Someone asked me how accurate my Sieg SX2.7 milling machine was in Z-axis. There problem was the head moving when you lock the travel.
My method for measuring this was to lightly clamp an engineers square to the table, fit a dial gauge into the collet chuck and align the measurement with the edge of the square.
I can hear the issues being raised now with the accuracy of the square being measured instead of the milling machine – we will come to part of an answer on that in a bit.
The measurements were made with a 50mm movement of the quill and a separate 50mm movement of the head.
The digital dial gauge was aligned to run down the edge of the square. You need to carefully align to avoid the ball running off the edge of the square.
Rotation of the dial gauge in the spindle – this was checked by repeating the measurement.
Fore/aft, quill = -1.06mm in 50mm down => head tilts forward.
Fore/aft, head = -0.072mm in 50mm down => head tilts forward.
This shows a difference between the two and I would suggest from the next measurement that both the head and quill are not aligned correctly in fore-aft.
I measured the accuracy of the head in fore/aft direction and then turned the measurement through 90° so that I could check the left/right title.
Left/right, quill = 0.026mm in 50mm down => head tilted to left.
Left/right, head = 0.026mm in 50mm down => head tilted to left.
I then held the position of the head and just locked the position of it, this moved the head to the left (looking at the machine) by 0.048mm
I need to think again about how to make these measurements more systematically with the equipment I have around me. This does show that you must lock the head before any alignment and machining tasks.