KEVOS® MAKEOS · Quality
Drill Bushing Position Check (Clearance, Drill Tilt, Hole Position Error, Chip Gap)
When a drill is guided by a jig bushing, how far can the hole start from its true position because of the clearance in the bushing and the gap below it, and is that within the position tolerance?
Check of hole position from a drill jig: the largest radial clearance between the drill and the bushing bore from their limits and the bushing wear you allow, the tilt the drill can take inside the bushing length, the largest offset of the drill point at the work surface after the gap below the bushing, the resulting position error as a diameter against the position tolerance, and the chip gap as a multiple of the drill diameter against your range.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Drill and bushing | ||
| Drill diameter | mm | Nominal diameter of the drill. |
| Drill lower deviation | µm | Lower deviation of the drill diameter from nominal, from the drill standard (drills are usually made slightly under nominal). |
| Bushing bore upper deviation | µm | Upper deviation of the bushing bore from nominal, from the bushing standard or drawing. |
| Bushing wear allowed before replacement | µm | How much the bushing bore may wear before it is replaced. |
| Bushing length (guided length) | mm | Length of the bushing bore that guides the drill. |
| Gap from bushing end to the work | mm | Distance between the end of the bushing and the surface of the work, left for chips. |
| Extra point wander at entry | µm | Any further radial wander of the drill point you expect at entry, for example from an uneven surface (0 if none). |
| Requirements | ||
| Position tolerance (diameter of the zone) | mm | Position tolerance for the hole, as the diameter of the tolerance zone from the drawing. |
| Least chip gap as a multiple of drill diameter | Least chip gap your practice allows, as a multiple of the drill diameter. | |
| Largest chip gap as a multiple of drill diameter | Largest chip gap your practice allows, as a multiple of the drill diameter. | |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
Drill and bushing
What this means: Drill diameter
- What it is
- The nominal diameter of the drill guided by the bushing.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 200 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 8 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Sets the chip gap ratio; the clearance comes from the deviations below.
- Get it right
- Use the drill size on the process sheet.
What this means: Drill lower deviation
- What it is
- How far below nominal the smallest permitted drill may be, the lower deviation of the drill diameter, in micrometres.
- Units and format
- Enter a number in µm: micrometres (µm, microns). The form accepts -1000 µm to 1000 µm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with -15 µm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A smaller drill increases the clearance and the position error.
- Get it right
- Drills are made slightly under nominal; take the value from the drill standard or measure the drills you use.
What this means: Bushing bore upper deviation
- What it is
- How far above nominal the largest permitted bushing bore may be, the upper deviation of the bushing tolerance, in micrometres.
- Units and format
- Enter a number in µm: micrometres (µm, microns). The form accepts -1000 µm to 1000 µm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 28 µm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger bore increases the clearance and the position error.
- Get it right
- Take it from the bushing standard or the jig drawing.
What this means: Bushing wear allowed before replacement
- What it is
- How much the bushing bore may wear larger before the bushing is replaced, added to the clearance.
- Units and format
- Enter a number in µm: micrometres (µm, microns). The form accepts 0 µm to 1000 µm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 µm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Wear adds directly to the clearance and so to the position error.
- Get it right
- Set a wear limit you can check with a plug gauge, and record it on the jig.
What this means: Bushing length (guided length)
- What it is
- The length of the bushing bore that guides the drill; a longer guide lets the drill tilt less.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 16 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Tilt falls in proportion to the bushing length.
- Get it right
- Measure the bore length that actually contacts the drill, excluding any counterbore or lead-in.
What this means: Gap from bushing end to the work
- What it is
- The gap between the end of the bushing and the work surface, left so chips can escape without being dragged back into the bushing.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The tilt error at the work surface rises in proportion to the gap.
- Get it right
- Too small a gap packs chips into the bushing; too large a gap lets the drill wander.
What this means: Extra point wander at entry
- What it is
- Any further radial movement of the drill point you expect when it starts to cut, for example on an inclined or rough surface.
- Units and format
- Enter a number in µm: micrometres (µm, microns). The form accepts 0 µm to 1000 µm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 µm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Adds directly to the offset at the work surface.
- Get it right
- Enter 0 for a flat, square surface; spot or counterbore an uneven one first.
Requirements
What this means: Position tolerance (diameter of the zone)
- What it is
- The position tolerance of the hole on the drawing, given as the diameter of the circular zone its axis must lie in.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.1 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the pass line.
- Get it right
- Leave part of the tolerance for the jig’s bushing position and the part’s location; do not give it all to the bushing.
What this means: Least chip gap as a multiple of drill diameter
- What it is
- The least chip gap your practice allows below the bushing, as a multiple of the drill diameter.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 10. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.5 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the lower edge of the gap range.
- Get it right
- Use your shop or jig standard; ductile materials with long chips need a larger gap than cast iron.
What this means: Largest chip gap as a multiple of drill diameter
- What it is
- The largest chip gap your practice allows below the bushing, as a multiple of the drill diameter.
- Units and format
- Enter a plain number with no unit. The form accepts 0 to 10. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Only moves the upper edge of the gap range.
- Get it right
- Use your shop or jig standard.
Method and formulas
- Largest radial clearance c = (bushing upper deviation + wear − drill lower deviation) ÷ 2.
- Largest tilt of the drill in the bushing ≈ 2c ÷ L_b (it can touch opposite sides at the two ends).
- Offset at the work surface e = c + tilt × gap + wander; position error as a diameter = 2e, against the tolerance zone.
- Chip gap ratio = gap ÷ D, against your range.
- Example: 24 µm of clearance lets the 8 mm drill tilt 3 mm/m, so the hole can start 42 µm off position, an error of 0.084 mm against 0.1 mm.
Limitations
- Geometric clearance and tilt only; drill flexure, a skewed work surface and the error of the jig's own bushing positions add to it.
- The hole may wander further as it deepens; the check covers the start of the hole at the work surface.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check whether a drill jig can hold a hole position tolerance
- Choose the bushing length and the gap below it
- Set a wear limit for bushings
Who it is for
- Tool and fixture designers
- Quality engineers
- Machinists
What you get
- Largest radial clearance in the bushing
- Drill tilt in the bushing
- Largest offset at the work surface and position error as a diameter
- Chip gap ratio
Interpreting the result
- A longer bushing and a smaller gap both cut the tilt error, but the gap is needed for chips: the check shows the trade.
- Bushing wear adds directly to the clearance, so a wear limit belongs on the jig's inspection record.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 3121–3125, 3131–3134. Topic reference; no source tables or recommended settings are reproduced.
Learn the Concepts Behind This Tool
These KEVOS® Library readings explain the method this tool uses, with worked examples.
Useful alongside · Library collection
Designing jigs and fixtures that pay for themselves: locating, clamping, mistake-proofing and the business case
In Manufacturing Operations and Design for Manufacture
What it covers: A good fixture makes the right way the easy way. How to locate parts on datums, clamp without distortion, build in mistake-proofing, design for welding and people, and judge the payback.
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