Return to main page
Measurement Over Wires / The Three Wire Method
I have found these programs very useful. A practical note here: The burr turned up when "single pointing"
threads on the lathe can often rise above the tops of the wires and thus interfere with the micrometer
actually contacting the wires for the "Measurement Over Wires" measurement. One should make a
measurement over the burrs to make sure the micrometer is contacting the wires. I still like this measurement
more than the others that use precision "Vee" shaped anvils since I feel assured that the proper sized wire is
not riding on the bottom of the groove as the anvils might be. A file can be used to remove the raised burr
in order to make a measurement.
The following two GW-BASIC programs are inverses of each other in that the first calculates the
"Measurement Over Wires" (MOW) given the "Nominal Thread Diameter" and the second calculates
the "Pitch Diameter" given the "MOW". I guess they are not quite exact inverses since the first inputs
"Nominal Thread Diameter" and the second outputs "Pitch Diameter" but if one assumes the "Unified Thread
Form" as these programs do then one can calculate one from the other if one knows the "Threads Per
Inch"(TPI). They were found most useful the way they are.
The advantage of these programs is that they allow one to use a range of wire sizes as long as they fall
within the range that lie on the 60 degree flanks of the thread. The first program tells what range this is.
These programs assume one knows what the TPI is since one usually knows his ahead of time. Usually I
run Program1 before running Program2 to get the wire sizes allowed before making a MOW and running
Program2. Getting the wire sizes could have built into Program2 but was not.
Usually wire sets for use in making the MOW of the "Three Wire Measurement" technique use wires sizes
that such a size that the wire sets exactly on the 60 degree flanks of the thread right on the "Pitch Diameter"
point of the thread. With this restriction than a table can be used to get the MOW however the advantage
of these programs is that one can use alternate sizes if the exact wire diameter is not available. In other words
if a larger wire is used that rides up higher in the groove of the thread, then the MOW is adjusted to compensate
for this. This assumes that the flanks of the threads are formed by a "V" that has straight sides to work exactly
correctly.
Program1
100 CLS
200 PRINT "*********************************************************************"
300 PRINT "** These Calc's Give Parameters For *Matched* Int and Ext Threads **"
400 PRINT "** **THUS: EXT/INT THREADS ARE EXACT FIT WITH ZERO CLEARANCE** **"
500 PRINT "** Calculations are based on the UN (ISO Unified Thread Form) **"
600 PRINT "** (To obtain clearance the program should be rerun with **"
700 PRINT "** alternate initial major diam's to give clearance between **"
800 PRINT "** major, minor, and pitch diameters of mating threads) **"
900 PRINT "*************** Written: Ken Knaell, Aug 1992 *****************"
1000 INPUT "Input 'Screw Size Number' : #",SN
1100 MD=.013*SN+.06
1200 PRINT USING "Major Diam For This Screw Size Calculates to: ##.####"; MD
1300 QQ=0!
1400 INPUT "Input A New Major Diam for Following Calculations(Return for Calcd):", QQ
1500 IF (QQ <> 0!) THEN MD = QQ
1600 INPUT "Input 'Threads/Inch': ", NN
1700 PT=1!/NN
1800 H=PT*(SQR(3!)/2!)
1900 PRINT USING "For This TPI The Thd Pitch Is: #.####, Sharp V 'H' Is: #.####"; PT; H
2000 PD = MD-H*3!/4!
2100 PRINT USING "The UN Pitch Diam. is: #.####, The UN Minor Diam. is: #.####";PD;MD-H*5!/4!
2200 WS=1!/(NN*SQR(3!))
2300 PRINT USING "Minimum, Best, and Maximum Wire Diameters are: #.####, #.####, #.####";WS*7!/8!; WS; WS*7!/4!
2400 'Note: Wire Disappears Below Top of Thd Before It Hits Bottom Of Form.
2500 'Note: Wire Hits Bottom of Thd Form at Diam = WS*3/4 (Smaller than WS*7/8).
2600 QQ=0!
2700 INPUT "Input actual Wire Diameter Used For Measurement Over Wires: ", QQ
2800 IF (QQ <> 0!) THEN WS=QQ
2900 MW=PD+3!*WS-(SQR(3!))/(2!*NN)
3000 PRINT USING "The Measurement Over Wires Should Be #.#### For The Wire Size Of #.####";MW;WS
3100 PRINT " ***** SUMMARY: EXTERNAL THREADS *****"
3200 SD = MD 'SD=MD-.216/NN
3300 DE=H*(5!/8!)
3400 PRINT USING "The Major Diameter is: #.####, The Thd Depth (H*5/8) is: #.####:";SD,DE
3500 FE=PT/8!
3600 FB = PT/4!
3700 PRINT USING "The Flat Top Width(PT/8) Is: #.####, The Flat Bottom Width(PT/4) Is: #.####"; FE,FB
3800 CF = H*(5!/8!)*(2!/SQR(3))
3900 PRINT USING "Compound Feed From Maj Dia With PT/4 (ISO Ext Std) Flat End Tool Is: #.####"; CF
4000 CS = H*(7!/8!)*(2!/SQR(3))
4100 PRINT USING "For Same PD and MD, Comp'd Feed From Maj Dia With Sharp Pt Triag Tool Is: #.####"; CS
4200 PRINT "** The Correct Feed To Obtain a Specific Pitch Diam Depends Upon **"
4300 PRINT "** Size of Flat on End of Tool And Stock Diameter. **"
4400 TC=H*(2!/SQR(3!))
4500 INPUT " Hit RETURN to continue to **SUMMARY: INTERNAL THREADS**", TRA$
4600 CLS
4700 PRINT " **** SUMMARY: INTERNAL THREADS ****"
4800 PRINT "NOTE: This Internal Thread is a Match Fit to the External Thd."
4900 PRINT "NOTE: All Three Radii of the Ext Thd Must Be Smaller Than These For Clearance."
5000 PRINT USING "The Major Diameter is: #.####, The Int Thd Depth (H*5/8) is: #.####:";SD,DE
5100 DT=MD-2!*DE
5200 REM The above is equivalent to : DT=MD-2!*.54127*PT or DT=PD-H/2
5300 PRINT USING "The Minor Thd Dia and Tap Drill For 100% Thread Depth Calculates To: #.#### ";DT
5400 D7=MD-2!*DE*.75
5500 PRINT USING "Tap Drill For 75% of ISO Thread Engagement Calculates To: #.####";D7
5600 DI=.54127*PT
5700 FI=.25*PT
5800 PRINT USING "Flat Crest and Base Width of Thd (ISO Int Std) Are: #.#### #.####"; FI; FI/2!
5900 IC=DI*(2!/SQR(3!))
6000 PRINT USING "Compound Feed From 100% TD Hole With PT/8 Flat Ended Tool (UN Int Std Thd Form) Is: #.####"; CF
6100 CSI = H*(6!/8!)*(2!/SQR(3))
6200 PRINT USING "For Same PD and Drill, Comp'd Feed With Sharp Pt Triag Tool Is: #.####"; CSI
6300 PRINT "The Correct Feed To Obtain A Specific Pitch Diam Depends Upon Size Of Flat On End Of Tool And Tap Drill."
6400 PRINT "A Smaller Hole and/or Smaller Flat on Tool Increases the Compound Feed By The Amount 2/sqr(3)=1.15 Times This Radial Alteration to Maintain Pitch Diam."
6500 PRINT "A More Pointed Tool Requires More Feed."
Program2
10 REM PGM TO CALCULATE PITCH DIAMETER (PD) FROM MEASUREMENT OVER WIRES (MOW).
20 INPUT "Input Threads/Inch: ",NN
21 WS=1!/(NN*SQR(3!))
22 PRINT USING "Minimum Wire Diameter: #.####";7!*WS/8!
23 PRINT USING "Best Wire Diameter: #.####:";WS
24 PRINT USING "Maximum Wire Diameter: #.####:";7!*WS/4!
30 INPUT "Input Wire Size: ",WS
40 INPUT "Input Measurement Over Wires: ",MW
45 H=SQR(3)/2!/NN : PD=MW-3!*WS+H
60 PRINT USING "Pitch Diameter Is: #.####";PD
70 MD=PD+H*3/4
80 PRINT USING "Major Diameter Should Be: #.####";MD
90 SD=MD-H*5/4
100 PRINT USING "Minor Diameter Should Be: #.####";SD
Return to main page