Cutter accuracy

[Neale]

My apologies - you've reminded me that I never really described my conclusion. After a lot of testing, I came to believe that apart from a very small amount of backlash there were no significant errors in my machine (and bearing in mind that this is a primarily woodworking CNC router and not a toolroom machining centre). In other words, it was working as well as my cheap C7 ballscrews, etc, allowed.

What I did conclude was that for whatever reason, my cutters did not cut to their nominal size. What I do now with a new cutter before using it for anything critical is to use it to cut a test piece - two parallel cuts along each edge of a small block a known distance apart as defined in gcode. I then measure the actual width of the material between the cuts and compare it with what I expected. That gives me the effective cutter diameter. For instance, my 4mm single-flute cutter cuts as if it were 3.80mm or somesuch. I just plug this number into the tool library in Fusion 360 and just carry on. Recently I have been cutting 3.5mm ply panels which use a tab and slot arrangement to make up shallow trays. Once calibrated, I get very repeatable and predictable dimensions so that tabs fit slots very nicely whether slots are horizontal or vertical, even when dimensions are changed to make other size boxes.

So, conclusion - my cutters do not cut to their nominal size, but once the effective cutting diameter is measured (by actual cuts, not by measuring the tool) that dimension is very repeatable.

Maybe I should just buy better quality cutters!

Hope this helps - I've stopped worrying about it now.