Weekly Hardware
Week 3:
Precision Linear Axis
This is the Precision Linear Axis that I built and tested.
The Process:
My objective was to fabricate a box rail and slider system with a radial clearance of 0.25mm. I was inspired by Professor Slocum's simple yet effective wood working technique of using paper as a spacer for achieving desired precision. I had to try this out for myself! For my prototype, the length and width of the slider were the same values as estimated from the apportioned error. (See Brainware 3)
After sketching the rail system (see Brainware 2), I headed to the hobby shop to scrounge material for the prototype. I was lucky to find some birch plywood that would work very nicely for the design. After making some markings, I used the table saw to cut out strips of the 6 pieces I needed. I measured the thickness of paper to be 0.002".
Next, I glued one of the side rails to the base, with a single sheet of paper in-between. I then aligned the 3.5" wide x 11.5" long slider to this rail. I then glued and clamped the other rail to the slider, with two sheets of paper in between. A sheet of paper was also placed in between the second rail and the base. This allowed for the two rails to be perfectly parallel with 0.002" of radial clearance between the slider and each rail. Once the glue dried, I removed the slider and shaved 1/64" off the width using the planing machine. Now, my total radial clearance was 0.002" + 1/128" =0.0098"=0.25mm, which was exactly what I wanted.
After this, I glued another layer of paper to the top of the two rails. I then glued and clamped the guard rails that would keep the slider constrained to the plane of motion.
Once the glue had dried, I applied some wax to the inner sides of the rail. After planing the outer edges, my precision linear axis was ready was testing! Overall, I was amazed by the accuracy achievable from the technique used. The slider operated extremely smoothly, with almost no angular deflection. The aspect ratio of the slider is about 1:3 which is in agreement with Saint Venant's Principle.
Repeatability Test:
In order to test repeatability, I clamped the box rail to a table, hot-glued a laser pointer to the slider, taped a sheet of paper to a wall 630cm away, and moved the slider out and into the rail (up to a fixed final position) while recording the position of the centre of the laser pointer on the sheet of paper. I repeated this 10 times. The results are as shown below.
The maximum spread was measured to be 3/32". Knowing the final distance between the laser pointer and the wall, the angular error of the precision linear axis was estimated to be 0.0217deg. I was overjoyed by this result.
Thus, a precision linear axis was successfully designed, fabricated and tested for repeatability.