Using a Custom Dovetail Fixture to Mill a Motorcycle Spark Arrestor

With a 26 year career as a General Machinist at Boeing, Kevin Bethards knows a thing or two about CNC. Owner of a PCNC 770 with a 4th Axis, Bethards has a small home machine shop where, using a custom dovetail fixture for his 4th Axis, Bethards milled a spark arrestor for a YZ250F motorcycle. “I came up with the idea for the dovetail fixture based on something we use at work, but I just scaled it down a bit.

The inspiration for this fixture comes from the folks at Techni-Grip. Check out their website at for a line of innovative workholding products and their tutorials on workholding methods. Filming the PCNC 770 machining the spark arrestor in the dovetail fixture, Bethards shows the process in a two part video series. Using Alibre Design and SprutCAM 7 software programs for CAD/CAM, Bethards custom designed the part to fit the exact specifications of the muffler. He explained, “At work, I’m in the emergent machine shop, We usually only make one part and I get a practice piece. That was my first try making the spark arrestor and I’m pretty happy how it turned out in the end.” Bethards first discovered Tormach on the CNC Zone website, “I was looking on CNC Zone and I saw the Tormach forum. People there had really good things to say about the mills and they were priced really reasonable for what you got. And everyone raves about your customer service.” He continued, “When I decided to buy a Tormach, I noticed there were other people in my area with mills. But they all have PCNC 1100’s. I wanted the smaller footprint and the 10,000 RPM spindle. I put the PCNC 770 in front of one of the garage doors and it gave me a little bit more space. Because I’m not milling as a business, the table size was fine for me. Plus, it runs on 110 VAC and that was a big thing too. I didn’t want to have to re-wire for a 220 outlet.” “I’ve made more things for my family and friends than I have for myself,” Bethards joked. “I was pretty busy with the spark arrestor project for a few days. Sometimes a month passes and I don’t do anything, but the last couple of weeks I’ve been using the PCNC 770 just about every day."