Where did this go wrong ? And What type of wood to use?

[Munroast]

I have made these before and providing you get your angles right they're fairly simple. I have been asked to make a couple of these so thought I would knock up one out of some scrap pine to make sure my jig and angles were bang on before using some better hard wood.

Measured it all out, 8 Sides, 430 high 265 across the base, painstakingly calculated and the accurately cut to 7.0° Mitre & 21.4° Bevel

But it all went horribly wrong, have recalculated it and recalculated it until I was blue in the face. And then after days of pacing about I spotted the mistake - can you ?

looks OK from the outside

But them angles are all wrong

And what would the best type of wood be to make these for outside here they will get a lot of rain and direct sunlight - I think she is wanting them varnished, money won't be an issue but they will need to be perfect and long lasting?

 

[foxhole]

Bevel should be 20 degrees ?

 

[Munroast]

Nope, bevel angle is bang on at 21.4°
The error is numerical but far far simpler for this Octagon cone.

 

[taityboy]

You have put 9 sides in.

 

[Munroast]

You have put 9 sides in.

Yep, I cut an extra one as one of the pieces wasn't quite right - then forgetting about this then struggled to join them all together, then blamed everything else for why it wasnt fitting. I even marked the dodgy bit with a ? mark, but failed even to see this
when using just 8 pieces to make the octagon it all goes together quite nicely

 

[Deleted member 174758]

Beech biscuits are unsuitable for exterior use - it will rot in time. Better to use hardwood ply loose tongues and a D4 PVA glue. I'd just go with a decent joinery grade redwood unless she's willing to pay for oak

 

[Munroast]

Beech biscuits are unsuitable for exterior use - it will rot in time. Better to use hardwood ply loose tongues and a D4 PVA glue. I'd just go with a decent joinery grade redwood unless she's willing to pay for oak

So would OAK be the wood to use ? If using loose tongues how succesful would gorilla polyurethane type glue be, it really is waterproof.

As it turns out she says just use what I have made, its pine off a bed headboard I got at the tip. Beech Biscuits and a waterproof PVA - it is to be black so if it is well painted it should last a few years, anyway she has been warned!


Here is the first one ready for painting - it would look quite nice varnished up. and it was so more successful using only 8 bits to form the octagon 9 didn't work

 

[Deleted member 174758]

So would OAK be the wood to use ? If using loose tongues how succesful would gorilla polyurethane type glue be, it really is waterproof.

Only thing I'd be wary of is the tendency of PU glues to blow the joints apart unless you clamp up really tight (Spanish windlass, maybe?). I think something like Everbuild D$ PVA would be a better bet, though

As it turns out she says just use what I have made, its pine off a bed headboard I got at the tip. Beech Biscuits and a waterproof PVA - it is to be black so if it is well painted it should last a few years, anyway she has been warned!

As I say - beech is not ideal for exterior use, it tends to ge black, start rotting and can swell. That's why Festool do Sipo Dominos in addition to the beech ones

Looking good

 

[Munroast]

Only thing I'd be wary of is the tendency of PU glues to blow the joints apart unless you clamp up really tight (Spanish windlass, maybe?). I think something like Everbuild D$ PVA would be a better bet, though

Needed to get inventive to clamp these things up
Used a mixture of glues, a waterproof/resistant type glue on the biscuits and PU on the rest - some of it is a bit mited up so don't know what will happen there.

 

[Deleted member 174758]

That's pretty much the only way you can do it - wood blocks and a band cramp (or Spanish windless) round the outside. When makling up hollow forms that way turners apply the blocks using basic PVA glue (D2 - the interior sort that reverses by using hot water or acetone) the glue thick paper onto the wood and then to glue the timber formers onto the paper. When you finish the blocks can be knocked odd and the paper removed using a sharp chisel before sanding or scraping. In the days before PVA the same task was done using hot hide glue, so I guess that hot melt glue could also be used these days

 

[foxhole]

Nope, bevel angle is bang on at 21.4°
The error is numerical but far far simpler for this Octagon cone.

Total need to be 360 degrees 18 cuts at 20degrees.You have 380+ degrees which is more than full circle hence it doesn’t fit .

 

[Nozzle]

Total need to be 360 degrees 18 cuts at 20degrees.You have 380+ degrees which is more than full circle hence it doesn’t fit .

But this is if you want the cuts to align to form a cylinder. For a cone, you can't just divvy it up equally.

I remember as a teen trying to make a 4-sided pyramid out of equaliteral triangles, the chamfer wasn't 45 for each corner. Needrf help from my maths teacher to figure it out... (no, I couldn't do it again!)

Nozzle