Time to practice! Start by make a copy of the Stage 1B Exercises Document through the button below, just like you did with the Stage 1A Exercises Document. Each exercise has a folder, a "reference" tab (a preview of what the final model should look like), and a tab or two for doing your exercise in. Solutions are also provided in the 1B Exercise Solutions Document to check your work afterwards.
In this exercise, you will be CADing and assembling a simple single stage gearbox. The goal of this exercise is to introduce how to model a very simple gear transmission. Additionally, you will learn how to use the the Shaft Featurescript, Replicate tool, MKCad parts library, and part configurations.
Warning
As this is the first exercise where MKCAD is being used, please be sure you have it added by following the tutorial in Stage 0! MKCAD is a parts library for FRC used for inserting COTS components into assemblies.
Note
Exercise 1 adds hardware (bolts and nuts) to the CAD models. You can read more about hardware standards on the Design Handbook page.
Part Studio Instructions
Navigate to the "Exercise #1 Part Studio" tab in your copied document and follow the instructions in the slides to complete the part studio.
0. Finished Part Studio.
1. Begin by creating the layout sketch for the gearbox. Draw the pitch circles for the 60T and 12T gears. Set the pitch circles tangent to constrain the center-to-center distance between the gears. Constrain the centers of the two gears to be vertical.
2. Add the outline of the motor, a 2.5" diameter circle, around the 12T gear that the motor is attached to. The layout sketch is now finished.
3. Create a new sketch for the motor plate. Using the layout as the reference, draw a 1.125" hole for the bearing and a 0.8" hole for the motor boss (the nub that sticks out from the motor). Note that depending on your manufacturing processes and tolerances, you may need to draw your bearing holes slightly larger or smaller than nominal (1.125").
4. Add two mounting holes for the motor. A good practice to constrain the holes is to draw and dimension a bolt circle, then draw the holes for the bolts on the bolt circle. This constrains the distance of the holes from the center of the motor with a single dimension. The bolt circle for the motor we are using is 2".
5. Add the four bolt holes for connecting the two plates. Use a center rectangle to create the construction geometry so that only two dimensions are required to constrain the holes.
6. Using centerpoint arcs, lines, and the sketch mirror tool, draw the outline for the plate around the holes and motor outline. The intelligent placement of the origin along the vertical line of symmetry allows you to use the right plane to mirror the plate outline.
7. Extrude the motor plate to be 1/4" thick.
8. Use the Spacer Featurescript to add a round 3/8" diameter, 5/8" long spacer.
9. Create the outer plate sketch on the face of the spacer. Use the Use sketch tool to copy the geometry and holes of the motor plate, but add a round cutout at the top.
10. Extrude the outer plate to be 1/4" thick.
11. Use the Shaft Featurescript to model the output shaft. Follow the settings used. You can learn more about the shaft retention property on the Design Handbook page.
12. Finished part studio. Name the key sketches and parts. Set the material of the plate and spacers to 6061 Aluminum. The shaft has its material automatically determined from the Shaft Featurescript.
Assembly Instructions
When putting together the assembly, you may need to use the "Lock Mate Inferences" functionality to properly mate the motor pinon. Watch the following video to learn how:
Locking Mate Inferences when Mating Assemblies
To lock mate inferences when you see the one you want to select, hold the Shift key when mousing. This is particularly useful for mating the motor pinion gear.
Using Shift key to lock mate inferences while mating.
Navigate to the "Exercise #1 Assembly" tab in your copied document and follow the instructions in the slides to complete the exercise.
0. Final assembly.
1. Insert the part studio into the assembly and fix the gearbox plate. Group mate the two plates together and then mate the spacer to the motor plate. Then, use the Replicate tool to replicate the spacer and its associated mate onto the other spacer locations.
2. Assemble the bearings and shaft using parts from the MKCad App.
3. Assemble the motor and motor pinion gear using parts from the MKCad App. You will need to use mate inference locking to fasten the motor to the motor pinion: See the above drop down to learn how.
4. Assemble the shaft spacer and gear using parts from the MKCad App. Configurable parts will have a blue grid icon in the instance list. Notice how you were able to change the tooth count of the gear from 40T to 60T after mating it. Using configurable components like this makes your models more parametric since you can change the component without needing to re-insert and mate.
5. Assemble the shaft retention bolts using the MKCad app parts.
6. Assemble the motor bolts, gearbox bolts, and nuts using the Onshape Standard Content and MKCad app parts. When inserting Onshape Standard Content, you can use the snap mode to automatically add the mates when inserting or insert the component and then manually mate it.
7. Finished assembly. Make sure to sort your parts into folders and name your replicate features.
Verification
If all is done correctly your assembly should have 19 Instances, and weigh approximately 2.3lb.
Configurable Parts
In this exercise you made your first gearbox. In doing so, you also used part configurations - a powerful tool that allows for variations of the same part. The gears that you inserted from MKCad were configurable - you were able to easily change the tooth count of the gear without needing to inserting a new component. Try and use configurable parts when they are available to make your models more parametric.
