encoder trouble

I have a cui amt102 encoder belt driven 1:1 to a spindle. The pyvcp rpm meter bounces at spindle stop, sometimes hold steady at 0.1 or 36.0, other times bounces around crazily. Also the rpm readings are way off
200 indicated = 410 true rpm
350 = 730
480 = 1010
The spindle speed is controlled manually by rotary switch. I suspect it has something to do with my iffy wiring job. Other possibilities ?

Does the encoder have a high definition? For the spindle is not a very good idea, because when the spindle is stopped it can be between two encoder pulses and giving you the bounce.

Regarding your values, they are almost the double of what you were expecting, so probably your scale is not correct.

I am assuming that you are using the encoder.n.velocity pin
as the source of the spindle speed.

Usually a low pass filter is needed to make the spindle
speed readings stable enough to be comfortably viewed.

There will always be some jitter in encoder velocity estimation
due to finite base thread to servo thread ratio (for software encoders)

You may be able to eliminate the noise when the spindle is stopped by setting the
encoder.N.min-velocity-estimate parameter (assuming this is a software encoder)

I was missing a basic concept thinking the RPM was being read from the index signal. Scale was set to 100 and the dip switches on the encoder are set to 48 ppr.

Yes, the encoder is being read in software. A fluctuating RPM display isn't a major issue at this stage as long it doesn't also mean poor thread cutting etc. I hope to try a few test runs today.