Lead-in chamfers for internal threads

[myyxlplyyk]

This is for 3D printed parts.

I was having a difficult time finding a way to do this, since adding threads to a hole then adding the chamfer doesn't work. I found a method online where someone first did the chamfer, then the threads, then reversed them in the timeline. It works with the normal boring metric and SAE threads, but it seems like there has to be a way that's not so cryptic. (this is with thread command, not hole command).

In the attached file, the threaded hole is a 1/4" BSP pipe thread using the thread command. The chamfer/thread/reverse method refuses to work on this. Also, when I try the hole command, and offset the threads from the top and add the chamfer to the smooth inner surface at the top (.5mm long), it also doesn't work. I haven't found any configuration with the hole command that works. 

In this file it's using the thread command. I can get my push to connect fitting in there, but I have to futz with it. I have to do a bunch and it would be nicer to not have to futz.

[g-andresen]

Hi,

1. create a thread > not modeled

2. Add chamfer

3. Edit thread > change to „modeled“

günther

[laughingcreek]

that doesn't work with his model.  

[myyxlplyyk]

@g-andresen @laughingcreek 

That method doesn't work on the pipe threads. (It does work on the regular threads). Here is the error:

Error: Chamfer9
<b>1 Reference Failures</b><br/>The edge reference is lost and this feature is using cached geometry.<br/>Edit this feature and select new edge references.
Compute Failed

[etfrench · Accepted solution]

Create a sketch on a vertical Origin plane.

Project intersect the body.

Draw the chamfer triangle.

Revolve it.

[MRWakefield]

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@myyxlplyyk wrote:

@g-andresen @laughingcreek 

That method doesn't work on the pipe threads. (It does work on the regular threads). Here is the error:

Error: Chamfer9
<b>1 Reference Failures</b><br/>The edge reference is lost and this feature is using cached geometry.<br/>Edit this feature and select new edge references.
Compute Failed

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Yes, that's what I found when I tried it. I suspected it was due to the crest and root radii so I made a copy of the Fusion BSP thread file and found that you could then add the chamfer. You do of course lose the rads but that might not be important in your application. To confirm it was due to the radii I added in <ThreadForm>8</ThreadForm> to the file (to denote Whit form) and the chamfer failed as it does with the standard BSP thread.

Hope this helps.

[etfrench]

You could also create this in another file, Insert Derive it, position it, then Combine/Cut to create the chamfered hole.

[myyxlplyyk]

Thanks, I'll just do that. I rarely do pipe threads so this is a fine solution.

[MRWakefield · Accepted solution]

While I was messing around with your model I noticed that you've used Offset Face to allow for the inaccuracies of 3D printing. There's a common misconception that you should offset all of the thread faces but unfortunately this leads to a distorted thread form. Note the difference between the thread form before and after the offset:

Before:

After:

Notice how the thread is much 'thinner', the crest is much sharper and the root radius much larger. This can compromise strength.

You can use offset face on a trapezoidal thread but you need to do it right otherwise you'll lose the thread profile. I made a post explaining the what, how and why of thread offsetting here if you're interested. An added issue when offsetting threads with rounded crests and roots is that the radii change size by the amount of the offset. You can clearly see this in the second screenshot above.

Ideally in a case such as this you're better off using a custom thread file with the offset built in, this way you keep the correct profile and don't have to concern yourself with the pitfalls of offsetting.

Hope this helps.

[HughesTooling]

Another way to do this is with another hole feature set to chamfer, something like this. Would be easier than the revolve cut if you have several holes.

[myyxlplyyk]

That is very helpful, thank you. I did notice when I did that (using "Q"), that the threads looked strange on this part. I wasn't too concerned because it's a hole for a push-to-connect fitting with pre-applied sealant. I was just going to be mashing it in there anyway permanently. I printed it with an orphan roll of PETG that was an off-brand I don't usually use and that didn't print very well anyway. The threads looked nasty but worked for installation of the fitting. I'll test the seal on one part and redo if necessary.

I have done "Q" on regular boring threads before and I don't recall them looking so strange. I do have trouble looking at threads with the cutaway view. I have used the offset command before and chose the faces, but again, me eyeballz give me trouble and when I found the "Q" method, it was "good enough/quick and dirty" for the moment.

However I do want to tighten up my skills with this and I will definitely reading that post.

[myyxlplyyk]

I have to try your suggestion at my machine as I don't 100% understand it yet, but thanks.

What I actually ended up doing is revolve a triangle to make a cone that set inside the opening of the hole. Then I just used it to cut the chamfer.

[MRWakefield]

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@myyxlplyyk wrote:

That is very helpful, thank you. I did notice when I did that (using "Q"), that the threads looked strange on this part. I wasn't too concerned because it's a hole for a push-to-connect fitting with pre-applied sealant. I was just going to be mashing it in there anyway permanently. I printed it with an orphan roll of PETG that was an off-brand I don't usually use and that didn't print very well anyway. The threads looked nasty but worked for installation of the fitting. I'll test the seal on one part and redo if necessary.

I have done "Q" on regular boring threads before and I don't recall them looking so strange. I do have trouble looking at threads with the cutaway view. I have used the offset command before and chose the faces, but again, me eyeballz give me trouble and when I found the "Q" method, it was "good enough/quick and dirty" for the moment.

However I do want to tighten up my skills with this and I will definitely reading that post.

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You're welcome. I obviously didn't know whether it was important in this particular case but it's always good to know the correct way to do things and the reason(s) for doing it that way. You can then make your own mind up whether any extra effort required is necessary.

[HughesTooling · Accepted solution]

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@myyxlplyyk wrote:

I have to try your suggestion at my machine as I don't 100% understand it yet, but thanks.

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I've attached my file to make it clear. I've also redone the design using a lot less timeline features and to give more clearance on the thread for 3d printing I've used a non uniform scale to increase the diameter by 10% (EDIT) looking at some threads I've done around this size I only needed 1% but you'll need to experiment a bit with this.

My finished design only uses 1 sketch and 7 features including adding the chamfer.

[myyxlplyyk]

Super big thanks.

Just FYI I appreciate all the help you give people here. I have read and learned from many of your answers.

You don't even want to see some of the timeline abominations of some of my more complicated designs and fixtures 😂😂

Soon I'm going to dig into using parameters.

[myyxlplyyk]

Ok, yeah. The chamfer solution was nice.