The Linear Toggle Clamp

• 4 - #8-32 hex nuts
• 1 - 2 inch #8-32 bolt
• 1 - 1.5 inch #8-32 bolt
• 2 - 1 inch #8-32 bolts
• 3d printer
• About 300 grams of PETG or similar plastic

To understand how a toggle clamp works, we need to break the system down into the four key components:

• The lever - the handle that you push to lock and release the clamp.
• The plunger - the horizontal shaft that presses against the object that is being clamped.
• The arm - the part that connects the lever to the plunger.
• The base - the device that prevents the machine from sliding and holds the plunger in place.

The three pivot points that connected these components are the connections between:

• the lever to the base,
• the lever and the arm, and
• the lever and the plunger.

As you push the lever forward, the arm translates this motion to the plunger. The base constrains the movement of the plunger to travel in a linear direction. The real genius of toggle clamps is how this design turns creates a mechanical advantage to clamp your workpiece tightly.

Mechanical advantage is the ratio between the force produced by the machine compared to the force that is applied to the machine. For example, if your machine produces 30 pounds of force while you apply two pounds of force, you have a mechanical advantage of 15. You can get this kind of amplification in force a large movement at the input of the machine results in a tiny movement in the output of the machine. For example, if you push your lever 15 inches and the resulting load on the lever only moves 1 inch, you also have a mechanical advantage of 15. It turns out the easiest way to calculate mechanical advantage is to look at the relative motion between the input and the output of your machine.

In my linear toggle clamp, the lever by itself gives is a 2 to 1 mechanical advantage when pushing the arm since the pivot point connecting the arm is halfway between the tip of the lever and the base. However, the overall mechanical advantage of the clamp gets much higher the lever gets close to its horizontal position.

As the lever moves from being in its vertical position toward its horizontal position, the plunger continues to move forward. However, when the lever is nearly at the horizontal position, the rate plunger moves only a tiny amount compared to the lever's motion. Because of the arm and the pivot points, moving the lever's results in only a tiny change in the plunger's forward position.

To see how this all works, I put together a Google Colab notebook that mimics this system's behavior.

• Colab Notebook to simulate a linear toggle clamp

Google Colab is a free system for running python codes online. The notebooks are similar to Jupyter notebooks but run in the cloud using shared resources. This notebook does a simple simulation of the system (see the drawing above) and creates two graphs. The graphs show the mechanical advantage and the plunger's position as a function of the lever angle. I have posted the graphs above, but you can play around with some of the system parameters if you like. The notebook also has a small animation script you can play with to view this system's motion.

I also generated this table showing the lever's angle vs. the mechanical advantage. The arm forces the plunger to go forward, effectively amplifying the force that is applied. As you can see in the table below and the graph above, the mechanical advantage gets very large in the last ten degrees.

It takes almost no effort to open and close the toggle clamp, but the force applied to hold the piece in place is substantial. Although adjusting the fit is simple, it is particulary useful when you are going to machine several identical pieces. You can open the clamp, remove the piece, replace it with a new workpiece, and lock it in place in a few seconds. If you have decided to make few dozen trivets as gifts or to sell, this feature is really nice.

I have added a plunger tip at the end of the plunger in addition to the lever-arm-plunger mechanism. This tip is screwed into the plunger. Similar adjustments can be made on commercial toggle clamps because this screw plays a critical part in operating the clamp. You can make tiny adjustments to the clamping force with this plunger tip. We will discuss this more a bit later in the instructions.

There is a great discussion of toggle clamps here if you wish to read more.

The engineering of this clamp took a little bit of trial and error. The challenge was to make the mechancial system work using 3d printed pieces. These are a few of the moving elements to consider in this design:

• The plunger needs to fit tightly into a track in the clamp's base. It needs to move smoothly, but not be loose.
• The length of the arm needs to be slightly longer than the length of the lever so the lever can be moved to the horizontal position. It is critical that the lever can move into this position so the system locks into place. At 90 degrees, the mechanical advantage of the system prevents it from unlocking.
• The pivot points much have enough clearance for the bolts, not be loose.
• The lever arm needs to attach in a way that allows it to lay flat and out of the way when the lever is placed in the horizontal position.

When designing mechanical parts that are created on a 3d printer, I usually add between 0.1mm and 0.4mm of space between parts that are designed to slide past each other. I created several prototype of the pieces to make everything fit nicely together. The big advantage of doing this project on a 3d printer was the ability to print parts in a few hours. I could test pieces and then throw them away if they didn't work perfectly.

All the design files for this system were created using Fusion 360. As I mentioned in the introduction, I designed this to be used to hold workpieces in place on XCarve CNC machine. This spacing of the screw holes in the base were made specifically for this purpose. Of course, you may wish to use this in a different way, so these are links to the original design files if you want to modify them:

• The Arm -- https://a360.co/3crUNgW
• The Plunger -- https://a360.co/3fjtk2V
• The Base -- https://a360.co/3sJiwix
• The Plunger Tip -- https://a360.co/39nDYC1
• The Lever -- https://a360.co/3cvfBUL

I have also included the STL files for the project if you want to print them directly.

The parts for my clamp were printed on a Prusa MK3 using PETG. Of course, any printer should work fine. If the fit isn't right, you might play with the Fusion 360 files a bit. I would recommend not using PLA because of the low tensile strength of that plastic.

I only used a 20% infill on my project. I probably would recommend going a bit higher - perhaps to 30% infill. I have noticed a few marks where the bolts that connect the base to the CNC machine. I used supports on the pieces to help keep everything aligned. After I removed the pieces from the printer, I carefully removed the supports and cleaned up any rough spots in the prints.

Putting the clamp together is really simple. You will need a few #8-32 bolts and hex nuts in addition to the plastic pieces.

1. Place a #8-32 hex nut into the plunger tip
2. From the opposite side of the plunger tip, insert a 2" #8-32 bolt. Using a screwdriver, adjust the bolt, so it is tightly locked into the plunger tip.
3. Place another #8-32 hex nut into the end of the plunger.
4. Screw the plunger tip into the plunger. You should screw the gap between the plunger tip and the plunger is about 1mm.
5. Place the plunger into the channel on the base of the clamp. It should slide smoothly in this grove. If it doesn't move freely, you may need to clean the print a bit more. If it is too loose, you might need to change the Fusion 360 files since 3d printers might have slightly different tolerances.
6. Since I didn't have a long enough bolt to attach the lever, I used two 1" #8-32 bolts. I inserted them on both sides and then used a screwdriver to tighten them. The holes on the lever and the base are tight enough to hold everything in place without hex nuts. The lever should move freely.
7. We can not attach the arm to the lever and to the plunger. The wider end of the arm goes around the lever. I used a single 1.5" bolt for this part and a hex nut. The connection between the arm and the plunger used a 1" #8-32 bolt and a hex nut.

Make sure everything moves smoothly. Your clamp should now be ready to use.

Clamping a piece of wood is pretty easy.

1. Place the workpiece between a fence and the clamp.
2. Screw the plunger tip, so it is retracted.
3. Put the clamp into the locked position.
4. Place the plunger tip, so it is touching the workpiece snuggly.
5. Tighten the clamp base so it can't move.
6. Open the clamp and then unscrew the plunger tip about one turn. This small adjustment will ensure the clamping force is strong enough to lock the piece into place.
7. Put the clamp back into the locked position.

Your workpiece should be securely clamped in place.

A linear toggle clamp is a good project for your shop and an excellent demonstration of simple machines. It shows how we can use simple mechanical principles can be used to solve real-world problems. It is also helpful for creating other fun projects. Please post any remakes and improvements in the design! Have fun!