trouble cutting a 8 TPI thread in SS304 - engine lathe

[draganm]

haven't had this problem before,and yes
1)I cut 304ss/316SS for 20 years and
2)I'm using the compound to move in at 29 degrees/ only cutting on 1 side of tool.
3)I'm taking very small cuts,like .002 at a time,but I'm in pretty far so it's a pretty wide cut for 8 TPI.

It seems to cut fine sometimes,then with no warning this happens. (material is jammed against holder and cold-welded to the insert. ( I can hear the lathe struggling so i back the tool out immediately,and that has saved the inserts ,just not my nerves.
oil or coolant,not much difference (coolant slightly better).

Anyone recommend a better tool? This is the TNMC-32nv,it is rated for 6 to 36 tpi pitch range. The chip is getting pushed back and under that shoulder that juts out over the insert. Once that happens, it has nowhere to go and builds up on top of the insert. It's an old tool,and I'm guessing there are better tools out there
thanks for any advice

 

[triumph406]

If you've cut 304 for 20 years, it's safe to assume you know what your doing.

I would venture to guess you have some crappy 304.

 

[kenton]

I agree that it is probably crappy stainless. But if your water based coolant is outperforming your cutting oil you probably need better cutting oil. I have had good luck with Mike-O-Cut in tough gummy stainless. I can tell a significant difference when tapping verses general cutting oil.

Ashburn Chemical-D-1332-16 Mike-O-Cut 87 Tapping & Drilling Fluid, 16 ounces: Amazon.com: Industrial & Scientific

 

[Mtndew]

The style insert should cut it no problem.
I think this is an issue with the coolant or oil. Do you have any Moly-Dee laying around? Any heavy way lube?
But if this is a godawful batch of stainless, then you might just have to muscle your way thru it. Or bit the bullet and buy a chunk of good stainless with a cert. No chinese crap.

 

[eKretz]

It looks like you are running too slow and getting built up edge/chip weld. What cutting speed are you running? Can you up the RPM?

If the stainless steel itself is crap, it might behoove you to switch to a cutting tool that you can sharpen yourself to a keener edge. It almost looks like you did a little touch-up work on that insert - is the TiN coating missing on that edge or am I seeing things? Maybe you didn't sharpen it to a fine enough edge?

 

[mTeryk]

Not disagreeing with what others have said but you might try a shallower depth of cut. At the end of a course thread, .002" infeed removes a lot of material.



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[Zeuserdoo]

As someone above said use molly-dee, if you have it, and a heavy cutting oil if you don't. I would also write off that used edge, and index the insert. You've got some serious BUE on that thing, and no amount of coolant or oil is going to make it cut good.

 

[draganm]

thanks for the replies. I didn't leave until 10pm last night and I'm sure you guys know how hard it is sometimes to walkaway from these things.

oil-I was using the old classic Relton RapidTap, ancient can from god knows when(has "won't harm the Ozone" written on the can). I have an old soup can of what looks like molly D, I'll try that next

material- also really old, piece of 4.0" pipe ASTM A312 from at least 20+ years ago, so unlikely china stock. This is a practice piece and gauge for the female end-caps. Then i have to cut the real deal on both ends of a 4 foot long piece of $900. pipe.

speed- I know I'm running way too slow. handbook recommends 255 fpm, which on a 4.5" OD pipe is 200rpm. I talked to the Engineer and i can increase the run-out/clearance groove area behind the thread, but I'm still nervous about stopping the tool at 8 pitch NPSM thread travel speed at 200rpm.

still disappointed in this tool geometry though. Here you can clearly see what's happening, chips is getting pushed into gap between and holder and insert. Once that happens, it jams the chip-flow, and you get that horrendous BUE pictured above



I did find this tool in my inventory, it's an excellent grooving tool. Kennamatel NSR-123B . I checked today and it's advertised as "threading and grooving" and it calls for insert NG3L, but i can't find a threading insert, only square nosed grooving.
Anyone know what the threading cert would be?

 

[draganm]

never mind, just found it. it's NT3L . Dam, $28. each from Kennametal

 

[BT Fabrication]

rapid tap sucks on stainless. it does just that and allows build up of it on the tool. only thing i found is a good high oil based coolant.

 

[dgfoster]

Can you flip the tool over and run in reverse? That might allow you to run faster without fear of hitting the shoulder.

Denis

 

[akajun]

Watch Joe Pye's youtube on threading in reverse with an internal threading tool. Really opened my eyes. I tried it and my threads come out much nicer, you can get your speeds up, and no worrying about threading to a shoulder and crashing.

 

[cudafamily]

If the swarf is jamming up like that I wonder if your holder pocket is worn/streatched. Unable to hold the insert firmly could be an issue.

 

[Illinoyance]

The tool holder is the problem. There is no support under the insert allowing it to deflect downward and create a gap for chips to enter and jam. Get a holder with solid steel BELOW the insert.

 

[gbent]

If you are making a gauge for stainless pipe, don't make it out of stainless! Specially not gummy stainless. It will gall and then you will be cutting it out of your piece of $900 pipe. I agree with threading in reverse, back side of center, right side up tool. No fears about running into a shoulder and you can see how the chip is behaving.

 

[draganm]

If the swarf is jamming up like that I wonder if your holder pocket is worn/streatched. Unable to hold the insert firmly could be an issue.

the holder has very little use, but like Illinoyance pointed out it's a flawed design. The entire load of the cutting force is carried by the retention screw. It would start all snug and tight, but would flex that screw down after a few cuts.I think that holder is for light work, not 8tpi in SS

If you are making a gauge for stainless pipe, don't make it out of stainless! Specially not gummy stainless. It will gall and then you will be cutting it out of your piece of $900 pipe. I agree with threading in reverse, back side of center, right side up tool. No fears about running into a shoulder and you can see how the chip is behaving.

i hear you, but eventually pipe and end caps will have ot go together anyway, all 304. I know not to force it and to use Antisieze with Molydbenum and silver paste.
I watched the upside down threading video, looks interesting but i'm saving that for later.

Anyway, i was able to cut the thread today. I didn't have the insert, so i took the NG3L and ground a threading point on it on the diamond wheel. I used MolyD from an old Timesaver Industrial tools jug (remember them?)I ran at 65rpm, not as fast as maybe ideal but a good speed considering you have to catch the threading dial at the right spot too and it's really going at 8tpi. Took .002 depth of cuts. It was pretty smoky, but i nailed the nominal pitch dia. within a .001".

Couldn't have down it without you guys, many thanks

Monday i get to do it 2 more times , but on an ID thread in the end caps. Then the 4 foot long pipe when it arrives with 2 more OD threads.
Thank goodness i have a huge pipe live-center to hold the other end. Fun!

 

[eKretz]

It can take a little doing to get used to manual threading at higher speeds. I recently threaded some parts with 8 pitch threads at 380 RPM into a 3/16" runout groove. The guy who took over the job to finish the last couple parts when I left was nervous and cut the speed in half. It can help to cut a back chamfer on the entry end of the runout groove to give you a little more space too.

The NT3 threading inserts are decent but if you really want a good TopNotch style threading tool, I like the NTUs a lot. They need a dedicated toolholder though. You also get 4 cutting edges instead of 2 for almost the same price as NT3s.

Threading - Top Notch™ Threading Inserts

 

[EPAIII]

Just an observation from an engineering point of view. The top of that holder seems to have no reason for sticking out so far. The downward force from cutting will try to rotate the insert away from that excessive overhang, probably opening the gap which the chip is getting caught in.

You might try grinding the top of the holder back so that it only protrudes as far as the mounting screw, keeping at least some slope to direct the chip upwards. That way when the insert rotates a bit on the mounting screw, the gap will not open - at least not as much. And this should not weaken the holder as the part of the overhang I am suggesting you remove is not supporting anything anyway. The insert is being pulled away from it.

 

[dian]

often problems go away if you sharpen/hone an insert on a diamond wheel. this is not for production, right?

 

[guythatbrews]

This is the TNMC-32nv,it is rated for 6 to 36 tpi pitch range.

Another thing that is not helping you is the 36 pitch part of the insert. It's way more pointed than it needs to be. The tip will break down for a nothing.

I no longer use the topnotch type stuff. There's way better stuff out there. But if you only need to make a few may not be worth it to tool up. But now that you have the hindsight bet you wish...

I like the Sandvik laydown stuff with chip control and your choice of angled shim. A lot stronger cutting edge. If you use an 8 TPI topping insert it will have the strongest tip consistent with the thread pitch.

The wrought pipe single points something awful. Hate it.

Lard oil was always the go to cutting oil of old. Bacon grease works, nothing is cheaper, and you can eat the bacon.