MACHINING TIMING BELT PULLEYS

[Brian Rupnow]

Okay, people, today we're going to take a look at making timing belt pulleys. Small timing belt pulleys are absolutely the cat's meow for the things I run with my model engines. The images seen in this post are downloaded cad models of the pulley I used on my overhead cam engine two years ago.

[Brian Rupnow]

This is the engine. Not a great picture of the engine, but a good picture of the timing belt pulley.

[Brian Rupnow]

Here we have the cad model of the same pulley. and we see that a 0.118" (3mm) rod fits exactly into one of the spaces between the teeth.

[Brian Rupnow]

Here we have the same cad model with two 3 mm diameter rods laid side by side in the pulley grooves. The measured angle between them is purely a resultant, and will change with the number of teeth in a given pulley diameter. It is interesting that the dashed "pitch line" runs exactly through the center of the 3 mm diameter rods. The diameter of this pitch line is 2.407", or 61.14 mm.

[Dan Rowe]

Brian, what is the belt series or name? Did you drill holes or use a milling cutter like cutting a gear?

Cheers Dan

[Brian Rupnow]

This gets easier for me if I pull it onto a drawing sheet. It is apparent that the outside diameter of the gear is .054" (1.37 mm] greater than the pitch diameter. If we break that down into radius differential, then it is .027" or 0.686mm.

[Brian Rupnow]

Dan, I didn't do either. I purchased the gear. I'm trying to figure out how to make a cutter so I can make one. The part number for this pulley is  A6A25-040NF091---Brian.

[Dave Otto]

Dan, I didn't do either. I purchased the gear. I'm trying to figure out how to make a cutter so I can make one. The part number for this pulley is  A6A25-040NF091---Brian.

3mm ball end mill?

[Roger B]

Interesting  These pulleys are obviously based on some concept. Reverse engineering it might be fun 

[Brian Rupnow]

If I wanted to make a single tooth cutter with a 1.5 mm tip radius and make many, many passes to cut each space between the teeth I could make one as shown (with some relief ground on the tip) .

[Jasonb]

The two external corners of the "U" should be rounded so you need more than a half round cutter, think they are usually hobbed or the cheap ones extruded.

https://gear.com.my/wp-content/uploads/pdf/page55.pdf

[Brian Rupnow]

Or--I could take a piece of 3 mm thick 01 steel bar, and first cut it like this---

[Brian Rupnow]

And then drill some holes like this---

[crueby]

Ah, the old drill-holes-then-turn-off-the-rim-trick, eh?!   (read in your best Get-Smart voice)

[Brian Rupnow]

And then go in and cut the material away between the holes, I might end up with something like this.  This is the part that becomes a real challenge. I'm good with drilling the holes as shown in the previous step, but I think that removing the material between the holes to end up with this would be beyond what I can do. After this step I would still have to profile both sides to end up with a 1.5 mm radius on the end of the points, then harden and heat soak the part.

[Jasonb]

You don't really need the rounded gullet on the cutter just do it with a milling cutter and index around for the number of teeth required, corner radius or bull ended cutter would do both.

Easier to form the 3mm half round while it is a solid disc than after cutting the teeth

don't take the cutter teeth to a point as they will wear, do like a gear cutter and relieve by hand

But still won't round the external corners

[Brian Rupnow]

This is what I would ultimately end up with as a cutter. I think this goes beyond what I am capable of making.  My thoughts were that if I made a cutter as shown, then I could cut use my rotary table and tailstock mounted on the milling machine bed and cut teeth an a 12" long piece of aluminum round stock, then part off pulleys to whatever length I needed. It isn't going to happen. I'm pretty sure you can't buy cutters like this either.

[Brian Rupnow]

Jason--Thank you. That is a great idea. I may build one of those and take it for a test drive after I get finished fooling around with my edger. If it worked, that would be a huge plus. If it didn't work, I wouldn't be out a lot of money.---Brian

[RoyG]

Brian,

You might find the discussion on this link to be informative re machining timing belt pulleys.   http://gearotic.com/ESW/FavIcons/index.php?topic=1493.0

I have made a few GT2 Timing Belt Pulleys myself, using a self-ground single point cutting tool - basically a similar design to the tool used by clock makers to cut clock gears.  Grind the single point tool so that it's profile fits the desired belt size (in my case GT2), with the necessary relief angles.  I made my pulleys from Aluminium, and used a piece of Silver Steel (Tool Steel) ground to shape, then hardened and tempered.  The tool shape will differ depending on the diameter of the pulley - as the diameter of a pulley increases, so the shape of the tip of each pulley tooth needs to change to accommodate the shape of the belt when it is flexed around that size radius.   https://www.modelenginemaker.com/index.php?action=post;topic=9401.15;last_msg=209963#

I had a made in USA GT2 ~25mm (I think) sized pulley on hand.  I hand ground the single point cutting tool so that it closely fit the tooth profile on that pulley.  I then used that single point cutter to cut three different sized GT2 pulleys up to about 48mm diameter.  (Sorry, I don't remember the exact dimensions as it was a few years ago.)  A ~25mm pulley cut with the single tool required only deburring and was a nice match to the genuine GT2 pulley.   The larger pulleys needed some hand filing to get the profile of the tips of the gear teeth so that the GT2 belt would run smoothly over the pulley.  It took only 5 to 10 minutes filing on each pulley, so not a lot of hand finishing - unless you're making heaps of them.

If you're talking about something small like a GT2 size belt, with low levels of power being transmitted, then achieving an exact tooth profile that matches the GT Patents is not necessary.  However, the larger you go, and the more power that you are trying to transmit, them more accurately you'll need to comply with the patents for the belt/pulley design to avoid rapid belt wear.

I hope that info is of some use.

RoyG

[gerritv]

Brian,
I can assure you that you have more than enough skills to make those cutters. I am nowhere near as proficient or experienced as you but managed to make 2 so far.

I built a Eureka relieving device per Ivan Laws book Workshop Practice #17. I also made the various supporting jigs. Lastly I made a small scale equivalent to the Cincinatti Universal Grinder cutter sharpening attached. All shown in the attached photo. The Eureka was my first project on large large, the offsets were done in a 3 jaw :-) My build thread is here

The above were used to make a cutter for T2.5 pulleys for my Ridders 2 Cyl Glass Engine (which is still not running but at least firing). I also made a cutter to cut grooves to replicate displays for a 1958 Univac II console (from Ontario Hydro) that we are restoring.

The other approach to look at is Deans method of making 4 tooth cutters.

[gerritv]

Re: aussieroy stating that the tooth profile changes depending on the pulley diameter, this is not the case with at least T2.5 pulleys. I would think it unlikely with the simpler GT2 profile as well. What is extremely critical is the outside dimension of the pulley relative to number of teeth.

One of Gates' patents has a good illustration: https://patents.google.com/patent/US4452594A
This one has more excruciating detail on the shape.

Searching for Toothed Belt patents assigned to Gates pulls up a lot of useful data.

I think using a cutter blank wider than 3mm will give you the ability to both shape the tooth and the radius of the tooth top. From my efforts with T2.5 the shape of the pully tooth is not necessarily identical to the belt tooth shape although at least the GT2 is a partial circle.

Gerrit

[Firebird]

Hi Brian

Sorry to join this topic so late.

A while back I needed to cut some timing gears for a project. I'm sure I posted it on here somewhere but I can't find it now. Anyway it would have had photos hosted by our dear friends PHOTOSUCKIT which are no longer available.

Basically what i did to cut T5 gears was to set up a grinding wheel in the lathe and re grind a slitting saw

If you click on this link there is a brief explanation

http://www.modelenginemaker.com/index.php/topic,3287.msg57962.html#msg57962

Click on the photo and it will take you to an old PHOTOSUCKIT account of mine where the images are still viewable.

Hope this helps

Cheers

Rich

[Brian Rupnow]

Thanks Rich.--Apparently I have an add blocker running on my computer and I would have to disable it to look at the Photobucket pictures.---Brian