Saturday, April 2, 2011

Part 6 - X Axis

With the base established, it was time to mount the X axis lead screws. To do this, the following bucket of parts was needed:

Off the shelf parts
4 x 1/2" ID sealed roller bearings
4 x 1/2" ID needle roller thrust bearings
4 x 1/2" ID collar clamps
4 x 3ft ACME 1/2-10 lead screws that will be trimmed to fit
24 x 1/2" SHCS
24 x T-Slot economy nuts
1 x NEMA 23 stepper motor

Fabricated Parts
4 x End Plates made from 0.25" aluminum plate
4 x Bearing blocks made from 0.375" plexiglass (just what I happened to have on hand)
4 x Motor Mounts made from 0.25" aluminum plate
2 x Low Profile Delrin 1/2-10 ACME Nuts made from 0.75" black Delrin

The material for the plates was purchased from the 8020 ebay store. The dimensions for the end plates are:

End Plate Dimensions
End Plate 3D isometric view

As simple as these plates are, they need to be reasonably precisely made in order for everything to fit properly during assembly. These were all machined on the micro mill which reduces some of the tedium of making the same part 4 times. The 8-32 holes were tapped by hand. The end plates support both ends of the lead screw and are attached to the 8020 extrusions using 6 x 1/4-20x0.5 SHCS.

The bearing blocks provide axial alignment for the lead screw. They were made from 0.375" acrylic sheet as that is what was on hand at the time. They have held up well for 2 years now but aluminum would have been the "pro" choice. Eventually 7 of these blocks will be needed so they might as well all be made at the same time. The indented sides serve no purpose and can be eliminated to reduce the amount of fabrication work.
Bearing Block Dimensions
Bearing Block 3D isometric view
After machining all the parts, the rotary bearings were press fitted into the bearing block taking care to only apply pressure on the outer bearing race to avoid damaging the bearings. A small amount of 3-in-1 oil will help the job go a little easier. A vice can be used to press the bearing into the bearing block using a 1.125 OD piece of tube to ensure only the outer race is impacted. The bearing pockets ended up pretty much size-on-size to the bearings making for a light but tight press fit.
End plate, bearing block and rotary bearing before assembly
After the bearing is pressed into place, the bearing block is bolted to the end plate using 4 x 8-32 x 5/8 SHCS. This is done for 2 of the end plates that will be located at the opposite end to the motor on the left and right side of the X axis. On the motor end, the arrangement is a little different.
End plate and bearing block after assembly
The NEMA 23 stepper motor standoffs have 8-32 UNC tapped holes on one end and clearance holes for a 8-32 screws on the opposite end. All other features are symmetrical. The parts were machined according to the following dimensions (click to enlarge):
NEMA 23 Motor Standoff Dimensions

NEMA 23 Motor Standoff 3D isometric view
The hole in the middle was a style feature and can be eliminated. After machining, deburring and tapping the part, the motors were attached.

The last parts were the Delrin nuts. Delrin is a self lubricating engineering plastic that is commonly used for leadscrew nuts. The following dimensions were used (click to enlarge):
Delrin Nut Dimenions

Delrin Nut 3D Model

The original design had 2 motors on the x-axis. Budget cutbacks reduced the number of motors to 1 using a belt drive to synch the left and right leadscrews. While not considered ideal by professional builders, the accuracy of machined parts achieved with parts made using the final router easily met specifications. However,the use of 2 motors is recommended to achieve maximum torque and speed on the X-axis. Remember that the X-axis drives the full weight of the gantry around so having 2 motors is advantageous and eliminates any concerns, real or imagined, regarding belt slippage or stretch.

X-Axis front end plate mounted


To be continued....

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