Putting a 1/4-20" mount on a NEMA stepper motor

[Rylee Isitt]

I have recently discovered some parts that may help some folks who are currently in the process of building/refining a stepper-motor driven stacking rig.

I would like to draw your attention to two parts being sold at servocity that will allow you to add a 1/4"-20 mounting hole to a NEMA stepper motor. 1/4"-20 is common for both camera equipment (tripod and rigging mounting holes) and Newport/Thorlabs/Melles Griot/etc "imperial" threaded optical equipment.

The parts in question are the NEMA motor mount, and the 1/4"-20 screw plate.

Those parts require some 6-32 machine screws to connect them to each other. I would suggest the 3/16" screws since in my experience the 1/4" screws jutted out ever so slightly, but that's not a huge issue either as you can cut/file the screws down or use washers as spacers. The NEMA motor mount comes with the screws needed to connect it to your motor, but they are short screws so may not be suitable for an attachment to the rear of the motor if the rear screws are expected to hold the motor together.

You want to attach the motor mount to the end of the stepper motor opposite the shaft. You can either attach it to the rear of the motor, or reverse the rotor and stick the mount on the front of the motor. With my motor, the rear screws held the motor together by reaching through all the way to the front screw holes, and the rear screw holes were unthreaded. The screws I had were not long enough to make it all the way through the motor to the front while also holding the NEMA mount in place. Rather than take a trip into town to buy longer screws, I opened up the stepper motor and reversed the rotor so that the shaft now stuck out of the rear of the motor. It is clear that my motor was meant to do this, but I cannot promise that all steppers are designed in such a way. This allowed me to attach the mount to the front of the motor without interfering with the motor shaft.

Once you've got a 1/4"-20 mount on your stepper, you can easily attach your motor to lots of stuff - camera rigging, clamps/plates, optomechanical equipment, Manfrotto magic arms, etc.

I tried sticking a stepper directly on a knock-off variable friction arm. This worked but was a bit fiddly. An actual Manfrotto Magic arm, or something of similar or better quality might be a great way to position a stepper into almost any place you need it.

I put mine on an Arca-swiss compatible (Sunwayfoto) discal clamp. Then I attached an Arca-swiss compatible rail in a vertical orientation to an L-bracket, in turn attached to a pair of cheap linear stages forming an x/y stage. The linear stages are screwed down onto a heavy metal base with rubber feet.

Once the motor on the clamp is clamped down to the rail, it can be moved in 3 dimensions, allowing it to be attached to a variety of microscopes and focusing blocks by just adjusting the motor position. The one linear stage on the base that moves the motor from side to side is useful as it applies tension to a timing belt that I use, with timing pulleys, to couple and gear down the motor to the focus knob. The second linear stage could be omitted, but I had it, and it adds weight to the base, so I used it.

I keep the contraption that holds the motor itself physically disconnected from the rig, and I use rubber feet. This helps prevent the transmission of vibration from the stepper to either the subject or camera.

I don't know how helpful this is... but there you have it. It might save you from having to make your own adapter.

I can post some photos later, when I get home...

[GemBro]

Great idea Rylee ... like to see the setup you have for this ...

At the moment my NEMA 17 mouting plate is resting on a spring damping plate (due to the current wobble issue) but also in a similar way you have done with the Arca/Swiss plates, on my inverted system ... I don't have 3 directions only 2 ... one for the stepper in/out from the focus control knob and then the focus block movement is done along the main inverted Arca/Swiss plate, to line up the stepper shaft axis ... the 3rd movment, I have winged it with padding (on top of the damping spring) at the moment but once I get the height right I won't need that movment as that will be permanent fix for my setup once the wobble has gone ... I would then probably drill a hole through the NEMA stepper mounting plate and then use a 1/2 20 screw and secure that to the Arca/Swiss plate's thread below ...

Bearing in mind there is no difference with StackShot (motor & focusing on the same rail system ... so advantages of seperation from the Rig? ... less settle down time, due to stepper movment vibration? ... maybe the fact mine is on a damper maybe reducing this already? ...

Regarding the stepper being off the Rig ... I know Rik has done this in the past and a few others ... but if there is error on anything then wobble will still be present ... so what's your thoughts and experience of it being seperate from the Rig? ...

I need to research this further ...

[Rylee Isitt]

Gem,

The pulley on my motor is not particularly well-centered because I enlarged the bore with a handheld drill. It's actually obviously crooked and you can see this in one of the images below. You want a drill press or lathe if you do this and spot-on centering.

However, this does not cause wobble as far as the focus block or camera are concerned. I could post a video of motor advancing through an image at 10x perhaps.

The block is a BHMJ and very sturdy... tugging on the fine focus knob doesn't move the camera around.

Does your camera wobble when you press gently on the focus knob without turning it? If your block is a bit less sturdy, you might want to take extra steps to make sure your connections are very well centered. However, I'd be a bit surprised if wobbling the focus knob would wobble the camera relative to the rest of the focus block. I am more inclined to think that the entire block is moving, or that vibration is conducting through the rig because of the direct coupling.

Here are photos of that part of my rig.

Here's the base that the camera sits on, you can see the timing pulley that operates it:


The NEMA 17 stepper mount with a 1/4"-20 mounting hole:


Attached to the motor:


Now with the motor on the Discal clamp:


Here's the motor stand:


And with the motor attached:


And finally, this is how the motor is coupled with the focus block via a timing belt, which is tensioned by adjusting one of the linear stages (the one that moves side-to-side) on the motor stand:

[Chris S.]

This helps prevent the transmission of vibration from the stepper to either the subject or camera.

Rylee, I'm a bit surprised that your stepping motor creates vibration that you would need to isolate against. You'll likely recall that I also use a timing belt drive, but have the motor mounted on the focus block. I don't see any vibration in live view, even with the screen zoomed in and looking through 100x objective.

My experience is limited to three or four models of stepping motor. Does yours behave very differently?

I do keep the motor's torque (current, really) turned way down--we don't need much torque for what we're doing, and this keeps the heat production down. Ditto for holding torque (meaning that with a StackShot controller, I don't use precision mode)--I don't need much of it, and it increases heat production. (Also, do I recall a thread in which you said you weren't sold on features like ramp speed? I find using a gentle ramp up decreases runout, the official term for wobbliness in the motor's drive shaft, which can contribute to a wobbly stacking experience.)

There are a number of focus blocks lurking around my work table, waiting for me to get around to motorizing them. I intend to mount all the motors on the focus blocks--I see no downside, and think it makes for a compact, robust integration.

(A tip for anyone who follows Rylee's nicely illustrated example and builds a timing belt drive: When tensioning the belt, avoid making it overly tight. This can transmit runout from the motor's drive shaft to the fine focus shaft, causing wobbliness. Try loosening tension until the belt begins to slip, then tighten a little bit until the slipping goes away.)

Cheers,

--Chris

[Rylee Isitt]

Chris,

It all depends on the drive mode. In the simplest drive mode, and the one that my controller uses, windings are either on or off. The vibrations are caused by the sudden start-stop-start-stop of the motor in this drive mode. I don't know if the vibrations would actually transmit through to the camera and cause problems as I have yet to make an adapter plate to directly attach the motor to the block.

These vibrations can be greatly reduced electronically by easing the transition between steps and there's a chance your controller board (assuming you use one) is doing this. It can done via PWM but I believe more advanced controller boards can regulate current in an analog way. One really great way is sinusoidal current drive. Microstepping would do it to, to a lesser extent. If you're using a motor controller board, you wouldn't necessarily know if it was doing this, but the feature should be advertised in the board's specifications.

That said, the wobbliness we're discussing here (as far as I know) is not in the direction of stacking, but rather Gem's rig seems to be moving around in all directions based on his description. I don't know for sure since I experience no wobbliness in my rig... Gem will have to chime in here.

I am driving my stepper at its full rated current, 740mA peak when both windings are on. I have not experienced any heat issues. I am working on a more advanced controller that can drive multiple motors and different types of motors, and might consider a motor driver board for this if I can find one that does what I need. It would offload a lot of complex code to a separate, ready-to-go board. I think you guys are slowly starting to win me over on that idea. Either that or I will program in PWM-based sinusoidal current drive or microstepping... time to think about it.

[Rylee Isitt]

Chris,

Here's a better article (Wikipedia) on the different drive modes.

The ones I use are the second and third mentioned (full step drive, and half stepping) but these cause vibration.

I will experiment with microstepping and/or sinusoidal current via PWM as folks who build my project may directly couple the stepper to the block and vibrations could make life difficult in that circumstance.