Measuring Threaded Holes

Transcription

1 Measuring Threaded Holes

2 It s a common question

3 In PC-DMIS I can use the pitch function within a circle feature so the probe would "follow" the thread. How are tapped holes measured in Calypso?

4 I have a part with several threaded holes that I need to report TP on.

5 what is the best method of checking the position of threaded holes, i would like to stay away from flex plugs due to cmm time.

6 We have been checking a thread hole and they screwed a pin in the hole and were measuring od of pin for T/P. I ve been trying to revise the program to check without pin to save some time. I am having trouble getting results.

7 Why the problems? Here s how a common thread is made:

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9 In a perfect world, pilot hole location (which h we don t care about) is the same as the pitch location (which we care about)

10 If you haven t noticed yet, this is not a perfect world Real parts can look like this:

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14 We can t directly measure the pitch diameter on a CMM. What s a CMM programmer to do?

15 The traditional accepted method is to use a Thread Location gage.

16 Threaded Hole Centerline Gages Flexible Solid Tapered Thread Lock Variable

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18 precision cylinder to be measured by the CMM. Split threads spring tight into the pitch diameter.

19 Let s Test it out! (Accura, 0.050mm tolerance)

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27 Run it 10 times in the same place to prove program repeatability

28 True Position Intersection

29 True Position Cylinder Bottom

30 True Position Circle

31 True Position Cylinder

32 True Position Cylinder Top

33 Good repeatability. Program is fine. Difference in answers show a tilt of the thread.

34 Run it 10 times and reload to prove fixture method...

35 True Position Intersection

36 True Position Cylinder Bottom

37 True Position Circle

38 True Position Cylinder

39 True Position Cylinder Top

40 Good repeatability. Fixture is fine. Same pattern of answers up the pin. Why the different answer from first test?

41 Run it 10 times and screw/unscrew flexible thread position gage to prove gage repeatability

42 True Position Intersection

43 True Position Cylinder Bottom

44 True Position Circle

45 True Position Cylinder

46 True Position Cylinder Top

47 Bad repeatability. What s Going On?

48 More testing! Just looking at INTERSECTION results Remove/Replace Plug (Duramax, 0.050mm tol)

49 True Position Intersection

50 Same stuff. Let s think about what s going on. (I hate that )

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52 Tighten up a little

53 A little more

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55 Ahhh So, based on how I tighten the plug, the center will move.

56 Just looking at INTERSECTION results Remove/Replace Plug, but Don t tighten! (Duramax, a 0.050mm 050 tol)

57 True Position Intersection

58 Wow! Outside of 0.050mm 050mm tolerance! Increase the tol to 0.200mm!

59 True Position Intersection

60 Tight flex plug 0.067mm 067mm repeatability loose flex plug (!) mm repeatability

61 We are in BUSINESS! Now, back to the original question: How do you best measure a threaded hole (without the plug)?

62 Calculating l Gradient/Slope (aka lead or pitch) 3/8 16 UNC = 16 threads/inch Gradient/Slope = 1/16 or or mm M10x1.5 = 1.5 mm of lead per rev Gradient/Slope = 1.5mm or

63 Measurement Strategy #1 A circle. Concept: Who Cares, Close enough.

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65 Measurement Strategy t #2 Symmetry between Two Circles, ½ Pitch apart Concept: Average of shifted circle centers will be real center

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67 Measurement Strategy t #3 One circle, Two Circle paths, ½ Pitch apart Concept: Same theory as Symmetry, but no construction step

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69 Measurement Strategy t #4 Calypso s Circle Macro Concept: The Thread routine in the software MUST be good.

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71 Measurement Strategy t #5 circle with helix path on pitch Concept: Center will be good, Who cares where it is measured, as long as it follows thread pitch.

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73 Measurement Strategy t #6 Cylinder with 3 pitch helix revolutions Concept: Measuring a cylinder to get an axis is better than just a circle.

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75 Measurement Strategy t #7 Cylinder with 5 pitch helix revolutions Concept: 5 revs must be better then 3!

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77 Measurement Strategy t #8 Cylinder with double-pitch helix. Concept: scan a lot on the thread it will average out to the true center.

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79 Measurement Strategy t #9 Cylinder with Linear Paths. Concept: scan a lot on the thread it will average out to the true center.

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81 Measurement Strategy t #10 Cylinder with reverse Helix. Concept: scan a lot on the thread it will average out to the true center.

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83 ¼ Pitch Helix Gradient

84 ½ Pitch Helix Gradient

85 1 Pitch Helix Gradient

86 1 ½ x Pitch Helix Gradient

87 2 x Pitch Helix Gradient

88 Measurement Strategy t #11 Cylinder with self-center Helix. Concept: stylus will push into the tread, close to pitch diameter.

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91 More testing! remove/replace plug. 5 runs, 5mm stylus 5 runs, 3mm stylus Plug Average is nominal. Try 3 different parts. (Accura, 0.200mm tol)

92 Part 1:

93 3mm stylus s

94 5mm stylus s

95 All 10 results

96 Nominal Upper Tol Range Average True Position Pin True Position Cyl 5 Helix at Plane True Position Cyl Doublepitch at Plane True Position Cyl Self Center at Plane True Position o Circle Helix True Position Symmetry True Position Double True Position Cyl 3 Helix at Plane True Position Circle Macro True Position Cyl Rev Helix at Plane True Position Cyl Linear at Plane True Position Circle

97 Part 2 :

98 3mm stylus s

99 5mm stylus s

100 All 10 results

101 Nominal Upper Tol Range Average True Position Pin True Position Cyl Linear at Plane True Position Cyl Self Center at Plane True Position Cyl Rev Helix at Plane True Position o Cyl 5 Helix at Plane True Position Double True Position Symmetry True Position Cyl 3 Helix at Plane True Position Circle Helix True Position Cyl Doublepitch at Plane True Position Circle Macro True Position Circle

102 Part 3 :

103 3mm stylus s

104 5mm stylus s

105 All 10 results

106 Nominal Upper Tol Range Average True Position Pin True Position Cyl Self Center at Plane True Position Cyl 5 Helix at Plane True Position Cyl Rev Helix at Plane True Position o Circle Helix True Position Symmetry True Position Cyl Linear at Plane True Position Cyl 3 Helix at Plane True Position Circle Macro True Position Cyl Doublepitch at Plane True Position Circle True Position Double

107 Now, you are thinking to yourself I just sat through 105 slides what is the conclusion?

108 What s Important to You and your company? Accuracy compared to Flex Plug? Repeatability? TIME?

109 Location Deviation (all Tests) Average Avg True Position i Pin True Position Cyl Self Center at Plane True Position Cyl 5 Helix at Plane True Position i Symmetry True Position Circle Helix True Position Cyl Rev Helix at Plane True Position i Cyl l3 Helix at Plane True Position Cyl Linear at Plane True Position Cyl Doublepitch at Plane True Position i Circle Macro True Position Double True Position Circle 0.099

110 Repeatability (all Tests) Range Avg True Position Symmetry True Position Circle Helix True Position Cyl Rev Helix at Plane True Position Cyl Self Center at Plane True Position Cyl 5 Helix at Plane True Position Circle Macro True Position Cyl 3 Helix at Plane True Position Cyl Linear at Plane True Position Pin True Position Double True Position Cyl Doublepitch at Plane True Position Circle 0.112

111 Inspection Time per Hole Time (sec) True Position Circle Helix True Position Circle True Position Circle Macro True Position Cyl 3 Helix at Plane True Position Symmetry True Position Double True Position Cyl 5 Helix at Plane True Position Cyl Rev Helix at Plane True Position Cyl Self Center at Plane True Position Cyl Linear at Plane True Position Cyl Doublepitch at Plane True Position Pin

112 Notes: Use LSQ fitting to eliminate the effect of junky threads

113 Notes: Use of the Flex Plug wore the thread geometry to some degree shavings on the plug were evident after use, especially with aluminum parts

114 Conclusion: There is no Perfect solution to gaging the location of threaded holes on a CMM.

115 Conclusion: If you have the time and money and can afford repeatability issues, use the flex plug. It s the only way to theoretically measure location based on thread face Surfaces

116 Conclusion: If you have the time, the best way to match a flex plug is self-center scan a helical cylinder up the thread with at least 5 revolutions.

117 Conclusion: If cycle time is important, just measure a circle with one revolution with a helix with the gradient of the pitch.

118 Conclusion: Use as large a stylus as possible. Measurements converged closer to the target center when a larger tip was used, regardless of measurement method.

119 Conclusion: Repeatability of ANY method on the CMM (with any one stylus size) is always better then repeatability of a flex plug.

120 Disclaimer: The conclusions displayed d here are representative of the data gathered in this testing. Before choosing a method of inspecting threads, it is recommended to test and verify your results on your parts.

121 You ve got Questions, We have answers. Measuring Threaded Holes