Threading is one of the most failure-prone operations in machining. These 5 practices prevent the most common issues.
Practice 1: Use Forming Taps Where Possible
Forming taps displace material instead of cutting:
- No chips in blind holes
- 30% stronger threads (work-hardened)
- 5-10× tool life vs cutting taps
- Better surface finish
Limit: material below ~36 HRC. Works in steel, aluminum, copper, brass.
Practice 2: Consider Thread Milling
Thread milling uses small rotating cutter in helical path. Advantages:
- One tool covers multiple thread sizes
- If breaks, easy to remove from hole
- Better in hardened materials
- Excellent for large/special threads
Trade-off: 2-3× longer cycle time.
Practice 3: Verify Pre-Tap Hole Size
The #1 cause of tap breakage is wrong tap drill diameter:
| Thread | Standard | Hardened Steel |
|---|---|---|
| M6×1.0 | 5.0mm | 5.2mm |
| M8×1.25 | 6.8mm | 7.0mm |
| M10×1.5 | 8.5mm | 8.7mm |
| M12×1.75 | 10.2mm | 10.4mm |
Practice 4: Use Thread Gauges
Production threading needs go/no-go inspection:
- GO screws fully into thread → confirms large enough
- NO-GO stops within 2 turns → confirms not oversize
Practice 5: Set Tap Life Limit
Don’t run taps to failure. Production guidelines:
- HSS-Co in 1045 steel: 100-300 holes
- HSS-Co in stainless 304: 30-80 holes
- Forming tap in aluminum: 1000+ holes
- Carbide tap in hardened steel: 50-150 holes
Index at 80% of expected life. Cost of one scrap part exceeds 100 taps.
See hardened steel tap guide and tap catalog.
