After having some trouble finding a clear guide on how to create the correct tooth profile for a HTD 5M timing pulley I have decided to create my own. This method will also work for a HTD 3M or 8M pulley, just change the pitch accordingly.
Note: If you want a pulley with a standard number of teeth it is often easier to just modify a CAD file provided by online retailers (ie. Misumi) rather than trying to roll your own.
First a few notes about the HTD tooth profile. The correct profile courtesy of SDP-SI is shown below
The belt has a 5mm pitch (peak to peak), a tooth height of 2.08 mm, a total belt thickness of 3.81 mm, a the tooth width is 3.05 mm, a radius of 1.49 mm and a valley radius of 0.43 mm. Note that the image below is one of many you will find online that shows an incorrect tooth profile! If you use the dimensions shown below to make the pulley in CAD it looks like that shown by SDP-SI with a flat valley.
A photo of a HTD 5M belt for comparison is shown below.
We now most of the information we need to construct the pulley.
To construct the pulley do the following:
- Select the number of grooves for your pulley. In this example we will choose 50 grooves.
- Refer to this handy timing pulley diameter calculator created by droftarts on his parametric pulley page on Thingiverse and find the outside diameter for your selected number of teeth. In this example for 50 teeth thats 78.43 mm. This will be the diameter of the pulley if you were to measure it with a caliper.
- In your cad software sketch a circle of this diameter
- Calculate the diameter of your pulley if you were to measure it with a caliper valley to valley by subtracting two times the tooth height of 2.06 mm. For example: 78.43 – 2 * 2.06 = 74.31 mm. Sketch this second circle diameter of 74.31 mm.
- Using a ‘2-point circle tool’ sketch a 3.05 mm circle at the top of your 74.31 mm diameter circle and fix it in place. This will form the valley of your tooth profile.
- Now for the tooth valley. Create a circle with a radius 0.43 mm (0.86 mm diameter) and using the tangent tool attach its tangent to the outer 78.43 mm circle and the 3.05 mm diameter circle you just created as shown below. This will form your valley radius.
- Zooming back out we now need to sketch the tooth pitch. For our 50 groove pulley we will have a tooth every 7.2 degrees (360 degrees / 50 grooves = 7.2 degrees). Sketch a construction line 7.2 degrees away from your first circle with the origin as your point of rotation. In the example below the angle is measured from the horizontal and so becomes 90 – 7.2 = 82.8 degrees.
- On this new line repeat steps 5 and 6 so that you have a full tooth profile as shown below.
- In this second last step we use the centre point arc tool to sketch the inner circles of the pulley profile and finish it off with a single straight line segment be tween the two 0.86 mm dia circles. Note that in order to get the lines to meet exactly you may need to turn off grid snapping using the icon on the bottom of the window.With this done our pulley profile is now complete.
- In our final step we just need to repeat this profile 50 times to form our pulley. Select the lines as shown in the previous image, choose the ‘circular pattern’ tool from the sketch menu (not the create menu!), choose the origin as your point of rotation and set the quantity to your number of grooves which in this case is 50.
And thats it, your done! Now you can go create your own custom pulleys for all sorts of applications such as making custom 3D printed strain wave gears.
You may have noticed that the diameter of pulley valley is 3.05 mm and not the 2.98mm dia specified for the belt. This is ok since there must be some play between the tooth for it to engage properly.
An example of the end result after 3D printing with a 0.4mm nozzle is shown below.
You can find a copy of the CAD file on grabcad here.