Eagle Suirface Grinder

Standard wheel size is 7" x1/2" x1.25" bore (180 x 13 x 31.75mm), however if you want to use a thinner wheel, you will need to make a new hub clamp disc, as on the Eagles these tend to be specific for the width of the wheel - important to check. Check the wheel rotation too, (should be anti-clockwise IIRC) as the hub clamp is held on with a normal right hand threaded nut.

As for wheel grit types, there is as much debate here as the HSS vs Carbide debate. In general, a harder steel needs a softer wheel, and vice versa. A 46 grit size is a good general standard to start with, in a 38A or WA (white alumdum) material in perhaps an H hardness.

Lots of good information direct from Norton in the following link:

be careful if you're new to grinding though, as IMO a grinder can be one of the msot dangerous machines in the workshop. Small light cuts (0.003" is a large cut), don't feed too fast (or too slow), and about 0.010"- 0.015" crossfeed will give a nice finish. And always find the high-spots first before you index a grind across the whole part.

IIRC the handwheel for feeding the knee is indexed in half thou' divisions.

Peter

Cheers for that Peter, I knew interwebb would come in handy one day. T.W.

The hardness refers to the strength of the bond holding the grit together, rather than the grit itself.

A hard material will dull or blunt the grit, and if it doesn't break off then the wheel will glaze and not cut, so a softer wheel allows the grit to break away easier and then effectively presents a sharp new cutting edge.

Softer materials don't wear the wheel so much, so can use a harder bond.

Peter

Why is this? I'm curious. Is it because lost grit gets embedded in a softer workpiece and then wears away the grinding wheel?

Best wishes,

Chris

It's because hard materials like hardened steel or tungsten carbide blunt the grains of abrasive before they have time to be released by the bonding agent to expose fresh sharp grains. The wheel then just rubs and builds up heat. The rate of grain release, or wheel breakdown, must therefore be inversely proportional to the hardness of the material being ground. Whether a wheel is 'hard' or 'soft' refers to how fast it breaks down and exposes new grit and not to the actual hardness of the grit itself which is what confuses people new to grinding.

A hard wheel breaks down slowly and a soft wheel breaks down fast. However the harder the material being ground the harder the grit must also be. In fact you can't effectively grind a material with a grit that is softer than the material being ground. Silicon carbide grit is harder and has sharper cutting edges than aluminium oxide grit so it is needed more on very hard dense materials like tungsten carbide but it is also weaker and breaks down quickly so perversely it's used on soft materials like non ferrous metals and stone. On hard steels it can give a very good surface finish and is sometimes used as a finishing wheel in a very fine grit rather than the more commonly used aluminium oxide grit for general purpose grinding and roughing.

I went into all this in a big way some years back when I was trying to find a suitable replacement wheel for the original one on my old Black and Decker valve refacer. It's a custom shaped dished 5" wheel that's no longer made so to put it mildly you're pretty much f***ed for anything suitable off the shelf. I even had a spare wheel when I bought the machine but not knowing they were no longer made and how rare it was I made a mandrel to put it on the mill for general grinding and doing something stupid one day I broke the bloody thing so when the one on the valve refacer wore out I was screwed.

I made an adaptor for the valve refacer so it could take a straight 5" wheel which then gave me some choice but nothing like what you'd have in a more common 6" wheel. Of course everyone said I wanted aluminium oxide for grinding steel. I tried all sorts of things. White aluminium oxide which gives good stock removal without too much heat buildup but a poor surface finish. A brownish wheel of a type I forget which was nigh on useless. Pink/gray aluminium oxide which is the norm for steel but still tended to chatter. I solved most of that with a finer grit, a new diamond and better dressing techniques but never quite matched what the OE wheel would do. That always used to give a mirror finish, never chattered and wasn't even that fussy about how it was dressed. When I tried to duplicate its performance with off the shelf wheels I became convinced it must have been made out of something magical.

Eventually I discovered that the original wheels were actually a special grade of silicon carbide although they were more blue than green. Happily someone has now started making them again and I'll get one once my current wheel is worn out but it does me for now.

Actually although grinding wheel choice can seem very much like a black art there are some very similar analogies to other types of machining once you understand the processes involved. A good example is picking a milling cutter to suit the material being machined.

1) Material hardness. The harder this is the tougher the cutter needs to be. In grinding terms the grit hardness of the common abrasives in ascending order are Aluminium Oxide, Silicon Carbide, CBN (cubic boron nitride), Diamond. 2) Tool life. Again directly related to material hardness, the tougher this is the more often you're going to need to sharpen or replace the tooling. Tough material = frequent sharpening = soft bond for a wheel that breaks down quickly and exposes new grit edges. 3) Material that clogs the cutter such as aluminium. You want a milling cutter with widely spaced teeth such as a two flute instead of multi flute and in grinding you want a wheel with widely spaced grains of abrasive and more porosity. Coolant/lubricant also make a similar improvement in both cases to wash away the swarf and keep the tool clean. 4) Desired surface finish. The better this needs to be the finer the feed and similarly the finer the grit. Again coolant always helps. 5) Stock removal. For high stock removal use deeper cuts and frequent sharpening. In grinding use coarser grit and softer bonds.

In a way it's really quite straightforward if you think of a grinding wheel as an abrasive cutting tool that's not so much different to a lathe tool or a milling cutter.

Yes - and your pc clock is an hour out.

Thanks, Dave and Peter. That makes good sense. Presumably the rate of wheel wear must be kept much lower than the rate at which material is removed from the workpiece, otherwise you'll lose your accuracy, is that right?

Best wishes,

Chris

Definitely not mine but maybe something to do with either of our news providers.

It looks okay to me. My clock is reading 10:52 pm. My previous message is shown on the news server as being posted at 10:35 pm. But your reply to my 10:35 pm message is shown as being posted at 9:44 pm. Are you sure it's not your clock, or your news provider's clock?

Best wishes,

Chris