I Learned the Hard Way: 6 Diamond & Core Bit Mistakes That Cost Me $2,000+ (And How to Avoid Them)

I run a small fabrication shop. We do a lot of custom metal, granite, and stone work. And let me tell you: the first year, 2017, was a dumpster fire of bad drill bit decisions.

If I remember correctly, I spent about $2,300 on bits that broke instantly, didn't cut, or were the wrong spec. The worst part? Most of it was avoidable. So I'm sharing the mistakes I made, in Q&A format, so you don't have to repeat them.

1. What's the actual difference between a diamond core bit and a standard metal drill bit?

This was my first big assumption failure. I assumed 'drill bit' was a universal term. It's not.

A diamond core bit (like a granite core bit or stone core drill bit) is for hard, brittle materials: ceramic, porcelain, granite, concrete. It's a hollow tube with diamond grit bonded to the rim. It grinds, it doesn't cut. It needs water or a constant flow to stay cool.

A standard twist drill bit (like a 'best drill bits for metal' search turns up) is for metal, wood, plastics. It has sharp flutes that carve material. No water needed.

Let me rephrase that: If you put a metal twist bit into granite, you'll destroy the bit in seconds. If you put a diamond core bit into steel, you'll gum it up. Learned that one the expensive way.

2. I need to drill a 200mm hole in granite. Do I need a 200mm core bit?

You need a core drill 200mm specifically designed for stone. Yes, size matters. I made the 'penny wise, pound foolish' mistake here.

Saved $60 by buying a cheap 200mm bit from a general hardware store. It was designed for brick, not granite. The first 10 seconds of drilling chipped the rim, and the bit wobbled so badly it cracked the stone slab. $450 wasted on the slab + $120 for the bit. Net loss: $570. Should've bought a proper diamond core bit from a supplier that specializes in stone fabrication.

My rule now: For any 1 1/4 diamond drill bit size or larger (that's roughly 32mm, or a 200mm hole for your sink), buy from a supplier who lists the 'hardness rating' of the bit. Most bits for granite will say 'for hard porcelain or granite.' If it just says 'for tile,' assume it's for soft wall tile only.

3. What should I pay for a good diamond core bit? (The diamond core bit price question)

This varies wildly. Based on major tool supplier price lists, January 2025:

• Small diamond core bits (6-20mm): $15-50
• Medium diamond core bits (20-50mm, like a 1 1/4 diamond drill bit): $40-100
• Large diamond core bits (100-200mm, like a stone core drill bit for 200mm hole): $100-350

But don't quote me on those exact prices. They change by vendor and time. The point is: you get what you pay for. A $20 'diamond core bit' for granite? I learned that lesson: it's not diamond, the grit falls off, and it's a waste of $20. The cost of redoing a job because a cheap bit fails is always higher than the upfront price of a decent one.

Tip: I'd argue the middle tier (like $60-80 for a 1 1/4 diamond drill bit) is the sweet spot for a fabrication shop. It's not the cheapest, but it has quality grit that lasts for dozens of holes.

4. I heard 'dry cutting' diamond bits exist. Can I use them for stone?

Yes, dry-cutting diamond bits exist for quick jobs on tile. But for stone, granite, and any deep drilling (like a 200mm core drill hole), you want wet cutting.

Here's the mistake I saw a newbie make in September 2022: He bought a dry-cut granite core bit to save on water setup. After about 5mm of depth, the bit started smoking. On granite, the heat builds up fast. The bit's diamond layer disintegrated. The stone cracked from thermal shock. Total waste: $180 on the bit + the stone. He ended up buying a wet bit anyway.

Put another way: Dry bits are for shallow holes in soft tile. Wet bits are mandatory for stone core drilling. If you're drilling a 200mm hole, you're going deep. Use water. It's cheap insurance.

5. How do I choose the 'best drill bits for metal' for my shop?

This is a different conversation from diamond bits. For metal, you care about:
Cobalt or HSS: Cobalt drill bits (M35 or M42) are the best drill bits for metal, especially stainless steel. HSS is fine for mild steel.
135° split point: This is my non-negotiable. A split point prevents 'walking' on metal. Standard 118° bits wander, especially on round surfaces.
Brand reliability: I've had good luck with Relton, and Greenlee for step bits. I avoid no-name bulk packs for critical work.

To be fair, I've used $10-cobalt sets from Harbor Freight that worked fine for a few holes in mild steel. But for consistent work on stainless or hardened steel, you pay for the quality.

6. What's one question nobody asks but everyone should?

This is the one I always include now: "Is my drill powerful enough?"

I once ordered a beautiful diamond core bit, 1 1/4 size, for a granite countertop sink hole. Checked the bit spec, the price, everything. But I didn't check my drill's power. I used a standard 18V cordless drill. The bit jammed, the drill's clutch overloaded, and it twisted my wrist (hurt for days). The bit overheated and the diamond layer burned off.

Solution: For any core drill 200mm or any diamond bit over 1 inch, you need a dedicated core drill rig or a powerful (1,000+ RPM) rotary hammer drill with a clutch. A standard variable speed drill will not handle the torque. I now have a separate drill set aside just for coring tasks. It's saved my wrists and my bits.

If you ask me, that's the classiest mistake: buying a great bit, but ignoring the tool driving it.

Prices as of January 2025; verify current rates with your supplier. This is based on my personal experience; your mileage may vary.