[damae]

In my retrofitting of a Series I Bridgeport, I have discovered how expensive timing belt pulleys are! For instance, I need two 18 tooth and one 26 tooth pulley, both H-series (.5" pitch) and 1" wide. Here's what I ended up spending:

16H100 $38
26H100 $60

I cheated by using metric size pulleys (38.0mm bore) that aproximate the correct Inch size (1.500"). If I ordered the correct Inch size, the cost soar to about $175 for those two pulleys!

And I ordered them with a simple bore, no set screw, no keyway, just a stright hole. I reason that these aluminum pulleys represent about $2 of aluminum (at the most) and perhaps 10 minutes of machine time on a 4 axis machine. So it seems like a part ripe for DIY'ing! What do you folks think?

I already blew $100 on those two pulleys and discovered I have need for several more of them, of various sizes. What is the simplest and most cost-effective way to build these? Can it be done easily with a 3 axis machine?

Perhaps I could use an indexer and cut one tooth at a time, manually indexing to the next tooth? I don't mind if this takes an hour or two to make a gear. Would this indexer do the job?

http://www.use-enco.com/CGI/INSRIT?P...&PARTPG=INLMK3

I'm sorely in need of some machining advice on how to do this. My gut feeling is that the best way to do this would be to simply use a 4-axis machine. But I'm kind of up against a boot-strap problem; I need the new pulleys to make my 4th axis.

Actually, I will be hobbling together my 3rd axis pulley too. Hopefully that one will last long enough to help me machine its own replacement!

So please chime-in and offer any comment you can. I'll take it all - from hair-brained idea to tried-and-true techniques.

Thanks in advance!

[damae]

I should add that the minimum radius at the root of each tooth is about 1mm (.040"). That means a .080" (2mm) diameter tool if I try to cut it from the top view. Of course, a tool that small has a cutting length much, much smaller than 1" deep. That's why I tend to think it would require machining on an indexer or 4th axis.

Please help me with some innovative solution. I'd even take interest in broaching if it's something easy to setup. Any idea is welcome!

[Verfur]

I belive an indexer/ fourth axes would do the job. I belive you would need a formed tool on a horazontal machine, so as to cut the bottom radi and the ramps on the teeth at same time. I cant imagen the time it would take to use a small ball nose end mill even after a rougher.

Let me know if you wind up with a way and how much you would want to mill a few more as I too think the prices are high from the hobby stand point.

John

[damae]

... I belive you would need a formed tool on a horazontal machine, so as to cut the bottom radi and the ramps on the teeth at same time. I cant imagen the time it would take to use a small ball nose end mill even after a rougher.

Let me know if you wind up with a way and how much you would want to mill a few more as I too think the prices are high from the hobby stand point.
....

I agree that ball end mills probably would be slow, because they would have to be of small diameter to get the small radius at the bottom of the groove. And the small diameter limits the depth of cut and feed speed.

That's why I was thinking I could use a rounded endmill like this one:

http://www.mcmaster.com/param/asp/PS...MainWidth=1068

This type of endmill could be the full width, as wide as the flat at the bottom of the groove. It's really just a flat end mill with a small radius at the outside edge. It should allow me to have a larger diameter tool (up to about 5mm or 0.2") and make deeper cuts.

So it seems like with some work, this setup might work. However, I am really hoping some machinist here has a hot tip for me about how to aproach this problem. For instance, I read this tip in another thread:

To chuck up a gear (or toothed pulley) in a lathe, first lay dowel pins between the teeth and then clamp onto the pins. This transfers force to the root of the tooth and prevents the tooth surface from getting marred up.

There are probably a dozen other gear/pulley making tricks out there! If you know of one, let me know! If you don't but have a crazy idea or comment, let me know that too! =)

Thanks!

[John S.]

Damae.

In alloy you can flycut each tooth with as simple cutter.
If you are worried about a flycutter lasting up then you could mill the bulk out first.

Here's a link to an excel spreadsheet that gives you all the info you need for cutting pullies.
The tip width and DOC is common for one size and the rest of the calculations are done when you enter the number of teeth.

http://homepage.ntlworld.com/stevens...%20pullies.xls

John S.

[damae]

Thanks for the spreadsheet John! For H-Series pulleys it recommends 0.165" cutter width by 0.120" depth of cut.

Can you elaborate for me what a flycutting setup would look like? Sorry for the ignorance. Until now, I thought flycutting was used only for surfacing flats, as in squaring up a piece of plate.

[John S.]

Crap drawing but dinner is ready and it's no contest

[mxtras]

Damea -

Have you looked into buying timing pulley stock and then cutting to width and boring it? I can't recall the cost - I may be off, but I thought the stock pricing was reasonable....?

Just a thought.

For the time it would take to make a pulley from scratch, I would shell out the cash, but my time is limited (except here at work :0).

A fly cutter is a good idea - you would have to feed slow for a smooth finish, so time would be a consideration.

Scott

[Verfur]

damae,

This is simaler to what I was saying only I would use a horizontal mill with a form cutter like mxtras is useing. the bottom of the vallys and the sides of the teeth are not flat, the bottom has a radi to match the OD. - depth of tooth and then the wall leads up on an angle. This makes the tooth look like a piramid on the top of a hill.

John

[ynneb]

Gear hobbing machine.
http://pacificsun.ca/~robert/travel2...h/Dscf3068.jpg

[damae]

Thanks to all for the excellent advice so far!

John, thanks for the sketch -- it's very clear and gives me a lot to think about. I'll need to buy a flycutting arbor, but that shouldn't be too expensive, I hope.

Verfur, I see you're right -- it's not a flat bottom, just a large radius, at the bottom of the groove. So I could only aproximate the bottom with a 5mm rounded endmill.Since trapeziodal belts are supposed to run with a small clearance at the bottom of the groove, perhaps I could get away with it?

Ynneb, thanks for the fascinating picture of the hobbing machine! A lot more machine than I want to be operating just to make pulleys. =) I'm still hoping for some kind of simple way to make these gears on my multi-purpose 3 axis machine.

Mxtras, the timing pulley stock could work for me if I have a source to buy it from. H-pitch pulley stock has proved hard to find. Do you know of anyone selling H-pitch stock? The upside? This is certainly justification for me getting a lathe! Hehe. (actually, it's unavoidable. I'll be stuffing a lathe into my garage eventually)

What about the spin/indexing fixture and method to make sure that I rotate the correct amount each time? I am hoping to cut the pulley blank on the mill, including the center bore, then fixture it from the ID. Any experience to offer here?

Thanks!
-D

[buscht]

damae, Here's another approach that might work.

This is just an example, you will have to adjust for your situation.

Assume you want a 1" thick pulley and you need 1/16" inside radii. This means you need a 1/8" dia cutter and the 1/2" cutting length on your cutter is inadequate.

Simply make 4 pulleys -1/4" thick. Use your CNC to put accurate dowel holes in each one, along with some bolt holes. After cutting all 4 parts, line them up with the dowels and bolt together.

[HuFlungDung]

Damae,
That little indexer you referred to in your first post most likely gives you very limited options. It might permit up to 24 divisions, or factors thereof, so it would not be very versatile for a wide range of tooth numbers on your pulleys.

Keep in mind that the most critical dimension of a timing belt pulley is the outside diameter. This is actually the pitch circle of the belt. If this diameter is off, the teeth of the belt will want to climb on the pulley teeth.

I've cut a few timing belt pulleys with a pair of slitting saws ganged on an arbor, spaced approx two teeth apart (straddling one tooth space, and cutting on the left side of one tooth and the right side of the next tooth). This generates a slight angle on the sides of the teeth. I usually just overcut the depth a bit, rather than worry about the fillet on the bottom corner, which serves no real purpose. The fillet on the top corner of the tooth is more important, but can simply be broken with a file, or radiused with a power rotary brush on your bench grinder.

[damae]

...
Keep in mind that the most critical dimension of a timing belt pulley is the outside diameter. This is actually the pitch circle of the belt. If this diameter is off, the teeth of the belt will want to climb on the pulley teeth.
....
I usually just overcut the depth a bit, rather than worry about the fillet on the bottom corner, which serves no real purpose...

Taking a closer look at my used timing belt pulleys I found pretty solid confirmation that the belt tooth is not intended to touch the root of the pulley groove. In the attached picture, you can see that all the contact surfaces -- the root of the belt groove and the sides of the trapezoidal teeth -- are shiny from wear. This pulley probably operated for years; my Series I BOSS3 machine is likely 25 years old. Yet the nylon fabric covering the tip of the belt's teeth are still soft and fuzzy, just like a brand new belt I have on hand!

So I will do as you suggest and just cut the groove deeper than needed. The ideal diameter is about 0.119" but I'll just use a 0.125" cutter and overcut the groove depth by .006"

[damae]

...Simply make 4 pulleys -1/4" thick. Use your CNC to put accurate dowel holes in each one, along with some bolt holes. After cutting all 4 parts, line them up with the dowels and bolt together.

Excellent Idea! I modeled it up and it looks pretty simple. This should be the best method given my current resources. It won't be the prettiest pulley, but hey, it should work!

My $100 custom pulley order from Misumi arrived today and the pulleys are fitted to the Z and X axis motors. This "layered" pulley will complete my Y axis, and get me up and running, mechanically. It's a stepping stone to building out my 4th axis and then making pulleys in one piece.

Thanks to everyone for the top-notch advice so far! I'll keep you updated when my schedule allows me to cut these parts and try them out!

If you're curious about the machine these pulleys are going into, you can check out my BOSS3 retrofit log:

http://www.cnczone.com/forums/showthread.php?t=13821

[mdreitzusa]

here is an article you might be interested in.http://www.metalwebnews.com/howto/gear/gear1.html

[dandumit]

I've cut a few timing belt pulleys with a pair of slitting saws ganged on an arbor

Hi ,
Even it's an old thread, I'm interested about subject. I have seen recently one movie on youtube about making timing pulley and was very fast (unfortunately I have lost the link)

It was about using this kind of tools :
http://www.unitedcuttingtool.com/Timing-pulley-hob.html
And an indexer between spinning axle of hob and 4'th axis.

Do you have any hints or examples ?

REgards,
Daniel

[fatal-exception]

I've used the fly cutter approach with a rotary table. It works well. In 10 minutes you could have a form tool ground, and 10 more minutes you could have the whole setup ready to go on your mill. Once it's all setup, cutting the teeth would be a breeze. They will all be the same and concentric with the axis of revolution. You could take multiple depth passes if required.

I guess this is just one of those things that you have to try if your skeptical about it. Once your on your way, you will wonder what you were worried about.

[rowbare]

Hoss has a video showing him cutting timing pulleys for his G0704: http://www.cnczone.com/forums/showpo...&postcount=232

There is also a link to one of Hoss' videos here, this fellow has written a special program for designing timing pulleys: http://www.cad2gcode.com/pulley/

bob

[machinistchest]

I`v often cut gears with a fly cutter and a super spacer/rotory table. I made a 90 degree flycutter that holds a 3/8" tool bit. It comes in handy when cutting form tools from drill rod. I just stick the holder in the lathe and turn the bit. I get a right hand tool on one end and a left hand on the other, then flame harden. I found this to be much faster than traditional grinding. I get minimal radii in corners as opposed to a grinding wheel breaking down on the corners.

Would a timing belt have a acme thread profile 28 included? ...MC