Servo - March 2014

body we made in Step 3.

Select the sketch with the tooth geometry, then select the Extrude command. If you are a beginner to SolidWorks, enter the value to match the thickness of the solid body you created in Step 3 (I'll enter 0.125 inches again). Make sure the Merge Results box is checked so that your two extrudes will become a single solid body.

If you are a more advanced user, in the section of the Extrude menu titled Direction 1, select Up To Surface and then click on the top surface of the first extrude. This will make sure your tooth is always the same thickness as the main body of your disk.

Make sure the Merge Results box is checked. Don't forget to radius any interior right angles to avoid a stress concentration there. This is often where a crack will start as your weapon delivers big hits!

Step 5. Let's create the hole for the weapon shaft. This feature will not affect the center of mass of the disk.

Create a new sketch on the same plane you have been using. Draw a circle centered on the coordinate system and dimension it according to the requirements of your design. I'll make it 0.5 inches. Then, subtract it from the solid body.

Select Through All to ensure that the hole goes through the disk, even if you decide to change the thickness later on.

Step 6. Next, we need to find the center of mass of the disk we have designed. Select the appropriate view from the View menu so that you are looking at the top of the disk on your screen. Then, go to the Tools menu and select Mass Properties (Figure 6A).

This will bring up a dialog box with information about different properties of the body you have created (Figure 6B). You can use this same tool to check the weight of your disk if you assign a material to the solid body.

For the purpose of balancing the disk, we want to focus on the center of mass information. You'll see that it shows an X, Y, and Z coordinate. It only needs to be balanced in the plane perpendicular to the axis of rotation. Since I drew my sketches on the top plane, the Y coordinate is not relevant for balancing the disk. The goal is to get the X and Z coordinates as close to zero as we can.

In order to balance the disk with more accuracy, it is helpful to show more decimal places. In the same dialog box, click Options and then Use Custom Settings. Increase the length resolution to four decimal places. Right now, my center of mass is at: X = 0.0345, Z = 0.0897. This may seem like a very small deviation from center but at thousands of revolutions per minute, the effects are significant.

Step 7. You can shift the center of mass either by removing material near the tooth (Figure 7A) or by adding material away from the tooth (Figure 7B; notice I used an ellipse to add material). Each time you add or subtract material, go back to the Mass Properties dialog box to see how the change affects the center of mass. It's a trial-and-error process, but just a few tries should get you very close.

Remember, you're trying to balance the weight of the disk so you want to offset the weight of the tooth as you add/subtract mass. Try to get the weapon balanced to three decimal places if you're working in inches (0.000).