Threaded Inserts: Types, Sizing, Callouts and Quoting

A tapped hole is the cheapest way to put a thread in a part, and for most holes in steel, stainless or aluminum that is assembled a few times it is the right answer. An insert earns its extra cost in a handful of situations.

Soft or weak parent material. Aluminum, magnesium, zinc castings and plastics strip at lower loads than the steel screw going into them. An insert spreads the load over a larger diameter of parent material, so the joint can take more torque before the thread pulls out.

Repeated assembly. A thread cut in aluminum wears every time a steel screw goes in and out. Service panels, test fixtures and anything a technician opens regularly are good candidates for a hardened or stainless thread that does not wear the same way.

Repair. A stripped thread in an expensive part can often be drilled out, retapped oversize and fitted with a helical coil or key-locking insert that restores the original thread size.

Material too thin to hold a thread. Sheet metal thinner than a few thread pitches has almost nothing to engage. Self-clinching nuts, rivet nuts and weld nuts give thin sheet a full-length thread.

Plastics and 3D prints. Threads cut or printed directly in plastic tolerate only a few assembly cycles and can creep under load. Heat-set and press-in brass inserts are the usual fix.

If none of these apply, a tapped hole with enough engagement is usually fine. The tapped holes guide covers how deep that needs to be.

The names get used loosely, so it helps to be precise in an RFQ. A dowel pin is not threaded at all, but it shows up in the same secondary operation because it is also hardware pressed into a machined hole.

The single most common mistake with inserts is modeling the hole for the screw instead of the insert. An insert almost always needs a larger or differently sized hole than a tapped thread of the same size, and the exact size comes from the insert manufacturer's installation data, not from a generic tap drill chart.

Helical coil inserts are installed in a hole tapped with an STI (screw thread insert) tap, which is oversize compared with a standard tap of the same nominal size. The drill under it is larger too. Coils are sold in lengths expressed as multiples of the nominal diameter, commonly 1D, 1.5D, 2D, 2.5D and 3D, so an M6 insert at 2D is about 12 mm long. The tapped depth has to be at least the installed insert length, plus room for the tang in a blind hole if a tanged insert is used. Tanged inserts need the tang broken off and removed after install in most cases; tangless versions avoid that step.

Key-locking inserts need a hole tapped for the insert's external thread, which is noticeably larger than the screw, and often a small counterbore so the keys sit flush. They also need more wall thickness around the hole than a coil does.

Heat-set and press-in inserts go into a plain hole, usually slightly smaller than the insert's knurled outer diameter and sometimes tapered to match the insert. Leave the hole a little deeper than the insert so displaced plastic has somewhere to go.

Dowel pins go into holes sized for the fit you want. A press fit in one part and a close slip fit in the mating part is the common arrangement, and the reamed or bored hole size is set from the pin's actual diameter tolerance. Blind dowel holes benefit from a way for trapped air to escape, or from a pull-out style pin if the pin may need removing. The engineering fits chart covers the hole and shaft tolerance combinations.

PEM (or equivalent) self-clinching fasteners are the standard way to add threads, studs and standoffs to sheet metal. They work because the panel material cold flows into a groove on the fastener as it is squeezed in. That mechanism sets a few rules that are worth designing around before the flat pattern is released.

The panel has to be softer than the fastener. The fastener must displace the panel, not the other way around. Standard 300-series stainless clinch hardware is widely used in aluminum and in mild or cold rolled steel, within the panel hardness limits the manufacturer lists. Stainless sheet is usually too hard for that hardware; stainless panels generally call for hardware made for the purpose, typically hardened 400-series or precipitation-hardened stainless types. Check the catalog for the exact panel hardness limit before specifying.

Each part number has a minimum sheet thickness. Below it, there is not enough material to fill the undercut and the fastener will push out or spin. The minimum is listed per part number and varies by thread size and fastener style.

The hole size is specific. Clinch hardware goes in a punched or drilled hole of a stated diameter, with a tight tolerance. An oversize hole, or a heavy chamfer on the install side, reduces the material available to clinch.

Keep the catalog minimum centerline-to-edge distance. Too close to an edge and the displaced material bulges the edge instead of flowing into the fastener. The same thinking applies near bends and formed features: hardware close to a bend line can distort during forming, or may leave no room for the press tooling after forming.

Choose which side it goes in from. Clinch nuts and studs are installed from one side, with the head or shank flush on the other. The drawing needs to say which face the fastener is pressed from, or every one can end up on the wrong face.

Inserts and coatings interact, so the order of operations belongs on the drawing or in the note. There is no single right sequence for every part, but these are the common rules of thumb.

Helical coil and key-locking inserts are usually installed after anodizing, plating or painting. The oversize thread is tapped before finishing, then the insert goes in afterward so the coating does not cover the insert thread. Coating builds up in the STI thread as well, so for thick coatings the shop may mask the holes or chase the threads before install.

Self-clinching hardware is commonly installed after plating or anodizing the panel, which avoids plating the fastener threads and trapping plating chemistry around the fastener. Powder coat and paint are handled both ways: install first and mask the threads, or finish first and keep coating out of the holes so the hole size and clinch are not affected. If your part is powder coated, say which you want or ask the shop to advise.

Heat-set inserts go in after printing or molding and before any painting or coating of the plastic part.

Dowel pins are usually pressed after finishing. Anodize and plating change the hole diameter, so a press fit sized for bare metal can become too tight after coating unless the hole is masked or sized to allow for it.

If you do not specify, the shop will pick a sequence that suits its process. That is often fine, but it is worth stating when threads must gauge after finishing or when coating inside a hole matters. The anodizing vs powder coating guide covers how much each finish builds.

A complete insert callout answers four questions: what hardware, how many, where, and anything about orientation or length that the part number does not settle. The manufacturer part number does most of the work, because it fixes the thread, the material, the length and the required hole. Use the manufacturer part number, such as a clinch nut designation from the hardware catalog, or write "or equivalent" if the shop may substitute.

For threaded inserts, give the installed length as a multiple of diameter or in millimeters or inches, and say whether it is free-running or screw-locking. For clinch hardware, name the side it is installed from. For dowel pins, give the diameter, length, material and how far the pin should stand proud of the surface. A leader to each location, or a hole table with a column for hardware, keeps the count and the positions in one place. The thread callout guide covers the thread designation itself.

Insert hardware is often inexpensive. The installation is where most of the price sits, and it scales with count and variety.

Count. Most inserts are installed one at a time, so price rises roughly with the number of pieces. That is why the total count matters to the quote.

Variety. Every different insert type, size or length is another tool, another setup on the press and another bin to manage. Standardizing on one or two sizes per part saves more than shaving hardware cost.

Insert type. Key-locking inserts cost more per piece and take longer to install than helical coils. Clinch hardware is fast once a press is set up, but special styles such as long standoffs or floating nuts usually cost noticeably more than a plain nut.

Hardware availability. Common sizes in stainless are usually on the shelf. Uncommon lengths, materials or proprietary designs can add lead time, and a specific part number with no equivalent allowed limits the shop's sourcing.

Access and sequence. Hardware near a flange, inside a deep pocket or on a curved face may need special tooling or an extra step. Installing after finishing means the part is handled again after it comes back from the finisher.

Hole accuracy. Dowel pin holes that must be reamed or bored to a precision fit take longer than drilled holes, and the positional tolerance between two pins can drive the setup.

The quote form has a dedicated checkbox for inserts, separate from the one for tapped holes. Ticking the right one, with the right count, is what lets a shop price the hardware and the install time instead of adding it as a change later.

Step 1: Upload the model and set the Process. Add the CAD file and check the Process on each part. Inserts appear under Secondary operations for CNC, Sheet Cutting, Sheet Metal, Die Casting and Metal Stamping. The form suggests a process from the file type, so make sure a bent part with clinch hardware is set to Sheet Metal.

Step 2: Tick Inserts. Open Advanced options on the part and tick Inserts under Secondary operations. Enter the total number of inserts, clinch fasteners, rivet nuts and dowel pins in Total count on this part, counting every piece rather than every type.

Step 3: Tick Threads / Tapped Holes separately if the part also has direct threads. Holes that get a plain tapped thread belong there with their own count. Holes that receive a helical or key-locking insert are counted under Inserts, and it helps to say so in the note so nobody counts them twice. The tapped holes guide covers what to write for those.

Step 4: Write each insert in the Note. For every insert type, give the part number or a description, the quantity, the location, and the details the part number does not settle: installed length, free-running or screw-locking, the install side for clinch hardware, pin protrusion, and whether to install before or after finishing. If you are happy with an equivalent, say so.

Step 5: Attach a drawing. Under Reference Files & Technical Drawings, attach a PDF drawing with the callouts, or a marked-up screenshot with each insert location circled and labeled.

The Inserts help text states the defaults the shop will use if you give no details: helicoils default to stainless steel 1.0×D, and dowel pins to stainless, full depth. Those are reasonable fallbacks, but they are often not what a design needs. A 1.0×D coil is the shortest common length, and in aluminum or magnesium a 1.5D or 2D insert is a frequent choice for more pull-out strength. The default says nothing about free-running or screw-locking, so name it if vibration matters. Stainless dowel pins resist corrosion but are softer than hardened steel pins, which matters for precision location that sees wear. Full depth reads as the pin pressed to the bottom of the hole; if the pin must stand proud to locate a mating part, give the protrusion height. Clinch hardware, rivet nuts and heat-set inserts have no stated default, so a part number or description is needed for those.

For 3D printed and injection molded parts, the process options do not include Secondary operations, so describe heat-set or press-in inserts in the part Note and on the drawing instead.

Here is a part Note for a CNC machined 6061-T6 aluminum housing, clear anodized, that a shop could quote without a follow-up question:

8X helical coil insert M4x0.7, stainless, free-running, 2D (8 mm long), in the cover bosses on the top face, install after anodize. 2X dowel pin 4 mm dia x 12 mm, hardened steel, press fit, 5 mm proud of the top face, holes sized for press fit after anodize. 4X M5x0.8-6H THRU tapped holes for the mounting feet, no inserts. Shop to supply all hardware, equivalents OK. Drawing attached, insert and pin locations on page 2.

On that part, Inserts is ticked with a count of 10 (eight coils and two pins), and Threads / Tapped Holes is ticked separately with a count of 4.

For a sheet metal cover in 1.5 mm 5052-H32 aluminum, powder coated, the same idea looks like this:

4X M3 self-clinching nut, stainless, PEM or equivalent, pressed from the inside face (marked on drawing), catalog hole size. 2X M4 self-clinching stud, 10 mm long, stainless, pressed from the inside face. Install hardware before powder coat and mask threads, or advise if you prefer a different sequence.

That part uses Process Sheet Metal, Inserts ticked, and a count of 6. If you are not sure which insert suits a joint, write the load and how often it will be assembled in the note, and ask the shop to recommend one.