SO3 Extrusion Tapping or Threading

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werkstuff
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SO3 Extrusion Tapping or Threading

Post by werkstuff » Sun May 17, 2015 5:16 am

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This first post is a bit more wordy than necessary, as a kind of caveat for the subject, so please feel free to just look at the videos and other options posted after it.



This intended to address the subject of Shapeoko 3 extrusions, regarding the tapped or threaded pockets for bolt-fastened connection of the aluminum extrusions to frame plates, as seen in shapeoko wiki and pdf assembly, so that it may be thorough, focused, easily found, and not add to clogging of other threads.

*A few posts in the SO3 Question/Answer thread have prompted me to this subject being one of value and interest for discussion:

1. The first was this post by someone who received extrusions with badly tapped threads.

2. The second was this one, where Mr. edwardrford addressed the issue, process, and progress of extrusion tapping for the SO3 to this point.

3. And, the third was the following post in the same thread where someone mentioned the application of trilobular thread forming bolts and the effective application to aluminum extrusions they have had.

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*There may not end up being much to this thread, as certain remedies for the primary issue do seem to have been put into place in the manufacture process of the Shapeoko 3. *However, the third post referenced did prompt me to inquire of the thread forming bolts which did seem to yield potentially interesting alternatives that may be of use to other Shapeoko users or of value for some integration into Shapeoko 3, etc. design or manufacture.

In searching I found the following demonstrations and version of thread forming trilobular bolt, with which I had no familiarity and have no affiliation.

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*The point of this post and thread is not to cast an unfavorable light in any direction, but simply exercise the benefit of collective and individual improvement for the benefit of everyone in a non-insistent manner.

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Last edited by werkstuff on Thu May 21, 2015 3:19 am, edited 2 times in total.

werkstuff
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Re: SO3 Extrusion Tapping or Threading

Post by werkstuff » Thu May 21, 2015 2:36 am

Another very good and fairly common option to improve this aspect of the Shapeoko 3, etc. would be hardened steel threaded inserts, 'timeserts', etc.

Anyone who has had stripped aluminum threads, often found in automotive applications, has probably come across something like this, or the more common cheaper fix generally known as hell-coil inserts. These, again, are very common and numerous in both variation and application.

Here are a few introductions from more small scale manufacture to more basic/minimum diy, which may be of help to either individual Shapeoko owners or at some point Shapeoko 3 design and manufacture, etc.






veng1
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Re: SO3 Extrusion Tapping or Threading

Post by veng1 » Thu May 21, 2015 1:18 pm

If the screw is sized correctly and the depth of material is sufficient for enough thread engagement, then there is no reason for threaded inserts in aluminum unless extreme strength is required.

As an example, a #4-40 threaded machine screw ( that's about an M2.5) drilled and tapped through 1/4" (~6mm) thick extrusion will often break the head off of the screw if the power driver torque is set too high. On the other hand, the contract manufacturer who is doing this has never broken the threads out. He requested that I change the screw size to #6 to stop breaking screws. This is based on experience of running about 500k-750k screws per year.

In this case, the extrusion is smaller than Edward's and fairly easy to machine and tap in a machining center.

The extruder, Alexandria Industries, specializes in secondary operations and my experience is that if they can do it, pay them to do it. The people that think hand tapping a screw boss is easy and they'd rather do it themselves may learn a hard lesson. Because of the open side, the tap can wander out of the screw boss fairly easily. These production quantities are much harder than they look to someone that doesn't have to actually do it.

cvoinescu
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Re: SO3 Extrusion Tapping or Threading

Post by cvoinescu » Thu May 21, 2015 2:26 pm

veng1 wrote:These production quantities are much harder than they look to someone that doesn't have to actually do it.
Oh boy, how true that is. Even for my much less impressive production runs than Edward's.
Proud owner of ShapeOko #709, eShapeOko #0, and of store.amberspyglass.co.uk

werkstuff
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Re: SO3 Extrusion Tapping or Threading

Post by werkstuff » Thu May 21, 2015 3:12 pm

veng1 wrote:If the screw is sized correctly and the depth of material is sufficient for enough thread engagement, then there is no reason for threaded inserts in aluminum unless extreme strength is required.

As an example, a #4-40 threaded machine screw ( that's about an M2.5) drilled and tapped through 1/4" (~6mm) thick extrusion will often break the head off of the screw if the power driver torque is set too high. On the other hand, the contract manufacturer who is doing this has never broken the threads out. He requested that I change the screw size to #6 to stop breaking screws. This is based on experience of running about 500k-750k screws per year.

In this case, the extrusion is smaller than Edward's and fairly easy to machine and tap in a machining center.

The extruder, Alexandria Industries, specializes in secondary operations and my experience is that if they can do it, pay them to do it. The people that think hand tapping a screw boss is easy and they'd rather do it themselves may learn a hard lesson. Because of the open side, the tap can wander out of the screw boss fairly easily. These production quantities are much harder than they look to someone that doesn't have to actually do it.
Being these are M6, with the threaded pockets circularly incomplete and aluminum, I think they would strip the threads in the extrusion first. It also appears there could be fairly minimal thread contact in general.

Unless I knew for sure that the inserts would not work, I would not think simply writing them off would be very good thinking, given the tapping process would not directly effect the final product, which would be essentially perfectly consistent threads every time, in a more durable material, and being replaceable as well.

And, with the thread forming screws, as can be seen in the first video above, the installation is not only much more effectively self-aligning, but also would take less time than installing regular bolts into already tapped holes, and might be able to be shipped installed in the extrusions, or even left to the assembly process done by customer who has made the purchase. So, with either option there would actually be less time, less labor, etc. which would probably quite easily translate to less cost, especially if you are considering loosing extrusions and sending out replacements.

chamnit
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Re: SO3 Extrusion Tapping or Threading

Post by chamnit » Thu May 21, 2015 3:58 pm

There is an engineering rule of thumb for threading. You need about 4 threads to get the maximum strength of a fastener for a given material. As long as you have that and not over-tighten and damage them, you should be fine.

Also threading a slotted hole is different than threading a fully-enclosed hole. The thread cutting pressures will naturally push the tap toward the slot opening, which is what's happening. My gut solution would be to insert a tight-fitting aluminum plug into the end of the extrusion before tapping. The plug will help keep the slotted hole from opening up and keep the tap straight. But, from what I recall, Carbide is going to have their suppliers thread the holes before receiving them. Any problems will be on their end.

Hans
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Re: SO3 Extrusion Tapping or Threading

Post by Hans » Thu May 21, 2015 6:06 pm

chamnit wrote:There is an engineering rule of thumb for threading. You need about 4 threads to get the maximum strength of a fastener for a given material. As long as you have that and not over-tighten and damage them, you should be fine.
Sorry if I'm on a pedantic rampage, but this isn't right. It works for a coarse grade 5 screw in structural steel, but even with a decent alloy these aluminum extrusions will need at least a dozen full threads before the female threads are as strong as the screw. The rule of thumb I usually hear is 1.5 times the screw's major diameter (9 threads for an M6x1 screw) but even that isn't enough in softer aluminum, or in an open thread port like we have here.
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chamnit
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Re: SO3 Extrusion Tapping or Threading

Post by chamnit » Thu May 21, 2015 6:29 pm

Hans wrote: Sorry if I'm on a pedantic rampage, but this isn't right. It works for a coarse grade 5 screw in structural steel, but even with a decent alloy these aluminum extrusions will need at least a dozen full threads before the female threads are as strong as the screw. The rule of thumb I usually hear is 1.5 times the screw's major diameter (9 threads for an M6x1 screw) but even that isn't enough in softer aluminum, or in an open thread port like we have here.
It's a rule of thumb for a given material. Perhaps you're right. It's probably more like 5-6 threads, but not more than that. 4 is the minimum to have a decent strength.

But, for aluminum, you'll never get the full strength of a threaded hole in it. If you have a long bolt with 12 or more threads, you'll yield the aluminum threads at some point along the length of the threaded hole, since the stiffness of aluminum and steel are so different. For full strength threaded holes in aluminum, you'll need helicoils.

Hans
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Re: SO3 Extrusion Tapping or Threading

Post by Hans » Thu May 21, 2015 7:17 pm

You're right, the first thread will fail earlier than a small-displacement model predicts because of bolt stretch. For a steel screw in steel, virtually all of the force is on the first 5-6 threads even if the screw is longer. In aluminum, about three times less stiff, the load is spread out over about three times more threads. There is an ISO standard for minimum thread engagement, but I prefer the empirical charts in Unbrako's engineering manual. At any rate, there's no reason anyone should accept just 4 good threads in their brand-new Shapeoko 3 extrusions. I'm glad it's being done by the extruder with rigid tools, because the threads have definitely been the weakest link in Bach 1 quality.
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werkstuff
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Re: SO3 Extrusion Tapping or Threading

Post by werkstuff » Thu May 21, 2015 7:51 pm

I don't think heli coils are the inserts that you want to go with as they are not solid, rely heavily on the threads that they... thread into, and tend to come out and fail structurally quite a lot.

Time-serts or other solid hardened, etc. steel are the type to go with. Take a close look at the second video I posted, and check around anywhere you would like, the hell-coil are cheap temporary fix compared to intentional, good, durable design that will last, be better replaced if necessary, and preserve the integrity of the material. There are at least a few good hardened inserts out there with just a quick search or two.

And, engineer rule of thumb will design things, but it will not be the one that fixes those same things. Ask anyone who works on their own cars or motorcycle, or professionals who do so for a living, or try it for yourself a little, you will find aluminum threads, especially the likes of these, will become easily compromised. With the insert, no movement is in contact with the aluminum anymore. Hardened steel inserts will likely be one of the best, most important, and overlooked upgrades for such an application. Because, if they fail... your screwed ...or, would that be ...not screwed ...you know what I mean.

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