Chipped corners on milling cutters

[RJKflyerParticipant @rjkflyer]

[RJKflyerParticipant @rjkflyer]

I’m newish to milling, and have been as careful as I can to follow all the recommended speeds (both rpm and feed) and cutting depths.

  

I’ve noticed that 2-3 of my 15-20 endmills have a very slight chip off of one of the cutting edges. Probably wouldn’t have spotted it until i looked through the magnifier.

 

Obviously, I’m wondering why, and expecting the obvious answers re speed/feed, but I’ve been careful – see above. Any other thoughts re technique? Might it be that on occasion i’ve not been careful enough bringing the cutter into contact with the workpiece?

 

Since the cutters appears to still work fine and offer a good finish, I’m minded not to do anything about them.

 

I should add that these are not expensive endmills – they’re carbide, 50 quid for 4,5,6,8,10mm set.

 

Thanks all! 

[JasonBModerator @jasonb]

Carbide is very fragile and easily chipped if you leave them laying on the bench or don’t handle with care.

 

At that price they are probaly a cheap imported carbide so could have been chipped before you got them, you wouldn’t get much change from £50 for just one 10mm Dormer carbide endmill!!

 

Best to keep the carbide for when you specifically need it and just use HSS for every day

 

Jason

Edited By JasonB on 23/08/2010 19:48:23

[RJKflyerParticipant @rjkflyer]

Hmmm, they’re all boxed and looked after, but I see your point, no pun intended.

 

Agree re price – I’m buying Hertel coated ones as i need the larger sizes although one of the ones with a (very small) chip is in fact a Hertel TiAlN coated.

 

However, now you mention it,  it’s also a solid carbide. Funnily enough not had any problem with long HSS ones which ought to be more vulnerable to chatter etc.

 

So, maybe i should steer clear of carbide as you say unless really needed…

[Anonymous]

Just as important as feed and rpm is the tooth load. Nornally one would start with an estimate of the tooth load, times that by the number of flutes and, via the recommended surface speed and hence rpm, you can calcualate the feed rate. As a conservative estimate for tooth load assume 1% of the cutter diameter. So, for a 10mm diameter cutter that would be 0.1mm, or 4thou per tooth per revolution. A sure way to damage cutter edges is to allow them to rub rather than cut.

 

Another way to damage edges, particularly for carbide, is allowing the cutter to recut the swarf, sounds horrible too.

 

As JasonB says carbide is unforgiving because while it is very stiff, that stiffness comes at a price, which is brittleness. For instance if you lower a cutter gently on to the workpiece to set a reference, but over do it by a couple of tenths then HSS will just dent the workpiece slightly. Carbide will chip instantly, even if the workpiece is relatively soft, like aluminium alloy. I learnt that the hard way (no pun intended!) using plain carbide cutters on a CNC mill. Overshoot the zero by a tiddly amount and the corners of the mill are chipped. If you rotate the cutter by hand at the same time as jogging down until you see a witness mark the sharp corners are fine.

 

Might any of this be a possibility to explain the chipping?

 

Regards,

 

Andrew

 

[John OlsenParticipant @johnolsen79199]

You may be able to improve the chipped cutters a little if you can back off the corner behind the chip, effectively putting a radius on the bottom corner of the cutter. You would need a diamond hone for that. I have done this to HSS cutters that have lost their corners. It means that any slot or step you cut with them ends up with a small radius in the corner, but this does not usually matter, and should actually be better from a stress raiser point of view.

 

regards

John

[RJKflyerParticipant @rjkflyer]

Yes, these are very helpful comments chaps, thanks. 

 

I do think actually, on reflection, I’ve done the damage with UNDERcutting/buffing (accidentally) just as Andrew suggests.Very good point, and a learning for me.

 

I can see the value of carbide, but to the right application: for example I needed to drill holes in the hardened stainless ‘rules’ of digital scales which I’ve added to the mill – even a Co drill wouldn’t touch it, but the carbide slotmill just ‘drilled’ through with ease…

[KWILParticipant @kwil]

Andrew mentions recutting swarf, if you have seen the industrial use of carbide tooling, flood coolant usually means a torrent to get the swarf away!

[Sub MandrelParticipant @submandrel]

I have read advice that suggests putting the tip relief John mentions on new endmills. I have used this tip to restore endmills. I find I always blunt the corners long before the edges.

 

Neil

Edited By Stub Mandrel on 25/08/2010 21:39:13

[John OlsenParticipant @johnolsen79199]

You may have noticed that a lathe tool will tend to be unhappy when it is cutting into a corner. They don’t like cutting on two sides. The corner of a milling cutter is in that situation most of the time when it is actually cutting.

 

 On the drilling holes into scales, I  hope you are not drilling through the area where the capacitive sensors live? You don’t want to short that out. This is the plastic strip down the middle of the scales. Mine came with some little pressed steel clamps that hold the end so no holes were needed.

 

regards

John 

[RJKflyerParticipant @rjkflyer]

Did indeed drill through the sensor layer, but at the extremities this has had no effect on the scale, as confirned by the manufacturer.

Still fancy fitting a decent DRO but that’s another thread for another time! For now the Chinese scales will do… Not bad for 125 quid, hmmm, but plus all my time to mill up the mountings.

[KWILParticipant @kwil]

RJK.  If you fancy a decent DRO, look at Newall Microsyn, very small and compact.

[RJKflyerParticipant @rjkflyer]

Absolutely, it’s the one I have spec’d for the moment I decide to go DRO.

[Anonymous]

A Newell Microsyn 2-axis DRO for my vertical mill is the best thing I ever bought for the workshop. Spent some of my redundancy money on it, and it has more than paid for itself, enabling me to win jobs that I wouldn’t have been competitive on without it.

 

Only had one wobbly with it in ten years. It got itself into an odd state that made no sense to me. A friendly tech support guy at Newell talked me through some hidden codes and setup functions that are not in the manual to fix the problem. In essence the unit had reconfigured itself as a lathe version. He mentioned that this could happen if there was a power glitch at power up. Oddly enough I HAD just noticed an electrical funny on a power up sequence just beforehand.

 

Best Regards,

 

Andrew

 

[KWILParticipant @kwil]

I have to confess all my DROs are Newall Microsyn, only the Bridgeport has Newall Spherosyn!

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