I'm making a pair of 1/4" x 6" plates out of stainless. Circular with a 3" bore in the center. Got em roughed out fairly well on the chopsaw. Kinda octagonal shaped. The center bore cut very nicely with a standard carbide brazed boring bar. The little short guys that come in a wood block like a drill index. I do have boring bars, but the short ones are really handy. Here's the trouble, when trying to turn the OD down in the lathe the interupted cut bit is creating a problem. Carbide cutting tools break on the cutting edge. I tried HSS and it lasted a little longer, probably got dull and then a piece of the HSS on the cutting edge came off. I realize I could probably rough grind it to the round OD, but it takes quite a while and wears the 8" grinding wheel pretty fast. Right now, I put it aside to wait for my pal to get home this weekend so I could use his plasma cutter. After the plasma cut it could be cleaned up on the grinder and then a little closer to the line with the disc sander. Fwiw, when I bought the metal they cut it to size with a plasma cutter. I have a band saw with good quality blades, but the few times I've cut stainless with it the $10. a shot bandsaw blade went away so I'm not doing that again. Maybe I'm missing the obvious here, but I'm open to suggestions....
Slow speed, small cuts will work best on interupted cut surface. The O.D. might have gotten work hardened from the heat produced from the chopsaw. You also might try grinding a small radius on the end of your cutting tool (1-16th radius ). Tommy
the Lathe will be your best bet. Sometimes it can be a ***** to cut stuff that's a little harder than normal. Try slowing the lathe down make sure your bit is sharp. Set it in the center or just a fuzz low. You might try getting your self a solid HSS tool bit, that way you can just keep sharping it until you get past the interrupted cut.
I would try coming at it from the side with a parting tool.That way you are below the interupted cut,just feed it slow.Or us a solid chunk of high speed doing the same thing with a good chip breaker on the edge.If you have some one with a mill use a rotary table to cut the diameter.That would work the best.Burl
sometimes leaving enoug material to do a plunge cut from the front will work. Because then it is not an interupted cut. Use a parting tool or a 60* threading tool to get in there. Then when you get the s**** off, finish to the line with a regular cut across the edge. Some stainless is really tough to work with. edit:Burl's got quick fingers!
Make sue your saddle/cross slide/tool post are good and tight on the lathe. I find interrupted cutting tends to bounce the tool back and forth if the slides are not nipped up a little with the clamp screws, and the tool takes a bigger bite than it should do.
+1 on the radius on the end of the tool and prolly slow speed slow feed... Stainless ****s in that situation. It cane be done though. Dave
carbide hates interupted cuts. It really shocks the cutting edge, I agree about putting a radius on your cutting tip that and slower rpm. You probally have a work hardened surface if you used the chop saw in that case the grade of your carbide may not be able to handle the hard rockwell. Good Luck
What if you come in, like a facing cut, but move in towards the chuck, in line with the lead screw, instead of moving the cross slide across the bed of the lathe. Hard to explain.
do you have an insert holder for a diamond shaped carbide insert? I am working on hardened ductile iron castings at the moment and they present the same problem being rough sandcast. I am using a diamond carbide insert angled towards the part at about a 35 degree angle,and feeding in sideways,slow speed,slow feed and it works. you have to come in from the side,plunging straight in will tear up your tools,because it's hammering on the edge of the tool. nice slow cut from the side will remove the rough edges cleanly.
You need a soft shock resistant grade of carbide if you're going to try an interrupted cut, especially in stainless. If you use HSS, I would suggest using the tool at full profile. IE don't grind it to a point, just put a small radius the leading corner (when looking down on it from above). You'll have grind a little clearance into the bottom edge as well, but not much at all. You want the cross sectional area to be as high as you can get it to absorb the impacts. Keep your overhang as low as you can get it, and check your speed. Sounds like the HSS either developed a built-up-edge (BUE) or you just annealed the cutting edge and it eroded away. Either way, the cutting speed (SFM) is to blame. Consult the Machinery's Handbook (AKA the metal working bible) to find the correct speed for your cutting tool/work piece material combination. When working stainless, make sure your DOC and feed are both heavy enough to get under the work hardened zone created by the previous cut. DOC should be at least 0.005", and the FPT should be at least 0.001", or you'll just succeed in making the piece harder and your tool duller without ever removing any material. Also, stainless wants a keen edge, don't hone the edge dull like you would with steel or cast iron. You gotta come at stainless like it's hella hard aluminum. Sharp tool, decent DOC and feed, and pay attention to the speeds to keep BUE at bay. However, unlike aluminum (where you just have to stay above a certain SFM), with stainless if you go too slow, you get BUE, if you go too fast, you melt the cutting edge, so you've got a much more narrow band of happiness to shoot for. Good luck. BTW, if you rough it down with a plasma, do yourself a favor and knock the first 15-20 thou off the diameter with a tool you don't like very much, 'cause it's gonna be hard as hell.
I will try and take a video tomorrow, hard to explain in words, also i'll write down the seiko insert number if you want, seems to be excellent at absorbing shocks. it gets hammered to hell doing those castings, but the cuts stay accurate.
Plazma hardened the SS, I experienced this years ago cutting thick plate and turning it down to Size wit ha plazma, ,thing that worked for me was cutting it about a inch larger the going in with a part off tool cutting towards the chuck, t******* to desired O.D , then grabing the O.D and parting out the I.D
Slow speed-high feed is the old story. Rex 95 shpuld cut it. If you can get some MolMax or BlackAlloy tools they are better than Rex 95. Good support and don't let it slide.
Ya but if its an interrupted cut the hi feed is gonna **** up the tool also.. Especially if its hard in spots or surface hard .First id have never cut it with a chop saw... Work hardening can be a mans worst enemy and trashes tons of tooling .... I could go on about about back yard machinists but we all have to learn somehow.. Its kinda funny .. My old man got me into this **** starting about age 13.. we never used carbide just good ol hi speed tool bits and now you cant hardly run a lathe without carbide? And a lot of people wouldnt know how to sharpen a tool bit anyways... Sometime I just have to shake my head and walk away ....... Dave
Can you find a rotory table & mill your OD in...? Carbide end mills will have no problem with an interupted cut.
newer carbide inserts are great stuff, i could do it the old way, but why not use the latest tech available? cutting tools have come a long way.
As has been said before, light cuts. Lower the speed and only take .010-.020 off per p***. Being a noob I have a tendency to wear out cutters as well. If it smokes or chatters you are going too fast or taking too much off. You might try annealing it as well. Cutting and machining sometimes tends to work-harden steel. When we have a rough cut on the lathe we just take it slow though.
Lotsa good info guys and many thanks. I'm doing not-too-bad on sharpening HSS cutting bits. Got a bunch of really old blanks and a bunch of new ones. Along with maybe a hundred already ground bits I can dig through for the odd shapes. (Got those when I was buying and selling lathes - always kept the tool bits except for the one that was on the lantern tool post to help hold it together for the sale etc.) I haven't reached the plasma cut area yet, but the chop saw cuts are probably work hardened as noted. I tried a carbide brazed V- thread shape cutting bit yesterday, but I'll need to undercut it a bit for clearance. I've used a V-shaped tool to cut aluminum disc's out of a larger piece of plate . . .both to save time and to save aluminum. The smaller pieces are usable other places. With the aluminum you need to go side to side and make a somewhat wide groove even with an undercut tool bit. With the stainless I'm cutting now, I need it to come in at 6" OD and it's just barely over that width now. The shop cut the stainless plate out of a larger piece. Woulda rather had a chunk sawn off a 'bar', but I gots what I gots. When you guys are talking of slow speeds, do you mean back-gears in or the belt on the slowest speed pulley groove? I did try cutting on 2nd groove from the fastest - which I use for auminum, but with the back gears in. Looks like what may work is to grind an undercut in my parting-off HSS blade and give that a try with some cooling oil. Thanks again, I'll let you know what happens....
Little cuts. But remember cut with oil only on Stainless especially in a manual lathe. Stainless will work harden if it gets hot. You should run stainless half speed of steel and half the cut. and of course it will take 2x plus to make. Stainless is probably the most dificult material to machine.
Absolutely, this is actually a trepanning operation, which can be done head on with a modified part off tool. Go easy on the break-through and you should be ok. Bob
For calculating the RPM quickly, use this formula RPM=[cs x 4]/d. Which is [cutting speed x 4]/ tool (or workpiece) diameter. Not knowing the alloy of stainless, use a cutting speed of 50. The diameter is 6" you say? So your RPM could be as low as 33 or high as 66 closer to the 3" hole. Good luck! Bob
I know when were working on stainless it gums up like crazy we find that cutters with a rake to them tended to get below the steel and provided a much better cut, that and the coating and grade of our carbide made a HUGE difference. We found that having a sharper tip initially helped but once that ****er went that was it. at least with a radius it will take more of a beating for much longer. Mind you I run a HS CNC mill so my work piece isnt exactly spinning like on a lathe but I think the same principles hold true. And Bobss396 formula for finding RPM is right on the money, I use that formula everyday and its alway close for starting out. Stainless is no fun to cut thats for sure but I imagine work hardened stainless must be absolutly nuts!!
I may not know what I'm talking about, but what about chucking it into a brake lathe and cut it with the drum surfacing tool? Would the stainless be too hard for this to make a manageable cut? What about cutting it with the disc tool that'd cut from both inside and outside? I don't know if these ideas are even feasible, but I thought I'd offer what I thought might work or something along that line.
Got one cut almost all the way through today. Didn't have that long in the shop. Lathe set at 45 RPM and I used a HSS parting tool about 1/8" wide set about 1/8" in from the edge. The cut was made into the face of the plate much like a trepanning tool. Kept it pretty well oiled down with cutting oil for steel by using a solder brush. The parting tool was initially tried with the standard rake that comes on them when new and cut fairly well until it got dull and starting dragging in the groove due to the groove is circular. I ground the other end down so there was clearance under the cutting edge and that cut fairly well until it got dull. Sounds like a bit of a rake may help, but I'm not going to put too much on it. Looks like 2-3 re-sharpenings may do it for one cut. Looking back I should have ordered a piece of 7" x 16" so that a plasma cutter induced hardened edge would not have been a problem. I thought the shop where I bought the metal would have taken it off a 1/4" x 6" bar, but they plasma cut it out of a plate. More tomorrow, until then, steady as she goes....
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