Tamiya`s choice of screws?

[Gazzalene 171]

Why do Tamiya use self tapping and machine screws for the same plastic? Some times a self tapper will hold a shock on and others a machine screw. Both into the same plastic.

I don't think one as more grip over the other.

[KEV THE REV 2319]

I think overall the machine screw is less 'brutal' on the plastic than a self tapper , plus if it is a critical part you can tap the hole to receive a machine screw . So for known problems on cars using self tapper - ie Monster Beetle you can fit machine screws from the start to prevent the problem further down the line . As to why Tamiya use ST - over MS I don't know , not really a cost saving thing , or maybe it is for Mr T .

[Juggular 4964]

I think self tappers are more forgiving.  If the plastic shrinks 0.3mm after coming out of injection molds, it's no big deal for self tapping screws.  It will make a hole as it goes in.  If the plastic shrinks less than expected?  Still no big deal.  Assuming that the plastic can withstand the tapping process. (I also cracked Black Foot chassis once, by over-tightening)  Bigger bite is brutal, but it's "good enough" for most soft plastic.  

But if the hole is 0.3mm bigger or smaller, that could be a big problem for the machine screws with less pronounced threads.  So, if the hole came out perfect in size, maybe Tamiya "prescribes" machine screws, maybe?  That'd be my wild guess...  DN01 uses machine screws. Perhaps because they used harder plastic.  Boy, was it difficult to use machine screws when few holes were smaller.  The friction was so much on fiber-reinforced plastic, it heated the screw enough to burn my fingers after unscrewing it.  

I've seen machine screws more often when screws get longer--because self tappers are somewhat tapered, the length is limited.  Also self tappers are easier to avoid cross-threading, at least to me.  You back out a turn, and you feel the screw sinking, and then you tighten.  But machine screws, you can't tell if you are cross threading or not.  If you repeatedly cross thread it, the hole is ruined.  So, I kinda prefer self tappers when possible.  

 

[nowinaminute 2815]

It doesn't really bother me either way most of the time except for one scenario:

When you buy hopup parts such as shocks that come with machine threaded ball studs and you have to screw them into holes that have already had tapping step screws in them. The machine threaded parts will pop out with the slightest of persuasion because the holes have been opened up too much.

A few times I've had to ditch the ball joints and go back to the stock step screws which is really annoying.

 

[Saito2 6686]

It appears Tamiya may mold the holes differently depending on fastener type as well. Machine thread screws seem to go into round holes. IIRC, a virgin, unused hole meant for a self tapper is molded in the shape of a triangle in some cases. Regardless of the fastener used, ABS is basically a inexpensive, mediocre material to use. I spoke with several engineers on the subject of plastic and they all agree. Now, in Tamiya's defense, for what many of their models are designed for and their intended use, ABS is fine. Tamiya has a certain durability and service life in mind when they build their pedestrian offerings. Tamiya doesn't build products thinking they'll be in use 30 years down the line (though they can be). Tamiya doesn't design products with Lipo or crazy brushless power in mind either. I'd guess that ABS is rather kind to their molds too. The engineers I spoke with claimed, while stronger, glass fiber reinforced plastics can erode molds quicker over time which makes sense.

Where Tamiya fails is older cars designed for competition. They used ABS in these areas too and it simply doesn't hold up under the rigors of competition. The Astute comes to mind. Newer, TRF style cars however definitely have improved with proper plastics. From the looks of the new CC02, its plastics appear promising as well. 

Much of the issue with any fastener going into ABS is stress fracturing. Without proper calculated clearances (tricky, as ABS does shrink as @Juggular mentioned) the resulting pressure of inserting the fastener will start the fracturing process that eventually leads to breakage down the line from impact or simply age. ABS gains its strength through mass so if the hole isn't supported with alot of material around it, it becomes even more vulnerable.

[nowinaminute 2815]

There seems to be a lot less ABS in Tamiya kits these days thankfully. It's not the worst plastic by any means and there's definitely worse out there but for RC use it's definitely middle of the road. When I send off stuff to get 3D printed, ABS tends to be the minimum acceptable material rather than the preferred choice.

I noticed there is less because I was making some ABS/acetone glue paste a while back for welding broken ABS parts so I went through some leftover sprues and it actually took quite a while to find any ABS. A lot of it was PC and PA etc.

[Saito2 6686]

48 minutes ago, nowinaminute said:

I went through some leftover sprues and it actually took quite a while to find any ABS. A lot of it was PC and PA etc.

True. a lot of what we think of as classic "Tamiya ABS" nowadays is actually more commonly PC.  While not as tough a glass reinforced, I'd probably give straight PC the durability nod over straight ABS, so it is somewhat of an improvement.