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The One True Bob
07-18-2004, 01:20 PM
First real bout with Stainess 316 today. What a disaster! Rat chewed finish at all speeds and feeds (manually fed, with speeds from 100 rpm to about 4000 rpm -- VFD's rock when coupled with a manual speed changer).

Set the carbide tool holder at complete, as dead as I can measure dead center, and then tried slightly over and under. Everything tight and lined up, all systems go. In fact, and really weirdly, I got alot of pretty good curly brown-blue chips going once the front surface of the work was "smooth", and all you could hear was the shhhhhhhhh of the metal parting as the carbide went through it, but the finish still sucked.

A possibility is might be that one of the balls in the front bearings is getting ready to eat itself; when I put the earphones to the front (main bearing) I get an occasional "hick". Thats the only thing I can find out of whack.

Ultimately toasted an entire box of bits, both sides. I'm really irritated, and out the $90 bucks for the inserts.

Best finish was with a HSS hand ground tool bit. Go figure.

Now, before everyone says 303 or even 304, I need the welding properties of the 316. I can't use the HSS bits because the steel will imbed and ultimately rust and ruin the finish (you know, the one I cant' get in the first place).

Anyone have any suggestions? I've seen some newer alloys that promise equal welding qualities and easy machining, but they aren't sold at the "small amount" shops like Online Metals.

Thanks..... As always, all help appreciated.

Bob

rklopp
07-18-2004, 02:59 PM
Give us some more details on what kind of inserts & toolholders you were using. If your setup is casting the chips against the just-machined surface, they may momentarily weld on and leave a little blob of metal. That wrecks the finish. Are you using lube? Mist or flood coolant might help if you have it. I prefer not to brush on lube with carbide inserts due to thermal shock issues.

The One True Bob
07-18-2004, 08:06 PM
The tool holders are a modified version of the Sherline QC setup, with inserts DPMT 21.51 2AVC29. They are tiny positive rake diamond inserts (see http://www.sherline.com/2258inst.pdf)

Slightly better results were achieved by cutting down a Sherline negative rake tool holder I still had laying around that uses larger DNMG 331's and mounting that to the cross slide of the Derbyshire.

To get the better finish, I ran it like mad while turning the cross slike with one hand and spraying Cool Tool on with the other. Sort of like patting the head and rubbing the stomach; I wasn't good at that and will have to come up with another solution.

I noticed that every so often there was a small (for lack of a better term) gouge in the circular tool path. Less than with the other set up.... Ridigity, maybe? Or maybe it is a bearing going....

rklopp
07-19-2004, 11:32 AM
A circular ridge could also be a built-up edge on the tool coming and going. A built-up edge effectively alters the size and geometry of the tool until it breaks off.

Mike Graham
07-19-2004, 05:27 PM
<font face="Verdana, Arial" size="2">Originally posted by The One True Bob:
[B]
First real bout with Stainess 316 today. What a disaster! Rat chewed finish at all speeds and feeds (manually fed, with speeds from 100 rpm to about 4000 rpm -- VFD's rock when coupled with a manual speed changer).
</font>

You mention an RPM range but don't specify the size of the workpiece. You also don't specify depth of cut. 316 can be a bear to get a good finish on - no doubt. Low speed and an agressive cut is usually your best bet. 20 thou depth of cut is about the minimum for the regular TNMG inserts that I use. This is on a biggish lathe, so since you mention 'Sherline' a few times this might not be a feasible option, but try for a low speed (say, 25sfpm) and the deepest cut your machine can manage, and KEEP IT COOL. Yes, you can get decent results with cutting oil if you're running slow enough, but I find that optimal results are found by using a water-soluble cutting fluid, and lots of it. You mention curly blue-brown chips, which is fine and dandy with flood coolant, but if you want a clean cut without flood then I'd advise you to drop the speed enough that you aren't discolouring the chips. That's right - go for silver. With flood, I like a peacock blue.



------------------
Mike Graham
Caledon, ON
http://web.295.ca/mike_graham

wierdscience
07-19-2004, 06:11 PM
For carbide on ss you have two choices as coolant goes,either none at all or flood it.Cool mist just don't get it.

I have never had any problems with HSS and rusting,its not a factor.

If you insist on using carbide,go for a negative rake uncoated insert, Ti and TicTin flake off and do little good.

[This message has been edited by wierdscience (edited 07-19-2004).]

The One True Bob
07-29-2004, 07:42 AM
Based upon suggestions here, and at rec.arts, I tried some 304L.

Welds nicely, I can get a non-ratlike finish. Best of all, after having argued with the 316 for what seemed like an eternity, even when using HSS the 304 feels (almost) like cutting brass! I think its possible I might have learned a bit about making grinding cutting tools and their geometry along the way. http://bbs.homeshopmachinist.net//smile.gif I've also learned about passivating, and carbide inserts, so all in all, this was a real successful deal!

The lathe, btw, is a Derbyshire 750 - very steady and solid, but with little room between the topslide and the centerline, making tool choices sometimes difficult to make. One of the things I've done is retired the modified Sherline QC setup for now; I think that was definitely a contributory problem. Of course, now I'll have to make something myself....

Thank you, everyone!

DR
08-08-2004, 08:21 PM
I cut lots of 316 and some worse SS. It's all in the machine's rigidity. We use CCMT carbide, dry, at about 300 SFM. Hot blue chips and chrome-like finish on the stock.

If you have to use a certain grade of SS buy the specialty stuff from Carpenter Technology. Do a google search, they sell online, I believe the first order is free shipping. It costs more, but as Bob found out sometimes the tooling costs are what kills you.

The specialty SS's that Carpenter and others sell are formulated for the easist possible machining. They skew the alloy composition so the elements making for hard machining are towards the bottom of the alloy spec, likewise those making easier machining are skewed to the max percentage. The bars are fully annealed also. Tremendously easier to machine than off the shelf SS. Available in all alloys and sizes and shapes. If you really want a treat try some of their 303, machines like brass.

The One True Bob
09-01-2004, 08:23 PM
<font face="Verdana, Arial" size="2">DR quoth most wisely:
...The specialty SS's that Carpenter and others sell are formulated for the easist possible machining. ...The bars are fully annealed also. Tremendously easier to machine than off the shelf SS. (snip)

</font>

Yea baby! Though a bit pricey, Carpenter's 304 just sort of falls right off the chuck under the tool, and HSS does real well with it. Nice finishes. Only drawback is that it seems to work harden real easily, which I don't understand after reading their materials, but hey, nothing is perfect. Resistance welds nicely, also. Thanks alot!

andypullen
09-02-2004, 11:05 AM
I've worked with alot of 316. It's tough, but not as bad as the nickel alloys and superalloys. Slow it down and keep it wet like the other posters have said. Don't try to force it off.

With carbide use a negative rake on the point of the tool, especially for roughing. A BR or BL type cemented carbide will work well for roughing if you can stand the stringy chips coming off. Micro 100's are good here. There are insert tools that will give you a similar geometry. The tip of the tool is the weakest part, so bury the shoulder first. A sharp point is good for finishing.