Make a New Part for an Old Machine





Introduction: Make a New Part for an Old Machine

About: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying posting things I have learned and done since I got my first ...

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.

Step 1: Materials and Tools

The photo shows various parts spread out on the top of the saw's motor carriage. From left to right: 5/16 inch bolt, 7/16 inch bolt, lock washer, Allen wrench for tightening the cam's set screw, motor carriage ball bearing, 5/16 inch nut and washer, hole in the casting for the cam bolt, and the old cam bolt.

  • 5/16 x 2 inch hex head bolt
  • 7/16 x 2 1/2 inch hex head bolt
  • Angle head grinder and cutting wheel
  • Hammer
  • Center punch
  • Fine measuring tool, like a digital caliper
  • Flat file
  • Round file
  • Vise
  • 1/2 inch Jacob's chuck on a motor
  • Drill press and various twist drills
  • Vise Grip locking pliers
  • Wrenches

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. 



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    As I have gotten the metal portion of my personal shop up and running, I often lose sight of the simple answer being best. Pretty is nice if it's for someone else, but pure function is more elegant to me on my own stuff. I like your solution especially for the hex head on the bushing allowing holding an adjustment to a very fine degree while the setscrew is tightened. Most supervisory mindset is "right now" without thought to someone else having to use it. Speed is only important (to my thinking) on continuous production lines. If given time to sleep on it, a simpler solution will often occur. In my field (electrical), I have that balance worked out over the years. But in machining, I'm still a 'prentice, a model builder. I just hope I can remember your technique when I need it. My metal working stuff is of similar age to your wood shop; 12" Atlas lathe, Horizontal mill and 7" Shaper. I have a problem with welding (breathing trouble), so probably would make the hole threaded and another screw from the back to tension the assembly. Classy work there; keep it up. Bi11

    1 reply

    Thank you, again. I would like to suggest three of my earlier Instructables for simple processes that yielded the needed precision on projects. I am suggesting them just because you might enjoy them. I needed to enlarge the arbor hole in a 10 inch sawblade for a friend's unique radial arm saw. I did it with a wood lathe and a Dremel tool. And, I decided to make my own center finder. Only a couple of simple steps were needed and it is almost foolproof. You might also enjoy the internal diameter micrometer I made so I could know what I needed to put new rings into a one lung 4-cycle engine to make a sidewalk go-kart for my kids.

    After my last reply to you, I remembered a man who did service work on copiers for a major office machine company. He always took a little extra time on each call to make sure the whole machine was in good condition and that he had really fixed the problem. He got pressure from his bosses for not making more service calls in a day, but he almost never got called back to tweak a machine he had just serviced. By contrast, the other guys in his department serviced more machines, but were frequently called back because something was still not right on a machine they had just serviced.

    As regards welding, some elaborate ventilation systems are available now. You could probably supply outside air to yourself almost as if you were an astronaut. I would probably go for a home brew version.


    An outstanding solution, keeping it simple. I would have spent most of the day setting up the lathe to cut an eccentric bushing. In the time I would have spent just setting up, you have the machine repaired and running. Sometimes we lose sight of the simple solutions. Outstanding thinking.
    Bill Hudson, Artificer wannabe

    1 reply


    Thank you. I weighed several possibilities before I thought of this one. I was not completely sure it would work. As a fallback measure, I thought I could cut the thinner portion of the original part a bit shorter and try again to weld a threaded stud onto it. If I were doing this while employed by someone, I would be fired for spending too much time sitting around thinking about a solution rather than producing a solution. I hope someone else will be able to use this idea someday.

    You are very ingenious, Phil. To do the hole was a clever idea.

    6 replies

    Thank you, Osvaldo. Maybe it will give someone else an idea about how to solve his problem someday in the future.

    Phil, do you think I could make a router using the motor of and old gloss cleaner?

    It has high speed and good force. It is in very good state, had a vacuum too.

    I have thought fasten it under a metal plate with legs, to use over the work table. Taking off the vacuum turbine, it is low noise, too.


    Many routers spin at nearly 25,000 revolutions per minute (rpm). It is not impossible that a vacuum cleaner motor could spin that fast. The new Dyson vacuum spins at over 100,000 revolutions per minute (rpm). Much depends on how fast your motor spins. Power is also important. I would not want a router less powerful than 1 horsepower. The chuck could be a problem. A collet chuck works much better than a 3-jaw drill chuck. The drill chuck begins to chatter and works itself loose very soon. I would say it may be possible, but there are some obstacles to overcome that may not make a vacuum cleaner motor a good choice. 

    Yes, all you say is right. I don't know the speed, but it is high, very high. The axis ends in a thread, a bit too little to attach with safety a chuck. Supposing the thread breaks while it is working, the chuck + tool would be a bullet.

    Maybe the alternative could be to make another vacuum cleaner, littler than original, and handy.

    Osvaldo, I did find an article at Instuctable that helps decide how fast a motor spins. It uses the musical frequency pitch of the motor while it is running and comparing it to a 440 cycle A note. I am not sure how helpful it really is. I cannot tell and A from a C#, myself. But, here is the link.

    Is the shaft on your motor long enough to cut more threads or to grind a flat or a keyway onto it?  

    I doubt the chuck and bit would go far if they broke off of the shaft. 

    Phil, very interesting your idea. I thought something like that many times but never tried, I suppose it is very effective, easy and a bit accurate. I never enter at Instructable's Answers, due to my poor English.
    The shaft is not thick, but has a length enough to enlarge the thread. I am thinking your phrase "I doubt the chuck and bit ...". I think the opposite, but surely you have more experience.

    Nice work, Phil; drilling an off-center hole for the eccentric. A problem, I suppose, is making and punching just the right point on such a small target. I bought an optical center punch that helps me do that, but it is still difficult:,180,42311

    We are travelling in parallel worlds. As you were doing your project, I was trying to find a way to drill concentric holes in a brass rod. And wouldn't you know it, they usually turned out eccentric! (like me, I guess).
    I came up with a method that worked, will post it soon as an Instructable.

    IMG_0401 (Large).JPG
    2 replies

    Thanks, Bill. I have drilled many holes that were meant to be centered, but became eccentric. If you have a reliable way to drill on center, we will all be grateful. The optical center finder looks like a very good tool.

    I made good use of my digital caliper on this project to check diameters, but also to scribe a faint mark inward from the edge of the larger bolt so I knew roughly where to make a dimple for the center of the hole. One of my bigger concerns was that my hole would not be parallel to the length of the larger bolt. What discrepancies there were in the end came to be worked out by hand with the file. If anything ever happens to the smaller diameter bolt, I can get a new bolt and put it into the hole. That is better than the choices if it were one solid piece and the thinner section broke off, as happened with the original.

    This radial arm saw was at a tool sale. No one bought it, even though it was priced at only $30. I bought it for my son-in-law.

    "One of my bigger concerns was that my hole would not be parallel to the length of the larger bolt"

    My Instructable will address this mutual concern of ours.

    Yay, Phil! Nice job fixing that exccentric. Montgomery Wards. A name out of the past. I still have a few PwerKraft hand tools left. I am surprised the name lives:

    1 reply


    Thanks. I still have a PowrKraft wood lathe I bought new at about age 13 or 14. When I was a young un a lot of what we wore and used came from Montgomery Ward's warehouse in St. Paul, Minnesota. The turn around from mailing the order until delivery in northeastern Iowa was about ten days. After high school I went off to college at Concordia in St. Paul. The Montgomery Ward store attached to that warehouse was a fifteen minute walk from campus. It was quite a wow moment when I found I could place a catalog order at a counter and pick it up 45 minutes later. The lathe came from a Ward's store 35 miles away where we also did quite a bit of our shopping, not from the St. Paul warehouse.

    Thanks. Describing it to you in the private message got me thinking about how I might make a new part, and I decided to try it.

    yeah making a eccentric anything is a tough job.Loks like you tackled it nicely

    I am just a poor country boy. What I lack in precision milling tools I need to make up with smart. If I were doing it again, I think I would set the center in just a little bit more and see if I could make the final drilling without tearing out to the side. I knew that could happen. Even at that, I think the part will perform just fine as it is.