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Making Rear Sprocket: Few Questions

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Ok,

I drew up a design for a sprocket and have a few questions. If I machine it out of 6061 aluminum, should I expect it to get eaten up extremely fast? If so, what should I be looking for, 7075? Also, does anybody have any information on what type of fixture I should look to make to set the sprocket up for machining? I was thinking a square block with the center bored out and the bolt holes drilled/threaded, and just bolting the sprocket stock to it? Any pictures of these fixtures that aren't set up for production jobs would be awesome. All my cutouts will have a chamfer on the edges and I want to be able to flip the stock over and machine both sides without screwing up the work's position.

Any help/ideas are appreciated.

Thanks,

Tim

Here is my drawing in mastercam. I was aiming at a design sort of like tags' sprockets.

sprocket.jpg

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I build things myself from aluminum and steel all the time. Often it's to save money, or because it can't be bought elsewhere, but many times it's just for the thrill of making it myself. If you've got a MetalSupermarkets close by talk to them. I buy my stuff there and they have other allows besides just 6061 and 7075. Just so you know, you can buy an RMA aluminum sprocket for $16.

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It looks like you have a class project going.

Are you going to CNC mill the sprocket ?

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At school, but making this for fun on my own time. I can get plenty of 6061 from the school, but I might have to dig a little deeper to find something a bit more strong.

And yes, the sprocket will be cut out on a cnc mill. Mastercam practically writes the program for you, you just tell it the details like what tools you're using and depths and whatnot.

I'm thinking of squaring up a peice of stock, then boring the center of the squared peice to size. Then drilling/chamfering my bolt holes. Thn I will take this square peice with holes and bolt it to some sort of fixture where I will machine my pockets and teeth. Thats the way I am thinking of it in my head, but am looking to see if there are other methods that are more simple/quicker.

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And yes, the sprocket will be cut out on a cnc mill. Mastercam practically writes the program for you, you just tell it the details like what tools you're using and depths and whatnot.

I know what you mean. I use MasterCam V.10 at work and love it.

If you can get your hands on a second, relatively thick plate (.500"+) of aluminum, you can hold this in the vise, or clamp it to the table. Ideally, the plate should be larger than the o.d. of the sprocket. You can then clamp the sprocket blank to this, then drill the bolt pattern in the sprocket blank. Remove the sprocket blank, then drill the same bolt pattern in the fixture plate, but for the tap-drill size.

Tap your bolt threads in the fixture plate. Then you can bolt the sprocket down and you'll have a repeatable/concentric X/Y axis. Your Z axis can be zeroed on the fixture plate. Now you can cut the O.D.,the teeth, mill the cutouts, and interpolate the inside diameter all in one operation.

I hope this helps.

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The 7075 is gonna make a lot structurally stronger sprocket. Personally, I wouldn't recommend 6061, if you're gonna use it on a big bike. You could always use the 6061 for proof material, though.

I assume you are planning on anodizing it ? If so, the surface hardness of the teeth after anodize will be comparable, with either material.

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I don't think you'll be standing the sprocket on end to do any milling. The vibration will be your first issue. What you need is a corner rounding end mill or possibly a special tool made, depending on what kind of profile you are wanting. Might even check with Iscar Tooling to see what they make for that. I don't think the 6061 will last long, but if you have the program made it's worth a try.

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You could always use the 6061 for proof material, though.
I don't think the 6061 will last long, but if you have the program made it's worth a try.

Do you guys run the CNC programs on plastics, etc for prototypes?

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Do you guys run the CNC programs on plastics, etc for prototypes?

Not normally, unless it's a real sophisticated program and/or high $ material. The programs can be proven out with both the MasterCam software and the graphics on the cnc machine tool before a cut is ever taken.

We do have a 3D printer that can produce a prototype out of ABS plastic using the file from a CAD program.

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Thanks for the replies. I'm definitely going to prove it out with 6061 to make sure it bolts up and everything works out since it's readily available. 76xtdvr that is a good idea, along the lines of what I was thinking. Making the holding plate larger will eliminate a lot of vibration

zwr250f, I have many endmills available, corner rounding, ball, etc. I was going to use a straight endmill for everything but the chamfering, what issues would there be with this?

And slowspokes, if I anodize the 6061 its hardness will be comparible to 7075? I didn't know anodizing is a hardening coat type of deal.

Thanks for the help,

Tim

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I was going to use a straight endmill for everything but the chamfering, what issues would there be with this?

For any chamfering, you can preset your Z axis high in your program, and then keep adjusting the Z setting in the mill to "sneak up" on the desired chamfer depths. Also, make sure the countersink angle for your sprocket bolt seats match your sprocket bolts. Probably an 82 degree c'sink cutter.

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Do you guys run the CNC programs on plastics, etc for prototypes?

No, between surfcam verify and Solidworks there is no reason to. Also, assuming your setup guy is half-assed competent they should be able to see any obvious errors on your part.

Many of the jobs we run are prototypes, we might run 30 jobs a week of quanities of no more than 2pcs/ea.

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T

zwr250f, I have many endmills available, corner rounding, ball, etc. I was going to use a straight endmill for everything but the chamfering, what issues would there be with this?

Tim

No issues that I know of, as long as you can fit the tool in there you should be fine. The sprocket itself is not thick, as far as worrying about taking deep cuts. However if you have relief around the teeth as many sprockets do you won't be able to machine them easily with a square endmill. It's possible but not desired.

good luck..post some pics when you get it done.

z

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Yeah I took note of the relief cutouts on most sprockets. I'm going to cut one out straight and see how it is after I run it for a bit. Next week I should have atleast one sprocket cut out. Monday I'm going to start working on my fixture.

I'll be sure to put pictures up of both.

Thanks,

Tim

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Colored Anodize (Soft Anodize) will not help hardness. The reason for 7075 is through hardness. It will be much harder for sand to destroy the sprocket.

Don

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Regular Anodize will not help the hardness much, but you can have it hard anodized. That will penetrate deeper into the surface, however, you will be limited to the color of light grey to black. I believe a few companies are offering hard anodize in maybe a green or brown now. We don't currently do that color, though.

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OK,

Haven't updated this thread in a bit. I have finihed my fixturing for the sprockets, and cut one out of particle type board today just to see how it ran. I ended up with three fixtures to complete the sprocket. The first fixture is for doing the bolt holes and the center pocket(for the hub). The second fixture I bolt the drilled square onto it and cut out the rest of the sprocket. The third fixture is a lathe peice that holds the sprocket so I can turn down the 1/4" stock to the proper thickness, and put a relief on the teeth.

Here is a picture of me cutting out the particle board sprocket..Not sure how long this would last on the bike haha.

sprocket3.jpg

In the picture above you can see me roughing out all the pockets. I come in with an 1/8" endmill to clean up all the pockets.

Here is the finished particle board sprocket.

sprocket2.jpg

sprocket4.jpg

And here is a crappy picture of the engraving I put in it. I have 6 "174's" engraved around it, since that is my number.

sprocket1.jpg

I'm still calling around to find a decent price on some stock, then I'll start cutting out some metal ones.

Let me know what you think,

Tim

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6061 is usually hardened after milling, T-6 is the hardening protocol. I believe 7075 also has to be hardened too, can't recall the method or protocol though.

I may be wrong, this is from memory from long ago, but 6061-T6 will bend long before it breaks, 7075 will up and break on you without warning. Don't know if that help as respect to wear, but it seems to me that the 7075 will last longer since it has more tensile strength and hardness.

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Dr.Thumper,

After looking on several companies websites, it seems most make there sprockets out of 7075, and things like triple clamps are machined out of 6061. So I'm thinking 7075 is the material of choice for the abuse that sprockets take. I may even cut one out of steel and run it for a little while to see how it holds up.

Thanks,

Tim

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