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micro endmills

Posted by joeaverage 
micro endmills
June 01, 2016 12:21AM
Hi,
have been milling simple boards a while now with good results. I prefer using small diameter endmills,
0.8mm for thru hole boards and 0.4 to 0.6mm for SMT boards where the smallest feature is SOIC at 1.27mm
pitch.
The cut from an endmill is typically better than any engraving bit I have tried, the downside is that they are
expensive and fragile. Additionally they only really offer any cut quality advantage when sharp. The fibergass
dulls them pretty quick and then they load up and shortly thereafter break.
I have been looking (longingly) at diamond coated endmills offered by a couple of manufacturers.
Amorphous diamond coated around $25US and CVD coated around $75US. Has anyone tried either
or both?
The manufactuer (Harvey Tools) suggests that the thicker CVD coating while lasting a lot longer can cause
problems in plastics because of edge rounding of the coating interfering with chip release. Any comments?
My project has high current requirements and so I'm using 12oz copper board, that is 0.42mm thick copper! Endmills
are really the only way to achieve a usable board as engraving bits with angled cut reduce .4mm tracks (gate drive/SMT control)
to near zero at that depth.

Craig
Re: micro endmills
June 13, 2016 04:47AM
Hi All,
have been experimenting over the weekend with the 0.4mm endmills I have on
hand made by Kyocera Tycom. I anticipated a real battle with the heavy copper
board I'm using. The results are suprisingly good.

At first I tried cutting in two cuts, one at 0.2mm and the final cut at full
depth of 0.5mm. Worked not too bad, the first cut showed some 'stringy
copper' but the second cut cleaned it up.

I then tried cutting in one pass ie the fulldepth of 0.5mm. To my suprise
it worked well. This is a pretty challenging slotting toolpath with a depth
of 125% relative to diameter. Anyone who has done any milling will tell you
that this is a bad idea, but it worked well.

I then tried flood cooling, my mill as small as it is was designed for metals
and is fitted with coolant pump etc. The results were even more impressive,
clean cut edges, no swarf build up in the flutes and seeming better tool life.

Eventually I broke the tool after about an hour which is pretty good for these
little endmills even in normal weight board. With the coolant flowing I could
not see the break ocurr or the cut prior to breakage. Looking under magnification
the cut appeared good so I'm guessing the cutter had not dulled overmuch,
the usual problem but rather looked like it 'twisted' off. I was feeding faster
with a 75% stepover at the time and may just have overloaded the tool.

I've come to realise that routing this heavy copper board is actually a
metal cutting operation where all the normal metal milling strategies
like coolant, swarf evacuation, speed and feed and cutter engagement are
important. The only real difference to a straight metal milling job is the
abrasive nature of the fiberglass in the chip stream.

These wee endmills I buy off Ebay for $4.50US, damn good value really.
An amorphous diamond coated endmill is $28.00US and a CVD coated endmill
about $80.00US. A diamond coated mill is going to have to last an awfully
long time before it stacks up dollarwise.

The feedrate I was using was 75mm/min slotting, 30mm/min plunge. I was feeding
at 125mm/min when the tool broke and guess I was asking a bit much. Using the
forumla 1% diameter per tooth per revolution with two flute cutter at 24000rpm
works out to 192mm/min. This formula is quoted with cutter engagements of 15-20%,
I'm being much more agressive with engagements of 80-125%. Given my agressive
cutting strategy maybe I have to be a bit patient!

Craig
Re: micro endmills
August 09, 2016 05:49AM
Hi All,
I have been experimenting some more and have learned some useful things.

I have run out of 0.4mm endmills but bought 30 0.5mm two flute Kyocera Tycom endmills
from drillman1 on eBay for $2.65US each, a bargain.

I have also started using Autoleveller software and the contribution to my success is remarkable.
What I had thought was flat was not, even when I had milled a pocket for the board.

I made two boards over the weekend, a small one with normal 1oz copper and another 150x150
one in the heavy 12oz copper board.

With Autoleveller I was able to cut just 0.05mm (50 micron) and achieve perfect isolation over
the entire board. Given that the copper is 35micron thick to get perfect isolation while taking such
a light cut is superb. This board has a Thin Shrink Outline chip with a pitch of 0.65mm and a spacing
between the pads of 0.3mm. I used a 30 degree engraving bit and very happy with the result. I have
always struggled with the little chips but am way more confident now.

The second board of course is a much different proposition with 0.42mm (420micron) of copper. I used
the 0.5mm endmill, the first pass at Z of -0.3mm and then a second pass at Z -0.5mm. Cutting speed was
200mm/min and plunge of 75mm/min with coolant. No trouble. I then took another ten passes at 0.1mm
stepover at 400mm/min and 150mm/min plunge again under coolant. With Autoleveller was able to achieve
4 hours cutting with just two endmills, great.

I have come to realise the difficulties I've had in getting useful life out of microendmills has been inconsistent
Z depth. Cutting any more fiberglass than you have to is going to seriously hurt toollife. Autleveller has
helped enormously.

I did buy a CVD diamond 0.5mm endmill from Harvey Tools for $80US plus shipping to New Zealand but
I haven't had to use it yet, to be honest I'm scared of breaking it! Under magnification the diamond surface
is quite rough, it looks like sugar crystals on a bun. I suspect and was warned that soft metals like copper
could lodge in the flutes and I could easily break it. When I buy some more board I might get some 1/2oz
board and try it on that first.

Craig
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