- Practice · 5 marks
Hole basis and shaft basis systems
Short answer
Hole basis: zero lower deviation hole (H), varying shaft. Shaft basis: zero upper deviation shaft (h), varying hole. Hole basis is preferred because holes require fixed dedicated tools (reamers, broaches); one H hole reamer serves many fits, reducing tool costs, while shafts are easily machined to any size.
Differentiate between the hole basis system and the shaft basis system of fits. Why is the hole basis system more commonly used?
In the hole basis system the basic size is the minimum limit (lower limit) of the hole, so the lower deviation of the hole is zero (letter H). Different fits are obtained by varying the size of the shaft. In the shaft basis system the basic size is the maximum limit of the shaft, so the upper deviation of the shaft is zero (letter h). Different fits are obtained by varying the size of the hole.
| Point | Hole basis | Shaft basis |
|---|---|---|
| Fixed member | Hole (H, lower deviation 0) | Shaft (h, upper deviation 0) |
| Varying member | Shaft | Hole |
| Example | 40 H7/g6, H7/k6, H7/p6 | 40 G7/h6, K7/h6, P7/h6 |
| Tools for the fixed part | Standard reamers, broaches and plug gauges are limited in number | Standard shafts, ground bars |
| Cost | Lower for general use | Higher: many hole sizes need many reamers |
| Used | Machine tools, general engineering | Where long drawn or ground bars of a standard size carry many parts, e.g. line shafting, textile and agricultural machines |
Why hole basis is preferred
- Holes are made by drills, reamers, boring and broaches whose sizes are fixed; one reamer size of H7 covers all fits.
- Shafts are easily turned and ground to any size, so the changing member is the cheaper one to change.
- Fewer tools and gauges are needed, so the cost of production and inspection is lower.
The shaft basis is used when a single shaft carries several parts with different fits (for example a bar on which several bearings, collars, and pulleys are fitted) because cutting steps on a shaft would weaken it and cost more.
✓ Answer checked by Claude Opus 5.5 (high effort)
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