The datasheet for the Simturn DX boring bar with threading insert seems to be suggesting an effective diameter of 5.5mm but it's clear this isn't how mine behaved. I needed more infeed than that in the end. So let's do a quick measurement on the machine. But how to do it?
One obvious(?) method might be to cut a slot or hole with the tool and simply measure the resulting diameter. Sounds fairly straightforward but this is a pointed cutter and not a very robust one at that.
I don't need ultimate precision here and the machine itself is hardly in watchmaking territory, having an achievable accuracy of around 20-30um on a good day. So let's be sensible here. Do this:
- Insert tool in spindle
- Hold crappy calipers in vise
- Touch off tool against fixed caliper jaws
- Rotate tool 180 degrees and close caliper jaws onto tool.
Subtracting the 10mm pin gauge (aka broken tool) gives an effective diameter of 9.57mm. Indeed this is somewhat less than the 11mm suggested by the datasheet. This may explain why I needed rather more infeed than expected.
I've now created an updated tool for this threadmill application, with an effective diameter of 9.57mm. Hopefully there will be less trial and error at the machining stage now.