect as much as possible of the dust

(You will never collect all of it, but a magnet in a plastic bag, or under newspaper does help to keep it off the machine and everywhere else, including the operator! )

Howard

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

Thanks for all the excellent tips and suggestions.

I'm acutely aware of the pitfalls when working with Cast Iron, having worked in an engine machining plant, and a Foundry for 7 years. I've just never machined it myself on a small scale.

Thanks again.

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

It's due to what cast-iron is made of, essentially Iron plus a hefty dollop of Carbon. When melted together and cooled the result is a matrix of Iron, Graphite and Iron Carbides (Cementite and or Pearlite).

Free graphite is a good lubricant, so cast iron is a good bearing material rather like Oilite. However, machining cast-iron breaks the matrix so the swarf is a broken down mixture of Iron, Graphite and Iron Carbide particles. Apart from being dirty Graphite is harmless. Iron shavings aren't a problem either. But Carbide particles are tough and abrasive. Whilst not in the same league as Tungsten Carbide, but Iron Carbide is hard enough to embed in most metals to form a kind of rasp. Like emery paper, half of it is soft and harmless, but not the grit.

Steel also contains carbides but machining it doesn't break tiny particles out - it stays trapped in the swarf.

Dave

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.

Best to wear a mask when machining cast iron, all those fine particles will damage your health if inhaled.

Never bothered with such things when I worked at Broadbent Machine Tools in the early '80s, where a lot of CI was machined. All the major castings for heavy duty lathes were mainly planed, bored & drilled dry.

The fine dust got everywhere & one of the electrical maintenance jobs to be undertaken during the summer shutdown was to strip & clean all the fluorescent light fittings in the main machine shop. It was a filthy job that took me around a week to complete - working off the walkway on top of the gantry crane, remove the tubes & reflectors, lower to the ground, wash and dry then replace. The dust was like black talc & required the use of detergent to get the parts clean. IIRC there were 6 rows of 8 foot doubles across the building & almost end-to end along the 150-200 yard length . In that regard it was fortuitous that I was only there for one summer shut down before being made redundant !

It was easy to spot colleagues who had just come back from holiday - they were the "pink" ones. After a couple of weeks they went grey from the ingrained dust that got deep into your pores.

Graphite is also abrasive - well man-made graphite is. We use cast iron grade coated inserts to machine it at work, as the CI coatings hold up better to the abrasion than the other options. Another horrid dust that gets everywhere, and when mixed with oil makes a very effective paint that takes days to wash out. My current colleagues do tend to wear nuisance dust masks, but HSE investigations concluded that the graphite dust didn't pose any particular health hazard. Best not to inhale any form of dust, though.

Nigel B.

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

I have machined several, including one from solid instead of cast to rough shape. I used all the magnets available and should have used a vacuum cleaner as well. The nicest type of cast iron to machine is tha ductile SG type, (spheriodal graphitic), none of the dreaded black dust.

Well, the machining itself went to plan, but boy, what a mess. Despite plenty of covering and magnets, the chips still went everywhere. The clearing up took three times as long as the machining itself. I shall be avoiding Cast Iron like the plague in future. Clearly, it's the only material for some jobs and castings, but if there's an alternative, that'll be for me.

What I don't understand is, the back plate I bought was a fully machined piece of Cast Iron, it just needed machining to suit my Collet Chuck. Why not supply it as a machined piece of Mild Steel? Cost presumably.

Hi, I too have machined a fair bit of cast iron, mostly grey and haven't bothered about covering up, just sweep down after and I haven't noticed any wear problems. it's just a bit of a black job, but barrier cream on your hands and soap and water when you've finished solves that, and if you think machining cast iron is a dirty job, have a go with a 9" angle grinder and cutting disc, been there done that a few times in my work.

Regards Nick.

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