A Collapsing Tap is a multi-piece machine tap whose thread-cutting chasers retract radially inward at the end of the cut, so the tap pulls straight out of a finished thread without reversing the spindle. Production shops run them at 200-800 surface feet per minute on turret lathes and dedicated tappers — roughly 3 to 5 times the cycle speed of a solid tap that has to spin backwards to unscrew. The mechanism solves the wasted reverse-rotation time on long internal threads and is standard kit on parts like coupling sleeves and pipe fittings, including those run on Landis and Geometric heads.
Collapsing Tap Interactive Calculator
Vary thread depth, pitch, retract RPM, and batch size to see reverse turns eliminated and cycle time saved by a collapsing tap.
Equation Used
The calculator multiplies thread depth by TPI to find the reverse spindle turns a solid tap would need to unscrew, then divides by retract RPM and converts minutes to seconds. A collapsing tap eliminates that reverse-rotation phase by retracting the chasers inward and withdrawing straight out.
- Inch/TPI units are used, matching the worked example.
- Solid tap retracts by reversing one full thread lead per spindle revolution.
- Retract RPM is steady and acceleration time is ignored.
- Collapsing tap withdrawal time is treated as zero added reverse-rotation time.
Inside the Collapsing Tap
A solid tap cuts a thread, then has to spin backwards the same number of turns to come out. On a 2-inch-deep 3/4-16 thread that's 32 reverse turns of dead time per part. A Collapsing Tap kills that overhead. Four to six chasers — the actual thread-cutting blades — sit in radial slots in the tap head. While the tap is cutting, a yoke or trigger ring holds those chasers locked at full diameter. When the head reaches the programmed depth, a stop collar contacts the workpiece face, the yoke trips, and a spring or cam pulls the chasers inward by 1 to 3 mm depending on size. The tap then withdraws straight out without unwinding a single turn.
The geometry has to be right or the part is scrap. Chaser lead must match the workpiece thread within ±0.0005 inch per inch — get this wrong and you bell-mouth the thread or strip the last turn. The trip timing matters just as much: if the chasers collapse 0.010 inch too early, the last thread is incomplete; if they collapse 0.010 inch too late, the chasers gouge the finished thread on retraction and you'll see a spiral scratch down the bore. After the cut, the operator pulls a reset lever (or a kickout pin on the next forward stroke re-engages the yoke) which snaps the chasers back to cutting diameter, ready for the next part.
Failure modes are usually mechanical wear in the trip mechanism. Worn yoke faces let the chasers collapse under cutting load — you'll see threads that taper smaller toward the bottom of the hole. Weak collapse springs leave chasers dragging on retract. Chipped chaser leads (the first 1.5 threads, where the cutting actually happens) ruin lead accuracy and need regrinding or replacement long before the rest of the chaser body is worn out.
Key Components
- Chasers: The 4-6 thread-cutting blades that sit in radial slots around the tap body. Each chaser carries a few threads of the target form — typically 4-8 threads with a 1.5-thread chamfered lead. Lead must match within ±0.0005 in/in or the thread bell-mouths.
- Yoke (Trigger Ring): Holds the chasers locked at cutting diameter against the spreading force of the cut. When the head bottoms against a depth stop, the yoke disengages and releases the chasers inward. Yoke face wear is the #1 cause of tapered threads on collapsing taps.
- Collapse Springs: Pull the chasers radially inward once the yoke trips. Typical inward stroke is 1-3 mm depending on tap size. A weak spring leaves chasers dragging on the finished thread during retract — you'll see scoring.
- Depth Stop Collar: Contacts the workpiece face or a fixture stop to mechanically trigger collapse. Adjustable in 0.001-inch increments on Landis and Geometric heads. Sets the actual thread length on the part.
- Reset Lever / Kickout: Returns the chasers to cutting diameter and re-engages the yoke after retract. On Geometric heads this is automatic on the next forward stroke; on Landis it's a manual lever. A sticky reset is the most common reason for a missed tap on a production cycle.
- Tap Body / Holder: Carries the chasers and houses the yoke and springs. Mounts in a floating holder that allows the tap to follow its own lead — the holder must give roughly 0.030 inch axial float so the chasers, not the spindle feed, control thread pitch.
Industries That Rely on the Collapsing Tap
You see Collapsing Taps anywhere a shop runs medium-to-large internal threads in volume — pipe couplings, hydraulic fittings, pump bodies, valve bonnets, oilfield tool joints. They earn their cost on threads deeper than about 1.5× diameter, where reverse-rotation time on a solid tap starts dominating cycle time. Below 1/2 inch they're rare; above 6 inches they become almost the only practical option short of single-point threading. Geometric and Landis built the bulk of the installed base in North America, and you'll still find their heads running production decades after install.
- Oil & Gas: Cutting box-end API tool joint threads on drill collar pup joints — Geometric DA-style collapsing heads on a Lodge & Shipley engine lathe handle 4-1/2 IF connections in a single pass.
- Hydraulic Components: Tapping SAE J514 ports on Eaton Vickers V20 vane pump bodies — the collapse cycle keeps tapping time under 8 seconds per port across a 6-port casting.
- Pipe Fittings: Running NPT 1-1/4 to 4-inch threads on cast iron couplings at Anvil International — Landis 5C heads on dedicated pipe-tappers cycling at 18-22 parts per minute.
- Valve Manufacturing: Cutting bonnet threads on Crane gate valves — collapsing taps in a turret station on a Warner & Swasey #4 chucker eliminate the spindle reversal on 2-inch threads.
- Heavy Equipment Repair: Re-tapping worn coupling threads on locomotive traction motor pinion sleeves at a Class I railroad backshop — Geometric collapsing heads in a horizontal boring mill spindle.
- Aerospace Ground Support: Threading hydraulic test stand manifold blocks for AN-style fittings — collapsing taps on a Cincinnati Milacron Sabre 750 mill cut 7/8-14 threads without thread-mill programming overhead.
The Formula Behind the Collapsing Tap
The number that matters most on a Collapsing Tap is cycle-time savings versus a solid tap. At the low end of typical use — short threads under 1× diameter deep — the savings barely pay for the head. In the sweet spot of 1.5× to 4× diameter at moderate-to-high RPM the collapsing tap can cut total threading time by 60-75%. At the high end, on threads deeper than 5× diameter, you're approaching the practical limit where chip evacuation becomes the bottleneck and additional speed gains flatten out. The formula below estimates time saved per part by skipping the reverse-rotation phase.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| tsaved | Time saved per part by eliminating reverse rotation | seconds | seconds |
| Lthread | Length of thread cut into the part | mm | inches |
| ntpi | Threads per unit length (pitch density) | threads/mm | threads per inch |
| Nrev | Spindle speed during retract phase of solid-tap cycle | RPM | RPM |
Worked Example: Collapsing Tap in a railcar-coupler rebuild shop tapping draft-gear pocket threads
A railcar-coupler rebuild shop in East Chicago is running 1-1/4 inch — 7 UNC threads, 2.5 inches deep, into AAR-grade cast steel draft-gear pocket housings on a Bullard Cut-Master vertical turret lathe. They're deciding whether to retrofit the tapping station with a Geometric DSA-3/4 collapsing head. Solid-tap retract speed sits at 400 RPM. Production target is 240 parts per shift.
Given
- Lthread = 2.5 inches
- ntpi = 7 threads per inch
- Nrev = 400 RPM (nominal retract speed)
Solution
Step 1 — at the nominal 400 RPM retract speed, count the reverse turns the solid tap must spin out:
Step 2 — convert that to time saved per part if those reverse turns disappear:
Step 3 — at the low end of the typical retract-speed range, 200 RPM (which is what you'd see on a worn machine or a heavy-duty cycle on tougher material), the same thread takes longer to back out:
That doubles the savings — on a slow machine the collapsing tap pays back even faster. Across 240 parts per shift that's 21 minutes back, or roughly one extra changeover window per shift.
Step 4 — at the high end of typical retract speed, 800 RPM (a fast modern CNC tapper retracting in rigid-tap mode):
Still real money over 240 parts — about 5.2 minutes per shift — but you can see the curve flattening. Above roughly 1000 RPM the savings drop below 1 second per part and the collapsing tap stops being the obvious choice unless thread quality or chip control demand it.
Result
Nominal time saved is 2. 625 seconds per part, which over 240 parts per shift is 10.5 minutes — about 4% of a shift returned as productive cutting time, enough to justify the head retrofit inside a year on this volume. The range tells the real story: at 200 RPM retract you save 5.25 seconds per part (the head pays for itself in months), at 400 RPM nominal 2.6 seconds, and at 800 RPM only 1.3 seconds where the economics start to tighten. If the shop measures actual savings well below 2 seconds per part on this job, suspect three things: (1) the depth stop collar is set 0.030 inch shallow so the head trips early and the spindle has to creep down further before retract — fix by re-shimming the collar, (2) the floating tap holder is binding axially and not letting the tap follow its own lead, which forces the operator to slow feed, or (3) chasers with worn lead-chamfer threads are pulling extra cutting load and forcing a slower forward feed to avoid stripping — pull the chasers and check the first 1.5 threads under a 10× loupe.
Choosing the Collapsing Tap: Pros and Cons
A Collapsing Tap isn't always the right answer. Solid taps, thread mills, and self-opening die heads all overlap with parts of its application range. The real decision turns on thread length, production volume, and how tight your thread tolerance has to be.
| Property | Collapsing Tap | Solid Machine Tap | Thread Mill |
|---|---|---|---|
| Cycle time on 2× dia thread depth | Fast — no reverse rotation, 3-5 sec saved per part | Slowest — reverse equals forward time | Slow on small threads, competitive on large |
| Thread accuracy (lead error) | ±0.0005 in/in with sharp chasers | ±0.0003 in/in (lead is built into tap body) | ±0.0002 in/in (lead is interpolated by CNC) |
| Tooling cost (1-1/4 inch UNC) | $1,200-2,500 head + $200-400 chaser sets | $80-200 per tap | $150-400 per insert mill + $25-60 inserts |
| Setup complexity | Medium — depth stop and reset must be tuned | Low — drop in and run | High — requires CNC programming and helical interpolation |
| Practical thread size range | 1/2 inch to 12+ inches | #0 to 4 inches | Any size the cutter fits, but slow below 1/2 inch |
| Blind hole capability | Excellent — tap retracts without unwinding through chips | Poor on deep blind holes — chips re-cut on reverse | Excellent — chips fall away during interpolation |
| Resharpening / regrind | Chasers reground or replaced individually | Whole tap reground or scrapped | Inserts indexed or replaced |
| Best volume range | 100+ parts per run | 1-100 parts | 1-50 parts or thread sizes outside tap range |
Frequently Asked Questions About Collapsing Tap
That's almost always yoke wear letting the chasers walk inward under cutting load as the tap penetrates. The chasers start at full diameter, but as cutting torque rises with depth, a worn yoke face lets each chaser creep 0.001-0.003 inch radially inward. By the bottom of the hole the effective pitch diameter has dropped enough that a go-gauge passes deeper than it should.
Quick check: tap a test piece, then pull a thread plug gauge through. If the gauge enters easy at the bottom but drags at the top, the yoke is worn. Replace the yoke or send the head back for rebuild — Geometric and Landis both still service their older heads, and a yoke rebuild runs a fraction of a new head.
Pick the thread mill when your volume is under about 50 parts per run, when you need pitch tolerance tighter than ±0.0005 in/in, or when you're cutting an exotic alloy where chaser life is short. The thread mill spreads the cutting load across many flute passes and won't bell-mouth — collapsing taps depend entirely on the chasers running concentric to the bore, and any fixture slop transfers straight into the thread.
Pick the collapsing tap when you're past 100 parts and the thread is a standard form (UNC, UNF, NPT, API). At that volume the cycle-time advantage — 3-5 seconds per part — outruns the thread mill's flexibility.
You can, but you'll fight it. The chasers cut their own lead, and if the spindle feed doesn't match that lead exactly, something has to give — usually the thread, in the form of a torn first thread or a stripped last thread. A rigid mount forces the workpiece, the tap, or the spindle to absorb the mismatch.
The fix is a floating holder with roughly 0.030 inch of axial give and a few thousandths of radial float. On a CNC lathe in rigid-tap mode you can run rigid if the control synchronises feed-per-rev to within 0.0002 in/rev of the chaser lead — anything looser and you need the float.
Three common causes, in order of frequency. First, the pilot bore is undersized — a standard collapsing tap expects the drilled hole within +0.005 / -0.000 of nominal minor diameter. A bore 0.010 inch undersize forces the lead-chamfer threads to take a cut they were never designed for, and they chip within 50-100 parts.
Second, interrupted cuts. If your part has a cross-hole or oil port intersecting the tapped bore, the chaser lead slams into the edge on every revolution and microchips. Either chamfer the cross-hole heavily or switch to a solid spiral-flute tap for that feature.
Third, coolant starvation at the lead. The lead is where 80% of the cutting work happens — if coolant isn't reaching the bottom of the chamfer, you're work-hardening the cast steel ahead of the cut and the chasers fail in tension, not abrasion.
Run a tap-only cycle on a sacrificial slug of the same material with the depth stop intentionally set 0.050 inch shallow, then measure thread length with a depth mic against the chamfer. Step the stop in 0.010 inch at a time until thread length matches the print. The collapse needs to happen 0.5 to 1 thread before the chasers bottom out — set it any tighter and you'll crash the chasers into the bore floor on the first part where stock thickness varies.
On Geometric heads the stop adjusts in 0.001 inch increments via the knurled collar at the rear of the body. Lock the collar with the setscrew after every adjustment — vibration will walk it loose inside a shift.
Heat is loading up the collapse springs. As the head warms through the run, the yoke and chaser slots expand at slightly different rates, and chip-pack inside the radial slots builds up enough to bind the chasers in their retracted position. The springs are sized for clean, cold operation.
Pull the head, blow the slots out with compressed air, and check spring free-length against spec — if any spring is more than 5% short of nominal free length, replace the full set. Don't replace one spring; uneven spring force pulls the chasers in unevenly and you'll see one quadrant of the thread cut deeper than the rest.
For threads under about 3/4 inch and thread depths under 1.5× diameter, yes — modern rigid-tap cycles on a Mazak or DMG Mori turret retract fast enough that the time saving from collapse is small. Above that size, no. Spindle inertia on a 4-inch tap means the reverse acceleration alone eats 0.5-1.0 seconds, and the cutting torque on a 2-inch UNC thread is high enough that rigid-tap synchronisation errors start damaging threads.
The crossover where collapsing taps clearly win is 1 inch diameter and up, threads deeper than 1.5× diameter, and runs over 100 parts. Below that the CNC rigid-tap is usually simpler.
References & Further Reading
- Wikipedia contributors. Tap and die. Wikipedia
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
