Fasteners are a design decision, not an afterthought
The joints in an assembly are usually decided last, once the geometry has been settled, and that order is backwards. A thread, an insert or a clearance fit is a feature that has to be machined, inspected and assembled, and the choice made at the very end of the design is the one most likely to force a drawing change after the first article is cut.
Treating the joint as a design input also exposes its cost early. A bolt into a tapped hole, a screw into an insert and a clearance hole with a nut are three different levels of machining effort on the same face, and the cheapest one that meets the function is worth choosing deliberately rather than by habit.
Threaded holes: cut, inserted, or bolted through
A tapped hole is the simplest option and the right one for most metal parts, provided the material and the depth suit it. Aluminium threads strip more readily than steel, so a shallow engagement in a soft alloy is a repair waiting to happen; the usual answer is a deeper thread or an insert rather than a larger bolt.
Where a joint will be opened repeatedly, or where the parent material is too soft to hold a thread, an insert carries the load over more material. It adds an operation and a purchased part, so it earns its place where the thread must survive many assembly cycles or where a stripped thread would scrap an expensive component.
A through-bolt with a nut is the most tolerant arrangement of the three, because the nut carries the thread and the machined part only needs a clearance hole. The price is access to both faces, which is a design constraint worth checking early rather than discovering at the bench.
Clearance holes, and the fit that matches the function
A clearance hole is a tolerance decision as much as a fastener decision, because it has to absorb the positional variation of both parts it joins. Three classes cover most work, and they are chosen for what the joint has to do rather than for quality.
A close fit locates the parts and can remove the need for separate dowels or shoulders, which makes it the right choice where alignment matters and the parts are light enough to position by hand. It leaves little room for error, so both parts must be held to their hole positions.
A medium fit is the general-purpose choice for most machinery. It assembles without force and still keeps the parts from wandering, which is why it appears on the majority of bolted joints in a frame or a housing.
A free fit absorbs positional error and makes assembly quick, which suits large, heavy or awkward parts that are hard to align. Specifying the fit rather than a nominal hole size is what stops a fitter from reaming a hole that was meant to be loose and turning a tolerant joint into a precise one.
Inserts for plastics and soft metals
A moulded or machined plastic part usually cannot hold a thread that will be used more than a few times, so a metal insert takes the load instead. The choice is between a heat-set insert pressed in with heat, a moulded-in insert placed before the plastic is shot, and a self-tapping insert cut into the plastic afterwards.
Each shifts work and cost. A moulded-in insert adds tooling and handling at the moulder; a heat-set or self-tapping insert moves the operation to the assembly floor and is easier to change late. Where the part is expected to be serviced, designing the boss and the insert together - enough wall around the insert, enough depth for the thread - matters more than which type is chosen.
Standard fittings versus custom machined hardware
Not every joint needs a custom part. A great deal of machinery is assembled from standard hardware - brackets, clamps, hinges, connectors - that has already been designed, tested and priced at volume, and reaching for a standard part rather than machining a bespoke one is often the cheaper engineering decision.
Studying how commodity hardware solves the same problems is a fast way to shortlist an approach: a mature, mass-produced fitting shows how the shape is driven by a specific load and environment rather than by appearance alone. Where a standard fitting meets the function, that is a machined part you do not have to design, make or inspect.
The judgement is the same as elsewhere: use a standard part where one meets the load and the environment, and machine a custom part only where the standard options force a compromise in geometry, tolerance or material.
Assembly sequence and access
A joint that cannot be reached with a wrench is a joint that will be assembled badly. Tool access, wrench swing and the order in which parts go together belong in the design review, because they are far cheaper to fix on screen than on the bench.
Where a fastener is awkward to reach, the design has a choice: move the fastener, add an access hole, or change the joint so the fastening happens before the parts are closed up. All three are legitimate; the failure mode is leaving the decision to the person assembling the first unit, who will find a way that works once and cannot be repeated.
What belongs on the drawing
A drawing that supports the joint lists the thread callout with its class, the depth of engagement, the clearance hole size and class, the insert type if one is used, and the torque if it is specified. It also names the mating part where the fit depends on it, so the machinist and the inspector are working to the same reference.
The most common omission is the fit class. A hole dimension with no fit specified gets made to whatever the machinist considers reasonable, and two parts made to different assumptions may not assemble. Stating the fit, the thread and the datum costs a line on the drawing and removes a class of dispute.
Thread and fit standards
Thread callouts are not free text. A fastener drawing refers either to the Unified Thread Standard or to the ISO metric screw thread, and the hole that receives it is a fit governed by the same dimensioning rules as any other feature (ASME Y14.5). Naming the standard turns a thread note from a habit into a specification.
Frequently asked
Should I specify a thread insert or a tapped hole?
Use a tapped hole where the material holds a thread and the joint is not opened often. Use an insert where the parent material is soft, where the thread will be reused many times, or where a stripped thread would scrap an expensive part. The insert costs more upfront and less in repairs.
What clearance hole size should I use?
It depends on the fit the joint needs. A close fit for location, a medium fit for general assembly, a free fit where positional error has to be absorbed. Specify the class on the drawing rather than a nominal size, so the hole is made for the function rather than by habit.
When is a standard fitting better than a machined part?
When a standard bracket, clamp or connector meets the load and the environment without forcing a compromise. A standard part is already designed, tested and priced at volume, so machining a bespoke equivalent only makes sense where the standard options do not fit the geometry, tolerance or material.