Machining 304 stainless

[ttabbal]

So, I mixed up and thought this was the easier to machine version (303). Owell, it was cheap I guess. I'd like to try cutting it just for experience, though I may wait a bit and practice more on something else. It sounds like I want to go hard on it, bigger DOC, faster speeds. A few places recommended >500 sfm with carbide tooling. For this 1" bar that's.. 1900 RPM?? Seems a bit much. My lathe will run that fast, but I have never done so.

Lathe is a PM1127, currently set to the high speed range. Mostly because it came that way and worked fine for what I was doing. Tooling is Chinese carbide, 1/2" holders CCMT060204 inserts. I could grind some HSS as well, if you want to offer some angles for that. I have some thick tap magic lubricant, wax, and kool mist 77 available. I could get some other lubricant though.

 

[Ray C]

With a CCMT, give it some lead-in angle. Speed approx 650 RPM. Feed 0.002 to 0.004" per RPM. DoC no greater than 20 thou. That will put you somewhere between in the ballpark to right on the money. I'm guessing a speed of 650, feed of 0.002 and 15 thou DoC will give you a nice finish.


Ray

 

[ttabbal]

Thanks! I'll give that a try. It sounds far more reasonable than some of the other stuff I found looking around.

 

[benmychree]

Next time get the 303, it is a joy to machine, the only drawback is that when tig welded it tends to undercut.

 

[Ray C]

This first image is the normal attack angle for a CCMT.


For really tough or harder materials, turn the toolpost an addional 5 degrees or so like this and reduce DoC a little:


FYI: Tough vs Hard materials are not one in the same. Stainless scratches easily, hardened metals do not scratch easily. Stainless is "bendable", hardened metal is not really "bendable". Stainless wears-out carbide inserts twice as fast as hardened metals. Anyhow, whenever you're working with hard or tough material, decrease the DoC and adjust the lead angle in the direction shown in the bottom picture. By adjusting the angle this way, it prevents the point of the cutter from digging into the part and getting sucked in.


Ray

 

[ttabbal]

Thanks again Ray! I tried a few adjustments and found 750RPM to result in the best finish and chips with the other settings as you mentioned. I found I could rough at 20 thou, and do the last couple passes at 10-15 thou for a nice finish.

I know you guys like pics... 1" turned down to 0.550. Drilled at about 300RPM and hand tapped. Took longer than I would like, but that's what I get for buying the wrong material. I'm not in a hurry so it's worth it just to learn more about adjusting to the material. Once I found settings that worked well, it didn't really take too long.

 

[Ray C]

Glad it worked. Other things to keep in mind. As the diameter is reduced, RPMs need to increase. According to the rough formula and SFM chart that I use, your speeds should have started at 650 and finished-up at 1100 RPM. I was not aware you were doing stock reduction or, I would have told you that upfront. FWIW, the general range for 300-400 series SS on the chart I use is 75 -130 where I happen to know that that 304 is really tough so, I estimated the SFM at 80.

When doing stock reduction, I change the speed when material starts to look like it's tearing instead of searing off. There's a fair amount of latitude in the calculations and it depends on material and type/condition of the insert.

EDIT: One other thing, when it comes to SS, chip control is very difficult to manage. Unless a dangerous situation is brewing, I don't go overboard to make chip control adjustment.s

Have fun...

Ray

 

[Ray C]

In case this is helpful to anyone, as a reminder, the formula for RPM for high speed steel cutters is 4 x SFM / Diameter. The SFM is looked-up in a chart (commonly available such as the Machinist's Handbook).

For carbide cutters, double or triple (2x or 3x) the number from the above formula. I always start with doubling it and rarely ever need to use 3x value. The reason for the great latitude is due to insert type and condition as well as condition of material. Just start with 2x from the HSS value.

Regarding SFM reference charts... There are many available and most have fairly different values varying up to 20-30% or more. Don't worry. Just pick one as a starting point and work up or down from there. You can also look at a couple different charts and use an average value for a starting point.

Ray