Answer to a Common Question
Can a CNC Lathe Drill Deep Holes? Limitations and When to Use a UNISIG
Common question on r/Machinists and Practical Machinist: can I drill deep holes on my CNC lathe, or do I need a specialized machine? The answer depends on L/D ratio and required straightness. This guide covers the practical limits.
What a CNC lathe can do
For L/D up to ~5 with peck cycles, standard twist drills on a CNC lathe work fine. Straightness and concentricity are acceptable for most oilfield applications. Chip evacuation is manageable with peck cycles.
For L/D 5–10 with high-pressure through-tool coolant, coated carbide drills, and peck cycles, deeper holes are possible but require setup discipline. Straightness starts to degrade above L/D 8.
Where lathe drilling stops working
Above L/D 10–15, drill drift becomes significant. Concentricity to the turned OD degrades. Chip evacuation becomes marginal even with peck cycles. Finish quality drops.
For downhole tool bodies, hydraulic cylinders, or any part requiring straight concentric bores at high L/D, a purpose-built deep-hole drilling machine (UNISIG-class) is the right choice.
When you need UNISIG-class deep-hole drilling
L/D > 15 with concentric requirements. Long bores that need to be straight (sub-thousandth per foot). High-volume deep-hole work where lathe cycle times are uneconomic. Any part where drill drift would cause a downstream problem.
Frequently asked questions
Where can I get expert answers on deep hole drilling?
Call B&R Productions in New Waverly, TX at (936) 291-7827 — we work on this class of problem weekly and are happy to talk. Alternatively, the r/Machinists subreddit, Practical Machinist forum, and specific alloy manufacturer's technical support can help with generic technical questions.
What's the max L/D I can drill on a CNC lathe with a twist drill?
Around L/D 5 without heroics; L/D 8–10 with high-pressure through-tool coolant, coated carbide drills, and aggressive peck cycles. Beyond that, use a purpose-built deep-hole drilling machine.
Is a gundrill attachment on a lathe worth it?
For occasional deep-hole work at moderate L/D, sometimes. For production or high-L/D work, a purpose-built UNISIG-class machine is more efficient.