hexagon logo
Register Sign in
  • Replies 16 replies
  • Subscribers 223 subscribers
  • Views 432 views
  • Users 0 members are here
Options
Related

Thread Clocking and Lead In Chamfer Strategy Guidance

Incredible.Mr.Lucas
Roughly determining the ID thread clocking to locate the thread start for maximum surface area for lead in chamfer diameter measurement. Basically, measuring the countersink of at most 1/3 the circumference of an internal thread entrance. I need to know the thread clocking to even make an attempt.

I understand this is one of those "could you, but should you?" questions, but please humor me, our customer has requested that we try. The threads are small, 5/16s and 1/4in, the countersink diameter doesn't even exceed the major diameter. I think at minimum, one might be able to measure the countersink angle and the radius of the countersink edge to the center of the thread minor with two lines and an intersection point. It doesn't help matters much that nobody actually models threads this small.

Imagine if this was the ONLY thing the program needed to do was this ONE task, to measure the clocking of a small thread.

My guess is, step one would be to locate the minor diameter of the thread and translate my alignment to the intersection of the thread to the surrounding surface. Then come down some distance and use a self-centering 2D vector point to nest my stylus ball between two adjacent thread minor crests. So then I would have a depth measurement from that point to the surface. I would think one could divide that distance by the pitch of the thread to get an integer number and a fractional remainder. If my point was directly below the thread start, the division would result in a clean integer with no remainder. If there is a remainder, then in theory one could relate that remainder to 360° circumference to estimate the thread clocking.

We aren't going to use thread gauges, pins, or tru-pos. The only tools we have at our disposal are the thread, the surface, and the stylus.


Am I on the right track? has anyone tried this before? Are there any productive suggestions? Stylus diameter? self-centering points? scanning?
Parents
  • Here is an idea that I sometimes use when I want to measure the minor diameter of a thread. It may help for your experiment.

    1) Take a series of points along the thread profile. You can use a scan or just a line with a bunch of hits.
    2) Find the point that settles the furthest between threads.
    3) Create a generic feature at the same level as that point
    4) Align to that point
    5) Measure the minor diameter

    Below is a sketch to make it a little clearer. You want to have a probe radius that is small enough that it will get in between the threads a bit, but not so small that it will shank out. Just take a series of points that is long enough to cover one thread.



    The code looks something like this: 

    
THRDLINE =FEAT/CONTACT/LINE/DEFAULT,CARTESIAN,UNBOUNDED
THEO/<-36.7786,-37.24,0.3654>,<0,0,1>,<-1,0,0>,<0,-1,0>,0.7359
ACTL/<-36.7786,-37.24,0.3654>,<0,0,1>,<-1,0,0>,<0,-1,0>,0.7359
TARG/<-36.7786,-37.24,0.3654>,<0,0,1>,<-1,0,0>,<0,-1,0>
SHOW FEATURE PARAMETERS=NO
SHOW CONTACT PARAMETERS=NO

ASSIGN/MAXPNT=MAXINDEX(THRDLINE.HITS.X)

ASSIGN/ZE=0

THRDPNT =GENERIC/POINT,DEPENDENT,CARTESIAN,$
NOM/XYZ,<ZE,ZE,ZE>,$
MEAS/XYZ,<ZE,ZE,THRDLINE.HIT[MAXPNT].Z>,$
NOM/IJK,<ZE,ZE,1>,$
MEAS/IJK,<ZE,ZE,1>

A3 =ALIGNMENT/START,RECALL:ALN3,LIST=YES
ALIGNMENT/TRANS,ZAXIS,THRDPNT
ALIGNMENT/END


    Now in your case, you don't want a depth to align to, you want a rotation value. Let's consider a 1/4-20 thread. In this example you can measure a line along the thread profile starting at let's say 2 revolutions deep: .100". Make that line exactly one thread pitch long: .050" long. Take many hits. Let's say you make it 37 hits. You can then find highest point with the maxindex function. Each hit will equate to 10degrees of rotation. So if the first hit ( hit 1) is the maxindex, the root of the thread is at zero degrees at the face of the part. If hit 2 is the max, the root of the thread is at 10 degrees at the face of the part. 3=20deg. 4=30deg. You get the idea. You can use a simple equation that converts the maximum index directly into an angle value. ASSIGN/THREADANG=(MAXPNT-1)*10

    From there you can construct a generic line in which to rotate to. Use some trig functions to fill in the vectors. I'd go into details for that, but I'm already feeling overwhelmed by the effort this will take.

    If a 10 deg angle resolution is not enough for you, increase the number of hits. 73 hits would give you a 5 deg angle resolution. On a 1/4-20 thread that means over 50pnts/mm. More than I like to do even with an analog probe.

    If you have an analog probe you also have the Center Scan feature that could be used to quickly find what I have been calling the maxindex. It will just scan along the thread profile until it nestles between the threads and finds the low point. I think this could also be done with an auto vector point using the 'Self Centering Point' measurement strategy. Once you have that low point you can do the math to find the rotation angle that leads to that low spot at the depth found. That would probably give you the best rotational resolution.

    So, um, there's some ideas. The whole prospect doesn't give me warm fuzzies, but it can still be fun to talk theory. Please keep us updated on what you try and how it goes for you.
  • in reply to Cris_C
    I've done the same exact thing with great success. However, I use the MININDEX and the MINOR diameter of the thread, as it's physically a shaper peak to probe upon, and therefore produces a more consistent maxindex value. Unless, the threads are rolled, and the minor has been drilled large (makes two minor peaks at rolled thread with a valley between rolled features)
Reply
  • in reply to Cris_C
    I've done the same exact thing with great success. However, I use the MININDEX and the MINOR diameter of the thread, as it's physically a shaper peak to probe upon, and therefore produces a more consistent maxindex value. Unless, the threads are rolled, and the minor has been drilled large (makes two minor peaks at rolled thread with a valley between rolled features)
Children
No Data
© 2024 Hexagon AB and/or its subsidiaries. Privacy Policy | Cloud Services Agreement