17 years ago
milling carbide inserts I use in my valve seat cutting system. I'd been
looking at 6" diamond wheels for about £70 to £100 for a while but the
Chronos 4" £40 made in China or somewhere similar one was tempting although
theoretically I wasn't able to run it fast enough on my mill at 1500 rpm. To
get the supposed ideal SFM meant 2500 rpm or more.
Took a drive to Dunstable to visit them and found a mildly cosmetically
damaged one on the shelf for £25 rather than a perfect one for £40. All it
amounted to was a small inclusion in the aluminium casting. I figured for
£25 it wouldn't be the end of the world even if it was utter shite like most
far eastern stuff is.
Made an arbour on the lathe to hold it with and got it mounted up on the
mill today in a 3/4" collet. Made a little fixture to hold the carbides with
and gave it a go. Well it's exceeded my expectations by a long way. It rips
through carbide bits at 1500 rpm like no tommorow and still puts a mirror
finish and a razor sharp edge on them. Only 1/2 thou cuts are recommended by
most diamond wheel manufacturers but I bumped that up to 1 thou and then
finally 2 thou and it just chews through it with no discernible stress or
cutting forces. Also surprising is the almost total absence of heat build
up. I can hack a couple of mm off a carbide in a few minutes in 2 thou cuts
and the carbide is still stone cold to the touch. Try that against a green
grit wheel and the carbide will be smoking in seconds.
I tried off hand grinding a couple of bits against the wheel and found it
infinitely easier than against a green grit wheel. The cut is so smooth it's
very easy to make a nice radius on a cutting edge and the finish is perfect.
In total today in a 5 hour session I sharpened about 40 cutting edges of
3/8" long square cutters taking maybe 10 thou off each one plus making a few
special cutters one of which involved taking 2mm off the depth. That amounts
to about half an inch depth by 3/8" length of carbide removed and the wheel
is unmarked. That's the equivalent of grinding an entire carbide square bit
to dust. If anything it improved as it bedded in more accurately to the
orientation of the arbour. On that basis this wheel will last me a lifetime
and still be unworn.
I'd been agonising for a few weeks about the ideal diamond grit to get.
Would 150 grit (the average you see advertised) remove the material ok but
leave a rough finish? Would 300 grit (which costs even more) give me a sharp
cutting edge but not remove any stock? Would a 6" wheel give me the right
SFM but a 4" wheel go too slow at 1500 rpm. Would a quality English or
American wheel be ok but a cheapo one made in itchifanni be shite and wear
out in no time? In the end I've been worrying about nothing. The Chronos
wheel is ultra fine to the touch compared to other wheels I've looked at but
still rips off stock faster at 1500 rpm than I could ever need it to. I have
no idea what grit it is because it doesn't say but it suits my needs
perfectly. I'd guess from the feel of the grit it's 300 or so but I could be
wrong. I tried it on steel at the end of the day and it rips through that
just as well. I know you aren't meant to use diamond wheels on steel but
sometimes you have to if you're shaping a brazed tip. Well it goes through
steel like butter. In fact it goes through carbide like butter.
It would be a bargain at twice the price. If you have a workshop that uses
carbide bits you need one of these. Go and buy one.
Dave Baker - Puma Race Engines