Making the cones
In this page we will cover:
- Constructing the work pieces, work holders and other simple jigs
- ways of measuring the outside and inside cone shapes
- Some tips on turning and boring the cones
For small cones it is effective to use solid rectangular blocks for the work pieces, whereas for larger cones less quality wood will be used if work pieces are built up.
Solid Work Pieces
Generally the work pieces for each cone are two rectangular solid blocks glued together with a paper joint, only one of which will become a part cone and used in the designed ribbon. The other will also become a part cone and it can be used to make a secondary ribbon but this ribbon will be different from your design. If you are not going to make this secondary ribbon then that block can be of waste wood. Figure 1 shows a cross section of a typical work piece, work holder and the cone that will result.
The cone centres (lathe axis) should be marked before gluing the two rectangular work pieces together. The distances from the joint surface are given as offsets in the "cone details table" generated in the "specify" page. A positive number indicates the centre is in the rectangular block that will be used in the ribbon and a negative means the centre is in the other rectangular block. Both centres are to be in the middle of the work piece width. When both parts of the cone are to be used in the designed ribbon (which I have termed the symmetric case),the end of each block that will be glued to the work holder is angled to provide a flat surface for this joint. The two identical blocks are glued with a paper joint to a wedge of waste wood. The cone centres (lathe axis) are exactly centred on the end faces of the wedge and should be marked before joining. No wedge is necessary if the addition of the two shape factors for the ends of the cone is 2. In both cases the assembled work piece will be aligned on a work holder by a small alignment dowel say 1/4in. (6mm) diameter which projects from the centre of the work holder face by 5/16in. (5mm). The centre of the hole which accepts this dowel is one of the centres marked above. Where the face of the work holder is not at right angles to the lathe axis the paper joint in the work piece must lie at right angles to the slope of the work holder face. This can be achieved by marking centre lines on the work holder and the work piece. See photo 1.
Built up Work pieces
There are at least two ways of building up a work piece, laminating or stave construction. The latter is very effective when the cone axis lies on the joint surface but can still be used if it doesn't. Then it requires that two of the staves for each cone be made up from 2 pieces (with a paper joint) such that the joint line is at the correct angle to the cone axis. This angle is given in the "cone details" table generated in the specify pages. Laminating however will use the joint surface as a starting place for the laminations. (see figures 2 and 3);
In both methods you can make the work pieces rectangular or slope them to follow the shape of the cone. Also the cone small end will need to be filled and drilled centrally for the alignment dowel.
Drilling alignment holes in the cone work pieces
To achieve repeatability use a simple jig (photo 2) to hold each cone work piece accurately and securely under the drill bit on a drill press. This hole is at right angles to the face of the cone work piece, but the dowel is not. Taper the projecting part of the dowel a little and the cone work piece will align on the maximum dowel diameter right at the angled face of the work holder.
I've tried mounting the cone work piece between centres but when the cone axis doesn't lie on the joint surface I could not ensure that the work piece was angled correctly nor could I get repeatability. Hence the use of a work holder for every cone. The work holders mount into a standard chuck, they also provide a means of locating the cone work piece centrally, at the correct angle and orientation. Unfortunately a work holder is required for every cone as the work holder will be cut into during the turning of the cone. A simple jig (see photos 3 and 4) enables the efficient production of the work holders.
Start with a stick long enough to make all of the work holders, wide enough for a work holder and thick enough to accommodate the chuck boss and the angled face.
Mark the edges and centre lines of each work holder and drill the centre holes to fit the locating dowel you are going to use. These centre lines will be used to circumferentially orient the cone work piece.
Cut the face angle along the whole stick using a table saw and separate each work holder
Mount each work holder in turn on the jig (Photo 2) and turn the chuck boss.
Fit a short length of dowel into the centre hole so that it projects about 3/16in. (5mm) out of the angled face and glue a cone work piece onto this angled face ensuring that the centre line on both the work piece and the holder are aligned (photo 1). Do not use a paper joint here.
Measuring the cone
Outside: small end
You can use the small end diameter or the small end chord length. photo 5 shows the position on the cone work piece where the measurements should be made. It is actually at the termination of the joint line. I find that the diameter is easier to measure at the small end.
Outside: large end
The diameter here can be difficult to measure particularly if there is significant "dish" in the design. The chord length is often a better choice. Either way the measurement should be taken at the termination of the joint line. See photo 5.
Using a template/gauge ensures the correct wall thickness at both ends of the cone and the correct cone angle and is much easier to use than trying to measure the wall thickness. It will also accommodate the curve on the inside surface that minimises the joint edge mismatch during assembly. Make the gauge out of something thin and stiff (for example: plastic, aluminium or steel) using the information given in the "Turning information" table. Initially the sides will be straight but will be modified when the first/test cone is turned. See figure 4
I highly recommend that you make at least 2 extra cone work pieces and holders. One to use as test piece and the other as a spare.
Do not turn the work piece into a cylinder as the over hanging end of the cone is a larger but unknown diameter than that at the chord (end of joint at large end). Instead turn the work piece as a cone from the beginning. You can estimate the cone shape by sighting against the inside template whilst the lathe is turning or use it to adjust your tool rest to the cone angle.
Make the cone shape a little steeper than it should be whilst you cut to a diameter a little larger than the required diameter at the small end. Then correct the slope of the cone as you turn down the large end. Check that along the cone axis the cone surface is straight.
If this is the first/test cone place a straight edge along the joint line (not along the cone axis). Hold the straight edge so that the gaps at each end of the joint line are approximately equal and estimate the size of this gap. Draw lines parallel to the sides of the inside template the same distance away from the template edge you estimated above. Mark and cut a smooth curve from the small end of the template, down to the line and back to the large end chord (see figure 4). This curve adjusts the inside surface to minimise the amount of cleaning up that will be needed.
Before beginning to turn the inside strengthen the paper joints by stretching masking tape over them as these joints can give way as the inside is turned.
Drill out the centre of the cone using a drill a little smaller than the small end of the template. Drill into the work holder a small distance. Bore out the remaining waste and use the template to determine when you have cut to the correct surface. This is when the line on the template that represents the large end chord lies coincidental with the end of the joint line and the inside surface touches the template along its length.
Problems that can arise
If you turn one end diameter a little too small then turn the other end the same amount too small. The allowances you made on the inner and outer ring radii will mean you can sand away the small end of the cone, thus effectively increasing the cone diameters.
If the paper joints should part whilst you are turning the inside, tape them tightly together again, without glue. They'll hold until turning is finished.
If the cone comes off the work holder, which it can occasionally for small diameter cones with thin walls (1/8in. 3mm.) do not despair. Bore out the work holder so that the cone (and it's masking tape) jams into place. Make sure it is running true and spot glue it with hot melt glue. If there is not enough wood in the work holder, or you run into the lathe jaws use a waste block to make the jam chuck. See photo 6.