A test bar is the most direct tool for measuring machine geometry. It replaces the drill with a known-straight reference, allowing the machine's alignment to be measured independently of tool condition, runout, and bushing wear. Every deep hole drilling machine should have a test bar that matches its spindle and bushing system.
Test Bar Types
Construction Types
| Type | Construction | Accuracy | Cost | Best For |
|---|
| Solid steel bar | One-piece hardened and ground steel | ±0.002 mm over 300 mm | $500–$2,000 | General alignment, most machines |
| Carbide-tipped bar | Steel body with carbide ends | ±0.001 mm over 300 mm | $1,500–$4,000 | High-precision alignment |
| Stepped bar | Multiple diameters on one bar | ±0.002 mm per step | $2,000–$5,000 | Multi-spindle alignment |
| Adjustable bar | Threaded segments, adjustable | ±0.005 mm | $1,000–$3,000 | Multiple machines, flexibility |
Size Selection
| Machine Type | Test Bar Diameter | Test Bar Length | Bushing Diameter |
|---|
| Gun drilling (small) | 10–20 mm | 500–1,000 mm | Match bar diameter + 0.01 mm |
| Gun drilling (medium) | 20–40 mm | 1,000–2,000 mm | Match bar diameter + 0.02 mm |
| BTA drilling (medium) | 30–60 mm | 1,000–3,000 mm | Match bar diameter + 0.02 mm |
| BTA drilling (large) | 60–100 mm | 1,500–4,000 mm | Match bar diameter + 0.03 mm |
Test Bar Specifications
Dimensional Requirements
| Specification | Standard Precision | High Precision | Measurement Method |
|---|
| Diameter tolerance | ±0.005 mm | ±0.002 mm | Micrometer |
| Straightness | 0.005 mm over full length | 0.002 mm over full length | V-block + indicator |
| Roundness | 0.003 mm | 0.001 mm | Roundness gauge |
| Surface finish (Ra) | 0.2 µm | 0.1 µm | Profilometer |
| Hardness | 58–62 HRC | 60–64 HRC | Hardness tester |
| Concentricity (ends to center) | 0.005 mm TIR | 0.002 mm TIR | Between centers |
Test Bar Material
| Material | Hardness | Dimensional Stability | Wear Resistance | Cost |
|---|
| Tool steel (O1, A2) | 58–62 HRC | Good | Good | Low |
| Alloy steel (4340, nitrided) | 55–60 HRC (nitrided surface) | Very good | Very good | Medium |
| Stainless steel (440C) | 55–58 HRC | Good | Good | Medium |
| Tungsten carbide | 90–92 HRA | Excellent | Excellent | High |
Test Bar Usage Procedure
Setup
| Step | Action | Detail |
|---|
| 1 | Clean the test bar thoroughly | Remove all oil, debris, fingerprints |
| 2 | Clean the spindle collet or holder | Wipe with lint-free cloth |
| 3 | Clean the bushing | Wipe ID clean |
| 4 | Mount test bar in spindle collet | Insert at least 50 mm |
| 5 | Tighten collet to specified torque | Do not overtighten |
| 6 | Verify test bar is seated | Check insertion depth mark |
Measurement Procedure
| Step | Action | Detail |
|---|
| 1 | Mount dial indicator on machine bed or magnetic base | Rigid mount, no movement possible |
| 2 | Position indicator at test bar OD near bushing | 10 mm from bushing face |
| 3 | Zero indicator and rotate spindle by hand | Read TIR |
| 4 | Record TIR at bushing position | — |
| 5 | Move indicator to test bar end (spindle end) | 50 mm from collet |
| 6 | Rotate spindle and read TIR at spindle end | — |
| 7 | Move indicator to intermediate positions | Every 100 mm along the bar |
| 8 | Rotate spindle at each position | Record TIR at each point |
| 9 | Check bushing fit — slide bushing over bar | Should slide freely without play |
Interpretation of Readings
| Reading Pattern | Indication | Corrective Action |
|---|
| Uniform TIR at all positions | Spindle runout only | Check collet condition, spindle bearings |
| Increasing TIR from spindle to bushing | Bushing off-center | Adjust bushing holder |
| TIR same at both ends | Angular misalignment | Adjust spindle head or bushing holder |
| TIR varies irregularly along length | Test bar bent | Replace or straighten test bar |
| TIR at bushing only | Bushing worn or misaligned | Replace bushing, realign |
| No TIR but bar does not fit bushing | Bushing undersize | Ream or replace bushing |
Alignment Verification Using Test Bar
Spindle-to-Bushing Alignment
| Step | Action | Detail |
|---|
| 1 | Mount test bar in spindle | Clean and secure |
| 2 | Measure TIR at test bar near bushing | Record value |
| 3 | Adjust bushing holder | Per alignment adjustment screws |
| 4 | Re-measure | Iterate until TIR < 0.01 mm |
| 5 | Slide bushing over test bar | Should slide freely |
| 6 | Rotate spindle with bushing engaged | TIR should remain < 0.01 mm |
| 7 | Lock bushing holder adjustments | Tighten lock bolts without disturbing alignment |
Bed Parallelism Check
| Step | Action | Detail |
|---|
| 1 | Mount test bar in spindle | As above |
| 2 | Mount dial indicator on carriage | Rigid mount |
| 3 | Traverse carriage along bed | Full working length |
| 4 | Read indicator against test bar OD | Measure deviation from one end to the other |
| 5 | Compare to tolerance | < 0.01 mm per 300 mm of travel |
Calibration Requirements
| Calibration Item | Frequency | Method | Tolerance |
|---|
| Test bar diameter | Annually | Micrometer at 3 points | ±0.002 mm of certified dimension |
| Test bar straightness | Annually | V-block + indicator on surface plate | < 0.005 mm |
| Test bar roundness | Every 2 years | Roundness gauge or between centers | < 0.003 mm |
| Dial indicators | Annually | Gauge calibration service | Per indicator spec |
| Surface plate | Annually | Calibration service | Per grade spec |
Test Bar Certification
A certified test bar should come with documentation showing:
- Diameter at each end and center (measured to ±0.001 mm)
- Straightness over full length
- Roundness at multiple positions
- Surface finish
- Material and hardness
- Calibration date and certifying body
Test Bar Care and Storage
| Practice | Reason |
|---|
| Store vertically in protective tube | Prevents bending and surface damage |
| Apply rust-preventive oil before storage | Prevents corrosion on ground surfaces |
| Never use test bar as a drift or pry bar | Damages precision ground surfaces |
| Clean immediately after each use | Coolant residue can cause corrosion |
| Handle with clean gloves | Fingerprints can cause corrosion |
| Never clamp a test bar without protection | Soft jaws or brass shims required |
| Inspect before every use | Check for nicks, scratches, corrosion |
| Re-certify after any impact | Impact can affect straightness |
Test Bar vs Laser Alignment
| Comparison | Test Bar | Laser Alignment |
|---|
| Accuracy | ±0.002 mm | ±0.001 mm |
| Setup time | 5–10 minutes | 10–20 minutes |
| Equipment cost | $500–$5,000 | $10,000–$30,000 |
| Skill required | Moderate | Moderate (laser) to high (interpretation) |
| Documentation | Manual recording | Automatic report generation |
| Long-bed capability | Requires long bars (expensive) | Easy — any length |
| Multi-axis measurement | Sequential | Simultaneous |
FAQ
What is a test bar used for on a deep hole drilling machine?
A test bar is a precision-ground reference bar that simulates a perfectly straight drill. It is mounted in the spindle and used to: verify spindle-to-bushing alignment, measure spindle runout independent of tool condition, check bed parallelism, verify bushing condition, and establish the machine's geometric baseline. Test bars provide a measurement that is independent of tool quality and condition.
How do I choose the right test bar for my deep hole drilling machine?
Choose a test bar diameter that matches your most common drill size and a length that spans from the spindle collet through the bushing to at least 100 mm past the bushing. The bar should fit the bushing with a clearance of 0.01–0.02 mm. Use a solid steel test bar (tool steel, hardened to 58–62 HRC) for general alignment work. For higher precision, use a carbide-tipped or nitrided test bar.
How do I measure spindle alignment using a test bar?
Mount the test bar in the spindle collet. Position a dial indicator at the bar OD near the bushing location. Rotate the spindle by hand and read the total indicator runout (TIR). Move the indicator along the bar to check alignment at multiple points. The spindle and bushing are aligned when the TIR at the bushing position is below 0.01 mm and the bushing slides freely over the bar.
How should I store a precision test bar?
Store the test bar vertically in a protective tube or case to prevent bending. Apply a light coat of rust-preventive oil. Never store horizontally without full-length support — the bar's own weight can cause a permanent bend over time. Keep in a climate-controlled area. Always clean the bar before and after each use. Never use the test bar for any purpose other than alignment measurement.
When should I use a test bar instead of a laser alignment system?
Use a test bar for routine alignment verification and after minor adjustments — it is faster to set up than a laser and sufficient for most alignment tolerances. Use a laser alignment system for: initial machine installation, after major machine relocation, when alignment tolerance is below 0.005 mm, for machines with bed lengths over 3 m, and when a documented alignment report is required for quality records.
A test bar is an essential alignment tool for every deep hole drilling machine. It provides a quick, reliable method for verifying spindle-to-bushing alignment — the most critical geometric relationship in the machine. With proper care, a test bar lasts for decades. This article reflects industry practice as of 2026.
