Lead Screw Machining
The first thing I did over the past week, was machine my lead screw on the Lathe. The desired dimensions and tolerances were as shown in the drawing below. After machining one end, I cut the lead screw to the appropriate length using the parting tool and machined the other end accordingly.
The two diameter measurements, one at the tip of the shaft and the other at the collet end are shown above. The desired h7 tolerance was 7.98mm. I was really content with the end result. A nice sliding fit was obtained.
Linear Slider Machining
After some metal work, it was time to head back to the Hobby Shop to continue wood working. The next build item was the linear slider.
Given the length of the slider, drilling a perfectly straight hole through the middle would have been challenging for two reasons:
1. Limitation in drill bit size and length
2. Drilling through end grain of wood is tricky since the fibres may not split apart equally. This could cause the hole centre to drift as the drill bit plunges through.
In order to overcome these challenges, I machined the block as two halves, split along the length of the block. A slot was cut on each half using the table saw such that when glued together the two slots would form a square hole of side 14mm all the way through the length of the block.
Once the glue had dried, I used the Milling Machine to drill the hole to mount the lead nut. The diameter of the lead nut is 28mm. The closes drill bit in the shop was 1-1/8 " size (28.58mm). The clearance is actually a good thing as it would allow me to adjust the height of the nut in the block, before fastening, to remove any vertical parallelism between the actuator mounts and the lead nut.
Next, I used the jointer to shave off a extremely thin layers off the block, such that it would slide smooothly through the rail. While I intended for a total clearance of 0.50mm, this was not achieved due to manufacturing error in the rail. While the two spacer plates of the rails are not perfectly parallel due to an unfortunate incident while gluing them last week. At some portions, the total clearance is 0.5mm while at others its almost 1mm. For the maximum case of 1mm, the max yaw angle of the slider is 0.27deg. The results in a table tip displacement of 2.3mm. The original apportionment of geometric error was 1.15mm. This result was unacceptable. Preloading the slider will be necessary!!
Personal reflection: Precision manufacture of large structures is really difficult! While I was able to achieve the required error when prototyping my smaller slider from week 4, a careless mistake on my part cost me greatly for the scaled up version.
Actuator Mounts Machining:
Finally, I machined the brackets to mount the self-aligning bearing at the top of the rail and the roller bearing at the bottom of the rail.
The pieces for the bottom bearing block were waterjetted. However given the $3/min cost of waterjetting, I found it not feasible to jet the other block. Instead I milled the pieces for the other block.
After tapping the holes for connecting the two pieces, the blocks were assembled. Slots on the bearing mounting plate allow for adjustment of the bearing height to minimize vertical parallelism. To minimize horizontal parallelism, a spacer block will be used to space both bearing block from the inner face of the rail before being secured to the backplate of the rail. As the blocks will be secured with a bolt and nut, by drilling clearance holes on the backplate, there is some room to make very small adjustments to the horizontal position of the block before tightening the nut.
In the coming week, I hope to add preload to the slider and perform a repeatability test of slider with the stepper motor.