The following description of my DIY focus rail is meant to be a general guide for using draw runners as a cheap linear bearing construction component, the idea is not my own it was given to me by my brother (who also has a background in mechanical engineering),
I had problems sourcing units of the right size, normally the units used for kitchen draws and the like are far too large, but at a local DIY supermarket I found some small units, these are made by a German company called STABILIT® part nr. 474.02.013, length is 182mm (closed) these are made from steel (no plastic parts) and are really smooth, with no noticeable play in vertical or horizontal directions , they are composed of two sets of six steel balls and are listed as having a maximum working load of 10kg, more than enough for most camera/lens combinations.
I decided to keep the construction as low-tech as possible, partly from the point of view of price but also so that others may repeat or at least use the basic idea in their own constructions.
The only parts I needed to buy were the tape measure, M10 threaded rod, bearing and the runners, everything else was made from what I had available.
The side plates, end plates and runner plates are all made from 12mm film faced plywood, you should be able to find this material at your local well-stocked DIY store, you won’t need much, so you should be able to find off-cuts if you are lucky, MDF could also be used, but would need to be of a dimension that allows screwing into cut surfaces, I pre-drilled all screw holes with a drill which had the core diameter of the screws used, this was to prevent splitting in this quite thin material.
Although there are several parts that I have turned on a lathe or milled it would be feasible to construct the unit using hand tools only, an electric drill(with stand if possible) and a jigsaw would suffice, also the “nut” that I turned from POM plastic could have been rectangular in shape, the difference would be mainly cosmetic, the unit should work just as well if care is taken with precision.
The following dimensions are for the unit I built and could easily be modified for other rails/runners:
Total movement: 164mm
The unit consist of the following parts:
Two side plates, no machining necessary.
Two end plates, plate at winding handle end is countersunk to retain a cheap Chinese 6000 2RS bearing (held in place by a turned metal ring, again this could just as easily have been a rectangular piece of sheet metal). The plate at the opposite end has a cut-out to allow movement of the carriage, this could possibly be replaced by two bars (one top, one bottom), untried, this could cause stability problems (less stiff).
Two carriage plates, on my construction these have been recessed to improve stability, but this is not absolutely necessary, just make sure that any screws that are used for attaching the runners are a tight fit.
A tripod attachment plate, made from 4mm steel plate, thickness is dimensioned to allow for a 1/4” Whitworth thread direct in the plate, this could be made from other material, just keep in mind that you will then have to find another manner of fixing a stable threaded section (insert?), I drilled some large holes in the plate to save weight, these holes have no other function and could be left out if required,
A camera fixing plate, made from 3mm steel plate, again this is not critical and could be made from other materials, the most important thing to remember here is that it is necessary to have risers of some kind ( in my construction 10mm aluminium) so that the carriage movement is raised above the side plates, I had to do some machining to the knob that I made with the 1/4”Whitworth thread for camera attachment, but this could just as easily have used a plastic gear wheel with a screw glued in position or something similar, you will have to use your own inventiveness here! To finish off the fixing plate a piece of 2mm black rubber sheeting was glued in place with contact adhesive.
The threaded rod (M10) needs no machining , just cutting to length and deburring, it is held in place through the bearing by two low-profile M10 nuts, one on each side, Loctite was used to secure the locking.
The plastic nut, (length 37mm, outside diameter 40mm threaded internally M10), turned from POM plastic, nylon or similar plastics could be used, this part could easily be made from rectangular material and drilled, if so make sure that the drilling is done on a pillar drill, it is extremely important that the hole for the M10 thread is exactly in the middle! The nut has two 5mm pins opposite each other extending from its middle line, these are inserted into 5mm holes drilled into the carriage plates, this transfers the nuts movement to the carriage, because the threaded rod is only supported by the 6000 2RS bearing and the plastic nut the connection is semi-floating, allowing the construction to take out any out-of-trueness in the moving parts, this works really well, and movement is exceptionally smooth, the length of threading in the nut helps to remove any play between the nut and rod,
A winding handle with a small knob was also turned from POM plastic, this could be replaced by a radio knob or similar, inventiveness again!
I finally added a tape measure to the top edge of the right hand side plate and a pointer to the camera attachment plate, this aids in moving the carriage in set increments, this can of course be done using the winding handle (1,5mm per revolution) but it is easy to lose count!
The total cost was about £10 for parts, you will have to be inventive if you wish to make a unit using this method but even if you had to buy everything you should be able to keep the price to an acceptable level, although of course sourcing things like the plastic could be a problem, so lateral thinking will be required!
And finally, the first result from a 10 image stack using the focus rail.