I almost finished the linear actuator I have been working on for the last day or so. I need a precision linear actuator for a project that I'm working on, needs to hold 0.01 mm. I originally tried an air cylinder and a hard stop, but using enough air pressure to just move the air cylinder proved to put too much pressure on the material I was trying to feed with it. So onto plan ''B'', a precision ball lead screw and a stepper motor. I just happened to have a few ball screws/nuts on the shelf that I picked up at an auction. This one is 20 mm dia, 10 mm pitch, by 1 meter long.
Due the the way the feeder has to operate, the ball screw can not turn, so I have to turn the nut. This requires that the nut be captured in bearings, both radial and thrust. Then also requires a drive mechanism.
So first the bearing housing starts out as a 4x4x5 inch block of aluminum. Block squared up and cover plate bolt holes and a pilot hole drilled for the pocketing operation.
Pocketing the pulley clearance.
Pocketing the roller bearing pocket, and the thrust bearing pocket below. By the time this is done the end mill will be reaching down 3.875 deep to pocket for the thrust bearing, then through for the screw clearance. I used a 1/2 x 3 inch end mill for the first part of the operation then switched to a 3/4 x 4 to reach the bottom.
Then to the boring head to final bore for the bearing. The bearing bore wasn't very pretty, no matter what I did it chattered.
I tried different feeds, speeds, and grinds. Nothing worked, so I just let it chatter. I think I need to build a better arbor to hold the boring head. Currently using a 1 inch straight shank in an endmill holder. I need to build a 30 taper shank that directly screws onto the head.
Then on to the cover plate/motor mount. 5/8 inch thick aluminum.
Once the work was done on the bearing side, then flip it over and do the work on the other side.
First, machine a 0.125 deep pocket in the MDF to locate the part. This absolutely locates the part and then it also requires minimal clamping to hold it in position.
Done with the motor side.
Drilling & reaming the drive pin holes and finishing the pulley to proper thickness by removing all but 0.030 of the hub.
The roller bearing installed in the housing and one thrust bearing in the bottom.
The drive pins (dowel pins) engage with the bolt holes in the ball nut. This is the only way I could come up with to drive the nut. The nut is hardened so no way to tap the holes, and no clearance for bolt heads on the back end. The bolt holes are about 0.255 diameter, so by slightly offsetting the 0.250 dowel pins in the pulley it has near zero backlash. The pulley is held in place by the thrust bearings.
Here is the exploded view of the assembly. That grease notch in the ball nut rides right over the rollers with no problem.
These four grub screws set the axial preload. They will get a drop of blue LocTite on final assembly.
And the assembled view
This little ''car'' will be attached to the ball screw to keep it from rotating.
And the rear view
I still need to bore the motor pulley to size, make a belt guard, and attach the car to the ball screw. I hope it will drill & tap, it's a bit hard.
