One component many DIY CNC builders have come across is the eccentric bushing.
You will find these on many linear rail systems for many types of CNC machines.
They are also used in many floating head systems on plasma cutter CNC machines.
While you can find these available online, I found it was much more convenient (and cheaper) to just make my own.
The bushing I will show you how to make is designed for a bearing with a 3/8" diameter hole, so I will be using a 3/8" bolt to keep the operation quick and simple.
If you are using another sized bearing, you will have to adjust the size of the bolt and possibly the canter hole to match your faster. For this bushing, I will be using a 1/4" bolt as my fastener.
Gather your parts.
You will need a bolt the size of your bearing. In my case 3/8". Notice the shoulder area on the bolt. I'm using one that has a shoulder at least the thickness of the bearing.
You will need access to the bearing that will be used.
The nut is optional. It is only needed to support the bolt if you are going to use a band saw to cut the bolt.
Place the bearing on the bolt and push it up against the head. Mark the bolt as shown here.
Use a fine tip marker or gel pen to make your mark.
Cut the bolt on the side of the mark closest to the head. The goal is to cut the bushing so that it is slightly shy of the thickness of the bearing.
Here, I am using a vise to hold the bolt while I cut it with a reciprocating saw.
Notice where the blade is in reference to the line.
Here, I am using a band saw to cut the bolt.
Notice the nut used to support the bolt.
Here, I am using a horizontal band saw.
The goal here is to create a bushing that ends up just shy of the full thickness of the bearing.
This is important, as it will make the cut much less critical.
Insert the bushing into the chuck as shown here. Tighten the two marked jaws, snug only.
Notice the orientation of the bushing. You need a flat end of the bushing to line up with one of the jaws. This will be the axis we will be offsetting.
Place your dial indicator on your lathe and point the tip at the center of the flat side of the bolt head, as shown here. Zero the dial indicator.
In this case I need to offset the bushing by .05"
I will want to adjust the jaws so that the bolt moves 180 degrees. As this indicates .05 on this dial.
Once you get the jaws reposition so you have a .05 offset, tighten all the jaws.
Drill the fastener hole. In my case I am using a 1/4" drill bit. The drill bit is fastened into a tailstock chuck and pushed into the rovoling part. Since
If you like, you can drill a small pilot using a center drill.
This will keep the larger drill bit from wandering.
Step 9 (Optional)
While not necessary, you can face off the end of the head end of the bushing. Here you can also add a small chamfer to the edge of the hole.
Remove the bushing from the lathe. You may need to hit the small end of the bushing with some sand paper or grinder to remove any burrs or flashing.
Insert the bushing into the bearing.
Insert your 1/4" fastener.
Add a nut to secure the fastener. You can snug up this connection, but make it loose enough that you can twist the bushing with a wrench.
As and option, you can use a washer between the bearing and the surface you wish to mount the assembly to.
The bearing must have a nut or properly sized washer between it and any thing its connected to.
To use the bushing, slightly loosen the nut securing the assembly. You want some resistance as you rotate the bushing.
Take a wrench that fits the bushing and rotate the bushing until you get the offset you want. In many cases you will be tightening the bearing up against a rail of some sort.
Once in position re-secure the nut holding the assembly.
I've created a video that will give a little different perspective of the process.
Please note that you only have to do the offset adjustments once. Subsequent parts can start with the drilling operation once the jaws are tightened.
There are other ways to make eccentric bushings or nuts. This one is done by Tom Verity.
You can use the same process to make other eccentric bushings.
The only difference will be the offset and the fastener size.
From left to right:
V-Bearing w 3/8" hole
- 3/8" bolt
- 1/4" fastener
- .05" offset
Skate Bearing w 8mm hole
- 5/16" bolt
- #10 Fastener
- .04" offset
R8Z Bearing with 1/2" hole
- 1/2" Bolt
- 1/4" Fastener
- .06 offset
You can also change things up by reducing the drill bit sizes and tapping the holes. This will allow you to secure the bearing assembly from one side only, thus reducing the size of its footprint.
Making Eccentric Bushing on a Drill Press
I can make bushings consistently all day long on my lathe. It is however, possible to make the bushing using a drill press and vise.
Start by finding the center point and marking it. Then, make another mark .05" from the center mark.
Using a punch, punch the outside mark.
Place the bushing in the vice, head side down as shown here.
Use a small drill bit to make a shallow pilot as shown here.
Then, move to your 1/4" bit and drill through the bushing.
One of the problems is that its possible for the bit to wander a little. Also, its a little hard to get the layout set consistently.
While this attempt was a fail, I have used the drill press to make successful bushings. I do recommend that you only use a .035-.04" offset to give yourself some wiggle room.
If you have to make a lot of these with the drill press, I recommend some sort of jig, so you dont have to layout the drill position.
Making an eccentric bushing with a 3-Jaw Chuck
One disadvantage of the four jaw chuck is that is problematic to make smaller bushings. For example, I needed to make a smaller bushing for a 6mm opening. My 4-jaw chuck would not work because the jaws could not close on something that small.
By using a small shim and a three jaw chuck you can achieve the same results.
Start with a shim that is the same thickness of the offset you need. Here I am using a .02" thick shim.
The right angle on the shim is to help hold it in place while I apply the tape (below).
Take a thin piece of tape and secure the shim in place on the bushing blank as shown.
Insert the bushing with the bushing aligned with one of the jaws, as shown here.
Tighten the chuck.
Drill the hole just like you would do with the four jaw chuck.
Face off as needed.
Here, I did the bushing three different ways. From left to right:
- Bushing tapped for #6 machine screw
- Single hole in bushing for #6 screw and nut
- Countersunk hole for #6 screw and nut
Here you can see the how I secured the bushings.
The tapped bushing requires the most work. It provides a smaller profile for tight applications. The screw can be a problem to tighten.
The single hole solution requires the least amount of work, but the profile can be problematic in tight quarters.
On the last bushing I countersunk a pocket for the head of the screw. This provides the lower profile and gives better grip on the bushing. Note that the pocket can be added off the drill press later if the single hole solution has too much profile for your application.
The three jaw chuck does work, but I find that attaching the shim to be time consuming. Something that could attach to the saw itself may prove more usefull.