I thought I share as I was looking for one and didn’t see any post recommending any with this particular feature.
Mind you it’s not cheap, but! While it won’t clamp onto the 65mm body of the router/spindle it fits perfectly on lower diameter boss on the bottom of mine and it might work similarly on the C3D router just fine.
But the main point is that the arm can be separated from the clamp, then it presents a 1/4" shank which can be clamped in an ER11 collet to check for spindle tram. That is something very hard to find.
I have to give props to @RichCournoyer for sharing way back when and always feel compelled to share since I struggled with a solution until I saw the light.
I now use a cheap dial test indicator with a dove tail indicator holder with a 1/4” shank that mounts in the spindle collet. As per Richard, I cut the indicator holder shank to an acceptable length, but works great and you might be able to find it cheaper.
I have magnetic bases. But I want it on the Z carriage and the collet nut is way too small to bear the weight of it comfortably. The rest of the spindle body is stainless+aluminum
What size is your base? The normal size magnetice bases are too heavy for the nut to hold. My NOGA arm has a teeny tiny magnetic base that is able to hang onto the nut.
I do have that too and the shank is different. It doesn’t have the 1/4" narrower section so unless you can turn it down it won’t fit into ER11 (an similar) collets.
