Rotary Table Digital Readout

I have one of the 6" rotary tables of generic design which can be mounted on edge as a dividing head.
For various reasons, it would be advantageous for me to have a digital readout for the table angle.
I had expected to find suitable kits readily available, as they are for linear readouts - but nothing came up in my searches.
I found a rotary encoder with remote display available, but the encoder is a 37mm high box which would require lifting the rotary table high off the mill bed and reducing available part and tool height capacity.

So - I had a look around to see what I could adapt. What I found was an inexpensive "digital level / angle finder", the small box type, with a tilting display - various brands available...


This was actually something I had for several years, but it never worked correctly after I dropped it. With nothing to lose, I set about dismantling it.
Inside, the 'heart' of the device consists of a pendulum and an inductive encoder mounted on a pair of PCBs.
The front half of the box houses the two AA batteries and the display, which is connected to the main PCB by a short ribbon cable. In the photo I cave cut the ribbon cable and battery wires to seperate the encoder board out.


The pendulum is attached to the encoder rotor with an M3 nut (on the back side) and is easily removed.

Mechanical installation to the rotary table requires a simple aluminium 'slug' to press into the bottom end of the table's hollow spindle, with an M3 hole tapped centrally.
To mount the PCB I cut a disc from some 2mm rigid plastic sheet (I happened to have some clear sheet, which is exceedingly brittle!). The disc is cut out to clear the smaller of the sandwiched pair of PCBs and holes drilled to mount the larger PCB with M2 screws and nuts.

The resulting assembly protrudes around 4mm from the underside of the rotary table, so I am making a simple rectangular aluminium sub-plate, recessed where required to clear the encoder PCB and to create a channel for the ribbon cable connecting to the display. The display will also be mounted on the sub-plate.

The origianl ribbon cable has 8 conductors connecting the encoder PCB to the display. I replaced this with a 10 way ribbon cable, which also carries the connections from the batteries in the display box to the encoder PCB.

The on/off switch is mounted on the encoder PCB. I attempted to create a remote switch, but without success. I suspect the switch on the PCB in not simply a normally open 'push to make' type.
I have decided to go for a slightly Heath-Robinson solution, with a pushrod in a groove in the sub-plate to operate the switch.

I'll post more photos soon. I just need to machine the sub-plate.

Questions:

• Do you want to use it in two or one orientation? Parallel to the X or Y table as A or B axis the pendulum thing would work.

If you want to use it flat as C axis not so much.

• required accuracy, 5000 line encoder good enough?
• Indirect reading with the encoder on the hand crank good enough? (does not account for backlash and does not see if worm is disengaged, but rel. cheap and easy to attach two small timing belt pulleys and a 5V encoder)
• Using the counts of a stepper motor good enough? https://www.homemodelenginemachinist.com/threads/arduino-rotary-table-for-dummies.26744/

I put a stepper motor for dividing on the thing and for what I want the indirect method to count the turns of the worm is "good enough", very nice for hole pattern drilling, or gear cutting.

I wonder if it is feasible to print (or mill) a timing belt ring gear to clamp on the circumfence of the table. I milled 5mm pitch HTD pulleys long time ago that was doable. Then a normal encoder could be attached to keep track with a timing belt.

The visual indication could be then done with this. https://www.touchdro.com/ (note that the owner of this company also posted a DIY guide, so who thinks it is too expensive can make it)

Greetings Timo

Hi Timo,

I am not using the pendulum. I have attached the encoder directly to the rotary table spindle, so there is no backlash and it will work in any orientation.

The encoder and readout gives 0.1 degree increments. I will check the accuracy and repeatability against the scale on the handle.

I can still use the handle of I need higher accuracy, but the DRO gives me an easy reference and the ability to set 0 at any position.

Peter Twissell said:

Hi Timo,

I am not using the pendulum. I have attached the encoder directly to the rotary table spindle, so there is no backlash and it will work in any orientation.

The encoder and readout gives 0.1 degree increments. I will check the accuracy and repeatability against the scale on the handle.

I can still use the handle of I need higher accuracy, but the DRO gives me an easy reference and the ability to set 0 at any position.

Click to expand...

Ah now I get it. The pendulum is not making the signal, it is just rotating the other disk. I thought you needed also some display.
The Stepper solution is very handy for doing things automatic or semi automatic.

Interesting project; I'll be watching.

Would this be an alternative (will need some modification, but has most of the parts):

That would be an alternative, but it's different to what I'm doing.
I am using a cheap device to give me a direct readout of the angle of rotation of the table to an accuracy of approx 0.1 degrees, which is adequate for most purposes.
The kit shown reads from an encoder on the worm, so it will not eliminate the effect of any mechanical backlash between the worm and the table.

Timo,
I am using the display from the original device.

This is the arrangement of the encoder PCB, mounted the my plastic disc and connected to the half of the original box which houses the display and batteries. The whole lot is sat on the aluminium base plate.


The underside of the rotary table with aluminium plug, tapped M3 for mounting the encoder rotor.

And the finished article.

Under the display can be seen the end of the pushrod which operates the on/off/zero button on the encoder PCB.

This is so smart! I get the concept but not the details quite yet.
Is the aluminum disc (1) basically attached to the rotating RT? What is the little peg (2) for?

Hi Petertha,
Thanks!
The aluminium disc is part of the original device. It is attached to the PCB and houses the bearings which support the encoder rotor. An M3 screw through the rotor and bearings attached the rotor to the table spindle.
The little peg is the end of the pushrod which operates the on/off/zero switch on the encoder PCB.

I've just been running some tests.
At first, I thought something was wrong with the encoder or its installation.
For the first 100 degrees or so of rotation, the readout aligned perfectly to the handwheel dial.
Over the next few turns of the handwheel, the readings drifted apart until they were 1.5 degrees different at the 180 degree mark.
Continuing round back to the start position, the reading came back into alignment.
I repeated the test, but this time used an additional means of measurement - a digital protractor - to see what was going wrong.
As it turns out, my readout is correct, but there is a lot of wear in one area of the worm wheel in the rotary table.
This discovery may explain some issues I have had in the past.

Although the readout is accurate to only 0.1 degrees, it is a lot better than the dial reading through the heavily worn gear.

You've gotta love it when you can finally come up with a solution to a problem you are having. Especially when the solution shows you hadn't been the cause of the problem.

ddmckee54 said:

You've gotta love it when you can finally come up with a solution to a problem you are having. Especially when the solution shows you hadn't been the cause of the problem.

Click to expand...

Absolutely! Beyond impressed.

I consider myself pretty handy, but I am beyond impressed not only by the project, but using it to identify a problem that wasn't apparent before.

Wonderfully done!

What a great idea!
Thanks for the write up.
Well done!
Scott

I was wondering what you did to get 360 degrees of rotation out of something that wasn't designed that way. But I guess when you took off the pendulum that problem went away didn't it. I've got a small rotary table stashed around here somewhere, I may have to "borrow" your idea.

The device has no to rotational limit, but the readout goes from zero to 180 degrees, then counts back down to zero over the next 180 degrees.
This is not a problem for me, as most of the parts I make are from my own drawings and I can draw parts with angles dimensioned to suit.
You are welcome to copy my process as you wish. That's why I posted it!