Bore a bronze bushing to the measured shaft diameter plus 0.05-0.10 mm, using a short rigid boring bar, roughing passes of 0.5-1 mm and a final finishing cut of 0.1-0.2 mm at fine feed. Because that clearance window is only 0.05 mm wide, a shop that holds only +-0.02 mm on each part cannot hit it by working to a drawing, so fit each bushing to its own measured shaft.
Part of Bearings and Bushings
Finishing a cast or drilled bronze blank on a lathe so its bore is round, straight, smooth and sized for the shaft it will carry.
The Clearance Window Is Narrower Than The Shop’s Tolerance
A bronze bushing wants a bore 0.05-0.10 mm larger than the shaft, which is about 0.002 x diameter (a radial clearance of about 0.001 x D, per Bearings and Shafts). Across the 25-50 mm shaft range that is exactly the carrier’s figure. Too tight and the oil film has no room to form; too loose and the film breaks and the shaft runs off-centre (see Lubrication Theory).
The window is 0.10 - 0.05 = 0.05 mm wide for a 50 mm shaft. That is narrow compared with what the carrier says an ordinary precision shop can hold: +-0.02 mm. So the tolerance on a part is not a safe way to specify a fit. The bushing is bored to the shaft you have, not to the shaft on paper.
What You Need
- A lathe with a short, rigid boring bar (see Metal Turning: the bar must be as short as possible, because a long, slender one deflects under cutting force and leaves the bore tapered or barrel-shaped)
- The finished shaft, already turned, ground and measured
- Inside calipers or a bore gauge, and a micrometer to read them against (Micrometers)
- Cutting fluid, and a burnishing tool for the last pass
Method: Bore To The Measured Shaft
Step 1: Chuck the blank so it runs true (see Workholding). Face both ends.
Step 2: Drill the starting hole. A drilled hole is typically 0.1-0.3 mm oversize and out of round; boring cures both.
Step 3: Rough bore in passes of 0.5-1 mm, leaving a few tenths of a millimetre.
Step 4: Measure the shaft journal at several points along its length and around it with the micrometer. Take the largest reading. Add 0.05-0.10 mm (about 0.002 x D) to get the target bore.
Step 5: Finish bore with a 0.1-0.2 mm cut at fine feed with cutting fluid. A second pass at the same setting, with no further infeed, removes the spring in the bar.
Step 6: Measure at room temperature. Cutting heats the work. Steel grows 0.012 mm per metre per degree (Precision Measurement), which for a 50 mm part is 0.0006 mm per degree: 20 degrees of heat is 0.012 mm, about a quarter of the 0.05 mm window. That coefficient is for steel; bronze has its own, not given here, so let the work cool.
Step 7: Cut the oil groove. The carrier gives a helical groove 1-2 mm wide and 0.5 mm deep along the bore, with a circumferential groove at mid-length for heavy loads, and says to keep grooves out of the load zone. A helical groove crosses the loaded bottom of the bore on every turn, so the two instructions cannot both be followed. For a bearing whose load always presses one way, cut a straight groove in the unloaded half instead.
Step 8: Burnish and deburr. The target is under 1.6 microns Ra. No instrument for Ra appears in this wiki, so judge by eye and by feel against a sample you trust, and treat the number as an aim rather than a measured fact.
Worked Example
Shaft measured 50.00 mm. Target bore 50.00 + 0.05 up to 50.00 + 0.10, so 50.05-50.10 mm; aim at the middle, 50.075 mm. Clearance window is 0.05 mm.
If the shop can only hold +-0.02 mm on the bore and +-0.02 mm on the shaft, then built to drawing the worst combinations are:
- bore at 50.095 and shaft at 49.98 gives 0.115 mm clearance, over the 0.10 limit
- bore at 50.055 and shaft at 50.02 gives 0.035 mm, under the 0.05 limit
Fitting each bushing to its measured shaft removes the shaft’s share of the error and leaves only the bore error to control.
Common Mistakes
- Using a long bar to reach deep. The bore tapers; the bearing carries load on one end only.
- Measuring warm. The apparent bore is bigger than the cold bore; the finished bearing is tight.
- Boring to a number from a drawing. The shaft in your hand may differ.
- Grooving through the loaded area. See Step 7.
Safety
The chuck and the swarf
A workpiece that comes loose from the chuck is a projectile (see Workholding). Never hold inside calipers in a spinning bore, never wear loose sleeves or gloves near the chuck, and know where the lathe stop is and how long the spindle takes to coast down before you reach in to measure.
What’s Next
- Learn next: Interference Fits - pressing the finished bushing into its housing without closing the bore
- Combine with: Bronze Bushings - sizing and choosing the alloy