We do a lot of 2D cutting in various thicknesses of MDF with 1/4" spiral downcut bits from onsrud. We are getting a significant amount of chatter on both the finished piece and the scrap piece. We cut at 13,000 rpm and a feed rate of about 1.5 ips. Anyone have any idea if it is the bit, the spindle, the speeds or anything else I should be looking at.