A gun drill with a standard point geometry is a compromise — designed to work across a range of materials. Point thinning and web thinning customize the point geometry for a specific material or application — reducing the cutting forces where they are highest (at the drill center where cutting speed is zero) and improving chip flow where it is most restricted (at the web where the chip space is narrowest). A properly thinned gun drill point reduces thrust by 20–40%, improves hole size control, and allows feed rates that would be impossible with a standard point — but only if the thinning geometry is correct for the material and application.
Gun Drill Point Fundamentals
Standard Point Elements
| Element | Function | Typical Value | Critical to Thinning |
|---|
| Outer cutting edge | Primary cutting — removes the largest chip volume | 40–60% of radius | Thinning does not affect outer edge — must preserve |
| Inner cutting edge | Cuts from center to outer edge — includes chisel edge at center | 40–60% of radius | Primary target of point thinning — reduces center forces |
| Chisel edge | Center of drill — where cutting speed is zero — material is extruded not cut | 0.1–0.5 mm width | Thinning reduces chisel edge width — reduces thrust |
| Cutting edge angle (outer) | Angle of outer cutting edge relative to drill axis | 30–40° | Must maintain after thinning |
| Cutting edge angle (inner) | Angle of inner cutting edge — typically steeper than outer | 20–30° | Often modified during thinning |
| Clearance angle — primary | Relief behind cutting edge | 8–15° | Must maintain adequate clearance after thinning |
| Clearance angle — secondary | Additional relief behind primary clearance | 15–25° | May need adjustment after thinning |
| Margin width | Narrow land on OD — supports drill in hole | 0.3–1.0 mm | Must not be removed or damaged during thinning |
| Web thickness | Thickness of drill body at the point | 20–40% of drill diameter | Web thinning reduces this at the point only |
Point Thinning Methods
Method Comparison
| Thinning Method | Geometry Change | Force Reduction | Best For | Limitations | Skill Required |
|---|
| Face thinning | Grind a flat or concave surface on the rake face near the center | Moderate — 10–20% thrust reduction | Steel — general purpose — standard materials | Reduces chip space slightly — not for deep thinning | Moderate |
| Flank thinning (point thinning) | Grind additional clearance behind the cutting edge at the center | High — 20–30% thrust reduction | Stainless steel — tough materials — high feed rates | Weakens the drill point if overdone — critical to maintain edge strength | High |
| Split point (notched) | Grind a notch on the cutting edge at the center — creates two cutting edges at center | Very high — 30–40% thrust reduction | Hard materials — high feed rates — small diameters (< 5 mm) | Complex to grind — fragile point — not for large diameters | Very high |
| S-point (curved) | Grind a curved transition between inner and outer cutting edges | High — 25–35% thrust reduction | General purpose — wide material range — medium diameters | Requires CNC or specialized grinding | High |
| Helical point | Grind the point with a helical clearance surface | Moderate — 15–25% thrust reduction | Large diameters — interrupted cuts — rough applications | Complex geometry — requires CNC grinding | Very high |
Point Thinning Parameters
| Parameter | Description | Typical Range | Effect |
|---|
| Thinning depth | How far back from the cutting edge the thinning extends | 0.3–2.0 mm (10–30% of diameter) | Deeper thinning = more force reduction — weaker point |
| Thinning width | How much of the cutting edge width is thinned | 20–40% of cutting edge width (from center outward) | Wider thinning = more force reduction — reduced edge strength |
| Thinning angle | Angle of the thinned surface relative to the original rake face | 5–15° | Steeper angle = more clearance — weaker edge |
| Transition radius | Radius at the transition between thinned and unthinned areas | 0.2–1.0 mm | Sharp transition = stress concentration — smooth transition = stronger |
| Remaining chisel edge width | Width of chisel edge after thinning | 0.05–0.20 mm | Smaller = less thrust — increased risk of edge chipping |
Material-Specific Thinning Recommendations
| Material | Recommended Thinning Method | Thinning Depth (% of diameter) | Remaining Chisel Edge (mm) | Notes |
|---|
| Low-carbon steel | Face thinning — moderate | 10–15% | 0.10–0.20 | Conservative thinning — material is forgiving — standard point works well |
| Medium-carbon steel | Flank thinning — moderate | 15–20% | 0.08–0.15 | Good improvement in feed capability — maintain edge strength |
| Alloy steel (4340, 4140) | Flank thinning — moderate to aggressive | 20–25% | 0.05–0.12 | Significant force reduction needed — balance thinning vs edge strength |
| Stainless steel (304, 316) | Split point or aggressive flank thinning | 20–30% | 0.05–0.10 | Must reduce center forces — work-hardening at center is severe |
| Cast iron | Minimal thinning — or none | 0–10% | 0.15–0.25 | Cast iron is brittle — thinning not essential — keep point strong |
| Aluminum | Face thinning — moderate | 10–15% | 0.10–0.20 | Light cuts — aluminum is soft — thinning provides modest benefit |
| Superalloy (Inconel) | Split point — aggressive | 25–30% | 0.03–0.08 | Maximum force reduction needed — very aggressive thinning — frequent inspection |
| Titanium | Flank thinning — aggressive | 20–25% | 0.05–0.10 | Work-hardens — needs aggressive thinning — monitor tool wear closely |
Web Thinning Methods
Web Thinning Techniques
| Technique | Description | Chip Flow Improvement | Strength Reduction | Best For |
|---|
| Notch thinning | Grind a U-shaped notch at the center of the web — widens chip space | Moderate — 10–20% improvement in chip flow | Moderate — notch acts as stress concentration | Steel — general purpose — moderate feed improvement |
| Center thinning | Grind a V-shaped or tapered recess at the center of the web | High — 20–30% improvement | Low–Moderate — gradual taper reduces stress | Stainless steel — tough materials — improved penetration |
| Parabolic web grind | Grind a parabolic curve from center outward — maximizes chip space at center | Very high — 30–50% improvement | Moderate | Small diameters — deep holes — maximum chip evacuation |
| Flattened web | Grind the web flat at the point — reduces web thickness at cutting edge | Low — 5–10% improvement | Low — minimal material removal | Large diameters — cast iron — where web is thickest |
Web Thinning Geometry
| Parameter | Description | Typical Range | Effect |
|---|
| Web thinning depth | How far back from the point the web is thinned | 1–5 mm (20–50% of diameter) | Deeper thinning = better chip flow — weaker drill body at point |
| Web thinning width | Width of the thinned area at the web center | 30–60% of web thickness | Wider thinning = more chip space — thinner web = weaker |
| Thinned web thickness | Remaining web thickness at the point after thinning | 10–25% of drill diameter | Thinner = better chip flow — risk of web cracking |
| Taper angle | Angle of the thinned section walls relative to drill axis | 30–60° | Shallower angle = gradual transition — stronger — steeper = maximum chip space |
| Blend radius | Radius at the transition from thinned to full web | 0.5–2.0 mm | Smooth blend reduces stress concentration — sharp corner causes cracking |
Force Reduction
| Thinning Application | Thrust Reduction | Torque Reduction | Feed Rate Increase Possible |
|---|
| Standard point (no thinning) | Baseline | Baseline | Baseline |
| Light face thinning | 5–10% | 3–5% | 0–10% |
| Moderate flank thinning | 15–25% | 10–15% | 10–25% |
| Aggressive flank thinning | 25–35% | 15–20% | 20–40% |
| Split point (small diameter) | 30–45% | 20–30% | 30–50% |
| Split point + web thinning | 35–50% | 25–35% | 40–60% |
Effect on Hole Quality
| Thinning Type | Hole Size Accuracy | Surface Finish | Hole Straightness | Notes |
|---|
| None — standard point | Good | Good | Good | Conservative baseline |
| Face thinning | Slight improvement | No change | No change | Minimal effect on hole quality |
| Flank thinning — moderate | Improved at higher feed | Improved | Slight improvement | Best balance for most applications |
| Flank thinning — aggressive | May degrade at low feed | May degrade | May degrade | Aggressive thinning requires higher feed to perform well |
| Split point | Best at high feed | Best chip formation | Best with proper setup | Requires higher feed to achieve best results |
| Web thinning only | No change | Improved chip evacuation | No change | Mainly extends tool life through better chip flow |
Grinding and Inspection
Thinning Equipment
| Equipment | Accuracy | Cost | Flexibility | Best For |
|---|
| Manual toolpost grinder — lathe | ±0.05 mm | Low | Moderate — skilled operator required | Low-volume — prototype — development — repair |
| Universal tool and cutter grinder | ±0.02 mm | Moderate | High — many geometry options | Medium-volume — regrind service — general shop |
| CNC tool grinder — 5-axis | ±0.005 mm | High | Very high — any geometry | High-volume production — consistent quality — complex geometries |
| Optical profile grinder | ±0.003 mm | Very high | Moderate — form-limited | Precision points — small diameters — critical geometries |
| EDM (wire or sinker) | ±0.002 mm | Very high | High — any conductive material | Very hard materials — PCD-tipped drills — complex forms |
Inspection Methods
| Method | What It Measures | Accuracy | Equipment | Frequency |
|---|
| Optical comparator — shadowgraph | Point angles — thinning dimensions — chisel edge width | ±0.01 mm | Optical comparator — 20–50× magnification | Every thinned drill — primary inspection method |
| Toolmaker's microscope | Cutting edge condition — chip breaker — clearance angles | ±0.005 mm | Toolmaker's microscope with reticle | Every thinned drill — secondary inspection |
| Profile projector | Thinning depth — width — transition radius | ±0.01 mm | Profile projector — 10–20× magnification | Setup verification — sample inspection |
| 3D optical scanner | Full point geometry — all dimensions | ±0.002 mm | 3D scanner — software analysis | Development — quality audit — complex geometries |
| Cutting test | Actual drilling performance — forces — hole quality | Practical assessment | Drilling machine — test workpiece | Every new thinning geometry — periodic verification |
Troubleshooting
| Problem | Symptom | Likely Cause | Corrective Action |
|---|
| Edge chipping at thinned area | Small fractures on thinned cutting edge | Thinning too aggressive — edge too thin — insufficient support | Reduce thinning depth — increase remaining chisel edge — increase transition radius |
| Drill breakage at point | Drill snaps at or near the point during entry | Web too thin — point too weak for entry impact | Reduce web thinning — increase web thickness at point — reduce feed at entry |
| Poor chip formation after thinning | Chips are long, stringy, or packed | Thinning geometry does not match chip breaker — wrong thinning type | Adjust thinning geometry — add or modify chip breaker — check thinning for material |
| Reduced tool life after thinning | Rapid wear at thinned area — chisel edge wears quickly | Thinned area too weak — insufficient material to support cutting edge | Reduce thinning aggressiveness — increase edge hone — check thinning geometry |
| Hole oversize after thinning | Diameter above tolerance | Asymmetric thinning — one cutting edge longer than the other | Verify thinning symmetry — regrind if asymmetric — check setup |
| Vibration after thinning | Chatter — poor surface finish | Unbalanced cutting edges — thinning asymmetry — incorrect clearance | Check thinning symmetry — verify clearance angles — balance cutting edges |
| Burr at hole entry (exit) | Material pushed out at entry | Chisel edge too large — thinning insufficient for the material | Increase thinning aggressiveness — reduce chisel edge width |
| Burning at drill center | Discoloration at center of drill point | Chisel edge too wide — center not cutting — rubbing generates heat | Increase point thinning — reduce chisel edge — increase coolant flow at center |
FAQ
What is point thinning on a gun drill and why is it needed?
Point thinning is the process of grinding additional clearance or a modified geometry at the center of the gun drill point — specifically in the region of the chisel edge and the inner cutting edge. It is needed because the center of the drill has zero cutting speed — the chisel edge does not cut — it extrudes and displaces material. This extrusion process generates high thrust force (typically 40–60% of total drilling thrust comes from the center 10–20% of the drill diameter), high heat (the extrusion generates significant heat at the drill center — the drill's center cannot evacuate this heat effectively), and poor chip formation (the extruded material at the center does not form proper chips — it is pushed to the side where the cutting edge can pick it up — this creates an irregular chip load on the inner cutting edge). Point thinning reduces the chisel edge width — converting some of the extruding area into cutting area — so more of the drill point actually cuts rather than extrudes. The benefits: reduced thrust force (20–40% reduction depending on thinning aggressiveness — this allows higher feed rates without overloading the machine or the drill), reduced heat generation at the center (less extrusion = less heat — the drill runs cooler — tool life increases), improved chip formation (the inner cutting edge can form proper chips instead of dealing with extruded material), and improved hole quality (lower forces and better chip formation produce more consistent hole size and surface finish). Point thinning is most beneficial for materials that work-harden (stainless steel, titanium, superalloys) — the extrusion at an un-thinned center work-hardens the material before the cutting edge reaches it — accelerating wear. Point thinning eliminates this extrusion, reducing work-hardening and extending tool life significantly.
How do I choose the right thinning method for my gun drill?
To choose the right thinning method for your gun drill: consider the material being drilled — for standard steels (low-carbon, medium-carbon, alloy), flank thinning (also called point thinning) is the best general-purpose method — it provides 20–30% thrust reduction with moderate complexity. The point is strong enough for most applications. For stainless steel and tough alloys, split point thinning is the best choice — it provides the maximum thrust reduction (30–45%) needed for these work-hardening materials. The reduced chisel edge eliminates the extrusion that causes work-hardening. For small diameter drills (under 5 mm), split point thinning is preferred — the small chisel edge on small drills concentrates very high forces — eliminating the chisel edge with a split point is essential for small-diameter gun drills. For cast iron and brittle materials, minimal or no thinning is recommended — the extrusion at the chisel edge can cause edge chipping in brittle materials — a standard point with a slightly larger chisel edge (0.15–0.25 mm) provides better edge strength. For aluminum and soft materials, face thinning is adequate — these materials cut easily and do not require aggressive thinning — face thinning provides a modest force reduction and improves chip flow at the center. For large diameter drills (over 20 mm), flank thinning is preferred — split point thinning on large drills creates a very wide notch that weakens the point excessively — flank thinning provides good force reduction while maintaining point strength. The rule of thumb: start with moderate flank thinning (15–20% of diameter thinning depth — 0.08–0.15 mm remaining chisel edge) for most materials — adjust aggressiveness based on results (if thrust is still high: increase thinning — if edge chipping occurs: reduce thinning).
Can I thin a gun drill point too much?
Yes — a gun drill point can be thinned too much. Over-thinning causes: point weakening (the thinned area removes material that supports the cutting edge — if too much material is removed, the edge can chip or break under cutting load — especially during entry where the impact load is highest). Reduced edge strength (the thinned edge has less cross-section to support the cutting force — the edge may micro-chip during drilling — the micro-chipping accelerates wear and degrades hole quality). Web cracking (if web thinning is too aggressive, the remaining web thickness at the point is insufficient to withstand the torsional load — the web can crack at the thinnest section — causing catastrophic drill failure). Loss of chip control (excessive thinning can disrupt the chip breaker geometry — chips that were properly formed with the standard point may become long and stringy after aggressive thinning — because the chip breaker is no longer effective at the modified geometry). Coolant flow disruption (the coolant hole position relative to the cutting edge is carefully designed — aggressive thinning can expose the coolant hole near the cutting edge — coolant pressure drops — the cutting edge loses cooling — tool life decreases). Signs that a point has been over-thinned: edge chipping visible under magnification (10× or higher) after drilling — small fractures along the thinned cutting edge. Reduced tool life (the drill wears faster than with a standard point — or than with a less aggressive thinning — the thinned edge cannot support the cutting load). Drill breakage at the point (during entry or early in the drilling cycle — the weakened point snaps). If these signs appear: reduce thinning depth by 20–30% — increase the remaining chisel edge width — increase the transition radius between thinned and unthinned areas. The optimal thinning balances force reduction with edge strength — it is material-specific and application-specific — test on sample material before committing to production thinning.
How does web thinning differ from point thinning on a gun drill?
Web thinning and point thinning serve different purposes on a gun drill. Point thinning modifies the cutting edge geometry at the center of the drill to reduce thrust force and improve cutting action at the chisel edge — it changes the shape of the cutting edge itself. Web thinning removes material from the web (the central body of the drill) behind the cutting edge to widen the chip space and improve chip flow — it does not change the cutting edge geometry directly — it changes the space available for chips to move past the point. The key differences: purpose — point thinning reduces thrust force and improves cutting at the center — web thinning improves chip evacuation through the point region. Location — point thinning is applied to the cutting edge and the clearance/rake face at the center of the drill — web thinning is applied to the web (the body behind the cutting edge) — typically starting at the cutting edge and extending back 1–5 mm. Effect on cutting — point thinning directly changes how the drill cuts at the center — web thinning has an indirect effect on cutting (better chip evacuation means chips do not pack at the point — reducing the force required to push chips through the point region). Effect on strength — point thinning removes material from the cutting edge support structure — weakening the edge if overdone — web thinning removes material from the drill body — weakening the web if overdone. When to use each: use point thinning when thrust force is too high — when the drill cannot penetrate at the desired feed rate — when the center of the drill shows excessive wear or heat damage. Use web thinning when chips pack at the drill point — when chip evacuation through the point region is poor — when the drill runs hot from chip friction in the flute — when drilling deep holes where chip packing is a problem. Most gun drills benefit from a combination of moderate point thinning and moderate web thinning — the point thinning reduces thrust — the web thinning improves chip evacuation — together they allow higher feed rates and longer tool life than either modification alone.
How do I regrind a thinned gun drill point?
Regrinding a thinned gun drill point requires careful attention to restore both the original point geometry and the thinning geometry. The regrinding procedure: inspect the worn drill — measure the remaining point geometry (cutting edge angles, clearance angles, chisel edge width, thinning dimensions — compare to the original geometry record — determine how much material must be removed to restore sharp edges). Set up the grinder — use the same grinding equipment and settings used for the original thinning (consistent geometry requires consistent setup — use the same wheel type, wheel speed, and coolant if used). Grind the primary clearance faces — restore the outer and inner cutting edge angles to the original values — remove the minimum material needed to reach unworn edge (typically 0.05–0.20 mm per face). Grind the thinning geometry — restore the face thinning or flank thinning to the original geometry (use the original thinning depth, width, and angle — the thinned area should match the original location and dimensions). Check the chisel edge width — verify the remaining chisel edge matches the original spec (if the chisel edge is too wide after regrinding, the thinning geometry was not fully restored — if it is too narrow, too much material was removed). Grind the secondary clearance — restore the secondary clearance angle behind the primary clearance — critical for clearance behind the cutting edge. Inspect the reground point — optical comparator or toolmaker's microscope — verify all angles and dimensions match the original — verify the cutting edges are symmetrical (both sides within ±0.01 mm). Test the reground drill — drill a test hole in the same material — check hole size, surface finish, and cutting forces — compare to the original performance. The most common regrinding error is removing too much material from the thinning geometry — each regrind removes the thinning geometry at the cutting edge — the thinning must be re-applied with each regrind. After 3–5 regrinds, the drill length has shortened enough that the thinning geometry may need adjustment — or the drill may need to be replaced. Keep a regrind log — track the number of regrinds per drill — the drill body thickness at the point decreases with each regrind — eventually the web becomes too thin for reliable operation.
Point thinning and web thinning customize the gun drill point for specific materials and applications — reducing thrust forces by 20–40%, improving chip formation, and allowing higher feed rates. Select the thinning method based on material: flank thinning for most steels — split point for stainless and superalloys — minimal thinning for cast iron. Apply thinning in moderation — over-thinning weakens the point and causes edge chipping or drill breakage. Combine point thinning (to reduce thrust) with web thinning (to improve chip flow) for maximum benefit. Regrind thinned points carefully — restore the original thinning geometry with each regrind — inspect with an optical comparator. Test thinned drills on sample material — verify force reduction, chip formation, and hole quality before production use. A properly thinned gun drill point transforms drilling performance — delivering higher productivity, better hole quality, and longer tool life. This article reflects industry practice as of 2026.
