Lathe Levelling - how level?

Your description is better than the one I wrote and discarded.

If you mechanically average the readings by zeroing the indicator at the minimum reading, turning the spindle to obtain the max reading and then backing out the slide to halve that number the indicator should read zero plus or minus equal amounts when you check eccentricity at any position along the ways. This is also a quick way to center round work in a 4-jaw.

The low-reading zero may have to be one or two pointer revolutions in from its stop.

The plunger spring causes an offset error if the rod is too flexible.

jsw

In article , snipped-for-privacy@yahoo.com writes

That's fine, if the bar has a constant diameter (or at least the two test regions are the same diameter). The post I replied to said it didn't need to be.

David

The bar does NOT need to be the same diameter at each of the test regions.

In our imperfect bar there will be some eccentricity and diameter difference at each of the measurement locations. On truly circular measurement locations the average of the minimum and maximum clock readings still precisely defines the centre about which the bar is rotating.

Jim

In article , snipped-for-privacy@yahoo.com writes

Then how does your "test" distinguish between (a) a perfect lathe with a bar of identical test areas, and (b) a lathe with say 10 thou misalignment of the bed between the test areas and a bar with a 10 thou difference in diameter between the two test areas?

David

You also have to take the diameter of the bar at the point of measurement into account. Suppose a bar 1 inch across but perfectly round and perfectly centered, the center is 1/2" from the measurements (both are the same in this case, and equal the average). If the bar is 2 inches across at another point, but still perfectly round and perfectly centered, the center is 1" from the measurements.

For Rollie's Dad's method add the two measurements and the diameter[8] of the bar - this total is twice the distance to the center.

[8] measured across the maximum/minimum plane at the point where the other measurements are taken - you should measure the diameter for each set of measurements, unless the bar is known to be parallel enough that you don't have to.

-- Peter Fairbrother

Think about a conical test bar running nearly true. To find the axis of rotation you have to add the diameter to the low reading, turning it into the high reading on the far side. Then the average of the two is the central axis. If the diameters are the same at both ends they cancel out since you really looking for a constant offset (the bar's radius) from the axis.

jsw

Oops, my antecedent is ambiguous.

The average of the near and far side high readings is the distance to the central axis.

jsw

.The average reading always relates to the true axis of rotation

- the actual local diameter and eccentricity do NOT change this result.

The basic premise of this method is that, if any truly circular object rotates about some arbitrary centre line, this centre line line is necessarily located equi distant from the points of minimum and maximum eccentricity and is independent of both the diameter of the object and the degree of eccentricity

I really don't know how to explain this more clearly - perhaps interested onlookers can suggest a more easily understood version

Jim

Sorry to reply to my own post but I need to don a hair shirt!

The method truly corrects for eccentricity but, because the measurement point is located at a point on average half the diameter away from the axis of rotation, it is NOT independent of diameter.

The basic premise ........... statement is correct but, because the measurement is made at a point (on average) half a diameter away from the axis, half the diameter is added to the clock reading. This means that, if the diameters differ, the clock reading should be corrected by half the difference in diameter.

Sorry about the confusion

Jim

Hmmmm, sounds very much like my hacked 5HP ABB 301 inverter (2 sets of series-connected electrolytics, neutral to the 1/2-way pads between 'em) - which I've had some fun and games with, it had been "drop tested" by DHL on its way to me, so it was a 200-piece jigsaw puzzle by the time it reached me... Only cost me £20 though! I've had to replace the LCD screen (£2 from a nice Chinese gent on Ebay, hooked the backlight up and it's a pretty blue colour now), do a lot of gluing and sticking, un-munge the connector from VFD to programming panel, but it's all working as intended now! I was a bit disappointed with ABB though, no service manual / circuit diagram available because it's "obsolete", only 10 years after production ended - I guess they didn't supply them to the military, then?

Have you hooked up the interlock switch on the C10 speed lever to "coast" the motor during changes? On the ABB I can set up 2 sets of acceleration / deceleration ramps under control of a switch, so I'll put in a nice slow ramp for speed changes, something fairly savage for E-stop / threading stop, although I'll need a braking resistor... All that will come once I've done a bit of renovation, poor thing had been neglected in a leaky shed :(

Re the headstock oil, I'm told that LHM fluid (for Citroen / Rolls-Royce suspension systems) is a fairly good choice, possibly with a hint of paraffin to reduce the viscosity a touch? Like the Holbrook, they have a fair bit of bronze in 'em so it's formulated to protect that as well as steel.

Cheers, Dave H.

Yes, that's exactly what I did

... snipped

I did when I tested the whole thing but I've been waiting for time to finish the job and do it all properly - a shortage of tuits.

On the ABB I can set up 2 sets of acceleration /