Article that appeared in the Australian Woodworker
Issue # 109
This is a
By Con Jansen
The idea was to turn a big bowl of about 8'6" (2.6m) diameter using only standard woodturning tools. Following a lively and extensive discussion it was decided to give the project the green light.
Making the Lathe
A special lathe was quickly designed as the majority of us had worked with machinery all our lives. We started with a two speed differential assembly from an 8-ton truck, as this would provide a very strong body and its large hub bearings would be able to handle the heavy load.
The differential was welded to inverted V-type legs. These legs were then welded onto a base of box form steel, which splayed out to a footprint of about 9'6" (2.9m) square. All the steel used was 12" x 6"x 1/2" (300 x 150 x 12mm) section, with the base sitting on shoes welded at the four extremities, giving the lathe good stability with a centre height of 4'6" (1.37m).
Because we had a 6-ton, 115hp farm tractor at our disposal, we decided to use it to our advantage to drive the lathe and positioned the tractor up onto the base to hold the whole unit down solidly. We estimated that with the tractor attached, the unit had a total weight of about 7-tons This worked well, even when it was set up and used on a lawn grass surface.
Photo 1. Completed Lathe without driver.
We connected the tractor drive using a modified utililty tail shaft attached to a four speed gearbox which was mounted under the differential centre. The drive was then transmitted vertically from the gearbox using a 3/4" pitch chain to the differential's input shaft.
We were able to use the left side brake on the differential as the clutch by attaching it to a hand brake lever assembly. The right side brake - also attached to another hand brake lever assembly - was used to bring the work piece to a stop once the clutch (left brake) was released. This system worked extremely well, allowing us to very gradually bring the workpiece up to operating speed. With all the ratios available we were able to rotate the workpiece from 15-500rpm in overlapping stages.
Photo 2. The completed base would be attached and driven by a tractor drive.
Laminating the Bowl
Because an 8'6" (2.6m) diameter tree was not available, it was necessary for us to laminate the bowl blank. Our saw miller member generously donated the required amount of Mackay Cedar, a very attractive local timber.
For the base of the project, a 3'6" (1.07m) solid piece of Mango wood was turned outboard on an ordinary lathe and the face plate, made from a 20" truck wheel rim was attached using six 1/2" (12mm) bolts. The finished base was then placed horizontally on a 200 litre drum so that the laminating process could begin.
A half size profile of the bowl was drawn onto a piece of ply mounted on the wall, so that we could plot our progress and calculate the lenth and width required for each successive layer. We decided to construct the bowl using layers consisting of 12 segments.
Photo 3. Weldbond Professional was used to glue the layers together.
Gluing the Segments
As the bowl was to be 2'6" (760mm) deep it would require 25 layers glued together. Mackay Cedar is a little tricky to glue and we decided after some testing to use Weldbond Professional. The company generiously donated all the required glue for the project for which we were very thankful. It proved to be easy to use and exceptionally strong.
Reinforcing the Segments
Each segment had two 3/8" (10mm) dowels inserted going down into the two layers below, at the same angle as the bowl profile. As we did not want any dowels showing, getting the angle right was very important as the wall thickness was to be less than 3" (75mm). At times, we used up to 60 clamps per layer to ensure that all the joints were tight and neat. In all, 298 separate pieces of wood were fixed together using 1,213 gallons (7.5 litres) of glue and 88" (26.5m) of doweling.
Photo 4. Up to 60 clamps were used at times to hold the bowl segments in place.
Attaching the Blank to the Lathe
Getting the bowl blank from horizontal to vertical position and onto the hub studs required lots of care and hard work as the laminated bowl consisted of 444 super feet (1m3)of wood and weighed approximately 1,452lbs (660kg)
Turning the Blank
The big moment came when we rotated the blank for the first time. It turned without any wobbling or vibration and there was only 3/4" (19mm) of runout - the result of careful measuring and some good luck.
The large diameter of the bowl together with the laminated construction created some real problems because our gouges were cutting along the grain -
Photo 5. The blank required lots of care and hard work to get it from horizontal to vertical.
something that does not happen with normal size turning. As we were using normal size turning tools, there was a real danger of long splinters of wood coming down the tool and injuring our hands. To prevent this happening, we welded guards onto the top of the chisels. This was only a problem when we were roughing down from the intial pine-apple surface to a rounded one. From then on it was simply a matter of using normal woodturning tools and procedures.
Photo 6. The very first cut of the blank.
Improvising and Adapting
Turning a bowl this size took some getting used to and included new experiences such as:
* The sheer size of the bowl turning in front of you.
* The time it took the bowl to do a full revolution.
* While working, everything in the turner's vision was
revolving except the tool rest, and the strange
sensation of sometimes standing inside the bowl
- a never to be forgotten experience.
A Big Bowl Needs a Big Toolrest
Our two freestanding tool rests were constructed using truck rims filled with concrete, having a cantilever tool post that could be adjusted to reach inside the bowl. This allowed two turners, one at the front and one at the rear of the bowl, to work simultaneously on the project.
The finished bowl was treated with Penetrol, as it has been proven that this product gives excellent results with Mackay Cedar. When told of our desire to use this product, the company generiously provided us with the required amount at no cost.
The almost finished bowl was displayed at our November 2002 Turnout, and proved to be of great interest to those attending, with quite a few turners having a go at it. We received a lot of praise from our visitors, who could not believe that our small group could achieve such a big project with no outside help. This proves that our group philosophy and motto of helping one another really works.
The group spent around 6 months on the project, sometimes working at night, giving us lots of experience, fun, satisfaction and an appreciation of
of each other's skills, as well as the generosity of all the people who contributed materials and gear that enabled us to complete the project.
The bowl will be on display at the Proserpine Museum. We have dedicated the bowl to our late member, John Doyle, who quite unexpectedly passed away before the finishing touches could be performed.
Photo 7. The almost finished bowl on it's way to the 2002 Turnout.