<p>The solution might be to countersink the opening at the threaded portion within the tube. There are various angles available for countersinks, although the more common angles are 82 degrees and 90 degrees.</p>

<p>Drive the countersink to the point where the wall thickness is zero, unlike the drawing below showing some material outside the beveled area.</p>

<p><a href="https://i.sstatic.net/WCXhM.png" rel="nofollow noreferrer" title="Cross section of center bit drilled holes"><img src="https://i.sstatic.net/WCXhM.png" alt="enter image description here" title="Cross section of center bit drilled holes"></a></p>

<p>For your purposes, you'd want the steepest angle possible, the 60 degree tool.  If you decided to purchase a countersink, pick a diameter slightly larger than the outside diameter of the wall thickness of the coupler. You could use a countersink of the same diameter as the outside diameter of the threads. </p>

<p>Center drills are available with 60 degree angles as well:
<a href="https://rads.stackoverflow.com/amzn/click/com/B00M54BPRQ" rel="nofollow noreferrer" rel="nofollow noreferrer">Amazon specific item</a>
<a href="https://i.sstatic.net/6edXI.jpg" rel="nofollow noreferrer" title="Center drill bits"><img src="https://i.sstatic.net/6edXI.jpg" alt="enter image description here" title="Center drill bits"></a></p>

<p>The second smallest center drill listed here has a 1.5&nbsp;mm center point with a 4&nbsp;mm drill point. If your coupling is larger than 4&nbsp;mm, the next size up will not work as well, as the center point is 2.5&nbsp;mm. You'd have to resort to a countersink only.</p>

<p>If you are near a machine shop or have a friend with a mill or even a decent drill press, those resources may be able to perform the countersink for a minimal (or possibly zero) fee.</p>

<p>I have a mini-mill and a collection of countersinks as well as center drills. I found my bag of unused couplers. They are for 5&nbsp;mm tubing and were flat on the threaded end. This is the result after a quick trip to the mill. It appears in the close up that I could have driven the center drill deeper into the fitting, and also cleaned off the swarf a bit as well. </p>

<p><a href="https://i.sstatic.net/t6WTO.jpg" rel="nofollow noreferrer" title="Chamfered inner hole backside of the pneumatic coupler"><img src="https://i.sstatic.net/t6WTO.jpg" alt="Chamfered inner hole backside of the pneumatic coupler" title="Chamfered inner hole backside of the pneumatic coupler"></a></p>

CC BY-SA 3.0
2017-01-15T23:11:06.893Z
That makes sense. :-) Thanks for the great answer. As it happens, I am finding some 60 degree center drill bits with 2mm tip on Amazon, so those should do the trick.
mhelvens
CC BY-SA 4.0
2017-01-15T22:42:35.867Z
2019-07-26T11:32:50.327Z
2019-07-26T11:32:50.327Z
fred_dot_u
<p>An alternative to chamfering the connector is buying a different type of connectors with a larger bore hole all the way through the connector and let the tube pass all the way through. These are used in my similar spool to extruder setup (reversed Bowden setup in OP's terminology).</p>

<p><a href="https://i.sstatic.net/cZi6Z.png" rel="nofollow noreferrer" title="Large hole pneumatic tube connector"><img src="https://i.sstatic.net/cZi6Z.png" alt="Large hole pneumatic tube connector" title="Large hole pneumatic tube connector"></a></p>

CC BY-SA 4.0
2019-07-26T11:25:54.503Z
2019-07-26T11:25:54.503Z
0scar
<p>I'm creating a reverse Bowden setup to guide my filament from spool to extruder, through a path which contains two <a href="http://amzn.eu/almYaNi" rel="nofollow noreferrer">couplers</a> in the middle as follows:</p>

<p><code>[spool] --- |#= --- =#| --- [extruder]</code></p>

<p>So I have to connect a tube to the <em>back</em> of a coupler (<code>---=#|</code>) and not just to the front (<code>---|#=</code>). That's the end that contains the screw thread, and it's not designed to take a tube. I can't manage to make it a smooth transition. When I try to push my 1.75&nbsp;mm filament through, it will often get stuck there. After it's through, the extruder seems to have no problem with it anymore.</p>

<p>Is there a trick to making this a smooth transition?</p>

<bowden><ptfe-tube>
Smooth transition between a PTFE tube and the back of a push-fit coupler
mhelvens
# Smooth transition between a PTFE tube and the back of a push-fit coupler

(Asked by: username_0 on 2017-01-15)

I'm creating a reverse Bowden setup to guide my filament from spool to extruder, through a path which contains two [couplers](http://amzn.eu/almYaNi) in the middle as follows:

`[spool] --- |#= --- =#| --- [extruder]`

So I have to connect a tube to the *back* of a coupler (`---=#|`) and not just to the front (`---|#=`). That's the end that contains the screw thread, and it's not designed to take a tube. I can't manage to make it a smooth transition. When I try to push my 1.75 mm filament through, it will often get stuck there. After it's through, the extruder seems to have no problem with it anymore.

Is there a trick to making this a smooth transition?

**Tags:** bowden, ptfe-tube

---

## 2 answers

### Answer by username_1 on 2017-01-15. Score: 1 | Accepted answer

The solution might be to countersink the opening at the threaded portion within the tube. There are various angles available for countersinks, although the more common angles are 82 degrees and 90 degrees.

Drive the countersink to the point where the wall thickness is zero, unlike the drawing below showing some material outside the beveled area.

[![enter image description here](https://i.sstatic.net/WCXhM.png "Cross section of center bit drilled holes")](https://i.sstatic.net/WCXhM.png "Cross section of center bit drilled holes")

For your purposes, you'd want the steepest angle possible, the 60 degree tool. If you decided to purchase a countersink, pick a diameter slightly larger than the outside diameter of the wall thickness of the coupler. You could use a countersink of the same diameter as the outside diameter of the threads.

Center drills are available with 60 degree angles as well:
[Amazon specific item](https://rads.stackoverflow.com/amzn/click/com/B00M54BPRQ)
[![enter image description here](https://i.sstatic.net/6edXI.jpg "Center drill bits")](https://i.sstatic.net/6edXI.jpg "Center drill bits")

The second smallest center drill listed here has a 1.5 mm center point with a 4 mm drill point. If your coupling is larger than 4 mm, the next size up will not work as well, as the center point is 2.5 mm. You'd have to resort to a countersink only.

If you are near a machine shop or have a friend with a mill or even a decent drill press, those resources may be able to perform the countersink for a minimal (or possibly zero) fee.

I have a mini-mill and a collection of countersinks as well as center drills. I found my bag of unused couplers. They are for 5 mm tubing and were flat on the threaded end. This is the result after a quick trip to the mill. It appears in the close up that I could have driven the center drill deeper into the fitting, and also cleaned off the swarf a bit as well.

[![Chamfered inner hole backside of the pneumatic coupler](https://i.sstatic.net/t6WTO.jpg "Chamfered inner hole backside of the pneumatic coupler")](https://i.sstatic.net/t6WTO.jpg "Chamfered inner hole backside of the pneumatic coupler")

> **Comments:**
> **username_0**: That makes sense. :-) Thanks for the great answer. As it happens, I am finding some 60 degree center drill bits with 2mm tip on Amazon, so those should do the trick.  

---

### Answer by username_2 on 2019-07-26. Score: 0

An alternative to chamfering the connector is buying a different type of connectors with a larger bore hole all the way through the connector and let the tube pass all the way through. These are used in my similar spool to extruder setup (reversed Bowden setup in OP's terminology).

[![Large hole pneumatic tube connector](https://i.sstatic.net/cZi6Z.png "Large hole pneumatic tube connector")](https://i.sstatic.net/cZi6Z.png "Large hole pneumatic tube connector")


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