Two concerns regarding your grinder conversion:
1. How much power is required?
2. What are the effects of increased wheel diameter?
If we're talking about the same grinder (Skil 3380), it's 2.1 amps with a rotational speed of 3450 RPM.
POWER
I would much prefer to leave the amp-to-horsepower conversion for the electrical motor geniuses, because I'm definitely not one of them. But to give a rough estimate, your motor is probably somewhere in the neighborhood of 1/4 HP.
Maybe 1/3 HP if it's a more efficient motor (doubtful).
The HP you require is going to depend upon the actual contact area the wheel has with the material being worked.
A good analogy would be the consideration of tire size on an automobile- a taller tire provides more ground contact, and a wider tire has an even greater effect. This is done in an effort to increase traction.
In the case of buffing, we want plenty of contact area to get the work done in a timely fashion. But unlike automobiles, we don't want "traction"; we want to purposely "spin our wheels"!:lol:
So we need to make sure our motor is powerful enough not to stall with a given wheel size (a combination of both diameter and thickness/width).
Concerning thickness, the great majority of us simply use the most common thickness, which is 1/2".
So is 1/4 HP enough for your needs?
I have an old 1/4 HP motor that's older than I am.
Using 6" diameter wheels, I can put two of them together (combined thickness of 1") and still
not stall the motor.
If I use the same (1") thickness with 8" wheels, I can easily stop the motor. But if I use only
one 8" wheel (1/2" thick), the motor does just fine.
So I believe for smaller work such as is typical of most knifemaking, you might be OK on power.
But like Ed has mentioned, you have to see what works and doesn't work for
you. By far, the determining variables are 1. differing pressures between individuals and 2. differing angles of approach.
The more pressure you apply, the easier it is to stall the motor. "Barely flattening the wheel" means different things to different people- it's subjective. Considering the type of object being worked (knives), applying enough pressure to stall the motor means you're also creating a dangerous situation if the motor
doesn't stall (grabbing and throwing).
If your angle of approach is such that more material is in contact with the wheel, then it's easier to stall the motor. An example would be holding the piece "in line with" the wheel vs. "across" (perpendicular to) the wheel.
Still, everything considered, your motor
should get the job done for a typical buffing scenario.
WHEEL DIAMETER
As mentioned above, a larger-diameter wheel will increase surface contact, but not nearly to the extent as increasing width.
So what's the big deal with wheel diameter?
You mentioned using the 8" wheel in order to increase access. I see your point, and while it's certainly possible, it's not recommended.
With the speed of your motor (3450 RPM), you'll have surface speeds well in excess of 7000 SFPM.
That type of speed is great for regular tool steels, brass pommels and bolsters, etc. It will certainly get the job done quickly.
But if you're working with stainless, you'll
probably find that it's too fast.
If you're working with handle material or titanium, you'll
definitely consider it too fast.
Again, this is an area where "different people; different results", so it's your decision, and your trial-and-error (or success).
For more information on the effects of wheel diameter and surface speed, here is a link (one of many) discussing grinder conversions:
http://knifedogs.com/showthread.php?19932-Which-Buffer-and-What-Speed
Good Luck,
Rob
