Splitting servo rails............

[JohnnyB]

Hiya Guy's,

Well, to be perfectly honest here I'm really really peed off!!!!!

For the past few days I've been trying to instal the servo's in my Uno Wot. Sometime ago, during the fuselage construction phase, I'd installed the "hardwood" servo rails, as supplied in the kit. Then a few days ago I went to screw in the Futaba self tapping servo screws. This servo was in the cut out in the wing. As I started turning the screw I heard that terrible sound of wood splitting. No, surely not?? So I carried on and the screw suddenly became very loose..... NO!!

Sure enough the servo rail had split. So I tried the other end of the servo - and the very same thing happened. So I had to strip out the rails and start again.

In the meantime, while the glue was setting (white PVA) I decided that I was ready to mount the ABS cowling that I'd spent ages paring away to make various appertures to fit over the engine. This has turned out OK, but again, when I went to screw in the little self tappers, the blasted little "hardwood" blocks, made as in the instructions, split.

So, I decided it was off to the model shop for some harder wood - SPRUCE.

I bought a strip of 3/8 X 1/4 Spruce and thought this would be the answer. But, today I found that this too split as soon as I put in the self tappers.

So - please, what is the answer to this problem?. Its not that the screws are too big, they are the normal size futaba servo screws, and I used these on the cowling as well.

So, far, I've replaced the wing servo rails 3 times and on the last occasion used the spruce, only for the same thing to happen.

I write this tonight wondering - what next??

John.

[andy watson]

drill pilot holes.

[260 Flyer]

And then apply a spot of cyano to the hole.

[JohnnyB]

Thanks Guy's,

Andy,

Sorry, but I forgot to mention that on each occasion I did in fact drill pilot holes as is normal practice for self tappers, I think.

Rob,

Yeah, sounds like a good idea. Probably give it a go in the morning with fingers tightly crossed......!!!

Cheers,

John.

[andy watson]

It is standard, but since you didn't mention it..............

Possibly your pilot holes need to be bigger if it is still splitting. 

My advice is from my DIY experience, rather than modelling.  Perhaps a good idea might be to test it on your wasted hardwood before commiting to the good stuff.

[JohnnyB]

Thanks Andy,

Yeah, I think your advice about testing on wasted hardwood is very sound. Also, trying different woods might be a good idea.

Really though, the whole experience kinda took me by surprise as I did'nt expect constantly splitting rails, especially while using hard wood.

Will report back on progress tomorrow.

Cheers,

John.

[001]

I think the answer lies in the choice of screw, and the grain direction of the wood,  the woodscrews supplied with servos normally are tapered , sometimes with an unthreaded shank below the head. If you drill the pilot holes too small, the theaded part grips but the shank acts like a wedge and forces the wood apart, if the drilled holes are too big the wood doesn't split but the screws drop out.

If you can find some parallel sided screws of the correct size and drill the bearers for them, then the bearers should be less likely to split. The socket head servo screws with captive washer sold by some specialists have untapered threads.

Also you need to look at the grain of the wood, if you are installing the bearers yourself, lie the grain at 90 degrees to the hole, otherwise the bearer will split more readily.

[Rick Tee]

I wonder what hardwood it is that's been supplied, i use beach as there is still some knocking around here from the pre plastic engine mount days, it has never split. I would expect spruce  to split though.

[Myron Beaumont]

Even Beech

[Rick Tee]

Lol, bad form old bean.

[Andy Butler]

I just finished my Uno Wot and had no problems (sorry). I drilled pilot holes and used the small self tappers that came with the kit. I invested in a new digital caliper from Maplins - absolutely brilliant. Make the pilot holes the same size as the srew shank and let the threads do the work. I think I used a 1.5mm drill in a pin vice to make the holes. Worked a treat on the engine mount too, just bigger screws.

Once you get it sorted, let me know how yours flies - I'm hoping to have the maiden flight on Sunday at the club field.

Andy.

[Former Member]

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[Alan Dunstan]

With spruce or basswood hardwood servo bearers I tend to glue (with PVA) good thin birch ply, NOT liteply (1/16 or less) top and bottom of the bearer. Then drill the pilot holes and gently screw in the self tapper to make the thread. If it feels like it is going to split stop and make the pilot hole slightly bigger. After doing this a few times you know what size pilot is needed for what size screw. Then after cutting the thread drip cyano into the hole. I then install the bearer, again with white glue. I started doing this after splitting spruce many many times and now I don't have a problem any more.

With lightweight gliders I have actually used balsa as the bearer again backed with thin birch ply and never had any problems. But always use white glue rather than cyano. You need the flexibility of the white glue to make this work. If you use cyano for speed it will always crack away as you start cutting the thread with the screw.

Alan

[Myron Beaumont]

Woodscrews are tapered

Self tappers are straight

Without your pilot

There'll be a break

[JohnnyB]

Thanks Guy's for taking the time to post replies to my unexpected dilema. I really appreciate all your comments.

I have now succesfully managed to mount all the servo's and they are very secure. Even the smaller cowl retaining blocks have taken the screws, although after stripping out the original split blocks, I did replace them with slightly larger and thicker blocks.

Just one question though - is it better to use beech rather than spruce as hardwood bearer/servo rails etc? Is it possible to visually tell the difference between the two woods in strip form? The model shop I frequent doe'nt actually label the various hardwoods, and then when I asked about the different wood types, he was'nt sure.

Andy B.

Good Luck with your Uno Wot flight over the week end. Its good to hear from another Uno Wot builder. At the moment I've still got to cover the fuselage and connect up the radio gear etc. Is this your first building project and are you a complete beginner as I am? How did you get on with fitting the cowl and how did you paint it?

Any chance of any pics?

All the Best,

John.

[Ernie]

Hi John

I use beech, or ply for any stressed timber part of the airframe. ie engine bearers, servo mounting. remember, if these bits go wrong, more than likely, it's a bin bag job...Spruce strip replaces balsa in larger models

Beech is very hard, try it with your thumb nail, It's light brown, almost gray in colour  and very close grained. Spruce is redder in colour, softer, and smells absolutely fabulous

ernie

[001]

I think that Spruce is a form of pine wood, and a softwood, it normally has a straight grain and could be slightly sticky to the touch. Beech is a hardwood and has a distinctive grain, a bit like a lattice, depending on the way it is cut.

(Balsa is also classed as a hardwood!)

[Terence Lynock]

Problem with resinous woods is that PVA dos'nt stick to it properly, clean the surface of the wood with cellulose thinners well before gluing up.

Found a way today of toughening up Balsa, Polyfilla do a brush-on fluid for hardening rotten wood, not the Epoxy based stuff that needs a hardener but an acetone based fluid that goes straight on and hardens in a few hours, sands well and I managed to bend a piece of soft 1.5mm  sheet  through a 90 degree 1/4'' radius  with the grain and no problems, it stays more plyable than CA,

                           regards,           Terry

[andy watson]

Just to clarify Johns point a little, wood is defined as hard wood or soft wood, but it has no bearing on how hard the wood actually is!

Hardwoods come from deciduous trees, and softwoods come from evergreen trees.

Sorry if you know all this- just another one of those little idiosycracies!

[Former Member]

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[Bob Cotsford]

generally speaking, if it's less than 18" long, it's beech or similar, if it's in 36" lengths it's spruce or obeche.

At least from the model shops I've used.

[Myron Beaumont]

Eric 

 re- "stops growing in the model box"   Are modellers hard or soft 'cos I'm SHRINKING  little by little

[Former Member]

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[Myron Beaumont]

Eric

I presume that advice is purely hypothetical 'cos I bet you've never tried it !  Not only that there ain't 'nuff time left to try it out  PS are you "batman "& sleep upside down ---I know you're dizzy - (Dont' we all!   )Whats this to do with servo rails anyway, I wonder ?

Guess who ?    & by the way gravity works the same way no matter whether you're the right way up or upside down  Are you telling me that if I hang upside down for 30 years I'll be taller ? Over to you - I suppose if I lie on my back I'll get thinner ONLY if I'm on my back ?  By the way I don't need to lose weight --I eat Complan to try to put weight on !!!!! ( every week)

[Former Member]

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