2020 Update: It has been pointed out by a comment (thanks Ivanov) that the tooth profile described here, while looking close, is not within spec for a HTD timing pulley. While its probably fine for use with a quick and dirty 3d printed prototype, use of this profile may may lead to damage of your transmissions with prolonged use!
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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.
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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.
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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.
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Grateful for the guide – but where did you get the tooth width of 3.05mm for this 5mm tooth belt? I’m adapting it to 3mm teeth and can’t find an equivalent dimension in the specs/
The value of 3.05 came from the image at the link below. Admittedly this is not an ideal source of information but it does work.
For a 3M belt try a value of 1.78mm as per the link below.
Perfect – thanks. I couldn’t find the source website. Thanks again for putting together a detailed guide!
But wait, that 3M diagram shows “2.50” as the small radius – that can’t be right…
As another follow up here, with clever use of constraints, you can actually avoid needing the small radius value – you only need the tooth width, tooth height, and the tooth spacing. With those constraints in place, the small radius for a 3M tooth ends up being 0.306mm. Hopefully that helps folks in the future.
What values would you change going to a 8mm pitch GT2 style pulley?
Dear Richard,
I have posted a parametric model based on your measurements and way of thought. Your post was at great help. Thanks 🙂
https://www.thingiverse.com/thing:3450523
Best Regards,
Daniel
Excellent tutorial, I had the HTD-5M pulley ready in no-time. Thank you for taking the time to post the tutorial online!
Not to offend anybody but this tutorial is wrong in general. You can clearly see that it is not made by mechanical engineer… Your tooth profile is totally wrong and the main reason is that you are trying to create a tooth wheel based on some internet drawings of tooth belt. It could work as some hobby 3d print guide but please do not use it professionally. Will damage your transmissions. You are missing some flat sections on the tooth exit and that will cause too much compression on the belt toot side walls. Also the way of dividing pitch circumference with degrees is absolutely wrong. You should use “pitch” which is measured as distance in millimeters from peak to peak. Again no means to offend anybody, it is simply wrong…. As a tutorial how to make some repetition of a profile on circle – it works.
Hi Ivanov. Thanks for taking the time to point out these mistakes. Your right in that I’m not a mechanical engineer and this guide was put together just as I was starting out learning F360. I would definitely approach things a little differently were I to do this again now. I will add a warning at the top of the post to make people aware that this profile is not correct and to use at their own risk. Thanks again!
“You should use “pitch” which is measured as distance in millimeters from peak to peak.” But I guess on the pitchline?
And what are according to you the profile specifications of HTD 3M / 5M / 8M?
Your reply is simply wrong. You are forgetting that the diameter was set exactly for the type of teeth dimensions from that doc sheet. So dividing 360 by the number of teeth to get the degrees between the center works perfectly. The calculation comes out spot on as I verified this.
Also, you are forgetting that peak to peak is not going to be the same from the straight line measure of the technical diagram because of the curvature of the circle of the gear.
On a curve, the peak to peak gets closer. Bend a belt for yourself and tell me if the peak to peak stays the same or gets closer, should be pretty obvious! Also, the drawings are not just some random ones by a 5 year old, but verified from the data sheets put out by the manufacturer, that should be plenty sufficient for 99.9% of applications out there for normal use.