Filing a Block to 18.00 ± 0.05 mm: When the High Spot Is Already Undersize
An ITI first-year trainee in a fitting shop is given a mild steel block 100 mm × 60 mm × 20 mm and told to file it to 18.00 ± 0.05 mm thickness, with opposite faces parallel and adjacent faces square within 0.05 mm over 100 mm. He clamps the block with about 8 mm projecting above the vice jaws and starts with a 300 mm bastard file, using crossing strokes at about 30°. After 25 minutes the file skids instead of cutting, the teeth are choked with grey pins, and marking blue on the surface plate shows a high patch at one corner. A vernier caliper reads 17.86 mm at that corner and 18.12 mm diagonally opposite; the other two corners read about 17.98 mm. The vice top is level with his chest, and he bears down heavily on the forward stroke.
1. Diagnose why the file is skidding and pinning, and explain how the vice height and clamping are contributing to the uneven cut.
Read the worked answer
The grey pins show pinning: soft mild steel is being cut with a coarse bastard file under heavy pressure, and the teeth are not chalked or carded. Once the teeth are choked, the file glides and skids instead of biting.
The vice top at chest height puts the work too high. The trainee's elbows are below the file, so he rocks the file instead of keeping it flat, and the heavy forward pressure adds to the rocking. Clamping only 8 mm above the jaws also lets the file foul the jaw edges and makes the block less stable. Correct height is about elbow level, with the work just clear of the jaws. Card the file, chalk it lightly, reduce pressure, and use a smooth file for the final 0.2 mm.
2. The trainee wants to keep filing the 17.86 mm corner until the blue shows an even surface. Evaluate this plan and give the correct measurement-and-correction sequence.
Read the worked answer
The plan is wrong. The lower limit is 17.95 mm, and the corner is already at 17.86 mm, so it is below size. Filing can only reduce it further; the block cannot be brought back to 18.00 ± 0.05 mm by removing more metal from that face.
Correct sequence: stop filing, wipe the block, and measure thickness with a micrometer at six points (four corners and centre on both faces). Record the minimum. If the minimum is 17.86 mm, the maximum finished thickness is 17.86 mm, so the block is nonconforming for this specification and should be rejected or reworked to a smaller size. In future work, leave 0.2–0.3 mm finishing allowance, blue the surface, file only the high spots, and check every few strokes rather than after 25 minutes.
3. Describe how you would check parallelism and squareness of the finished block, and state two measurements that would confirm the block is within tolerance.
Read the worked answer
Clean the surface plate and remove burrs. Check squareness by standing the block on the plate and pressing a try square against each adjacent face; a feeler gauge should not enter more than 0.05 mm at any point over the 100 mm length. Check parallelism by measuring thickness with a micrometer at the four corners and centre of both faces; the difference between the largest and smallest readings must not exceed 0.05 mm. If a dial test indicator is available, sweep the top face while the block sits on the plate and read the total variation. Two confirming measurements are: maximum thickness minus minimum thickness ≤ 0.05 mm, and the largest try-square gap ≤ 0.05 mm over 100 mm.