I have seen shops spend weeks chasing straightness problems when the root cause was a spindle that was 0.03mm out of alignment with the bushing. Checking alignment takes thirty minutes with a test bar and a dial indicator. Here is the procedure I use.
What I Check and In What Order
I check alignment in a specific sequence because each step depends on the one before it:
- Spindle condition (runout at the taper)
- Spindle axis parallel to the way axis
- Bushing holder concentricity with spindle
- Bushing ID concentricity with spindle
- Workpiece axis alignment with spindle
If I skip a step, I waste time chasing a problem that does not exist.
Checking Spindle Runout
Before I check alignment to anything else, I verify the spindle itself is good. I mount a test bar in the spindle and check runout at two points: near the spindle nose and 200mm out.
| Measurement Point | Acceptable | Marginal | Action Required |
|---|---|---|---|
| At spindle nose | Under 0.005mm | 0.005-0.010mm | Over 0.010mm: inspect taper or bearings |
| 200mm from nose | Under 0.010mm | 0.010-0.020mm | Over 0.020mm: spindle bearing or bar issue |
If the near and far readings are significantly different, the spindle axis is not straight — a bearing or housing issue, not a toolholder problem.
I use a dial indicator with 0.001mm resolution for all alignment checks. A 0.01mm indicator is not precise enough for this work.
Spindle Axis to Way Axis Parallelism
The spindle centerline must be parallel to the way axis in both the horizontal and vertical planes. If the spindle points even slightly off-axis, the hole will drift progressively.
I check this by mounting a test bar in the spindle and running an indicator along the bar parallel to the ways.
| Check Direction | Acceptable per Meter |
|---|---|
| Horizontal (X axis) | Within 0.010mm/m |
| Vertical (Y axis) | Within 0.010mm/m |
I have written about the broader alignment procedure in Machine Alignment for Straight Holes.
Bushing Holder Concentricity with Spindle
The bushing holder — the fixture that holds the guide bushing — must be concentric with the spindle axis. If the holder is off-center, no bushing will align correctly.
I check this by mounting an indicator on the spindle and sweeping the bushing holder bore:
- Acceptable TIR: under 0.015mm
- Marginal: 0.015-0.025mm
- Action required: over 0.025mm — adjust or shim the holder
If the bushing holder is adjustable (many gun drilling machines have this feature), I adjust it. If it is fixed, I shim it or indicate it in place before tightening.
Bushing ID Concentricity
Once the holder is aligned, I install the bushing and check the ID concentricity. The bushing itself can be out of spec — I have received new bushings with 0.02mm runout.
- Acceptable TIR: under 0.010mm
- Marginal: 0.010-0.020mm
- Action required: over 0.020mm — replace bushing
A bushing that shows high runout even after the holder is aligned has an ID that is not concentric with its OD. Replace it. For bushing selection details, see Gun Drill Entry Bushings Guide.
Workpiece Alignment
The workpiece axis must align with the spindle axis. On a lathe, this means the chuck or collet is running true. On a dedicated gun drilling machine, it means the workpiece support steady rests are set to the same center height as the spindle.
I check workpiece alignment by indicating the OD of the workpiece near the chuck and at the far end:
| Condition | Near Chuck | Far End |
|---|---|---|
| Acceptable | Under 0.020mm TIR | Under 0.050mm TIR |
| Marginal | 0.020-0.040mm | 0.050-0.100mm |
| Action | Adjust chuck or collet | Adjust steady rest |
For long workpieces, an out-of-alignment far end causes the drill to enter the hole at an angle relative to the bore centerline. This produces a bell-mouth entry and progressive deviation.
The Common Alignment Mistakes I See
Skipping the spindle check. I have seen operators indicate the bushing and workpiece, spend an hour adjusting, and never check whether the spindle itself is running true. The spindle taper collects dirt and can easily be 0.01-0.02mm out.
Using a worn test bar. A test bar with center wear gives false readings. I indicate the bar itself at two points first to verify it is straight.
Tightening in the wrong sequence. On machines with adjustable bushing holders, the tightening sequence affects final alignment. I snug all bolts, indicate, then torque in a cross pattern.
Forgetting thermal effects. A cold machine that was aligned hot will show different readings. I do alignment checks after the machine has been running for at least 30 minutes.
How I Document Alignment
I keep a log for each machine with the following recorded monthly:
- Spindle nose runout
- Spindle axis to way parallelism (both planes)
- Bushing holder concentricity
- Bushing ID after installation
- Notes on any adjustments made
Trending these values over time tells me when a spindle bearing is wearing or a way is losing alignment. I cover this in Machine Calibration Frequency.
Key Takeaways
- Check components in order: spindle → way parallelism → bushing holder → bushing ID → workpiece.
- Guide bushing concentricity to spindle should be within 0.02mm TIR. Beyond that, expect hole deviation.
- Use a 0.001mm resolution indicator for alignment work. A 0.01mm indicator is not accurate enough.
- Document alignment readings monthly. Trending catches developing problems before they cause scrap.
- Thermal expansion matters. Align the machine hot, not cold.