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Building a New Lathe

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Neubert1975:
very nice casting quality.
Mine often gets porous and/or often ends up being to soft for turning in the lathe.

vtsteam:
Thanks Neubert.  :beer: If too soft, it probably means you're using extrusions or other unknown scrap. Try using only aluminum castings for scrap. They have a high silicon content.

Porosity means different things to different people. Occasional very tiny bubble dots (needing magnification to see), are usually the result of dissolved gas -- often ascribed to melting in steel or iron crucibles. But many scrap castings have steel in them already (pistons for instance have internal steel reinforcing clips) or rust washed over them from outside junkyard storage, used motor oil sludge, varnish, piston rings, etc. so it's pretty much impossible to avoid ALL ferrous contamination unless you're using virgin ingot material and a non ferrous crucible. Or you can add a clay and sand liner to a steel or iron crucible. I don't usually bother doing that for my own aluminum castings, and nothing I've built has fallen apart yet. But to each his own in that department -- many internet casting discussions go back and forth repeating the same technicalities, discuss de-gassing, etc. Not many using involved methods actually build much of interest to me or detectable superior quality from what I've seen. But again, the game is the important part, not necessarily the result.

If by porosity you mean bigger bubbles and hollows, the cause is often too moist greensand, or failure to skim the melt, or some other sand contamination. Pouring too hot, is also a cause. Unless you're pouring super thin castings, or lost foam, aluminum should just be fluid, in my opinion, and not super-heated. Venting with a wire can help fill difficult cavities, though I often forget to, and usually things work out.

vtsteam:
I've spent a good part of the morning thinking about XL timing belt profiles in order to make a cutter, and there's just no solid info on the likely tooth shape for a pulley the diameter I'm shooting for.

For making a cutter, there's the old grind it and check against an existing pulley method, more or less okay for a fly cutter, but I'm hoping to turn a profile. I do have info on the straightened belt tooth profile (rack), but that doesn't tell you what the pulley's curve will do to it.

First thoughts: straight belt profile shows a 25 degree offset from vertical for the bevel. Since I'm bending things 4 degrees per tooth in a 90 tooth circle, Maybe I should split the difference and call it a 2 degree reduction in the angle, so say 23 degrees for the pulley bevel(s).

Then  there's the width of the bottom of the tooth next. Straight line belt shows .054" there. Does it compress and shorten any? Probably. How much  on a 90 tooth circle? I don't know.

Finally I just decided to go with the plain rack proportions for the cutter -- 25 degrees, and .054" tooth end width, and .050" depth. And just file the slight tooth radii. It IS for a rubber belt after all, not solid gears. We'll try the KISS method and see how S that turns out to be.....  :loco:

awemawson:
John Stevenson did a treatise on cutting XL pulleys - I think his web site is still up and kept alive by his son Adam.

vtsteam:
Can't find the site. Tired of research anyway, the day's a waistin'. Out to the shop......

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