This article is part of the CNC Tolerances & Quality Control Guide: GD&T, CMM, Cpk, PPAP on CNX Precision.
CNC drilling is the most common hole-making operation in modern manufacturing. It is fast, flexible, and highly repeatable. However, a good hole requires more than a spinning drill. Speeds, feeds, geometry, and coolant all matter. This guide explains the process from start to finish. It also includes a reference table you can use on the shop floor. You will learn about drill types, cutting parameters, and best practices. By the end, you can specify better holes and avoid costly scrap.
What Is CNC Drilling?
CNC drilling uses a rotating cutting tool to create a cylindrical hole. The machine controls position, depth, feed, and speed. Therefore, holes are consistent from part to part. Unlike manual drilling, CNC drilling eliminates operator guesswork. It also supports peck cycles and chip-breaking routines. These features make deep holes safer and more reliable.
Drilling is usually the fastest way to make a hole. It removes a full disk of material in one pass. Therefore, cycle time is short. However, drilling leaves a rougher surface than boring or reaming. Typical drilled holes hold about IT10 to IT12 tolerance. If you need tighter tolerances, follow drilling with finishing operations.
Indeed, hole accuracy depends on several variables. Machine positioning sets location accuracy. Drill geometry sets roundness. Feed rate affects size and finish. Coolant pressure controls chip evacuation. Therefore, every variable must be controlled together. Change one parameter and hole quality may shift. This is why process planning matters.
Common Drill Types for CNC Drilling
Choose the drill that matches your material and hole depth. Each type has a specific role in CNC drilling.
- Solid carbide drills: the best choice for production and tight tolerances.
- HSS drills: cost-effective for small batches and low speeds.
- Indexable insert drills: efficient for large diameters and stable machines.
- Coolant-through drills: essential for deep holes above 3xD.
- Gun drills: made for holes deeper than 20xD with straightness demands.
- Spade drills: flexible for large holes with replaceable blades.
Drill geometry also matters. A 118 degree point suits soft steels. A 135 degree point reduces thrust on hard materials. A 140 degree point handles stainless steel well. Helix angle controls chip flow. Low helix suits aluminum; high helix suits deep holes in steel.
Coatings extend drill life. TiN suits general steel work. TiAlN handles heat and abrasive materials. For stainless and Inconel, look for AlTiN or specialized nano-coatings. Moreover, edge preparation matters. Honed edges resist chipping. Sharp edges cut freer in aluminum. Choose the coating after you know the material.
Hole specifications usually list diameter, depth, location, and finish. Some drawings also demand straightness and roundness. Therefore, read the complete callout before choosing a drill. A 0.05 mm tolerance needs carbide and a rigid setup. A 0.5 mm clearance hole may accept an HSS drill.
Key CNC Drilling Parameters
Cutting speed and feed decide tool life and hole quality during CNC drilling. The table below gives practical starting points with carbide drills.
| Material | Cutting speed (m/min) | Feed per rev (mm) |
|---|---|---|
| Low-carbon steel | 80–120 | 0.10–0.25 |
| Alloy steel | 60–90 | 0.08–0.20 |
| Stainless steel | 40–70 | 0.06–0.15 |
| Aluminum | 150–250 | 0.15–0.35 |
| Titanium | 20–40 | 0.05–0.12 |
| Cast iron | 80–140 | 0.10–0.25 |
Start at the low end of each range. Then adjust based on chip shape. Short, tight chips indicate a good setup. Long ribbons or blue chips mean too much heat. In addition, use a peck cycle for holes deeper than 3xD. Pecking breaks chips and helps coolant reach the cutting edge.
The table covers carbide drills. HSS drills run at lower speeds, roughly half the carbide values. For example, HSS in alloy steel runs at 30 to 45 m/min. Therefore, choose the tool material first. Then set the parameters. This order avoids expensive mistakes on the shop floor.
Peck depth depends on the material. For steel, use a peck of 0.5xD to 1xD. For aluminum, deeper pecks work because chips are small. For stainless, shallow pecks prevent work hardening. In addition, reduce the feed on the final peck. This technique cleans the hole bottom and improves the entry.
CNC Drilling Best Practices
Follow these steps for clean, accurate holes. First, spot drill or center drill before the full hole. A spot ensures the drill starts on location. Second, check the hole depth ratio. For holes above 5xD, consider a pilot hole or step drilling. Third, use through-tool coolant whenever possible. It flushes chips and cools the cutting zone. These steps form the backbone of good CNC drilling.
Deep holes need a planned approach. Start with a short pilot drill. Then follow with the full-length drill. For holes above 15xD, consider gun drilling. In addition, coolant pressure must be high enough to push chips out. Otherwise, chips pack and break the drill. Our team programs these steps carefully for every deep-hole job.
However, workholding matters too. A rigid setup prevents the drill from wandering. Furthermore, check the spindle runout. Excess runout enlarges the hole and wears tools unevenly. For threaded holes, drill slightly oversize before tapping. For press fits, remember that drilling alone rarely meets the tolerance.
Surface finish inside the hole depends on speed, feed, and coolant. In general, lower feed improves finish but may cause work hardening in tough alloys. Therefore, balance finish and cycle time. Measure the first part of each batch and record the tool life.
At CNX Precision, CNC drilling is part of our daily work. Our 3/4/5-axis machines drill, tap, and ream in one setup. This approach reduces handling and improves positional accuracy. We also machine deep holes with gun drilling when straightness matters.
Coolant concentration affects hole quality. Emulsions between 6 and 10 percent work for most steels. For aluminum, lower concentration prevents foaming. In addition, filter the coolant. Fines from previous jobs can clog coolant-through drills. Consequently, clean coolant means longer tool life and better finishes.
Measure drilled holes during production. Use plug gauges for pass or fail checks. Use CMM or air gauges for statistical control. Record tool life and adjust feeds as tools wear. This routine keeps quality stable across long runs.
Frequently Asked Questions
What tolerance can CNC drilling hold?
In general, standard drilling holds about IT10 to IT12. However, carbide drills with good setup can reach IT9. For tighter fits, add reaming or boring after drilling. Position errors also add up. Therefore, spot drilling and rigid fixturing are essential.
How deep can a CNC drill go?
Standard drills handle 3xD to 5xD without special tooling. Coolant-through drills reach 10xD to 15xD. Gun drills can exceed 20xD with excellent straightness. Beyond 20xD, consider gun drilling with a dedicated machine or attachment.
Why do drills wander on entry?
Drills wander when the point enters at an angle or the setup lacks rigidity. Spot drilling, a rigid bushing, or a pilot hole fixes the problem. Also check for worn margins and uneven lips. A center drill with a 90 degree point works well for most entry conditions.
Need reliable holes in your next part? Send your drawing to CNX Precision. We apply proven CNC drilling parameters to every job. Request a free quotation today.
For related information, see our guide to cnc machining service and 5-axis cnc machining and cnc milling vs turning and cnc machining tolerances.