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Speeds, Feeds, and Fluids - The Science of Smooth Cutting

Getting your cutting parameters right makes the difference between struggling through each hole and drilling like a professional. This chapter covers the practical formulas and techniques for setting optimal speeds, feeds, and lubrication for consistent results.

Setting Speed - The Simple Formula You'll Actually Use

Speed settings depend on material and hole size. I use a simple rule of thumb: start with a baseline of 700 RPM for a 1/2-inch hole in mild steel. Then adjust for hole size - smaller holes need faster speeds since the outer edges travel less distance per revolution.

My Baseline Rule

700 RPM for a 1/2" hole in mild steel

That's it. That's your starting point. Now adjust from there.

Adjusting for Hole Size

Smaller hole = faster speed (cutting edges travel less per revolution)

Quick math:

Hole Size RPM Multiplier
1" 350 0.5x
1/2" 700 1x (baseline)
1/4" 1400 2x
1/8" 2800 4x

The pattern: Double the RPM when you halve the diameter.

For example, moving from 1/2-inch to 1/4-inch holes, I double the speed.

Adjusting for Material

Softer = Faster:

  • Aluminum: 2x the steel speed
  • Brass: 2x the steel speed
  • Plastic: 3-4x the steel speed

Harder = Slower:

  • Stainless: 1/2 the steel speed
  • Cast iron: Same as steel (maybe slower)
  • Tool steel: 1/3 the steel speed

Next, adjust for material. For soft materials like aluminum, double the speed. For harder materials like stainless steel, cut it in half. This approach has served me well for most fabrication work.

Real-World Calculation Examples

Let's work through some actual setups:

Example 1: 1/4" Hole in Aluminum

  1. Start: 700 RPM (1/2" in steel)
  2. Adjust for size: 1400 RPM (1/4" is half of 1/2")
  3. Adjust for material: 2800 RPM (aluminum = 2x)
  4. Set drill press near 2800 RPM

Example 2: 3/8" Hole in Stainless

  1. Start: 700 RPM (1/2" in steel)
  2. Adjust for size: ~930 RPM (3/8" is 3/4 of 1/2")
  3. Adjust for material: ~465 RPM (stainless = 1/2)
  4. Set drill press around 450-500 RPM

Example 3: 1" Hole in Mild Steel

  1. Start: 700 RPM (1/2" in steel)
  2. Adjust for size: 350 RPM (1" is 2x of 1/2")
  3. No material adjustment (it's steel)
  4. Set drill press to 350 RPM

Quick Reference Chart with Material Multipliers

Material Speed Multiplier Notes
Aluminum 2x Watch for built-up edge
Brass 2x Free cutting
Mild Steel 1x (baseline) Most forgiving
Stainless 0.5x Keep feeding
Cast Iron 1x No coolant needed
Plastic 3-4x Clear chips often

Feed Pressure by Feel

No numbers here - it's all in the feel! Best tip is to let the drill bit do the work! You want to cut the hole with the least amount of pressure possible.

The Right Pressure

  • Steady chip flow
  • Drill advances smoothly
  • No forcing required
  • Can maintain by hand

Too Much Pressure

  • Drill stalls
  • Motor bogs down
  • Chips pack in flutes
  • Drills break

Too Little Pressure

  • Drill rubs (not cutting)
  • Work hardens
  • Drill dulls quickly
  • Excessive heat (smoke is bad)

Remember: There's no prize for drilling fast. A steady, controlled cut beats rushing every time. Speed is nothing without control!

Cutting Lubricant

Before drilling, apply cutting lubricant. Various types exist, but I have had good results with water-based lubricants. They stay in place, clean off easily, and do not smoke like oil-based alternatives.

Types and When

Water-based (like Anchor Lube):

  • Stays in place
  • Doesn't smoke
  • Easy cleanup
  • My current favorite

Cutting oil:

  • Better for tough materials
  • Smokes more
  • Messier
  • Traditional choice

WD-40:

  • Emergency only
  • Better than nothing
  • Not a real cutting fluid
  • Aluminum only
  • You should be using Kroil anyways!

Application

Drill press:

  1. Apply before starting
  2. Add more every few seconds
  3. Flood for deep holes
  4. Brush on is fine

Key: Keep it wet. Dry cutting = dull drills.

Reading the Chips - Your Real-Time Feedback

While drilling, watch the chips (aka: swarf) coming off to gauge progress. You want some curl and length to them. If they are very thin, the drill is not loading heavily enough. For steel, getting chips that curl but break regularly indicates good speed and feed rate.

What Good Chips Tell You

Steel chips:

  • 6's and 9's = perfect speed and feed
  • Consistent curl and break = optimal parameters
  • Slight straw color = acceptable heat
  • Breaking every 1-2 curls = correct brittleness

What steel chip problems mean:

  • Powder/dust = speed too high or drill dull
  • Long strings = feed too light
  • Blue/purple = overheating, reduce speed
  • Thin wisps = not enough feed pressure

Aluminum chips:

  • Tight spirals when cutting well
  • Should evacuate easily from flutes
  • Shiny surface, not oxidized
  • May form longer strings (normal)

Stainless chips:

  • Tighter curls than mild steel
  • Must maintain continuous cutting
  • Should break regularly
  • No discoloration if speed correct

Cast iron chips:

  • Powdery, granular appearance
  • No real curl formation
  • Dark gray to black color
  • Creates dust, not strings

Continuous thin chips that break apart easily suggest proper drilling parameters.

The Chip Color Guide

For steel:

  • Silver = too slow
  • Straw = perfect
  • Blue = acceptable
  • Purple/black = too fast

Remember: Your chips don't lie. Learn to read them and they'll tell you everything you need to know about your drilling operation.