I was aware of this library but hadn't explored further. I'll take a look and try it out.

By definition, voltage is the potential to supply current.  Higher voltage has the ability to supply more current, faster... especially in a PWM controlled way which is what the A4988 is doing.

That short definition of voltage and current was surprising useful. I do not have an electronics background and it hit the spot with improving my very basic understanding. 👍

Stationary Noise

Yes, the full current setting is constantly being applied even with the motor stopped.  In fact from my tests, I found that the motor will be drawing more current when stopped than when at full speed (even multi-thousand rpms).  In your project, you should look into controlling the Enable pin and turning off the motor when not opening/closing a gate.  If you run it anywhere near max A4988 current, you will find it gets very hot besides wasting a lot of electricity.

🙄😊 Such an obvious thing to do and yet didn't occur to me. Thanks for the suggestion, I will definitely implement it.

The guillotine design gates I have are the same as those shown in the reply from @zander.

I never did this as I went with an Oneida system and used the heavy gauge large radius pipe after I saw what happened to my brothers shop. Remember things like fine saw dust, flour, other fines can build up static charges from rubbing up against other fine particles. At some point it will discharge, 99%+ with no damage BUT eventually you may experience the 1% so give the excess charge an easy path to ground.

@zander I think I will look into this a little further with my engineer friend. This area was his living for many years, so I’m sure he will have an opinion. But I’ll shelve it for now so as not to detract for the current project.

Yes, an impressive project and perfect for a dronebotworkshop project!

Anyway... I do know that steppers skip steps, so your limit switches are redundant.

Skipping Steps

I failed to fully qualify my statement.  I'm a little out practice of discussing technical aspects since my leaving the forum.  @davee comments are more expletive.  Stepper motors are completely reliable (as you surmised with hard disks and 3D printers).  They miss steps under one mechanical condition - being asked to supply more torque than the driver's current setting can provide.  As @davee mentioned this happens in several situations in your design.

1. can't overcome friction of the sliding mechanism (bad)
2. has reached the stop and still has steps commanded (good)
3. trying to drive it too fast and the inertia of the design is requiring too much torque trying to accelerate it  (bad)

This last problem can be handled by using AccelStepper (Arduino Library) to accelerate instead of simply trying to drive it instantly to some set speed.  If this is your problem, you might find that if you try different speeds, you'll reach a threshold that starts skipping steps.  Also as @davee suggested, using micro stepping also helps.  

Voltage

By definition, voltage is the potential to supply current.  Higher voltage has the ability to supply more current, faster... especially in a PWM controlled way which is what the A4988 is doing.  @davee can explain it far better in EE terms, but in practical terms using the A4988 or DRV8825, I've always seen better performance at higher voltage.  The A4988 can utilize up to 35 volts.  The DRV8825 up to 45V.  I don't really understand why they post voltage on steppers since it is entirely controlled by the stepper driver and current limit is the only significant value on the stepper motor.

Something else I noticed yesterday was wining coming from the motor when powered up but not moving. I varied the current limiter on the bench power supply and it did change the frequency of the hum.

Stationary Noise

Yes, the full current setting is constantly being applied even with the motor stopped.  In fact from my tests, I found that the motor will be drawing more current when stopped than when at full speed (even multi-thousand rpms).  In your project, you should look into controlling the Enable pin and turning off the motor when not opening/closing a gate.  If you run it anywhere near max A4988 current, you will find it gets very hot besides wasting a lot of electricity.

Suggestion

I didn't see any design, but if you really meant a guillotine like design, you have to convert rotational to linear movement... requiring some mechanism.  A crank, connecting rod is cumbersome.  A geared (rack and pinion) can present a contamination problem.  Since you mentioned having a 3D printer, making a design using curves shouldn't be a problem.  Something like a direct drive like this is simple mechanically.  Just a thought.

I never did this as I went with an Oneida system and used the heavy gauge large radius pipe after I saw what happened to my brothers shop. Remember things like fine saw dust, flour, other fines can build up static charges from rubbing up against other fine particles. At some point it will discharge, 99%+ with no damage BUT eventually you may experience the 1% so give the excess charge an easy path to ground.

The company I used to work for was owned by a company that designed and developed intrinsically safe products. I'm still in contact with one of the engineers, also retired, and will sound him out about this.

Thanks for your suggestions, I will keep returning to this until all possible avenues have been exhausted. I will, however stay with the current scheme to keep the project moving along. 

I did try a few experiments with current and step frequency when I first tried the motors, but all gave inconclusive results. I think I will try out your suggestions that I can, presently, and then repeat the results and publish them for public perusal.

I haven't tried micro stepping as yet, but intend doing so at some point. If this might give an indication of problem area, I'll try it sooner than later.

Something else I noticed yesterday was wining coming from the motor when powered up but not moving. I varied the current limiter on the bench power supply and it did change the frequency of the hum. 🤔

If the pipe is NOT PVC then wrap the pipe in braided copper. Putting it inside leads to jams and does a poor job of grounding.

Good luck.

Ha, read this in a final peruse around just before my bed time, which got me thinking where I got the inside the pipe thought from.  Just what I don't need as now I may be going to dream of blasted pipe explosions.  Anyway this is from a quote from a woodworking forum with some links should anyone be daft enough to want to wade through them.   Whether the opinion has any merits I don't venture an opinion as I'm not going to be doing any grounding in my shop.

So I quote:

"An Electrician did a video about installing an extraction system and he specifically said not to put the wire on the outside as it won't do anything (has to go inside)."

https://www.youtube.com/watch?v=812Ez9OT0As
https://www.youtube.com/watch?v=9oFCOlYXx5c

Ok end quote, and bloody heck is that the time 😮 

If the pipe is NOT PVC then wrap the pipe in braided copper. Putting it inside leads to jams and does a poor job of grounding.

Good luck.