#1  
Old 02-16-2014, 08:09 PM
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Default Indispensable tool issue - i'm flummoxed!

I know it's a fairly simple tool...but i haev an issue and i can't figure it out. hoping i can explain it on here.

So...i created a board that is round, 42 inches diameter. I have created my tool with a hinge attached to the board.

i have taken into account the thickness of the board

I have taken into account the slight gap between the base of the timber and the hinge

I get an exact 21 inches to where the base of the brick should sit.

Yet I have a gap between the tool and the first course of bricks as seen here.



The board is exact in size, no issue there.

If i was to continue using this tool, the size of the first mortar joint would be huge, as seen below.



I even measured the height as it would be at the top of the dome using this tool and yep, its exactly 21 inches. see below.



So if its 21 inches at the top, why am almost an inch short at the base...and that is the same gap all the way around the base....
it's definitely centred.

i'm flummoxed graham...

EDIT - i should also note, the hinge isn't secured to the floor at the end of it, it is pretty well right under the timber...prob a mm off centre but that has been accounted for.
Attached Thumbnails
Indispensable tool issue - i'm flummoxed!-2014-02-17-10.29.20.jpg   Indispensable tool issue - i'm flummoxed!-2014-02-17-10.29.28.jpg   Indispensable tool issue - i'm flummoxed!-2014-02-17-10.30.29.jpg   Indispensable tool issue - i'm flummoxed!-2014-02-17-10.30.37.jpg   Indispensable tool issue - i'm flummoxed!-2014-02-17-10.30.51.jpg  


Last edited by alexi82; 02-16-2014 at 08:21 PM.
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  #2  
Old 02-16-2014, 09:52 PM
cobblerdave's Avatar
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Default Re: Indispensable tool issue - i'm flummoxed!

G'day
You Have compensated for the thickness of the base board, BUT this makes it only true when in the vertical position, any other position it will be to short.
The IT needs to be adjustable to complete the dome accurately.
To set it for every row of bricks, a simple guide is to cut A 1/2 shape of your dome. Then fit it every row and adjust the IT for the next.
Another thing you will find that if you work off the IT it will tend to dislodge the brick you just laid. Design it to take a spacer say a piece of ply with you can slip out and then move the IT cause its now clear of the brick.
There are other ways of doing this but this is the simplest one I know.
Regards dave
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Last edited by cobblerdave; 02-16-2014 at 10:04 PM. Reason: Wrong term
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Old 02-16-2014, 11:34 PM
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Default Re: Indispensable tool issue - i'm flummoxed!

I can't believe I didn't puck that up! Sheesh!
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Old 02-17-2014, 12:40 AM
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Default Re: Indispensable tool issue - i'm flummoxed!

*pick that up rather! Thanks heaps. Again
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Old 02-17-2014, 05:41 AM
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Default Re: Indispensable tool issue - i'm flummoxed!

Alex,
The center of the IT should hit at half the thickness of the brick or the angle of the tilt on the dome is off center it becomes cumulative on each course. Currently your IT center is hitting the top of the brick. IMHO

Last edited by UtahBeehiver; 02-17-2014 at 05:47 AM. Reason: typos
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Old 02-17-2014, 03:24 PM
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Default Re: Indispensable tool issue - i'm flummoxed!

Quote:
Originally Posted by UtahBeehiver View Post
Alex,
The center of the IT should hit at half the thickness of the brick or the angle of the tilt on the dome is off center it becomes cumulative on each course. Currently your IT center is hitting the top of the brick. IMHO
i get what you are saying, i'm just trying to think how to practically build the tool to suit this.

would you just position the main arm of the tool inline with the centre of the brick, and have the angle iron or whatever it is that supports it on top of the arm?
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Old 02-17-2014, 05:01 PM
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Default Re: Indispensable tool issue - i'm flummoxed!

Quote:
Originally Posted by alexi82 View Post
i get what you are saying, i'm just trying to think how to practically build the tool to suit this.

would you just position the main arm of the tool inline with the centre of the brick, and have the angle iron or whatever it is that supports it on top of the arm?
If it helps here are a couple of pics of my IT. Worked fine for me. I did lose the clamp part very quickly as it was not required. If you want to make it further adjustable it is only a matter of cutting the threaded rod and putting a threaded tube in.
Attached Thumbnails
Indispensable tool issue - i'm flummoxed!-ver-1-30-3-13-2   Indispensable tool issue - i'm flummoxed!-ver-1-30-3-13-6  
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Old 02-17-2014, 06:26 PM
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Default Re: Indispensable tool issue - i'm flummoxed!

Nice. I actually have just been outside thinking about it and have figured it out. This is what i made. Off set the angle iron, bent it a bit. I have got a couple of wing nuts and a groove in the timber so i can move it up and down and tighten it easily. so I can place it against my semi circle timber form, measure it, then adjust and reattach. works a treat!

thanks for the help guys
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Old 02-18-2014, 02:14 AM
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Default Re: Indispensable tool issue - i'm flummoxed!

I'm struggling to put this into simple sentences but here goes.

1) A properly constructed IT does not move the just-laid brick when you lift it off. The bottom edge of the L bracket must not fall below the plane of the hinge. If this is done correctly the L bracket is pulled away from the brick as the IT is lifted.
2) If the IT is properly constructed so that the hinge point is level with the middle of the face of the first course of brick, it does not have to be adjustable.
3) What make sit difficult is that when the first course of bricks are laid on top of the floor, the centre of the bricks is not the bit that would touch the hemisphere. So the top edge is further out from the arc of the hemisphere than it should be.
4) if the hearth bricks are laid inside the dome, the first course of bricks can be cut so the bit rising above the hearth floor is exactly half a face brick, then using the IT to do a hemisphere works properly.

I wish I could get sketch-up to work so I could illustrate what I mean. A fixed IT has worked twice for me, so i'm kinda convinced I'm right.
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Old 02-18-2014, 02:29 AM
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Default Re: Indispensable tool issue - i'm flummoxed!

the only reason ive made it adjustable is to account for the thickness of the board it is sitting on...but i suppose if you start at the right distance at the first course then it would continue up...

not sure i am getting what you mean about the l bracket...well i do but i can't picture it... be great to see a pic of yours or to get sketchup working!

otherwise might be settling for what ive got or a timber form...

I should add, that when i mocked up a couple of chains of bricks, 3 high, the angle was perfect to the shims I had already measured...so i figured it was correct.
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