vulcan32
Member
If you want to change the hole size on the knobs a 6mm reamer will give better results than a 6mm bit. Here's an example
6mm Cutting Diameter 6 Blades HSS Machine Reamer Milling Cutter | eBay
These don't cut as aggressively as a bit and have a flat end so it's easy to get a good fit. Reamers do cost more than bits, the one I've linked to is comparatively inexpensive . They can usually be found locally at any machine shop supply.
Thank you for the suggestion- using a reamer would be prudent.