Appearance
An undersize bore is deceptive. Unlike an oversize hole — which is usually scrapped — an undersize bore can sometimes be reamed to size, but at the cost of an additional operation and the risk of misalignment. Worse, the most common cause of undersize — built-up edge — often signals a process that is on the verge of catastrophic tool failure.
Understanding Hole Undersize
Measurement and Definition
| Term | Definition | Typical Allowance |
|---|---|---|
| Nominal diameter | Specified bore size (print dimension) | — |
| Lower tolerance limit | Minimum acceptable diameter | Per ISO 286 (H7, H8, etc.) |
| Undersize | Actual diameter minus nominal | Negative value |
| Taper | Diameter increase or decrease along length | Direction matters for corrective action |
| Localized undersize | Diameter drop at a specific depth | Often indicates BUE or chip packing |
For an H7 bore in the 10–50 mm range, the allowed deviation from nominal is typically +0 to +25 μm. If the bore comes out undersize by even 5 μm, it is below the lower tolerance limit and technically out of specification.
Oversize vs. Undersize: Different Root Cause Profiles
| Factor | Oversize | Undersize |
|---|---|---|
| Most common cause | Tool geometry imbalance | Built-up edge |
| Coolant pressure effect | Too high | Too low |
| Cutting speed relationship | Too high | Too low |
| Feed rate relationship | Too low | Too high |
| Guide pad effect | Wear → oversize | Burnishing → undersize |
| Correctability | Usually scrapped | May be reamable |
Root Cause Category 1: Built-Up Edge
Built-up edge (BUE) is the most common cause of undersize holes in deep hole drilling. It is also the most dangerous because it precedes tool failure.
How BUE Causes Undersize
When workpiece material welds to the cutting edge:
- The welded material changes the effective cutting geometry
- The built-up material increases the effective cutting diameter slightly
- However, the material deposited on the edge alters the cutting action, increasing friction and pushing the tool away from the cutting axis
- The result is a bore that measures undersize
BUE Diagnostic Signs
| Sign | Indication |
|---|---|
| Bore undersize by 5–15 μm consistently | Continuous BUE on cutting edge |
| Bore undersize intermittently (every few holes) | BUE builds up and breaks off |
| Surface finish deterioration | BUE particles embedded in bore surface |
| Increased power consumption | Higher friction from BUE |
| Tool wear accelerates | BUE breaks off, taking tool material with it |
BUE Root Causes and Solutions
| Cause | Mechanism | Corrective Action |
|---|---|---|
| Cutting speed too low | Low temperature allows material to weld | Increase cutting speed by 15–20% |
| Poor coolant lubricity | Inadequate oil concentration or wrong type | Use oil-based coolant with EP additives; increase concentration |
| Coolant pressure too low | Heat not removed from cutting zone | Increase coolant pressure to recommended range (20–80 bar) |
| Uncoated tool | Carbide has high chemical affinity for workpiece | Use coated tool (TiAlN, AlCrN, DLC) |
| Sticky workpiece material | Low-carbon steel, stainless, aluminum have high BUE tendency | Increase speed, use sharp edge geometry, increase coolant lubricity |
| Dull cutting edge | Worn edge increases friction temperature | Regrind or replace tool |
TIP
The fastest diagnostic test for BUE-caused undersize is to increase cutting speed by 15–20% and run one test hole. If the bore diameter increases, BUE was the root cause. If diameter is unchanged, look elsewhere. This test takes minutes and can prevent unnecessary machine downtime.
Root Cause Category 2: Coolant System
Insufficient Coolant Pressure
Low coolant pressure is the second most common cause of undersize holes:
| Pressure Condition | Effect on Bore Size |
|---|---|
| Pressure too low | Chips not evacuated, heat accumulates → BUE forms → undersize |
| Pressure adequate | Chips evacuated, cutting zone cooled → correct bore size |
| Flow rate insufficient | Even at correct pressure, inadequate volume reduces cooling |
The Tungaloy troubleshooting guide directly identifies insufficient coolant pressure as a cause of hole diameter inaccuracy and recommends raising pressure above 1.5 MPa (15 bar) as a starting point.
Coolant Type and Lubricity
| Coolant Issue | Effect | Corrective Action |
|---|---|---|
| Water-miscible coolant (low oil content) | Lower lubricity promotes BUE | Use oil-based coolant for precision deep hole drilling |
| Coolant concentration too low | Insufficient EP additive action | Maintain concentration per manufacturer spec |
| Contaminated coolant | Suspended chips abrade guide pads, alter diameter | Filter to ≤10 μm |
| Coolant temperature too high | Viscosity drops, lubricity reduced, BUE risk increases | Add chiller or increase tank capacity |
Root Cause Category 3: Tool Geometry and Condition
Cutting Speed and Feed Parameters
| Parameter | Effect on Bore Size |
|---|---|
| Cutting speed too low | Promotes BUE → undersize |
| Feed rate too high | Increases burnishing force from guide pads → undersize |
| Combined: low speed + high feed | Strongly promotes undersize — the worst combination |
The Tungaloy guide recommends an undersize corrective action of reducing cutting speed by half — but only when the cause is excessive burnishing torque. This is the opposite of the BUE correction, which requires increasing speed. Correct diagnosis is essential.
Guide Pad Burnishing
Guide pads support the gun drill or BTA head against the bore wall. The burnishing action of the pads can reduce the bore diameter:
| Guide Pad Issue | Effect on Bore Size |
|---|---|
| Excessive burnishing force | Pads compress and spring-back the bore surface → undersize |
| Incorrect pad geometry (too wide) | Increased contact area → more burnishing |
| Chamfer angle too small | Reduced cutting action, more compression |
| Back taper insufficient | Pads rub along the entire hole length |
Corrective actions:
- Reduce feed rate (reduces burnishing force on pads)
- Verify pad width is appropriate for the bore diameter
- Check back taper (insufficient taper increases pad contact)
- Consider two guide pads instead of three
Regrinding Quality
| Regrinding Issue | Effect | Corrective Action |
|---|---|---|
| Incorrect relief angle | Altered cutting edge support → diameter change | Verify relief angles against tool drawing |
| Asymmetric regrind | Uneven cutting forces → diameter variation | Inspect under magnification |
| Damage left on cutting edge | Promotes BUE at the damage site | Ensure complete removal of damage |
Tool Margins and Back Taper
Worn tool margins and insufficient back taper can both produce undersize bores:
| Issue | Effect |
|---|---|
| Worn margins | Reduced clearance, increased friction, heat, diameter reduction |
| Insufficient back taper | Pad/margin contact along excessive length → thermal expansion reduces bore |
| Excessive back taper | Reduces guidance, can cause oversize |
Back taper should be approximately 0.02 × diameter per 100 mm of length for standard gun drills.
Root Cause Category 4: Cutting Parameters
Feed Rate Effects
| Feed Rate | Effect on Bore Size | Mechanism |
|---|---|---|
| Too high | Undersize | Excessive guide pad burnishing compresses bore surface |
| Optimal | Nominal size | Balanced cutting and burnishing forces |
| Too low | Oversize | Tool deflection, chatter |
Cutting Speed Effects
| Cutting Speed | Effect on Bore Size | Mechanism |
|---|---|---|
| Too low | Undersize | BUE formation |
| Optimal | Nominal size | Clean cutting action |
| Too high | Oversize | Tool deflection, thermal effects |
Entry and Exit Effects
| Issue | Symptom | Corrective Action |
|---|---|---|
| Feed too high at entry | Undersize in first 2–3× diameter | Reduce entry feed to 50–70% of normal |
| Pilot hole misaligned | Undersize on one side | Verify pilot hole concentricity |
Root Cause Category 5: Machine Setup and Workpiece Factors
Workpiece Clamping
| Issue | Effect | Corrective Action |
|---|---|---|
| Unstable clamping | Workpiece moves, tool forces change → undersize | Clamp workpiece firmly |
| Insufficient support | Vibration → altered cutting action | Add steady rests |
| Thin-walled workpiece | Elastic deformation during cutting → spring-back → undersize | Use internal support or reduce clamping force |
Elastic Spring-Back (BTA Specific)
In BTA drilling, the workpiece undergoes elastic deformation as the tool passes through:
- The BTA head's guide pads exert radial force against the bore wall
- The workpiece wall elastically expands under this force
- After the tool passes, the wall springs back
- The final bore diameter is slightly smaller than the tool diameter
Spring-back magnitude depends on:
- Workpiece wall thickness (thinner walls → more spring-back)
- Material elastic modulus (lower modulus → more spring-back)
- Guide pad radial force (higher force → more spring-back)
For thin-walled BTA drilling, compensation requires either:
- Using a tool ground slightly oversize to account for spring-back
- Reducing guide pad burnishing force
- Increasing feed rate (which reduces specific burnishing time)
Systematic Troubleshooting
Step 1: Characterize the Defect
| Measurement | What It Reveals |
|---|---|
| Bore diameter at entry, middle, and exit | Location and pattern of undersize |
| Diameter trend across multiple holes with same tool | Progressive wear vs. BUE (BUE is intermittent) |
| Surface finish in undersize zone | BUE produces rough finish; burnishing produces smooth |
| Compare with new tool vs. reground tool | Regrind quality factor |
Step 2: BUE Verification Test
The fastest diagnostic step:
- Increase cutting speed by 15–20% for one test hole
- Measure bore diameter
- If diameter increases → BUE was the cause
- If unchanged → look at coolant, guide pads, or spring-back
Step 3: Elimination Sequence
| Order | Check | Quick Verification |
|---|---|---|
| 1 | BUE on cutting edge | Visual inspection of tool under 10× magnification |
| 2 | Coolant pressure at tool tip | Pressure gauge — is it within recommended range? |
| 3 | Coolant type and concentration | Refractometer for emulsions; oil analysis for straight oils |
| 4 | Cutting speed and feed | Are parameters within recommended range for the material? |
| 5 | Guide pad condition | Measure pad height; check for wear or burnishing |
| 6 | Back taper | Measure drill diameter at tip and 100 mm behind tip |
| 7 | Regrind quality | Compare reground geometry to original tool drawing |
| 8 | Workpiece wall thickness | Thin-walled parts may need spring-back compensation |
Step 4: Corrective Actions by Symptom
| Symptom | Most Likely Cause | First Action |
|---|---|---|
| Consistent undersize, all holes | BUE, low coolant pressure, excessive burnishing | Increase speed 15–20% (BUE test) |
| Undersize with rough surface finish | BUE | Increase cutting speed, improve coolant lubricity |
| Undersize with smooth finish | Guide pad burnishing, spring-back | Reduce feed, check pad geometry |
| Intermittent undersize (some holes) | BUE building up and breaking off | Check coolant, tool coating |
| Undersize at entry improving along length | Entry feed too high | Reduce entry feed |
| Undersize in thin-walled parts | Elastic spring-back | Increase feed, oversize tool |
| Progressive undersize through tool life | Guide pad wear | Replace or regrind tool |
Case Studies
Case 1: BUE-Induced Undersize in Stainless Steel
| Parameter | Value |
|---|---|
| Process | Gun drilling, 6 mm × 300 mm in 316L stainless steel |
| Defect | Bore 0.012 mm undersize, rough surface finish |
| Root cause | Built-up edge on outer cutting corner |
| Diagnosis | Speed increased from 25 to 30 m/min → bore returned to nominal |
| Correction | Increased coolant pressure from 60 to 100 bar |
| Result | Undersize eliminated, surface finish improved |
Case 2: Spring-Back Undersize in BTA Thin-Wall Drilling
| Parameter | Value |
|---|---|
| Process | BTA drilling, 80 mm × 1,200 mm in thin-wall steel tube |
| Defect | Bore 0.020 mm undersize, smooth surface finish |
| Root cause | Elastic spring-back after guide pad burnishing |
| Diagnosis | Wall thickness 8 mm — insufficient to resist pad radial force |
| Correction | Tool ground 0.025 mm oversize at final diameter; feed increased 15% |
| Result | Bore within tolerance |
Case 3: Coolant-Related Undersize
| Parameter | Value |
|---|---|
| Process | Gun drilling, 10 mm × 500 mm in 4140 steel |
| Defect | Progressive undersize over several holes, then sudden tool failure |
| Root cause | Coolant pressure dropped from 80 to 40 bar due to pump wear |
| Diagnosis | Pressure gauge at spindle showed intermittent drops |
| Correction | Replaced coolant pump seals, installed pressure monitoring |
| Result | Bore size stabilized |
FAQ
Q: What is the most common cause of undersize holes in deep hole drilling? Built-up edge (BUE) on the cutting edge is the most common cause. The welded material alters the cutting geometry, increases friction, and pushes the tool away from the cutting axis, producing an undersize bore.
Q: How can I tell if built-up edge is causing undersize? Increase cutting speed by 15–20% for one test hole. If the bore diameter increases, BUE was the root cause. Also inspect the tool under magnification — BUE appears as welded material on the cutting edge.
Q: Can coolant pressure cause undersize holes? Yes. Insufficient coolant pressure allows heat to accumulate at the cutting zone, promoting BUE formation and chip packing. Both conditions can produce undersize bores.
Q: What is elastic spring-back in BTA drilling? Spring-back occurs when the workpiece wall elastically deforms under the radial force of the BTA head's guide pads. After the tool passes, the wall springs back, producing a bore slightly smaller than the tool diameter.
Q: Can a reground tool produce undersize holes? Yes. If the regrind alters the cutting edge geometry — relief angles, tip offset, or edge sharpness — the cutting force balance changes. An asymmetric regrind is a common cause of undersize.
Q: What is the difference between oversize and undersize root causes? Oversize is typically caused by tool geometry imbalance (inner cutting edge too large), excessive coolant pressure, or low feed rate. Undersize is typically caused by BUE, insufficient coolant pressure, high feed rate (excessive burnishing), or elastic spring-back.
Q: Can undersize holes be reamed to size? In some cases, yes — but only if sufficient stock remains. The reaming operation introduces its own alignment risk and adds cycle time. It is always better to correct the root cause than to add a secondary operation.
Q: How does built-up edge form in gun drilling? BUE forms when workpiece material welds to the carbide cutting edge due to high temperature and pressure combined with chemical affinity between the tool and workpiece materials. It is more common in stainless steel, low-carbon steel, and aluminum.
Q: What cutting parameters should I check first for undersize? Check cutting speed first — speed that is too low promotes BUE. Then check feed rate — feed that is too high increases guide pad burnishing. The combination of low speed and high feed is the worst case for undersize.
Q: How do guide pads affect bore size? Guide pads burnish the bore wall as the tool rotates. Excessive burnishing force from incorrect pad geometry, wear, or high feed rates can reduce the bore diameter. Conversely, worn pads reduce support and can lead to oversize.