ER milling collect chuck typical accuracy


Jan 19, 2021
I'm new to milling, so far the only thing I did has been watching videos. I'm not fixated about precision although that is a long term goal for me.

I've got cheap ER16 and ER25 collect chucks for my MT2 milling machine which is integrated in the 250 x 550mm lathe.

On the milling spindle I have between 0.01 and 0.02mm total runout on the edge of the taper.
Fitting the chucks on with moderate tension I read some 0.03 - 0.04mm on the collet chuck taper. That is pretty much the same for both of them.
This improves a bit if I rotate the chuck in the spindle, but I did not spent a lot of time with this approach. A test bar into the ER16 has a little more than 0.05mm TIR not far from the collet.

Are these errors typical in this class of machines? Can give me trouble in producing "reasonably" parts?
Is it a normal practice to indicate a chuck for the best position and possibly mark it?
While not desirable, you can still mill with that amount of runout. It will resulting increased wear on the milling cutters and a slightly larger apparent diameter of your tools. 0.05mm TIR is 0.025 (.001") non-concentricity which, for most operations, is within the acceptable error. If it bothers you, you can compensate with your tool diameter. Even with micromachining (less than 1 mm tools), you should still be OK.
In Dec, I got an MT5 ER-40 chuck, and depending on the rotation angle it has 0.000,3" to 0.001,5" runout (on the ER-40 tapered clamping surface). So I found the point of least runout, and marked it based on the mark on my lathe spindle. Now I can assemble it onto the spindel, as good as it goes, easily.