Rules Of Thumb

 

John W. Cobb

 

2004


Table Of Contents

 

 

Introduction. 3

Production Time for Bowls. 3

Bowls Base Diameter 3

Platters Base Diameter 4

The Thirds Rule for Boxes. 4

Conclusions. 8


 

Introduction

 

This article is intended primarily for beginners, but experienced turners may find information of interest here as well.  In the paragraphs that follow, I will describe rules of thumb that turners use to make the process easier and faster.

 

Production Time for Bowls

 

Here is a rule that I learned years ago from my friend Richard Raffan of Canberra, Australia. 

The time that it should take to turn a standard bowl, i.e. not a decorative one, is the product of the maximum diameter in inches by the height in inches.  So, for a bowl that is 10 inches in diameter and 4 inches deep, the time it would take a professional turner to make is 40 minutes.

 

When pricing work, it is helpful to know how long work should take to make as opposed to how long it actually takes.  You then use the time it should take as the basis for a wholesale price.

 

Bowls Base Diameter

 

The diameter of the base of a ‘standard’ bowl should be between 1/3 and ½ the maximum diameter of the bowl.  If you make the base any less than 1/3, then the bowl starts to look and become unstable.  If you make the base more than ½, then the whole thing starts to look dumpy and ill formed.  I like to make my bases about 40% of the maximum diameter for utility bowls.  Note that 40% is close to the golden mean (38.2%).


 

Platters Base Diameter

 

The diameter of the foot ring of a platter should be between 65% and 75% of the platter diameter.  This gives the platter a nice solid feel.  Any less than 65% and the platter begins to be easy to tip over by pressure on the rim.  Any more that 75% and the platter begins to look dumpy plus there will not be enough room to make a pleasing transition from the underside of the rim to the foot ring. 

 

If the platter is to be mounted on an expanding chuck, keep the foot ring at least ¾” wide to avoid splitting it during turning.

 

The Thirds Rule for Boxes

 

This is a very useful approximation for box turners.  When designing a box, we know that if we make the height of the body equal to the golden mean (0.618) times the total height, then the box should have pleasing proportions.  (I say should because the visual effect of the end result also depends heavily on what you do to the masses of the body and lid).  Measuring with rulers and micrometers is a terrible time sink; it is much more efficient to place lines by eye whenever possible.  The thirds rule provides us with just such a technique.

 

Let us consider an example.  Let’s say that we have a box blank that is 5 inches in length and 3 inches in diameter.  Let us further suppose that the box has a chucking spigot 2 inches in diameter and 1/8 inch long at each end.  Also, let’s assume that the spigot will be 3/8” long and that we have a narrow parting tool so that the parting waste is 1/8”.  So, the total height of the box will be 5 inches minus ¾ inches or 4 ¼ inches.  The height of the lid will be 1 5/8 inches and the height of the body will be 2 5/8 inches. 


 

The length of the blank between the chucking spigots on each end is 4 3/4 inches.  If we place the parting cut mark 1/3 of this distance from the tailstock end, then the lid height will be just less than 1 5/8 inches.  This is very close indeed to the theoretical value and 1/3 is easy to visualize and estimate.  Working in millimeters is much easier and more accurate.  Consider the same example worked out in millimeters:

 

The blank is 127mm in length.  There are chucking spigots 50mm in diameter and 3mm in length at each end.  The parting cut will waste 3mm and the spigot will be 10mm long.  The total height of the box will be 108mm.

 

The theoretical height of the lid, according to the golden mean is 41mm.  The height of the body would then be 67mm.

 

The length of the blank between the chucking spigots is 121mm.

 

Using the thirds rule, the parting cut will be placed 1/3 of 121 or 40.33mm from the tailstock end.  We will round this to 40mm.  This is within 1mm of the theoretical measurement, quite accurate enough.  I know that I cannot work to a tolerance any finer than 1mm.

 

As I said in the beginning of this article, this technique is an approximation.  It is legitimate to question the accuracy of this technique.  The technique will be exact for precisely one set of measurements and it will vary depending upon the spigot length and the width of the parting cut.

 

Consider the case where the blank is 107mm in length. 

There are chucking spigots 50mm in diameter and 3mm in length at each end.  The parting cut will waste 3mm and the spigot will be 10mm long.  The total height of the box will be 88mm (107 – 19).

 

The length of the blank between the chucking spigots is 101mm.

 

The theoretical height of the lid, according to the golden mean is 33.6mm.  The height of the body would then be 54.4mm.

 

By the thirds rule, the lid will be one third of 101mm which is 33.67mm and the body will be 54.33.  This result is nearly perfect.


 

In contrast, consider the case where the blank is 69mm in length.

As before there are chucking spigots 50mm in diameter and 3mm in length at each end.  The parting cut will waste 3mm and the spigot will be 10mm long.  The total height of the box will be 50mm (69 – 19).

 

The length of the blank between the chucking spigots is 63mm.

 

The theoretical height of the lid, according to the golden mean is 19mm.  The height of the body would then be 31mm.

 

By the thirds rule, the lid will be one third of 63mm which is 21mm and the body will be 29mm.  This result is not perfect.  In fact, there is a 2mm error or about 10% of the lid height (2/19).

 

Consider a third case where the blank is 144mm in length.

As before there are chucking spigots 50mm in diameter and 3mm in length at each end.  The parting cut will waste 3mm and the spigot will be 10mm long.  The total height of the box will be 125mm (144 – 19).

 

The length of the blank between the chucking spigots is 138mm.

 

The theoretical height of the lid, according to the golden mean is 48mm.  The height of the body would then be 77mm.

 

By the thirds rule, the lid will be one third of 138mm which is 46mm and the body will be 79mm.  This result is not perfect either.  In fact, there is a 2mm error or about 4% of the lid height (2/48).


 

 

To see that the spigot length and parting cut width affect the outcome, consider the nearly perfect case above with a longer spigot and a wider parting cut.

Let the blank be 107mm in length. 

There are chucking spigots 50mm in diameter and 3mm in length at each end.  The parting cut will waste 5mm and the spigot will be 16mm long.  The total height of the box will be 80mm (107 – 27).

 

The length of the blank between the chucking spigots is 101mm.

 

The theoretical height of the lid, according to the golden mean is 31mm.  The height of the body would then be 49mm.

 

By the thirds rule, the lid will be one third of 101mm which is 34mm and the body will be 46.  This result is not perfect at all.  There is a 3mm error which is about 10% of the lid height.  The lid on this box will be larger than that called for by the golden mean.  The box may look top heavy depending upon how the detailing is done.

 


 

Conclusions

 

What we can conclude from all this is as follows:

 

Given a particular spigot length and parting cut waste, there is one box height where the thirds rule gives perfect or nearly perfect results.

 

For boxes shorter in height, the error will increase rapidly.

 

For boxes taller in height, the error will increase but not as rapidly as for shorter heights.

 

For the range of heights where jewelry boxes are normally made, the error due to using the thirds rule is acceptable and can safely be ignored if we keep the spigot length in the 3/8” range and the parting cut narrow.

 

For a graphic portrayal of these facts see the following figures:

 

Figure 1

Shows box height verses error for a spigot length of 10mm and a parting cut width of 3mm.

 

 

Figure 2

Shows box height verses error for a spigot length of 12mm and a parting cut width of 3mm.

 

 

Figure 3

Shows box height verses error for a spigot length of 16mm and a parting cut width of 5mm.