Why Leitech Thread Depth Gages?

We searched forums, engineering communities, and quality  control groups to understand how manufacturers actually  check thread depth. What we found was eye-opening.  

One quality inspector with 15 years of experience wrote:  "Most of my experience has been in medical and aerospace  manufacturing and they would NEVER base a thread depth  on turns."  

Another engineer documented getting FOUR different  measurements from FOUR different people — using FOUR  different methods — on the same threaded hole.  

A machinist forum moderator summarized it perfectly: "I've found there really isn't any standard way to  check thread depth."  

There is. But first — here's what most shops are doing instead. 

The most common method in North American manufacturing.  Screw a thread plug gage into the hole, count full  rotations as you back it out, multiply by pitch.  

It sounds logical. It isn't measurement.  

Here's the problem: Every threaded hole has a chamfer  at the entry. Counting starts when the gage first  contacts thread — which includes the chamfer and partial threads that don't contribute to functional  engagement, depending on your designed datum surface. The reference point is inconsistent,  operator-dependent, and shifts every time a different  person performs the check.  

One inspector counts 10.5 turns. Another counts 11.  On a 1.5mm pitch thread, that's 0.75mm of difference  — enough to fail an aerospace assembly or cause a  fastener to bottom out in a medical device.  

And when an auditor asks for documented, traceable  depth data? "I counted the turns" doesn't pass an  ISO 1502 or AS9100 audit. There is no data.  There is no traceability. There is no record.  

Turn-counting is an estimate dressed up as a measurement. 

Measure a bolt's total length. Zero your calipers.  Screw the bolt into the hole until it stops. Measure  what sticks out. Subtract. 

This method shows up on every machinist forum, in  every shop, on every continent. It's clever. It's  resourceful. And it's fundamentally flawed.  

Here's why: Bolts have chamfered tips and lead-in  threads. The reference point — where the bolt  "stops" — changes based on the bolt, the chamfer,  the material, and how much force the operator applies.  Two people using two different bolts on the same hole  will get two different numbers.  

More importantly: this method measures how far a bolt  goes into a hole. That is not the same as measuring  functional thread engagement from the first full  thread ridge — which is what your engineering drawing  actually specifies per ASME Y14.5.  

It's not NIST-traceable. It won't pass an ISO 1502  audit. And it measures nominal hole depth — not  functional thread depth.  

One engineer documented it perfectly on a quality  forum: his shop and his customer both used this  method — and got completely different results on  the same parts. The customer rejected them. 

A workaround is not a measurement system. 

A standard thread plug gage modified with a notch or  step ground in to represent minimum depth. If the  notch disappears below the part face — pass.  If it doesn't — fail.  

This is a legitimate quality control method. It's  traceable, it's standardized, and it's used in  serious manufacturing environments including  aerospace and defense.  

But it has three significant limitations:  

First: Pass/fail only. No numerical value. No  actual depth reading. No data for SPC. No number  to put in your quality record. Just a visual  indication that the hole is either deep enough  or it isn't.  

Second: One notch per depth requirement. Need to  check five different thread depths on the same  thread size? You need five different gages.  Multiply that across a product line and you have  a drawer full of single-purpose tools, each  requiring its own calibration cycle, its own  storage, its own replacement cost.  

Third: It only checks minimum depth. It doesn't  tell you HOW deep the thread is — only that it  meets the minimum. Over-tapping — drilling and  tapping 15% to 40% deeper than necessary — goes  completely undetected. And as the Leitech savings  calculator above shows, that undetected over-tapping  is costing your operation real money every single day.  

A pass/fail check is better than nothing.  A direct numerical reading is better than pass/fail. 

A depth micrometer or digital caliper with a depth  rod is a legitimate precision instrument. Accurate,  traceable, and widely available in every gage lab  and on most shop floors.  

So what's the problem?  

First: It measures hole depth — not functional  thread engagement. A depth mic measures from the  part face to the bottom of the hole. That includes  the chamfer, the partial threads, the full threads,  the thread runout, and the drill point at the bottom.  Per ASME Y14.5, your engineering drawing specifies  minimum full thread depth — not hole depth.  These are not the same number.  

Second: It's a separate operation from size verification.  You still need a thread plug gage to verify functional  size. So now you have two tools, two operations, two  chances for handling errors, and two separate data  entries. In a high-volume production environment,  those extra seconds add up to hours of lost time  per shift.  

Third: Blind holes are awkward. Getting a depth mic  to seat properly on the bottom of a small blind hole  — especially a threaded one — requires skill and  consistency that varies between operators.  

The depth mic is a great tool. It's just not the  right tool for simultaneous, traceable, functional  thread size AND depth verification on a production floor.  

That requires something purpose-built for exactly  that job. 

The CMM is the gold standard of precision measurement.  Programmable, highly accurate, fully traceable, and  capable of capturing complex geometry that no hand  tool can touch.  

It is also completely impractical for production  thread depth inspection.  

CMMs are expensive — a basic unit starts at $50,000  and goes up from there. They require programming,  climate-controlled environments, skilled operators,  and significant setup time per part. They are designed  for first article inspection, PPAP, and laboratory  environments — not for verifying 99 threaded holes  on a V12 engine block coming off a production line  every few minutes.  

One engineer on a quality forum was asked to inspect  minimum effective thread depth on an 18mm internal  thread using a CMM — without sectioning the part.  The responses ranged from "grind the head off a screw  and probe it" to "tell them to count threads."  Nobody had a clean answer because CMMs aren't  designed for this.  

For first article and PPAP — absolutely use your CMM.  For 100% production inspection of functional thread  size and depth simultaneously — you need a Leitech COMBI Gage.  

Fast. Traceable. Direct numerical reading.  One check. Every part. Every shift. 

Everything the other five methods attempt — size  verification, depth measurement, traceability,  numerical data, variable depths, audit-ready  documentation — the Leitech COMBI Gage delivers  in a single operation.  

As the GO member verifies functional thread size,  the calibrated telescoping sleeve contacts the  part face and delivers a direct numerical reading  of functional thread depth — simultaneously.  One check. Two measurements. Zero guessing.  

NIST-traceable from the first full thread ridge.  Calibrated to FED-STD-H28 and ISO 1502.  Available in Standard, Hi-Resolution, Digital,  and Motorized configurations for every production  environment from shop floor to gage lab.  

This is not a workaround. This is not an estimate.  This is not a modified bolt or a counted turn.  

This is a measurement.