Six Core Drilling Problems Every Trade Knows - And How to Stop Them Happening

Every trade has a job that should take ten minutes and somehow swallows an afternoon. For a lot of us, that job is a core hole.

You've marked it up, the customer is hovering in the doorway, and forty minutes later you're stood in a cloud of brick dust with a core that's stopped cutting, a drill that's twice tried to take your wrist with it, and a hole that's maybe 30mm deep.

Nine times out of ten it isn't the operator. It's the core. Here are the six problems that come up on UK sites again and again - what's actually going on, and what to put on the end of the machine instead.

1. It cut beautifully for thirty seconds, then just stopped

This is the big one, and it's almost always glazing.

A diamond core doesn't cut with the diamonds alone - it cuts because the metal bond holding them wears away at roughly the same rate the diamonds do, constantly exposing fresh, sharp grit. Get that balance wrong and the diamonds go flat while the bond stays put. Now you're rubbing a polished steel ring against a wall and wondering why nothing's happening.

Usually it's because the bond is too hard for what you're drilling. Hard material, hard bond, no abrasion, no self-sharpening. Heat builds, the segment glazes, and progress dies.

The field fix: re-dress it. Run the core into something abrasive - a breeze block, an aerated block, a sharpening block - for a few seconds to strip the bond back and re-expose the diamonds. Cut a bit, re-dress, cut a bit. It works, and every experienced driller has done it.

The proper fix: stop buying a core that only suits one material. A genuinely universal dry core like the Marcrist DCU750X is built to stay sharp across brick, block, cast concrete, stone and screed, so you're not re-dressing between every hole. The CCU850X does the same job with a self-cooling barrel design and covers everything from plasterboard to reinforced concrete on one core.

2. Engineering brick eats cheap cores for breakfast

Class A and Class B blues are where budget cores go to die. Three holes, maybe four, then it's a very expensive steel washer.

The maths matter more than the sticker price. A core that costs half as much and lasts a quarter as long is not a saving - it's two extra trips to the merchant and an hour of your day. Cost per hole is the only number worth looking at.

Marcrist tier their dry cores deliberately, and they're honest about it. The DCU350 is a B-B speed/life rating: a sound, budget-conscious core for soft to medium materials. For repeated Class B brick, Marcrist themselves point you at the DCU750X because it lasts considerably longer. That's the right way round - buy the grade the job actually demands.

And if hard brick is your daily bread, look at the PC850 Diamond Soft Percussion Core. This is the one that surprises people. Conventional dry coring makes you switch the machine out of hammer before the diamonds engage; the PC850 is designed to keep percussion running through both the pilot and the diamond phase. Marcrist report drilling times of around 2.5 minutes on holes that previously took 14 minutes or more, and up to 300 holes on general building materials in testing. If you're an electrician or plumber punching large-diameter holes through hard masonry all week, that's your afternoon back.

3. The dust - and it's not just a mess problem

Ask any fitter what they hate about dry coring indoors and it's the dust. Brick dust in the carpet, in the customer's kitchen, in your lungs.

That last one is the serious bit. HSE describe silica as the biggest risk to construction workers after asbestos, and cutting, drilling and grinding brick, concrete, stone and mortar is exactly how respirable crystalline silica gets made. The UK workplace exposure limit is 0.1 mg/m³ averaged over an eight-hour shift, under COSHH - and HSE's guidance for handheld drilling and coring is clear that you control it at source with on-tool extraction and/or water suppression, with RPE on top. Not a mask on its own.

Here's the part that sells itself, though: extraction isn't only a health control, it's a performance upgrade. Marcrist's dust extraction / water feed unit (½" BSP female to ½" BSP male) pulls dust away through the core, and that airflow cools the segments. Marcrist state that cores run with dust extraction drill faster and last longer - up to 25% more life on the PC850 - and that the cooling effect lets you dry-drill materials you'd otherwise need water for, including lightly reinforced concrete.

Cleaner house, cleaner lungs, more holes per core. There's no argument against it.

4. It won't start where you marked it

The bit skates across the face of the brick, chatters, bounces, and by the time it bites you've scored a 40mm arc across the customer's new facing brickwork.

Two things fix this, and on a dry core it's the mechanical one that does the heavy lifting. The DCU750X runs block segments, so locating the hole isn't the segment's job - it's the pilot's. Marcrist's own DCU750X kit ships with a 200mm pilot drill bit and a ½" BSP 3-jaw chuck short adaptor for exactly that reason: the pilot goes in on your mark, the core follows it, and the bit never gets the chance to skate. Where you need more reach, the extended 3-jaw chuck adaptor kit does the same job - just note that it is ½" BSP, so it pairs with the dry cores and will not fit 1¼" UNC wet or combi cores such as the CCW850X and WCU850X.

On the wet and combi side the geometry carries more of the load: those cores run a rooftop / easy-start segment shaped to bite on first contact rather than skitter, and for hand-held combi work a combi core guide rod keeps the start honest.

Technique still counts. Light pressure and low speed until the core has scored a full ring, then commit. Leaning on it at full revs from the first second is what causes the bounce in the first place.

5. It snatches - and hand-held coring is how people get hurt

A core binds in the hole, the bit stops, and the drill doesn't. If you're up a stepladder holding a heavy machine at shoulder height, that's a genuinely dangerous moment, and it's one of the most common complaints on UK trade forums.

Binding comes from a few places: worn segments that no longer cut clearance for the barrel, debris packing the kerf, too large a diameter for the machine's power, or simply letting the drill drop off axis mid-hole.

What helps is a core that runs true. Marcrist's cores are dynamically balanced, and on the semi-wet CCW850X that's specifically credited with minimising vibration and making hand-held drilling much easier. Beyond the bit: keep both hands on it, use the side handle, match the diameter to the machine rather than hoping, keep the core square, and get on a proper platform rather than a ladder. If the hole is big and the concrete is reinforced, rig-mount it - that's what stands are for.

6. Rebar

Steel doesn't have to be a disaster, but it does need a change of approach. Standard advice from the drilling world holds up: drop the motor speed, ease the pressure by around a third, and let the core work through rather than forcing it. You'll usually feel it - the motor bogs and the slurry changes colour. Forcing it at full pressure is how segments get torn off the barrel.

For heavily reinforced concrete, wet drilling and the right bond do the heavy lifting. The WCU850X uses a cobalt bond with a high-concentration segment, rated A++++ for speed and A++ for life, in 1¼" UNC from 42mm to 350mm - that's the one for highly reinforced and prestressed concrete, granite and natural stone. For rig-mounted work in the hardest material, the WF850X runs titanium-coated diamonds and a self-sharpening design through flint concrete and reinforced concrete.

Quick pick: which core for which job

Browse the full range on the diamond core drill bits page.

One last thing: check the thread before you order

Half the "it doesn't fit" calls come down to this. Marcrist dry cores are typically ½" BSP (F) and the wet cores are 1¼" UNC (F). If your machine's spindle doesn't match, you don't need a different core - you need a core drilling adapter. Thirty seconds of checking saves a return.

The bottom line

Almost every core drilling problem traces back to the same root cause: a core that wasn't built for the material it's being asked to cut. Get the bond right, get extraction on the machine, and use the technique the material demands, and coring goes back to being a ten-minute job.

Marcrist have been making diamond tooling in Britain since 1979, and the reason their cores turn up in so many vans isn't the sticker price - it's the cost per hole.

Shop the Marcrist diamond core drill bit range →

Frequently asked questions

Why has my diamond core drill bit stopped cutting?

It's almost certainly glazed. The metal bond has stopped wearing back, so no fresh diamond is being exposed. Re-dress the core in an abrasive material like a breeze block for a few seconds to bring the edge back - and if it keeps happening, the bond is too hard for the material you're drilling.

Can I dry drill reinforced concrete?

Lightly reinforced, yes - with the right core and dust extraction fitted. The airflow through the core cools the segments and does the job water would otherwise do. Heavily reinforced or prestressed concrete needs a wet core such as the WCU850X.

How do I stop a core bit wandering when I start the hole?

Start it mechanically rather than relying on the segments to locate for you. Dry cores like the DCU750X run block segments, so use the 200mm pilot drill supplied in the kit - or the ½" BSP extended 3-jaw chuck adaptor kit if you need more reach. Wet and combi cores use a rooftop / easy-start segment that bites on first contact, and a combi core guide rod steadies a hand-held start. Whichever you are using, start slow with light pressure until the core has scored a complete ring before you commit.

Is a more expensive core bit actually worth it?

Judge it on cost per hole, not cost per bit. A budget core that gives you four holes in Class B engineering brick works out far more expensive than a premium core that gives you a hundred and doesn't cost you the time.