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Turn a Sphere

Four Methods, in Brief


A wooden sphere may be a toy, an item in a larger design, or simply an object on which to hone your turning skills. In any case, getting the spherical shape exactly right (or close enough) is a bit of a challenge.


This article gives a brief description of four ways to do it. These methods all begin with a uniform cylinder mounted either between centers or held by a scroll chuck.


1. Use a template as an aid to forming the spherical surface. A method for making accurate templates is described in another article on this website.


2. Make a series of straight-line cuts between indicator marks so that you gradually creep up on the spherical surface. [Sören Berger method]




3. Use cuts made into the cylinder with a parting tool to define the surface at several points. [Doc Green method]


4. Use a sphere-cutting jig to mechanize the process.








The Shadow method, the one that I prefer, is described in detail in the next article. It uses an overhead light to cast a shadow of the workpiece straight down onto a platform attached to the ways of the lathe. A circle drawn on a piece of card stock is taped to the platform, centered under the workpiece. To turn the sphere, you remove wood appropriately until the shadow fits inside the circle.


The Ghost method, described below, is more a technique for refining the shape of a sphere than turning one from scratch. It involves mounting a near-sphere between cup centers and turning away the shadow or ghost that appears when you look across the top of the spinning blank. The axis is then shifted and the process repeated. After about three shifts, the sphere will usually be acceptable.


Note: methods 2 and 3 above require some minor calculation involving dimensions. To avoid fractional inches, make your measurements either in decimal inches or millimeters. My preference is millimeters. (1” = 25.4 mm)



A Good Place to Start


For almost any method, a good place to start is with a blank held in a scroll chuck and turned to the diameter you want your sphere to be. Advantages of this method are that the tailstock end of the blank is accessible, and at the end, you have one less nubbin to remove after taking your sphere off the lathe.


Here’s a tip: If your method requires marking a diameter on the end of the blank, mark that diameter by making a cut across the end of the blank just deep enough to be able to see and measure. This is much easier and more accurate than trying mark with a pencil, and very little wood is lost in the process.


1.  Using Templates


This idea is simple: turn away wood until the blank matches an outside template whose radius is the same as that desired for the sphere.


Considerable wood must be removed before you can meaningfully begin to use a template. A quick way to “knock off the corners” is shown in the following photos. This method does not cut all the way to the surface of the sphere; some room is left for error, but not much.

Once you begin to use the template, it’s just a matter of being patient and not taking off too much wood at any point. Work from right to left so as to leave as much wood supporting the blank as possible. Do the major portion of the sanding before you reduce the diameter of the nubbin too much.

At the end, the nubbin (or nubbins if you work between centers) may be cut away and its footprint sanded to conform to the surrounding surface. Another technique is to use the ghost method, described below.

Note: My preference is to use a sanding pad held in a collet chuck mounted on my lathe. If you do not have a collet chuck, see the article on this website that shows how to make a sanding pad holder that mounts in a scroll chuck.





Removing the Ghost – Turning between Cup Centers


First of all, a cup center is simply a short cylinder having a dished-out face so that it will make contact with a sphere only at the rim of the cylinder. The cup center at the headstock can be held by a scroll chuck, faceplate, or screw chuck. The one on the tailstock side must fit over and seat on the tail center in such a manner that it cannot wobble from side to side.

       

The near-sphere is initially mounted between the cup centers so that the axis of rotation is at right angles to what it was previously. When it spins at a moderate RPM and you look across the top, you will see a faint image of the nubbins superimposed on the round part of the sphere. The idea is to then turn away the faint image (the ghost) without touching the more solid-looking round part. This will remove the nubbins.

Next, the rotation axis is shifted to one side by perhaps 45° and you will again see the ghost. This time, however, it will be only a fairly narrow blur or fuzziness next to the solid image. Turn the fuzziness away until the outline at the top of the sphere becomes more distinct. Then shift the rotation axis to the opposite side and repeat the process.


My observation is that this method is a bit hit-or-miss. The blank, because it is not perfectly spherical, will not fit the cup centers precisely so that getting it centered is a bit of an issue. Further, it is easy to go too far in removing the ghost, which only creates a bigger ghost when the axis is shifted for the next step. And all the while, your sphere is getting smaller.


The last step is to sand the sphere while it is between the cup centers, shifting the axis every now and then and working through the grits. By feeling for the high spots, it is possible to improve the shape considerably during the sanding process.


Fortunately, a sphere that is to be used for purely decorative purposes does not have to be perfect. There can be considerable variation across various diameters, as measured by a caliper, and the sphere will still look and feel spherical.



2.  The Sören Berger Method


Begin with a cylinder whose diameter is the same as that of the desired sphere. Mark the center line and the two “ends” of the sphere.

Do the math to find the distance from the ends that the marks for the first cuts should be made. (Multiply the diameter by 0.293.) Locate and mark these lines. Also, find the diameter of the tenons at each end of the sphere. (Multiply the diameter by 0.414.) Form these tenons, which serve as indicator marks, being as accurate as you can.

Make straight cuts between the marks on the surface and the corners where the tenons meet the body of the cylinder. After these two cuts, the cross section of the blank will be an octagon.

More math: calculate the distance for locating the second series of lines. (Multiply the diameter by 0.1075.) Mark these lines. Also find the new diameter for the tenons at the ends of the cylinder. (Multiply the diameter by 0.2.) Turn the tenons down to the new diameter.

Make the second series of cuts. The resulting cross section will be a polygon having 16 sides or facets.

Next, draw lines by eye (or measure, if you prefer) that divide each facet in half. Then make a series of curved cuts between these lines to give the roughed-out sphere.

  

Do the rough sanding so smooth out any major irregularities and then turn away the nubbin on the tailstock side. Sand the footprint of the nubbin. Then reduce the other nubbin to a small diameter and separate the sphere from the blank.

Do the final sanding to remove all traces of the nubbins and refine the surface. My preference is to use a lathe-mounted sanding pad as shown above in the template method.


Here’s a link to an excellent video in which Mr. Berger takes you through the entire procedure:


https://www.youtube.com/watch?v=R4s5LziiS08


In the video he uses a special caliper to locate the lines. While the caliper is convenient, it is not essential. You can use a bit of arithmetic and a dial caliper to do the same thing, as shown in the diagrams above.


Note the way he uses a pipe cap as an aid in sanding. That tip alone is worth the price of admission.



3.  Define the Surface with a Parting Tool


This is a method I devised for the Fixtures and Chucks book. It uses a parting tool to make benchmark cuts along the cylinder to define the sphere. The idea is the same as that used by spindle turners to establish critical diameters along the length of a spindle.


Begin with a uniform cylinder whose diameter equals that of the sphere you wish to turn. Mark the center line and “ends” of the sphere. Divide each half into two equal parts, and then divide the end quarters into two equal parts.

To gauge the cuts, you can measure either the depth of the cut or the diameter that results. Choose your method and then do the math. Only two calculations have to be made.

Make the cuts. Position the parting tool to the side of the line on the side farthest from the center line. Make two additional cuts to define the ends of the sphere.

Begin by removing the waste wood between the benchmarks. Also, turn away enough wood to give some working room on each side beyond the ends of the sphere. Be careful not to destroy the benchmarks or to lose the ends.               


Make curving cuts between the benchmarks, but keep the benchmarks.              

Continue shaping the sphere. At this point you will be depending largely on your ability to visualize the correct contour. Then take the surface down until the benchmarks just disappear.

           

Reduce the diameter of the nubbins as you continue the spherical surface toward the ends. Smooth and refine the surface by sanding. Then part off the sphere. Use a sanding pad to deal with the footprint of the nubbins and refine the surface as you see fit.



4.  Mechanical Jigs


A search for “sphere cutting jig” will yield a multitude of commercially available jigs for turning spheres. Also to be found are descriptions and plans for making your own, as well as videos showing the various jigs in operation.


A jig makes it easier to reproduce spheres of the same diameter and, hopefully, give a more-nearly spherical surface in less time. However, you must still do the sanding by hand and there will be at least one nubbin that will have to be finished off the lathe.


Given below are two links and photographs of what appear to be high-end sphere cutting jigs. You can read all about them.


Carter Products

http://www.carterproducts.com/turning-tools/perfect-sphere









Lignotec

http://www.lignolathe.com/en/sphere_turning/index.htm


The Shadow Method


This technique is described in detail in the following article.


  [Back to Top]           [Go to the Shadow Method.]

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