Round Chest Lid (need true-ing help)

[pjm699]

I have made a curved lid for a (pirate's style) chest by routing one side of each piece of wood with a dovetail bit to give the curve using 22 strips of timber - but....due to the grain of the wood (read my incompetence / inaccuracy) the assembly is not entirely true - only three of the corners rest on a flat surface at any one time.

I am hoping that by doing something cunning with the end pieces I can pull it back to true (it's about 4mm out over a 560mm length)

What i need is for someone to tell me what that cunning thing might be - can anyone help, thanks Phil

 

[xy mosian]

Hi pjm699, I am not sure that you will be able to pull out a built in twist like that, others may disagree. Without knowing about your timber sizes or your assembly technique, I would think your twist could have been a great deal worse. Personally I would look at planing the edge pieces to get the curved assembly to sit flat on a board, then make end pieces to fit. After all the pieces would be approximately 2mm narrower at one end than the other. If that is too much for you to bear, then add extra pieces, vertically, that is in line with the 'chest' sides and make tham such a width that the 2mm variation, end to end, will hardly show. I would think 25mm at the widest should do it, but the seven times rule may come into force and something as low as 15mm may be sufficient.

I am interested what others, with more experience than I, think.

xy

 

[pjm699]

Thanks for that,

is how I have made the chest but the top comprises 22 battens which are each 23mm wide at the top - rather than the 9 shown to make the curve smoother.

I think I will go for the planing option for twist correction first- hopefully I can use a plane with crossed fingers!

Anticipating my skills - I made the lid first and the square bits will be made to fit to that, Phil

 

[xy mosian]

If it helps to have a guide line to plane to :- Sit the curved section on a flat board and wedge the high corners until you are happy with, say the top being level. From your description these wedges will be in the order of 2mm thick. Take a piece of timber the same thickness, or thicker than the wedges and using this as a pencil rest, draw a line the length of the lid on both long sides. That will give you a line to plane to. When planing check the edge you are planing against the other edge, a straight edge on a piece of timber will do this. Remember you are trying to plane the edge back to a flat surface which includes the other edge.
I apologise if this is old hat to you.
Have fun.
xy

 

[Chems]

If it was me and I wasn't 100% plane happy, I'd set out some sandpaper on a flat surface and just sand until it laid flat. I agree with XY that the variation you have got is impressive and nothing to be ashamed of at all and that you would struggle to pull that twist out and somehow use the end boards to lock it in place.

 

[Shane]

I've got to agree that that amount of twist in that type of construction is nothing to be ashamed of, and I would just plane flat as mentioned previously. You could plane a half the amount from the lid as will as the box if you wanted, depends what looks best

 

[goldeneyedmonkey]

All the advice given seems spot-on. I'd definitely plane or sand it, it's not going to be noticeable. Can I just ask XY (or anyone else who cares to comment), what's the 7 times rule? I've not come across this before I don't think.

Cheers. _Dan.

 

[pjm699]

Life got in the way - again - last night.
Guides marked and currently building up courage to start shaving - looks a bit messy but it mostly filler on the surface.

Wine(ish) bottle for scale (yet another project!)

Thank you for all your help gentlemen - hopefully a finished item before too long!

Yay- worked out the image insertion - have to go to the BBCode and it's the middle of the three options with some of the spaghetti at the ends removed.

 

[Chems]

Looks really good.

To post pictures, get the straight url for the image which looks like: http://farm6.static.flickr.com/5189/561 ... 1507be.jpg making sure it always ends in .jpg

Then put that inside the image tags like this:

 [IMG]http://farm6.static.flickr.com/5189/5612908200_f8211507be.jpg[/IMG]

 

[xy mosian]

goldeneyedmonkey, The seven times rule? I'll try to pass on the explanation I was given forty-odd years ago by a product designer. It is based on an aspect of human perception, so anything could happen.
Imagine a cabinet with two doors which do not overlap. If the fit is close but the door edges are not truly parallel it will stick out like a sore thumb. However if the nominal gap between the closed doors is seven times the error, then the apparent non parallel effect will disapear. So in random numbers. If the doors are a tight fit but there is a gap at one end, or in the middle, of 0.5mm, then making the nominal gap 3.5mm, the gap will appear parallel. Basically hiding the error.
The suggestion is that a nominal width to error ratio of higher than seven will not be apparent, but a width to error ratio of less than seven will be apparent.

That's about the best I can do I think. Steve Mettham was the name of the guy who told me of it and I thought everyone knew. If I can track him down I'll ask him if it was all invention.

Hope this helps. xy

 

[goldeneyedmonkey]

Cheers XY, I had to read through it a few times before it sank in properly (nothing to do with your explanation mind, I'm just not the best at picturing things in this manner, I need to physically see things to grasp at the logic usually!) But I see what you mean, I do vaguely remember being told something similar by an 'old timer' once. I don't think he labelled it as the seven times rule, I think he just said "make it bigger" .

Thanks for taking the time to explain. _Dan.