Drilling Glue-In Anchor Holes On A New Climbing Route

Drills & Drill Bits

Choosing The Right Drill and Drill Bits

For drilling natural stone we need a drill bit that is capable of being hammered whilst under rotation at high speed and this immediately precludes normal masonry bits fitted to chucks that only rotate.

SDS Drills & SDS Drill Bits

The system in use worldwide is called SDS, was designed by Bosch in 1975 and is a quick-release concept that enables the user to just insert the drill into a snap-in clutch. The name SDS comes from the German “steck, dreh, sitzt” (insert, twist, fits). Bosch publicise the system on the international market as Special Direct System, although in German-speaking nations it is more commonly known as “Spannen Durch System” (Clamping System).

The concept was designed to provide a more efficient system of delivering a hammer blow to the end of a drill bit whilst it was rotating. Because of the machined hollowed-out section in the shank end of the drill, the bit is able to move to and fro freely but within limits, when struck by the continuous hammer mechanism in the drill body. There are also two slots, which are machined opposite one another to lock the bit into position. This whole concept makes for a far more comfortable working environment for the user and less wear and tear in the chuck.

The SDS system is an extremely durable, powerful and versatile development for power tools. It has become almost the de facto standard for light industrial power tools and for good reason.

It is important to note that there three types of SDS bits and neither are interchangeable with other conventional drill bits, or indeed each other. SDS Plus and SDS Max are not interchangeable, for example and it is SDS Plus that is used for the diameter and depth of rock climbing anchors.

All SDS Plus drill bits have five common parts, but may have different cutting heads. The Shank end is inserted into the drill chuck and has two sets of grooves; wide and short, that provide alignment for the bit as it reciprocates in and out of the chuck and the other longer, narrow set, for applying rotation to the bit. The Land holds the cutting edges of the bit on its outside diameter, while the Flutes facilitate the removal of rock dust to prevent jamming as the hole is being drilled. The Head and Tip perform the cutting along the side and tip of the bit.

SDS Drill Bits Explained

Key Features of A Typical SDS Plus Drill Bit

SDS Plus drill bits have two-bit heads to choose from: cruciform or crosshead (4 cutting heads) or straight (2 cutting heads). The cruciform is intended for drilling in reinforced concrete where the potential contact with the reinforcing bar (rebar) could occur, in which case straight head bits can jam with a resultant wrist injury to the person holding the drill.

Crosshead bits have considerably less surface skate making it easier to start the hole and notch the rock for recessed glue-in anchors. Rock is harder than concrete so using this type of drill bit tends to eventually result in heat damage to the bit head. Purple-black discolouration observed around the head is an early warning that the head may snap off within the hole. In case this happens, then a small magnet fixed on a metal rod can be used to tease the broken head out. Drill bits snap and wear, so always take several bits of each diameter required to the crag, it is too critical an item not to carry spares. Occasionally bits may jam and reverse rotation won't free the bit. In these situations, detach the drill from the jammed bit and place an adjustable wrench over the square sides, tighten the head and carefully apply torsion back and forth to loosen the bit.

Bit length should be sufficient to over-drill the intended embedment depth and allow extra so the drill can always achieve the target depth without contacting the rock surface. On overhanging rock, using longer bits can reduce the effort in drilling when at arm's reach or where placements are obstructed.

It’s important to ensure the hole you are drilling is in the correct location and aligned at right angles otherwise, the fixing will not sit correctly against the rock surface and drilling full depth without checking could produce an unuseable hole that then requires patching. Pushing the drill is counterproductive since it prevents the hammer from striking back and forth against the shank of the drill bit however, occasionally pull the bit back to help clear backlogged dust. There are several ways of indicating when the required hole depth is achieved; a tape mark wrapped around the bit, using a proprietary depth gauge / stop fitted on the side of the drill or referencing a mark on the bit itself. Hilti bits, for example, have red rings painted on the Land, near the base of the Shank that provides a good visual reference.

It may be a surprise to learn that the diameter specified is suited for the fixing and not reflective of the actual diameter. For example, 12mm SDS Plus drill bits measure 12.4mm normally and the fixing should be no larger than 11.8mm. When drilling with a hand-held drill the hole created is roughly triangular with curved sides that, at the widest point, measure 12.4mm. This is known as a Reuleaux triangle after the German engineer Franz Reuleaux, and is an effect that can be adopted to drill almost square holes. Using cruciform drill bits with four cutting points creates a polygon profile and holes tend to be ‘rounder’ as a result.

Matching the correct bit diameter and length to the fixing is crucial and this is of particular issue for North American developers using European fixings. For glue-in bolts, this can be less of an issue for certain examples eg using a ½" bit (12.7mm) for a 12mm glue-in that does not feature an interference fit. For expansion anchors, however, there must be no deviation from the required hole diameter.

Drill Bit Size Chart

Cordless Electric Hammer Drills

This format offers the best balance between weight and performance with the majority of all bolts placed using this method. Hilti, Bosch, Makita, DeWALT and Metabo are just a few well-known brand names from which to choose and the following aspects should be compared when choosing upon a particular model.

Operating Voltage

Normally advice when choosing a cordless hammer drill for bolting would be to select a minimum operating voltage of 24v or preferably 36v due to run time being the sum of voltage x battery capacity (Ah).

The higher the voltage and battery capacity in Ah then the more holes that can be drilled per battery. However, there are a growing number of manufacturers producing 18v cordless hammer drills with brushless motors that are considerably lighter yet provide sufficient performance without requiring an impractical number of battery packs being taken to the crag. The GBH 18 V-EC 18v hammer drill from Bosch is one serious contender to the 24v or 36v drills already available.

Impact Energy

The energy the drill generates for hammering is measured in Joules and in practical terms; equates to how fast a hole can be drilled, not how many. Cordless power drills produce around 1.7J to 2.0J, with mains powered drills nearer to 9.0J. Impact energy is particularly important for drilling harder rock types such as granite or volcanic tuff.


This is important if you intend to use spinning methods to remove old expansion bolts. While tapping the wedge bolt to disengage the clip considerably decreases resistance to rotation of the bolt shaft, drills with brushless motors or low operating voltage may not have sufficient power to turn the shaft. This can be overcome by avoiding spinning altogether by adapting hydraulic punch press drivers however, these tools are expensive and the common approach is to use ball screws that rely on spinning the old bolt first.


The total energy available to drill is calculated by multiplying Volts times Amp hours. This unit otherwise called Watt-hours is both important for real-world performance on the crag and for compliance with restrictions on lithium batteries when flying. All major airlines will restrict the carriage of lithium-ion batteries according to the Watt-hour rating and number of batteries. An ideal battery has the highest capacity for the lowest weight and size but is acceptable for carriage on a commercial passenger flight.

Bosch produces an impressive 18v lithium-ion pack rated at 6 Amps that only weighs 0.6kg yet fits in the palm of your hand. Many developers own cordless angle grinders as they also re-bolt old routes, so look for brands that use standardised battery packs within a range of cordless power tools. Having a single type of pack that will run both your drill but also a cordless angle grinder has many advantages; packs can be shared at the crag, which may reduce the number required but also saves money in eliminating the need for another line of batteries and charger for the other tool.

Drill Weight

Heavier drills absorb more of your checked-in luggage allowance when travelling overseas and obviously require more effort to use and carry on the walk into the crag. For anyone bolting on lead, drill weight becomes especially relevant for obvious reasons, but in consideration of what the gear loops on your harness are rated to carry if using a standard climbing harness. How an attachment cord is rigged should also be considered.

Newton’s third law (for every action, there is an equal and opposite reaction) accounts for why controlling drill bit skate is easier with a heavier drill – a consideration for those who use glue-in anchors and thus need to create notches for countersinking the fixing. Crosshead drill bits help to compensate for lighter drills that may skate more readily.

Overall care of Your Drill and Drill Bits

Often overlooked but important is the use of grease to lubricate the SDS grooves; this reduces wear, improves drilling efficiency and reduces power draw from the battery when in operation.

Battery chargers normally feature a temperature cut-off to prevent batteries from overheating therefore do not charge or leave batteries in direct sunlight. Batteries not used for a while will typically exhibit decreased performance until subjected to several discharge-recharge cycles.

Charging is assumed to be performed using a mains power supply so for anyone expecting to use a portable generator, or power invertor, this may not be possible due to the modified sine wave these forms of power generation produce.