Machining cast iron

I am machining some 1" dia cast iron bar for a friend and I am having difficulty in getting that super smooth finish that can be found on parts supplied by manufacturers, also the amount of dust created when cutting through the crust eventually chokes up the screw threads on my Myford 3 jaw chuck.

A) how do I get that super finish ?.

B) how can I stop the dust from penetration the innards of the chuck ?.

C) will some lubrication help. ?.

Thanks, Ken.

A small magnet on the tool or post might help?

First of all do not lubricate, the carbon in the cast iron is sufficient.

What depth of cut are you taking and with what kind/shape of tool?

If it is that dusty a vacuum cleaner will pull a lot of it away.

NO lubrication, it will just turn into grinding paste, and cast iron doesn't need lubrication when it is machined. I don't usually have trouble with the dust in the chuck, but I do cover the bed between the head stock and the saddle with news paper. Yes you are going to get dirty, if you blow your nose, use a paper hankie, it will be black.

Ian S C

Hi Kenneth,

Assuming no 'cold' spots and once you are under the skin/crust - try a slower turning speed and a slightly round nosed HSS tool. If you have used the same tool to cut under the surface (I use a carbide tool for this part of the work) then it will most likely need to be re-sharpened to get anything like a decent finish. Cast iron dust is dirty stuff and does tend to get everywhere, so a regular clean up with (paint) brush and vacuum cleaner helps to control it.

No lubrication is required.

Regards,

IanT

Hello Kenneth,

​Cut cast iron dry, there is more that enough carbon content to provide any lubrication needs. Get under the skin of the material with a decent degree of feed, use a modest speed, not necessarily low back gear either, and then turn the waste of within the capacity of the machine to cope with comfortably. I have found a sharp pointed tool will tear off the waste much more satisfactorily than a tool with a broad face, those can chatter.

​Scratching about on the surface makes dust, wears the cutting edge on the tool and leaves a poor finish

Swarf in the chuck is inevitable, as soon as you are done take it completely to pieces and clean it scrupulously.

​The super finish comes at the end stages using light cuts having roughed the thing to shape, a little polishing with a scotchbrite pad will remove the final rough edges and leave a nice surface.

​A magnet will just turn into a ball of dust I'm afraid and leave everything it shouldn't magnetic when it is taken away; it will have no useful effect really

Regards
Brian

It is the graphite content in the cast iron which provides a kind of self lubrication for the cutting tool, any more grease is just ineffectual at best and a nuisance at worst. I always cut this dry just like brass. One of those "showery" materials unfortunately so any guards you may have devised will come in useful for keeping your floor and clothes clean and avoid any hot chips going down your t-shirt.

Really fine grit papers are great for finishing, I use a shop vacuum to clean up most of it, comes to be the final word on swarf removal in general, not that expensive to obtain one from a supermarket these days but i have a henry hoover. Magnetic doodads are best left used for picking up items that have fallen into awkward spaces.

I normally wipe down the bed ways with paper towels and leadscrew and regrease with some proper slideway lubricant. Probably the best defense you have against damage are some felt or rubber bed wipers for the vee ways.

Once you've cut cast iron you soon get used to these things and it's a pleasant material to machine eventually . "Meehanite" is a brand of close-grain cast iron which is a good one to look out for if you can get it. Lovely finish

Michael W

Edited By Michael Walters on 25/10/2016 10:27:24

I cut cast iron from the back, using an upside down tool, and running the lathe at slow speed (backgear)

This means the mess doesn't fly about all over the place and I sook it up with a vacuum as it accumulates on the lathe

For a nice final cut finish though you may still have to run the lathe at full speed on the correct side with a sharp tool, or as mentioned, a wee rub with some fine grit paper can also do the job.

There are different grades of cast iron.

I have been turning some 90mm dia cast iron which is a pleasure to machine (neglecting the dust) cuts like butter with carbide tool and polishes to a beautiful finish with emery paper. Unlike some cast iron bars, used for window sash weights which are a B%%#####$$$ to machine.

Paul.

Kenneth

When machining materials such as cast iron which create a deal of dust it is a wise precaution to wear a mask so preventing inhalation.

Emgee

one inch dia cast iron ,when using HSS tools turn at a speed of 200/220 rpm,if surface is "as cast" use use carbide tipped tool for roughing at about 500/700 rpm. finish turn with HSS tool ,flat top i.e. no top rake, a radius on the tip will improve finish, too big a rad will cause chatter,and will also depend on the size of a lathe,a large lathe can turn with a large radius as its mass will tend to absorb chatter, if you need to work up to a sharp corner then its very small rad so reduce the feed. On the last couple of finish cuts resharpen the tool on the grinder and carry on ,do not increase speed to improve finish ,all that will happen is that the edge of the tool will get rubbed off.during my apprenticeship I machined a lot of cast iron and just got used to it ,ok its a bit dirty,but nice to machine,do not oil up your chucks keep them dry and clean with brush, clean your machine before turning steel as soluble oil can turn cast iron into cakes of rust,don't blow your nose into good handkerchiefs the iron from your nose turns to rust and rots the handkerchief.After retirement I have made some cylinder liners for stationary engines from solid the largest was 6.5 by about 17/18 inches,bored through at about 5 inches generating about 130 lbs of swarf I did not bother with a brush just shovelled it into a wheelbarrow, my generation did not bother about dirt or the cold working conditions just got on with it. Just keep excess iron swarf from slide ways ,i have seen machine tools where a lifetimes use has made the exposed slipways look as if they had been scratched by a giant cat.

Just because you can machine CI dry doesn't mean that you have to do so - coolant does have the advantage of keeping the dust down. You do need to clean the coolant tank out soon after finishing, though, as any swarf that makes it's way into the tank eventually sets like concrete.

Nigel B