Runout is one of those problems that is easy to ignore because the symptoms look like other issues. A hole comes out 0.03mm oversize and the natural instinct is to check the drill diameter or the bushing clearance. Half the time, the culprit is tool runout.
Here is what I have learned about measuring runout, understanding its effects, and controlling it.
What Runout Actually Does to the Hole
When a gun drill has runout at the tip, the cutting edge orbits around the spindle centerline instead of rotating on-axis. This orbit increases the effective cutting diameter.
The math is straightforward: if the drill tip has 0.02mm TIR runout, the hole will be approximately 0.02mm larger than the drill diameter. In practice, it is not always exactly 1:1, but it is close enough that any measurable runout at the tip will produce an oversize hole.
Runout also causes:
- Out-of-round holes. The orbit is not perfectly circular. The hole becomes lobed — slightly triangular or oval depending on the tool geometry.
- Uneven edge wear. One side of the cutting edge takes more load and wears faster. This accelerates tool failure.
- Chatter and vibration. The oscillating cutting force excites vibration in the drill shaft, which degrades surface finish.
- Straightness deviation. The unbalanced cutting force pushes the drill sideways on entry.
I cover the straightness effects in detail in Drill Walking Systematic Diagnosis.
How Much Runout Is Acceptable
The acceptable runout depends on the hole tolerance. For most gun drilling work, I follow these guidelines:
| Drill Diameter | Acceptable Tip Runout (TIR) | Effect on Hole Size |
|---|---|---|
| Under 6mm | Under 0.005mm | Under 0.005mm oversize |
| 6mm to 12mm | Under 0.008mm | Under 0.008mm oversize |
| 12mm to 20mm | Under 0.010mm | Under 0.010mm oversize |
| Over 20mm | Under 0.015mm | Under 0.015mm oversize |
If the hole tolerance is IT7 or tighter, I aim for the lower half of these ranges. For IT8 and above, the standard ranges are adequate.
The key is measuring runout at the drill tip, not at the spindle nose. A spindle with 0.003mm runout can produce a drill tip with 0.015mm runout because of toolholder stackup, drill shank straightness, and the length leverage.
How to Measure Gun Drill Runout
I measure runout in two places: at the toolholder and at the drill tip.
At the toolholder. Mount a dial indicator on the machine and measure the drill shank near the collet. This tells me the combined runout of the spindle, the toolholder, and the collet.
At the drill tip. This is harder to measure on the machine because the tip is inside the bushing. I measure it off the machine with the drill mounted in a V-block and an indicator on the tip.
| Measurement Location | What It Tells Me | Acceptable |
|---|---|---|
| Spindle taper (test bar) | Spindle bearing condition | Under 0.005mm |
| Toolholder OD | Toolholder condition | Under 0.005mm |
| Drill shank at collet | Combined spindle + holder + collet | Under 0.008mm |
| Drill tip (off-machine) | Drill straightness + tip grind quality | See table above |
If the tip runout is high but the shank runout is low, the drill shaft is bent or the tip grind is off-center. If both are high, the problem is in the spindle, holder, or collet.
Common Causes of Runout
In my experience, these are the most common sources of excessive runout in gun drilling:
| Cause | How Often I See It | Fix |
|---|---|---|
| Dirty spindle taper or collet | Very common | Clean the taper and collet bore with a clean rag and solvent. Do this every tool change. |
| Worn collet | Common | Replace collet. Collets wear after 500-2000 tool changes depending on quality. |
| Drill shank out-of-straight | Occasional | Straighten or replace the drill. This happens when drills are stored loosely or dropped. |
| Spindle bearing wear | Rare | Machine repair. You will see runout at the spindle taper first. |
| Toolholder damage | Occasional | Inspect the holder taper and drive keys. A dropped holder will produce runout. |
The most common cause is also the easiest to fix. I have seen operators spend hours chasing oversize holes when the problem was a chip caught in the spindle taper.
How Runout Accelerates Tool Wear
When the cutting edge runs out, it does not take a uniform cut. One side of the tip cuts more material per revolution, and the other side cuts less. The overloaded side wears faster, which increases the runout further, which overloads the edge even more.
This positive feedback loop is why runout-related tool failure often happens suddenly. The drill runs acceptably for 50 holes, then starts producing oversize holes, then breaks within the next 5-10 holes.
I track this by recording tip runout every time a reground drill goes into service. If a drill that started at 0.005mm runout is at 0.012mm after regrind, the regrind service is introducing runout. I cover regrind inspection in Gun Drill Regrind Inspection.
Practical Runout Troubleshooting
When I see an oversize hole that I suspect is from runout, I do this:
- Clean the spindle taper and collet. Run a test hole. If the size improves, the problem was contamination.
- Swap the collet. If the size improves, the collet was worn.
- Measure the drill tip runout off-machine. If it is over 0.01mm, straighten or regrind the drill.
- Check the toolholder. If everything else checks out, the holder is damaged.
Steps 1-3 solve 95% of runout-related oversize problems. Step 4 is rarely needed but catches the remaining cases.
Key Takeaways
- Runout at the drill tip directly increases hole diameter by approximately the TIR value. A 0.02mm runout produces a 0.02mm oversize hole.
- Measure runout at the tip, not just at the spindle. The tip runout is what matters and it is always higher than spindle runout.
- A dirty spindle taper or collet is the most common cause of excessive runout. Clean the taper at every tool change.
- Runout creates a positive feedback loop with tool wear — once it starts, it accelerates.
- Track tip runout on reground drills. If runout increases after regrind, the regrind service is the problem.
- Steps 1-3 (clean, swap collet, measure tip) solve 95% of runout issues. Do those before looking deeper.