Should I give up on M6 SS inserts?

[Charley White]

After getting the blessing for 6mm screws from Voile, I went with that size for my DIY switch plates. At the time, Tognar and at least one of the "inserters" sold that size insert (snowboards), so I thought I could migrate in six. Now, seems not. I can find some via Amazon, but they are @$5 a pop. (BTW, I don't care about slotted drive and open-bottom: I have a great solution.)

"Suck it up, Princess," and go the way of the world, or have a source for me?

[jfb]

I like the "suck it up, Princess" but you might try Fastenal. Online or bricks and mortar.

After getting the blessing for 6mm screws from Voile, I went with that size for my DIY switch plates. At the time, Tognar and at least one of the "inserters" sold that size insert (snowboards), so I thought I could migrate in six. Now, seems not. I can find some via Amazon, but they are @$5 a pop. (BTW, I don't care about slotted drive and open-bottom: I have a great solution.)

"Suck it up, Princess," and go the way of the world, or have a source for me?

[Charley White]

I like the "suck it up, Princess" but you might try Fastenal. Online or bricks and mortar.

Thanks, John, but same EZ LOCK mfgr at $6.73 a pop instead of the $5 via the revenue-seeking, profit-eschewing Amazon. It's almost Halloween. Should see a wide choice of pink tutus on Craigslist soon---cheap.

[jfb]

Thanks, John, but same EZ LOCK mfgr at $6.73 a pop instead of the $5 via the revenue-seeking, profit-eschewing Amazon. It's almost Halloween. Should see a wide choice of pink tutus on Craigslist soon---cheap.

I'm set for tutus but I've got my eyes open for an elegant leotard.

[jasonq]

I make my own inserts, I have a lathe, but its not too hard to do without one. Here's the skinny.
two methods, mod and scratch.

both:
search the internet for std screw dimensions and tap drill sizes.


Mod:
find inserts with the right outside thread, but the inside thread smaller than what you want. The inner thread's outer diameter needs to be smaller than inner diameter of what you want. For inner M6, 10-24, 10-32, or M5 should work. smaller is fine. Now make sure the inner diameter of the outer thread is large enough to fit the M6 thread. for M6, it looks like M8 or 5/16-18 will work, but a bit tight, M10 or 3/8-16 will work no problem. You want the inserts to have thru holes, not blind.

Now, go buy those inserts, a plug tap for M6, tap handle, and a tap drill for M6, and a bottoming tap for the outside thread (to tap the ski). And get a coupling nut for the size of the outer thread (basically really long nut). Take two inserts and put them into the coupling nut, tighten them against each other so they can't spin. Put coupling nut in vice or something. Drill out the inserts with the M6 tap drill. Now tap that hole M6. repeat as many times as needed. you are done.

scratch:
basically the same as above, but instead of starting with inserts of a different inner thread size, just buy threaded rod stock. It's way, way, way cheaper. Cut to the length you want with hack saw, dremel wheel, whatever. The tricky part is drilling the tap hole centered. That's where a lathe really comes in handy. I suggest buying one steel insert, and drilling out the inner threads as above. Those guide the drill so its easy to stay centered. Don't tap that insert. Instead insert it and a slug from the thread rod into the coupling nut, and use the insert as a drill guide. tap and you are done.

Both cases, i recommend putting a little counter sink on the inner threads to help start the screw. I also recommend cutting a slot across the bottom side. when you install them into the ski with epoxy, it will fill that slot and help keep the insert from screwing out when you remove the bindings. In non metal skis, you can shape that slot so the insert is self tapping if you want.

If you use brass inserts or brass rod, this is really easy. If you use steel make sure you lube the tap with oil. if it is hardened steel, like starting with set screws instead of rod, you'll probably break the tap. 303 stainless should be fine, but not as easy to work with as brass. I've had zero problems with brass inserts in 10+ years of doing this.

[polemonium]

Why not use the 5mm inserts that binding freedom sells? Of all the reports on inserts that I've read about, none seem to have revolved around breaking a screw, suggesting 5mm is strong enough. That's also the size Voile and K2 used in tele skis AFAIK.

I don't know what your switch plates involve, but if you've already drilled them with clearance for a 6mm screw, there should be ways to use a 5mm screw in them, like countersinking the holes and using a flathead 5mm screw. (Edit: that was if you were mounting the switch plates to inserts in the skis. If the switch plates are tapped for M6 thread, then drill some new holes in them and tap for M5?)

[Charley White]

Interesting stuff, jasonq, thanks for that. I'd like to try tapping SS, but I know me & my job queue, so might be a while. I DO note, though, that if I went with brass, I can get 6mm inserts straight from McMaster for about 50 cents ea. Little long, but I can fix that quickly. I assume--maybe obvious--that you mount with brass screws...and no strippages.

Why not use the 5mm inserts that binding freedom sells? Of all the reports on inserts that I've read about, none seem to have revolved around breaking a screw, suggesting 5mm is strong enough. That's also the size Voile and K2 used in tele skis AFAIK.

I don't know what your switch plates involve, but if you've already drilled them with clearance for a 6mm screw, there should be ways to use a 5mm screw in them, like countersinking the holes and using a flathead 5mm screw. (Edit: that was if you were mounting the switch plates to inserts in the skis. If the switch plates are tapped for M6 thread, then drill some new holes in them and tap for M5?)

[Edit to add: inserts I seek are to mount plates to ski, so 6mm would be merely to let all screws be the same thread.]

A number of reasons. Most recent is my plate "inserts" are 6mm ss nuts--just nuts. They are located within thin plywood ~ thickness of the nut, then overlain by multiple sheets of carbon fiber. Ignoring the friction/adhesion of the epoxy putty surrounding the nuts in the wood holes, the only uplift resistance is applied by the CF to the nut area of "OD minus ID." Just intuition in calculating this, but that area just looks much more with a 6mm than a 5mm. Before this "final design" move, I'd used 1/4" t-nuts, which required some very slight reaming of my hammerhead shims, and were way too heavy. But I'd become very fond of dealing with those 1/4" screws at the tailgate. Also, when I looked at the 5mm machine screws that came with my Switchbacks, the heads looked notably smaller than the wood screws...and why I emailed Voile for 6mm permission. The last, unquantifiable, reason I have now is because that old choice seems to have been taken away.

(OK, and since this is a very small audience thread I'll allow I have an aversion to any company that trots out the wife & kids ("family business") to induce choosing them over the pirates ("unlicensed knockoffs") of their proprietary....threaded metal connectors!? Then, going over to those pirates what I see is a graphic of my daughter--dressed for the shoot as a pole-dancer--riding one of their wonderful massive inserts to ecstasy. What a choice for voting-with-dollars!

[jasonq]

charlie,
I mount with SS screws (10-24 in my case ~=M5, but coarser thread) into brass inserts. The thing to keep in mind is that brass threads are stronger than threads in the ski's binding plate (some metal sandwich skis might be stronger, then delame is the issue). And you can have more threads engaged between the screw and the insert vs a binding screw which really doesn't get much if any strength from the threads in the core (some hardwood core skis are an exception). And strangely a non-hardened nut can often hold more load than a hardened nut. The reason is if the threads can yeild slightly, more threads come into contact with the screw as the load increases. With a full hard nut, instead the first thread fails, and other thread takes the load, and fails, etc. The other thing to keep in mind is the number of threads engaged. In general the 1st 3 threads threads do all the work (mechanical engineering rule of thumb). So with M6, the thread pitch is 1mm, so the first 3mm will do most of the work. if you can get your mounting screw 6mm deep, you basically have a 2x safety factor on pull out if those first 3 threads fail.

at 50 cents each buy some of the brass inserts, put them in a junk ski or a 2x4 and see if you can torque a screw hard enough damage them. then do a crowbar test, bolt something down and try to pry it off. I've done this test before and pulled the top sheet away from the core before stripping screws. But cheap and easy to do yourself.

[Charley White]

I am going to do this, Jason. May I ask why you don't use brass screws? I remember someone--dana, maybe--raising "anodic index," back when T'tippers started adding inserts. W'pedia sets the critical diff ~ 1.5v and I note brass to ss looks like under that. My only other design reason is that I would want my screws to wear long before my inserts if I planned to swap a lot. Which i don't, so really am just curious.

I remember hearing from a ME friend that the first few threads carried all the load, and that was part of my inclination to use nuts i/o t-nuts. Why design for a space launch tolerances when all you are is a high-stance telemarker? Also funny is my first switch plates when I started goofing around with this had brass 10-24 for my Voiles. Never really used them, though.

charlie,
I mount with SS screws (10-24 in my case ~=M5, but coarser thread) into brass inserts. The thing to keep in mind is that brass threads are stronger than threads in the ski's binding plate (some metal sandwich skis might be stronger, then delame is the issue). And you can have more threads engaged between the screw and the insert vs a binding screw which really doesn't get much if any strength from the threads in the core (some hardwood core skis are an exception). And strangely a non-hardened nut can often hold more load than a hardened nut. The reason is if the threads can yeild slightly, more threads come into contact with the screw as the load increases. With a full hard nut, instead the first thread fails, and other thread takes the load, and fails, etc. The other thing to keep in mind is the number of threads engaged. In general the 1st 3 threads threads do all the work (mechanical engineering rule of thumb). So with M6, the thread pitch is 1mm, so the first 3mm will do most of the work. if you can get your mounting screw 6mm deep, you basically have a 2x safety factor on pull out if those first 3 threads fail.

at 50 cents each buy some of the brass inserts, put them in a junk ski or a 2x4 and see if you can torque a screw hard enough damage them. then do a crowbar test, bolt something down and try to pry it off. I've done this test before and pulled the top sheet away from the core before stripping screws. But cheap and easy to do yourself.

[polemonium]

Also, when I looked at the 5mm machine screws that came with my Switchbacks, the heads looked notably smaller than the wood screws...and why I emailed Voile for 6mm permission. The last, unquantifiable, reason I have now is because that old choice seems to have been taken away.

Your switch plate is different than I was envisioning and can understand the desire for a larger nut in that application. Not sure I understand your last sentence about the old choice - if it's the Voile M5 insert machine screws, you can actually still get them from the Voile website under "discontinued" (very not obvious). M5 screws are pretty standard and I would guess the heads are strong enough for a reasonable use.

I would prefer steel to brass screws for strength of head and resistance to rounding-out of the socket or philips driving surfaces, but this is a guess.

(OK, and since this is a very small audience thread I'll allow I have an aversion to any company that trots out the wife & kids ("family business") to induce choosing them over the pirates ("unlicensed knockoffs") of their proprietary....threaded metal connectors!? Then, going over to those pirates what I see is a graphic of my daughter--dressed for the shoot as a pole-dancer--riding one of their wonderful massive inserts to ecstasy. What a choice for voting-with-dollars!

IIRC, there was some TGR dustup between jondrums (the Binding Freedom guy, who designed the inserts originally) and the Quiver Killer guy, which is now behind them. Not like my choices are that important, but I do like that the BF guy gives advice on TGR, and I especially like that he maintains a library of high quality paper templates for binding mounting. I've already used these several times for normal (non-insert) mounts.

[jasonq]

charlie,
brass screws are really rather weak relative to steel. I would be really worried about them snapping off. the issue is brass is not very tough in an engineering sense, and screws have a nature stress concentration due to the threads. this is a bigger deal on screws than nuts. nuts don't get bent sideways, and if they do, they have larger cross section than screws. Also, in terms of thread strength, the nut thread has more cross sectional area than the screw (defined by the root of the thread, which is the OD for the nut, but the ID for the screw, so the nut has more area). Not to mention the screw also gets torqued in. I can't comment on the galvonic interaction between SS and brass, other than to say i have had no issues. As far as wear, remember brass/steel is a very common journal bearing combination.

if you are very worried about insert wear, one trick i have used it to get the smallest brass tacks i can find. file a groove down the length of the insert threads. The just install the inserts with wood glue. Then to lock the insert in place, tap the tack down the groove. To remove the insert, pry out the tack, then use an ez-out to back out the insert.

the weight weenie in my has thought about titanium screws...

[polemonium]

I have no experience with titanium in a ski application, but in bicycles ... I'm leery of titanium as a fastener or in places where its diameter is constrained and it's subject to bending loads. It's strong, but not as stiff as steel, so it bends more and that can lead to fatigue. On bicycles, for example, titanium pedal spindles and bottom bracket axles typically have a weight limit and have broken under big guys. It's good for frames though because the designer can make the tubes fatter, so the overall structure resists bending.

[Charley White]

So logical, the relative importance of strength in the screw to the nut, as you explain it, Jason. The bonus here is there is a much greater choice of screw length is SS vs brass. And, neat trick on grooving the ext threads for the tack. I'm actually not too worried about insert thread wear, particularly with the M6 as its quite easy to feel the threads starting right and not suffer a cross-thread at the start.

... Not sure I understand your last sentence about the old choice - if it's the Voile M5 insert machine screws, you can actually still get them from the Voile website under "discontinued" (very not obvious). M5 screws are pretty standard and I would guess the heads are strong enough for a reasonable use.

That was just a nod to the possible effect of psychology, not logic, driving my desire. When I first looked, but wasn't buying, 6mm inserts were available and I didn't care much about them. Now they are not when I go to buy and I want them. Curiously, I note that Tognar still sells the 6mm screws if not the inserts.

[Charley White]

Here's a drift (since you two really know what you are talking about and we're not bothering anyone

Can either of you explain regular binding screws... why the threads flare so wide--but thin--in the middle, then choking back so narrow near the head? They are so different from regular wood screws they make themselves look necessary; but they are so impossible to buy from other than a shop where you have to bribe with beer, I'm highly suspicious the design is merely a flourish and scarcity creating, not engineering driven. A while ago I switched entirely to #12 SS sheet metal screws for direct-to-ski mounts. The core is larger so thread surface less but it just seems great enough. Add to that the fact they never corrode and I've never snapped a head while extracting as I have with "real" (unrusted) binding screws (even just wood glue sealed) and I just don't get binding screws.

[polemonium]

I don't really know what I'm talking about - never been a ski tech, and I'm not an engineer, just a scientist with opinions about materials.

I don't know much about the spec for binding screws or why the narrow thread section near the head. There is an ISO spec for them, so it's not just a shop conspiracy, but I have no idea who actually makes binding screws. According to a screw thread on TGR they are 5.5mm x 1.8 pitch. This is very close to #12 sheet metal screws in thread diameter and pitch. I've seen Nordic binding screws with even deeper threads, probably for mounting to XC skis without a reinforced area? Binding screws are nothing like wood screws as you noted. Most alpine-type skis have either a strong layer near the top, or a reinforcement layer in the binding mounting area, and my guess is that a lot of the pullout strength comes from this thin layer in addition to the core, so analogous to how sheet metal screws work. Both are self-tapping.