I am looking to sharpen drill bits, wood turning tools and what ever else you would sharpen in the wood shop. I am told that a 6" is okay but should really get a 8". I am looking at Ryobi brand bench grinders. How long would the big one last to the small on? Is there better sharpening quality on the bigger than smaller since there is a smaller radius in the angle. I like the 8".

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    If you are going to use a grinder -- not necessarily a good idea; sharpen more often and you won't have to remove that much metal, but it can be useful for starting from a truly blunt edge -- consider a liw-speed grinder. That will reduce the risk of overheating the metal and destroying it's temper. Note that at equal rpm, a larger wheel's edge is travelling faster than a smaller one.
    – keshlam
    Nov 4 '16 at 18:22

This is unfortunately one of those things where there'll be many differences of opinion, for some people it's 8" all the way and things smaller are nearly toys. But for tool sharpening and the occasional metalwork shaping job 6" is most definitely adequate. In fact for a lot of leisure woodworkers even smaller than this is perfectly usable, just somewhat slower.

It's worth remembering that as far as sharpening goes grinding is only necessary every now and then unless you're a woodturner. For bench chisels, plane irons and anything else you might run over a grinder's wheel to shape the edge, grinding is not a daily, weekly or even monthly task for the majority of users. It may not even be necessary once a year given the slow rate of wear for the typical woodworker!

Is there better sharpening quality on the bigger than smaller since there is a smaller radius in the angle.

Some people argue that there is, but in most cases it makes no practical difference in the edge produced.

Larger grinders are generally more powerful (which is good for a couple of reasons) and can often be faster as well, and while there's a perception that this is a Good Thing too, it often isn't in sharpening. With just about any grinder we already have to take pains not to overheat the tips of chisels when shaping them (sometimes called 'blueing' because the steel turns blue when it has overheated). As should be obvious the faster the grinder the more care you have to exercise, generally speaking.

In addition to the outright speed of a grinder another reason an 8" needs to be used more carefully than a 6" because for any given speed the surface of an 8" wheel is actually moving much faster — so at the same RPM an 8" grinder is more likely to overheat a tool than a smaller one, everything else being equal.

It's beyond the scope of this Answer but you should also look into the various wheel types available, how fast and how cool they cut can vary quite a lot (but not uniformly, CBN wheels are generally the best of both worlds but you do pay for the privilege).

Closing note: also remember that you don't absolutely need a grinder. They are very handy, but as far as care and maintenance of woodworking tools goes you can quite happily do without one, as was fairly common until quite recently.

With diamond plates being freely available now and coarse ones cutting so quickly some of the jobs that used to be "I'm gonna need to use a grinder for this!" can actually be accomplished by hand within a reasonable timeframe, q.v. the complete re-grind of a chisel in this previous Answer.

Also see:
Bench grinder, tool sharpener or sharpening stone?
How does one aggressively sharpen chisels and plane irons when damaged?

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    Agree with your closing note, but my grinder sees kitchen knives, chisels, drill bits... The convenience means the difference between having sharp and dull tools. (Yes, this is a character failing.) I have to admit that 'need' is a strong term, but I'd say I 'very strongly want'... And I second the notion of getting a low speed (~1700 rpm) model. Nov 6 '16 at 2:30
  • Hey, I'll take one of those freely available diamond plates! :) Also, grinders are handy for things other than just sharpening - they're pretty good at... grinding. ;)
    – FreeMan
    Nov 8 '16 at 14:36
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    @FreeMan Freely available = commonplace. Sorry it doesn't mean free. Although the cheap ones, which can be decent users, are very affordable.
    – Graphus
    Nov 9 '16 at 1:19
  • Aw, darn... I was hoping for a freebie... :/ :D
    – FreeMan
    Nov 9 '16 at 13:39
  • @FreeMan On the plus side do you live near a Harbor Freight? They do a set which I believe costs less than your typical large coffee from Starbucks, which I think is a bargain even if they only last six months (but they'll likely last you years).
    – Graphus
    Nov 10 '16 at 8:34

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