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
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 pit... show full transcript
Worked Solution & Example Answer: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
The addendum is equal to the module:
extAddendum=extmodule=4extmm.
Step 5
5.1.5 The dedendum
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The dedendum is typically equal to 1.25 times the module:
extDedendum=1.25imesextmodule=1.25imes4=5extmm.
Step 6
5.1.6 The clearance
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The clearance can be calculated using:
extClearance=extDedendum−extAddendum=5−4=1extmm.
Step 7
5.1.7 The number of teeth
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Answer
The number of teeth can be calculated as follows:
ext{Number of Teeth} = rac{ ext{Pitch Circle Diameter}}{ ext{module}} = rac{104}{4} = 26 ext{ teeth}.
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
Indexing is calculated using the formula:
ext{Indexing} = rac{40 ext{ (for example)}}{26 ext{ (number of teeth)}} = 1 ext{ full turn of the crank and} rac{21 ext{ holes}}{39 ext{ hole circle}}.
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 workpiece 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 workpiece. Set graduated dials to zero.
Move the cutting tool a short distance off the workpiece and feed the compound slide say 0.06 mm.
With the turning, engage half nuts at the correct time on the chasing dial, until the cut is in progress.
Withdraw the cutting tool to quickly disengage the half-nut lever. Return the crank to the correct angle or zero the slide,
Check with thread gauge to see if thread pitch is correct.
Repeat with successive cuts until threading is complete. Remember to bring the feed-screw collar 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.
