Specifying a hole for a press fit involves giving the hole a slightly smaller diameter than the bushing’s outer diameter and controlling that diameter with an appropriate tolerance so you get the correct interference. Here are the key points:
1. Determine Your Required Interference
- Nominal Size: If the bushing’s OD is exactly 16.00 mm (nominal), then your hole must be under 16.00 mm.
- Interference Range: A typical starting point for an interference fit at around 16 mm diameter might be on the order of 0.01 mm to 0.03 mm total interference (though the exact amount depends on factors like operating temperature, load, material properties, and the manufacturer’s recommendation).
- Material Considerations: Aluminum is more compliant (softer) than steel or cast iron. This can allow slightly larger interference without damaging the bushing, but it can also “relax” over time. If in doubt, consult the bushing manufacturer’s recommended press-fit tolerances.
2. Choose a Fit or Tolerance Scheme
You can specify the hole dimension in either:
- ISO Tolerance System (e.g., H7, F7, etc.), or
- Direct Limits (e.g., 15.98 +0.00/−0.02 mm).
Example Using Direct Limits
- Hole Diameter: 15.98 mm
- Tolerance: +0.02 mm / −0.00 mm
This would mean the hole can be anywhere between 15.98 mm and 16.00 mm. The worst-case scenario (15.98 mm hole with a 16.00 mm bushing) yields 0.02 mm interference, while the “loosest” scenario (16.00 mm hole and 16.00 mm bushing) gives almost zero interference. In practice, you typically design so there is always at least some minimal guaranteed interference (e.g., 0.01 mm).
Example Using ISO Fit
If your bushing OD is 16 mm nominal (treated like a “shaft” in ISO terms), you can choose a hole tolerance class that ensures an interference. For instance, an F7 or E7 hole with a 16 mm “shaft” can provide a light to medium press fit. The exact fit notation depends on the bushing’s actual manufacturing tolerance.
3. Add a Chamfer or Lead-In
Always include a small chamfer (e.g., 0.5 mm × 45°) on the hole entrance. This helps align the bushing and prevents shaving or galling the edges of the aluminum when pressing the bushing into place.
4. Callout in the Drawing
A typical drawing callout for a press fit hole might look like:
Ø15.98 +0.02/−0.00
Press fit bushing (16.00 OD)
0.5 × 45° chamfer
Or, if you use ISO fits:
Hole Ø16 mm (F7)
Press fit bushing (16 mm, h6)
0.5 × 45° chamfer
(Exact class depends on the bushing tolerance.)
5. Check Your Final Assembly
- Verify the Actual Bushing OD: Measure a sample of the bushings to confirm their real OD and tolerance.
- Confirm the Hole After Machining: Check that the aluminum hole matches your specified tolerance before press-fitting.
- Press-Fit Procedure: Use a proper arbor press or hydraulic press and ensure alignment. Lubrication can help reduce galling.
Summary
To specify a hole for a 16 mm OD bronze bushing in an aluminum part:
- Make the hole diameter slightly smaller than 16.00 mm—on the order of 15.97–15.99 mm.
- Provide a tight enough tolerance so you always maintain the desired interference (e.g., 0.01–0.02 mm).
- Add a chamfer to guide the bushing.
- Confirm real-world measured sizes (bushing OD and hole ID) to ensure the press fit functions as intended.