Shop-Made Tools: Making Your Own Special Cutting Tools¶

Author: Johannes A. Nilsson

What Are Shop-Made Tools?¶

Shop-made tools are cutting tools, holders, and fixtures you make yourself instead of buying. Sometimes the tool you need doesn't exist. Sometimes it costs $500 and you need it for one job. Sometimes you can make it better than what's for sale. This is about taking raw materials and making the exact tool you need.

Why Make Your Own Tools?¶

Real Example: You need to cut a 0.437" radius on 50 parts. A form tool costs $300 and takes 3 weeks to deliver. Or you can grind one from a HSS blank in an hour for $5.

That's why we make tools.

Fly Cutters: Your First Shop-Made Tool¶

A fly cutter is the simplest tool you can make that gives professional results. It's basically a spinning arm with one cutting tool that makes super smooth, flat surfaces.

What Is a Fly Cutter?¶

Imagine a single-point lathe tool spinning in your mill. That's a fly cutter. It takes a wide, shallow cut and leaves a mirror finish. Way better than face milling with an end mill.

Making a Basic Fly Cutter¶

What You'll Need¶

2-3" diameter steel round (1018 works fine)
HSS tool bit blank (3/8" or 1/2" square)
Set screws
Basic lathe and mill

Step-by-Step Process¶

Make the Body
- Chuck up your steel round in the lathe
- Face both ends flat
- Turn a shank to fit your spindle (R8, MT3, whatever you have)
- Leave the head 2-3" diameter
Mill the Tool Pocket
- Put the body in your mill vise
- Mill a pocket at 45° angle for your tool bit
- Make it a snug fit - you want maybe 0.001" clearance
- Pocket should be 1/2 to 2/3 through the body
Add Set Screws
- Drill and tap for set screws (usually 1/4-20 or 5/16-18)
- Two screws at 90° to each other
- This locks your tool bit in place
Balance It (Optional but smart)
- Drill a hole opposite the tool pocket
- Removes weight to balance the cutter
- Reduces vibration at higher speeds

Grinding the Cutting Tool¶

This is where the magic happens. You're making the actual cutting edge.

Basic Geometry¶

Top rake: 5-10° positive (slopes down from cutting edge)
Side clearance: 10-15° (so only the edge touches)
Nose radius: 0.015-0.030" (prevents sharp corners that break)

How to Grind¶

Use a bench grinder with a fine wheel
Keep the tool cool (dip in water frequently)
Grind the angles freehand or with a tool rest
Polish the cutting edge with a stone
The smoother the edge, the better the finish

Using Your Fly Cutter¶

Speed: Run it slow - 100-300 RPM for steel, 300-600 for aluminum
Feed: Go easy - it's taking a big bite
Depth: 0.001-0.005" for finishing, up to 0.020" for roughing
Overlap: 80-90% overlap between passes

Boring Bars: Custom Hole Sizes¶

A boring bar is a tool for making precise holes to any diameter. Think of it as an adjustable reamer that you make yourself.

What's a Boring Bar?¶

It's a steel bar with a cutting tool at the end. You stick it in a hole and it cuts the inside to make it bigger and more accurate. Essential when you need a hole size that doesn't match any standard tools.

Simple HSS Boring Bar¶

Starting Material¶

Steel rod (4140 is good, cold roll works too)
Diameter = 50-65% of your smallest hole
Length = 4x diameter maximum (longer = more chatter)

Making Process¶

Prepare the Shank
- Turn to fit your boring head or tool holder
- Or leave round for collet holding
- Make it concentric - runout here means runout in your hole

Grind the Cutting End

Side view:       Top view:

   |\              _____
   | \            |     |____
   |  \           |_____|
   |___\

Relief angle     Clearance prevents rubbing

Key Angles¶

Front clearance: 10-15°
Side clearance: 5-10°
Side rake: 5-10° positive
Keep the point sharp!

When to Make vs Buy¶

Make Your Own¶

Odd sizes (like 0.7835" for that weird bearing)
Long reach applications
Special profiles (like back boring)
Learning experience

Buy Commercial¶

Standard sizes you'll use often
Carbide insert bars for production
When rigidity is critical
Coolant-through designs

Form Tools: Making Complex Shapes¶

Form tools cut specific profiles in one pass. Need a special radius? Odd angle? Custom groove? Make a form tool.

Understanding Form Tools¶

Instead of moving the tool to create a shape, the tool IS the shape. One plunge cut or pass creates the entire profile.

Making a Simple Radius Tool¶

Let's say you need a 1/4" radius on the edge of parts.

Process¶

Start with HSS Blank
- 3/8" x 3/8" x 3" HSS tool bit
- Or larger depending on your radius
Grind the Inverse Profile
- Your tool needs a 1/4" CONCAVE to cut a 1/4" CONVEX
- Use radius gauge to check
- Or make a template from shim stock
Add Relief Angles
- Front relief: 5-7°
- Side relief: 2-3° each side
- Too much relief = weak tool
- Too little = rubbing and heat
Test and Adjust
- Try on scrap material
- Check with radius gauges
- Adjust until perfect
- Stone the cutting edge smooth

Form Tool Tips¶

Go slow: Form tools take big bites
Use cutting oil: Lots of it
Light cuts: 0.002-0.010" per pass
Sharp tools: Resharpen frequently
Rigid setup: Any flex ruins the profile

Special Holders and Arbors¶

Sometimes the challenge isn't the cutting tool - it's holding the work. Custom arbors and holders solve weird work-holding problems.

Between Centers Turning Arbor¶

For turning work that's already bored.

Arbor Description¶

Steel shaft with 60° centers on both ends
Work slides onto shaft
Dog drives the work
Perfect concentricity

Making One¶

Turn between centers (yes, to make a between centers arbor)
Get diameter 0.0005" under your bore size
Polish to size
Cut flat for drive dog
Harden if needed

Expanding Arbor¶

Holds work from the inside of a bore.

Design¶

   Slots cut lengthwise
   |  |  |
   v  v  v
  [========]----
   ^ Taper ^

Draw bolt pulls tapered plug
expanding the slotted section

Making Tips¶

3-4 slots evenly spaced
1° taper over working length
Don't over-expand (they can crack)
Size for 0.005-0.010" expansion range

Tool Materials Explained¶

High Speed Steel (HSS)¶

What HSS Is¶

Tool steel that stays hard when hot

Why Use HSS¶

Cheap and available
Can grind to any shape
Resharpenable forever
Good for learning

HSS Best For¶

Lathe tools, boring bars, form tools

Carbide¶

What Carbide Is¶

Tungsten carbide - way harder than HSS

Types¶

Brazed carbide: Carbide tip silver-soldered to steel body
Insert tooling: Replaceable carbide inserts in steel holders

Why Use Carbide¶

Stays sharp longer
Cuts faster
Handles interrupted cuts
Better finish

Carbide Best For¶

Production tools, hard materials

Heat Treatment Guidelines¶

O1 Tool Steel (easiest DIY):

Heat to bright cherry red (802°C)
Quench in oil (motor oil works)
Clean and polish
Temper in oven at 204°C for 1 hour
Should be straw yellow color

When to Send Out for Heat Treatment¶

Complex shapes (less warping)
Critical tools
Air-hardening steels
When you need consistent results

Safety First¶

Testing New Tools¶

Always¶

Start at 1/4 normal speed
Take a 0.001" test cut
Stand to the side
Wear face shield AND safety glasses
Have someone else there if possible

What Can Go Wrong¶

Tool breaks: Sharp pieces fly
Tool grabs: Work gets yanked
Tool bends: Ruins work and maybe crashes
Bad heat treat: Tool goes soft mid-cut

Prevention¶

Calculate cutting forces
Always test on scrap first
Start conservative
Keep documentation
Learn from failures, the best way to learn

Your First Projects¶

Project 1: Simple Fly Cutter¶

2 hour project
Immediate results
Under $20 in materials
Use it forever

Project 2: 60° Threading Tool¶

Grind from HSS blank
Practice tool geometry
Save $50 vs buying
Great for learning

Project 3: Boring Bar Set¶

Make 3 sizes
Learn different geometries
Solve real problems
Build up your confidence in your capability

Economics of Tool Making¶

Example: Radius Form Tool¶

Buy It¶

$300+ custom ground
2-3 week delivery
Single radius only

Make It¶

$5 HSS blank
2 hours labor
Any radius you want
Ready today

Break Even¶

One job

Hidden Benefits¶

No waiting for delivery
Exact size you need
Learn valuable skills
Solve problems in-house
Impress the Boss with your demonstrated capability

Record Keeping¶

Document Each Tool¶

Keep a document with:

Sketch or CAD of the tool
What it's for
Material used
Grinding angles
Speed/feed that worked
Problems encountered

Why¶

You or your protégé will need to make another someday

Getting Started¶

Basic Tool List¶

To start making tools you need:

Bench grinder (6" is fine)
HSS tool blanks
Dial caliper
Protractor or angle gauge
Safety equipment
Most of all, Patience and Perseverance

First Purchase¶

HSS Blank Set¶

About $30 gets you:

Various sizes of square blanks
Ready to grind
Make dozens of tools
Learn without breaking the bank

Final Advice¶

Making your own tools is a journey into the world of Machining. Your first fly cutter will probably look butt-ugly but if it cuts, it's good. Your tenth will be a work of art.

Start simple. Make tools that solve real problems in your shop. Every tool you make teaches you something - about working out geometry, about materials and their behavior, about problem solving.

And there's no feeling quite like using a tool you designed and made to solve a problem that had you stumped. That's not just machining - that's actually engineering, craftsmanship, and creativity rolled into one. You will be a hacker of physical space and time.

Remember: The best tool is the one that gets the job done safely. Pretty comes with practice, but pretty isn't necessary when you are starting out.