Modified 3.5"-5 O.D. Buttress Thread

[nitemare]

i am currently machining a modified O.D. 3.5"-5 buttress in DOM tubing. by modified, i mean that the customer has changed the major Ø and minor Ø to accomodate their customers mating part. the mating part is a plastic cap that is used on a hydraulic tank for the fill tube. the parts are subject to below freezing temps, so that is where the modification came in. anyway...having trouble with a nasty burr at the first thread - can not get rid of the darn thing. i've tried to re-machine the lead in angle after threading 18 passes, and then re-cutting with 4, 6, or 8 passes to attempt to clean out the threads. i've even gone as far as trying to machine a linear step at the start to minimize the lead in chamfer size, but to no success. result is always the same - a razor sharp edge on the first thread. any one have any ideas on how i can resolve this matter? also, i'm using a kennametal nt4rb insert. they recommend that i use a negative 15° modified flank infeed. anyone heard of this? i've done modified infeed angles, but i'm a bit confused on the -15° infeed...any input would be greatly appreciated.

[star-turn]

I would say you need to put a quick/blunt start on the 1st thread.

do this with a grooving/part off off tip a little wider than the thread form - re run this tool in a s/cut cycle going 1 to 1.5 of the thread depth from the start of the thread - also start half a pitch in from where you started the s/cut tool.

ie.. 5mm tpi s/cut tip started at z 10. start groove tool at z 7.5
finish at z -5.0
this will remove the feather edge.

then repass with s/cut tip as original s/cut cutting to deburr sides.

should be left with a nice start to the thread.

ST

[nitemare]

not quite sure what you mean here. pardon the stupidity, but i'm a bit slow on the uptake. i have a 3.5 inch, 5 threads per inch o.d. thread. so, my pitch is .200", and the thread is 45°, 7°; the 45° is the leading egde. all i need to do is knock off the burr at the start of the first thread. again, sorry for the lack of understanding. thought maybe i was not explaining correctly.

thanks

[star-turn]

No worries,

to remove the feather edge ( razor sharp first thread ) you need to blunt/quick start the thread

you can do this as mentioned by using an external groove tool in a screw cutting cycle or mill it on with live tooling.

both will remove the sharp edge.

ST

[Geof]

The kind of start you are looking for is what is called a "Higbee". I think that is the correct name/term and if you Googled this you might find something. What you do is program a flat tool, such as a grooving tool to cut down the center of nhe thread and do a rapid pullout. This removes the feather edge that is at the start of just about any thread.

And I have to admit I have never done it successfully on a lathe but when thread milling it is a snap.

[nitemare]

HIGBEE! It sort of works...i still get a bit of a sharp edge where the first full thread is at the major Ø...here's what i have:

N7T0303(THREAD O.D.)
(3.5 - 5 BUTTRESS)
(KENNA NTB4RB KC5025 INSERT)
(HOLDER NSR 164D)
G97S450M3
G0G55X3.6Z.4M8
G76P000500Q0R30
G76X3.2015Z-.96P1200Q283F.2
G0X10.Z7.
M1
N8T0404(HIGBEE)
(USE 5MM ISCAR GROOVER)
(GIMF 502 IC9054)
G97S450M3
G0G55X3.6Z.4M8
G76P030500R0Q0
G76X3.2Z-.2P1220Q461F.2
G0X10.Z7.
T0400
M1
N10T0303(RE-THREAD O.D.)
(BURR CLEAN-OUT PASSES)
G97S450M3
G0G55X3.6Z.4M8
G76P050500Q0R0
G76X3.2015Z-.96P1200Q1200F.2
M9
M5
G0X10.Z7.
M30

Any thoughts or suggestions, let me know.

Thanks to all...appreciate the help.