Rounded knife used are "R" size; such as wood 18MM to be processed off 6.3MM, is the choice of R6.3MM
The same is true for anything else; understanding R is how many MM you want to process
All models handle diameter 6.35mm

​ Part No. Size(inch) D(mm) R(mm) B(mm) H(mm) Round knife 1/4 X 1/4 19 3.17 3.17 7.00 Round knife 1/4 X 5/16 20.94 3.97 3.97 8.00 Round knife 1/4 X 3/8 22.5 4.76 4.76 9.00 Round knife 1/4 X 1/2 25.7 6.35 6.35 11.0 Round knife 1/4 X 5/8 28.88 7.96 7.96 12.0 Round knife 1/4 X 3/4 32.05 9.52 9.52 15.0 Round knife 1/4 X 7/8 35.20 11.1 11.1 16.0 Round knife 1/4 X 1 38.4 12.7 12.7 18.0 Round knife 1/4 X 1-1/8 43.58 14.29 14.29 20