very BASIC turning questions for n00b

[SRT Mike]

We've been doing CNC milling at our shop for a few years now - everything from basic 2D parts to 3D profiling, thread milling, etc.

We have some work that would be much better done on a lathe - and we just bought an LC40 from a company near by. It has live tooling and 2 turrets...

...and we know almost nothing about turning

They say there are no dumb questions, but I'm gonna throw some stupid ones out there...

1) How do you know the angle to set the tool bit in relation to the part? Lets say you are turning down the OD - you have lets say a square insert toolholder. Are you using the tiny corner of the square? The flat side of the square? Do you want the tangent line of the tool bit point to be parallel to your parts surface? Or do you hit it at an angle?

2) There are so many different bits out there - weird shapes, diamond, round, square, octagon, etc. Milling is easier - you pick your diameter and go with 2 flute for AL, 4 flute for steel. You start to get into more 'exotic' mills for special jobs like 6 flute high helix, etc. But what about turning - are there actually common needs for all the different insert shapes or is it like milling, where you will generally use a certain kind of bit and only venture into the whacky shapes when you are doing a specific job or turning a specific material, etc? Are there any rules of thumb for inserts like there are for endmills (i.e. 2 flute in AL, carbide produces longer life and better cuts, etc)??

3) It looks like a lot of the tools I see dont have much reach at all. The lathe has something like a 24" max turning diameter, and the turret is huge. How in the heck would you ever get a tool down inside a part with such a gigantic turret? You'd always use a boring bar 100% of the time for an ID cut?

4) One part is like a teardrop type shape (curved tapering diameter). Would you use the same sort of insert bit to turn a taper vs. just turning down an OD?

Anyone got a rundown on the insert bit geometries and what they are used for?

5) Last but not least, I see parting tools on various sites - by the time you get the parting tool into the holder, it seems to only stick out an inch or so, which mean you could only part off a 2" OD part. What do you do when you want to part a piece that's... let's say... 6"? Do you turn a flat portion down to 2" (or whatever the biggest parting tool is you can find) then part it from there?

Sorry I know these are beginner questions but my only lathe experience was years ago on an old manual Okuma and I don't remember hardly anything about it!

Any good "intro to CNC lathe" books around?

[paulC]

May be a good place to start.
http://www.jjjtrain.com/vms/
Paul

[Geof]

Here is another link stolen from a different thread. I have looked at nearly all the lathe pages and they are good information; simplified but not too simplified.

http://technology.calumet.purdue.edu/met/mfet/275/

To try and answer some of your questions (you do have rather a lot ).

If you are wanting to do turning and facing with the same tool it is necessary that the nose of the tool is less than 90 degrees because you need clearance behind the tip in both the X and Z direction. You do not want the side of the tool that is parallel to the direction of cut to touch the work...that way lies serious chatter.

If you are doing heavy hogging and particularly if it is an intermittent cut than a nose greater than 90 degrees may be preferred. This means that the cutting edge approaches at an angle and intermittent cuts do not strike the entire face of the tool they hit at the outer edge and then the chip expands across the cutting edge.

If you are turning something that decreases in diameter as you move along Z then you need even greater clearance and a narrow angle diamond insert may be the best.

Yes for ID cuts you use a boring bar; sometimes you might use an opposite handed boring bar for an OD cut on a large diameter.

Parting cuts? Six inches is a bit of a challenge; I have done 4 inches in leaded steel with a 1-1/2" hole through it fairly easily but never 6". The problem with parting is having the chip binding in the deep groove with disastrous consequences...in a big machine you suddenly find the part rotating with a sheared off chuck of parting tool holder more or less welded in the cut.

A trick on big part offs on nasty material is to go in halfway with two parts separated by a land slightly less than the width of the parting tool; you cannot easily do a partail width cut because the toolholder is thin and flexes. When you haver done the two side cuts then you go down the center removing the land between the other two cuts and continue on to the final part off.

If you have specific questions post pictures (jpg's please I am technologically challenged). I may or may not get back over the next few days as I will be seeing the sights in Boston starting Friday.

[HuFlungDung]

Try to even find a partoff blade that will reach 3"! For some reason, there is a gap between part-off tools for the ordinary man, versus partoff tools for the he-man I searched high and low once, and couldn't find anything longer than the standard blade (which is about 6" overall) except for something that was like 20" long and parted off 3/8" wide

A big turret diameter is an advantage, because it gets the other boring bars back out of the way of the chuck. My lathe has about a 16" turret, and I run a 12" chuck, so I am hard pressed to use any more than 4 boring tools on a single job, and if I need to do some OD turning, I've only got room for 3 boring tools. Setup can be a challenge.

CNMG is the 80 degree diamond shape insert. WNMG is a trigon shape. Both of these are quite versatile and dare I say "common" for general use, as they will turn and face in one position. TNMG is a good choice for turning near a tailcenter, if you get the style holder that only has a single sided pocket (Kennametal Kenloc).

I prefer T shaped (60° triangle) for general boring, as I find that the tendency to chatter is somewhat reduced because the point is 'more pointy' (30° clearance on the trailing edge) than the C shape or W shape which barely clears the part (only 5 degrees clearance on the trailing edge). I'm assuming the worse case where the bar is small, and extended as far as possible to make the cut, etc.

[Geof]

Originally Posted by HuFlungDung .....except for something that was like 20" long and parted off 3/8" wide ......

And the machine you use it on stands ten feet high and swing 60 inches .

Maybe I exagerrate, a Herbert 9C-30 would do it and they were only 6 or 7 feet high.