The Driving School - harvey's academy of driving

2024-10-27 09:06:05

Not enough material in my experience as I noticed it when I tried to tap the hole. The tap just slid straight in more or less. If you wanted to drill them out it would mean the next size up (In most cases M4)

I could go on for hours on this, but I’m constantly being distracted by my partner and child so may come back and fill in the bits I’ve missed... and doesn’t make sense...

I did a little test this evening on some parts that I never use, my usual way is to just screw a machine screw directly in, when screwing and unscrewing I listen out for that audible click, unscrewing before screwing back in. So I’ve tapped which indeed does go in nicely, but I’ve also cracked open the helicoils I bought a while ago and very much like the firm positive feel it has, only thing is I’m finding is keeping them in the same plastic when unwinding, broken off that little tab too when securing them in.

with this in mind.. A self tapping screw if installed correctly will give a sound joint, and with a coarse thread cut form has a good resistance to pull out. However, you have to get the tension just right for the joint to not work loose by using the base material as the elastic part of the assembly to retain tension. This is almost impossible, since the material is very weak and we’re just not that consistent. Even using the same tool every time, same dry or lubed conditions the variation of feel will inevitably cause you to over tighten. Once overtightened the thread is plastically deformed and can no longer maintain any kind of bolt tension and obviously, because it’s a sharpened self tapper it’s very good at displacing the plastic material once you reach this point. So the big down side to self tappers is over working and unsuitability to repeated service. The advantages are low cost and retaining wall section if you are size or weight limited.

I don't mind screws coming loose over time, the vibrations a buggy undergoes while being thrashed means you've gotta expect summat to come loose eventually.

I just want to add that I got the following 3racing thread taps from rcjaz.com and I'm very happy with them. They also come in the right size for most Tamiya kits:

I'm curious to hear others' experiences using machine screws in place of self-tapping screws in models they use regularly and run hard.

I've noticed this too. I generally don't have loosening issues with self tappers in harder ABS/PC plastics in my personal experience. Its the softer PA plastics that I have more issues with. I tend to battle cracked plastic over time more than loosening however which is why I use the hot screw method for self tappers in harder plastics. Many times, the more intricate moldings around screw bosses in Tamiya vehicles do not have adequate amounts material around the hole to hold off cracks originating from simple initial screw insertion, let alone the impact forces it sees in use. I'm curious if there's enough material in these three sided holes to drill to size for the correct tap and install a machine screw. It would be nice to stop both loosening and cracking with one procedure.

I just looked at the kit instruction manuals I have and bought the length of screws that were commonly used, 10mm is most popular so bought more of them, 18mm not so common so bought fewer of them (well actually 20mm as WF didn't do 18mm).

I think this is a sign they waved the white flag on manufacturing consistency, if you size a hole for self tap but can’t maintain the form or shrinkage then the end result is an assembly you either can‘t fit or break which means complaints and returns, sounds like the three tang idea ensures you’ll always get the assembly and the inconsistencies just result in a large variation of assembled strength, which let’s be honest most of us either wouldn’t notice or might just think they botched themselves by over-tightening.

When I get time to do a shopping list I might do a bulk order and split them up so can pass them on at cost to other people lazy like me in std size sets.

A steel screw with a dab of grease will tap the thread even in the carbon reinforced plastics. I just screw them in gradually (you can feel the heat building up as it cuts in). If I'm using an aluminium bling fastener then I'll cut the thread with a steel screw first, then install the aluminium screw.

Any tips on where to buy screw sets from? Is there a one stop shop for all the most used sizes in one set? Or do you just by individually? I’ll also need to buy some taps.. M3 and possibly an M4, can you recommend a particular brand? I imagine since it’s for plastic tool life of even the cheapest will be far longer than my useful life...

I used the self-tapping screws included in the DF03 and DT02 kits I bought when getting into the hobby and had some problems with the screws loosening after extended driving sessions on bumpy dirt. In particular, the self-tappers holding the gearboxes to the chassis tubs loosened the most. There didn't seem to be any easy way to keep them in, so I was tightening them after every run.

We treat RC cars unfairly as real mechanical assemblies and expect a little too much of the plastic. The huge divide in mechanical properties of the materials in these kits you’ll never achieve a permanent fastened joint. The plastic will plastically deform long before the fastener reaches 75% yield, even with very small fasteners the small section screws would also suffer from other issues relating to the head of the fasteners but that’s another story.

I bought mine from Banggood, ended up getting steel. They have a good range of different materials and head types, and you get a box with dividers with a good number of each size. Not that expensive either and quality is fine.

Thanks, I’m just lazy I want a nice multi compartment pill box of every size I’ll ever need but never use.. I’ve seen a few places do kit specific sets of stainless screws. I need a good old fashioned pick’n’mix with a nice little scoop.

Thanks for the link, I’ll compare this to more local suppliers and see how they compare. I have a load of paints to buy so a one stop shop might be the place to go.

So what to do? In the automotive industry if you expect to get a good fastened joint in anything other than steel and especially if you’re depth limited you need thread inserts, helicoils for aluminium or keenserts or similar, for carbon and plastic parts. In plastic you’d have a fairly large insert that can be screwed in, pressed in, moulded in, tanged for anti rotation, glued, electro welded or vibration welded and any combination of the afore mentioned... the thread condition then becomes metal on metal and can reduce to 1xD depth engagement. The insert advantage is either creating a larger threaded joint to the substrate than the final fastener or an intermediate material better suited for fusing to the substrate. These are then ideal for multiple assembly use. Downside.. a model kit designed for self tapping will most likely not have the cross section required to sustain the insert.

A threaded joint to remain fastened you need to maintain a good stretch.. The bolt needs to remain under tension and within normally 75% of the fastener material proof stress (the point at which it would start to yield), this is naturally dependent on several factors such as the materials used for the fastener and the parts being fastened plus frictional losses. If you set a fastener by measuring its stretch, then you take out of the equation friction and therefore torque is largely irrelevant (other than to diagnose assembly errors) the stretch is a calculated elongation to a load under the proof stress of the material. In theory a properly designed threaded joint you won’t need any kind of additional thread locker or secondary locking. (The amount of preload you set will also have to factor in the working load to avoid fastener yield...)

The only downer is that tamiya now don't fully form the holes in places. Instead of having a proper hole for the thread, they've put three vertical columns of plastic inside the hole, presumably to make it easier to thread screws directly into plastic with using a tap and help prevent splitting.

I just want to add that I got the following 3racing thread taps from rcjaz.com and I'm very happy with them. They also come in the right size for most Tamiya kits:

Wherever I can I use tap and machine threaded screws to replace self tappers, for both M3 and M2 size screws. The only downer is that tamiya now don't fully form the holes in places. Instead of having a proper hole for the thread, they've put three vertical columns of plastic inside the hole, presumably to make it easier to thread screws directly into plastic with using a tap and help prevent splitting. My memory might be failing but I seem to remember this on places where M4 size screws would be used (front hubs where ball joint screw in?). I also use high tensile socket cap screws in favour of the stainless screws, as I've found the heads prone to rounding out on stainless socket cap screws when tightening them right up.

I do find the machine screw and self tapper thing weird. I am pretty sure that the plastics used can take machine screws with no issue if the hole is correct size. I have been using machine screws in place of tappers for ages now.I have never pulled a machine screw from any of Tamiyas plastic and I have tried with spare parts etc. This is m3 by the way. On some models that have had regular tear downs and the screw seems loose in plastic I use a touch white acrylic glue. sets well but gives way when you need to undo.

i'll use BSI IC-2000 tire glue when the screw goes screwy. and they will unscrew when you need them to. i've tried silicone other CA's they'll work alright, but the BSI IC-2000 is Black, but if you use accelerator it will craze white on you.

I don't mind screws coming loose over time, the vibrations a buggy undergoes while being thrashed means you've gotta expect summat to come loose eventually.

So, has anybody built a new off-road kit that has ABS or polycarbonate chassis parts, replaced the self-tapping screws with machine screws, and then driven it hard? I mean something like 30-50 battery packs with a few cycles of cleaning and maintenance thrown in? Do the machine screws ever loosen and strip out like self-tappers? Most of my models are on-road, so I can't comment directly. The few TTs and Ms where I did this seem to be fine, but they don't see the bumps and shocks of an off-road model.

I’ve personally had a few kits from these guys, haven’t had a problem myself with the screws and they even made up a screw kit for me that wasn’t on their system, I just asked.

For screws I used Westfield Fasteners, they have most the common sizes, where a kit required 15mm, they did 16mm which is ok, and for 18mm I ordered 20mm.

Zip code 76262 is located mostly in Denton County, TX. This postal code encompasses addresses in the city of Roanoke, TX. Find directions to 76262, browse local businesses, landmarks, get current traffic estimates, road conditions, and more.

So if you’re thinking of replacing a self tap with a machine screw and use of an insert is not an option you should really check the size of the pre tap size and the depth if you can get 2xD and the hole size is on the lower limit of the recommended taps then it’s an ideal candidate. You can use thread lockers but really without bolt stretch you’re just delaying the inevitable work loose and making servicing more of a bind for cleaning, don’t forget the torque feel is affected by the cleanliness of the joint so when you apply to a bore with old loctite in there your feel will be that the bolt is tighter than it is in reality and will work loose even quicker.

Over the years there has been some debate about using machine screws vs. self-tapping screws, and there has been some distinction made between thread-forming and thread-cutting taps. The engineering perspective has been that self-tapping screws are designed for softer materials with larger, coarser threads, and then there's been the empirical perspective that machine screws are better since they don't seem to back out. The argument seems to be that machine screws in soft materials run the risk of stripping out. I don't think the debate has ever really been settled with some apples-to-apples comparison and some hard driving. Most people agree a thread-forming tap is the right tap since it doesn't leave a hole too loose for the fastener.

I haven't run the new threaded build yet (DT02), so won't comment on that, but having run my DN01 through about 20 packs, on mixed terrain, no problems so far, none of the machine screws have loosened (I check after each run). I've had a couple work loose on my original self tapper DT02, and one that will not tighten at all, ended up taking the screw out, running a thin layer of super glue down the hole, let it set, then put the screw back in, very tentatively, didn't do it up too tight.

Later I started substituting machine screws for self-tapping screws in TT01E and M-chassis builds, and I liked how they went in and provided a definite "stop" when they were fully inserted. They also didn't seem to loosen during use, but on-road cars see different usage than off-road cars. Most of the mid-grade kits like DN, DB, TA, TB, FF, and CR chassis include machine screws by default, but the plastics are also nylon instead of ABS or polycarbonate.

Agreed. In plastic components, I'll use a SS cap screw (2.5mm hex) to cut threads, then replace it with an Al or Ti screw (2mm hex button head). The heat generated from threading a ferrous screw into plastic parts seems to aid the plastic in conforming to the fasteners.

I'm curious to hear others' experiences using machine screws in place of self-tapping screws in models they use regularly and run hard.

I’d like to give the machine screws a go, I think in most model build cases they use self tappers because the manufacturing is cheaper, the screws are cheaper, the wall sections are small and they actually like you having to buy replacement parts.. could I get away with using the self tappers and never need to replace them? Absolutely yes because I don’t drive the cars much or hard

Machine screws for the same nominal size (measured OD) will have a larger core diameter, so handle higher clamping loads have more material for a decent sized assembly tool to fit whilst retaining a lower head height (again comparing like for like) They have more threads per mm and so can distribute load along more of the tapped hole (though in practice you need to de-pitch a fastener to achieve this in full and prevent the lead thread being disproportionately loaded) They will have a more rounded thread form meaning they are less likely to machine away the tapped hole. Rule of thumb for machine screws is 2 x thread diameter is required if the screw is to be serviceable (steel screw into aluminium housing) so with plastic you’d want to scale this.. in the models I’ve built (only 3) I don’t think I’ve seen more than a couple of screws that have 2xD engagement So it would be unfair to expect 1- bolted joints to remain fastened and 2- multiple assemblies not to degrade the joint. As you mentioned, thread forming has the advantage of adding some compressive stress to the joint so increasing it’s tensile capacity, but on this scale other advantages of Increased root stress concentrations don’t really impact because of the plastic nature, the most important thing is getting the right tap drill diameter. I don’t know what the hole sizes are for the self tappers in these model kits but if they are larger than the minimum size for thread cutting then I’d question the suitability of thread cutting. With a machine screw you’ll have more margin to play with in regards to over tightening and feel, but you’ll be even less likely to achieve a proper bolt stretch. They lend them selves to using adhesive thread lockers and multiple builds.

I've built two DT02 Sand Vipers this year, the first was my first build after a 30 year break from the hobby, so just followed the instruction manual, the second was finished today, never touched a self tapper, in fact that screw bag wasn't even opened. I've used a M3 0.5 thread tap and A2 stainless M3 0.5 hex head screws, the fit and security of the screws is far superior to self tappers, no more of that "ooh, just one more turn, ah ****, too much".

Just a general tip ,(most of you probably do already), tap threads in Tamiya kits, use machine screws rather than self tappers.

For screws I used Westfield Fasteners, they have most the common sizes, where a kit required 15mm, they did 16mm which is ok, and for 18mm I ordered 20mm.

Just a general tip ,(most of you probably do already), tap threads in Tamiya kits, use machine screws rather than self tappers.

bearing

The Driving School - harvey's academy of driving

You Might Also Be Interested In