Machine spec sheets list dozens of numbers. Not all of them are equally important. Here’s how I read a spec sheet and what I focus on.
Critical Specs
These specs determine whether the machine can do the job:
Drilling diameter range. The minimum and maximum diameter the machine can drill. The effective range is narrower than the spec range because tooling availability limits the actual range.
Drilling depth. The maximum depth the machine can drill in one pass. This is determined by the feed axis travel and the drill tube length.
Spindle speed range. The minimum and maximum RPM. The speed needed is determined by the drill diameter and material. A machine with a 6000 RPM max is limited to drills over about 3mm diameter in steel.
Coolant pressure. The maximum coolant pressure the system can deliver. Gun drilling needs 1000-2000 psi typically.
Coolant flow. The maximum coolant flow rate. Gun drilling needs about 3-4 L/min per mm of tool diameter.
Important Specs
These specs affect productivity and capability but aren’t deal-breakers:
Spindle power. Higher power allows higher feed rates and larger diameters. A 15kW spindle is typical for mid-size gun drilling machines.
Feed force. The maximum feed force the machine can apply. Higher force allows higher feed rates.
Machine weight. Heavier machines are generally more rigid and produce better holes.
Guide bushing capacity. The range of guide bushing sizes the machine can accept.
Less Important Specs
These specs are often listed but rarely relevant:
Rapid traverse rate. How fast the axes move when not cutting. This affects non-cutting time but is rarely the bottleneck.
Positioning accuracy. The accuracy of the positioning system. Deep hole drilling accuracy is more dependent on alignment and tool guidance than positioning.
Tool change time. Only relevant if the machine has an automatic tool changer.
Specs That Can Be Misleading
- Maximum drilling diameter. The machine might be able to drill a 50mm hole but only at very low feed rates. The useful range is usually 50-70% of the maximum.
- Spindle power. The power rating might be at the motor, not at the tool. Belt and gear losses reduce the power at the tool by 10-20%.
- Coolant pressure. The pressure rating is usually at the pump outlet, not at the tool tip. Line losses reduce the pressure at the tool.
How I Compare Machines
When comparing machines, I look at:
- Drilling diameter and depth (must match my work)
- Spindle speed range (must reach the speeds I need)
- Coolant pressure and flow (must clear chips effectively)
- Machine rigidity (heavier is better)
- Spindle condition (for used machines)
The other specs are secondary. A machine with good numbers in these five areas will drill good holes.
Here’s how I prioritize the spec sheet categories when evaluating a machine:
| Spec Category | Why It Matters | What to Look For |
|---|---|---|
| Drilling diameter and depth | Determines what parts the machine can handle | Useful range (50-70% of max), not just the maximum spec |
| Spindle speed range | Required RPM depends on drill diameter and material | Max RPM sufficient for the smallest drills I run |
| Coolant pressure and flow | Essential for chip evacuation and tool life | Minimum 1500 psi; 3-4 L/min per mm of tool diameter |
| Machine rigidity | Affects straightness and surface finish | Machine weight, bed construction, base width |
| Spindle condition | Runout directly affects hole quality | TIR under 0.005mm; no bearing noise at operating speed |
Key Takeaways
A spec sheet tells you the machine’s capabilities, not the machine’s quality. Two machines with identical spec sheets can produce very different holes. The build quality, alignment, and condition matter more than the numbers on the spec sheet.
I use the spec sheet to narrow down the options, then evaluate the machine in person before making a decision.