Internal Thread Crusher: How It Works and When to Use It

• 1
  What an Internal Thread Crusher Actually Does
• 2
  Hole Size, Lobes, and Feed: The Three Numbers That Matter
• 3
  Where Forming Wins and Where It Breaks Down
• 4
  How to Inspect a Formed Internal Thread
• 5
  Machine Setup and Common Failure Modes
• 6
  Step by Step: Setting Up a Forming Operation
• 7
  Internal Thread Crusher vs Cut Tap: Which Fits the Job

An internal thread crusher is a forming tool, not a cutting tool. Its lobes have no cutting edges. The tool is fed into a pre-drilled hole, and the lobes cold-work the metal outward into the minor diameter. The material flows into the thread profile instead of being removed. No chips come out of the hole.

That single difference drives most of the engineering consequences. Because the grain structure is deformed rather than severed, the thread roots carry a compressive stress layer. Fatigue life in the threaded section usually improves. The surface finishes better too, often landing in the Ra 0.8–1.6 μm band straight off the tool.

The trade-off is process control. Forming a thread requires the hole to be the right size before the tool ever touches it. Too small, and the lobes stall or snap. Too large, and the crests come out incomplete. On a CNC machine with rigid synchronized feed, holding that window is routine. On a manual setup with a floating tap holder, it is not.

The tool also needs a bottom clearance. Forming lobes push material ahead of them, so a blind hole needs a deeper drill depth than a cut thread would. Many first-time failures on a new part trace back to this one dimension.

• 1
  No flute, no chipEliminates chip evacuation problems in blind holes.
• 2
  Cold-worked rootAdds compressive residual stress at the thread root.
• 3
  Tighter hole toleranceDrilled hole diameter becomes a critical feature.

The pre-drill diameter for forming is not the same as for cutting. Forming taps and crushers need a larger hole, because the material has to flow into the flanks. A typical rule is to take the nominal pitch diameter and subtract roughly half the thread pitch, then verify against the tool maker's chart. Get this wrong by 0.05 mm and the thread class moves.

Lobe count changes the load per lobe. A three-lobe tool on a small hole spreads forming force over fewer contact points, which raises torque. A six-lobe tool spreads it further and reduces peak load, but the lobes are smaller and less able to move heavy material. For holes under Ø6 mm in stainless, three or four lobes is common. For aluminium, more lobes run cooler.

Feed and speed follow the forming window. Synchronized feed must match the thread pitch exactly, or the lobes rub instead of roll. Cutting speed typically runs 20–40% below what a cut tap would use in the same material. Stainless 304 and 316 sit at the low end; 6061 aluminium runs near the top.

Cooling matters more than most people expect. Forming generates heat by deformation, not by shear at a cutting edge. A good lubricant reaches the lobes and keeps the material from tearing. On titanium and Inconel, form tapping is often the wrong process entirely.

• 1
  Pre-drill diameterLarger than a cut-tap drill; verify per tool chart.
• 2
  Lobe countMore lobes, lower peak torque, smaller lobes.
• 3
  Sync feedMust equal pitch; rigid tapping is preferred.

Aluminium is the easy case. 6061, 6061-T6, 6082, and 7075 all form well. 7075 is stronger and less ductile, so the pre-drill window narrows, but it still forms cleanly with good lubricant. Die-cast ADC12 is more variable because of porosity. A pore sitting under a lobe can open into a torn flank.

Stainless is where the process earns its keep in medical and food equipment. 304, 316, and 316L form threads with a smooth, gapless root that is easier to clean and less likely to trap product residue. 17-4PH in the solution-treated condition forms well; after aging to high hardness, it does not.

Copper and brass form beautifully. C36000 free-machining brass is almost ideal, though the lead content matters for some end uses. Beryllium copper forms but needs attention to dust control during any subsequent grinding.

The materials that stop the process are the hard and the gummy. Tool steel above roughly 35 HRC will not flow. Titanium alloys like TC4 (Ti-6Al-4V) tend to gall and seize on the lobes. Nickel alloys such as Inconel work-harden faster than the lobes can move material. In those cases, a cut tap or thread mill is the right answer.

• 1
  Best fitAluminium, brass, low-carbon and stainless steels.
• 2
  Use cautionDie castings with porosity, high-hardness alloys.
• 3
  AvoidTitanium, Inconel, hardened tool steel.

Go and no-go gauges still work, but they tell you less than they do on a cut thread. A formed thread can pass the gauge and still have a torn crest that shows up later in a leak test. Visual inspection under low magnification catches most of this.

Pitch diameter is the number to watch. Forming tends to push the pitch diameter slightly large at the start of the hole and slightly small at the bottom, because the material has to flow further as the lobes progress. If the part has a critical thread class, check at both ends.

The minor diameter should come out fully formed. An incomplete minor diameter means the pre-drill was too large. A chipped or galled lobe leaves a repeating mark at every lobe spacing, which is easy to spot once you know the pattern.

If the thread is going into a pressure or fluid path, a thread root that is smooth and continuous is worth more than a slightly tighter tolerance band. That is often the real reason a shop switches to forming in the first place.

• 1
  Check both endsPitch diameter drifts along the hole depth.
• 2
  Watch the crestTorn crests pass gauges but fail leak tests.
• 3
  Repeat marksLobe damage shows at every lobe spacing.

Rigid tapping is close to mandatory. A tension-compression holder that lets the tool float will not hold the feed, and the lobes will rub. On a CNC mill or lathe with synchronized spindle and feed, the process runs reliably. On older machines without sync, results vary part to part.

Tool runout is the other silent killer. A crusher running 0.03 mm off center loads one lobe harder than the rest. That lobe wears first, then breaks. Indicating the tool holder before every run is cheap insurance.

The most common failure is a snapped tool in a blind hole. It usually traces to a pre-drill that is undersized, a bottom clearance that is too shallow, or a feed that drifted out of sync. When it happens, the part is often scrap, because a broken forming tool is work-hardened and hard to drill out.

Lubricant choice matters more than on a cut tap. A high-pressure forming lubricant with extreme-pressure additives keeps the lobes from tearing. Water-soluble coolant can work, but concentration should sit at the high end of the range.

Cycle time is usually shorter than cut tapping. No reversal dwell, no chip clearing, and higher feed rates are possible once the process is dialed in.

• 1
  Sync tappingRigid feed, no floating holder.
• 2
  Runout under 0.02 mmIndicate the holder before the run.
• 3
  High-EP lubricantPrevents galling and torn flanks.