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Tormek IT replied that according to our hosting service, we had an attack by a botnet.

My money weas on a DDOS (distributed denial of service) attack. All too common these days. Insecure smart devices making it easier.

Zinc machining must be a fairly new technology.

Not sure what you mean Ken. New to Tormek, or new in general. Casting zinc alloys has been around for many decades, see alloy wheels on cars, and alloy engines and parts. I think that it was more expensive than steel fabricated parts which may have been a disincentive. Also new alloys have been developed with more strength and resistance to fracture and this may have also been a contributing factor.

Perhaps someone on speaking terms with the Tormek people could ask them what their decision making process was.

As others have said the is excess dust when sharpening dry, and wet is more efficient. In addition the wheel last much longer since the water reduces the impact on the grit. The same also applies to CBN.

The only down side to me is having to clean up after a sharpening session. I don't see that as a real bother, just part of the process.

Lawnmower blades are very forgiving and any sharpening will be infinitely better than how they come from the factory. I have typically used an angle grinder in the past. However a flat single cut file will produce as fine an edge as you will ever need. I would not even bother with the tormek to be honest.

I'm not sure I understand what you mean. But do you mean that aluminum oxide won't cut mentioned steels?

Yes and no. It is a matter of hardness. More specifically MOHS hardness. There are different forms of harness but it is MOHS hardness which determines the abrasive for each material. In the MOHS hardness cscale each material is given a number form 0 to 10, 10 being diamond and 0 being talc. For an abrasive to be able to sharpen it has to have a higher MOHS hardness that the material being sharpened.

For steels it depends on the composition and the temper. Different steels of ther same composition will have different harnesses depending on the temper. Knives often consist of several different steels so you may easily be able to sharpen the casing but have difficulty with the core.

Aluminium oxide has a range of MOHS hardness depending on the impurities but in general it can harden carbon steels but not so much high speed steel. So it will vary. When I was sharpening a very hard Japanese knife, my HSS thicknesser blades or my HSS drill bits the SH wheel glazed rapidly. It did cut the steel but was very slow and the wheel had to be constantly graded. It proved to be extremely inefficient. The issue was that the steel had a higher MOHS hardness than those steels. It may have been that the steel fell into the range of the softer to harder particles within the SG wheel this glazed rapidly.

The glazing happens when the steel being sharpened breaks the grit down since it is harder than the grit and you end up with a smooth wheel unable to cut the steel. This is an indication that the wheel is too soft.

Bottom line is that for efficient sharpening you need a harder grit that the material being sharpened.

I may try the existing stone in the meantime just to see what happens

You can use the SG so long as the jigs et all allow you to use it. The only difference is that the hollow is slightly deeper due to the reduced radius.

Many times DE-250 (grit 600) is the final step

The DE is 1000 grit.

Is there any practical or theoretical pros with using diamond (or CBN) as the last step of sharpening super steel's like m390, rather than traditional aluminium oxide stone?

Your abrasive needs to be harder (MOHS) than your steel or you will simply glaze your wheel. This is the process whereby the sharpening process rounds over your grit and leaves it blunt, thus making the wheel less effective and deforming rather than scratching the steel. I am assuming that you are dressing the SG otherwise you are leaving it glazed and not working efficiently.

The composite wheel is a lower grit and will not leave as fine an edge as the leather wheel. On the other hand honing on the composite wheel will be faster that on the leather wheel. As for customer experience it all depends as they say. The composite wheel will leave a toothier edge and for many operations will cut more easily as a result. For very fine and precise "food craft" even the leather wheel will leave too coarse an edge.

For general sharpening your average kitchen knife the composite wheel will be fine.

new drive wheel seems of much higher quality than the old one

Yes. They upgraded it significantly. The old plastic wheel would crack over the years.

if I let go then reapply pressure to the holding points on the wheel.

I hold the three arms to the wheel as I adjust. I think that is how you are supposed to use it.

during shipment the machine was damaged so the the motor is misaligned

Just what I was about to suggest. Definitely not right!!!

Agree completely with the previous answers. Just a couple of additional thoughts. Tormek say that the SG when fine graded gives you 1,000 grit. If you want to then you could then hand sharpen on the 3,000 stone then strop as you normally do since you would not be changing the bevel. What I do on high end knives is to strop on the Tormek wheel then hand strop with a quality green compound. On my nice chisels and plane irons is to do a few strokes on my 5000 grit ceramic stone and then hand strop. It is maybe 30 sec extra but seems to give me a slightly better edge. I also often refresh the edge on my woodworking tools on the 5000/hand strop which keeps it fresh and go back to the Tormek for major sharpenings. 

I'm looking for some cheap tools to practice on

If you do a search on AliExpress for "wood turning tools" then you will see quite a selection for some very reasonable prices. Make sure yu select the appropriate tools for the type of turning you want to do.

Yes, it does sound like something is scraping. The shaft may be slightly bent. Check the body and wheel to see if you can see any wear marks where it could be scraping. Not sure that it will really do any damage but do check with Tormek.

This could work - https://forum.tormek.com/index.php?topic=5335.0