[Hamzter]

Hi All,

I'm currently making some parts for a friend, which involves cutting a small tapered channel in a piece of mild steel (EN1A/230M07), this tapers from 2.0mm to 0.8mm, I'm using Ø0.6mm 2 flute ballnose carbide AlTiN cutters, but I'm having trouble with breaking them midway through the program. Here's the settings I'm using:

Machine - CNC SX3
DOC - 0.1mm (0.004")
Feed - 32mmpm (1.26ipm)
Feed per tooth - 0.0037mm (0.00014"/tooth)
Speed - 3600rpm (spindle max)
No coolant

The code takes approximately 40 minutes, first part I made worked great, but the second part the cutters only lasted 8-10 minutes each, and I ran out of cutters before I finished the part. Does anyone have any idea why that might have happened? The cutters all broke midway through a straight path, which suggests to me they're going blunt rather than overloading. Cutters were held in an R8 collet with runout ~0.02mm (0.008") at the tip, I've ordered a suitable ER collet to try and improve that. Backlash on the machine is about 0.07mm (0.003"), though I've tuned the backlash compensation to virtually eliminate that.

Thanks for any suggestions.

[AARONT]

It all looks pretty reasonable. Do you have air blast to get the chips out so you're not recutting the chips? I bet your main issue is the run out. .008" is a lot for run out on a half inch end mill much less a .6mm end mill. Think of it this way your run out is 33% of your tool diameter. Thats why your tool is breaking.

[Kiwi]

0.02mm = 0.0008"

[victorofga]

try to roughing first with a tapered endmill.. with a flat end on

bitsbits net has engraving bits with different side angle and different tipsizes..
on ebay also a lot..

you also can grind a spotdrill if you don't have time to get the tapered tool..

that backlash too much for this tiny tool..
with other words 0.07 backlash for a 0.6 mm tool same like 1.2 mm backlash for a 10 mm tool...

that can break tools easily...

[AARONT]

Good catch kiwi. I thought it sounded a little crazy.

[Hamzter]

Haha, thanks for the correction, must proof read more

I'd be a little surprised if the backlash was the cause, there's so much friction in the ways the table won't be rattling around, especially with such tiny cutting forces. I'd planned out the tool paths so there's a small non cutting movement on each pass that will ensure that the 'kick' the backlash compensation gives isn't during cutting, which could definitely break the cutter. However that's got me thinking, maybe the stiction in the ways is causing it to surge, either fatiguing or simply overloading the cutter? Maybe a bit of oil is due on the ways just before cutting...

I'm having more luck with roughing out with larger cutters though, I'm now roughing out with a 1.0mm, and finishing off with a 0.8mm cutter, they seem to be a whole lot tougher, I've successfully made 2 more parts without breaking a tool, so I'm happy with that For the reference of others, here's the final feeds and speeds I'm using:

Cutter - 0.8mm carbide AlTiN ballnose 2 flute
DOC - 0.1mm
Speed - 3600rpm
Feed - 46mmpm
Feed per tooth - 0.005mm
No coolant

Cutter - 1.0mm carbide AlTiN ballnose 2 flute
DOC - 0.2mm
Speed - 3600rpm
Feed - 41.6mmpm
Feed per tooth - 0.005mm
No coolant

Thanks for the help!

[RCaffin]

Your runout is a bit high for such a fine cutter. Try resetting the cutter and collet to minimise it.

Try misting with a light oil occasionally, to keep the cutter from clogging up. This is NOT cooling: this is stopping the stuff from sticking. A very good way to bust a cutter.

Cheers