The Fundamentals of Custom Chucking archive

Monday 19 October 2009

John Lovatt looks at custom chucking and mounting options

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Many articles have been written on work holding but for this article, the aim is to link practical turning techniques with elements of practical application. So pick an application, get into your workshop, bedroom, or wherever you do your creative thing, and have a go.

The plethora of mechanical chucks and the variety of jaws available today has generally made work holding easier, however, it has introduced a reliance on simple jaw holding to the detriment of traditional skills.

This presents an unseen, self-propagating barrier to tackling many projects, where as many older methods can be incorporated into what can generally called "chucking."

The objective of this article is to demonstrate how the performance of proprietary work holding equipment can be better utilised. Hopefully you will discover the sense of achievement and satisfaction that can be experienced from practicing and incorporating these ideas. Beyond simple bowls, there's a whole other woodturning world out there!

This article picks up on last issue's Fundamentals of Indexing, but takes the method forward to show how versatile such a simple self-made mount can be in turning the main parts of virtually any type of spoked wheel such as ships, wagons, spinning wheels and toys for example, even picture frames and mirrors too.

The self-made chuck jaw mounted boss does not necessarily need to have integral indexing if your chuck has the required indexing holes.

Making a wheel rim

1. Turn the core mount. See the indexing article in issue 192, and consider the tip in Step 3 for choosing the size of the boss.

2. Make up a segmented form (4, 6, 8, or more pieces) to create a wheel rim. For strength, stepped lap joints, dowels, or other techniques are necessary.

2a. One of the easiest ways to create a strong joint is to bond two overlapping layers. Then, turn the form in two stages; firstly by holding the form externally whilst the external diameter, bore, and one face are turned then by holding it in the bore and turning the remaining face.

If you have suitable extension plate jaws for your chuck, you could miss out part of this but the inner rim disc will still be required. However, there are advantages to the method shown here: it is easy to turn; offers better concentricity; and there is virtually no risk of getting any heart stopping metal to metal contact.

3. Saw two discs in plywood or MDF - 112mm (1/2in) thick would be ideal for most applications, but this will vary dependant on the wheel size. Make one of the discs to a diameter about half the rims finished depth. This will be the backing disc.

The second disc needs to be cut a little larger than the proposed inner diameter of the wheel rim. This is because it will be turned later to match the inner rim diameter. Both discs require a good registration bore - use the core mount boss as a gauge to ensure

a good fit.

4. Bond the segmented form to the backing disc as concentrically as you can. (The backing disc can be used as the assembly platform for the segments).

5. Fit the core mount to the chuck jaws and screw the segmented form and backing disc on to the core mount. Two screws will suffice for small work, but increase to four for large wheels.

6. Turn the outside diameter of the rim, then the front face, then finally the bore. Turning the bore will involve cutting a little way into the backing board - this does not matter as the board is a sacrificial item. A segment is also known as a Felloe (Felly in the USA), and the completed segments form the rim or felloes.

7. Part off the backing disc close to the as yet unturned piece - if this is carefully done the rim will not fall, but will be easily detached by hand. For safe working, turn down the feather edge left by the parting tool.

8. Reverse the backing pad on the location boss to act as a registration face or axial stop. Using longer screws, fit the inner rim disc over the location boss. Skim the disc edge to create a jam fit for the rim - the rim makes the perfect gauge to get it right.

Clean up the remains of the backing plate and turn the face of the rim to width.

Drill index holes. This features a split hub design where it is desirable to have the rim tenon as an integral part of the spokes.

Making a wheel hub

9a. Turn the extended centre section of the workpiece roughly to size as indicated by the lighter background. Size the taper to a firm fit using the core mount as a gauge.

9b. Hold the workpiece on axis when you come to forming the location recess, the taper must be a good fit with no wobbles. If it is not good enough, make the plain end stub diameter to suit one of your chuck jaws. For the location boss, size is not important because it will act as a gauge to ensure a good fit when you come to turning the location recess. Finally part off.

10. This is the cut away of internal shapes.

11. Fit the core mount back in the chuck then drive the tapered end of the workpiece into the tapered socket. Form a snug fit recess to accommodate the location boss.

12. Assemble the two parts and turn to a final shape. Bore through for the axle: if you get any slippage occurring during the drilling use tape to bind the two parts together.

12a. This is an example of the split boss assembly method - notice that the spokes (or staves) have integral tenons at both ends. This allows the spokes to be set in the rim (or felloes) as well as lying in one half of the hub. The other half of the hub is then added to complete the assembly.

12b. If the hub boss is made as one piece, the rim tenons will need to be separate in the form of dowels, and the rim end of each spoke bored out accordingly.

Mounting without extension jaws

Extension plates undoubtedly have a useful role to play in woodturning but like all proprietary equipment, they have their limitations. If you are unlikely to make many large items, particularly bowls, the expense of such tooling must be taken into account. Fortunately, the underlying principles described in making wheel rims also apply to other items. To demonstrate this, lets take a look at a bowl that features a flared foot ring.

1. A faceplate ring is probably the most used and most useful accessory available for initial work mounting. It provides the means to mount all sorts of add on self-made work holding devices. Here, it is providing the initial mount to turn a dovetail recess to match the jaws.

2. Take the piece off the chuck and remove the faceplate ring. Hollow out and turn the rim then to finish, apply lacquer or oil if you choose. Avoid over-tightening jaws (a common fault). To ensure a more efficient grip, make the dovetail angle slightly less than the angle of the jaws.

3. This is the start of making the bowl mount, and again MDF is the chosen material (thickness is dictated by the size of the bowl and by its rim form. Generally speaking, 18mm (3/4in) would accommodate most projects). Fix the faceplate ring to a sawn disc of MDF and mount to the chuck. Then form a shallow 3-4mm (1/8-5/32in) dovetail recess to suit your jaws.

4. Remove the disc and faceplate from the chuck and remove the faceplate from the disc. Remount the disc to the chuck jaws, and, using the faceplate as a gauge, carefully create a recess with a diameter that provides a snug fit with no movement - about 3mm (1/8in)-4mm (5/32in) deep. This is the register that will ensure the disc will always be on a concentric axis with your lathe when you use the disc again for another project.

5. Fit the faceplate ring to the disc, assemble to the chuck jaws and check for concentricity.

Woodworkers Institute

Tagged In:

John Lovatt , fundamentals , chucking , mounting , options

Glossary Rollover a term to view its definition

Faceplate , Woodturning

Handy Hints

1. For step 3, if you have a saw-tooth or Forstner bit of about the size of the core mount boss, bore a hole in a piece of scrap material to act as a female gauge before turning the boss. This way, discs can simply be bored before cutting discs. Mark out and drill screw holes to suit the core mount face: you may need to drill four holes if the wheel rim is very large. Ensure the holes are on equal axes
2. For step 8, a slight taper lead on the edge of the disc edge helps. Carefully reduce the diameter using a flat scraper until the rim is a snug driving fit – if you go too far, resort to paper packing