200mm on a 250/T8

[William]

So I am a brand new T8 owner.  I am looking at expanding my stone collection.  I have found that pricing wise, buying a 200 mm stone is considerably less expensive.  Any problems using one on a T8?

[Elden]

Welcome William.
The simple answer is yes in regard to the standard SG-250 and SG-200, in my opinion. The full size of the larger stone is 250 mm diameter and it is wider as well. To some people a wheel measuring 200 mm on a T7 or T8, is pretty well worn out. One with a 175 mm diameter is Very worn out. If one does mathematical comparison of the prices, for what you get, the 250 mm is well worth the cost.
Someone else can give you a better answer than I in regard to the exotic wheels.

[Ken S]

I totally agree with Elden. I would go one step further. I would not recommend purchasing any more grinding wheels. That's honest advice that you probably don't want to hear. Learn how to use your new Tormek with its Original wheel. Allow yourself the luxury of really learning how to sharpen with the SG-250. Keep your learning process simple and focused.

Ken

[Twisted Trees]

A third agreement, depending on what you are sharpening, as a wood turner where concave bevels are a problem, I would be throwing out wheels at 200mm.

Ken is spot on with proper use of the grading stone the original wheel will sharpen pretty much anything, but the Tormek like any tool does require some skill and practice from the user. Do that bit first, when you find that everything is good but maybe not quite sharp enough to split atoms, or you are sharpening so much quantity that the time to change wheels is offset by the 30 seconds per unit a different stone would make then you will also know the value of 250 over 200.

[Ken S]

William,

I did not mean to sound discouraging. Like the rest of us, you are quite capable of mastering the Tormek. I hope you will; Tormek sharpening is a satisfying skill. You will become skillful more quickly without the distraction of extra equipment. Once you have a solid foundation of skill, you will find that that knowledge transfers quickly to other grinding wheels. You won't be disappointed.

Ken

[John_B]

I too have lusted after additional wheels but to date I have resisted.

I am able to achieve remarkably sharp edges, easily in the shaving range, using the standard wheel.

I do use the FVB for honing as I find it much easier than maintaining the proper angle by hand. Watching the latest Tormek training video they achieve great results by hand; the product of much practice.

[RichColvin]

William,

I performed the analysis on using a 250mm stone down to a useable minimum size (180mm).  That is in the attached chart.

As noted, that consumes 1,182 cm3 of grindstone.  Using a T4 stone from 200mm to 180mm provides for 239 cm3 of grindstone -- only 20%.  So, unless the T4 stone is <20% of the cost of the T8, it is cheaper to get a T8 stone.

There is more information about getting the most from your grindstones at:  http://sharpeninghandbook.info/Grindstones-StoneLife.html

Good luck.
Rich

[Ken S]

Rich,
Here is the maximum wear recommendations from the handbook:


"Lifetime of the Stone
Mounting: Put the grinding wheel onto the shaft and turn the EzyLock anticlockwise by hand. When sharpening, the grindstone will automatically tighten to the right torque. You need no tools.
It is not possible to predict how much grinding can be done or how many hours a grindstone will last. It depends upon the tools being ground and how much each tool is ground. As an example, a professional grinder has ground 2.000 food knives on the ∅ 250 mm (10") stone, by which time it had worn to 200 mm (8"). If you shape gouges in HSS steel, it wears the stone more quickly. When sharpening an already shaped tool the stone wear is very limited.
We recommend that the stone is changed before it wears down too much. You should not allow it to be less than approx. 180 mm (7") in diameter in order to ensure a satisfactory grinding performance. The smaller T-4 stone should not be less than 150 mm (6")."


While 180 mm is the recommended minimum diameter for 250mm grinding wheels, 150 is the minimum recommended diameter for 200 mm grinding wheels.
Regardless of how the math may work out for cost, grinding becomes less efficient with wheel wear.
Surface Feet per Minute (SFM) = RPM of the shaft (100 RPM) x the circumference of the grinding wheel.

Incidentally, the higher RPM (120) of the T4 essentially cancels out the reduction caused by the smaller diameter.

Ken