XV Pilot 1936 Posted April 16, 2014 Hi All. For a while now I have had a theory about machine thread screws and plastic parts, but I didn' t want to mention it on the forum before speaking to a mechanical engineer, because it goes against accepted wisdom in the RC community. However having spoken to not one but three engineers, it turns out I was right, so here goes: REPLACING SELF-TAPPING SCREWS WITH MACHINE THREAD ONES IS PROBABLY NOT AN UPGRADE! IT IS LIKELY A DOWNGRADE! It all comes down to the relative strengths of the screw versus the material being screwed into, and the ratio between the screw's core and its threaded portion. In the case of a metal screw threaded into metal, both materials are usually of comparable strength, and the threads on the screw can be quite fine. Making them too coarse means that you are sacrificing core material in favour of thread, weakening the screw unnecessarily. However in the case of a metal screw threaded into a plastic chassis, the screw material is a lot stronger than the chassis, so it makes sense to allow the threaded portion to take up more of the screw's cross-section, as you don't need as strong a core, but you do need the screw to engage as much of the soft chassis material as possible so that it doesn't strip and pull out. So, with this in mind, high-end kits that use lightweight aluminium hardware threaded into hard fibre-reinforced plastics are perhaps well-suited to using machine screws, but to swap out the self-tappers in your TT01 for steel machine screws, just because they have pretty hex heads, is weakening your car. We see many examples of this in the engineering we come across in our daily lives. Woodscrews have coarse threads because wood is usually softer than metal. Drywall screws have huge threads because the plaster core of the drywall is a lot softer than the screw. Household appliances, cars, in fact pretty much anything designed by a halfway-competent engineering team will have examples of coarse threads in soft stuff and fine threads in hard stuff. So why do we look upon Tamiya self-tappers with such scorn? They do a better job than many so-called "Upgrade" aftermarket screw kits! 4 Share this post Link to post Share on other sites
TWINSET 1793 Posted April 16, 2014 So why do we look upon Tamiya self-tappers with such scorn? They do a better job than many so-called "Upgrade" aftermarket screw kits! I think it's the heads of the screws most people dislike, not the thread profile, although a decent screwdriver eliminates most of the issues A lot of US companies use fine-pitch 'machine' screws into plastic as they use a harder plastic (graphite etc) than Tamiya does on a lot of their lower end kits. Share this post Link to post Share on other sites
SteveU30 1011 Posted April 16, 2014 I think it's OK on shelf queens where you are looking for the screw heads to look pretty but I would never deviate from the correct type of screw on a runner for exactly the reasons you mention. Share this post Link to post Share on other sites
MadInventor 3884 Posted April 16, 2014 Quite right. I do however replace the standard tamiya machine thread screws with hex heads for ease of maintenance. Also there are examples (E.g Hotshot mechanism box) where tamiya put machine screws into plastic as standard. In these case I usually use a tap to cut the thread first, as machine screws are not designed to 'self tap', even into plastic. I've also found that with the heads of after market hex bolts, the stainless steel ones are quite soft in comparison with the 'black' non stainless screws. Share this post Link to post Share on other sites
Gruntfuggly 562 Posted April 16, 2014 Empirical evidence suggests to me that machine screws stay in plastic parts - the self tappers come out on their own. That being said, I usually replace self tappers with machines screws after they start coming out on their own - not before... Share this post Link to post Share on other sites
mr crispy 2293 Posted April 16, 2014 Seems to me the trade off is a greater surface area on the machine screw against the self tapper leaving more material to bite into. One things for sure, the most important thing is to make sure you don't over tighten them! The times I've encountered spinning screws in 2nd hand chassis... Grrrr! Share this post Link to post Share on other sites
speedy_w_beans 4272 Posted April 16, 2014 My overall experience with DT02 and DF03 ABS chassis with self-tapping screws and screw pins was pretty lousy. Half the self-tapping screws in the DT02 would loosen on their own after running about 30-40 packs through the car -- they'd have to be tightened after every run, or most of the plastics would have to be replaced. Most of the lower gearbox/tub screws on the DF03 started loosening on their own after a similar number of packs, plus the chassis cracked under the steering posts. On the other hand, my DF02 and TT01 ABS chassis with self-tapping screws both held up pretty well to a lot of driving. I can't explain why; the DF02 saw as much if not more abuse than the DT02 and DF03. The TT01 I can kind of rationalize because it's an onroad car and doesn't see stresses from offroad driving. The cars that have held up best over time for me are the ones that don't use ABS/self-tappers and rely on glass-filled polyamide or carbon reinforced polyamide and machine screws. Hard material + machine screw = excellent. So for modern chassis I'm more keen on TRF201/DN01, DB01, TB03/TB04, TA05V2/TA06, CR01, etc. Basically the mid-level kits, not the bottom end or top end. I do have to give some love to Grasshopper, Mad Bull, Wild One, and Lunch Box, but that's more because of their aesthetics and character, not because of their materials and hardware. Are the self-tappers included in Tamiya kits thread-cutting or thread-forming screws? To me the issue with Tamiya self-tapping screws is repeated stresses causing the threads in the ABS plastic to enlarge, which means the screw no longer has any friction with the hole and it loosens on its own. Tightening it down doesn't fix the problem; it actually makes the threads in the plastic larger even more. The hole is basically ruined. It seems like using a machine screw in ABS plastic doesn't cut the threads, but rather forms them, and it's possible to maintain the friction between the screw and hole better. I'm not worried about the strength of the screw; I've never sheared one in any chassis I've owned. I'm more interested in the screw staying put. I have not built an ABS chassis with machine screws yet, but I'm pretty close to just doing it because I hate self-tappers so much. On the topic of screw heads (separate from machine vs. self-tappers), overall I don't have a problem with the Japanese JIS cross-head found on almost all Tamiya screws. Note I didn't say Phillips; there is a difference. Using a high-quality JIS screwdriver bit it's almost impossible to strip the head. I do like hex drive screws better -- they stay on the tip of the tool on their own, and I don't have to press the tool into the fastener to stay engaged while installing or removing them. But to call the Tamiya JIS screws junk, well, that may be more of tool issue than a screw issue. 1 Share this post Link to post Share on other sites
Percymon 2184 Posted April 16, 2014 Also there are examples (E.g Hotshot mechanism box) where tamiya put machine screws into plastic as standard. I think in that example its more to do with the availablility of screws at that length - machine screws are readily available, self tappers are probably almost impossible to find/source at economical rates Share this post Link to post Share on other sites
