Kevin Rackham

When it came to making a dome for the Pansy, I weighed up getting a casting, which I considered too expensive and they still need machining. So, I looked round to find what material I had in stock to make the  dome.

I did have one piece of solid brass that I could have machined it out of, but I thought that the amount of machining to be carried out would have been excessive and very wasteful. Therefore, I opted to make it out of three pieces that I had in stock, two aluminium blocks one already had a large hole through it, one solid and a bit of a brass shell case for the middle section.

I then got out my various ball turning attachments. I checked and none of them would make a ball to 3.25” diameter, which was the diameter of Pansy’s dome.

I decided to make a large ball turning attachment for the Myford lathe. As this was made during the lockdown at the beginning of 2021, everything had to be made from what I had available, as I couldn’t just go and see one of the local engineering firms that usually supply my bits and pieces. The criteria I set my self was as follows:

1) It had to be as big as I could get away with on the Myford.

2) It had to be rigid enough so that I could get a good surface finish on the workpiece.

3) The cutting radius had to be easily set, as some of the other radius turning tools that I have are a pain to accurately set the radius on.

4) It had to be able to be mountable on the Quick-change tool post, for ease of setting the tool at centre height I found two Uniballs from a scrapped mobility scooter steering to make the pivot points. I could then design and make everything to fit these.

The main frame was made from 20mm square mild steel bar. The 16mm square piece that is held in the tool holder was tenoned, Loctited and screwed into the back bar. This was also reduced in width to fit the quick-change tool holder.

The Uniballs were spaced away from the back bar by two identical spacers. These are held in position by two 10mm countersunk screws.

The inner frame back was made from 16mm square mild steel. The hole for the tool steel was drilled and reamed to fit ¼” HSS tool steel as this is what I had. The tool bit is held in place by a 5mm grub screw.

The top and bottom arms are made from 25mm x 10mm mild steel plate. The two pivot holes were set exactly ½” & 1½” from the end, this allows easy setting of the radius (photo 1, above).

It is also possible to set the radius using a depth micrometer for greater accuracy. The end of the top and bottom arms were relieved slightly to allow bigger radii to be machined. I made a couple of spacers so the Uniballs cleared the top and bottom arms. The ends of the arms are recessed to fit the frame back and this holds everything square and is held in place with 6mm countersunk screws and Loctited for good measure.

The handle was made from a piece of round bar, threaded at one end to fit to plastic knob that I had found. The bar was then heated to red hot and bent to right angles. The handle fits into a hexagon bar with a grub screw. The hexagon bar was drilled through on all faces so that a suitable orientation for the handle was achievable.

A problem was found when I started to use the tool, in that the tool could go past the centre point and hence was then rubbing the tool rather than cutting. The temporary solution, the G-clamp (can be seen in photo 3 below), this was holding a block in place so that the tool cannot go back past the centre axis.

After the steam dome was finished this bar was fixed permanently (photo 2 above), this also allows greater flexibility on the sweep of the radii using the tool.

Now onto the dome itself. The solid bit of aluminium was put in the chuck and the centre drilled and bored out to clear the inner dome. A step was also machined on this end to accommodate the brass centre section.

Once this was done the block was reversed in the chuck and held on the inside of the block. If I had held it on the outside the chuck jaws would have hit the ball turning attachment. The bar was faced to length and drilled and countersunk for the fitting screw. And I also roughed out the shape using an HSS tool.

The ball turning attachment radius was set and then set up to the lathe axis and the tool to centre height and touched on the end of the block. The cross slide was then locked and the leadscrew dial reading was noted. The saddle was wound away from the job. Cuts would be applied by moving the saddle to the noted reading on the leadscrew dial. Machining the section of dome (Photo 3, above).

The centre brass section was to be machined next. As the brass tube is very thin-walled if this was just clamped in the chuck it would distort the brass. Therefore, a piece of aluminium was bored out to fit the outside of the brass tube and then a hacksaw cut was put through the ring. This was used to clamp the tube in the three-jaw chuck. The bore was machined out and the end faced to fit the top and bottom section of the dome. The tube section was reversed and faced to length (photo 4, below).

The bottom section was bored to fit over the inner dome boiler bush and faced to length. I had previously made a gauge out of aluminium for making the top section of the boiler cladding for clearance over the boiler and under the side tanks. This gauge could be used again to work out the distance for the curved recess for the base of the dome. The curved base was fly cut on the milling machine (Photo 5, below, the gauge is also shown).
A mandrel was machined to press the dome base onto. (because of the fly-cut section there was no way this could have been reversed and held in the chuck jaws). The mandrel was also threaded, and a plate made to hold the dome base in place. The outside diameters were roughed out and the step machined accurately to fit the brass section. The bottom flared profile was roughed out using a ball nosed end mill by eye (photo 6 below). And finally finished with files, scrapers and emery cloth.

The three sections were pressed together using Loctite and a final fine cut taken on the outside diameter to blend it all in. The finished dome was left on the mandrel for painting.

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