1/8 BSPT thread on tube ends

[Kiwi 'smith Dean]

Hi Guys, i just wondered if someone could help me out with a short program, if you have done this before and already have a short program on file there? I have had this lathe for 8 years and never had to cut taper threads before. Always use G76.

Fanuc Oi-TD control.

The steel tube that i have to turn a taper and thread the ends on is 0.400" diameter, and i'm wanting to set maximum thread diameter to about 0.396" in the highest part of the taper. (OD male thread)

I know it's 55 degree tools for threading, 28 TPI. Wanting the thread length to run about 0.460" long.

Thanks, Dean.

 

[wmpy]

Hi Dean,

You just add a R value to your 2nd G76 line. I believe for an external pipe thread it would be negative R. The R value is a radius value, not a diameter value.

Are BSPT threads 16:1 taper like NPT threads? If so, then you calculate R as follows:

((Z length of threading)/16)/2

Take the total length of the threading movement- you must consider your starting point. So, if you're threading to Z-0.4 and starting at Z0.24, you use 0.64 as your Z length. Then you divide by 16 to get 0.04. That is the change in diameter. Then you cut that in half to get the change in radius. So, in this example, you'd have R-0.02

Then for the programmed X diameter in your G76 line, you're telling it the diameter at the big end. I find it's always best to move the X offset up and take some test cuts. I've never been able to nail a tapered thread on the first try unless it's one I've done before.

Good luck!

(This is all from my head, and it's been a long day, so if I'm wrong about something, someone please correct me!)

 

[Kiwi 'smith Dean]

Thank you WMPY, i'll give that a go when i get a chance and report back with my findings, just in case anyone else is stuck in a similar situation.

Regards, Dean.

 

[sinha]

Hi Dean,

You just add a R value to your 2nd G76 line. I believe for an external pipe thread it would be negative R. The R value is a radius value, not a diameter value.

Are BSPT threads 16:1 taper like NPT threads? If so, then you calculate R as follows:

((Z length of threading)/16)/2

Take the total length of the threading movement- you must consider your starting point. So, if you're threading to Z-0.4 and starting at Z0.24, you use 0.64 as your Z length. Then you divide by 16 to get 0.04. That is the change in diameter. Then you cut that in half to get the change in radius. So, in this example, you'd have R-0.02

Then for the programmed X diameter in your G76 line, you're telling it the diameter at the big end. I find it's always best to move the X offset up and take some test cuts. I've never been able to nail a tapered thread on the first try unless it's one I've done before.

Good luck!

(This is all from my head, and it's been a long day, so if I'm wrong about something, someone please correct me!)

That is correct.
Both NPT and BSPT have the same taper. One just needs to divide the total Z-travel by 32.

 

[Kiwi 'smith Dean]

Thanks to WMPY and Sinha for your replies. Finally got to write a small program and run this today. After a bit of trial and error on the X value for depth of cut I got it where it needs to be. Maybe just a note here for someone else doing this, be aware of what your female threaded part that you are using for a test-fit actually is - either threaded BSP or BSPT. If it's a female BSPT that has had the tap run in further than normal it may give you a much looser thread fit, compared with going into a BSP threaded hole.
Anyway, just in case this helps out anyone else who needs to run this thread, here is the program that worked in my machine and control (Fanuc Oi-TD). Disclaimer: It may not work perfectly in your machine, due to variations in wear offsets, workpiece material/hardness, etc. Use your own risk and/or with due care.

O219 1/8 BSPT MALE THREAD STUB FOR Ø0.410 TUBE – WITH SLIGHT CHAMFER 0.450” LONG
N10 G50 S2000 Max turned length = 0.566” 30/01/2021
N20 G28 G0 U0 W0 (HOLD TUBE IN ER40 COLLET, OR SIMILAR. ENSURE @ 0.850” STICKOUT LENGTH)
N30 T0303 (VBMT 0.4mm NOSE RADIUS INSERT – approx. 0.016”)
N40 G96 S250 M3
N50 G0 X0.420 Z0.100 M8
N60 G71 U0.006 R0.005
N70 G71 P80 Q140 U0.010 W0.001 F0.010
N80 G0 X0.304 Z0.020 (FOR SLIGHT CHAMFER AT START)
N90 G1 X0.366 Z-0.036 F0.001 (actual measurement at minor end of taper should measure 0.368”)
N100 G1 X0.396 Z-0.466 F0.002
N130 G1 Z-0.566 F0.003
N140 G1 X0.420
N150 G70 P80 Q140
N160 G0 X0.430 Z0
N165 G1 X-0.020 F0.002
N166 G1 Z0.100 F0.010 M9
N170 G28 G0 U0 W0
N180 T1111 (55˚ TIP. Note: distance from side of insert to tip center is approx. 0.064”)
N190 G97 S300
N200 G0 X0.410 Z0.500
N210 G0 X0.410 Z0.100 M8
N220 G76 P020055 Q0008 R0003
N230 G76 X0.349 Z-0.514 R-0.0191875 P0264 Q0080 F0.0357142 (for 28TPI, 1.789910608 degree taper)
N240 G0 X0.410 Z1.000 M9 (1:16 taper on diameters)
N245 M30 (Note: start thread at X0.359 and work down in 0.002” steps until correct fit is achieved.)


Dean.