Appearance
A manufacturer of aluminium window frame extrusion dies (H13, 48-52 HRC, 200 x 100 x 40 mm die plate, 4 bearing bores of 8 mm x 20 mm per cavity) gun-drilled the bearing bores with AlCrN-coated carbide (8 mm, Vc = 25 m/min, f = 0.015 mm/rev, oil at 60 bar). The entry side was counterbored to 16 mm for the approach angle. The bores were hand-polished to 2-4 Ra with 6 micron then 3 micron diamond paste on a wooden lap. Straightness verified by gauge pin.
Extrusion Die Bearing Bore Drilling
Extrusion die bearing bores are the precision holes in the die plate that determine the cross-sectional shape and the surface finish of the extruded profile. The bearing length (the length of the parallel section of the bore) controls the back-pressure and the flow rate of the extruded material. The bearing bore must be gun-drilled with a straightness of 0.001 in/ft or better, and the bore surface must be polished to a mirror finish (2-4 Ra) to reduce the drag of the extruded material flowing through the bearing and to prevent surface defects (weld lines, die lines, and surface roughness) on the extruded profile.
The bearing bore is typically 1-20 mm diameter and 5-60 mm long (the bearing length-to-diameter ratio ranges from 0.5:1 to 5:1, depending on the profile geometry and the material being extruded). The die plate is heat-treated to 48-52 HRC before drilling. The bearing bores are drilled on a CNC gun drilling machine with the die plate clamped to the machine table. The gun drill parameters for hardened H13 are: carbide gun drill with AlCrN coating, Vc = 20-30 m/min, feed f = 0.01-0.02 mm/rev, oil coolant at 50-80 bar. After drilling, the bore is inspected by inserting a gauge pin (the pin must pass through the full bore length without resistance). The entry side of the die plate is then counterbored to create the approach angle (the transition from the die pocket to the bearing bore). The counterbore is typically 2-4 times the bearing bore diameter, with a 45-60 degree included angle. The bearing bore is then hand-polished to a mirror finish by running a wooden or brass lap through the bore at 500-1000 rpm, starting with 6 micron diamond paste and finishing with 3 micron paste.
Gun Drilling Parameters Comparison
The table below compares gun drilling parameters for the most common extrusion die materials.
| Parameter | H13 (48-52 HRC) | D2 (58-62 HRC) | S7 (46-50 HRC) | Stainless 420 (48-52 HRC) |
|---|---|---|---|---|
| Cutting speed Vc | 20-30 m/min | 15-22 m/min | 22-32 m/min | 18-25 m/min |
| Feed rate f | 0.01-0.02 mm/rev | 0.008-0.015 mm/rev | 0.012-0.022 mm/rev | 0.01-0.018 mm/rev |
| Coating | AlCrN | TiAlN | AlCrN | TiAlN |
| Coolant pressure | 50-80 bar | 60-100 bar | 50-70 bar | 50-80 bar |
| Surface finish after drilling | 8-12 Ra | 6-10 Ra | 8-14 Ra | 8-12 Ra |
| Polished finish | 2-4 Ra | 1-3 Ra | 2-5 Ra | 2-4 Ra |
Bearing Length-to-Diameter Ratio by Application
The bearing L/D ratio is selected based on the profile geometry and the extruded material. The following table provides typical ranges.
| Application | Material | Bearing L/D ratio | Typical bearing length | Back-pressure characteristic |
|---|---|---|---|---|
| Aluminium window frame | 6063-T5 Al | 1.5:1 to 3:1 | 12-24 mm | Medium |
| Aluminium heat sink | 6063-T5 Al | 2:1 to 4:1 | 16-32 mm | Medium-high |
| Plastic pipe (PVC) | Rigid PVC | 3:1 to 5:1 | 24-40 mm | High |
| Plastic profile (UPVC) | UPVC compound | 2:1 to 4:1 | 16-32 mm | Medium-high |
| Aluminium handrail | 6061-T6 Al | 0.5:1 to 1.5:1 | 4-12 mm | Low |
FAQ
Why is the bearing bore hand-polished instead of machined to final finish?
Hand-polishing produces the mirror finish (2-4 Ra) required for extrusion die bearing bores that cannot be achieved by gun drilling alone. The gun drilling process leaves spiral feed marks (typically 8-12 Ra) that act as surface defects on the extruded profile. Hand-polishing with diamond paste on a wooden lap removes these marks and creates a smooth, directionless surface that reduces drag on the extruded material. The hand-polishing also allows the die maker to feel the bore surface and detect any localised defects (such as tool marks from the gun drill) that must be removed.
What causes die lines on aluminium extrusions and how does bearing bore quality affect them?
Die lines are longitudinal scratches on the extruded aluminium surface caused by defects on the bearing bore surface. Any burr, tool mark, or rough spot on the bearing bore will drag across the soft aluminium (heated to 480-520 C) as it exits the die, creating a visible line on the profile. Die lines are the most common surface defect in aluminium extrusion and are directly attributable to bearing bore quality. A properly polished bearing bore (2-4 Ra or better) will produce a die-free surface for runs of 5 000-10 000 kg of extruded aluminium before the bore surface degrades and requires repolishing.
How is the bearing length calculated for flow balancing in multi-cavity dies?
In multi-cavity dies, each cavity must have the same material flow rate to produce identical profiles from all cavities. The bearing length for each cavity is adjusted to balance the flow: a shorter bearing length reduces back-pressure and increases flow rate; a longer bearing length increases back-pressure and reduces flow rate. The bearing lengths are initially calculated by finite element flow simulation and then fine-tuned by test extrusion and measurement of the profile weight per unit length from each cavity. The adjustment increments are typically 0.5-1 mm of bearing length per iteration.
What is the role of the approach angle in the extrusion die?
The approach angle (the transition between the die pocket and the bearing bore) guides the material flow from the large distribution cavity into the narrow bearing bore. A 45-60 degree included angle provides a smooth transition that prevents flow separation and turbulence at the bearing entry. The approach angle is created by counterboring the entry side of the die plate to a diameter 2-4 times the bearing bore diameter. The counterbore shoulder must be concentric with the bearing bore within 0.05 mm to ensure uniform material flow around the circumference of the bearing.
How does the bearing bore affect the surface finish of the extruded product?
The bearing bore surface finish is directly imprinted on the extruded product surface. A mirror-polished bearing bore (2-4 Ra) produces an extruded surface with a bright, smooth appearance (Ra 0.4-0.8 microns on the aluminium). A rough bearing bore (8-12 Ra from gun drilling alone) produces a matte or streaky surface on the extruded product. For architectural aluminium profiles that require a bright anodised finish, the bearing bore must be polished to 2 Ra or better to achieve the desired surface quality after anodising.
Data are based on published research and industry experience as of 2026.