Hi Clive,

I use socalled white wheels on my bench grinder, 60 grit would be Ok but 100 grit would give a finer edge.

Thor

The grit size is determined by the number of holes per inch in a mesh. So allowing for the diameter of the wire mesh the grit size is slightly smaller than might be thought.

While the finish obtained by grinding is, of course, related to grit size, it's not a particularly close relationship. The grit is randomly located on the wheel, so over a revolution or so any peaks tend to get ground down. A grit of 46, or 60, is often recommended as general purpose for a surtace grinder. A coarser grit is less likely to clog and need repeated dressing.

It's a myth that the tool surface is directly replicated in the workpiece. It has an effect but it's not a one to one relationship. Consider using a Coventry die chaser as a cutting tool, so a nice regular V pattern. If the feedrate is exactly matched to the thread pitch then we'll cut a nice thread. But suppose the feerate is 1.5 times the thread pitch. Subequent teeth will tend to knock off the peaks left behind. Similarly if the feedrate is less than the thread pitch then the peaks left behind will be smaller than might be expected. Given that the roughness of the tool after grinding is likely to be pretty random in terms of peaks and troughs the finish obtained should be better than one might expect. After grinding HSS lathe tools I use a cheap diamond hone for two reasons. One, to remove any tiny burrs left on the cutting edge by the grinding process. Second, to remove a thou or two from a flank if I need a tool of a specific width ot shape.

When I started dabbling in model engineering in the early 70s I bought all the standard ME text books mostly from MAP. In those honing of tools usually gets a mention in passing, but no more.

I'm off to fit more spokes to my traction engine wheels, so fire away, I won't be offended.

Andrew

MAP = Model and Allied Publications