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Internal roll formed threads. Inspecting Pitch Diameter and Minor Diameter don't account for missing material at thread peaks.

crispy

Plastic
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Jul 26, 2013
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Oregon, USA
Searched and can't find this topic. I've also been searching the web, including the use of ChatGPT, and can't find anything that addresses this.
When roll forming internal threads, gaging the pitch diameter and minor diameter don't seem to be enough to verify that the thread form is good. By good, I mean the threads don't have deep valleys at the peaks. If I look at threads under a microscope, it seems like I should be able to see the bottom of the valleys that end up at the thread peaks, even in small 8-32 threads. The ID of the ridges of these valley peaks are a bit wavy, and every now and then there are cracks in these peaks. These threads pass GO/NO GO pitch diameter and minor diameter gages, but it seems to me that this is not adequate for inspecting roll formed threads. Besides destructive inspection to really measure the valleys in the thread peaks, I see other method to add to the inspection plan.
How do others deal with this? These particular threads are in A356-T6 castings, post heat treat. Maybe we should just require that they be cut threads.
 
Is this for your own product or something you're making for a customer? Did they specify formed threads? That's kinda' how formed threads are. Why were formed threads specified? The right solution will depend on the need.
 
I'm missing a piece somewhere. If they pass the minor diameter you know that the peaks of internal threads will not interfere. And if the pitch diameter passes you know that the mating fastener will fit (provided it also is within specs) so how does the shape of the peaks you're looking at become an important feature?
 
I suspect he's concerned about a lack of adequate material at the peaks, effectively enlarging the minor diameter for functional/strength purposes. I.e., "not good" threads despite fitting inside the min/max profile envelope. Although I suppose you could argue that if the valleys along the peaks are deep enough, they could violate the profile envelope.
Thread form profile doesn't really address cracks, galling or similar things which would be other reasons for "not good" threads.
 
I suspect he's concerned about a lack of adequate material at the peaks, effectively enlarging the minor diameter for functional/strength purposes.
That's exactly the issue. All because it passes a minor diameter GO/NOGO check doesn't mean there's a useful amount of material there to do the job. Here's some chicken scratch to show what I mean. (I hope the copy paste works.)
1699375818586.png
The left thread is a good thread, the right one is an exaggerated result of roll forming, but it is what I'm seeing with these threads. Obviously they drilled the hole too large.
We don't specify a process for the threads. We are going to now.
It just seems silly to me that the industry doesn't do more to make sure roll formed internal threads are functionally adequate. I imagine in higher volume situations, a shop will choose roll forming and drill the holes as large as they can and still pass the GO/NOGO checks to save time and make more $$$. This situation just seems like it is set up for failure.
I could be wrong about that, and that is the reason for this thread. Aside from destructive testing, there doesn't seem to be another way of inspecting roll formed internal threads.
In the end, since these are internal A356-T6 threads, and it is an NAS socket head screw going into these threads, that missing peak may not be contributing a whole lot to our pulling out issue.
To answer another question, this is for our own product, so we can specify what we want.
Thanks for the replies.
 
There was an article about the whole subject of formed threads in Modern Machine Shop. Man it has to be ten years ago now. What you drew is exactly what they identified as being common and normal for formed threads. Those cupped valleys are why formed threads aren't acceptable in medical devices: a spot to trap or hide contaminants.

The only place the material has to go is either into that area which becomes the thread peak, or for the material to compact. To compact the parent material would be a lot of force and would almost certainly grab and break the tap. Therefore, all your tolerance between 100% thread form and whatever you get, is held in the material at that thread peak. That's your tolerance zone for whatever the drill created.

As you already realized: the holes probably ended up slightly oversized and you had less material available to fill the void. First stop would be to make sure the drill size is what the manufacturer recommends for their forming tap. After that try stubby drills if they're not already, and obviously carbide. Stab them at the max recommended rate and no pecks. Don't want any extra dwell time in the hole to oversize it.
 
There was an article about the whole subject of formed threads in Modern Machine Shop. Man it has to be ten years ago now. What you drew is exactly what they identified as being common and normal for formed threads. Those cupped valleys are why formed threads aren't acceptable in medical devices: a spot to trap or hide contaminants.

The only place the material has to go is either into that area which becomes the thread peak, or for the material to compact. To compact the parent material would be a lot of force and would almost certainly grab and break the tap. Therefore, all your tolerance between 100% thread form and whatever you get, is held in the material at that thread peak. That's your tolerance zone for whatever the drill created.

As you already realized: the holes probably ended up slightly oversized and you had less material available to fill the void. First stop would be to make sure the drill size is what the manufacturer recommends for their forming tap. After that try stubby drills if they're not already, and obviously carbide. Stab them at the max recommended rate and no pecks. Don't want any extra dwell time in the hole to oversize it.

That (and because they are shallow bottoming holes that I am roll forming) I usually skip drilling, and just pocket the holes with an endmill. But I am only doing 2-4 holes in the parts I do, and having a perfect hole is worth the extra few seconds of cycle time.
 
I’ve had minimal success roll tapping without first reaming the hole, I found that more than +/- .001 on hole size to make a large difference in quality. This was on 4-40 thru 1/4-20, I’ve never used one larger than that.
 
That (and because they are shallow bottoming holes that I am roll forming) I usually skip drilling, and just pocket the holes with an endmill. But I am only doing 2-4 holes in the parts I do, and having a perfect hole is worth the extra few seconds of cycle time.
Interesting that you pocket them. Probably not possible on 8-32 but, a big thread...

I like that.
 
You say you have a pulling out issue. I doubt changing to a cut thread will solve your problem, but its worth a try. IME with die castings, a roll formed thread with an ugly minor diameter was still stronger than a cut thread. Try using some lube on your screws and see if that helps your pulling out problem.
 
Below is from the Balax catalogue. Aluminum castings are less dense than wrought material, cold forming will compress the material and create a stronger thread.
Do you have any specifications the threads are to be inspected to? AS8879 is common for aerospace products.

An industrial MRI machine can check that thread, if you don't exceed the high/low envelope, the thread is dimensionally acceptable, depending on the inspector...
1699405664042.png
 
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That (and because they are shallow bottoming holes that I am roll forming) I usually skip drilling, and just pocket the holes with an endmill. But I am only doing 2-4 holes in the parts I do, and having a perfect hole is worth the extra few seconds of cycle time.
I am usually doing 3/8-16 and 1/4-20 roll formed threads, in aluminum and brass. Hole is pocketed .365" deep and I thread form right down to .360" depth. They are for stainless studs, and I need to get the best thread holding power I can get.

I use Nachi, thread forming taps.
 
I'm missing a piece somewhere. If they pass the minor diameter you know that the peaks of internal threads will not interfere. And if the pitch diameter passes you know that the mating fastener will fit (provided it also is within specs) so how does the shape of the peaks you're looking at become an important feature?


Strength


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I am Ox and I approve this post!
 
My understanding is that thread percentage doesn't affect strength as much as one might think. The only thing I don't like about formed threads is that a careless assembly person can get a screw started, not in the thread, but in the crease at the crest, and then ram the thing in. Worse with power tools. FWIW, I like the Balax taps best as I think they have the tap geometry figured out a bit better than others. If you have a pull-out problem, it's probably time to look into the size and pitch for the application because what you have might be marginal at best.
 
% of thread makes a LOT more difference on a formed thread than a cut thread.


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I am Ox and I approve this post!
 








 
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