With a good number of hours under my belt, a print quality issue began to show up on the new printer. You can see it evident in the side walls of the battery holder pictured below.
A closer view (photo below) shows lots of gaps and a general poor finish.
I entertained a number of possible causes: Too much "retract" when executed, resulting in extrusion not resuming flow in time for printing. I even entertained ambient temperature changes being an unlikely but possible cause of the problem, as Winter was setting in, and considered increasing the hot-end temperature as one of the things to try. But before changing anything, I set another print under way, with a slow print speed (20mm/sec), and had a close look at the printing process in action. (I've made a cut-away fan-duct just for easy inspection, which I must post about.)
On observing the "gaping" happening a number of times during the test print, it was obvious that the large gear that drives the hobbed bolt was slowing down and sometimes stopping, when it should have been turning at a steady rate. Closer observation of the smaller gear revealed the issue. The extruder motor shaft was slipping within the small gear.
I knew from past experience that simply tightening the grub screw was most likely not going to be a long term solution, as had the grub screw been resting against the "flat" on the motor shaft then it wouldn't have slipped even when it had worked a little bit loose. The reversing action of the motor would have resulted in more of a clicking noise than full slippage.
The "flat" was easily extended using a flat needle file. The bearing of the stepper was protected from filings with some 'blue tack'. The shaft was gripped in a small vice. (See photo below.)
I then reprinted the battery holder and the difference in print quality was immediate (photo below). The plastic flow was consistent and the print finish excellent.