Bought one of the above a couple of weeks ago for sharpening the square 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 s**te 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 s**te 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