Appearance
A manufacturer producing 6 mm × 500 mm bores in 304 stainless steel at 50,000 holes per year uses brazed carbide-tipped gun drills at $85 each, regrinding 8 times with a tool life of 80 m per regrind. Annual tooling cost is $7,800 including regrinding. Switching to solid carbide gun drills at $145 each with 20 regrinds and 120 m per regrind reduces annual tooling cost to $4,200 — a 46% reduction — while improving hole tolerance from IT9 to IT8 and surface finish from Ra 1.2 µm to Ra 0.6 µm.
Construction Differences
The fundamental difference between solid carbide and carbide-tipped gun drills lies in the construction of the cutting portion and shank.
Solid Carbide Gun Drill
A solid carbide gun drill is manufactured from a single piece of cemented carbide. The cutting head, flute, coolant hole, and shank are ground from one monolithic carbide blank. The carbide blank is typically brazed to a steel driver (drive shank) at the rear for mounting in the machine spindle.
| Feature | Solid Carbide |
|---|---|
| Cutting portion | One-piece solid carbide |
| Flute | Ground into solid carbide |
| Coolant hole | EDM-drilled through solid carbide |
| Shank | Carbide body brazed to steel driver |
| Number of joints | 1 (carbide-to-steel at rear) |
Carbide-Tipped (Brazed) Gun Drill
A carbide-tipped gun drill has a steel shank (alloy steel tube or rod) with a carbide cutting head brazed onto the front end. The steel shank provides the bulk of the length, while only the tip is made of carbide.
| Feature | Carbide-Tipped |
|---|---|
| Cutting portion | Carbide tip brazed to steel shank |
| Flute | Ground into steel shank (or formed) |
| Coolant hole | Drilled through steel shank |
| Shank | Steel; integral with the shank |
| Number of joints | 1 (carbide-to-steel at tip) |
Hybrid Construction
Some gun drills use a hybrid construction where the carbide tip extends further back along the flute area (extended carbide head) before transitioning to a steel shank. This provides additional rigidity near the cutting zone while keeping cost lower than full solid carbide.
Stiffness and Straightness Comparison
The stiffness of a gun drill directly affects hole straightness, surface finish, and achievable depth-to-diameter ratio.
| Property | Solid Carbide | Carbide-Tipped |
|---|---|---|
| Young's modulus | 550–650 GPa | 550 GPa (tip) / 210 GPa (shank) |
| Relative stiffness | 2.6–3.1× steel | 1× steel (shank-dominated) |
| Torsional deflection at 20×D | 0.02 mm typical | 0.05–0.08 mm typical |
| Max L:D ratio (stable) | Up to 200:1 | Up to 100:1 |
| Hole straightness | 0.02–0.08 mm/m | 0.05–0.15 mm/m |
| Tolerance grade | IT7–IT8 | IT8–IT9 |
The higher stiffness of solid carbide (2.6–3.1× the modulus of steel) reduces deflection under cutting loads, resulting in superior straightness. For high L:D ratios exceeding 100:1, solid carbide is strongly preferred because the cumulative deflection of a steel-shank drill at extreme depths produces unacceptable hole deviation.
Diameter Range
| Diameter Range | Solid Carbide | Carbide-Tipped | Recommended |
|---|---|---|---|
| 0.5–1.0 mm | Available | Not available | Solid carbide only |
| 1.0–3.0 mm | Available | Limited (minimum ~1.4 mm) | Solid carbide |
| 3.0–8.0 mm | Available | Available | Either (depending on volume) |
| 8.0–20.0 mm | Available | Available | Carbide-tipped (cost-effective) |
| 20.0–40.0 mm | Available (expensive) | Available | Carbide-tipped (significantly cheaper) |
| > 40.0 mm | Very expensive | Available | Carbide-tipped or indexable |
For diameters below 3 mm, solid carbide is the only practical option because the carbide tip of a brazed drill cannot be manufactured small enough to accommodate the coolant hole and flute geometry. For diameters above 8 mm, carbide-tipped drills become increasingly cost-advantaged because the carbide volume required for a solid carbide drill grows with the square of the diameter.
Tool Life and Regrinding Economics
Regrinding Capability
| Factor | Solid Carbide | Carbide-Tipped |
|---|---|---|
| Regrinds per tool | 15–20 | 5–10 |
| Regrind limit factor | Flute length reduction | Carbide tip wear length |
| Material removal per regrind | 0.1–0.2 mm at tip | 0.2–0.5 mm at tip |
| Diameter change per regrind | 0.002–0.005 mm | 0.005–0.010 mm |
| Regrind cost | $20–$40 | $15–$30 |
| Specialised equipment | CNC gun drill grinder | CNC gun drill grinder |
Solid carbide gun drills can be reground 15–20 times because the entire tool is carbide — regrinding only shortens the tool length. Carbide-tipped drills are limited to 5–10 regrinds because the carbide tip has a finite length; once the tip is ground back to the steel shank, the drill cannot be reground further.
Cost Per Hole Model (6 mm Gun Drill in 304 Stainless)
| Cost Element | Carbide-Tipped | Solid Carbide |
|---|---|---|
| Initial tool cost | $85 | $145 |
| Regrinds per tool | 8 | 18 |
| Total lives per tool | 9 | 19 |
| Tool life per life | 80 m | 120 m |
| Total metres per tool | 720 m | 2,280 m |
| Tool cost per metre | $85 / 720 = $0.118 | $145 / 2,280 = $0.064 |
| Regrind cost per metre | 8 × $25 / 720 = $0.278 | 18 × $30 / 2,280 = $0.237 |
| Total cost per metre | $0.396 | $0.301 |
| Cost per hole (500 mm) | $0.198 | $0.150 |
The solid carbide drill delivers a 24% lower cost per hole despite costing 1.7× more initially, because it lasts 3.2× more total metres.
TIP
The cost advantage of solid carbide gun drills increases with production volume and diameter. At diameters below 3 mm, solid carbide is the only practical option. At diameters above 20 mm, carbide-tipped drills are typically more economical because solid carbide becomes prohibitively expensive. The breakeven diameter for cost-per-hole varies with production volume — at 50,000 holes per year, solid carbide is cost-effective up to approximately 12 mm diameter. At 5,000 holes per year, carbide-tipped is more economical above 6 mm.
Surface Finish and Hole Quality
| Quality Metric | Solid Carbide | Carbide-Tipped |
|---|---|---|
| Surface finish (Ra) | 0.4–0.8 µm | 0.8–1.6 µm |
| Tolerance | IT7–IT8 | IT8–IT9 |
| Straightness | 0.02–0.08 mm/m | 0.05–0.15 mm/m |
| Roundness | 0.005–0.015 mm | 0.010–0.025 mm |
| Burr at exit | Minimal | Moderate |
Solid carbide gun drills produce superior surface finish and tighter tolerances because the rigid carbide body eliminates the vibration and deflection that create surface irregularities. For applications where the gun-drilled hole is the final surface (no secondary reaming), solid carbide can eliminate the need for a separate finishing operation.
Selection by Workpiece Material
| Workpiece Material | Recommended Type | Rationale |
|---|---|---|
| Carbon steel (1018, 1045) | Either | Both perform well; choose by diameter and volume |
| Alloy steel (4140, 4340) | Solid carbide (preferred) | Rigidity required for consistent straightness |
| Stainless steel (304, 316) | Solid carbide | Work-hardening demands rigid support |
| Stainless steel (precipitation-hardened) | Solid carbide | High cutting forces require maximum stiffness |
| Inconel 718 | Solid carbide | Extreme cutting forces; deflection unacceptable |
| Titanium Ti-6Al-4V | Solid carbide | Low elastic modulus of workpiece requires rigid tool |
| Aluminium 6061 | Either (carbide-tipped economical) | Low cutting forces; steel shank sufficient |
| Aluminium 7075 | Either | Similar to 6061 |
| Cast iron | Either (carbide-tipped economical) | Abrasive but low cutting forces |
| Tool steel (> 45 HRC) | Solid carbide | High hardness requires rigid support |
| Composites (CFRP) | Solid carbide (diamond-coated) | Abrasive wear demands carbide throughout |
Application Selection Decision Framework
| Condition | Recommended | Primary Reason |
|---|---|---|
| Diameter < 3 mm | Solid carbide | Only practical option |
| Diameter 3–8 mm, > 10,000 holes/year | Solid carbide | Lower cost per hole; better quality |
| Diameter 3–8 mm, < 5,000 holes/year | Carbide-tipped | Lower initial cost |
| Diameter 8–20 mm, high volume | Solid carbide (if quality critical) | Better tolerance and finish |
| Diameter 8–20 mm, moderate volume | Carbide-tipped | Best cost-performance balance |
| Diameter > 20 mm | Carbide-tipped | Solid carbide too expensive |
| L:D > 150:1 | Solid carbide | Steel shank deflects excessively |
| L:D 50–150:1 | Solid carbide (preferred) | Better straightness |
| L:D < 50:1 | Either | Both can achieve acceptable straightness |
| Tightest tolerance (IT7) | Solid carbide | Maximum rigidity |
| Moderate tolerance (IT9) | Carbide-tipped | Sufficient quality at lower cost |
| Hard material (> 40 HRC) | Solid carbide | Rigidity prevents chipping |
| Soft material (< 200 HB) | Either | Lower cutting forces |
| CNC production | Solid carbide | Consistency over long runs |
| Manual or older machine | Carbide-tipped | More forgiving of misalignment |
| Regrinding capability available | Either | Both benefit from regrinding |
| No regrinding service available | Carbide-tipped (throwaway) | Lower replacement cost |
Troubleshooting
| Problem | Solid Carbide | Carbide-Tipped |
|---|---|---|
| Tool breakage (catastrophic) | Reduce feed; check alignment; verify runout < 0.005 mm | Check for braze joint weakness; reduce peck |
| Rapid flank wear | Upgrade coating; reduce speed | Regrind sooner; check coolant concentration |
| Chipping at cutting edge | Increase hone edge prep; reduce feed variation | Check braze integrity; reduce feed |
| Poor hole straightness | Check bushing clearance; reduce peck depth | Switch to solid carbide for better stiffness |
| Oversize hole | Check bushing wear; reduce runout | Regrind to correct diameter; replace bushing |
| Surface finish deterioration | Regrind at first sign of wear; check coolant filtration | Reduce speed; check for vibration |
| Braze joint failure | Not applicable | Reduce cutting temperature; improve coolant flow |
| Coolant hole blockage | Check filtration; increase coolant pressure | Check chip size; adjust peck cycle |
| Inconsistent tool life | Standardise regrinding process; verify coating quality | Standardise tip brazing quality; verify batch consistency |
| Vibration chatter | Increase stiffness; reduce overhang | Switch to solid carbide |
| Chip packing in flute | Increase coolant pressure; check chip breaker geometry | Increase coolant flow; adjust feed |
FAQ
What is the difference between solid carbide and carbide-tipped gun drills?
Solid carbide gun drills are made from a single piece of cemented carbide, providing maximum stiffness and rigidity. Carbide-tipped gun drills have a steel shank with only the cutting tip made of carbide, offering lower cost and greater toughness. Solid carbide drills are 2.6–3.1× stiffer than steel-shank drills, producing straighter holes with better surface finish.
How many times can a solid carbide gun drill be reground?
A solid carbide gun drill can be reground 15–20 times before the tool becomes too short for effective use. Each regrind removes 0.1–0.2 mm of material from the tip. In comparison, carbide-tipped gun drills can be reground 5–10 times, limited by the length of the carbide tip brazed to the steel shank.
When should I use solid carbide instead of carbide-tipped?
Use solid carbide for diameters below 3 mm (the only option), for L:D ratios exceeding 100:1, for hard materials above 40 HRC, for tight tolerance requirements (IT7–IT8), and for high-volume production where the lower cost per hole justifies the higher initial investment. Above 20 mm diameter, carbide-tipped is more economical.
Are solid carbide gun drills worth the higher cost?
Solid carbide gun drills cost 1.5–2× more than equivalent carbide-tipped drills, but deliver 2–3× the total tool life and 24–46% lower cost per hole in production volumes above 10,000 holes per year. The breakeven point depends on the number of regrinds achievable, tool life per regrind, and the value of improved hole quality (reduced scrap, eliminated secondary operations).
What diameter range is available for each type?
Solid carbide gun drills are available from 0.5 mm to approximately 40 mm diameter (above 20 mm they become very expensive). Carbide-tipped gun drills are available from 1.4 mm to 50 mm diameter. Above 8 mm, carbide-tipped drills are significantly cheaper than solid carbide. Below 3 mm, solid carbide is the only practical option.
Which type produces better hole quality?
Solid carbide gun drills produce superior hole quality due to their 2.6–3.1× higher stiffness. They achieve IT7–IT8 tolerance, Ra 0.4–0.8 µm surface finish, and straightness of 0.02–0.08 mm/m. Carbide-tipped drills typically achieve IT8–IT9 tolerance, Ra 0.8–1.6 µm surface finish, and straightness of 0.05–0.15 mm/m.
Can carbide-tipped gun drills handle hard materials?
Carbide-tipped gun drills can handle materials up to approximately 40 HRC. Above this hardness, the steel shank's lower stiffness allows micro-deflection that causes edge chipping and poor hole quality. For materials harder than 40 HRC, solid carbide gun drills are strongly recommended.
What causes braze joint failure in carbide-tipped drills?
Braze joint failure is caused by excessive cutting temperature softening the braze alloy (typically melting above 600°C), or by cyclic fatigue from interrupted cuts. Solutions include increasing coolant flow, reducing cutting speed, and ensuring the braze joint has adequate thickness (0.05–0.15 mm). Proper brazing quality control is essential.
How does diameter affect the cost comparison?
The cost of solid carbide drills scales with the square of the diameter (more carbide volume), while carbide-tipped drills scale linearly (only the tip is carbide). The cost breakeven point is approximately 8–12 mm for most production volumes. Below 6 mm, solid carbide is cost-competitive or cheaper per hole. Above 20 mm, carbide-tipped is significantly cheaper per hole.
Which type is better for high L:D ratio drilling?
Solid carbide gun drills are strongly preferred for L:D ratios exceeding 100:1. The higher stiffness prevents the cumulative deflection that causes hole deviation at extreme depths. At L:D ratios above 150:1, solid carbide is effectively the only option. Carbide-tipped drills are limited to approximately 100:1 maximum L:D in stable conditions.
Summary
The choice between solid carbide and carbide-tipped gun drills depends on diameter, production volume, material hardness, and tolerance requirements. Solid carbide gun drills provide 2.6–3.1× higher stiffness, IT7–IT8 tolerance, Ra 0.4–0.8 µm surface finish, and 15–20 regrinds per tool. They are the preferred choice for diameters below 3 mm, L:D ratios above 100:1, hard materials above 40 HRC, and high-volume production where lower cost per hole justifies the higher initial investment. Carbide-tipped gun drills offer lower initial cost, greater toughness, and economical performance for diameters above 8 mm, moderate production volumes, and less demanding tolerance requirements. The cost breakeven point is approximately 8–12 mm diameter for typical production volumes.