Work Holding Secrets: Because Flying Parts Kill People¶

Author: Johannes A. Nilsson

The First Rule of Machining¶

If it ain't clamped, it ain't staying put. I don't care if it weighs 500 pounds - cutting forces will move it, throw it, or turn it into a projectile. Good work holding is the difference between going home with all your fingers and explaining to the ER doc why there's a part embedded in your skull. Always consider that that forces of inertia and entropy are out to get you.

The Physics you didn't learn in shop class¶

Cutting forces go three ways:

Down - Into the table (usually fine)
Sideways - Trying to slide your part (the sneaky bastard)
Up - Trying to lift your part (the killer)

Your clamps need to fight all three. Gravity only helps with one.

Basic Clamping Rules¶

The 3-2-1 Principle¶

This is how you make a part stay put without distorting it:

3 points define a plane (bottom of part)
2 points stop rotation (side of part)
1 point stops the last degree of freedom

Over-constrain it and you'll bend your part. Under-constrain it and it'll move and possibly kill you.

Clamp Placement¶

Good Clamping:

Close to the cut - support where forces happen
On solid sections - not thin walls
Even pressure - multiple light clamps always beat one gorilla clamp
Low on the part - less leverage for lifting

Shit Clamping:

Only at the ends - part bows in middle
Over unsupported areas - crushes the part
Too much pressure - distorts everything
High on the part - creates a lever

Vise Work¶

The most common work holding, and the most commonly fucked up.

Vise Basics¶

Jaw Preparation:

Keep them clean: one chip changes everything
Keep them parallel: check with an indicator)
Keep them true: mill them if needed
De-burr everything: sharp edges dig in wrong

Part Preparation:

De-burr all edges
Clean all surfaces
Check for warping & distortions
Support thin sections

Parallels: Your Best Friend¶

Those hardened bars aren't just spacers:

Lift the part for tool clearance
Spread clamping force
Keep the part square
Tell you if the part is seated (tap test)

The Tap Test: After clamping, tap each parallel. They should both ring the same. If one's dead, your part is bent or not seated.

Vise Tricks¶

The Penny Trick: For thin parts, put a penny (or thin parallel) under one end. Tighten vise until part touches both jaws, then remove penny. Now you've got even pressure without bending.

Soft Jaws: More on these custom tools you can fab to make your life better later, but they're magic for:

Holding finished surfaces
Odd shapes
Repeat work
Delicate parts

Stop Blocks: Use them. Every time. Repeatability matters more than you think.

Soft Jaws: A Superpower¶

If you're not making soft jaws, you're working too hard.

When to Use Them¶

Round parts in a vise
Finished surfaces you can't mark
Odd shapes that won't sit flat
Production runs of the same part
Thin walls that regular jaws would crush

Making Soft Jaws¶

Start with aluminum (6061 is perfect)
Machine in place (critical for accuracy)
Cut the profile 0.001-0.002" undersize
Add relief cuts where needed
Drill and tap for stops if needed

Pro Tips¶

Number them - Soft jaws are married to specific jobs
Save them - Future runs are easier
Machine both sides - Double your utility
Add features - Built-in stops, part ejectors, etc.

Custom Fixtures: When Vises Aren't Enough¶

Sometimes you need to build your own work holding.

Fixture Plates¶

Start with a flat plate:

Aluminum for most work
Steel for heavy cutting
Mic-6 for critical flatness

Add features:

Tapped holes for clamps
Dowel pins for location
Relief cuts for chips
Coolant channels if fancy

Toggle Clamps¶

The fast-change artist:

Repeatable pressure
Quick loading
Adjustable
Don't trust the ratings (use 2x safety factor)

Vacuum Fixtures¶

For thin, flat parts:

Great for sheets
No clamp marks
Limited cutting force
Needs good seals
Expensive and complex to set-up

Magnetic Work Holding¶

Magnets are magic, but respect them.

Magnetic Chucks¶

Pros¶

Fast setup
No clamp interference
Even holding
Great for grinding

Cons¶

Only ferrous materials
Limited holding power
Chips stick everywhere
Can magnetize your part or tooling

Safety Rules¶

Never trust magnets alone for heavy cuts
Always use backup clamps for roughing
Check holding power before cutting
Demagnetize parts and tools after (unless you want chips forever)

Avoiding Distortion¶

The part was straight until you clamped it. Here's why:

Causes of Distortion¶

Over-clamping - gorilla syndrome
Uneven support - part rocks
Heat buildup - expansion fighting clamps
Residual stress - material wants to move
Point loading - clamps too small

Solutions¶

Jack Screws¶

Support the middle of long parts

Adjustable height
Multiple contact points
Takes the bow out

Step Clamps¶

Spread the load

Use the right height
Keep them level
Multiple small better than few large

Indicating While Clamping¶

Watch the distortion happen

Indicates part while tightening
Stop when movement starts
Find the sweet spot

Advanced Techniques¶

The Sacrificial Plate Method¶

For through-holes and full profiling:

Attach part to aluminum plate (superglue, VHB tape, or screws)
Clamp the plate, not the part
Machine through into the plate
Replace plate as needed

The Ice Fixture¶

For tiny or delicate parts:

Freeze part in ice block
Machine while frozen
Melt to release
Seriously, this works!

Superglue Fixturing¶

The controversial method:

Use good CA glue
Clean surfaces perfectly
Heat to release (150°C)
Great for small, flat parts
NEVER trust for heavy cuts!

Work Holding for Specific Operations¶

Drilling¶

Prevent spin: drill grab is real
Support the exit side: prevent blowout burrs
Clamp near the hole: minimize flex

Tapping¶

Rock solid holding: taps don't forgive
Perpendicular setup: crooked threads suck
Easy chip clearing: or broken taps

Profiling¶

Access all sides
Minimal clamp interference
Consider order of operations

Finishing¶

Soft contact only
Even pressure
Quick changes
No marks allowed

Common Fuck-ups¶

The Part Moved¶

Insufficient clamping
Forgot to tighten something
Chips under the part
Coolant hydraulics

The Part Flew¶

Climb milling with weak setup
Grab during drilling
Clamps too high
Trust in magnets alone

The Part Distorted¶

Over-clamping
Uneven support
Heat with no expansion room
Released internal stress

The Golden Rules¶

If you're scared of the setup, fix it - Trust the Gut Brain. Your gut knows!
Two pounds of clamp for one pound of cut - Overkill saves lives
Check after first cut - Parts move, clamps loosen
Plan the whole job - Don't paint yourself into a corner
Clean everything twice - One chip will ruin everything

Investment Priorities¶

Start cheap, upgrade what you use most:

Good vise - I like Kurt brand, or get some clone equivalent
Parallel set - Import is fine to start - A Starrett set is bought once and used for life
Step clamp kit - Get a decent one, or possibly two - You can't have too many
123 blocks - Incredibly versatile
Soft jaw material - Always have some ready
Bessey Clamps - Another thing you can never have enough of

Final Words¶

Work holding is where machining gets creative. There's always a way to hold the weird-ass part the engineer designed - you just might need to invent it. Start simple, build your bag of tricks, and remember: the best work holding is the one that lets you go home with the same number of fingers and eyes you arrived with.

And if you hear that sound - that tiny "tink" of a part shifting - STOP. That's the sound of shit about to go real wrong. Re-clamp it, check it, and try again. The part can be replaced. Your face cannot.