Surface Finish¶

What those little squiggly lines on prints mean, and how to achieve them without going insane.

Surface Finish Basics¶

What the Numbers Mean¶

Ra (Roughness Average): The most common spec

Measured in microinches (µin) or less commonly in micrometers (µm)
32 µin = Pretty rough (visible tool marks)
16 µin = Normal machined finish
8 µin = Smooth (barely feel tool marks)
4 µin = Mirror-ish (with the right light)

Quick conversion: 1 µm ≈ 40 µin

Reading the Symbols¶

\_/\_
32 = 32 microinch max roughness

\_/\_
32 = Must be machined to 32 µin
✓ (no grinding allowed)

\_/\_
16 = Must be ground to 16 µin
G

If you see multiple numbers, the top one is usually what matters.

How to Actually Achieve Different Finishes¶

125 µin - "Just Make Chips"¶

Any tool, any speed
Roughing passes
Who cares, it's probably gonna get painted or powder-coated

63 µin - Standard Machined¶

Sharp tools
Normal feeds and speeds
What you get when you're not trying

Mill settings:

0.003-0.005" per tooth
Don't dwell
Conventional milling usually better

Lathe settings:

0.006-0.010" per rev
Sharp tool
Positive rake

32 µin - Nice Machined Finish¶

Okay, now you need to pay attention!

Mill:

Fresh tool (not brand new - slightly broken in is actually better)
0.002-0.003" per tooth
Climb mill if your machine doesn't suck
Spring pass (repeat the final pass)
Coolant or cutting oil is a must

Lathe:

0.002-0.005" per rev
High spindle speed
Polished tool
Light final pass (0.001-0.002" DOC)

16 µin - Smooth as a Baby's ass¶

We are getting serious now!

Mill:

Fly cutter with polished HSS
200-400 SFM
0.001" per tooth max
Multiple spring passes
We're flooding on the coolant

Lathe:

Polished HSS or polished carbide
0.001-0.002" per rev
Maximum RPM your lathe can handle
0.0005" final pass
High pressure coolant aimed directly at your cutting edge

8 µin or Better - Mirror Time¶

It usually requires secondary operations:

Grinding:

Fresh dressed wheel
Light passes
Lots and lots of coolant
Let the wheel do the work

Lapping:

Cast iron lap
Diamond compound
Figure-8 patterns
Check every 30 seconds

Hand Work (yeah, really):

400 → 600 → 800 → 1200 grit
Always with oil
One direction only
Thoroughly clean between grits

Measuring Surface Finish¶

Fingernail Test (Free)¶

Run your fingernail (if you have them) perpendicular to lay:

Catches hard = 63 µin or worse
Catches slightly = 32 µin
Barely feel it = 16 µin
Can't feel = 8 µin or better

Surface Comparators ($50)¶

Little blocks with different finishes
Visual and touch comparison
Good enough for 90% of work
Get one that matches your work

Profilometer ($$$$)¶

Drags a tiny stylus across the surface
Actually measures real Ra value
Only for when Boss or consumer demands proof
Oh, and don't drop it!

Factors That Affect Finish¶

Tool Geometry¶

Nose radius: Bigger = better finish (to a point)

0.015" = General purpose
0.031" = Better finish
0.047" = Best finish (but weak)

Relief angles: Less = better finish, more rubbing Rake angles: More positive = better finish on aluminum

Speeds and Feeds¶

General rule: Slower feed = better finish

But also:

Too slow = rubbing and work hardening
Too fast = torn surface
Find the sweet spot

Speed: Usually faster = better (until it isn't, experience will forecast)

The Material Matters¶

Aluminum:

Loves sharp tools
High speeds
Watch for built-up edge

Steel:

Consistent feeds
Good coolant and plenty of flow
Honed cutting edge

Stainless:

Sharp tools are mandatory
Never stop feeding
High Sulfur content cutting oils, or Diesel Engine spec. motor oil in a pinch

Cast Iron:

Run dry
Higher speeds
Negative rake

Machine Condition¶

Spindle runout: Kills finish Way wear: Causes chatter Backlash: Shows up as lines Rigidity: Everything matters more

Common Problems and Fixes¶

Chatter Marks¶

Those horrible washboard patterns:

Fixes:

Change speed (up or down 20%)
Increase feed
Reduce tool stick-out
Change number of flutes
Sometimes run WITHOUT coolant

Built-Up Edge (BUE)¶

Aluminum welded to your tool:

Fixes:

More coolant
Higher speed
Polish the tool
Different coating
Give up and use HSS

Feed Lines Too Visible¶

Fixes:

Smaller feed rate
Larger nose radius
Overlap passes more
Spring pass
Hand blend if allowed

Inconsistent Finish¶

Check:

Tool wear (measure every 10 parts)
Coolant concentration off or coolant needs to be flushed and changed
Machine warming up
Material hardness variation

Shop Finish Tricks¶

The Crocus Cloth Special¶

For lathe work:

320 grit crocus cloth
A few drops of oil
Back the tool off slightly
Hold cloth against spinning part
Move slowly along length
Instant improvement

WARNING: Fingers + spinning things = potential for a very bad time. Be careful!

Fly Cutter Magic¶

The secret to mirror finishes on mills:

Stone the cutting edge
Run at 300-400 SFM
0.0005" per tooth
Width of cut = 70% of diameter
Multiple passes at full depth

The Scotch-Brite Finish¶

Job calls for a "brushed" finish?

Get part to 32 µin
Red Scotch-Brite pad
Straight lines, one direction
Consistent pressure
Use Kroil (or WD-40 if you are cheap) as lubricant

Speed Kills (Chatter)¶

Sometimes slower is better:

Try 50% speed
Heavy feed
Drown in coolant
Reducing speed often works when nothing else does

Psychology¶

Under Promise, Over Deliver¶

Consumer wants 32 µin? Aim for 16.

Easier to defend
Looks better
Same effort usually

The Sample Game¶

Always make a sample piece first:

Get consumer approval
Keep it as reference
CYA when they complain

Know When to Say No¶

Some finishes aren't possible on certain:

Materials (cast iron won't mirror)
Geometries (deep pockets)
Machines (worn ways)

Better to say no than make scrap.

Equipment That Helps¶

Must Have¶

Sharp tools (duh)
Good clean coolant
Surface comparator
Selection of polishing stones

Nice to Have¶

Profilometer
Multiple nose radius inserts
High-pressure coolant
Power draw bars (more consistent)

Secret Weapons¶

Microscope (USB scopes are $50 now)

See what's really happening
Catch built-up edge early
Understand your failures
Impress/terrify parts consumers

The Reality¶

Most prints over-specify surface finish. That 16 µin callout on a bracket? Nobody's checking. But when it matters, it REALLY matters:

eg:

Seal surfaces
Bearing fits
Sliding surfaces
Cosmetic parts

Know the difference and save yourself time.

Quick Reference¶

Ra (µin)	Feed (IPR)	Visual	Where Used
250	Whatever	Rough as hell	Hidden surfaces
125	0.010-0.015	Visible tool marks	Non-critical
63	0.006-0.010	Light tool marks	General
32	0.003-0.005	Smooth, some marks	Close tolerance
16	0.001-0.003	Very smooth	Bearing surfaces
8	0.001 max	Mirror-ish	Seals, critical

Remember: Good surface finish is 20% settings, 30% tool condition, and 50% giving a damn.