Metal Crumbles

[mmatisoff]

I sharpened the tools for my Taig lathe with the help of an experienced machinist. One question I have is should I be getting shaving from the metal I turn or is it okay if the metal crumbles? It does the same thing on brass and steel even after adjusting the speed for each metal.

 

[mcostello]

Pictures of the crumbles would help a lot.

 

[tornitore45]

Picture of the tools would be helpful as well.

The only metals you could say they crumble is cast iron, bronze and some brass.
Other metal should give a long curls or "commas" shaped chips.
Tool shape, rake angle, chip breaker, lubrication, feed and speed are all factors in shaping the chip.

 

[OrangeAlpine]

Sometimes a chip breaker will produce tightly curved, broken chips. Is this what you have? If so, that is good.

Bill

 

[wildun]

Should I be getting shaving from the metal I turn or is it okay if the metal crumbles? It does the same thing on brass and steel even after adjusting the speed for each metal.

What is the finished work look like?
First things to check out are tool height (important) and the clearence under the tool cutting edge,- it could be caused by rubbing, often caused by the tool being too high.
Also check the rigidity of the toolholder and work.

 

[mmatisoff]

I'm just learning to turn metal so far. The turned piece is somewhat coarse, not smooth. I've attached two pics: one showing tool next to the work piece, and the other showing the "crumbles." Any help would be most welcome.

 

[jwcnc1911]

So many factors can affect chip shape. Is this manual or one of the little CNC machines? You probably won't have a consistent enough feed to produce a ribbon chip on that machine. If the surface finish is good run it. Tool geometry does affect chip shape but in this case ID say it's more feed related... If the machine can take it and you can give it feed it faster. In my experience 9 times out of 10 the solution is counterintuitive. Most people want to slow feed rate down when it actually needs feeding up. If you're getting chatter feed it up. At work, we say "crank the pot!"

 

[///]

I've had similar results from a length of 'mystery metal', and the resulting surface finish was very poor. That stock has been moved to the scrap bin.

Have you tried the tool on some Aluminium?

 

[Anko]

You must check the tool geometry, tool height, speed of the spindle, feed, and also the deep of the cut. Also the type of metal will affect.

I will try a high RPM and a very small cut, adjust the feed until you feel that the tool is cutting good.

with silver steel (drill rod unhardened), aluminion, brass and mild steel you should have no problem with this little test, but with chrome-nikels aloys I will not be so shure.

hope to help

Saludos

 

[Woodster]

Looking at your pics, i would say that your tool geometry is way off and tool nose radius is far to big. Tool nose radius should be about .1mm to .4mm max (.004" to .016") For a good general purpose turning tool, go for 5deg top, front and side rake/relief. Minnimum depth of cut should be the tool nose radius and max feed per rev should not exceed tool nose radius (For finishing cuts). Rough cutting tool should have .8mm max tool radius.

 

[Tin Falcon]

http://www.wpi.edu/academics/me/CAMLab/research%20areas/chipctrl/intro.htm#model
the above link should help some.


do you know your speed and feed.
Tin

 

[Hopper]

+1 on try less of a radius on the tip of the tool. I find that for small diameter in a small lathe, a very sharp tool with a point that maybe is radiused with a few rubs on the oilstone works best.

The best book I have seen for hard tack tips on grinding tools for smaller lathes is "The Amateur's Lathe" by LH Sparey. As he points out, the rules of tool grinding for bigger more solid industrial lathes don't always apply to small modeling lathes.

He calls for more side and top rake than usual but with no back rake. A total 'angle of keeness' of 55 degrees between the front and top surfaces. There are many other ways of doing it but after years of using bigger lathes in toolrooms and machine shops, I found Sparey's ideas perfect on my small lathe at home.

 

[Wizard69]

+1 on try less of a radius on the tip of the tool. I find that for small diameter in a small lathe, a very sharp tool with a point that maybe is radiused with a few rubs on the oilstone works best.

Actually I've found that lots of honking on an oils stone can do wonders for cut quality. Of course you have to have the right geometry for the material at hand. Part of this may also be due to my lack of a true tool grinder with a suitably fine wheel. The point is some material respond real well to sharp tools.

The best book I have seen for hard tack tips on grinding tools for smaller lathes is "The Amateur's Lathe" by LH Sparey. As he points out, the rules of tool grinding for bigger more solid industrial lathes don't always apply to small modeling lathes.

He calls for more side and top rake than usual but with no back rake. A total 'angle of keeness' of 55 degrees between the front and top surfaces. There are many other ways of doing it but after years of using bigger lathes in toolrooms and machine shops, I found Sparey's ideas perfect on my small lathe at home.

The interesting thing here is I have a number of references to lathe operations and they all seem to promote their own cutter designs as gospel. I don't have Sperry's book but I do understand what you are saying, even with my 9x20 I have to take care with how I feed and sharpen the tool. There is a huge difference in what works on a 300 pound machine as opposed to a one ton machine.

 

[jwcnc1911]

I don't think geometry is the problem. Corner radius is not the culprit here. I see tools with corner radii up to 40mm run all day every day. It's feed related. I see round inserts in 24mm dia turning daily - in 5160. It's material or feed/speed.

Crank it harder and tell us the results.

 

[wildun]

That steel you have cut appears to me to be some sort of free cutting grade which will probably crumble anyway.

Also, if you used the tool (as at the attitude shown in the photo), the angle or attitude to the work which the tool is set at is a little wrong, as it is pointing into the work, perhaps making it dig in.
Whereas it should really be going in at right angles or perhaps trailing slightly the other way, - alternatively it can have the angle ground into the front cutting edgel.

I feel that big radii on cutting tools may work well on big rigid expensive lathes, but not on our little home workshop jobs however!

 

[jwcnc1911]

I hadn't considered the size of the machine... It's most definitely feed related. Rigidity is definitely a factor. Geometry is definitely a factor. Speed is definitely a factor. No one element is the smoking gun. The more corner radius the greater the cutting force so I would agree on a smaller machine that corner radius needs to be smaller. I've never used anything that small.

The material also looks like hot rolled... if it is feed the hell out of it and see what happens. I guess my main point is experiment and see what you get and what works best for you. You'll find what you like but only after you try it. Try new tool geometry focusing on clearance and relief. I would only worry about radius size if I had to have a sharp shoulder.

And ditto the "every one preaches their tool geometry as gospel," I certainly do but there is always more than one way to skin a possum, the only thing that matters is you cook him right after you skin him.

I'm going back to '88 but like the Nike slogan, Just do it.

 

[Hopper]

And ditto the "every one preaches their tool geometry as gospel," I certainly do but there is always more than one way to skin a possum, the only thing that matters is you cook him right after you skin him..

Thm:




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[mmatisoff]

I'm getting closer to getting the cutting angle correct on my mini lathe tools. One more question. When I move the newly sharpened tool backward, I get nice long curls of brass. But when I go forward, I get crumbles, or the tool doesn't move at all. Again, thanks for assistance..

 

[wildun]

Remember, the feed rate, lead angle,rake and depth of cut especially are all very different when rewinding (guess thats what you are meaning). Most brass will crumble if you are turning using zero rake or negative rake and normally zero rake is best for brass and cast iron - at least that's what I was taught and have found over the years to be correct. Having said that, there are a lot of different types of brass with different cutting qualities.

Main thing is, concentrate on the finish how good is your finish? - the shavings are scrap after all!