Making Gear For Paper Shredder

[Grandpop]

Please bear with me, this will be long and photo heavy if I can figure out how to do this correctly.

I have had a 12 sheet crosscut paper shredder for about 10 years. Was shredding 6 years of old financial / tax stuff when I jammed it up and heard the gears jump. Took it apart and found this plastic helical / straight gear was the problem – it had 3 teeth torn off.



Searching the internet and calling the OEM only confirmed that the gear could no longer be purchased. Equivalent paper shredder was about $200, which seems pretty expensive for a stupid plastic gear. So looked around on the hobby machine shop pages, others, and Youtube videos, I found what appeared a simple method for making a gear hob, plus it looked easy to cut the teeth with the tools I already had. Keep in mind that while I used to be a tool & die maker in another lifetime, I never attempted to make a gear before. Decided to try it, and if it didn’t work I could go buy the new shredder then.

Since the 9 tooth straight gear was still good, I was originally going to just make a band for the 37 tooth helical portion. I don’t have the special equipment to cut a helical gear, so decided to approximate it as just an angled straight gear profile. The theory is that you make a 5 tooth gear hob where the center tooth cuts the most, but the 2 teeth on either side continue to refine the shape of adjacent gears till they match the elliptical shape automatically. Only trick is to get the spacing between teeth correct, and the width of the teeth correct. So I did the math for a 37 tooth and roughed out a O-1 gear hob.



Went back and measured the depth of each tooth with a pin, was using the dial indicator to give me the pitch between teeth. Worked pretty good! I was felly really good about this plan right about now.


That is it worked great till I held the plastic gear up to my new hob and discovered that the spacing was at least .010 per tooth too small! Apparently the formulas I used were for straight tooth gears only, and the angled gear I need had a cosine error that I ignored. I had about 3 hours into the hob, so decided to cannibalize it and just go to single tooth fly cutter type gear hob. Was able to use the original hob and just make the diameter a little smaller - ended up with a nice little 4 tooth cutter ready for the propane torch heat treat / quench.


Next problem was that the 5C collet spin indexers all only come with 36 holes for 1° indexing, and don’t come with the sides milled to hold in a vise. So first I took the indexer apart and clamped it to an angle plate with 123 blocks under each end of the spindle. That allowed me to clean up the first side, then flipped it over and did 2nd side to same dimension. After the rough I took a nice finish cut on each side (no photos). Now I need to make an indexing plate with 37 holes in it. Found a piece of 3/16 plate in the scrap pile and sawed it into a rough circle. Had read before on the hobby sites that it worked to push the disk up against chuck jaws with a center if you took light cuts, so tried it. I could only take .010 deep cuts, but did it no problem. Only took about 20 minutes to make it round and too size.



I had to reverse the jaws so could now hold it on the OD to bore the ID a light tap fit for the spindle.



Next I set it up in the mill and swept the ID, then used the readout to center-drill, drill, end mill to location and near size, counter sink and ream all 37 holes. That took about 2 hours alone! But I now have a finished indexer 37 hole plate.



Now when I put the indexer back together I am finally ready to make the gear itself.



Still not certain this would work; I set up the indexer on the plastic gear to make sure this will work. This should give you an idea of how I am going to try to cut the angled tooth portion.




Will finish this later...

 

[4GSR]

Well it won't be perfect, should be get you halfway there. You may have to do some filing and fitting to get it to work!

 

[Grandpop]

I was originally going to just make a shell that I could press over and key onto the good portion of the straight gear, but I didn’t see any good way to hold that all together. I am going to make both portions on the gear. First thing I needed to do was make up a little disk to fill in the undercut they had on the plastic gear. I need this so I can use this surface on the old gear as a stop when I go to set up the new gear in the indexer. Then I quickly whip a new gear blank. Was a bit sketchy to bore that .236 hole all the way through, but found a boring bar and took lots of light spring cuts.



Everything I read about the gear hobs said they don’t work well for tooth counts below 20 teeth, and I didn’t have room for the shank to run out anyway, so I decide to just approximate the 9 gear tooth profile with a bunch of various diameter bores that are plunged straight down. I set the old gear up and measure 16 different pin sizes between the gear tooth back to the center hole, then draw it all up in the AUTOCAD. Since I could not find all of the end mill diameters to match, I did some adjusting for the end mills I had (I will use 9 different sizes). I make up a giant chart of the locations for each diameter / tooth, and then start by drilling the very bottom of the gear tooth.


This is after the 3 largest end mills; I am going smaller each time so less to remove with the smallest end mills.


After 3 more end mills:


And after the last three:


Still not a perfect gear profile, but you can see the tangent points at several places along the tooth profile in the above photo. I started by trying to file the raised areas down with small needle file, then finally figured out that that a hand scraper would do this a lot more quickly. That went so well that it only took about an hour to finish up all of the teeth. Looks pretty close to the original!



Now on the helical impersonation gears! At first I set it up on 25°, eyeballed the center of the .236 bore, and cut three teeth in rough. That didn’t work – the straights / flats between the teeth were not parallel, and the teeth looked tapered.



After some thinking, I decide that I need to the cutter to be on center in the middle of the tooth, not the outside edge of the tooth. I have to scribe some lines and go set up center again with a 20X eye loupe. That means the teeth I roughed in are in the wrong spot, so I have to loosen the collet and rotate them till they match the new cutter location. That was much better, but the straights were still not parallel. I decide to just rotate the indexer/vise around a bit, and tried to indicate the surface at .005 taper per the .480 wide profile. That was better still but not quite correct, so I re-indicate to .008 taper. That made parallel straights / flats, so I cut all the way around with a roughing cut, one index at a time.



I need an easy way to check the tooth depth, so I just slide the old gear into the bore of the new gear and feel for the mismatch. I move the cutter over to the plastic and move the table till the cutter just touches, then re-zero the readout. I back up .005, and try the next cut all the way around.



After that second cut I place the plastic gear back on, and not sure why, but the profiles match up really well. Am I done? Can I measure this to be sure? I mark the teeth for three pins to use like measuring a thread, and measure the plastic versus the aluminum gear. They are within .001 of each other; I am done cutting. It will either work or it won’t. I file some of the little burrs off and try to smooth out the edges, but for better or worse, this is done.




I try the gear back in the shredder, and it seems to rotate pretty well. I put it all back together, and it rotates OK – a little bit of noise, but I can rotate it all by hand. I figure if the straight vs helical error is a problem, then the hard helical drive gear will eat away the soft aluminum gear and it will quiet down. The aluminum gear goes from the helical motor shaft and drives the big white one. For now this works really good, so back to shredding the next 6 years of stuff!



I have since shredded 2 more years of stuff, and this works / sounds good. I guess after about 30 minutes of run time I will pull it apart and see what the gears look like, but for now, this works good enough. Thanks to all of the folks that shared their knowledge on the various sites, I was able to pull this off!

By the way, the look in my Sweetie's eyes was priceless when she came home and found me shredding again...

 

[intjonmiller]

By the way, the look in my Sweetie's eyes was priceless when she came home and found me shredding again...

I love that.

So many times they have no idea what we've accomplished, or what we've had to work through to get there. I sometimes try to explain but my wife's eyes glaze over, the way mine do when I hear about drama with her sister or whatever. But every now-and-then we get to work on something that interests them and it's fun to get that reaction. I saw where my wife filled out a paper for our daughter's teacher at the beginning of this last school year and in the volunteer opportunities section she wrote that I "can build just about anything anyone needs." While that's a stretch, I certainly appreciated her approval.

 

[brino]

Cool! I have never seen or heard or thought of making a spur gear by plunge cutting with end mills.
Interesting approach.

I probably would have made it as two pieces and pinned it together, being afraid that my last cut on the second gear would be wrong and I'd have wasted all that time.

Also, great job of photographing and posting. I know that can take a bunch of time and effort too!

Thanks,
-brino

 

[Andre]

I would've cut a small relief at the shoulder end of the small gear, then used the lathe carriage with a ground form tool to broach the tooth profile. A rotary table bolted to the lathe faceplate works pretty good for this kind of stuff, I broached a timing belt pulley in a similar way. Lots of ways to peel the monkey.

Nice work!

 

[Grandpop]

I tbought about lathe broaching to clean up the teeth of the spur gear, but I didn't have a way to index it, and didn't want to spend the time to build an index plate. I never though of the rotary table bolted to the faceplate- I will have to try that sometime.

One thing I learned is that it is pretty easy to duplicate a part that you have in your hand, but I don't think I could have made this from scratch from just a drawing!

 

[JimDawson]

Wow, I'm impressed. Great job!

 

[Grumpy Gator]

Nice work. I know the feeling of "If I can hold it I can build it."
*G*

 

[planeflyer21]

Very well done!

Definately best keep your fingers, clothing, and hair clear of the intake on that now.