JimG
Western Thunderer
When I got my CNC milling machine, my original, primary intention was to make driving wheels for locomotives in S scale, until I got waylaid into building coaches. However, I want to start making some locomotives, and one of the ones I hope to start is a Caledonian 498 class - the dock tank designed by MacIntosh. Dave Lockrie posted an excellent picture of one in the Caledonian Railway Association forum
http://www.crassoc.org.uk/forum/viewtopic.php?f=8&t=165#p975
In that same thread, Mike Williams also supplied an extract from a Caledonian plate showing the detail of the wheels with their large balance weights.
I used this picture as a guide to milling out a centre for the wheel and the results of the first attempt have been promising.
First, I dug an ancient bit of half hard brass strip out of the scrap box to use for a blank, set it up in the mill and faced off an area to 3mm thick.
Next came the first shaping of the boss and the balance weight using a 2mm diameter cutter...
Next came the final shaping of the boss, and the flaring of the spoke ends next to the boss. For this I used a 2mm diameter ball end mill. This cutter finished the boss to size, and produced a curved fillet around the boss to reproduce the flare on the end of the spokes.
Next came the main cutting operation, with a 1mm diameter slot drill cutting out all the apertures between the spokes, then making the main cutouts round the wheel to release it from the main strip. Four small tabs were left to retain the wheel in the strip to avoid any possible problems with the wheel moving and causing damage if it was cut right out.
You can see the waste bits in the apertures still in place since the cutter had not quite cut right through, but had left a very thin layer which retained the waste.
I then used a piercing saw to cut through the tabs round the circumference, and I pushed out the thin metal holding the waste bits, and I was left with the wheel centre.
You can see the remains of one of the tabs on the left hand side of the wheel centre. Eagle eyed readers will also note the centres drilled for the axle and the crankpin - a step that I omitted to take a picture of.
I had machined the boss to be 1mm thick to be turned down in the lathe to be 0.8mm thick - primarily to get a turned finish on the boss - although the milled finish came out better than I had expected. So the centre was drilled out and push fitted onto a mandrel on the lathe and the b oss turned down to 0.8mm thick, and the outer diameter was tidied up to get rid of what remained of the tabs.
I took the edge off the spokes with a scraper to get closer to the shape of the prototype's spokes and made a comparison with the prototype.
I've got close but I'm not quite there yet. I am going reduce the thickness of the boss - possible down to 0.5 - 0.6mm, and use a larger radius ball end mill to get a larger radius on the spoke ends. I'm also going to thin the spokes slightly.
I'm also trying to work out the best metal to use. I looked at the 3mm and 1/8" brass strip I had to hand and it was definitely not flat - the bit if old scrap was the flattest bit I had. I might have to source some clockmakers' plate which I believe to be flat.
Jim.
http://www.crassoc.org.uk/forum/viewtopic.php?f=8&t=165#p975
In that same thread, Mike Williams also supplied an extract from a Caledonian plate showing the detail of the wheels with their large balance weights.
I used this picture as a guide to milling out a centre for the wheel and the results of the first attempt have been promising.
First, I dug an ancient bit of half hard brass strip out of the scrap box to use for a blank, set it up in the mill and faced off an area to 3mm thick.
Next came the first shaping of the boss and the balance weight using a 2mm diameter cutter...
Next came the final shaping of the boss, and the flaring of the spoke ends next to the boss. For this I used a 2mm diameter ball end mill. This cutter finished the boss to size, and produced a curved fillet around the boss to reproduce the flare on the end of the spokes.
Next came the main cutting operation, with a 1mm diameter slot drill cutting out all the apertures between the spokes, then making the main cutouts round the wheel to release it from the main strip. Four small tabs were left to retain the wheel in the strip to avoid any possible problems with the wheel moving and causing damage if it was cut right out.
You can see the waste bits in the apertures still in place since the cutter had not quite cut right through, but had left a very thin layer which retained the waste.
I then used a piercing saw to cut through the tabs round the circumference, and I pushed out the thin metal holding the waste bits, and I was left with the wheel centre.
You can see the remains of one of the tabs on the left hand side of the wheel centre. Eagle eyed readers will also note the centres drilled for the axle and the crankpin - a step that I omitted to take a picture of.
I had machined the boss to be 1mm thick to be turned down in the lathe to be 0.8mm thick - primarily to get a turned finish on the boss - although the milled finish came out better than I had expected. So the centre was drilled out and push fitted onto a mandrel on the lathe and the b oss turned down to 0.8mm thick, and the outer diameter was tidied up to get rid of what remained of the tabs.
I took the edge off the spokes with a scraper to get closer to the shape of the prototype's spokes and made a comparison with the prototype.
I've got close but I'm not quite there yet. I am going reduce the thickness of the boss - possible down to 0.5 - 0.6mm, and use a larger radius ball end mill to get a larger radius on the spoke ends. I'm also going to thin the spokes slightly.
I'm also trying to work out the best metal to use. I looked at the 3mm and 1/8" brass strip I had to hand and it was definitely not flat - the bit if old scrap was the flattest bit I had. I might have to source some clockmakers' plate which I believe to be flat.
Jim.