Threading NPT NPS thread on SS316L Annealed material.

[KingKB]

I am trying to do OD threading on SS316L annealed pipe. wheni was threading on SS316L non annealed pipe everything was good. but after i bought material with annealing things get worst. the chips get stuck on the insert tip and because of that i cannot have consistency in the threads depth. i have tried G92 and G76 cycles both. cannot get rid of chattering . if i reduce the rpm to 500 the chip sticks to the insert and there is variation on threads depth. if i increase the speed to 1000 or 1300 there is chattering . i am using VM7 grade vardex insert ER14tpi. threading is to be done 16mm npt on 1 side and 16mm on the other followed by NPS thread totallong 58mm. Please help friends.

 

[William Lynn]

Anytime I ran stainless, I made sure my coolant solution was strong. Too weak would cause issues. I never threaded it at 500 RPM, always slower.

 

[guythatbrews]

You'll get better answers by providing more info.

What machine? Coolant or no? What is the overhang from the chuck jaws, especially on the side with two threads? ER 14 is missing some numbers. Is it for sure an NPT insert? Are you cutting the major diameter profile first and the threading? And the most puzzling of all, what the heck is 16mm NPT?

 

[ondori]

300 series stainless is not heat treatable, and are only harden-able by cold working (i.e. during machining).

The L in SS316L means it's low carbon content, but also still meets all the specifications of SS316.

at 16mm (3/8 NPT??), 500rpm would be ~80SFM, which is sufficient. I wouldn't go any higher.

Is your tool center height correct?
Is your setup rigid?
Is your depth of cut too high?

Coolant is ideal for 300 series stainless, but for a thread this small, you could even run it dry. We do it all the time.

However the wrong coolant can be worse than dry cutting.
skip the coolant, grab a bottle of tapping fluid, or any heavy weight oil (like waylube 68) and use that for a test run.

 

[KingKB]

300 series stainless is not heat treatable, and are only harden-able by cold working (i.e. during machining).

The L in SS316L means it's low carbon content, but also still meets all the specifications of SS316.

at 16mm (3/8 NPT??), 500rpm would be ~80SFM, which is sufficient. I wouldn't go any higher.

Is your tool center height correct?
Is your setup rigid?
Is your depth of cut too high?

Coolant is ideal for 300 series stainless, but for a thread this small, you could even run it dry. We do it all the time.

However the wrong coolant can be worse than dry cutting.
skip the coolant, grab a bottle of tapping fluid, or any heavy weight oil (like waylube 68) and use that for a test run.

Depth is 16mm and 58mm on the other side. . at 58mm 14mm is npt and rest followed by NPS.
Tool centre height is perfect and i cannot use without coolant. however about rigidity i am going to fix a revolving centre on the back side during the next trial today evening. i will post results tomorrow

 

[gmc1724]

Depth is 16mm and 58mm on the other side. . at 58mm 14mm is npt and rest followed by NPS.
Tool centre height is perfect and i cannot use without coolant. however about rigidity i am going to fix a revolving centre on the back side during the next trial today evening. i will post results tomorr

Depth is 16mm and 58mm on the other side. . at 58mm 14mm is npt and rest followed by NPS.
Tool centre height is perfect and i cannot use without coolant. however about rigidity i am going to fix a revolving centre on the back side during the next trial today evening. i will post results tomorrow

VM7 inserts like higher rpm, I'm guessing you're threading 1/2" or 3/4" npt, I'd start off at 1600rpm with a high coolant concentration, the slower you go the more build up you'll get on the tip.

 

[KingKB]

Threads are 3/4 npt . i used VM7 but chattering problem and the chip gets stuck to the insert. used 1600 rpm also. today am going to try VTX insert grade. lets see what is the result after VTX and revolving centre

 

[gregormarwick]

First and foremost, define your thread properly so people know what you're talking about. There is no such thing as a 16mm NPT, and I think you're using "depth" to mean the length of the thread? When talking about a thread, most people assume "depth" to mean the height of threadform, analogous to "depth of cut" - in the case of 14tpi NPT this is around 1.45mm...
14tpi leaves two options for NPT - 1/2" or 3/4", and 16mm long suggests 1/2", as it is halfway between L2 (13.556mm) and L4 (19.85mm) of that thread.*
If you'll forgive me for making assumptions, your apparent lack of knowledge of these threads leads me to ask if you know that these are gauged threads, and if you have gauges for them? It's possible to make an NPT by measuring pitch diameter at specific locations, but it's quite difficult...

Secondly, make sure you're using the right insert. ER14tpi is not an insert designation. Are you cutting the NPS and the NPT with the same insert? You should not be. Full profile NPT inserts have the taper angle built in, so an NPT insert will not crest an NPS properly, and vice versa. The deviation is likely very small at 14tpi, but it remains true all the same.

Thirdly, make sure your rough turning pass is leaving very minimal material over the thread major. Just enough for the insert to crest, but not more. Like around 0.2mm on diameter over the thread major - this is tricky to get right on an NPT because of the taper.

Lastly, years ago I did a job that had huge quantity of NPT threads cut on 316 pipe, from 1/2" to 4". I started out with Vardex inserts (I forget the grade and geometry, but it was definitely appropriate for the material) and had the exact problem you are having - edge built up followed by the point of the insert breaking off. I switched to Sandvik's 266 system and got quite literally 10x more parts per edge, changing nothing else. That was the job that made me switch wholesale to Sandvik for threading and ditch everything else.

*Edit, I missed your last post stating 3/4", sorry about that.

 

[KingKB]

First and foremost, define your thread properly so people know what you're talking about. There is no such thing as a 16mm NPT, and I think you're using "depth" to mean the length of the thread? When talking about a thread, most people assume "depth" to mean the height of threadform, analogous to "depth of cut" - in the case of 14tpi NPT this is around 1.45mm...
14tpi leaves two options for NPT - 1/2" or 3/4", and 16mm long suggests 1/2", as it is halfway between L2 (13.556mm) and L4 (19.85mm) of that thread.*
If you'll forgive me for making assumptions, your apparent lack of knowledge of these threads leads me to ask if you know that these are gauged threads, and if you have gauges for them? It's possible to make an NPT by measuring pitch diameter at specific locations, but it's quite difficult...

Secondly, make sure you're using the right insert. ER14tpi is not an insert designation. Are you cutting the NPS and the NPT with the same insert? You should not be. Full profile NPT inserts have the taper angle built in, so an NPT insert will not crest an NPS properly, and vice versa. The deviation is likely very small at 14tpi, but it remains true all the same.

Thirdly, make sure your rough turning pass is leaving very minimal material over the thread major. Just enough for the insert to crest, but not more. Like around 0.2mm on diameter over the thread major - this is tricky to get right on an NPT because of the taper.

Lastly, years ago I did a job that had huge quantity of NPT threads cut on 316 pipe, from 1/2" to 4". I started out with Vardex inserts (I forget the grade and geometry, but it was definitely appropriate for the material) and had the exact problem you are having - edge built up followed by the point of the insert breaking off. I switched to Sandvik's 266 system and got quite literally 10x more parts per edge, changing nothing else. That was the job that made me switch wholesale to Sandvik for threading and ditch everything else.

*Edit, I missed your last post stating 3/4", sorry

First and foremost, define your thread properly so people know what you're talking about. There is no such thing as a 16mm NPT, and I think you're using "depth" to mean the length of the thread? When talking about a thread, most people assume "depth" to mean the height of threadform, analogous to "depth of cut" - in the case of 14tpi NPT this is around 1.45mm...
14tpi leaves two options for NPT - 1/2" or 3/4", and 16mm long suggests 1/2", as it is halfway between L2 (13.556mm) and L4 (19.85mm) of that thread.*
If you'll forgive me for making assumptions, your apparent lack of knowledge of these threads leads me to ask if you know that these are gauged threads, and if you have gauges for them? It's possible to make an NPT by measuring pitch diameter at specific locations, but it's quite difficult...

Secondly, make sure you're using the right insert. ER14tpi is not an insert designation. Are you cutting the NPS and the NPT with the same insert? You should not be. Full profile NPT inserts have the taper angle built in, so an NPT insert will not crest an NPS properly, and vice versa. The deviation is likely very small at 14tpi, but it remains true all the same.

Thirdly, make sure your rough turning pass is leaving very minimal material over the thread major. Just enough for the insert to crest, but not more. Like around 0.2mm on diameter over the thread major - this is tricky to get right on an NPT because of the taper.

Lastly, years ago I did a job that had huge quantity of NPT threads cut on 316 pipe, from 1/2" to 4". I started out with Vardex inserts (I forget the grade and geometry, but it was definitely appropriate for the material) and had the exact problem you are having - edge built up followed by the point of the insert breaking off. I switched to Sandvik's 266 system and got quite literally 10x more parts per edge, changing nothing else. That was the job that made me switch wholesale to Sandvik for threading and ditch everything else.

*Edit, I missed your last post stating 3/4", sorry about that.

Sorry for the problems you had to face . actually i am new and will take care next time while asking question as in to put in proper details. however i thank you very much for guiding me to use sandvik for threading. i will contact the sandvik dealer for help. really appreciate your post and suggestion. thanks a lot