More on threading!

[JIM DEWOLF]

Sure like all the comments on threading.Great stuff. I use the rythem system myself but then I have had some practice which is what it takes to stop on a dime.
Anyway here's my question. I have to cut a Buttress thread which has a 45 degree backside and a 90 degree face. What angle do I set the compound to? This is internal 3 mm pitch. Let you guys do the thinking for me.
Jim

[halfnut]

Toss up between about 3 different ways.

Could set to cut against 90 just skinning the 45, set compound to 44 or 46, or 441/2 or 45 1/2. Depends how the compounds graduated. I sure hate to cut 46 or 44 degree angles in mill, confusing sometimes, 1 degree that side of 45 or is it the otherun, look at print again, turn the print upside down etc.

If you were in hurry and thread didn't need to be the worlds best I'd set compound in line with cross slide, keeping zero with cross slide and plunge straight in with compound.

If it needed to be a bit nicer I would set compound at 90 degrees from cross slide, then you would have to keep track of settings each time on cross slide. Could take skin cut on 90 side to clean up that way, making it look nicer.

Got a trading buddy that needs some buttress threads cut for a screw on an old mill vice, one side is around 45, but the other side is concaved with a radius in root. I keep telling him to find a chunk of steel, we'll cut it. This one might be fun, non critical, so I have lee way.

[JIM DEWOLF]

Thanks halfnut. You got me thinking! I'm just going to forget the compound and plunge straight in. it's a 2 1/2" bore in aluminum so should be no problem, unless someone else has a better idea.
Jim

[Thrud]

Jim

Just remember that the deeper the cut the more the bit actually cuts, so to minimize the chip load on the bit the cuts should get progessively smaller and smaller. This is part of the reason the compound is set to skim one side and cut with the other. I would set the tool so the buttress ends up as a 45.1* or so angle. It will produce a cleaner thread as well

Also "V" shaped chips are a bitch to clear out of the cut properly (especially with a buttress thread) and produce about three times normal stress on the tool.

CNC machines alternate plunges from each side which evens the wear on the threading tool - almost impossible on a manual machine even with digital readouts.

Good luck, take your time!

Dave

[JIM DEWOLF]

thrud,
Thanks for the input. I was wondering why you would recommend 45.1*. I was going to grind my tool to conform with the thread, that is 45*on one side and 89.5* on the other so I am only cutting the 45* face. And as you say, decrease the cut each pass. I would set the compound 90* to the cross feed to give me a little sideways adjustment. I'm cutting the full 45* so need you to explain your system. I guess I am missing the point. One thing I don't need is to screw this thing up as I will have a good 4 hours into it in milling time.
Jim

[Thrud]

Jim
Jim,

May the Good Lord smack you in the head before any snafu's creep in and bugger the other four hours work!

Perhaps a sacrificial offering to the "lathe gods" to insure the threading is to Mil-Spec on the real thing would be in order.

I spent a week making a perfect copy of my spindle out of Stainless for a dividing head only to thread it wrong in the end. Measure twice, thread once. I have a 7"x16" gear change chart for my lathe now...

Good luck
Dave