minimizing runout

THE LATHE, aka: LS-76, and the LJ-10 and LJ-12 tape machines

minimizing runout

Postby Andrew » 27 Apr 2013, 21:37

Fortunately, there is a way by which to measure and compensate for, to some degree, the amount of "runout" demonstrated by the carefully made (flat) turntable platter. This is similar to measuring the runout of a disc brake. The brake is mounted vertically and the platter, horizontally. In the case of the vertically-mounted brake, the runout is lateral (i.e., horizontal). In the case of the horizontally-mounted turntable platter, the runout is vertical.

The entire L. J. Scully steel weldment is first to be leveled using a spirit level. With cutter removed and microscope, off, tone arm secured, carriage and feed nut, also, somehow, secured (preferably, removed), one end of THE LATHE can be held up while someone else adjusts the height of the feet that are screwed into the bottom of the weldment legs. One rotational direction raises a leg, the other, lowers it. If working alone, however, a brick and a 2" x 4" board can be used as a fulcrum and lever, with the brick placed on the floor, in front of the middle of the crossbeam, leaving just enough space to allow insertion of one end of the board - your lever. Of course, you will also need a place on which to stand (; since you'll be pushing down on the other end of the board to raise THE LATHE, while readying it for using something like a large power amp as a counter-weight, placed on the raised end of the board. Have the anvil (or power amp, or free weights) resting on a nearby chair and have one foot on the lever while the other is on the ground, then use your hands to seize and position the designated counter-weight, and carefully swap the force from being applied to the lever by your foot to being applied by the counterweight... Though pressure only is applied (upwards) to the underneath of the weldment's crossbeam (while downward force is applied to the top of the brick by the back of the board), it's best for the load end of the board to be draped by a hand towel so as to protect the finish of the black anodized weldment.

Using the now-balanced floating lever suspension, adjust the height of the table's feet, repeating the lifting, adjusting, lowering, and re-measuring of "true," as needed, on one or both sides, in order to fine-tune the bubble's deflection within the spirit level capsule). Then the A-table (which is the aluminum chassis comprising the lathe bed and turntable axle sleeve) is leveled, as it floats on rubber bushing Lord mounts, and each mounts' height is adjustable. The spirit level is consulted for this leveling, as well.

Then you "level" the 16", 40 lb., aluminum and composite, selectable-diameter, air-channel platter, after it's placed on the vacuum turntable. First, the spirit level is used while one adjusts the tightness of the three mounting bolts that hold the platter on the turntable, near the spindle. (The vacuum turntable is bolted to the top of an aluminum pipe that is inserted vertically from above into an oil-filled, sleeve. The thick-walled turntable axle rests on top of a 7/16"-diameter chrome-plated, steel thrust ball, which is suspended in the bottom of the sleeve by a brass plate, with a little hole in the center. A plug, in the bottom of the axle sleeve can be removed for removal of the locking screw and the leveling screw, brass plate, and thrust ball, but oil change is forcibly entailed.) The platter bolts should all be screwed in enough that their heads are below the surface of the platter and the bolts are just tight, but they are not to be torqued hard. Then you put a depth feeler on the 15" diameter position of the 16" diameter platter and, by hand, slowly revolve the platter while watching the reading of the depth feeler on its dial indicator.

The goal for the L. J. Scully (son and grandson team of original (John J.) Scully company, of Bridgeport, Connecticut) is probably similar to others: +/- 1 mil (i.e., +/- 0.001") runout. That is, the amount by which the measured radius of the platter rises and falls from the target height, which is perfectly parallel to the ideal spirit level, should be no greater or lower than 1 thousandth of an inch. This (2 mil p-p) runout is not excessive for dampening with silicone fluid and the counterspring. However, increased runout may have a noticeable effect on the groove depth in a "once-around" pattern. (This can also happen with a balanced turntable holding a blank disc that may have uneven thickness to its lacquer coating. The best flatness of lacquer is achieved with a liquid curtain application to conveyor belt-fed blanks, rather than the more common technique of pouring lacquer in liquid form onto a spinning blank (Al) disc. The only change in thickness of the coating on the liquid curtain discs is at the very outer edge due to the Meniscus effect, alone. Whereas, the centrifugal spreading of the lacquer being poured on spinning blanks makes a wide ring of thickness at the outer edge and one which, itself, is not always of consistent thickness around the ring.)

Wherever the depth feeler measures an increase in runout, you slightly tighten the nearest bolt (to the depth feeler). #656 presently has no more than +/- 1,0 mil runout, and #660 has +/- 0,5 mils runout.



- Andrew
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