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View Full Version : Tool grind for turning a 304 stainless 'octagon'



Tony Ennis
08-01-2016, 12:16 PM
I'm making a garden-variety index plate. My donor material is 304 stainless. (Not really my first choice, long boring story...)

The plate was square; I want it round. I cut the corners off in the bandsaw. Now it's a 3rd grader's version of an octagon.

I don't want to slam my carbide inserts with with this thing. Is there any particular grind for HSS I should use for interrupted cuts in 304?

This is my first experience with 304, parts of my adventure have been... traumatic. :D

BCRider
08-01-2016, 12:26 PM
Assuming you have marked out the circle to size that you want you could start with taking the corners off the octagon to make it a 16'agon... whatever that would be called.

I've worked with a few different mystery stainless metals. It was all from scrap yards so I have never known the proper numbers for any of them. Some pretty much require carbide to cut and some has been extremely gummy and wanted to build up rapidly on the tool.

For the gummy stuff I ground the top of the HSS with more relief both back from the edge and angled back to me on the handle. There's a name for that second angle but I don't recall it. Basically make the top of the cutter angle down towards the rear and deeper into the HSS bar to angle back to the body and away from the work.

If you're 304 is more along the lines of the really tough stuff that is hard as nails then my only "secret" was to turn the speed down and keep the tool cutting. It seemed to take the edge off promptly if I tried to cut too lightly.

gzig5
08-01-2016, 01:05 PM
I think just about any grind should work as long as you are taking light cuts until the corners are rounded. I'd suggest decent top rake and a bit of radius on the nose.

http://www.steves-workshop.co.uk/tips/toolgrinding/toolgrinding.htm

mars-red
08-01-2016, 02:26 PM
I've had good luck grinding trepanning tools for stuff like this, but I've not tried trepanning stainless yet.

enginuity
08-01-2016, 03:30 PM
I'm making a garden-variety index plate. My donor material is 304 stainless. (Not really my first choice, long boring story...)

The plate was square; I want it round. I cut the corners off in the bandsaw. Now it's a 3rd grader's version of an octagon.

I don't want to slam my carbide inserts with with this thing. Is there any particular grind for HSS I should use for interrupted cuts in 304?

This is my first experience with 304, parts of my adventure have been... traumatic. :D

I cut a lot of 304 and 316 stainless.

The problem is going to be surface speed. How slow can your spindle go? If you have a garden variety off shore lathe you may be forced into carbide.

304 is a strange material depending on how it is manufactured. I have cut stuff that is easier than 303 and stuff that is harder to cut than 316. I break a lot of hex bar off with carbide inserts at reasonable speeds - but you need a good quality insert from a quality manufacturer (ie Walter) designed specifically for stainless. I understand that this may not be accessible to you in the home shop. Properly coated carbide works very well in this application - despite a lot of misinformation around the web properly selected carbide does just fine on interrupted cuts. The Walter grade for heavy interrupted cut is a WSM30 grade. Forget about breaking a chip once you are cutting on a full diameter unless you have the HP and rigidity for a really heavy cut. Deal with the stringy chips with pliers - don't grab them with your hands (don't ask me how I know).

A standard HSS ground tool works ... as long as the material hasn't hardened when you previously cut it and you can keep the spindle speed down (you are looking at an absolute maximum of 30 sfm for uncoated HSS tools on a good machine). Keep it cool (lots of coolant) make sure your tool has a reasonable radius (if it doesn't the tool will just chip). Don't dwell in the cut - rubbing will cause you significant future grief. A high cobalt HSS tool is preferred - take appropriate precautions when grinding if using it. You really have one shot with an HSS tool, if it is not cutting and rubbing you will be amazed how quickly it hardens the workpiece. At this point carbide is your only option.

The good news is once you get adjusted to working with it, it's not too bad. That said, I actually prefer cutting 316 to 304 due to the significant batch to batch differences I find in 304.

strokersix
08-01-2016, 04:10 PM
A disc or belt sander would get you really close. Even a bench grinder if you are desperate. OD is non-functional so you really only need it to look good and not have sharp edges.

Tony Ennis
08-01-2016, 04:31 PM
My lathe isn't powerful but it does have back gears. I also have access to carbide if I need it. I think I'll try HSS first, and punt to carbide if necessary.

boslab
08-01-2016, 04:48 PM
Hexadecagon (6+10), could you trepan it?, just a thought, I rend to keep chopping bits off till I can't get a bite myself, seems to work, I then use a HSS tool, not as brittle as a tip
Mark

BCRider
08-01-2016, 06:53 PM
....A standard HSS ground tool works ... as long as the material hasn't hardened when you previously cut it and you can keep the spindle speed down (you are looking at an absolute maximum of 30 sfm for uncoated HSS tools on a good machine). Keep it cool (lots of coolant) make sure your tool has a reasonable radius (if it doesn't the tool will just chip). Don't dwell in the cut - rubbing will cause you significant future grief. A high cobalt HSS tool is preferred - take appropriate precautions when grinding if using it. You really have one shot with an HSS tool, if it is not cutting and rubbing you will be amazed how quickly it hardens the workpiece. At this point carbide is your only option.......

Enginuity, that whole post of yours was solid gold info. THanks.

The part I quoted sure sounds like the piece of SS a buddy brought over for me to make a thumb screw to use on a nice tripod he had. The original had broken so it was up to me to make him a replacement. Before I understood what he had given me I did let it dwell and it rubbed and hardened. It was so bad that a freshly ground HSS would not cut through and instantly took off the edge. And the few brazed carbides I had at that time were clearly "both" the wrong type as the edges simply pushed in and crumbled. So I had to cut off the roughly 1 inch piece I'd started on and begin afresh. This time it was super slow and enter the cuts fairly aggressively just as you're describing. I've still got the left over piece of it with notes about what it's like felt penned on it.

Oddly enough it accepted the knurling and threading with a die just fine. It was obviously still tough but no issues with it other than treating it like any other higher grade steel.

Based on what you provided I'm going to add "303SS or 304SS" to the markings on it. I'll use it some day but I'm sort of waiting for something worthy. Such as holding up the truck with a 1/4 inch bolt made from the stuff.... :D

Tony Ennis
08-01-2016, 11:18 PM
Thanks all.

I tried the HSS and the edge died in short order. I decided to switch to carbide. It did ok of course, but the interrupted cuts eventually hammered one cutting edge into powder.

The back gears did the trick. Once I turned down past the interruptions I was able to engage the leadscrew. The finish on the edge is pretty good not that it matters a bit.

Turning 304 is not the scary beast I had feared it would be; even when I hardened it, I could get under the hard spots with the carbide, even on my Atlas.

The big lesson here is to take the time to cut as many corners off as you can before putting the work on the lathe. Turning the octagon round was very loud, hard on the lathe, time-consuming, and basically unpleasant.

Thanks all for the words. Much appreciated.

RB211
08-02-2016, 10:10 AM
Thanks all.

I tried the HSS and the edge died in short order. I decided to switch to carbide. It did ok of course, but the interrupted cuts eventually hammered one cutting edge into powder.

The back gears did the trick. Once I turned down past the interruptions I was able to engage the leadscrew. The finish on the edge is pretty good not that it matters a bit.

Turning 304 is not the scary beast I had feared it would be; even when I hardened it, I could get under the hard spots with the carbide, even on my Atlas.

The big lesson here is to take the time to cut as many corners off as you can before putting the work on the lathe. Turning the octagon round was very loud, hard on the lathe, time-consuming, and basically unpleasant.

Thanks all for the words. Much appreciated.
Pictures please

Tony Ennis
08-02-2016, 10:15 AM
Pictures please

I can post when I'm done. You don't wanna see the action shots. I have another 1/4" of 304 to take off (per side) which will take a while. Next is to fit the blank to the spindex and mark the hole circumference. Finally I'll mark the holes, hand punch them, and drill them. What could possibly go wrong!

This is an hour for you guys, an educational week for me.

gzig5
08-02-2016, 10:17 AM
I'm surprised you killed your HSS bit in such short order. I've been turning 304 a lot lately with HSS and no problems with sharp bits. This includes turning down 1" hex to round and threading to make bushings and parting off hex nuts. No issues with the interrupted cuts at 55-100 rpm. Haven't had to touch up the tools either. Guess I got lucky, but you have more surface speed with the larger diameter so that make have come into play.

browne92
08-02-2016, 10:20 AM
Deal with the stringy chips with pliers - don't grab them with your hands (don't ask me how I know).

You too, huh? :rolleyes:

Tony Ennis
08-02-2016, 10:21 AM
I'm surprised you killed your HSS bit in such short order. I've been turning 304 a lot lately with HSS and no problems with sharp bits. This includes turning down 1" hex to round and threading to make bushings and parting off hex nuts. No issues with the interrupted cuts at 55-100 rpm. Haven't had to touch up the tools either. Guess I got lucky, but you have more surface speed with the larger diameter so that make have come into play.

I am unskilled. I am not in denial.

enginuity
08-02-2016, 10:28 AM
I'm surprised you killed your HSS bit in such short order. I've been turning 304 a lot lately with HSS and no problems with sharp bits. This includes turning down 1" hex to round and threading to make bushings and parting off hex nuts. No issues with the interrupted cuts at 55-100 rpm. Haven't had to touch up the tools either. Guess I got lucky, but you have more surface speed with the larger diameter so that make have come into play.

30 sfm on 1" hex material is about 100 rpm. My experience has been this is the maximum for uncoated HSS on 304 / 316.

If you are making a 4" indexing plate (not sure on the exact diameter that is being done) even 50 rpm is about 55 sfm, which is too fast. This is probably why Tony had to use carbide.

Carbide is a very interesting material in that it has been around since the second world war, but didn't really replace HSS until coatings were discovered and developed. It's amazing how such a small amount of coating can drastically improve the cutting performance.

enginuity
08-02-2016, 10:31 AM
I am unskilled. I am not in denial.

Physics is hard to beat some days ;)

enginuity
08-02-2016, 10:33 AM
You too, huh? :rolleyes:

I'm not that old and have already developed the skill to open a bandage with one hand while applying pressure. Who designed those bandaid packages anyway? You'll bleed to death before you get it open.

Tony Ennis
08-02-2016, 10:40 AM
Hmmm from what enginuity said, I calculate that I need about 20 RPMs for the blank at its current diameter (5.5") Last night I was turning too fast, even when I was doing well. I'll slow it down further this evening. We'll see what happens. I would like to get a substantial chip off and get to 5" dia a little quicker.

RB211
08-02-2016, 11:50 AM
Actually, the ugly action shots are the most interesting


Sent from my iPhone using Tapatalk

BCRider
08-02-2016, 12:18 PM
I'm not that old and have already developed the skill to open a bandage with one hand while applying pressure. Who designed those bandaid packages anyway? You'll bleed to death before you get it open.

You're using the wrong tools for the job in that case. As machinists you should know that if it's bleeding that rapidly the proper course of action is to stitch it together with safety wire.

gzig5
08-02-2016, 01:48 PM
I am unskilled. I am not in denial.

Maybe, but not what I was inferring.:)

Paul Alciatore
08-02-2016, 02:48 PM
Safety wire? I use upholstery staples.




You're using the wrong tools for the job in that case. As machinists you should know that if it's bleeding that rapidly the proper course of action is to stitch it together with safety wire.

Tony Ennis
08-02-2016, 11:58 PM
Update - Slowing the sfm down was a winner. I moved the belts to the slowed position and then engaged the back gears. I was able to keep the motor RPMs pretty high, ensuring torque. While I wasn't moving a lot of metal, it came off easily and consistently. The shavings were long and curly - basically a pain. For just a moment I found the right depth of cut and they broke into small pieces.

I hit the diameter to -.002" which is well within what I wanted. The hole for the spindex hub is pretty much spot on. Goes right on and has maybe the slightest play.

More To Come.

enginuity
08-03-2016, 12:51 AM
Thanks for posting Tony - I'm glad you finished the turning.

jimsehr
08-03-2016, 01:21 AM
I ran thousands of these out of 316 ss. see pics The trick was to get under the interrupted cut to a solid part of stock and use lots of oil. The end size plus minus .001 . if I didn't get under the interrupted cut the edge of carbide broke down.
http://i86.photobucket.com/albums/k106/jims_03/P1080190_zps6e3ad9c6.jpg (http://s86.photobucket.com/user/jims_03/media/P1080190_zps6e3ad9c6.jpg.html)
jimsehr

The pic shows using depth mic to check size of part. The depth mic has a 025 flange on it to check part like depth of groove slots. hard to see in pic.

garyhlucas
08-03-2016, 08:48 PM
This thread reminds me of a few years back when our machinist was complaining that the old but absolutely like new Southbend Heavy 10 wouldn't cut stainless. He was laid off because we simply didn't have enough work and I needed a stainless part. My boss came out in the shop and watched me peeling off a nice heavy chip at slow rpms in back gear. He said "I thought our lathe couldn't cut stainless?" I said it was actually our machinist that couldn't cut stainless!

sch
08-03-2016, 09:16 PM
About 10-12 yrs ago while doing a year at the CC machine shop, the instructor was using ~4" SS hex to make "flag poles". I don't know the type as it
was donated stock from the huge, now defunct, Saginaw spinoff from GM factory. They turned it round and made socket joints on the ends to get a 15-20' long
pole when joined together. Instructor was using a variant HSS 1/2" bit called "Spitfire". The students would spend all day knocking off the hex angles and boring the sockets
on 60" lengths. RPM was slow but perhaps 100 rpm or so. There were some pyrotechnics at the cutting edge, which seemed to last a long time (hours).

Spitfire specs: http://www.blackalloy.com/spitfire.htm