Anchor layout errors on core drilled walls are rarely caused by the drill. They come from the survey reference, the rig anchoring and the fact that a 100 mm core wanders more than a 25 mm hole. Fix the setup and the layout holds.

Tolerance chain

Stage	Realistic tolerance
Survey set-out	+/-5 mm
Marking through to the wall	+/-3 mm over 3 m
Rig anchoring drift	+/-4 mm
Core wander (100 mm bit)	+/-3 mm at 200 mm depth
Anchor fixing position	+/-2 mm

Stacked worst case is about 12 mm, which is why specifying +/-5 mm for a 100 mm core in a 150 mm wall is unrealistic. State a tolerance the chain can actually deliver, or add a slotted base plate.

Set-out that survives the wall

1. Establish a single datum line and a single vertical reference; every mark references both.
2. Mark with a laser on the wall, then punch and re-measure; do not trust a chalk line over 3 m.
3. Mark the hole centre and a 200 mm cross, so you can check drift after the first 50 mm of cut.
4. Photograph the marked wall before drilling. Disputes are resolved by that photo.

Rig setup

• Anchor the rig with two bolts minimum, on the same side as the feed force.
• Check the rig is square to the wall in both axes with a digital level, not a bubble.
• Take the slack out of the anchor before the bit touches the wall; most first-hole drift is the rig settling.
• For deep cores, re-check squareness every 150 mm of depth.

Piloting large cores

For cores above 75 mm, drill a 12-16 mm pilot through first, then core. The pilot gives the core bit a fixed centre and removes the wander that dominates the tolerance budget. In reinforced walls, the pilot also finds the rebar before you commit a large diameter.

When the layout is tight

A tolerance budget you can actually hold

Anchor accuracy is the sum of small errors, not one big one. Writing the budget down before setting out turns a vague promise of "plus or minus a few millimetres" into something the crew can check at each step.

Contributor	Typical allowance	How it goes wrong
Drawing and model transfer	2-3 mm	Scaling from a PDF instead of setting out from coordinates
Set-out on the slab	2-3 mm	Chalk line measured from an uneven wall instead of a reference datum
Rig placement and anchor	2-5 mm	Vacuum base creeping on a dusty surface
Bit drift at entry	2-4 mm	Starting without a pilot or a guide plate
Hole diameter vs anchor	As specified	Drilling at nominal bit size with a worn bit

A 10 mm positional tolerance is realistic on a well-run site; 5 mm is achievable with a pilot and a fixed rig, and 3 mm needs a jig and a survey check. Promising 3 mm with a hand-held rig and chalk lines is how a first fix becomes a rework order.

Set-out, fixtures and checks that hold position

• Set out from a fixed datum line marked on the slab, not from the nearest wall. Walls move, datums do not.
• Mark each hole with a cross plus a circle at the specified diameter, and number it against the drawing. A single dot gives the operator nothing to check against.
• Use a guide plate or a temporary ply template on the first hole of each group, then transfer the pattern from the finished hole rather than re-measuring every time.
• Anchor the rig with a vacuum base on a clean, dry surface, or a mechanical anchor on a rough one. A vacuum base on dusty concrete is the most common source of drift.
• Pilot at 8-12 mm through the full depth, check position with a rod and a square, and only then open to final diameter.
• Record the as-drilled position of the first and last hole of each run. Two checks per run show whether drift is accumulating along the line.

On post-tensioned or heavily reinforced slabs, the layout check is not optional and the scan comes before the mark. Set-out accuracy is only worth anything if the hole is in a place where it is allowed to be.

What to hand over with the as-drilled record

The deliverable for an anchor layout job is not a finished wall, it is a record that lets the next trade trust the holes. Four items cover it, and any of them missing turns a clean job into a site argument.

• An as-drilled mark-up showing hole number, diameter, depth and actual position relative to the datum.
• A photograph of each group before the rig moved, with a tape or staff in frame for scale.
• Bit diameter and condition at the time of drilling, since a worn bit explains an oversized hole later.
• Any hole relocated for reinforcement or post-tension obstruction, with the reason and the approval reference.

Hand that over at the end of each pour or wall, not at the end of the project. Layout tolerances are argued about most often when the fixings are being installed months later, and by then nobody remembers which hole was moved or why.

If the structural engineer needs +/-3 mm, coring is the wrong process. Options: core oversize and grout the anchor in place, use a slotted base plate, or cast-in the anchors. Choosing coring and then arguing about tolerance on site costs more than either alternative.