[bishb25]

Hi All,

I've noticed a problem when trying to measure turned threads. When I measure threads using the 3 wire method the threads measure nominal according to the chart that came with the wires. But our go/no-go ring gauges don't fit the threads. I have to run the threads to the minimum measurement over wires in order to make the go gauge fit. I've checked the thread profiles on our optical comparator and they seem to check out as far as the Vee being 60 degrees and perpendicular to the axis of the bar. Can anyone explain why the gauges and the wires differ so much?

Thanks

[juergenwt]

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[juergenwt]

Outer Dia.? Burr on top of thread from cutting thread?? Root Dia. and Radius? Angle of threading tool when single pointing. Are you feeding in at 29 1/2 deg? Pitch OK? This happens quite often. Trust the thread gage.

[bishb25]

These are mostly CNC cut threads. Major dia. is turned within limits given in Machinery's handbook. We're using seco insert 60 deg partial profile threading tools. The tool tip has a small radius but the wires don't touch the root when measuring (unless it's the thread gauge that's touching the radius). What I need to know is can I trust the 3 wires for those odd threads I don't happen to have ring gauge for?

[basalt]

You can trust the 3 wire measurement providing your thread profile is correct--the tool is ground to exactly 60 degrees, it has zero back/side rake, it is exactly on centre and is perfectly perpendicular to the axis. The next thing to do is stay away from maximum material condition, get inside the tolerance/allowance. When I get to the proper depth or a little more, with the compound set parallel to the lathe axis, I move the tool back and forth with the compound, to clean up both sides of the thread as I creep up on my final size. If all this is well, the ring gage should fit. In 30 years of cutting threads this way I have never had one not fit or come back. Here are the formulas I use--

Depth of thread Unified National--
External 0.61343/Threads Per Inch [taken from exact nominal Outside Diameter].
Internal U.N. 0.54127/T.P.I. [taken from an Internal minor diameter that is-- Nominal O.D. - 2(0.54127/T.P.I.)]

Best Wire Size = 0.57735/T.P.I.

Measurement over Wires = O.D. - 1.5155/T.P.I. + 3 X Wire Size used [not necessarily the best wire size]

[juergenwt]

You are cutting your threads on CNC. If you are using the correct cutting tool for the thread you are cutting than your thread should be ok if measured with 3 wires. The problem is that depending on the material you are cutting and the sharpness of your cutting tool and also your cutting speed - you could have a ridge pushing up on top of your thread and in that case the three wires would do you no good. Because you are using CNC I would assume you are making a production run of many parts. I would advise you to buy a set of GO-NO GO thread rings.

[Delw]

You CANNOT use thread wires to check anything but thread pitch DIA. no ifs ans or butts about it. you can use any size thread wires that fit, you just need to calculate it out.
a thread gage ( go and No go) will check everything and functionaliblity.

if your using a cnc and have problems with bad threads you dont know how to machine threads. ( its simplier than anything on a cnc)
heres the problems and can be. more than likely are.

you thread profile is garbage.
your inserts are not correct
you RPM is way too fast
your machien is trashed and doesnt repeat.
or you dont know how to machine a thread.

best way to machine a thread is cut od to finish size.
thread
then cut od again at same size
thread again.

now you have a perfect thread with out any deflection. providing you have a good machine and your tooling isnt shot.
dont forget allways use g97 for threading (never g96)
leave it a min of .200 from start of thread.

now aside from this you never said what your lead was ie TPI. or what type of material it is, let alone size and how far you are from the face of the chuck.
These also play factors.

if your comparator shows profile being good. then I would guess that your not using the proper dims for the thread wires you have and getting a false/wrong reading.
some where you screwed up in the calculations.

Delw