Self-Tapping Vs M3 Vs Heat

[magnumb]

I've heard various opinions on ways to prevent the hard plastic Tamiya uses from cracking over the years.

Some say to heat the self-tapping screw with a soldering iron for a few seconds then thread in and others say to use your own M3 hardware with an M3 tap. The self-tapping seems fine for a while, but after a couple disassembles and age it starts cracking/splitting. 

Any other ideas? 

 

[Rinskie]

This is timely for me as I decided to switch over to M3's on my stuff. I have had my eye on the M3 forming taps for a while and when I found one in stock (not really as it came from Japan) I ordered it. I should see it tomorrow or Wed so I don't know anything about it yet but I am told that's the way to go as it doesn't cut and remove plastic but forms it to the M3 threads. We shall see. I ordered a replacement chassis so I will have fresh plastic to form. If nothing else, it will look cooler and allow me to use my beyond awesome MIP drivers. 

[TurnipJF]

If the aim is to prevent the plastic from cracking, you might be better off using a conventional fluted tap to cut the thread. 

A thread forming tap squishes the thread into the plastic just like a screw would. This makes it easier to insert the screw afterwards, and is a good idea when using soft aluminium screws that don't like being driven straight into hard plastic, but doesn't solve the cracking issue. 

A conventional tap cuts away the excess plastic as it makes the thread, so that the screw doesn't put any more stress on the edges of the hole than it has to. This goes some considerable way to solving the cracking issue. 

[KEV THE REV]

I've used M3 & M4 fluted taps here and there where necessary / repairs etc sometimes backed up with a nut on the other end if it's an open end thread , and no complaints so far  . Also good for cleaning up nuts if they are not available as replacements or rare items .

[Kowalski86]

Previously I would use food-based silicone grease to lubricate the screws, then install them very carefully, and of course keep it away from any significant sun light in storage. The soldering iron trick is largely for the hinge-pin screws that go into suspension arms.

Now I try to avoid models that use ABS/Tappers altogether.

[skom25]

There is one answer and I do not see any reason to do this differently, except situation when you want to use e.g. Self Tappers, to build vintage car.

Buy Tamiya M3 tap and silicone grease. Put grease on Tap and just screw it slowly. No heat and perfect thread for machine screws.

Iron, self tappers etc. sounds like solution for issue which does not exist.

[svenb]

@ChrisRx718 like to  grease his and listen to the squeaking noise as they go in

😳

[Saito2]

https://www.stanleyengineeredfastening.com/-/media/web/sef/resources/docs/other/threaded_fasteners_for_plastics.ashx

Some reading, I believe another member posted, for when this question arises.

 

 

[Saito2]

(puts on fire suit)

Most of my research on the interweb points to the use of self tapping screws for plastic. The situation seems not only to deal with ease/speediness of assembly but also the strength of the joint that self tappers provide in softer materials over machine screws.

Tamiya, in general uses plastics and these plastics are designed with self tapping screws in mind. The issues with cracking I've found over the years deal more with part design vs fastener selection. Typical ABS and PC gain strength through mass. Many of the small parts on RC cars lack this whether due to part design constraints or economical concerns (using less material saves money). While Tamiya started using CAD in the latter half of the 80's and is clearly shown conducting strength analysis/impact test on various structures in one of their videos from the era, it was probably nearly impossible  to calculate every conceivable stress imposed on a fastener-related joint in every car they produced at the time. Nominal usage scenarios aside, crashes, flips, impacts alone affect RC buggies in seemingly countless ways. Rather than work around all that by simply using tougher materials like high-end Kyoshos, Associated, etc. Tamiya still relies on ABS/PC construction for most of their line (though more reinforced plastics are making their way in).

All that being said, we have the scenario where we have a material and hole designed for a self tapping fastener but said part does not have the luxury of enough surrounding materiel for adequate strength in our use case. Micro fractures from initial fastener insertion that wouldn't affect a typical joint with more surrounding material and less stress applied, now becomes the start of a part failure somewhere down the line. Heating self tappers prior to insertion reduces the chances of the aforementioned micro fractures while still allowing the use of the superior self tapping screw thread pitch and design made for use in plastics.

With any fastener, back threading prior to inserting is always recommended. Machine screws will cross-thread but self tappers are aggressive enough to cut new threads if one is not careful.

That's why I use self tappers. YMMV. They are toy cars and if some enthusiast prefer tapping/forming holes and using machine screws that's cool too.

[Kowalski86]

2 hours ago, Saito2 said:

With any fastener, back threading prior to inserting is always recommended. Machine screws will cross-thread but self tappers are aggressive enough to cut new threads if one is not careful.

This advice needs to be repeated more often, simple back threading will make your Tamiyas hold up much longer as you service them. Plus, while Tamiyas screws are JIS (vs hex), they're good quality.

[Old_Corny]

I've just purchased some fluted taps and used them on the TC-01. All seems good so far - fingers crossed!

+1 for back-threading when assembling.

[alvinlwh]

I drive my M screws in with a electric screwdriver. It heats up melting the material slightly, and when it cools down, forms around the thread. Granted that my RC are only around 3 years oldest, I never had a problem with cracking. The only time I had a problem is when I crashed directly into a wall, multiple times. I know of someone else also using the same method on the forum, but not sure how old his RC are. 

[alvinlwh]

Just a heads up, Ali has the tread forming too on for cheap for Black Friday, delivery included if total order (all items, not necessarily from same seller) is over £8. As long as you are not the "Ali/China = bad crowd", this is quite a good deal. 

I am tempted to order some but decided I cannot be bothered forming threads. 

 

[Kowalski86]

44 minutes ago, alvinlwh said:

I am tempted to order some but decided I cannot be bothered forming threads. 

I think it's better to just use the included screws, outside of abuse, known design faults with older models, or overtightening, cracks generally don't become a thing until 20 years or so have passed.

[alvinlwh]

7 hours ago, Kowalski86 said:

I think it's better to just use the included screws, outside of abuse, known design faults with older models, or overtightening, cracks generally don't become a thing until 20 years or so have passed.

If I can get 20 years abuse out of a car, RC or not, I will be very happy. 

[Rinskie]

So after using a forming thread on my TT02R I can say without a doubt I will do it again on the next build. It was a pleasure not having to worry about apply pressure while twisting self tappers. It made my $150 TT02 feel and look like a $200 kit. 

[Quailane]

The answer to this question is very simple. Use the self-forming tap as deep as it will go, and use machine screws that are as long as can fit in the hole. I have even used this to repair vintage plastic pieces that had damaged self-tapping threads. This is the way that minimizes stress in the plastic. Just go slowly with the tapping and keep it lubricated for deep holes. 

DO NOT CUT THE THREADS! Tamiya themselves warn against this. The space in the untapped hole is enough. The problem is the shape. The self-forming tap shaped the empty space into the correct shape for the screw to fit perfectly. Cutting away plastic weakens the connection considerably.

[Rinskie]

And if what @Quailane said is not enough, it's a fun process. It slowed down the build to a nice methodical pace. I'm a convert! I put a little dab of grease the tap for each hole. 

 

[skom25]

On 11/19/2024 at 12:47 PM, Saito2 said:

Tamiya, in general uses plastics and these plastics are designed with self tapping screws in mind

It applies only to rather low grade models and even not all of them. Most of the parts in e.g. cheap DT-02/3 are perfect to form thread and use machine screws. I do not see reason, to use self tapper in reinforced parts of higher end cars.

Self tappers are probably perfect for plastic, if you screw them once and forget.

I assembled two models using self tappers and during rebuild I switched to machine screws in both on them, when I replaced parts. Threads made using self tappers were bit loose after just few times I screwed/ unscrewed them, when formed threads were absolutely perfect after many iterations.

There is no reason to use self tappers, except of vintage cars etc.

Tamiya quite often even instructs to use thread forming tool in manual.

My conclusion:

If you want to do things right, buy thread forming tool and prepare parts using grease, to avoid heat and friction.

If you want just to build car, because there are 100 other waiting for build, just use anything which is in kit and forget.

 

[Kowalski86]

46 minutes ago, skom25 said:

Threads made using self tappers were bit loose after just few times I screwed/ unscrewed them, when formed threads were absolutely perfect after many iterations.

The trick is to reverse thread before tightening the screws in, otherwise self-tapper screws will damage the existing threads.

[Saito2]

7 hours ago, skom25 said:

My conclusion:

If you want to do things right, buy thread forming tool and prepare parts using grease, to avoid heat and friction.

If you want just to build car, because there are 100 other waiting for build, just use anything which is in kit and forget.

My conclusion:

If you want to do things right, use whichever fasteners or methods you choose because there's no "right" or "wrong" way to fiddle around with these toy cars.

[Smokescreen38]

So…hold on. People are finding that vintage cars will start to crack from just sitting on a shelf at room temperature after a certain amount of time?  Does it ever happen sooner than 20 years or so? 
 

****. 

[Kowalski86]

17 minutes ago, Smokescreen38 said:

So…hold on. People are finding that vintage cars will start to crack from just sitting on a shelf at room temperature after a certain amount of time?  Does it ever happen sooner than 20 years or so?

There are far too many variables to give a good answer, so I wouldn't worry about it. I've had a few very old runners (Grasshopper, Hornets, Frog) that didn't have any cracks.

Tamiya ABS plastic does dry out with sun exposure, but other than a few specific, dated design flaws (hinge pins screws) it generally doesn't crack while sitting around if they've been assembled correctly (no over tightening, loctite, etc).

The only plastic that I would concern myself with is anything that uses "PC-ABS", like a few of the previous M chassis. That stuff is extra stiff and really doesn't seem to hold up.

[Olly84]

7 hours ago, Smokescreen38 said:

So…hold on. People are finding that vintage cars will start to crack from just sitting on a shelf at room temperature after a certain amount of time?  Does it ever happen sooner than 20 years or so? 
 

****. 

Yes, my Lancia 037 on the TA03RS Chassis, bought and assembled in 2002 did cack. Many plastic chassis parts with self tapping screws just split open. I have never driven this car in anger an it just sits on a shelf and looks pretty.

No problems at all with a just slightly younger TA05. 

 

[magnumb]

12 hours ago, Smokescreen38 said:

So…hold on. People are finding that vintage cars will start to crack from just sitting on a shelf at room temperature after a certain amount of time?  Does it ever happen sooner than 20 years or so? 
 

****. 

Yes, without a doubt. Noticed this on several of mine that are just sitting. And if you plan on maintaining them and wanting to keep them for a long time you can almost guarantee they will split/hairline crack even with back threading. It's just the nature of the plastic....most of the time it's really of no concern, but going forward i'm just going to end up using a tap and machine screws. It just bothers me seeing them crack and i don;t really like using a Phillips screwdriver anyways.