Waste From Square Blanks

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KDM

KDM

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Square Mould Turning Wastage

The calculation for the amount of material wasted when turning a square moulded blank into a cylindrical shape yields a really surprising number. Here's the science part:

Assuming the desired cylindrical blank is the same length as the initial right cuboid moulded blank, the length can be disregarded for percentage wastage calculations.

To calculate wastage when turning blanks of a given size down to resultant cylinders of a given diameter, we may use the following simple equations:
b = s^2
c = π (d / 2)^2
w = (b - c) / b * 100
​
Where:
s is the length of side of the square section of the right cuboid moulding.
d is the desired diameter of the cylinder
b is the cross sectional area of the mould
c is the cross sectional area of the cylinder
w is the percent wastage.
​
Example

We start with a right cuboid moulding 22mm of square side. We aim to turn this into a cylindrical blank of 15mm diameter.

The cross sectional area of the initial blank is
b = 22^2 = 484mm^2
​
The cross sectional area of the cylinder is
c = π (15 / 2)2 = 177mm^2
​
This is a wastage of
w = (484 - 177) / 484 * 100 = 63%
​
This is pretty staggering. Even for a blank of 15mm sides (the bear minimum) and wanting to turn this down to a 12mm slimline, we still lose a massive 50% of the initial material.

Conclusion

Using circular moulds as small as possible where we can is likely to save at least 50% on the raw casting compound and increase the amount of use we get from the cutting tool before re-sharpening!
 
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Further Thoughts on Waste From Centre Drilling

Centre drilling the blank produces even more waste. To take into account the waste from centre drilling, the equations given above are extended / modified as follows:

b = s^2
c = π ((d / 2)^2 - (k / 2)^2)
w = (b - c) / b * 100
​
Where:

s is the length of side of the square section of the right cuboid moulding.
d is the desired diameter of the cylinder
k is the diameter of the hole in the cylinder
b is the cross sectional area of the mould
c is the cross sectional area of the cylinder with the hole cut
w is the percent wastage.
​
Example

We start with a right cuboid moulding 22mm of square side. We aim to turn this into a cylindrical blank of 15mm diameter with a 7mm centre hole.

The cross sectional area of the initial blank is as before

b = 22^2 = 484mm^2

The cross sectional area of the cylinder with the hole cut out is

c = π ((15 / 2)^2 - (7 / 2)^2)= 138mm^2

This is a wastage of

w = (484 - 138) / 484 * 100 = 71%

In the example of the blank of 15mm sides turned down to 12mm with a 7mm hole, this is a loss of 67% of the material.

Conclusion

Using circular moulds as small as possible with the centre hole moulded into the blank is likely to save at least 65% on the raw casting compound and increase the longevity of your drill bit and chisel.
 
My eyes glazed over after the first equation....:eek:
So what your saying is use round moulds as close to the diameter of your pen as possible...Right?::confused: Whew...I didn't know I needed all that math to figure that out!:biggrin:
 
Is this now the new Pen Math for Dummy's Forum? ... as I fall down many, many times trying to figure out why my head is spinning.

Actually, I do believe the formulas show all of us turners just how much "money" we are having to waste as we turn great wood into worthless chips and dust. No matter how one looks at it there is a lot of waste, either in sawing or turning.

Now someone get busy and figure out how many inches of wood are cut out by the table saw blade. Take for instance, cutting .75"x.75"x8' square strips of Cocobolo or some other expensive wood using a Forrest II table saw blade vs a thin kerf blade. Then one begins to understand the idea behind using a thin kerf blade. I do believe the expense of the blade is saved in short order. :eek:
 
1TJTurner said:
My eyes glazed over after the first equation....:eek:
So what your saying is use round moulds as close to the diameter of your pen as possible...Right?::confused: Whew...I didn't know I needed all that math to figure that out!:biggrin:
Click to expand...

It was obvious that square moulds were fairly wasteful. I just wanted to know how much!

TBH, the square mould hasn't lost its place: there are still layers and patterns and stuff you can't do with a small round mould. And I really like turning them round...
 
You thought that was bad lisen to this. I was watching a video on bowl turning few nights back. The guy was a tall guy and he had to muscle this log on to a heavy duty lathe, and I am not talkig about Mini like we use I am talking about a 850 lbs. as he said it lathe. then, by the time he was finihsed with the bowl just for being green turned the darn thing was about maybe 1/6 of the original size. now that's WASTE.
 
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