<h2>It depends</h2>
<p>Most inserts have two possible diameters: the <em>maximum outer</em> diameter and the <em>minimum outer</em> diameter. Like, the <em>maximum</em> diameter is the diameter that is over <em>all</em> the retention parts - the star section in top and bottom - and the <em>minimum</em> diameter is the diameter of the thin <em>shaft</em> in the center, as you will see easily in the model here.</p>
<p><a href="https://i.sstatic.net/H30jc5HO.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/H30jc5HO.png" alt="A modeled M3 insert of example setup" /></a></p>
<p>You want your model to <em>melt</em> around the retention lugs of the extending sections. So taking the diameter of the necked-down &quot;minimum outer diameter&quot; area as the hole diameter means that just the lugs are pushing aside melting walls during insertion. That is the nominal size your holes should be for a given insert.</p>
<p>Now wall thickness comes into play: the more solid your walls, the more solid the part will <em>grip</em> on the insert. A fully solid part has no way to push material aside, so melting an insert into a nominal hole might result in some material getting pushed out the top or bottom - which makes a slightly larger hole needed. But if you have a relatively thin wall, such excess material would be pushed into the infill, so you can even use a <em>smaller</em> diameter for the inserts.</p>
<p>For more about inserts in plastics, I suggest watching <a href="https://youtu.be/G-UF4tv3Hvc?" rel="nofollow noreferrer">CNC Kitchen on inserts</a> as well as his <a href="https://youtu.be/2wRc1KbEAU8" rel="nofollow noreferrer">Helicoil experiment</a></p>

CC BY-SA 4.0
2024-08-26T19:18:21.533Z
2024-08-26T19:18:21.533Z
Trish
<p>To use a knurled heat insert threaded nut on a 3D print you are first required to have a hole of a slightly smaller diameter than the insert. So that the heat insert can be inserted into the print and still have sufficient material around the edge for the knurls to bind into.</p>
<p>How is the size of the holes calculated?</p>
<p>For example, if you have two M3 threaded inserts. One with an OD of 4.2 mm and one with an OD of 4.6 mm, is there a formula that can be used to determine the optimum hole size.</p>

<3d-models>
How do you calculate the hole size required for a threaded insert for a given OD?
Aaargh Zombies
# How do you calculate the hole size required for a threaded insert for a given OD?

(Asked by: username_0 on 2024-08-26)

To use a knurled heat insert threaded nut on a 3D print you are first required to have a hole of a slightly smaller diameter than the insert. So that the heat insert can be inserted into the print and still have sufficient material around the edge for the knurls to bind into.

How is the size of the holes calculated?

For example, if you have two M3 threaded inserts. One with an OD of 4.2 mm and one with an OD of 4.6 mm, is there a formula that can be used to determine the optimum hole size.

**Tags:** 3d-models

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## 1 answers

### Answer by username_1 on 2024-08-26. Score: 4

## It depends

Most inserts have two possible diameters: the *maximum outer* diameter and the *minimum outer* diameter. Like, the *maximum* diameter is the diameter that is over *all* the retention parts - the star section in top and bottom - and the *minimum* diameter is the diameter of the thin *shaft* in the center, as you will see easily in the model here.

[![A modeled M3 insert of example setup](https://i.sstatic.net/H30jc5HO.png)](https://i.sstatic.net/H30jc5HO.png)

You want your model to *melt* around the retention lugs of the extending sections. So taking the diameter of the necked-down "minimum outer diameter" area as the hole diameter means that just the lugs are pushing aside melting walls during insertion. That is the nominal size your holes should be for a given insert.

Now wall thickness comes into play: the more solid your walls, the more solid the part will *grip* on the insert. A fully solid part has no way to push material aside, so melting an insert into a nominal hole might result in some material getting pushed out the top or bottom - which makes a slightly larger hole needed. But if you have a relatively thin wall, such excess material would be pushed into the infill, so you can even use a *smaller* diameter for the inserts.

For more about inserts in plastics, I suggest watching [CNC Kitchen on inserts](https://youtu.be/G-UF4tv3Hvc?) as well as his [Helicoil experiment](https://youtu.be/2wRc1KbEAU8)


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