Sharpening questions

[transatlantic]

I've just purchased a Record RSBG8, and a few things occurred to me.

1. Why do standard bench grinders run at such a high rpm? surely you only need that kind of speed if you're removing a lot of material? ... in which case, wouldn't you just use a lower grit?
2. Why aren't variable speed bench grinders a standard thing? I'm know they cost a little more, but wouldn't being able to run at a lower rpm be safer, cooler and most of all, extend the tools life as you're not removing as much material?
3. Why do most turners use grinders instead of waterstones/diamond stones etc? ... I've seen a few people use stones for skews, but that is about it.

 

[chipmunk]

1) Single winding induction motors are quiet and cheap - and fast. You need to use a light touch to remove a small amount of material or buy a slow-speed grinder with two windings like the Creusen for example.

2) Variable speed induction motors require more poles and an inverter. Very expensive. Brushed motors can be easily made variable speed but have lower torque, are often noisy and less reliable.

3) The main difference between turners and other woodworkers is that for us the tool is moving and cutting faster and therefore blunts quicker. That's why HSS is the norm rather than Carbon steel. You need a quick way to resharpen and razor sharpness isn't a priority really - just sharp enough.

So, learning to sharpen effectively and quickly but without wasting too much metal is a priority for turners.

HTH
Jon

 

[Phil Pascoe]

^^^ as he said. If you were to use a perfect, razor sharp turning tool the edge would last about two seconds - by which time it would have cut the same amount of wood as a bench chisel would have in about 15 minutes. I touch up a skew occasionally for a final cut though.

 

[duncanh]

I regularly use a diamond hone to refresh the edge on my gouges as it saves me putting the tool in a jig, switching on the grinder and grinding away my expensive tool. For the final cut I usually use a freshly ground tool.

 

[chipmunk]

I use a diamond hone for skews and scrapers where it's very easy to stick to the bevel angle but gouges are quite tricky IMHO to touch-up reliably.

I used to be an advocate of free-hand grinding until I made my own grinding jigs and now find that it's quicker and more consistent for me to use them for sharpening.

I have tried touching gouges up with a hone but, again for me, a quick wipe round of the gouge in a jig on a fine grinding wheel even as the grinder is speeding up is more consistent and just as quick. It is also then indistinguishable from a full sharpen.

Others may be disagree and find that they come to a different conclusion. It's whatever works for you that counts.

Jon

 

[Paul Hannaby]

Probably worth bearing in mind that the peripheral speed on an 8" grinder will be 33% faster than a 6" grinder so they will generate more heat and remove steel faster if both had a standard induction motor, which will have the same RPM.
I always preferred an 8" wheel and I recently switched from a standard grinder to a slow grinder (running at around 1500 rpm instead of 3000 rpm). It's still plenty fast enough for sharpening. It may take a little longer to reshape tools but I don't do that often anyway.