I talked to Clausing today. One question I had was how much of a cut the lathe ought to be able to handle without chattering. The guy I talked to didn't recall such a spec. Later, I asked if there was spec on spindle runout et al, and he said that there was a sheet that gave that information, which he was happy to fax to me, and did.
It turned out to be the "Clausing Test Report, 5900-series Lathe", a one-page form that was filled out and provided to the purchaser of each lathe. There are 15 tests listed. Most are for various kinds of alignment and runout, and are all less than 0.001" both near the headstock or tailstock as appropriate, and also at the end of a 12" bar held in one or the other.
As for runout, on hearing my long sad chatter stories, Clausing suggested tightening the spindle bearing take-up nut by 1/4 turn, even though the spindle passes the spein-spindle-by-hand test.
The next to last test, the "Running test for smooth operation", is the answer to my original question. The test specifies that the lathe must be able to cut a 1.5" diameter bar of CRS with 5" protruding from the3-jaw chuck (and no tailstock use), running at "high speed" while taking a 0.125" depth of cut (0.25" diameter reduction) and feeding 0.0026" per turn, without chatter.
What is "high speed"? If their tools were HSS, the surface speed should be in the range 80-120 fpm for optimum performance. To achieve 120 fpm in a 1.5" round bar, one must spin the bar at 306 rpm, which is above the fastest backgear speed and below the slowest normal speed, so this seems unlikely.
If the tools were instead carbide, the optimum speeds are a factor of 5 faster, which puts one at 1,528 rpm, which is at the upper end of the5914's range (the max being 1900 rpm), which makes sense. This sounds like smoking blue chip territory for sure. But I will try it. While standing well to one side.
Also of interest is the cross feed screw backlash, 0.004" max. It was0.025" on receipt of the lathe, and is now 0.010" after replacement of the T-Nut (the original screw being retained). The compound slide backlash also meets this 0.004", after replacement of both screw and bronze nut; it had been about 0.050".
Actually, what is the claimed speed range of the 5914? This must have been published in a brochure or catalog page somewhere. The scale on the variable speed control wheel appears to be wider than the actual range. For one thing, the scale goes to 2000 rpm, but one hits a mechanical stop at 1900 rpm. Probably the same control wheel assembly was used on multiple lathe models, each with slightly different gear/pulley ratios, with limit stops adjusted to match.