Friday, February 12, 2010
Persistance Is It's Own Reward
Yesterday I poured , yet, another housing. I am happy with the result. As you can see, it is a bit different from the original one. I added a bump out on the back for the inlet instead of the flat back. This seemed to be a good idea, but it introduced a few new problems. The first was the issue of shrink defects. I actually poured another one a few days ago and it had a moderate divot on the rear bump. It was a usable casting as is. The defect was more visual than anything else. It upset my eyes to look at it, so I decided to make another. The biggest problem was that the bump made it, nearly, impossible to chuck it in the lathe. It needs to spin on center in both the horizontal and vertical planes to properly finish it. A huge PITA. My solution to both problems was to drill the back of the pattern and use a removable dowel for a sprue. This allows extra metal in the area that is prone to shrink and, since I drilled it on center and perpendicular to the mounting face, the sprue can be used to chuck the housing in the lathe. It worked out just fine and the machining process was a snap. My big problem now is to find the proper bit to drill for the 3/4 NPT fittings. You don't just go to The Borg and pick up a 29/32" blister packed, Chinese cheepo bit off of the pegboard. I think I will try to bore it out. We'll see.
Well, as usual, I screwed up and ruined my beautiful casting. I tried to bore it out and something went awry. The hole ended up egg shaped and too large at the same time. So much for experimentation. I went straight to Ebay and ordered the proper 29/32" bit. I should have it by the middle of next week. At least I have the casting process down to a science. I'm busy tomorrow, so maybe Sunday I will give it another shot. I think I will go ahead and pour a couple of housings, while I'm waiting for my bit to arrive, so I can have a spare. I'm normally not a very patient person so maintaining my cool has been a challenge. So far there has been little cursing and no thrown tools/parts. I can last for only so long, though. I've lost track of the number of pours that I've made.
Today is Sunday. I poured another housing today and it came out very nice. Probably the best, so far. Maybe I'm getting better or luckier. I'll take it however I can get it. I faced it off, bored the cavity out and generally cleaned it up. I will resist any urges to try to drill the inlet and outlet openings until my drill bit arrives. I can ram up and pour one of these things in about 2 hours, but I'm not anxious to do it again. I still have to make the pulley and a mounting bracket. The pulley will be cast aluminum and I will probably cut the bracket out of 1/4 or 3/8 CRS.
The first pic is of the sprue being trued up so I can turn the casting around and chuck up the resulting protrusion and face the housing and bore it to size. The next pic is of the semi finished pump. I just have to drill and tap for the 4 studs that will hold it together and the inlet and outlet. The sprue will be cut off since I no longer need it.