A wobbling core bit makes an oversized hole, wears segments unevenly and eventually destroys the arbor. Wobble is not one fault; it is a symptom with four usual causes, and replacing the bit first is the expensive way to find the real one.

Four causes, in the order to check

Order	Cause	How to confirm	Fix
1	Arbor or spindle wear	Dial gauge on the arbor, no bit fitted	Replace arbor, check spindle taper
2	Bent barrel or damaged shank	Spin the bare bit on a flat bench	Replace barrel, do not straighten
3	Uneven or glazed segments	Inspect segment height and colour	Dress or replace the bit
4	Rig or stand misalignment	Check anchor, column and bit axis	Re-anchor, align, tighten

Measuring runout

1. Remove the bit and measure spindle runout with a dial gauge; more than 0.1 mm means the machine is the problem.
2. Fit the bit and measure at the rim of the barrel; more than 0.3 mm points to the bit.
3. Run the bit in air at working rpm and watch the shadow or a pencil held close; a visible orbit means wobble.
4. Cut a test hole in scrap and measure the diameter at top and bottom. An oversize hole at the top only is usually rig alignment.

Signs in the hole

• Hole 3-5 mm oversize for the whole depth: barrel or shank.
• Hole tight at the bottom and open at the top: rig or stand tilt.
• Vertical scoring on the wall: a single segment doing all the work.
• Ringing or chatter at start: arbor play or a loose chuck.
• Segments worn on one side: misalignment or a bent barrel.

Prevention

Store bits upright, never drop a barrel on concrete, and check the arbor every few hundred holes. Keep the rig anchored square and re-check after every move. Dress segments before they glaze rather than after the bit starts ringing.

Tolerances that pass and fail, by diameter and by purpose

"It looks straight" is not a criterion. Runout has a number, and the acceptable number depends on what the hole is for, not only on how big it is.

Diameter	Acceptable runout at the crown	What it protects
25-50 mm	0.5 mm	Anchor bolts and small fixings that demand round holes
50-100 mm	1.0 mm	Through-bolts, conduit, drain connections
100-200 mm	1.5-2.0 mm	Pipe penetrations with sleeves and seals
200 mm+	2.0-3.0 mm	Large sleeves, core samples for testing

A core sample for strength testing is held to a tighter runout than any plumbing penetration, because a tapered or oval specimen invalidates the test. Sorting the purpose before accepting the number prevents a sample being taken with a wobbling rig and quietly discarded later.

When the spindle is the cause, not the bar

Most crews change the bit first, then the bar, and only then look at the machine. That order is backwards: the spindle is the reference everything else is measured against, and a spindle that is out will make every bit and every bar look faulty.

• Check the spindle with a dial gauge on a known-good bar, at the coupling face. If the reading varies with rotation, the fault is in the spindle or the gearbox bearing.
• Look for play by hand with the machine off: grab the spindle and try to move it radially and axially. Any perceptible movement is a bearing issue, not a bit issue.
• Check the mounting face for burrs and old slurry crust. A 0.2 mm burr under the flange shows up as millimetres at the crown of a long bar.
• Run the machine without a bit and listen. A whine or a rhythmic knock at speed points to the gearbox, and no amount of bit swapping will fix it.
• Log spindle runout when a machine is commissioned and re-check after any drop or heavy stall. Two numbers months apart show whether wear is progressing.

A machine with spindle runout beyond its own specification should be taken out of precision work, not compensated for with a new crown. It will still drill drains; it should not be drilling samples or anchor patterns.

What ignoring runout costs over a project

The cost is rarely one bad hole. It shows up as a pattern across a week of work, and it hides inside numbers that look like ordinary wear.

• Diamond consumption up 30-50% because one side of the segment does most of the cutting.
• Rework on anchor patterns where the as-drilled positions drift beyond tolerance after 20-30 holes.
• Oversized holes needing larger anchors or resin injection, each of which is a variation order.
• Slower cutting because a wobbling crown spends part of each revolution cutting air on one side.
• Water and slurry losses at the collar, since a wobbling crown throws water wider than a steady one.

Bench-checking the rig takes ten minutes at the start of a project. The alternative is discovering the pattern when the fixings will not torque up, which is the worst possible time to learn it.

Charge diagnosis as a separate line. A wobble found in ten minutes is a cheap repair; a wobble run for a week is a new barrel, a new arbor and a recut hole.