Large thread pitch diameter

[[Ja...]]

This is a first for me. I just received a heads up I’ll be needing to pick up the pitch diameters on some rather large threads in the next few weeks. The sizes are 5.375” Ø x 12, and 8.500” Ø x 4. There will be both male and female threads of each size. Does anyone have experience in finding pitch diameters on a CMM?

[[Wo...]]

If I have a probe that's within the range of a measuring wire size for that pitch I use self centering points and formulas to measure the pitch diameters.
To find the right probe size I usually look up wire sizes from : http://theoreticalmachinist.com/Threads ... erial.aspx

It usually an additional check to gages.

[[Ke...]]

When I can not find a probe that matches the prescribed wire size,and I will use the Tangent function to simulate the 3 wire method.You can easily program in multiple locations, using Patterns.

[[Ja...]]

Wojciech Malig - Thanks for the link. That'll come in handy. Unfortunately for this particular application it's not going to help as it stops well short of the thread sizes I need to check.

Keith Forche - Your idea is sounding the best so far. Would you give a detailed description as to how you do this? Or is there a webpage somewhere that I could refer to?

[[Wo...]]

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I generally use it to find the right size probe to the given pitch - regardless of the diameter. Let's say I have 9.000-12 thread - I will look up any -12 pitch thread and get the range of wire sizes that can be used. From then it's just the matter of inputting the right formulas in Result Element for example:
I will measure a three self-centering points evaluated as midpoint and find either minimum or maximum value (depending if OD or ID thread)
Pull the probe radius that measured that particular point, double it to get the diameter, and then convert to inches:

(getProbeRadiusFromActual("Point_Thread.1")*2)/(25.4)

Calculate the constans:
3*( getActual("Probe_Diameter.1").actual)-(0.86603*0.083333) - the 0.08333 being the pitch (-12 in this case)

Measure a self centering point on the opposite side of the thread offset by half the pitch.

Create a distance between the opposite points ( evaluated as mid-points)

Create another Result Element adding the distance result to the probe diameter

getActual("Distance_Ref.1_X").actual+ getActual("Probe_Diameter.1").actual

Substract the constans from the above sum using another Result Element

getActual("MidPoint_Dia+Probe_Diameter.1").actual- getActual("Constans.1").actual

You need to either substract or add up the probe diameter and costans depending if its internal or external thread.

I bet there is an easier way to do it - this is just how I've been handling it and the results are always very close the PD measurement over the wires.

Good Luck!
Wojciech

[[Ke...]]

For 1 simulated threadwire check:
Scan lines on both sides of a flank.
Bring in both of those lines as Feature 1 & Feature 2, in the Tangent construction.
Enter the nominal pin size; and then toggle the result selection until the diameter location is in the appropriate location.

Repeat this process, until you have 2 Tangent constructions on one side of the thread, and 1 Tangent construction on the opposite side of the thread.

On the side with 2 Tangent constructions: create a 3D Line between the center of the Tangent centerpoints.
Create a Perpendicular from the 1 Tangent construction (opposite side of part) and the 3D Line.
Use a Result Element to calculate the Actual Perpendicular Length +/- the nominal pin size.

It's up to you how many locations you check, and exactly where you put the Tangent constructions.

On a longer thread, I may only verify a few locations spaced along the length. I may also select a probe diameter that is *close* to the prescribed pin size and take self-centered midpoints along the full length of the part, and pull those into lines or cylinders to provide a reference check for Straightness and/or Cylindricity.

[[Ja...]]

Sorry for the delay in responding. Too busy. Thanks for the help. I'll give these a try and see what works best for me.