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Long curly ragged-edged blue chips

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bfk

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#1
No, not the latest food fad.

Most of the machining I've done has been on aluminium. Last night, armed with my new MikeyModel™ HSS bit, I tossed some 5/8" steel rod (Home Depot's finest) on my Sherline lathe and had at it. With a bit of experimentation I was able to take off a consistent .015 and get a decent finish. It was definitely noisier than working in Al.

The chips coming off were of the long loosely curled type. One edge was ragged and the colour was a lovely blue. I know the blue means that the chip is taking the heat away, which is good, but I wonder if it means there's too much heat being generated. I also don't care for the rotating razor sharp rat's nest. Not sure what the ragged edge tells me; well it doesn't tell me much, but maybe it does for someone more experienced.

I'm going to try some more experiments if I can get some time this evening, but I thought to shorten the experimentation time by asking those more knowledgeable than myself.
What should I try first?
 

4gsr

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#2
Welcome to Chinese steel...I'm sorry, I didn't mean that. It's more than likely from one of the mini-mills here in the states.

The steel you get from HD and other box stores is probably the lowest grade out there being a 1018/26 grade or Chinese grade C15/C20. Could even be A-36,, too. It does not possess any elements to improve machinability. And yeah, that's the kind of stringy chip it makes. Even on a bigger lathe making bigger chips, it still stringy and a mess to deal with.
 

NEL957

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#3
Long stringy chips are cause from wrong angles on tool and the best way to stop it is with a chip breaker. Sometimes there is a slight grove cut just behind the cutting edge and the chip will be drive into the trough and naturally start to curl. I would start with the angles because angles make all the difference. This PDF will give the basic angles needed to get good finish and proper chips.
Good Luck and keep at it, it will come and when it does it will be a joy.
Nelson
 

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mikey

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#5
I'm with Ken - buy some decent steel to experiment with. The stuff they sell in my local hardware store is 1018, the worst stuff to cut with any tool.

Blue chips mean you are taking a pretty decent cut on the lathe and most of the heat is going out with the chip, which is what you want if you are trying for maximum metal removal rates. On the other hand, you can also take a lighter cut with the same feed rate and produce amber chips, which is what I look for when cutting steel.

The stringy chips are a function of depth of cut, feed rate and material properties. On a Sherline lathe, you have several options. One, take a shallower depth of cut and increase feed rate and that will hopefully produce chips instead of stringers. Two, try using cutting fluid as benmychree suggested; this will change the form of the chip a lot and I suspect you will get tighter coils with it. Three, use a different steel. Four, try a lower lead angle (tip turned more toward the chuck); this amounts to reducing depth of cut and increasing feed just by virtue of altering the lead angle. I'm sure there are other options that I haven't thought of yet but try these and see what happens.
 

bfk

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#6
Thanks, folks.
I lowered the lead angle and slightly lowered the depth of cut. I had been using cutting fluid, but tonight I was much more liberal with it. Combining those with a feed rate that was about as fast as I can crank it and a slightly lower rpm and lo and behold, no stringers. The straw/amber chip colour isn't as pretty as the blue, but I can live with that. Maybe I'll take a drive over to the metal supplier tomorrow and get some steel that isn't 20% bird droppings and see if that gives a better finish.

This forum is really great!
 

mikey

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#7
Yeah, lead in the mix will work better for machinability than bird dodo. :)
 
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