Tapping is the cheapest and simplest way to put a thread in a part, and it's the wrong answer the moment the assembly gets taken apart more than a handful of times or the base material is too soft to hold up on its own.

Quick Summary

• Tapped holes cut threads directly into the part material — cheapest option, but threads in soft metals like aluminum can gall or strip under repeated assembly.
• Heat-set inserts are melted into plastic (typically with a soldering iron or ultrasonic horn), giving plastic parts a durable metal thread that outlasts the plastic itself.
• Helicoils are coiled wire inserts screwed into an oversized tapped hole — the standard fix for stripped or unreliable threads in aluminum, and the default choice in aerospace assemblies.
• A print calling out a thread should specify the standard, class, and depth explicitly (e.g., "1/4-20 UNC 2B x 0.500 MIN FULL THREAD") — leaving any of that off invites ambiguity the same way a vague tolerance callout does.
• Minimum thread depth in aluminum is roughly two times the nominal thread diameter; going shallower risks stripping under normal torque.

When Is a Plain Tapped Hole the Right Call?

When the part material is hard enough to hold threads on its own — steel, most stainless grades, hard aluminum alloys under light duty — and the assembly won't be taken apart more than 10–20 times. Tapping is the cheapest threading method available, works with standard thread sizes and pitches, and needs no secondary hardware. If cost is the top priority and the design doesn't ask for anything unusual, a tapped hole is usually the right default.

When Does a Tapped Hole Start to Fail?

Two situations cause most of the problems: soft base material, and repeated assembly cycles. Aluminum and plastic threads deform a little every time a screw is driven in and backed out — eventually the ridges that grip the screw wear down to nothing, and the hole no longer holds torque. This shows up as a screw that spins freely instead of tightening, usually after fewer disassembly cycles than anyone expected.

What Are Heat-Set Inserts, and When Are They the Right Fix?

A heat-set insert is a small metal sleeve, usually brass, with external knurling and an internal machine thread. It's melted into a plastic part — with a soldering iron for one-off or low-volume work, or ultrasonically for production — so the surrounding plastic reflows around the knurled exterior and locks the insert in place once it cools. This gives a plastic part a genuinely durable metal thread instead of relying on the plastic itself to hold up under repeated screwing.

The design rules matter here: the hole needs to be sized to the insert's specific knurl diameter (guessing causes loose or split parts), the surrounding wall or boss should run roughly two to three times the insert's diameter so it doesn't crack during installation, and the hole should be a little deeper than the insert itself so there's somewhere for displaced material to go.

What's a Helicoil, and Why Does Aerospace Default to It?

A helicoil is a coil of diamond-section wire, screwed into a specially oversized and tapped hole, that forms a smooth, hard, precision internal thread. Instead of the screw bearing directly against the parent material, it bears against the coil — which spreads load across more threads and resists the galling and stripping that plague tapped aluminum specifically. It's also a legitimate repair option: a stripped tapped hole can often be re-tapped oversized and fitted with a helicoil to restore it to its original thread size, rather than scrapping the part.

This is why helicoils show up constantly in aerospace and aluminum structural work — repeated assembly and disassembly over a part's service life is the norm, not the exception, and a coil designed to spread load survives that far better than aluminum threads cut directly into the part.

What Actually Belongs on the Thread Callout?

Beyond deciding which threading method to use, the print needs a complete callout: thread size and pitch, standard (UNC, UNF, or metric), tolerance class, and minimum thread depth or engagement length. "1/4-20 UNC 2B x 0.500 MIN FULL THREAD" tells a machinist everything needed; "tap for 1/4-20 screw" leaves the depth and class up to interpretation, which is exactly the kind of ambiguity that causes a part to come back wrong.

Frequently Asked Questions

How deep does a tapped hole need to be? In aluminum, roughly two times the nominal thread diameter as a minimum — a 1/4-20 thread needs at least 0.500" of engagement. Going shallower risks the threads stripping under normal torque, since there isn't enough engaged thread to distribute the load.

Can a stripped tapped hole be fixed without scrapping the part? Often, yes — re-tap the hole to the next standard oversize and install a helicoil or similar thread insert. This is a standard field and shop repair, not a workaround of last resort.

Do heat-set inserts work in metal parts, or only plastic? Only plastic. The whole mechanism depends on the surrounding material melting and reflowing around the insert's knurled exterior — metal doesn't do that. Metal parts needing a durable insert use helicoils or a similar mechanically-driven insert instead.

Is a helicoil stronger than a plain tapped thread in the same material? For repeated assembly cycles, yes, meaningfully. The coil spreads load across more thread engagement and resists the wear pattern that causes plain aluminum threads to gall and loosen over time — which is exactly why aerospace assemblies default to it rather than tapping directly into structural aluminum.

Need help specifying the right threading method for a part? Our Unified Thread Standard reference covers UNC, UNF, and UNEF sizing, or upload your drawing and we'll help you land on the right callout.

Written by James Wright, eMachineShop

Most hole-related manufacturing problems trace back to three things: depth-to-diameter ratio, thread callouts, and whether the hole was designed for the tool that will actually cut it.

Quick summary: Standard drilled holes should stay within roughly a 4:1 to 6:1 depth-to-diameter ratio for reliable results without specialized tooling. Threaded holes need a clearly specified thread standard and adequate thread engagement length. Hole diameter should match standard drill sizes where possible to avoid custom tooling costs.

What Depth-to-Diameter Ratio Is Safe for Standard Drilling?

Beyond roughly 4–6x the hole diameter in depth, standard twist drills start to wander and struggle with chip evacuation. Deeper holes need gun drilling or peck-drilling cycles — both slower and costlier.

Should You Specify Standard or Custom Hole Diameters?

Standard drill sizes (fractional, number, or letter drills) are stocked and fast. A custom diameter close to — but not matching — a standard size often means a boring operation instead of a simple drill cycle.

How Should Threaded Holes Be Specified?

Call out the full thread standard (e.g., ¼-20 UNC), the thread class, and the minimum thread engagement depth. An underspecified thread callout is one of the most common sources of RFQ back-and-forth.

What About Hole Placement Near Edges or Other Features?

Holes placed too close to a part edge or to each other risk breaking through into an adjacent feature or weakening the wall between them. Standard practice keeps hole edge-to-edge spacing at least one hole diameter.

FAQ

What’s the minimum wall thickness between two holes? A common rule of thumb is at least one hole diameter of material between hole edges, though this varies by material and part rigidity.

Do blind holes cost more than through holes? Often yes — blind holes require depth control and sometimes a flat-bottom finishing pass that through holes don’t need.

Source: ASME Y14.5-2018. Related: CAD Help Guides

Once a part is mostly flat, mostly the same thickness, and doesn’t need tight 3D tolerances, sheet metal fabrication almost always beats CNC on cost — sometimes dramatically.

Quick summary: CNC machining removes material from solid stock, which costs more in both raw material and machine time than forming or cutting sheet metal to shape. The crossover point is usually geometry-driven: flat, simple parts favor sheet metal; complex 3D parts favor CNC regardless of cost.

Why Is Solid-Stock Machining More Expensive by Default?

CNC starts with a block and removes everything that isn’t the part — that’s wasted material and machine time. Sheet metal starts close to final thickness and is cut or formed to shape, wasting far less material.

What Kind of Part Favors Sheet Metal?

Enclosures, brackets, panels — anything that’s essentially a flat shape with bends, punched holes, or simple cutouts. These are sheet metal’s core strength.

What Kind of Part Still Needs CNC, Regardless of Cost?

Parts with 3D pockets, precision bores, or tight positional tolerance on machined features. Sheet metal can’t replicate that geometry no matter how the cost comparison shakes out.

Is There a Volume Threshold That Changes the Answer?

At very high volumes, other processes (stamping, injection molding) may beat both — but between CNC and sheet metal specifically, the crossover is almost always about geometry, not quantity.

FAQ

Can a part be partly sheet metal and partly CNC? Yes — sheet metal weldments are sometimes CNC-finished at critical mounting features, combining both processes.

Is sheet metal less precise than CNC? Generally yes for tight tolerances, though modern sheet metal fabrication (laser-cut, precision-formed) can hold reasonably tight tolerances on flat features.

Related: Sheet Metal Fabrication

Two parts can look identical on a drawing and still come back with quotes 40% apart. That’s not shop inconsistency — it’s usually a tolerance, finish, or material callout that isn’t actually identical once you look closely.

Quick summary: Quote variance between similar-looking parts almost always traces to differences in tolerance, surface finish, material grade, or setup count — not to arbitrary shop pricing. Here’s where to look first.

Are the Tolerances Actually the Same?

A drawing revision that tightened one dimension — even a single hole position — can add a secondary operation the earlier version didn’t need. Under ASME Y14.5-2018, every toleranced feature is an independent cost driver; “mostly the same part” doesn’t mean “the same cost.”

Is the Material Grade Identical, Not Just the Material Family?

6061-T6 and 6061-T651 aluminum aren’t interchangeable in machinability or price, even though both are “6061.” The same applies across steel and stainless grades — a spec sheet difference that looks minor can change cycle time.

Does One Part Need an Extra Setup?

A feature on the back face of one part, absent on the other, can mean a second fixturing pass. That’s not visible at a glance on a 2D drawing — it shows up in the programming.

Is the Finish Callout Really Equivalent?

“Machined finish” and “machined finish, deburred, no visible tool marks” are not the same spec, even if they read similarly. The second one is a manual finishing step.

FAQ

If two RFQs come back with different prices for what I believe is the same part, what should I check first? Compare tolerance callouts feature-by-feature, not just the title block — most variance hides in a single dimension.

Does file format (STEP vs. drawing PDF) affect quote consistency? It can — a shop quoting from a 3D model alone may miss a note that only appears on the 2D drawing. Send both when possible.

Source: ASME Y14.5-2018. Related: Comparing CNC Quotes: Tolerance and Finishing Line Items · AMT’s What Drives the Cost of Precision CNC Aerospace Parts?

Most parts don’t need ±0.001”. Most engineers spec it anyway, because it feels safer than asking whether the assembly actually requires it.

Quick summary: ASME Y14.5-2018 defines tolerance as a functional control, not a quality signal. Standard CNC work comfortably holds ±0.005” without special processing; going tighter should be a deliberate decision tied to fit or function, not a default.

What Does Standard CNC Tolerance Actually Achieve?

Most 3-axis CNC work holds ±0.005” reliably at normal speed and cost. That’s tight enough for the overwhelming majority of mechanical assemblies — brackets, housings, mounting features.

When Does ±0.001” Genuinely Matter?

Press fits, sealing surfaces, and features that stack tolerance across an assembly with no room for variance. In those cases, the tight tolerance is functional and should stay exactly as specified.

What’s the Cost Difference in Practice?

Tighter tolerance usually means slower feed rates, more frequent in-process inspection, and sometimes a secondary finishing pass — real cost, not a rounding error on the quote.

How Do You Decide?

Trace the dimension back to what it controls. If nothing downstream depends on that level of precision, ±0.005” (or looser) is very likely sufficient.

FAQ

Is ±0.001” ever the “safe” choice with no downside? No — it always costs more in machine time and inspection, even when it’s achievable.

What’s the standard CNC tolerance if I don’t specify one? Shop defaults vary; ask your machinist, or see our tolerance and finishing line items guide.

Source: ASME Y14.5-2018. Related: GD&T Reference Guide · AMT’s Why Does Every Aerospace Print Say ±0.001”?

Quantity decides this more than anything else does: roughly 1–50 units points toward 3D printing, 1–500 toward CNC machining, and 500–1,000+ toward injection molding — and getting that threshold wrong is the single most common reason a part costs way more than it should.

Quick Summary

• 3D printing wins at very low quantities (1–50 units) and with complex, organic, or hard-to-machine geometry — no tooling cost, but per-unit price barely drops with volume.
• CNC machining is the sweet spot from roughly 1–500 units, with constant per-part cost, full material properties preserved, and the tightest achievable tolerances of the three.
• Injection molding only pays off once volume clears roughly 500–1,000+ units — the mold itself costs $3,000–$30,000+, but each part afterward is pennies to produce.
• A staged approach — 3D print to prototype, CNC to validate function, injection mold for final production — is standard practice, not a compromise.
• Material and geometry constraints differ sharply: CNC can't reach every internal cavity a printer can; molding needs draft angles and uniform walls that neither of the other two require.

What Actually Decides Which Process Is Right?

Quantity is the first filter, but it's not the only one. Geometry, material, tolerance, and timeline all push in their own direction, and the honest answer is usually "it depends on which of these your project cares about most."

How Many Units Do You Actually Need?

This is the cleanest signal available. 3D printing is most economical for 1–50 units — there's no tooling investment, but the cost per part barely improves as volume increases, since every part still gets printed individually. CNC machining works well from roughly 1–500 units — no tooling cost either, and the cost per part stays essentially flat regardless of quantity, since each part is cut independently from stock. Injection molding flips the model entirely: the mold itself is the large upfront cost, but once it exists, each part costs only material and a few seconds of cycle time — which is why it's uneconomical below roughly 500–1,000 units and dominant above it.

Does Geometry Rule Out Any of These?

Yes, in both directions. 3D printing has essentially no geometric limit — internal channels, organic curves, lattice structures, and undercuts that would be impossible to machine or mold are routine for a printer, because it builds up material layer by layer instead of removing or injecting it. CNC machining is constrained by tool access: a cutting tool has to physically reach every feature, so deep internal pockets, undercuts, and certain internal geometries are off the table or require multiple setups. Injection molding needs draft angles so the part can eject from the mold, plus reasonably uniform wall thickness to avoid warping and sink marks — design rules neither of the other two processes impose.

What About Material and Tolerance?

CNC machining preserves the full mechanical properties of the material, since the part is cut from solid stock rather than melted or built in layers — this matters a lot for metal parts and anything load-bearing. It also holds the tightest tolerances of the three, commonly down to a few thousandths of an inch, which makes it the right call for press fits, bearing bores, and anything that has to mate precisely with another part.

3D printing's tolerance is looser (commonly ±0.1–0.3mm) and mechanical properties can vary by print orientation, since layers bond to each other differently than a single solid piece of material would. Injection molding's tolerance depends heavily on material shrinkage during cooling — commercially typical tolerances run wider than CNC's — and it's genuinely the only one of the three that's a poor fit for tight-tolerance, precision-fit features regardless of how good the tooling is.

Which One Should Come First in a Product's Life?

Often more than one, in sequence. A common pattern: 3D print early concept models to check fit and form quickly and cheaply, move to CNC machining once the design needs to be functionally tested in the actual material, and only commit to injection molding tooling once the design is locked and the volume justifies the upfront mold cost. Treating these as complementary stages rather than competing choices avoids paying for injection tooling on a design that's still likely to change.

Frequently Asked Questions

Can I switch from 3D printing to CNC machining partway through a project without redesigning? Sometimes, but not always cleanly. A shape designed to take advantage of 3D printing's geometric freedom — deep internal channels, organic curves — may need real redesign to become machinable, since a cutting tool can't reach what a printer could build directly. It's worth planning the eventual production process early rather than assuming a printed prototype will translate directly.

Is injection molding ever worth it below 500 units? Occasionally, if the part is extremely simple and the mold is correspondingly cheap, or if a customer specifically needs the exact surface finish and material behavior that only molding produces. But as a general rule, below a few hundred units the upfront tooling cost rarely pencils out against CNC's zero-tooling, constant-per-part pricing.

Does 3D printing get cheaper at higher volumes the way molding does? Not much. Since each part is printed individually regardless of how many identical parts are queued, the per-unit cost stays relatively flat rather than dropping the way injection molding's does once the mold is paid off. This is exactly why 3D printing loses its cost advantage past small batch sizes.

Which process is best for a metal part that needs to bear real load? CNC machining, in almost every case. It preserves full material properties from solid stock, and while metal 3D printing exists, it's a specialized and typically more expensive route reserved for geometries that genuinely can't be machined any other way.

Not sure which of the three fits your part? Upload your design and we'll help you land on the right process for your quantity, material, and tolerance needs.

Written by James Wright, eMachineShop

The process that’s right for one prototype is often the wrong process for 5,000 units — and the reverse is just as true. Knowing when to switch is a cost decision, not a maturity milestone.

Quick summary: CNC machining and 3D printing dominate prototyping because they require no tooling investment. Injection molding and other tooled processes dominate production because per-unit cost drops dramatically once tooling is amortized — but only past a volume threshold that justifies the tooling cost.

Why Is CNC or 3D Printing Usually Right for Prototypes?

No tooling investment. You can iterate a design multiple times without paying for a mold each time you change a feature.

Why Does Injection Molding Take Over at Higher Volumes?

Mold tooling is expensive upfront, but the per-unit cost afterward is very low. Past a certain volume, that trade pays for itself — often faster than most people expect.

How Do You Know When You’ve Crossed the Threshold?

Compare total cost at your expected volume: CNC/3D print cost scales roughly linearly per unit; injection molding has a large fixed cost plus a low variable cost. The crossover point depends on part complexity and material, but it’s a calculable number, not a guess.

What Happens If You Switch Processes Too Early or Too Late?

Switching to tooling too early risks paying for a mold before the design is finalized — expensive if it changes. Staying on CNC too long into production means paying a higher per-unit cost indefinitely.

FAQ

Can a design go straight from sketch to injection molding, skipping CNC prototyping? Technically yes, but it’s risky — any design flaw discovered after the mold is cut is expensive to fix.

Is there a hybrid approach? Yes — some programs use CNC-machined “bridge tooling” to produce a few hundred units while production tooling is still being built.

Related: Injection Molding Services

Every individual part can measure perfectly within its own tolerance and the assembly still won’t go together — because tolerance stack-up isn’t about whether any one part is correct, it’s about what happens when several correct parts’ tolerances compound in the same direction.

Quick Summary

• Tolerance stack-up is the cumulative variation that builds up across a chain of mating dimensions in an assembly, not the tolerance on any single part.
• Every part in the stack can be perfectly in-spec on its own and the assembly can still fail to fit, if the tolerances happen to compound in the same direction.
• Worst-case stack-up analysis (adding every tolerance at its limit) is the most conservative approach; statistical methods (like root-sum-square) reflect real-world variation more realistically but require more analysis.
• The more parts in a mating chain, the more the stack-up matters — a two-part assembly is a much smaller risk than a five-part stack.
• GD&T with proper datums, rather than a chain of independent linear tolerances, is one of the most effective ways to control stack-up directly.

What Is Tolerance Stack-Up, Actually?

When several parts mate together — a shaft through a series of spacers into a housing, for instance — each part’s individual tolerance contributes to the total variation in the final assembled dimension. If every part in that chain happens to land at the high end of its tolerance range, the total variation can be significantly larger than any single part’s tolerance alone, even though every part individually passed inspection. eMachineShop’s own mating parts guidance puts the core rule plainly: the lowest possible dimension of the female part needs to stay greater than the largest possible dimension of the male part, accounting for both parts’ tolerance ranges at once, not just their nominal sizes.

Why Can an Assembly Fail Even Though Every Part Is In-Spec?

Because “in-spec” only guarantees that one dimension falls inside its own tolerance band — it says nothing about how that variation interacts with the variation in the next part, and the part after that. A five-part stack where each part is off by a few thousandths in the same direction can add up to a gap or interference that no individual inspection would catch, because no individual part actually failed its own inspection.

What’s the Difference Between Worst-Case and Statistical Stack-Up Analysis?

Worst-case analysis assumes every part in the chain lands simultaneously at its tolerance limit, in the direction that makes things worst — it’s the most conservative approach and guarantees the assembly works even in an unlikely worst case, but it can force tighter, more expensive tolerances than statistically necessary, since that exact worst-case combination is often statistically rare. This is exactly why it’s worth knowing what the standard default tolerance actually is before assuming every part needs a custom, tighter spec — the baseline is often adequate for most of the parts in a stack, with only the true fit-critical dimensions needing anything tighter.

Statistical methods, like root-sum-square, instead treat each part’s variation as following a distribution and calculate the likely combined variation rather than the absolute worst case. This usually allows for looser, cheaper individual tolerances while still keeping the assembly reliable in the vast majority of real cases — the tradeoff is that a small percentage of assemblies could theoretically still fail, which needs to be an acceptable risk for the application.

How Does the Number of Parts in the Chain Change the Risk?

Directly and significantly. A two-part mating interface has a much smaller stack-up risk than a five-part chain, simply because there are fewer tolerances compounding. This is one of the underappreciated reasons to consolidate parts in an assembly where possible — fewer mating interfaces means fewer opportunities for tolerances to compound into an interference or gap problem.

How Does GD&T Help Control This?

A chain of independent linear tolerances lets each part’s dimension wander independently, with no explicit control over how features relate to each other across the assembly. GD&T, properly applied with clear datums, controls the relationship between features directly — position, concentricity, parallelism relative to a shared reference — which constrains how the variation can combine rather than leaving every part free to drift independently in whatever direction its own tolerance allows. Every dimension having a tolerance, and tolerances applying to the full length of a feature rather than just a sample point, are foundational GD&T rules that exist specifically to close the ambiguity gaps a plain linear-tolerance chain leaves open.

Frequently Asked Questions

Does tolerance stack-up only matter for precision assemblies? No — it applies to any assembly with more than one mating part, though the consequences are more visible in precision work. A loose consumer product might absorb some stack-up without anyone noticing; a bearing assembly or optical mount won’t.

Is a tighter tolerance on every part in the chain always the safest fix? It’s the most conservative fix, but not always the smartest one. Tightening every part’s tolerance drives up cost across the whole chain; a stack-up analysis often reveals that only one or two dimensions in the chain actually need to be tightened to solve the problem, while the rest can stay at standard tolerance.

How do I know if my assembly needs a formal stack-up analysis rather than just tolerancing each part reasonably? If the assembly has three or more mating parts in a critical dimensional chain, or if a failure to fit would be expensive or unsafe, a formal analysis is worth the time. For a simple two-part interface with generous clearance, reasonable individual tolerancing is often sufficient without formal analysis.

Can a stack-up problem be fixed after parts are already made, or does it require a redesign? Sometimes a shim, spacer, or adjustable feature can absorb the accumulated variation without redesigning the mating parts themselves — this is a common practical fix. But it’s cheaper by far to catch the stack-up risk during design than to add a compensating feature after parts are already in production.

Working through a multi-part assembly and want a second opinion on where tolerance stack-up might bite? Upload your drawings and we can talk through the mating dimensions before they’re locked in — our pre-order checklist and cost-reduction guide are both worth a pass first, since “use the loosest applicable tolerance” is exactly the instinct that keeps a multi-part stack-up problem from becoming a multi-part cost problem too.

Written by James Wright, eMachineShop

ITAR controls who is allowed to access certain defense-related technical data and hardware. CMMC governs how that data has to be protected once you have access to it. They’re related, but they answer different questions.

Quick summary: ITAR (International Traffic in Arms Regulations, 22 CFR Parts 120–130) is an export-control regime restricting access to defense articles and technical data to authorized U.S. persons and registered entities. CMMC (Cybersecurity Maturity Model Certification), built on the NIST SP 800-171 security controls, governs how Controlled Unclassified Information must be protected in a contractor’s IT environment. A company can be ITAR-registered without being CMMC-certified, and vice versa, depending on what data it handles.

What Does ITAR Actually Control?

ITAR restricts the export — including disclosure to foreign persons — of defense articles and associated technical data. Registration under ITAR is a prerequisite for legally handling that category of information at all.

What Does CMMC Actually Control?

CMMC is a cybersecurity certification framework, built on NIST SP 800-171, that verifies a contractor’s IT systems and processes meet defined security-control levels for protecting Controlled Unclassified Information (CUI).

Can a Company Have One Without the Other?

Yes. ITAR registration is about legal eligibility to handle defense technical data. CMMC is about whether your systems are secure enough to protect that data once you have it. A company can be ITAR-registered while still working toward CMMC certification, or vice versa depending on contract requirements.

Why Do Both Matter to a Customer Choosing a Machine Shop?

If a program requires a shop to receive ITAR-controlled technical data, the shop needs both — the legal eligibility under ITAR and, increasingly, the security posture required by CMMC — to be a viable long-term partner.

FAQ

Does ITAR registration automatically mean CMMC compliance? No — they’re governed by different regulations and evaluate different things (legal eligibility vs. cybersecurity posture).

Which one applies to unclassified technical data specifically? Both can, depending on the data’s classification — ITAR governs export control of defense articles/data broadly, while CMMC governs protection of CUI specifically.

Sources: 22 CFR Parts 120–130 (ITAR); NIST SP 800-171 (basis for CMMC controls). Related: eMachineShop is ITAR Registered and JCP Certified — see Our Company.