R?

I know you say specific for 1660 and I don't have one but I have a couple of similar machines in this range that have the same problem as yours.

My dial comes with 3 gears that need changing to suit a given pitch, tide table and also depends on the seasons.

I have now given up, not in frustration but in the fact i can thread faster leaving the half nuts engages and using a swing up tool.

I do so wish i had discovered then 40 years ago, it would have saved so much hassle.

Take a read of this thread.

**LINK**

If no-one has the proper details already tabulated it is relatively easy, albeit tedious to work things out from scratch rather than resorting to trial and error.

With the leadscrew stationary measure the distance the saddle moves for one turn of the dial with each gear fitted. One turn of the dial allows any thread whose pitch divides exactly (ie whole numbers, no decimals or fractions) into the distance travelled to be cut by re-engaging the half nuts at whole turn intervals. The same rule applies for part turns of the dial so, for example, if you want to use three graduations the pitch has to divide exactly into one third of the measured distance.

I suspect that your dial has three sets of graduations one even numbered, probably 4 or 8, one set of 3 and one set of 5. I'm told the simplest metric set-up for one set of graduations is 12 with a 4 mm pitch leadscrew and 4 gears.

Clive

Even using whole turns of the thread dial by always engaging the half nut on the same graduation mark won't allow you to cut all metric threads with a single indicator gear. You are still limited to thread pitches that divide exactly into the saddle travel for one turn of the dial.

This problem is inherent to metric threads being rationalised directly by pitch rather than turns per unit length like imperial. The thread dial is an analogue calculator which adds the effects of spindle travel and saddle movement to indicate the point where the tool into alignment with the thread. Obviously if the spindle and feed screw are stationary moving the saddle brings the tool into and out of alignment with the thread. The TDI being geared directly to the screw acts like a mechanical dial caliper where a rack and pinion system converts linear movement to turns of a dial. Hence the TDI dial is effectively calibrated in units of thread pitch which clearly only works for a whole number of divisions on the dial. When the spindle is turning the alignment point effectively moves back and forth along the bed hitting the same point once per rotation. As the spindle drives the leadscrew the dial indictates the distance by which the alignment point has shifted during any fraction of a spindle turn. Again this is only useful if one turn of the dial represents a whole number of pitches.

When actually screwcutting the dial is stationary on whatever mark you have selected. Theoretically you could always start the cut from the same place on the bed and same rotational position of the spindle but that is impractical for a human using the half nuts. In practice you engage the half nuts at a fairly random position on the bed and the dial calculates the rotational offset needed to bring tool and thread into alignment at that point so you can engage the half nuts. Electronic leadscrew drive systems can handle odd fractions of a turn but humans need whole numbers and not too many at that.

Clive

Engage anywhere John ?