The key is to
Measure at the Bore!

Wheel thickness is also referred to as “wheel width” and when ordering wheel adapters or I.D. grinding quills the “wheel width” has to be stated accurately to securely lock the grinding wheel in place. 

Wheel types come in a wide variety of shapes and sizes, so remember the width of the wheel at the bore where the adapter or quill contacts the grinding wheel is the measurement point. 

Look at the two types of grinding wheel examples shown below.  The cup wheel is wider on the O.D. than the straight wheel, but it’s thinner at the bore.  Always measure the width of the wheel at the bore then pick the appropriate adapter or quill to fit.