Every 300 mm of extension adds the chance for runout to double. On deep cores the bar, not the bit, decides whether the hole comes out straight.

Alignment chain

Component	Check	Tolerance
Stand base to wall	Square in two axes, digital level	0.5 degree
Column verticality	Plumb over full stroke	1 mm per metre
Carriage to column	No play at full extension	Zero visible slop
Extension bar	Runout at 50 mm from bit	Under 0.3 mm
Bit to bar	Concentric, tight	No wobble at start

Mounting sequence

1. Anchor the stand, take the slack out of the anchors, re-check squareness.
2. Fit the first extension bar, spin it by hand and watch the tip; a bent bar shows immediately.
3. Fit the bit, then check runout with a dial gauge or a fixed pointer at 50 mm from the tip.
4. Run the machine at low rpm with no load for 10 seconds; listen and watch for vibration.
5. Core 50 mm, stop, re-check squareness and runout, then continue.

Where runout comes from

• Bent extension bar from a previous drop or over-torque.
• Worn arbor thread or a cross-threaded connection.
• Carriage play at full stroke, which shows only on deep holes.
• Anchor drift under load, common on old or cracked walls.

Deep core practice

Retract every 40-50 mm to clear slurry and let the segment cool. Keep water flowing; a dry deep core welds the segment to the wall. For holes beyond 500 mm, core from both sides and meet in the middle rather than pushing one long slug.

Where runout actually comes from

An extension bar looks like a simple tube, but every joint in it adds an error, and the error is amplified by distance. A 300 mm bar with 2 mm of misalignment at the coupling can throw the crown 8-10 mm off at the wall.

Source of runout	Typical contribution	How to check
Bar bent in storage	1-3 mm over its length	Roll it on a flat bench, watch the gap
Coupling thread wear or grit	1-2 mm	Thread by hand, feel for wobble before tightening
Mating faces not clean	0.5-1 mm	Wipe both faces, look for slurry crust
Rig spindle misalignment	2-5 mm at the crown	Dial test or a straight rod through the spindle
Core running out under load	Grows with feed pressure	Watch the crown while feeding, not at idle

Runout at idle tells you almost nothing. The number that matters appears when the crown is under feed, because that is when a bent bar or a worn coupling deflects.

Setting the bar so runout stays inside tolerance

• Clean both mating faces before assembly. Slurry crust is the most common single cause of a wobbling bar, and it is invisible until the hole goes oval.
• Thread the bar by hand until it seats, then tighten. Cross-threading under power damages the coupling and adds permanent runout.
• Support long bars with a steady or a guide collar. An unsupported 500 mm bar will sag and whip regardless of how straight it is.
• Check runout with a dial gauge on the bar at the crown end, both at idle and under light feed. Log both numbers.
• Never use an extension bar to compensate for a misaligned rig. Fix the spindle first; the bar only transfers the error further out.

A useful working limit is 1 mm runout at the crown for a 100 mm core and 2 mm for diameters above 200 mm. Beyond that the hole goes oval, the segment wears on one side, and the anchor or pipe that has to fit will not.

What runout does to the finished hole

The consequences are not cosmetic. Runout turns a round hole into an oval, widens the effective diameter at the top and bottom of the cut, and loads one side of the segment far harder than the other.

• Ovality of 3 mm on a 100 mm hole fails most fixing specifications even though the nominal diameter was drilled.
• One-sided segment wear can halve crown life and is often misdiagnosed as "bad diamond".
• A hole that is wider at entry and narrower at depth will not accept a through-bolt cleanly.
• Vibration from runout transfers into the wall and can crack plaster or tile at the edges of the opening.

All four are visible in the first 30 mm of the cut. Checking the hole with a square and a caliper at that point costs a minute; discovering the ovality after a 400 mm hole costs the hole.

Storing bars so they stay straight

A bar that is straight on the bench and bent on the wall was bent in the van. Store extensions horizontally in a rack or a tube, never upright against a wall where the end takes the load, and never under the core boxes. A ten-minute rack saves the runout checks that would otherwise happen on every hole, and a bar that has been dropped gets measured before it is used again, not after.

Check the bar after every job. A bar with 0.5 mm runout that "still works" is the reason the next deep hole comes out 4 mm off at the bottom.