A Better Bump Knob

In customer reviews on weed trimmers there are frequent requests for a better, tougher bump knob. The photo shows one I made from steel.

Bump knobs use a reverse thread or left hand thread bolt with a square head. I used the bolt from my plastic bump knob. (The bolt is held in the plastic knob with only a light washer pressed into place. The bolt easily pulls out. Once it does, it tends to pull out frequently.)

My steel bump knob allows me to tighten something as tight as I want it.

Materials

• 1/8" steel
• 1/16" steel
• 1/2" angle iron

Tools

• Metal saw or cutter
• Welder
• Drill and bits
• Ball peen hammer
• Grinder

I tried to duplicate the plastic bump knob as much as possible. That means the square head of the bolt cannot turn, but is locked in position as if it were in a wrench. Yet, the head can move up and down inside the knob.

I did not take photos when I built my knob, so I am using an image I made in Google Sketch-Up. Cut a steel disc from 1/8" steel the same diameter as the plastic bump knob. Drill a hole in the center. The hole on my bump knob was 7/16" in diameter.

I cut and welded two short pieces of 1/2" angle iron so the bolt is centered in the hole, but loose enough to move up and down. (The red blurs indicate weld bead.)

See the second image. The blue lines indicate the position of two additional pieces of angle iron fitted around the bolt head.

I used steel about 1/16" thick for the cover of the bump knob. To get the dome shape, I rested the steel on a piece of pine and pounded with a ball peen hammer. Then I cut out the part that had the best dome shape.

I was able to weld the dome to the angle iron pieces so the welds are inside the knob.

Finally, I cut a strip of #20 gauge steel to fit between the flat bottom piece and the domed top. I tacked it at the start and bent it over to fit, welding tack welds as I went.

When I was finished, the knob did not run out of balance too badly. I did grind on the outside of the knob to smooth the welds and provide more balance.