When i first started in the sport i had quite a few instances of BZM 2 shoe clutches tearing the linings off and burning up very quickly. Mostly due to misuse the first few times but the last pair of clutches just let go under barely any load at all. So i wanted to make a clutch that was fairly "idiot" proof and much more robust then the ones available at the time.
Since then new clutches like the 2A and the BZM delta (which get a bad rep but i like them, they work well for me being a slower rider) have come up which sort of answered my problems, but for the sake of do it yourself'ness (is that even a word?) i wanted to finish my home made one.
The design was shamelessly ripped off from Frank Giannini's which consists of KTM 50SX pro clutch shoes on a custom aluminium hub. I chose this style because the shoes are easy to buy locally at most dirtbike stores, and they were cheaper then the bzm ones at the time. The other main bonus was the clutch material covered nearly 360 degrees. So the hook up should be awesome. This is Gianninis original style.
A problem with this design however is that they use special cone washers to act as the clutch spring and using various stack shapes and thicknesses is how you set the engagement RPM. they also heat up and wear out after a few hours use and need to be replaced which isnt so good. So an ongoing part of the project has been to find a spring equivalent. The other change i made to gianninis design was to make the hub small enough so that the shoes can fit into a standard 80mm bell instead of the 82mm one they came with (i believe this was the case? maybe someone can educate me there) so thats one less job i had to do. boring out clutch bells is a nightmare.
Now making the clutch itself! Unfortunately i didnt take any photos of the process itself cause it was made at a mates house using his much more substantial machinery, but ill give a quick run down on how i did it anyway if any one cares for the process. first some pictures!
To make the piece i started out with a 60mm round of 2024 aircraft aluminium 12mm thick. it was bored for a 10mm center hole in the lathe. After this i then made a little jig to hold the round piece in the rotary table for machining. The hub is a sweat fit on to it and an m6 bolt clamps it solid.
After this was done i mounted the rotary table on the mill (all 50 something kilos of it) and clamped the jig in the 3 jaw chuck mounted on it. If you havent seen one before they look like this. it allows you to rotate your work piece accurately, usually to at least within a degree.
this allowed my to then accurately mill the 3 faces at 180 degrees to one another. i was able to get all 3 heights milled exactly the same via the use of the digital readout attached to the mill. they look like this - it allows the position of the cutting tool to be positioned within a thousandth of a millimeter.
with all 3 faces milled to size i was then able to drill the holes. i basically copied them off the original hub i already had so combined with the DRO it should have been easy.
EXCEPPPTTT there is only a readout on the Z axis (up and down) and X axis (side to side). the rotary table was mounted on the Y axis (front to back) which is the only way it would fit on the mill table because it was so huge. so i couldnt use it to drill the holes making the job alot more drawn out.
SO i had to do it the slow way which was putting the hub in the milling vice and manually lining it up 3 times to each 3 sides. to do so required the use of a dial indicator to make sure the flat on the clutch hub was perfectly level with the mill table. i was able to get it to 0.02mm. this is what a dial indicator looks like..
With the flat of the clutch hub level with the mill table i was then able to set the DRO up. So we know were to drill the holes in the hub we have to know where the edge of the hub itself is. To do that i should have used an edge finder - when it touches the work it lights up letting you know that it is positioned exactly at the edge of the part - the radius of the finder itself. handy!
Well unfortunately i couldnt find any batteries for it. so i had to hit up the interwebs at about 4am to find out what else i could do. fortunately the center finder kit also had an edge finder tool in it that worked great. i never knew what that bit was for until now!
With the DRO set i could then drill the holes for the clutch shoes. the center one is 5mm tapped to m6 for the clutch spring retaining bolts to sit in and the outside two are about 8.5mm for the 8mm pins in the shoes to sit in.
That was a bit of a nail biting moment because id been working on the thing for about 5 hours at that point. and if the holes didnt line up i had to start over. fortunately they all came out PERFECT. I LOVE DIGITAL READOUT. (in the healthy way of course)
The other three sides came out much quicker and all 3 came out equally perfect. Next job was to throw the hub back in the lathe and turn a bit off the face of it because it was a little thick and bore a recess in the front to allow the crank nut to sit further on the crank.
Finally the keyway! seems not many people know how they are made, so here it is. Its done using a large arbour (mechanical press) a small hub that fits inside the hole youve drilled (in our case 10mm) with a guide in it for the keyway cutter (the broach) and the keyway cutter itself (3mm wide for a 10mm hole).
The keyway cutter itself is tapered so it cuts deeper as its pushed through. There is a shim you put in behind the cutter to make the keyway a little deeper if neccessary. it wasnt needed here.
Wishing i took pictures of the build of it now, gawd dammit. but ah well. Definitely learnt alot through the making of it. Especially the usefulness of a DRO and rotary table. If i needed to make one again i could definitely make one start to finish in about 2 hours max instead of 7 or so. Providing there was a Y axis read out of course ;)
Next job to do on the clutch is get new cone washers and retaining bolts because mine are ruined. then work out how to replace the cone washers.
Main idea at this stage is to obtain some 8mm ID springs from somewhere (possibly a scooter) and then use a threaded stud instead of a bolt with a nylock nut on top to adjust the preload.
And thats all for now. Next time i mention it, this baby will be in my 911 and working :)
Next blog = hopefully some CNC parts.
No comments:
Post a Comment