I have an application where a set of large rollers pulls product through a process. The machine is drive by a 1750 rpm 5hp motor, but is geared down and the final shaft rpm is 0.2RPM. For some products, we run this process as low as 0.1RPM or as fast as 0.75RPM.

The process rate is typically stated in inches per minute and the operator is physically measuring this rate with a tape measure over the course of a minute. I want to measure the RPM at the axle and use this as the reference, or ideally put together a device that can compute and display the actual linear rate based on this RPM.

Can you recommend any bolt-on RPM measuring gauge that will function between 0- 1 RPM?

 

I would make it with a digital input in combination with a proximity sensor and some metal knock on the axis that has a certain and defined length. You can also make a vane on the axis and let that pass a photocell.
In the PLC you detect the upgoing edge of the digital signal. You start a counter and wait until the digital input has a edge down signal. The time between the upgoing and downgoing of the signal is linear with the speed (if it did not change during the time measurement).
If I would have to make it, I would use a Pro-face GP4116 touchpanel connected via Modbus RTU to a Proconel PM16 digital input module. Programming in Pro-face GP Pro EX. You can also use a GP4201TAD touchpanel with the PM16DI connected to the RS485 modbus RTU interface and on the ethernet port you can for example put the measured value that is read-out by another system via Modbus TCP.
You could also connect an analog output module and send the measurement out as for example a 4-20mA signal.
Lots of possibilities and solutions.
Pro-face touchpanels: https://cascade.net/en/visualisation/
PM16DI input module: https://cascade.net/en/modular-plcs/