La holgura afecta la unión de caja y espiga.

In this context, holgura usually means play, slack, clearance, or looseness—especially in a technical or woodworking context.

It does not usually mean the more general idea of comfort or roominess here. In joinery, holgura refers to extra space where parts should fit more tightly.

Caja y espiga is the standard Spanish term for a mortise-and-tenon joint.

• caja = the mortise (the slot or hole)
• espiga = the tenon (the projecting piece that fits into it)

So unión de caja y espiga means mortise-and-tenon joint.

Because caja y espiga names the type of joint, while unión is the actual noun meaning joint or connection.

So the structure is:

• la unión = the joint / the connection
• de caja y espiga = of the mortise-and-tenon type

This is similar to English phrases like a dovetail joint or a tongue-and-groove joint, where one part names the object and the other part names the type.

Because afectar normally takes a direct object without the personal a when the thing affected is not a person.

So:

• La holgura afecta la unión... = correct
• La holgura afecta a la unión... = generally not the normal choice here

In Spanish, the personal a is mainly used before specific people (and sometimes pets or personified beings), not before things like la unión.

Spanish uses afectar in different ways depending on style and meaning, but in technical language it is very common to use it directly:

• Esto afecta la resistencia.
• La humedad afecta la madera.

This sounds natural in technical or formal Spanish.
You may sometimes hear afectar a in other contexts, especially with people or broader effects, but in this sentence afecta la unión is perfectly normal.

Because caja y espiga works here as a fixed technical label for a type of joint.

In Spanish, when naming categories or technical types, articles are often dropped:

• unión de caja y espiga
• junta de dilatación
• sierra de mano

If you said de la caja y la espiga, it would sound more like you are talking about a specific mortise and a specific tenon, rather than the name of the joint type.

The sentence is talking about the concept of clearance affecting the joint as a type or as one specific joint.

So the singular is natural:

• La holgura = the clearance / looseness
• afecta la unión = affects the joint

If you wanted to talk about several cases, you could make it plural:

• Las holguras afectan las uniones de caja y espiga.

Unión is correct and natural, especially in technical writing.

Depending on context, Spanish might also use:

• ensamble
• ensamblaje
• junta

But unión de caja y espiga is a clear and natural way to say mortise-and-tenon joint.

In woodworking texts, the exact preferred term can vary a bit by region, by author, and by how technical the text is.

This is the normal Spanish order:

• subject: La holgura
• verb: afecta
• object: la unión de caja y espiga

Spanish word order is flexible, but this version is the most straightforward and neutral.

You could change the order for emphasis, but it would sound less neutral:

• La unión de caja y espiga se ve afectada por la holgura.

That means roughly the same thing, but it is more passive in style.

Unión is pronounced roughly oo-NYON.

A few useful points:

• The written accent on ó shows the stress: u-nión
• The ni before a vowel sounds like ny in English canyon
• So it is not pronounced like oo-nee-ON

Approximation:

• unión → oo-NYON

Yes, in other contexts holgura can mean room, margin, or ease, and that can be neutral or even positive.

For example:

• Trabajamos con holgura = We worked with plenty of margin / comfortably.
• Hay holgura en el presupuesto = There is some slack in the budget.

But in a joinery sentence like this one, it usually suggests undesired looseness or excess clearance.

Yes. It sounds like normal technical European Spanish, especially in carpentry, joinery, or engineering-related writing.

It is concise, formal, and matter-of-fact. A Spanish speaker in Spain would understand it naturally, especially in a technical context.