5.1 When a broken gear was measured, it was found to have an outside diameter of 112 mm - NSC Mechanical Technology Welding and Metalwork - Question 5 - 2016 - Paper 1
Question 5
5.1 When a broken gear was measured, it was found to have an outside diameter of 112 mm. A replacement gear must be made to mesh with a rack that has a straight-line... show full transcript
Worked Solution & Example Answer:5.1 When a broken gear was measured, it was found to have an outside diameter of 112 mm - NSC Mechanical Technology Welding and Metalwork - Question 5 - 2016 - Paper 1
Step 1
5.1.1 The module (round off to a whole number)
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Answer
To find the module, use the formula for circular pitch:
extCircularpitch=mimesextπ
Given that the circular pitch is 12,567 mm, we can rearrange this to find the module:
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Answer
The pitch-circle diameter (PCD) can be calculated using the relationship:
extPCD=extOD−2m
Substituting the known values:
extPCD=112−2(4)=112−8=104extmm
Step 3
5.1.3 The cutting depth
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Answer
The cutting depth can be calculated using:
extCuttingDepth=m+0.157extmm(forstandardcutting)
Substituting the module value:
extCuttingDepth=4+0.157=4.157extmm
Step 4
5.1.4 The addendum
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Answer
The addendum is the height of the tooth above the pitch circle and is given by:
extAddendum=m=4extmm
Step 5
5.1.5 The dedendum
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Answer
The dedendum is calculated using:
extDedendum=m+1.25extmm=4+1.25=5.25extmm
Step 6
5.1.6 The clearance
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Answer
Clearance is given by:
extClearance=0.25imesm=0.25imes4=1.0extmm
Step 7
5.1.7 The number of teeth
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Answer
Finally, the number of teeth can be determined by:
extNumberofTeeth=mPCD=4104=26extteeth
Step 8
5.2 Calculate the indexing required for the number of teeth of the gear in QUESTION 5.1.7.
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Answer
The indexing is calculated using the formula:
Indexing=n40, where n=26ext(numberofteeth)
d therefore,
Indexing=2640extisapproximately1.54extturns
Step 9
5.3 Explain, step by step, the procedure to cut an external metric V-screw thread with a pitch of 2 mm on a centre lathe using the compound-slide method.
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Answer
Set up the work piece in the lathe and turn the part to be threaded to the major diameter of the thread.
Set the compound slide to the correct angle (30°) to the right and set the tool up accurately in the post.
Consult the index plate of the quick-change gearbox for 2 mm pitch and move the levers accordingly.
Start the lathe and set the cutting tool so that it just touches the work piece. Set graduated dials to zero (cross feed and compound slide).
Move the cutting tool a short distance off the work piece and feed the compound slide by 0.06 mm to start.
With the turning, engage half nuts at the correct line on the chasing dial, which should be left in progress.
Withdraw the cutting tool to quickly disengage the half-nut lever. Return the carriage to the starting position.
Check with thread gauge to see if thread pitch is correct.
Repeat until the desired thread is complete. (Remember to bring back the cross-slide back to zero for each cut).
Each successive cut is set by means of the compound slide. Check thread with ring gauge for correct fit.
