Short Slot Fabrication and Drilling Parameters

Not every hole on a board is round. Connectors, switches, mounting hardware and high-current terminals often need an elongated opening, and the usual way to make one is to route it or drill it as a slot rather than drill a larger circle.

Slots are simple to draw and harder to produce. This article covers the case that causes most of the trouble — the short slot, where the length is no more than twice the width — and the process measures that keep it on position.

What Counts as a Short Slot

A slot is defined by its length and its width. When the length is more than about two or three times the width, the tool can be fed along a path and the result is a routed channel with a predictable shape. When the length is close to the width, the geometry is much less forgiving, and the entry and exit of the tool dominate the result.

The conventional definition of a short routed slot is a length no greater than twice the width, whether the slot is plated or non-plated. This is the case where the drill has very little distance to travel, so the hole cannot stabilise itself, and the finished position can drift noticeably from the position that was designed.

Why Slot Position Drifts

A round hole is guided by the material around it: once the drill has entered, the walls keep it centred, and the exit position is close to the entry. A short slot has almost no guiding length, because the tool travels the length of the slot and stops. Any asymmetry in the entry, any deflection from the surface, or any deviation in the tool path is reproduced directly in the finished feature.

The practical consequences are not symmetric around the board. A slot that has moved by a fraction of a millimetre may still accept a component lead, but a connector that needs to be inserted by an automated placement machine will not accept it, because the machine aligns to the nominal position. Boards that fail this way have to be populated by hand, which is exactly the cost the design was trying to avoid.

CAM Preparation and Rotation

The first measure is taken in the CAM data. Rotating the slot geometry about its own centre, so that the slot is not aligned with the axes of the drilling machine or with the grain of the glass weave, spreads the cutting load around the tool rather than concentrating it on one side. A small rotation, of the order of five degrees, is enough to achieve this and is simple to implement as a CAM rule.

The rotation is applied at the data preparation stage, so the fabrication drawing should describe the slot by its nominal position and dimensions rather than by a rotated geometry, and the fabricator applies the compensation. A drawing that already contains a rotated slot may be rotated again, and the two rotations may not cancel.

Slot Drill Selection and Tool Life

A short slot is cut with a slot drill rather than an ordinary twist drill, and the flute length matters. A short flute length increases the stiffness of the tool, which reduces deflection, but it also limits how deep the tool can go before the shank contacts the surface. A flute length of roughly four and a half millimetres is a common choice for boards in the usual thickness range, because it covers the depth required while keeping the tool as rigid as the geometry allows.

Tool life is the other half of the selection. A slot drill removes material along its full width rather than cutting a circular path, so the cutting load per revolution is higher than for a round hole of the same nominal size. Tools wear faster, and a worn tool produces a slot that is undersized and further off position, which is why the slot drills have to be changed on a schedule rather than on failure.

Drilling Parameters for Slots

The parameters for a slot are not the parameters for a round hole of the same diameter. Because the tool is engaged over a larger contact area, the feed and spindle speed must be reduced to keep the cutting force and the temperature within what the tool and the laminate can take. Reducing the drilling parameters, the spindle speed and the feed rate to roughly sixty percent of the values used for an equivalent round hole is a workable starting point.

Under-cutting the parameters is safer than over-cutting them. A slot drilled too aggressively will produce a rough wall, resin smear and, in the worst case, a broken tool left in the panel. The same conservatism applies to the entry and exit: a backing material under the panel supports the laminate as the tool breaks through, which reduces both burring and positional error.

Chip Relief Holes for Small Slots

When a slot is small, the chips produced by the cut have nowhere to go. They pack into the slot, the tool rubs rather than cuts, and the heat and the side load both rise. The remedy is to drill additional openings alongside the slot so that the debris has an escape path. These chip relief holes are placed tangentially on either side of the slot, so that the slot and the two holes form a connected opening.

Sizing them is a straightforward calculation. The relief hole diameter is derived from the slot length, taking half of the length minus a small allowance, with a floor of about 0.25 millimetres so that the relief hole itself does not become a slot. For slots below about one millimetre in width, the relief holes are effectively mandatory, because the chip clearance problem is worst exactly where the aspect ratio of the slot is smallest.

Plated and Non-Plated Slots

A plated slot is a connection between layers, so its wall must be plated exactly as a round hole is, and the plating has to cover the full perimeter of the slot including the ends. That is more difficult than plating a cylinder, because the plating solution has to circulate into a shape with two closed ends. The plating steps used for via filling follow the same principle of getting chemistry to a place where the flow is poor.

A non-plated slot is a clearance feature and is generally easier, because the wall does not need a continuous conductive coating and the tolerance on position is usually set by the mechanical part rather than by a solder joint. Where a slot is used for isolation between two conductors, the requirement is on the remaining material rather than on the opening, and the drawing should say so.

Specifying Slots on the Fabrication Drawing

The drawing should identify every slot separately from the round holes, with its length, width, position and whether it is plated. Slots whose length is less than twice their width should be listed as short slots and called out as such, because that is the information the CAM department needs in order to apply the rotation. Marking them at the drilling stage, after the data has been prepared, is too late to change the geometry.

The tolerance on a slot should also be stated explicitly rather than inherited from the general hole tolerance. A component that is inserted by hand can accept a slot at the general tolerance; a slot that has to accept a machine-inserted terminal, or that defines the position of a board in a chassis, usually needs a tighter one. Where a design keeps slipping on slot position, the design guidelines that keep a board manufacturable and the routing rules for slots and board edges are the two references worth checking before the data is released.

FAQ

When does a slot stop being short? The usual boundary is a length of twice the width. Beyond roughly three times the width the tool can travel along a path and the position becomes much easier to hold.

Why rotate the slot in the CAM data? Rotating it away from the machine axes spreads the cutting load around the tool and reduces the tendency for the slot to drift. Around five degrees is enough, and the fabricator applies it rather than the designer.

Should chip relief holes be added to every slot? Only to small ones, where the chip clearance is the limiting factor. For slots below about one millimetre in width they are effectively necessary; for larger slots the debris clears on its own.