stainless steel 303 lathe part

[seawolf217]

I have a part that I'm running on a haas lathe. We start with 10mm blanks ~.393". I then have to turn the first 2 inches of the part to .25"...

Right now I'm using a roughing tool with a CNMG432 insert. I'm taking one rough cut at ~ 1600 rpm and .01 feed leaving about .007" for the finish tool. I take two passes with the finish tool - one cut to size and another spring cut. The tolerance on this part is +-.0003". Finish tool is cutting ~1600 rpm at .005 feed CNMG431 insert.

The roughing tool lasts for about 100 parts which is good enough for us. The finish tool lasts forever since its taking a small cut.

The issue that I'm having is that I'm not getting a good finish on the parts. I'm not sure if they are deflecting or work hardening from the roughing pass. I'm using flood coolant so i don't think it is heating up too much... Should I change my feeds/speeds?

[ctate2000]

Try ditching the spring pass. If you have trouble holding size without it try making two finish passes at .0035. I suspect you are roughing up the finish with the spring pass. The insert geometry is critical with that shallow cut. You may not be forming a proper chip. Seco makes a great chip breaker for light radial depths, I have used it for finishing 303 with excellent results. It is the FF1.

Are you running unsupported?

[g-codeguy]

I agree with Tate. The spring pass is causing the problem. Try to increase RPM for a better finish. Use a 35 degree profile tool with .008R if necessary with a ground edge to eliminate chatter. Lot less tool pressure. A VNGP-330.5 will have less tool pressure than a VNGG-330.5 insert. Your idea of roughing in one pass is a good one.

[davereagan]

I'm not a lathe guy, but for the finish pass, 1600 rpm at .250" diameter is barely above 100 sfm. 303 is free cutting. I'd scream it at 5000 rpm if your lathe and chuck allow it. There is a minimum heat required to get a good shine.

[ctate2000]

5000 rpm sounds great in theory, I suspect the part will start whipping at that speed.

[davereagan]

With .25" diameter x 2" long, I assumed he had a live center. Would you still expect problems at that speed with a venter?

[seawolf217]

i think my machine is locked at 3000 rpm max... i think there is a way to change a setting to allow it to go to 6000 but even at 3000 the machine vibrates too much and I don't know if I'd be able to hold the tolerance...

[g-codeguy]

Originally Posted by seawolf217 i think my machine is locked at 3000 rpm max... i think there is a way to change a setting to allow it to go to 6000 but even at 3000 the machine vibrates too much and I don't know if I'd be able to hold the tolerance...

I agree. If the machine is vibrating too much, the part can be up to .003 out-of-round. We got rid of one used lathe we had purchased for that very reason. .0015 was as good as we could get by slowing RPM down (without pumping more money in it). Had to slow the 3 Daewoo Lynxes down to 2000 and sometimes slower before we got the spindles realigned. Out-of-roundness would take up all the tolerance..and more on some jobs...from whip. .002 and less tolerances.

[PixMan]

Originally Posted by g-codeguy I agree with Tate. The spring pass is causing the problem. Try to increase RPM for a better finish. Use a 35 degree profile tool with .008R if necessary with a ground edge to eliminate chatter. Lot less tool pressure. A VNGP-330.5 will have less tool pressure than a VNGG-330.5 insert. Your idea of roughing in one pass is a good one.

This is good theory, but you don't have to switch your toolholder. I suggest you try a Valenite CNGP43.007 SR grade VP9605. The shape of the insert behind where the cutting happens is of no consequence. This insert is a very upsharp edge with ground periphery, and that's what reduces cutting pressure.

BTW g-codeguy...a VNGP and a VNGG are the same. One (VNGP) is the ANSI designation, the VNGG is the ISO designation for the same insert. I think you meant a VNGP will have less cutting pressure than a VNMG. This is true.

Lastly, when the OP says the "machine is locked at 3000rpm", that usually means there's an active G50 limit. Execute a command such as "G50 S4000" to increase it. If it doesn't go higher, then the machine is maxed out at something less than 4000.

[g-codeguy]

[QUOTE=BTW g-codeguy...a VNGP and a VNGG are the same. One (VNGP) is the ANSI designation, the VNGG is the ISO designation for the same insert. I think you meant a VNGP will have less cutting pressure than a VNMG. This is true.[/QUOTE]

That's funny. The VNGP and VNGG inserts we have in stock are not the same. The GG's are flat like an MG (but of course sharp) whereas the P's are higher at the point and are more prone to 'stringing' chips instead of breaking them up.

[PixMan]

That last character of the 4-letter ANSI designation is the only one of the 4 that can be subjective, depending upon the manufacturer of the insert. If you look at the ANSI shape option for "TYPE" (position 4), the P and the G can both be upsharp edges. The G shape shows a generic chipbreaker top-form geometry with rounded "root", while the P shape shows straight angles to the sharp tip.

So I guess we're both right.

[seawolf217]

Do you mean there is a G50 S3000 as the default? I didn't put one in the program since i was programing the RPM directly - G97 S1800... I'll have to check it out when I get a chance... If I get a part with a low enough tolerance I'd like to run it up a lot higher, because right now even at 2000 the machine begins to vibrate a lot...