Cracking at thread flanks?

[SmokeWalker]

I’m getting this mysterious cracking on my threads in C37 alloy steel bolts I was single point threading on my manual lathe. Any ideas why? Tools were HSS and cutting fluid was used. Changing the sfm didn’t make much of a difference.

 

[Conrad Hoffman]

I'm guessing you were plunging straight in. Probably need to come in at 29 degrees, make the tool sharper and maybe more relief angle. It's very easy to roll the edge of a tool while grinding it, giving way less angle than you thought you had, which makes it rub. What's the cutting fluid? I'd want a decent oil, probably animal or veggie based. My go-to on the manual lathe is Monroe Cool Tool II or Crown Buttercutt.

 

[ChipSplitter]

Can you post more pics? It could be a wide range of things such as:

tool off center
compound not set at 29.5 degrees
wrong tool or dull tool
lack of rigidity in tool or workpiece
spindle running in reverse.....

 

[DDoug]

Cracking or tearing ?

 

[SmokeWalker]

I don't know. What you see in the pics is all I can see.

 

[SmokeWalker]

I was plunging. I can do all those things. I was using Tap Magic Xtra Thick. Lard oil is good for turning, but this was good too.

 

[Mark Rand]

You say changing the SFM didn't make much of a difference. What was the SFM?

Also, what is your infeed?

 

[SmokeWalker]

You say changing the SFM didn't make much of a difference. What was the SFM?

Also, what is your infeed?

.005 on diameter per pass. I was about 40 SFM.

 

[jim rozen]

At 0.005 plunge per pass, that's a lot at the end of the threding cycle. You had a lot of tool engaged in the work.

Suggest you change over to 29 degree compound, feed with the compound, and the last few cuts should be 0.001 or less.

 

[ChipSplitter]

40 SFM with HSS isn't helping you any either.
If you can justify it, get a carbide laydown or topnotch setup and run more like 200 SFM.

 

[Bobw]

What the heck is C37. I figured it was some low carbon steel, and you were from Europe.

Nope. Some Magnesium-ish thing?

Either way. to me at least, looks like you are tearing. Kind of like a mild crap steel,
or even an annealed alloy steel. Tools a little dull, speed a little low, and it all
just looks like shit.

I don't know the alloy, but either speed or sharpness, or both should give you a better
looking thread. Rigidity could also play a part, she looks like she's kind of long.

 

[ChipSplitter]

I hazard a guess that it's a Grade 5 bolt which is 1045ish. Threads sure look like it.

Seems some days it cuts nice and chips break up, then the next day you take the paint off the lathe doors from the chips wrapping around the chuck.

 

[MattiJ]

What the heck is C37. I figured it was some low carbon steel, and you were from Europe.

Nope. Some Magnesium-ish thing?

Either way. to me at least, looks like you are tearing. Kind of like a mild crap steel,
or even an annealed alloy steel. Tools a little dull, speed a little low, and it all
just looks like shit.

I don't know the alloy, but either speed or sharpness, or both should give you a better
looking thread. Rigidity could also play a part, she looks like she's kind of long.

C37 as Rockwell hardness C37? That would be something like grade 8 bolt.

100% agree with others, what OP is having is usually called tearing.
Two ways around it: LOT more speed, heavier infeed or slower and smaller infeed.
For something like 1018 the ”lot more speed” approach is usually only valid for CNC.
Threading at 900 SFM calls for good reflexes if you do it on manual lathe

 

[AD Design]

Agree with others that it looks like tearing instead of cutting. How are your relief angles? Any sign of tool rubbing under magnification? Tool height at or just below center of axis? HSS is my personal choice over carbide in a manual lathe. I'll stone the top and sides to get a keen edge if the material shows tearing. What others have suggested about cutting fluid, less depth of cut, and increased speed (within safe limits to disengage half nuts) is good advice.

 

[michiganbuck]

Come in at 29-30 so only shaving one side if the thread, as mentioned by Conrad .
I like a bit or insert with some positive top, back or side cutting edge rake angle, any one realy helps.
Amount of part hanging out...short or with the tail best.
and that the tool holder more centered on compound and cross slide. I see many YouTube with the bit/insert way far to the left so to make the holding of bit/insert more tippy to the down force of cutting...
Sharpness of bit or insert..sharp enough to shave your finger nail..
Guess we should add how the chuck jaws are holding.. set deeper in the chuck might ofset any taper jaw effect...

 

[DDoug]

What lathe ?
Maybe post a few pix of the overall set-up

 

[Mtndew]

RPM is too slow and it's tearing.

 

[CarbideBob]

Cracking or tearing ?

Does not look like either to me. I see material reweld.
Tearing itself is quite rare in metal cutting. This condition much more common.
I think most call this tearing so perhaps I'm just fussy.
The material that looks like poop was not there before the cutting tool came along.
It came from a teensy tiny bit earlier around the part. Spacing of the lines and cut depth per pass plus tool geometry tell you the where.
More top rake, more clear, more speed, more lube, coated tool, sharper edge. Pick your poison or better a few of them.
I guess you just call all this "cleaner cutting".
Bob

 

[James H Clark]

Get a good carbide insert and run it a lot faster than you want to.

JH

 

[DDoug]

What lathe ?
Maybe post a few pix of the overall set-up

Nothing yet ?

A few pix would go a long way.