Planning the railroad (or how to make a saw carriage that moves)

I've decided that the best thing to model first would be the saw mill and shop areas of the logging railroad. But rather than having a stationary saw mill, I'd like to make an O scale mill where all the moving parts move.  For the most part, getting the belts and wheels to rotate is fairly straight forward, the only complicated part is properly positioning everything (wheels can be made from styrene, or purchased, and the belts can be rubber bands).

What's hanging me up is the back and forth motion required for the saw carriage.

You see, the saw carriage would hold a large log in place and push it through the circular saw and then would be pushed back to repeat the process.  This process was powered by the belt system and operated by a lever that would be controlled by the saw operator.  Thus, I need a simple back and forth operation in the span of a length of lumber.  Typically, lengths would be 8, 10, 12, 16, and 20 feet.  I want to be able to cut 16 foot piece of lumber which in O scale ends up being 4 inches.the carriage needs to move twice that length plus about 2 inches to clear the saw, so I need a 10 inch movement total.

After thinking about it, I have four solutions:

 

Option 1 is to have a vertically mounted DC electric motor power a large wheel (actually 10 inch diameter, not 8 inch).  This wheel would then move a piston via the piston arm.  The result would be turning the single rotational direction of the motor into a back and forth operation at the piston.  The piston itself would be mounted to the underside of the carriage.  To adjust speed I would have to adjust the voltage to the motor.  This option is simple, but I would need a lot of space to make it work, about 10 x 22 inches.  I can be done, but that's pretty big!

 

 

Option 2 would work for a higher rpm motor because rather than directly driving the wheel via a friction wheel, this set up uses a worm gear.  The downside is that the wheel would in fact need to be a large gear with hundreds of teeth.  It is possible, but would be time consuming to construct something like this, and would probably have to be done with a large plate of steel. Also, it doesn't reduce the necessary area for this machine to work as I would still need a 10 x 22 inch space.

 

Option 3 is much more compact and utilizes what is called a "self reversing screw".  Rather than explain it's operation, here's a video from ABSSAC Industries:

 

 Rather than using a wheel or gear to make a piston-type motion, the self reversing screw works by running in a channel carved into a rod.  When the block gets to the end of the channel, it is forced into the second channel, running the other way.  This operation is bullet proof, except I would need to find someone who can make such a machine in either metal or through 3D printed plastic.

 

And finally here is Option 4.  Rather than using mechanical means to reverse the screw, I could use electric means via a time delay polarity reversing circuit.  Ever couple of seconds (or however long the time is set for) the circuit board will reverse the polarity headed to the DC motor, thus causing it to run in the opposite direction.  The motor would power a long screw which would basically be a piece of all-thread that is 3/8 or 1/2 inch in size and long enough for the saw carriage to move.  The all-thread would be moving a nut up and down it's length which would then be attached to the log carriage.

Options 3 and 4 take up much less space, requiring about a 2 by 12 inch area to work properly.  option 4 may just be the cheapest of the 4 presented.