[Ubarch]

Hello,

So I'm trying to make a Stirling engine to practice metalworking. The design I am working with requires a "heater" which should not be aluminum. I chose to make this part out of cast iron. I've just finished about 4 hours of toil with it and have virtually zero progress. I must be doing something wrong, and I hope someone can help me here.

I'm working on a Sherline lathe. My work is a 6" long, 1.5" diameter piece of cast iron bar stock. The first thing I did was attempt to cut off a 1.5" piece to work with, so I bolted down the cutoff-bit that Sherline sells (the thin, flat piece that looks like a straight razor) into its angled holder, and everything seemed like it should work. The chuck was holding one end, and the other was held by the free-spinning cone piece whose name I cannot rememeber. I managed to drill out an indentation in the iron to let the cone secure the work, but this had to be done by hand and it was very slightly off center. I was able to bring the lathe up to full speed without the vibrations seeming very bad. I also didn't have any real cutting oil; all I had was Safeway-bought general purpose oil. I oiled up the oitside fo the bar and the cutting bit.

At this point, I attempted a cut. There was a lot of squealing, and noise, and I was able to get one or two chips to fly off the bar, leaving little burning-oil smoke trails. I stalled the motor twice trying to do this. I decided that perhaps the cutting bit was a little off from the centerline of the work; this would certainly explain things. I repeated this process, cutting from both sides (making sure to flip the cutting bit over), and eventually manufacturing two aluminum shims to get more height. None of this produced different results, and I noticed that the cutting area of the bit had oxidized, and was worn down. I gave it a new edge with a rotary tool, and tried again; same problem. At this point I gave up and sawed the piece off with a hacksaw. That sucked.

Now it was time to face the piece. I had a small amount of sucess here, but nothing that could be called progress. The iron piece oxidized and wore down the cutting bit in the same manner, and I was only able to cut a small amount of iron. I was also able to observe that the bit was almost exactly on the centerline. After stopping the lathe, I observed that the cut areas exhibited visible galling. At this point, I decided that I was too ignorant of iron-working to continue without damaging the equipment.

Can anyone shed some light on why I couldn't cut iron? The book that Sherline pushes, "Tabletop Machining" has a few paragraphs on machining cast iron, and there is no mention of it being difficult in the least; only that it does not deform while being worked and that the shavings are messy. I inferred from this that the Sherline lathe should be able to cut cast iron.

Any help is appreciated.

ubarch

[Bubba]

ubarch,
Sounds like you are running it to fast! For 1.5 inch diameter, I would look at turning it in the region of 200 to 250 rpm! I have always cut cast iron dry

[Ubarch]

Hmm.. . 2-250 rpm is about 3-4Hz. I think I was actually around that range. I tried a bunch of speeds. I'll try 225rpm the next time I attempt to machine the piece, but do you think it could be anything else?

[Me2]

Ubarch,
You mentioned 1.5" cast irom bar stock. Where did you get your stock, if its something window sash weights throw it away. These are extreamly hard and into the range of white iron. Cast iron will tend to be hard on the outside due to chilling and the non percipitaion of carbides during the transformation stage. Usually a heavy cut to get under the skin will do the job. Cast iron will usually machine like butter and requires no cutting lubricant. You would have better luck buying cast iron stock from your local bearing supplier although this can be rather pricey.

[Mcgyver]

ubarch, hang in there, you'll get it. just remember to do everything the exact opposite of what you posted .

my suggestion is that you pick up a senior high school machine shop text – while the content is for slightly bigger machines, they usually present the basics in a way that is easy for beginners to follow. Once you start to learn the basics like determining the cutting speed, feed, depth of cut and tool geometry the solution to problems like this become intuitive.

On top of everything Bubba and Me2 said, which is excellent advice I’d add with regards to the trouble parting:

- Never ever part off with a centre (that cone shaped thing) its dangerous and probably won’t work as you are forcing the sides of the cut into the parting tool. If you can’t hold the piece though the chuck, use a steady rest. If you don’t have a steady it’s the band saw, failing which it’s a vice and hacksaw (as you found out - that’ll get you saving for the band saw)
- parting gives lots of people the shivers. One of the biggest problems is clearance between the sides of the part and tool and the danger of binding and/or a chip getting caught. The Brits often favor holding the tool upside down and at the back of the lathe for this reason. I wouldn’t attempt to part 1.5” in one shot with a light lathe/tool – make the part 1.5 or 2 parting tools widths wide. Feed in a bit on one, back out, move over, feed the other, and so on. Its no fun when the parting tool grabs and things go bang which its very liable to do on a 1.5” part with light parting tool.
- Where geometry comes into play is in making sure the parting tool is ground correctly. You need rake & front clearance – side clearance is usually built into the blades. Make sure its perpendicular to the lathe axis, both horizontally and vertically
- Look at the end of the parting tool. How wide is it, 100 thou? How often on that size lathe do you take a .100 dept of cut? I bet not often. The point is that parting is a very heavy cut with an extremely light duty, flexible tool. Forget theory, you’ve got to slow everything down to the point that the machine and setup can handle the cutting force. As the weak link is the slender parting tool, this holds true for bigger lathes as well.
- Its perfectly acceptable to hand drill the centre hole – you are using a centre drill, right? There are ways to lay this out and you should have centre punched before drilling. Once in use, it will work its way just fine. However, if the work is really eccentric, you are taking all the challenges of the big cut described above and compounding it by making it an intermittent cut.

Nice cast iron machines beautifully – let us know how you make out

[HuFlungDung]

Ubarch,
Just noting your comment about running 3 to 4 hz on your spindle. This is quite likely in what I would call the 'gut-less' power range of your motor, if it is using a frequency drive to control the speed. If you have some kind of gear reduction to get the motor speed up while keeping the spindle speed down around 200 to 300 rpm, it will give your machine a bit of mechanical advantage to actually cut without stalling out so easily.

[Ubarch]

The bar stock came from Small Parts corporation, and is listed as:

CAST IRON, ROUND - Class 40, Grey ASTM-A48

I'll try to respond to some of Mcgyver's points:

-The original idea was to make a deep cut in the car such that it would be easy to finish off with a hacksaw. However, I see your point.

-I'm fairly sure the parting tool was factory new. I've actually "found" this set. It was purchased years ago with surplus funding for my lab, along with a mill, and most of the acessories sold. The tool was unscratched, in the original box when I found it, and had an edge ground onto it. The block that holds this piece of Sherlines is angled, and this leads me to believe that the angle should have been corrent. This being said, on my next attempt I will explicitly check the points you mention.

-I could not figure out a way to drill a hole in the center of the piece while it was still 6" long. Maybe I should have tried to hold it in the lathe chuck and drill it very, very slowly with the object that the centre attaches to. Note that for 1.5" bar stock, the chuck jaws are opened up too far to let the machine spin unless spacers are inserted under every working piece of the machine. I have a steady rest, but nothing to raise it to the appropriate height. I have spacers for most other pieces.

Thanks so much for the advice. I'm going to try the following things tuesday night:

1.) I have a copy of Home Machinst's Handbook. I'll read up to see if there are any technical subtleties I missed.

2.) I'll try a very slow speed.

3.) I'll regrind the lathe tool I seem to have damaged. I'll try to get to the grinder we have, but it's in another building and I may have to default to a rotary tool.

I'll post again if there is still no success, and try to take some pictures. That may help.

Thanks again!

[Mcgyver]

and no coolant! you are fine to handrill the centre drill hole - perfectly acceptable. it will seat itself on the centre. another way is to layout and centre punch the end (to give the centre drill something to start on, hold it in the three jaw, support the other end with the centre drill, and very slowing drill the centre hole, either way works.

good luck and let us know

[Ubarch]

I blew my nose this morning and it was all full of iron particles.

I wasn't even aware that I was making dust. Next time, mask.

[One of Many]

Cutting cast iron, you should be using carbide for all cutting tools and dry. It sounds like the partoff tool you were using was HSS? That edge may only last a few turns. Same with turning the OD of the bar. That brings in the need for a silicon carbide grinding wheel to keep your cutting edge in good shape. Don't use a standard Aluminum Oxide wheel.

As has been stated in fair warning. Never use a live center while parting off material. At least never go all the way through, but on cast iron it could snap before it gets there. The chances of the loose material jamming between the spinning part, cutter and live center can break things in a flash. Unless you are good at catching flying metal objects between your teeth! Heheh!

I'm sure your machine will cut it at about .02-.04 per pass. Given the proper tooling and setup. Just very slow within the envelope of the lathes capacity.

DC

[RotarySMP]

One question not yet raised is whether parting 1.5" cast iron bar is not sinply beyound the capacity of the sherline? Don't be afraid to use the hack saw and then face.

[ESjaavik]

Cutting cast iron, you should be using carbide for all cutting tools and dry. It sounds like the partoff tool you were using was HSS? That edge may only last a few turns.

Carbide and Sherline? I don't think the two go well together. Carbide is brittle and Sherline is not stable. No offense to Sherline, no machine in this price/size class is. If you must, at least go for the least brittle kind of carbide.

HSS have served me well for many years on small machines that does not easily hold a good cut. Cutting CI try to get "under the skin" in one cut (or as few as possible). Keep the speed down. After this first cut, check that your tool is still sharp.

But parting off a 1.5" CI piece on a Sherline? Yes, I might be mad enough do it just to prove what a correctly set up HSS tool can do. But I could probably cut off 5 pieces with a hack saw in the same amount of time. And I would feel sorry for the poor machine while doing it. For situations like this a hand crank can come in handy. Lower speed and more feel for the load. One case where this is the way to go is when making ribs in an air cooled cylinder. A hacksaw will be inconvenient for such a job.

[One of Many]

After this first cut, check that your tool is still sharp.

And the second cut and the third cut, and..........

Having an edge failure during a cut with inferior material/cutter compatibily may be no worse than risking a brittle cutter material. Even more so on light duty equipment. The emphesis should be put on the proper grind to keep the cutting pressures low enough to survive long enough to be less hassle and frustration to the user.

If Sherlines have the Aluminum dovetail bed, it may be best to avoid cast iron all together.

Granted, before carbide in the by gone era, all that was available was HSS and some amazing work was accomplished with very attentive operators knowing when not to push a rapidly dulled tool. I would think it highly unlikely the home shop ametures would ever cut enough material to gain that level of expertise.

My near 30 years of shop experience warrants carbide as a reasonable risk, but then again I use reasonable equipment.

DC

[Ubarch]

...So now I'm hearing contradicting advice. On one hand, Mcgyver says nice cast iron machines beautifully, and the Sherline book seems to imply that this is also true. On the other hand, One Of Many and ESjaavik say that it probably should not be done. Note that the task of parting is already complete; I finished it with a hacksaw sunday night. At this point I'm just trying to face.

My lathe tools are indeed high speed steel. I could order some carbide pieces, but it would delay my project even further. I would also need a carbide boring tool to do this correctly, which Sherline does not sell. To do it incorrectly, I suppose I could use a regular carbide tool as a boring tool. I am at an impasse.

[ESjaavik]

...So now I'm hearing contradicting advice. On one hand, Mcgyver says nice cast iron machines beautifully, and the Sherline book seems to imply that this is also true. On the other hand, One Of Many and ESjaavik say that it probably should not be done. Note that the task of parting is already complete; I finished it with a hacksaw sunday night. At this point I'm just trying to face.

Then I was misinterpreted. Sorry English is not my native tongue.
I agree that CI (without chill) machines beautifully. But parting 1.5" diameter of it is too much for such a tiny machine. Consider that you are putting on a cut with the full width of your parting tool! You would never put on that much cut when surfacing (or you would get the same problem).

Any machining of CI should be preceded and succeded by a thorough wipedown of your machine. The dust contains particles, carbides being the worst of them, which together with oil/grease makes a grinding slurry. They can also embed in aluminum if it is not anodized. A paper* towel covering the bed and slides can help keep it out. I don't let that stop me, but I always clean the machine thoroughly before, during and after to keep the grit out as much as I can. But my machines all have CI bed/slides, even the laptop lathe and mill.

(*Paper, because it easily tears if the chuck or workpiece grabs it.)

My lathe tools are indeed high speed steel. I could order some carbide pieces, but it would delay my project even further. I would also need a carbide boring tool to do this correctly, which Sherline does not sell.

Did you ever consider there may be a reason they don't? Stick to your HSS tools, learn how to hone+grind them and always keep them as sharp as possible. Am I sounding like an echo? Then it's because this is important.

[pauluk]

One quick point, don’t stand in line with a parting off tool, stand off to one side. On light machines they can easily dig in and break, and the bits of shrapnel fly a fair way.

[Mcgyver]

What, you expected everyone to agree? this is the dark art of metalworking. There are a multitude of factors and considerations at work whenever you machine something - differences in opinion are often just a person prioritizing these items to their paradigm. This where book learning has its place; learn the factors and how the interact, then you can prioritize to suit your own situation.

don't cut CI on a Sherline? I think where this is coming from is that on the outside of cast iron there can be a hard skin. ideally with CI you want the first cut to get underneath that skin. If you lathe is so light that you can't take a heavy enough cut to get under that skin in one pass, it will be frustrating as the edge of your tool won't last long. Compounding things is the fact that carbide is better suited to taking off this tough outer skin because of the relative hardness of material and tool, yet with very light cuts, on the irregular surface of a casting means an interrupted cut – this will quickly chip carbides.

I don’t get the aluminum bed part - maybe is because the out layer of cast iron can be abrasive – mostly because of sand that might be there, and AL scores easily? I wouldn’t worry too much about it, when you are roughing out cast iron you should have some paper towels (don’t use a rag!) down to keep the grit out of the ways, regardless of what the ways are made of.

I’d go at it with HSS – model engineers have been doing this in light lathes for years with HSS, for personal experience its just not a biggie. Yes, until you are through the outer layer, it will beat the crap out of the tool requiring a lot of touch ups, but who cares? It cheap to touch up a HSS bit in the grinder

Once the complication of the outer layer is removed, its just like any other machining job – you can’t induce a bigger cutting load that either the rigidity or power of you machine is able to handle. All this means is use depth of cut and feed rate that the Sherline can handle. I don't own a Sherline, but have cut ferrous in a Unimat, so that has to be worse from a rigidity/power view point. In fact, Unimat and a nice piece of 2” extruded CI are both sitting in the garage…..I wonder if its up for it

After you’re through the outside, it’s a piece of cake with HSS or carbide. I disagree with One of Many’s about cutting CI with HSS as a sort of lost art – some of the most brilliant metal work turned out is from the model engineering community and it is mostly done with HSS in light lathes. Then again, you will get a longer edge life with the carbide, so use a positive rake carbide if you want – no reason you can’t, but obviously you touch on carbide’s main advantage, high removal rates. Although, imo the lower cutting force of the more acute HSS geometry, despite more frequent sharpening, will better suite a light lathe.

Finally, the piece you bought is likely extruded and is grey or ductile cast iron. This stuff is butter, doesn’t have the abrasive properties in the out layer that a casting does and does not irrc have the same out skin chilled/hardness issues

Having said all that, I agree that a big honking piece of CI in an little tiny lathe is pushing the envelope, but machining isn’t about the machine ‘can’t’ to this, its about figuring out how to do it.

[One of Many]

...So now I'm hearing contradicting advice. On one hand, Mcgyver says nice cast iron machines beautifully, and the Sherline book seems to imply that this is also true. On the other hand, One Of Many and ESjaavik say that it probably should not be done. Note that the task of parting is already complete; I finished it with a hacksaw sunday night. At this point I'm just trying to face.

My lathe tools are indeed high speed steel. I could order some carbide pieces, but it would delay my project even further. I would also need a carbide boring tool to do this correctly, which Sherline does not sell. To do it incorrectly, I suppose I could use a regular carbide tool as a boring tool. I am at an impasse.

Don't give up. Try it both ways and see which gives you results you expect. Learning to adapt to the present conditions is part of the joy in a project well done. Soon it will become second nature to avoid the little pains or a major catastophy you can see looming before you get there. These guys are right. Nice cast iron machines nicely and some of it is a crud. It is also very abrasive. Unfortunatley in order to make progress you must do what you can with what you have.

The contradictions are based on each ones own experience. Look around at all the Sherlines out there and the extraordinary work that has been produced on them. I think that has more to do with care and comprehending their limitations than what is actually cut on them. Us lifer's don't have the patience to dittle around as much as pure motivation of the ameture. We can read the situation a lot better when it is at hand.

Parting off in the lathe has to be one of the riskiest operations to begin with. The weaker the machine the chances for a mishap go up. Keeping the cut as close to the chuck as reasonably possible helps. If the cutter grabs due to lack of rigidity in the rest of the machine, the work holding aparatus will not be strong enough to hold it either. That is just the nature of these small machines. The work takes a lot longer to accomplish. No matter what material it is, you need to gain the insight, feel and sound of the difference in proper cutting and a present or developing problem. The tough part is, it is hard to adapt without having any tricks up your sleeve to resolve problems or know where the root causes are. Be that feeds and speeds, material or cutters, not to mention weak setups that multiply the issues.

If your HSS tools continue to fail, either go to nicer cast iron or more compatible cutter material. Cleanup is critical after any abrasive is machined. Even flying grit from emery left unattended to will do long term damage.

DC

[Ubarch]

Success!

Slowing the RPM down to about 4Hz made a huge difference. I was able to face the piece, and then begin removing metal from the center. The lathe still chatters some, and the cuts are not smooth. Part of this is probably due to my lack of a grinder. I have to use a dremel tool to sharpen the lathe tools. Once I finished facing the piece, I could really only take off about 3 mil at a time. I have to reduce my 1.5" diameter bar to approximately 0.75", so this will take a long, long time.

I took some pictures this time, but i cannot post them now. Thanks to everyone that helped me.

[jderou]

Out of curiosity, cause I have a feeling I may be in this boat sometime, how deep is the hard outer layer. Can you grind it off as you would on case hardened parts?