Appearance
A manufacturer of hydraulic cylinders produces 50 mm × 1,500 mm bores in 4140 steel (28–32 HRC). The initial process uses a BTA drill to 49.2 mm, then finishes with an HSS reamer at 20 m/min and 0.15 mm/rev. The reamer produces Ra 1.6 µm surface finish at IT8, but tool life is limited to 40 m of machining and diameter consistency drifts beyond tolerance after 25 bores. Switching to a carbide-tipped BTA reamer with TiAlN/AlCrN multilayer coating and carbide guide pads, with reaming allowance reduced to 0.35 mm per side and speed increased to 45 m/min, achieves IT7 tolerance with Ra 0.6 µm and tool life exceeding 200 m.
Reamer Types for Deep Hole Drilling
Deep hole reaming tools are selected based on diameter range, depth-to-diameter ratio, production volume, and tolerance requirements.
| Reamer Type | Diameter Range | L:D Ratio | Tolerance | Surface Finish | Application |
|---|---|---|---|---|---|
| Gun reamer | 1.9–30 mm | Up to 300:1 | IT7–IT9 | Ra 0.8–1.6 µm | Small diameter; extreme depth |
| BTA solid reamer (brazed) | 12–65 mm | Up to 100:1 | IT7–IT8 | Ra 0.6–1.6 µm | Small to medium bores; moderate volume |
| BTA indexable reamer head | 20–350 mm | Up to 100:1 | IT6–IT7 | Ra 0.4–1.2 µm | Medium to large bores; high production |
| Carbide-tipped reamer (replaceable) | 16–300 mm | Up to 50:1 | IT6–IT7 | Ra 0.4–0.8 µm | High precision; production environments |
| PCD modular reamer | 5–50 mm | Up to 30:1 | IT6–IT7 | Ra 0.2–0.8 µm | Aluminium, non-ferrous, composites |
Gun Reamer
Gun reamers use the same external chip removal principle as gun drills — coolant is delivered through an internal hole, and chips exit through an external V-groove. The single-edged design provides self-centering action. Gun reamers are the preferred choice for diameters below 20 mm where L:D ratios exceed 30:1. They are limited to lower feed rates compared to BTA reamers but offer superior straightness in small diameters.
BTA Reamer (Brazed Solid)
Brazed BTA reamers have carbide cutting edges and guide pads brazed into a steel head body, then ground to final diameter. The multi-blade configuration (typically 3–6 blades) with opposing guide pads provides stable cutting action. The internal chip evacuation system removes chips through the centre of the drill tube, preventing chip contact with the finished bore surface.
Indexable Reamer Head
Indexable reamer heads use mechanically clamped carbide inserts and replaceable guide pads. They offer the advantage of quick insert changes without removing the head from the drill tube. The insert seats are precision-ground, and replacement accuracy can reach ±0.01 mm. Indexable heads dominate production applications above 20 mm diameter due to lower cost per hole and reduced downtime.
Cartridge-Type Reamer
Cartridge-type reamers use replaceable insert cartridges that contain both the cutting edge and its clamping mechanism. This design allows the entire cutting assembly to be replaced when worn, preserving the main body. Cartridge-type reamers are used for large diameters (>40 mm) where frequent geometry changes are required for different materials or tolerance adjustments.
Reamer Geometry Parameters
The geometric parameters of a reamer determine its cutting action, surface finish, and hole quality.
| Parameter | Typical Range | Effect on Performance |
|---|---|---|
| Chamfer angle (lead angle) | 15–45° | Smaller angles reduce cutting forces; larger angles improve alignment |
| Primary relief angle | 6–12° | Affects edge strength and wear resistance |
| Secondary relief angle | 15–25° | Provides clearance for the margin |
| Margin width | 0.1–0.5 mm | Wider margins improve burnishing but increase friction |
| Back taper | 0.01–0.05 mm per 100 mm | Reduces friction on trailing edge; prevents binding |
| Number of blades | 3–8 (depending on diameter) | More blades improve roundness but require higher torque |
| Helix angle | 0° (straight) to 15° | Straight flutes for most deep hole reamers; helical for chip evacuation |
Chamfer Angle Selection
The chamfer angle (lead angle) is the most influential geometric parameter for reaming performance. A 45° chamfer distributes cutting forces more evenly along the cutting edge, reducing burr formation and improving hole entry. A 15° chamfer produces lower radial forces but concentrates wear at the leading corner. For deep hole reaming, a 30–45° chamfer is recommended to balance tool life and hole quality.
Relief and Margin Design
Primary relief behind the cutting edge should be 6–12° for carbide reamers and 8–15° for HSS reamers. The margin — a narrow cylindrical land (0.1–0.5 mm wide) — provides burnishing action and dimensional stability. Excessive margin width increases friction and heat generation, while insufficient margin reduces the reamer's ability to maintain diameter.
Back Taper
Back taper is ground on the trailing portion of the reamer to reduce friction against the bore surface. A typical back taper of 0.01–0.05 mm per 100 mm of length is sufficient to prevent binding while maintaining adequate support. Excessive back taper can cause the reamer to cut oversize at the trailing edge.
Guide Pad Design for Reamers
Guide pads are essential for BTA reamers, providing support, damping, and bore wall burnishing. The pad geometry directly affects hole straightness and surface integrity.
| Parameter | Typical Value | Influence |
|---|---|---|
| Angular position | 90° and 180° from cutting edge | Counterbalances cutting forces |
| Pad lag length | 2–10 mm behind cutting edge | Clearance for chip flow |
| Chamfer angle | 12–45° | Larger angles reduce hole deviation |
| Back taper | 0–8.8 µm/mm | Reduces deviation less markedly than chamfer |
| Pad width | 3–10 mm (proportional to diameter) | Contact pressure and heat dissipation |
| Number of pads | 2–6 | Stability and damping |
| Contact area | ~1.2% of projected area | Wear concentration at front edge |
Pad Angular Position
Guide pads are positioned at ~90° and ~180° clockwise from the cutting edge in a two-pad configuration. As diameter increases, additional pads (3–6 total) are added in front and rear sets to improve stability. The 90° pad carries the highest load and wears faster than the 180° pad due to the asymmetric cutting force distribution.
Chamfer Angle and Hole Deviation
Research by Katsuki et al. demonstrated that hole deviation decreases with increasing guide pad chamfer angle. A chamfer angle of 12° produces the most axial deviation, while 45° chamfer minimises deviation. This is because the chamfer acts as a lead-in that gradually engages the bore wall, reducing lateral cutting forces.
Pad Lag Length
The pad lag — the distance from the cutting edge corner to the leading edge of the guide pad — must be sufficient to allow chips to clear before the pad contacts the bore surface. A lag of 2–10 mm is typical, with longer lags used for materials that produce large chips. Insufficient lag causes chip packing between the pad and bore surface, leading to scoring and oversize holes.
Pad Material Selection
Guide pads for reamers are typically made from a harder grade than the cutting inserts:
| Pad Material | Hardness | Application |
|---|---|---|
| WC-Co carbide (standard) | 1,300–1,600 HV | General purpose; steel and cast iron |
| PCD-coated carbide | 6,000–8,000 HV | Stainless steel, aluminium, abrasive materials |
| Cermet (TiCN-based) | 1,500–1,800 HV | High-temperature applications |
| CVD diamond coated | 8,000–10,000 HV | Aluminium, composites; non-ferrous |
Carbide Grade Selection for Reamer Inserts
| ISO Class | Workpiece Material | Recommended Grade | Coating | Application |
|---|---|---|---|---|
| P | Carbon steel, alloy steel | IC907, KCU05A, IN2005 | TiAlN or TiAlN/AlCrN | General reaming; good wear resistance |
| M | Stainless steel (austenitic) | IC907, IC806, grade 1024 | TiAlN or AlCrN | Thermal protection; anti-BUE |
| K | Cast iron | IC507, uncoated WC-Co | Uncoated or TiN | Abrasion resistance; sharp edge |
| N | Aluminium, non-ferrous | PCD, IC07 uncoated | Uncoated or DLC | Sharp edge; low cutting forces |
| S | Superalloys (Inconel, titanium) | 3132, IC806, grade 1024 | AlCrN or TiAlN | Hot hardness; diffusion resistance |
| H | Hardened steel (>400 HB) | PCBN, IC806 | TiAlN or AlCrN | Maximum wear resistance |
Grade Characteristics
For reaming, the carbide grade must balance wear resistance against the risk of chipping. Unlike drilling, reaming removes a small stock allowance, so edge toughness requirements are lower. Grades with higher cobalt content (10–16%) provide edge toughness for interrupted cuts, while lower cobalt (6–10%) grades maximise wear resistance for continuous finishing.
Submicron and ultrafine-grain carbide substrates (grain size 0.2–0.5 µm) are preferred for reaming applications because they allow a sharper edge geometry while maintaining edge strength. ISCAR IC907 (submicron grade with TiAlN coating) and Kennametal KCU05A (advanced substrate with multilayer coating) are representative grades that perform across a wide material range.
Coating Selection for Reamers
Coating selection is critical for reamer performance because the cutting speed in reaming is typically lower than drilling, and edge sharpness is essential for surface finish.
| Coating | Hardness (HV) | Oxidation Temp | Friction Coefficient | Reamer Application |
|---|---|---|---|---|
| TiN | 2,600 | 600°C | 0.55 | General purpose; low-temperature reaming |
| TiCN | 3,000 | 400°C | 0.40 | Steel; wear resistance |
| TiAlN | 3,000 | 900°C | 0.45 | Steel, stainless; general reaming |
| AlCrN | 3,600 | 1,000°C | 0.40 | High-speed reaming; dry machining |
| TiAlN/AlCrN multilayer | 3,700+ | 950°C | 0.35 | Production reaming; best all-round |
| DLC (diamond-like carbon) | 2,000–3,000 | 350°C | 0.10–0.20 | Aluminium; non-ferrous; anti-adhesion |
| CVD diamond | 8,000–10,000 | 700°C (air) | 0.05–0.15 | Aluminium; composites; abrasive materials |
TIP
TiAlN/AlCrN multilayer coatings are recommended as the first choice for production deep hole reaming in steel and stainless steel. The nano-layered structure prevents micro-crack propagation, achieving 20–50% longer tool life compared to single-layer TiAlN coatings. For aluminium and non-ferrous materials, DLC or PCD coatings eliminate built-up edge and achieve the best surface finish. When reaming with coolant above 50°C, AlCrN coatings maintain hardness better than TiAlN due to their higher oxidation resistance (1,000°C vs 900°C).
Multilayer Coating Advantages
Recent research (2024) on TiAlN/AlCrN multilayer PVD coatings demonstrates that the alternating nano-layers (individual layer thickness 2–10 nm) provide:
- Higher hardness (3,772 HV) compared to single-layer TiAlN (3,000 HV) or AlCrN (3,600 HV)
- Improved fracture toughness through crack deflection at layer interfaces
- Reduced coefficient of friction (0.35 vs 0.45 for single-layer TiAlN)
- Extended tool life of 20–50% in production reaming applications
Reaming Allowance and Cut Distribution
Reaming allowance — the amount of stock left for the reamer to remove — is the most critical process parameter for deep hole reaming success.
| Hole Diameter | Steel (recommended allowance per side) | Stainless Steel | Aluminium | Cast Iron |
|---|---|---|---|---|
| < 10 mm | 0.10–0.20 mm | 0.15–0.25 mm | 0.10–0.20 mm | 0.15–0.25 mm |
| 10–25 mm | 0.15–0.30 mm | 0.20–0.35 mm | 0.15–0.30 mm | 0.20–0.35 mm |
| 25–50 mm | 0.25–0.40 mm | 0.30–0.50 mm | 0.20–0.40 mm | 0.30–0.50 mm |
| 50–100 mm | 0.35–0.60 mm | 0.40–0.70 mm | 0.30–0.50 mm | 0.40–0.70 mm |
| > 100 mm | 0.50–0.80 mm | 0.60–1.00 mm | 0.40–0.70 mm | 0.60–1.00 mm |
General Rule
The reaming allowance should be 2–3% of the finished diameter for diameters below 25 mm, reducing to 1–2% for larger diameters. Too little allowance causes the reamer to rub rather than cut, leading to rapid wear, work hardening, and loss of diameter. Too much allowance overloads the cutting edges, causing deflection, taper, and oversize holes.
Two-Pass Reaming
For tolerances tighter than IT7 or hole depths exceeding 100× diameter, two-pass reaming may be required. The first pass removes 70% of the stock at higher feed, and the second pass removes the remaining 30% at reduced feed for final sizing and surface finish. A minimum of 0.10 mm per side should be left for the finish pass.
Cutting Parameters for Deep Hole Reaming
Cutting parameters for deep hole reaming differ from standard reaming due to the extended contact length, limited coolant access, and chip evacuation constraints.
Cutting Speed
| Workpiece Material | Hardness | HSS Reamer Vc (m/min) | Carbide Reamer Vc (m/min) | PCD Reamer Vc (m/min) |
|---|---|---|---|---|
| Carbon steel (1018, 1045) | < 200 HB | 15–25 | 40–70 | — |
| Alloy steel (4140, 4340) | 28–36 HRC | 12–20 | 35–60 | — |
| Stainless steel (304, 316) | < 250 HB | 8–15 | 25–50 | — |
| Cast iron (grey) | < 220 HB | 20–35 | 50–80 | — |
| Aluminium (6061, 7075) | — | 30–60 | 60–120 | 150–300 |
| Titanium (Ti-6Al-4V) | — | 8–15 | 20–40 | — |
| Inconel 718 | — | 4–8 | 15–30 | — |
Key principle: Reaming speeds should be 50–70% of the drilling speed for the same material. Higher speeds improve surface finish but accelerate flank wear. Lower speeds reduce tool life through built-up edge formation.
Feed Rate
Feed per revolution (fr) for reamers is typically higher than for drills of the same diameter:
| Reamer Diameter | Steel | Stainless Steel | Cast Iron | Aluminium |
|---|---|---|---|---|
| < 10 mm | 0.05–0.15 mm/rev | 0.04–0.10 mm/rev | 0.08–0.20 mm/rev | 0.08–0.20 mm/rev |
| 10–25 mm | 0.10–0.30 mm/rev | 0.08–0.20 mm/rev | 0.15–0.35 mm/rev | 0.15–0.35 mm/rev |
| 25–50 mm | 0.20–0.50 mm/rev | 0.15–0.35 mm/rev | 0.25–0.60 mm/rev | 0.25–0.60 mm/rev |
| 50–100 mm | 0.30–0.80 mm/rev | 0.25–0.50 mm/rev | 0.40–1.00 mm/rev | 0.40–1.00 mm/rev |
Feed has less influence on surface finish than speed, so using the highest practical feed reduces cycle time without significantly compromising hole quality.
Coolant Pressure and Flow
| Reamer Diameter | Coolant Pressure | Coolant Flow Rate | Application |
|---|---|---|---|
| < 10 mm | 50–150 bar | 10–40 L/min | Small diameter; gun reamer |
| 10–25 mm | 30–100 bar | 40–120 L/min | Medium BTA reamer |
| 25–50 mm | 20–70 bar | 120–300 L/min | BTA reamer; indexable head |
| 50–100 mm | 15–50 bar | 300–800 L/min | Large BTA reamer |
| > 100 mm | 10–30 bar | 800–2,000 L/min | Heavy-duty BTA reamer |
Through-spindle coolant (TSC) is essential for deep hole reaming. The coolant must flush chips from the cutting zone and provide lubrication at the guide pad-bore interface. For BTA reamers, coolant pressure must be sufficient to transport chips through the internal chip tube without settling.
Tolerance and Surface Finish Capabilities
| Reamer Type | Achievable Tolerance | Surface Finish Ra | Straightness |
|---|---|---|---|
| HSS reamer (standard) | IT8–IT9 | 1.6–3.2 µm | — |
| Carbide-tipped reamer | IT7–IT8 | 0.8–1.6 µm | 0.1–0.2 mm/m |
| BTA reamer (brazed, ground) | IT7 | 0.6–1.2 µm | 0.05–0.15 mm/m |
| BTA indexable reamer head | IT6–IT7 | 0.4–1.0 µm | 0.05–0.10 mm/m |
| Adjustable reamer (KOMET REAMAX) | IT6 | 0.4–0.8 µm | 0.03–0.10 mm/m |
| BTA broaching and boring cutter | IT6 | 0.2–0.6 µm | 0.02–0.08 mm/m |
Factors that influence achievable tolerance include machine spindle runout, pre-drilled hole straightness, reamer alignment (guide bushing), reaming allowance uniformity, and coolant delivery. For consistent IT7 tolerance, spindle runout should not exceed 0.005 mm TIR and pre-drilled hole straightness should be within 0.1 mm/m.
Coolant Delivery and Chip Evacuation
Coolant Types
| Coolant Type | Concentration | Lubricity | Cooling | Application |
|---|---|---|---|---|
| Soluble oil emulsion | 5–10% | Good | Excellent | General reaming; steel, cast iron |
| Straight cutting oil | 100% | Excellent | Good | Stainless steel; titanium; superalloys |
| Synthetic coolant | 3–8% | Moderate | Excellent | Aluminium; high-speed reaming |
| High-lubricity oil | 100% | Excellent | Moderate | Reaming with PCD; burnishing |
For BTA reaming of steel and cast iron, soluble oil emulsion at 6–8% concentration is the standard choice. For stainless steel and titanium, straight cutting oil provides the necessary lubricity to prevent adhesive wear on guide pads.
Chip Evacuation Monitoring
Chip form and colour provide real-time feedback on reamer performance:
- Steel: Tight curls or segmented chips indicate proper cutting. Blue chips indicate excessive speed.
- Stainless steel: Loose, stringy chips signal proper cutting. Packed chips indicate insufficient coolant flow.
- Aluminium: Small, well-broken chips are ideal. Built-up edge causes rough surface finish.
- Cast iron: Fine powder is normal. Large flakes indicate tool wear or excessive feed.
A sudden change in chip colour or form during reaming is the first indicator of tool failure or coolant blockage. Continuous chip monitoring should be implemented for production deep hole reaming.
Troubleshooting Reamer Selection Problems
| Problem | Likely Cause | Corrective Action |
|---|---|---|
| Oversize hole | Excessive reaming allowance; spindle misalignment | Reduce allowance to 0.2–0.4 mm/side; check alignment |
| Undersize hole | Insufficient allowance; reamer worn | Increase allowance; replace or recondition reamer |
| Poor surface finish | Excessive speed; inadequate coolant | Reduce Vc by 20%; increase coolant flow |
| Bell-mouth entry | Misalignment; excessive feed at entry | Use guide bushing; reduce feed at entry |
| Tapered hole | Non-uniform allowance; reamer deflection | Pre-drill to consistent diameter; use guide pads |
| Tool chatter | Insufficient rigidity; excessive overhang | Reduce overhang; use damping guide pads |
| Guide pad galling | Adhesive wear; insufficient lubricity | Switch to PCD-coated pads; increase coolant oil content |
| Chip packing | Insufficient coolant pressure | Increase pressure by 20%; check chip tube clearance |
| Edge chipping | Grade too brittle for material | Switch to tougher grade (higher Co content) |
| Rapid flank wear | Speed too high; wrong coating | Reduce Vc; upgrade to TiAlN/AlCrN multilayer |
| Reamer breakage | Feed too high; chip blockage | Reduce feed; ensure chip clearance before restart |
| Diameter drift | Gradual reamer wear; inconsistent allowance | Set up SPC; schedule reamer reconditioning at 0.01 mm wear |
FAQ
What is the difference between a BTA reamer and a gun reamer?
BTA reamers use internal chip evacuation — coolant flows through the annular gap between the drill tube and bore wall, and chips exit through the centre of the tube. They are used for diameters above 20 mm with L:D ratios up to 100:1. Gun reamers use external chip evacuation through a V-groove and are preferred for diameters below 20 mm with L:D ratios exceeding 30:1. BTA reamers offer higher feed rates and better surface finish because chips do not contact the finished bore surface.
What reaming allowance should I leave for deep hole reaming?
The recommended reaming allowance is 2–3% of the finished diameter per side for diameters below 25 mm, reducing to 1–2% for larger diameters. For a 50 mm bore in steel, 0.35–0.40 mm per side (0.70–0.80 mm on diameter) is typical. Too little allowance causes rubbing and rapid wear; too much allowance causes deflection and oversize holes. The pre-drilled hole must also be straight and concentric to within 0.1 mm/m.
What carbide grade is recommended for reaming stainless steel?
For reaming austenitic stainless steel (304, 316), ISO class M grades such as ISCAR IC907 or IC806 with TiAlN coating are recommended. Kennametal KCU05A and Sandvik grade 1024 with TiAlN coating are also suitable. The grade should have a submicron substrate for edge sharpness combined with a tough binder phase to resist chipping from work-hardened material. AlCrN coating is preferred when coolant temperature exceeds 50°C.
What coating is best for deep hole reamers?
TiAlN/AlCrN multilayer PVD coating is the best all-round choice for production deep hole reaming in steel and stainless steel. The nano-layered structure delivers hardness up to 3,772 HV, oxidation resistance to 950°C, and 20–50% longer tool life compared to single-layer TiAlN. For aluminium and non-ferrous materials, DLC or CVD diamond coatings provide the lowest friction and best anti-adhesion properties.
What tolerance can BTA reamers achieve?
BTA reamers consistently achieve IT7 tolerance (H7) in production, and adjustable reamer systems can reach IT6. A brazed BTA reamer ground to final diameter typically holds ±0.01 mm on diameter. Indexable reamer heads with precision-ground insert seats achieve ±0.015 mm. Factors that degrade tolerance include spindle runout, pre-drilled hole straightness, and reaming allowance variation.
How are guide pads configured on BTA reamers?
BTA reamers typically use 2–6 guide pads arranged at specific angular positions around the head circumference. The standard two-pad configuration places pads at ~90° and ~180° clockwise from the cutting edge. Additional pads are added for larger diameters. The pads have a chamfer angle of 12–45° at their leading edge, a back taper of 0–8.8 µm/mm, and a lag of 2–10 mm behind the cutting edge.
What coolant pressure is needed for deep hole reaming?
Coolant pressure requirements depend on reamer diameter and L:D ratio. For BTA reamers 25–50 mm diameter, 20–70 bar is typical. Below 10 mm, pressures of 50–150 bar are required to overcome capillary resistance. Above 100 mm, pressures as low as 10–30 bar may suffice since the annular gap cross-section is large. Coolant flow must be sufficient to maintain chip transport velocity above 5 m/s through the chip tube.
When should I use indexable vs brazed reamer heads?
Indexable reamer heads are preferred for diameters above 20 mm in production applications where quick insert changes and adjustable diameter are needed. They offer lower cost per hole over the tool life and eliminate the need for re-tipping. Brazed (solid) reamer heads are preferred for diameters 12–65 mm where the highest concentricity and surface finish are required, or where the reamer diameter is non-standard and needs to be custom-ground.
What causes oversize holes in reaming?
Oversize holes are most commonly caused by excessive reaming allowance — the reamer deflects under load and cuts more material than its nominal size. Other causes include spindle misalignment (runout > 0.005 mm), insufficient back taper causing the reamer to wedge, guide pad wear allowing lateral movement, and inconsistent pre-drilled hole diameter. Reducing the reaming allowance to the recommended range and verifying spindle alignment typically resolves oversize conditions.
Can I ream a hole that was gun-drilled?
Yes, gun-drilled holes can be reamed to improve tolerance and surface finish. However, the gun-drilled hole must have consistent diameter within 0.05 mm and straightness within 0.1 mm/m. The reamer should be guided by the pre-existing hole using a guide bushing at the entry. A reaming allowance of 0.15–0.30 mm per side (depending on diameter) should be left after gun drilling. The reamer must be aligned to the gun-drilled hole axis within 0.01 mm to avoid step formation.
Summary
Deep hole reamer selection requires matching the reamer type (BTA reamer, gun reamer, or indexable reamer head) to the diameter, depth, tolerance, and material requirements. BTA reamers with carbide-tipped cutting edges and guide pads are the standard for production reaming above 20 mm diameter, achieving IT7 tolerance with Ra 0.6 µm surface finish. Reaming allowance of 1–3% of diameter per side is critical — too little causes rubbing, too much causes oversize holes. TiAlN/AlCrN multilayer PVD coatings provide the best all-round performance for steel and stainless steel reaming. Guide pad geometry, particularly the chamfer angle and angular position, directly affects hole straightness and surface integrity. Cutting speeds for reaming should be 50–70% of drilling speeds, with coolant pressure adequate for chip transport through the tube bore.