This bolt has an eccentric cam. The threaded portion has been added by welding 5/16 inch threaded rod to the smooth part of the bolt 5/16 inch in diameter. I was timid with my welding because I did not want to ruin any more threads than necessary. As a result, I got a poor joint. It is better than it was during the last four decades in the life of the PowrKraft (Montgomery Ward) radial arm saw on which this part is used to adjust the bearings that hold the motor carriage on the track. Someone had twisted the bolt off where the threads began. Getting a new part would be impossible. I decided to try making a new part with fairly basic tools.
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Step 1: Materials and Tools
- 5/16 x 2 inch hex head bolt
- 7/16 x 2 1/2 inch hex head bolt
- Angle head grinder and cutting wheel
- Center punch
- Fine measuring tool, like a digital caliper
- Flat file
- Round file
- 1/2 inch Jacob's chuck on a motor
- Drill press and various twist drills
- Vise Grip locking pliers
Step 2: File the 7/16 Inch Bolt
I filed around the 7/16 inch bolt just under the head to make that part the same diameter as the rest of the smooth part. See the second photo. When finished, the bolt will fit all of the way into the hole for the cam bolt.
Step 3: Cut the 7/16 Inch Bolt to Length
Insert the 7/16 inch bolt from the bottom of the carriage hole. Mark where the bolt comes through the top surface of the carriage hole. Cut to length leaving a squared and flat end. (Actually, on this application the bolt needs to be cut just below where it comes through the casting. That provides tension when the nut on top is tightened that helps keep the cam from turning. There is a set screw, but it alone does not keep the cam from turning.)
Step 4: Mark for Drilling
I want to drill a hole 5/16 inch in diameter through the length of the 7/16 inch bolt. The center for the hole will be 5/32 inch or slightly more inward from one edge of the bolt's body. The punch mark may not appear to be off-center, but it is.
Step 5: Begin Drilling
Clamp the 7/16 inch bolt's head firmly in a locking pliers so it is relatively easy to keep the head of the bolt very flat on the drill press table. Begin drilling with small drills a bit less than 1/8 inch.
Step 6: Expand the Hole
I used each twist drill in my set to make the hole larger and larger in very small steps. I wanted to keep the hole parallel to the sides of the bolt as much as possible. In the photo you can see the hole is definitely off-center.
Step 7: File Work
At the end of the drilling, the side of the bolt tore away. I smoothed the ragged edges with a file.
Step 8: Make It Fit
Before long the 7/16 inch bolt fit easily into the hole in the motor carriage. I had to do a little work with a round file to make the 5/16 inch bolt fit into its hole in the 7/16 inch bolt. Notice how the 7/16 inch bolt makes an eccentric cam.
In the lower left portion of the photo you can see an Allen wrench that tightens a set screw to hold the cam adjustment in place once set. The screw just to its right holds the shroud that covers the bearings.
Step 9: Finished
The bearing in the front of the photo is mounted on the new cam bolt I made. I decided to omit the lock washer so the nut can catch as many threads as possible. I will use some Locktite to keep the nut secure instead.
The original cam bolts were adjusted with a large screwdriver and then the set screw was tightened with an Allen wrench. The new cam bolt I made can easily be adjusted with a 5/8 inch socket wrench and a very gentle hand. Then lock down the set screw. (The set screw is not all that keeps the cam from turning. The cam portion is a little shorter than the top of the casting. When the nut above the ball bearing is tightened, tension from the nut helps bind the cam so it does not turn.)
At first the idea of making a cam bolt with such close tolerances and careful machining seemed impossible. But, with some basic tools and a little patience applied in logical steps, I was able to make a perfectly functioning cam bolt.