What’s the Difference?
Cutting taps and form taps both create internal threads, but they do it in completely different ways — and they require different drill sizes. Confusing the two is one of the most common (and costly) mistakes on a shop floor.
Cutting Taps (Chip-Producing)
Cutting taps work exactly like you’d expect: they have flutes that cut and remove material to create the thread form. The chips are either pushed ahead (spiral point/gun taps) or pulled back (spiral flute taps).
When to Use Cutting Taps
- Brittle materials — cast iron, brass, bronze, brittle plastics
- Through holes (spiral point) or blind holes (spiral flute)
- Large threads — form taps above 1″ are uncommon
- Low-volume or manual work — more forgiving of speed/feed variation
- Hard materials — form taps need ductile material to work
Advantages
- Works in almost any material
- Lower torque requirement
- Available in every size and thread form
- Easier to resharpen
Disadvantages
- Produces chips (chip management/evacuation needed)
- Threads are weaker than form-tapped threads (cut grain structure)
- Tap can break if chips pack in the flutes
- Surface finish is rougher
Form Taps (Roll Taps / Chipless Taps)
Form taps have no flutes and no cutting edges. Instead, they have lobes that displace and cold-form the material into the thread shape. The metal flows into the thread form rather than being removed. Think of it like a tiny rolling mill inside your hole.
When to Use Form Taps
- Ductile materials — aluminum (especially structural alloys like 6061, 7075), mild steel, copper, low-carbon steel, stainless steel
- High-volume production — form taps last 3-10x longer than cutting taps
- Chip-sensitive operations — zero chips means no chip packing, no re-cuts, no stuck taps
- Stronger threads needed — cold-forming creates continuous grain flow, producing threads 10-30% stronger than cut threads
- CNC environments — consistent, predictable, excellent for unattended operation
Advantages
- No chips — the #1 reason shops switch. Zero chip evacuation problems.
- Stronger threads — cold-worked grain structure, not cut
- Longer tool life — 3-10x more holes per tap vs. cutting
- Better surface finish — burnished thread flanks
- No re-cutting on reversal — the tap can’t grab chips on the way out
Disadvantages
- Higher torque — displacing material takes more force
- Material limitation — doesn’t work in brittle materials (they crack instead of flowing)
- Requires proper lubrication — tapping fluid or oil is critical
- Different drill size — larger hole required (see below)
- Not available for all sizes — uncommon above 1″ diameter
The Critical Difference: Drill Sizes
This is where shops get burned. Form taps require a larger pre-drilled hole than cutting taps for the same thread.
Why? A cutting tap removes material — it needs enough material to cut a full thread profile. A form tap pushes material — it needs less material in the hole wall because the metal will flow into the thread shape.
⚠️ The Most Dangerous Mix-Up in Threading
Form tap drill + Cutting tap = Stripped threads. The hole is too big. The cutting tap will produce shallow, weak threads that can fail under load with no visible warning.
Cutting tap drill + Form tap = Broken tap. The hole is too small. The form tap has to displace too much material, torque spikes, and the tap snaps — usually stuck in your part.
Example: 1/4-20 UNC
| Parameter | Cutting Tap | Form Tap |
|---|---|---|
| Drill Size | #7 (0.2010″) | #1 (0.2280″) |
| Thread Engagement | ~75% | ~65% |
| Hole Difference | 0.027″ — that’s the margin of error | |
That 0.027″ difference doesn’t sound like much, but it’s the difference between a thread that holds and one that strips.
Real-World Application: Structural Aluminum
In aerospace and structural aluminum fabrication, form taps are often the standard, not the exception. When you’re running thousands of 6061-T6 or 7075-T6 parts on a CNC, form taps make sense:
- Aluminum is ductile and forms beautifully
- Zero chips means no chip packing in blind holes
- Cold-formed threads in aluminum are significantly stronger
- Tool life in aluminum with form taps can exceed 10,000 holes per tap
- Unattended CNC operation is more reliable (no chip birds nests)
Some shops doing structural aluminum run 100% form taps with zero cutting taps in the building. When you’re in that environment, the form tap drill chart is second nature. But when a machinist from a general job shop walks in, they’ll reach for the cutting tap drill chart — and that’s where threads get stripped.
Best Practices
- Label your drill charts. Post separate charts for cut and form, clearly marked.
- Color-code your taps. Many manufacturers use different ring colors — learn your shop’s system.
- Program it into the CNC. The tool table should call out tap type AND matching drill.
- Train new hires explicitly. This isn’t obvious to someone who learned on cutting taps.
- Use our Tap Drill Size Lookup Tool — it has a cut/form toggle built in.
Quick Reference: When to Use Which
| Scenario | Use |
|---|---|
| Aluminum (any alloy) | Form tap ✅ |
| Cast iron | Cutting tap |
| Mild/low-carbon steel | Either (form preferred in production) |
| Stainless steel | Cutting tap (300 series can be formed) |
| Brass / Bronze | Cutting tap (too brittle to form) |
| High-volume CNC | Form tap ✅ |
| Manual tapping | Cutting tap |
| Blind holes in ductile material | Form tap ✅ (no chips to pack) |
| Thread size > 1″ | Cutting tap (form taps rare above 1″) |