After last week's post, I decided the only way to recover was to start over. I put that piece of cast iron with the improperly chamfered side into my stock collection and started over. Cutting off a blank with the 4x6" bandsaw and then squaring it to 1.80 on a side goes fairly quickly, so by Tuesday it looked like this. Roughly square at 1.80" on a side and 3.25" tall.
This time, instead of doing everything to make it an octagon, I'll just cut it on the big lathe until it's a cylindrical bar that's 1.750 +/- .005 diameter. The next step was to mark the center of the top end, then drill with a big center drill to accommodate the live center on my big lathe. I could have milled the ends of the bar so that they'd be perpendicular to the sides, but I did that on the lathe before centering and chucking the square bar in my four-jaw chuck.
With the sides of the block 1.80" (they probably differed from each other by a few thousandths) and the desired diameter of the cylinder being 1.750", how much do I remove? I drew the two shapes up in CAD and it told me that from the tips of the square to the rim of the circle is 0.397". That's roughly 19 passes cutting .020 (radius) at a time, plus a final cut or two to sneak up on the final size. Each pass takes about four minutes, which argues that with some clean up time, it should take about two hours. My lathe has power feed, so I can put everything in position for the next cut and engage the gears. Do "something else" for the majority of the cut and then go stop it at the critical point. Four minutes is just enough time to not get much done.
The final cylinder is supposed to have heat dissipating fins that are 3/32" wide, spaced 3/32 from each other, and each 3/32" ring is 1/4" deep in radius. The actual shape with most dimensions is like this:
I cut all of those features today and started making the 1.000" cylinder bore, getting through the largest single cutting bit I have: 9/16". The rest of the diameter will be cut with a boring bar with emphasis on getting it the right diameter, smoothed and looking like a cylinder bore should look.
If you haven't noticed the dimensions, from that flat, square, surface that the cylinder is sitting on to the left end of the part is 3/8". There "should be" enough material there to make that smaller end on the left and part it off once the bore is done.
Getting the bore right is important stuff and while I've done it on two engines, it still demands close attention. With luck, I get it completed this week.
Starting to look like a cylinder!Delete
I wonder...if a floating cylinder hone would be a good way to take off the last half thousand in the bore?ReplyDelete
Are you using carbide or HSS on this?
... carbide or HSS on this?Delete
Both. Turning to 1.750 diameter was with carbide. All of the spaces between the fins and the wider one at the left end were cut with HSS.
The cylinder bore was drilled out to 1/2" with HSS bits and now I'm enlarging it to 1.000" with a carbide insert boring bar.
I had been thinking about a three stone brake cylinder hone I have for the final touches, but, yeah, I should use something like that.
Nice work, SiG!ReplyDelete