|||||You Spin Me Round - Dead or Alive||]|
Ok, so I've set up my milling station to hold eccentric workpieces in a turntable (I'm not going to mistake calling it a true "rotary table" since it doesn't have a crank or any kind of indexing functionality), so now I can start reworking my oscillator cam in my sickle pruner described in my previous post. First, the mill:
That's a 4-jaw independent chuck for a Harbor Freight wood lathe. I originally had big plans for making a digital dividing head with it, but that was a bit too ambitious and I needed something to work right away, and don't really need to index anything, just turn around an offset axis. It mounts with a 3/4" 10 thread, so I just drilled a hole into one side of some scrap square tubing and bolted it down with some machine bushings to let it rotate stiffly by hand. Slap it in my 2-axis vise and now I have a crude 4-axis mill. Yes, it's a HF drill press; yes, it's a drill chuck; but I managed to rig a perpendicular draw pin to slide into the 2MT tang removal slot, so it self-tightens and the runout is low enough so that I can use a larger-diameter end mill without problem and get a fairly decent finish. For prototyping purposes, it's accurate enough to get the job done.
And now, the job...
Here is the reciprocating blade head and cam mechanism for my hydraulic Concord grape pruner:
The cam is upside down in order to show the drive pin; flip it over and the pin rides in the slot running perpendicular to the blade assembly. The camshaft mates with a hydraulic gear motor with a spline shaft and rotates around the axis of the flower-shaped hole. That surface came from a salvaged drive gear from another piece of tilling equipment that got the power train upgraded, leaving me two motors, a working gearbox and a scrap gearbox, which was fun to break down to pull out the seized gear. You can see where I plug welded it on the backside to a mild steel disc, leaving a raised boss on the other side with a clean shoulder, no filet welds on the mating side so it can rotate more freely.
The same was done to make the drive pin using a bolt with the hex head cut off. This is the problematic part. First, the plug weld failed and cracked and was repaired multiple times. I eventually fixed that problem by having someone use a more powerful welder to get better penetration, as well as run a clean fillet weld around the working side of the pin as well. However, this strengthening of the joint only revealed the weakness of the material itself. It would run fine for a few days, only to seize up once the inside radius wore down to where it locked in the channel.
So, now instead of welding up another composite piece, I'm going to mill the sucker out of a single slug. This eliminates much of the joint weakness and, more importantly, allows me to use higher-carbon alloys I just can't weld with my equipment, so that I can effectively heat treat it to harden it. I'm going to get a few blanks from online sources, most likely from speedymetals.com, because they sell tool steel at 1" lengths so I can afford even to go that route. I'm no metallurgist, I've only every tried case hardening with Cherry Red before (and that didn't work out so well), and even with the incredibly informative and helpful materials info library on the speedymetals website, I could still use some advise on selecting the right alloy to use for good machinability and wear resistance. By simply selecting metals recommended for use in cams, I have come up with the following candidates:
1) 4140 annealed cold roll
2) Class 40 grey iron
3) O1 tool steel