Quick Answer

For a 400mm thick wall, use a 450mm (18") core bit, or a 350mm bit with a continuous threaded tube extension so the machine stays clear of the wall. As a rule, the bit length should be slightly longer than the hole depth plus the distance between the wall face and the machine body. When holes are deeper than one bit length, extend the barrel with a threaded tube rather than buying a one-piece mega-bit.

Core Bit Lengths Available

Bit Length	Max Practical Hole Depth	Typical Use
100mm (4")	80mm	Tiles, thin toppings
165mm (6.5")	150mm	Floor slabs
200mm (8")	180-190mm	Standard slabs and walls
350mm (14")	330mm	Thick walls
450mm (18")	430mm	400mm walls, one pass

How to Extend Depth With Threaded Tubes

• Choose a continuous threaded tube with the same thread as your bit (1/2 BSP, 1-1/4-7 UNC, M16).
• Screw the tube between the machine spindle and the bit barrel.
• Add tubes in sections to reach depths over 600mm.
• Use a pilot drill or guide to keep the extended assembly square.

How to Choose: Step by Step

• Measure the total depth: wall or slab thickness + machine clearance.
• Pick the shortest bit that covers that depth — shorter bits are stiffer and wander less.
• For depths over one bit length, add a threaded extension tube instead of forcing a longer pass.
• Match the thread to your machine spindle and check the diameter stays constant through the extension — our core drill bits come in standard threads.

Safety Tips

• Long assemblies flex — anchor the rig and drill slowly when starting.
• Withdraw the bit often to clear slurry in deep holes.
• Check the tube threads are clean and tight before each job.

Frequently Asked Questions

Q: Can I join two 200mm bits to drill 400mm?

A: Not by stacking barrels — use a proper continuous threaded tube drill extension designed for the spindle thread.

Q: Does a longer bit cut slower?

A:Longer bits need more power and can deflect, so they cut slightly slower and are harder to keep square. Use the shortest length that reaches.

Q: What diameter extension tube do I need?

A: The tube must match the bit's internal barrel thread and diameter — check both before ordering. JOY TECH can supply matched bit + tube sets for deep holes.

Conclusion

Match bit length to hole depth plus machine clearance, and extend with threaded tubes for deeper work. A correct length choice keeps holes straight and bits long-lived. Contact JOY TECH for matched core bits and extension tubes in bulk.