Facing off with carbide tip tool causes hardening of workpiece?

I've just been facing off to length some 1/2 inch square bar (Columns for a Stuart James Coombes). Faced off and centre drilled a couple with an HSS knife tool and no problem at all. I needed to set up left and right hand carbide tip tools, so used the facing off to check the tool height.

Faced off a length off the bar and broke the centre drill. Decided I'd been careless. So faced off the other end and took more care with the centre drill. Snapped the tip off. (Using a No.2 size centre drill) . The tips are the typical triangular indexable types, been using them for years.

Reverted to the HSS knife tool, no problem at all.

My assumption is that the carbide tip tool has "twisted off" the material as it got it centre, rather than shearing it off, causing it to harden.

The bar was sold as EN1, but it might be EN3.

(PS I'm not using the round bar in the James Coombe castings kit, because I have a specific design in mind for the columns...which I thought would be good turning practice and hopefully will look really smart if I get it right)

Steve

Steve

You've not been near the drinks cabinet have you?

Unusually your description is very confusing.

You centre drilled with knife tool?, 'the No2 size bit was the usual triangular insert?

Are these carbide slocumb bits? but not what I would call triangular.

A better explanation would help me if possible

Ian P

I'd say you are a bit above ctr height with the carbide tools which leaves a conical bump on the end of the work which is pushing the Ctr drill to the side. Or not actually going all the way past centre so teh radius on te insert is also leaving the same shape bump

Let me try that again!!

I've just been facing off, to length, some 1/2 inch square bar (Columns for a Stuart James Coombes).

Faced off with an HSS knife tool, then centre drilled without a problem (Using a No.2 centre drill).

Next I set up a carbide tip tool, the indexable triangular type, adjusted the height, and then faced off a length of the square bar. When I centre drilled the bar I broke the centre drill. Decided I'd been careless. So I faced off the other end and took more care with the centre drill. Snapped the tip off.

Reverted to the HSS knife tool, no problem at all.

My assumption is that the carbide tip tool has "twisted off" the material as it got it centre, rather than shearing it off, causing it to harden.

I'll re-check the tool height, sounds best explanation, though the material didn't show any signs of a dimple and the drill didn't show any signs of wandering off. Pretty sure it's not key steel.

In terms of EN1/3 not hardening,  would that hold true if I literally twisted a length until it sheared into two pieces? I'd have expected the sheared ends to be harder than the bar in it's natural state.  So not hardening through machining, but by dint of being stretched beyond it's elasticity.

Hopefully, the Neurons fired in the right order this time, instead of some random sequence!

Steve

Edited By Steve Withnell on 23/03/2014 17:55:33

hi steve,

you are not alone!

ive centred axles for miniature locos in batches then usually on the last one the blessed tip of the centre/slocombe drill breaks off in the work. no logical reason why other than a bit 'gun ho' as the jobs progress.

this has happened twice in 30 years. i still cannot fathom why it should happen. i am a little more careful these days and less haste etc...

cheers,

julian

Steve,

It doesn't explain why you have this problen, but the solution should be simple:

"Catch" the centre, using a Watchmaker's Graver.

... It only takes a few seconds with a Lozenge-shaped graver [*]

... if you don't have a Hand-Turning Rest, just put a length of rod in the toolholder

[ Please remember to wear good eye-protection ]

MichaelG.

.

[*] Edit: also known as a Diamond graver; but be aware,

that's the shape of the tip, not the material. 

Edited By Michael Gilligan on 24/03/2014 07:05:38

Could also be a bit of dodgy steel, esp. if EN3b. I ve had this a couple of times lately, maybe a bit of scrap ball-bearing got in the mix.

Weird things can happen around the centre area because you have almost infinite torque, zero speed and rapid changes of direction (360degrees) in a tiny workzone

HSS is more flexible and forgiving but carbide breaks

I drill a depression/hole in the centre before facing off, if the job allows it

Edited By Ady1 on 24/03/2014 11:21:19

My favourite centring tool is one of those carbide tipped dead centres, I ground half the tip away so it can now be pushed in and pops a centre hole instantly in the workpiece like a d-bit drill

Very few of the above replies address the difference between facing using an HSS tool and a carbide tipped tool. I would agree, Steve that a possible culprit is work hardenning of the surface due to the fact that the carbide tips are not as sharp as a well honed HSS tool.

Michael, while EN1/3 cannot be hardened by heat treatment I think it can be work hardenned, or am I talking out of the top of my head?

Russell.

hi ya, try it the other way round centre drill first then face off, unless your piece of material is sawn/cut at a really bad angle this is quite a safe way of keeping your slocam drills intacked.in my personal experience ive never known eni/3 to work harden,but anything is possible ian c

I feel that you are making too fine a cut, and the tool is rubbing on the work, this gets worse as you near the centre of the bar. With a carbide tool you need to take a good size cut, and feed it in fairly hard. Me, I'd stick to HSS.

Ian S C