Click on the loco to go back to the 25NC home page

25NC Front Truck

Where to start? Well according to Keith Wilson who writes in the Model engineer, make something people can recognise and you can cart around. Well he recommends the front truck so it seemed like a good point for a kick off.

Click here to have a look at the detailed drawings.

So just break myself in I thought a bit of easy turning would be in order so a start was made on the wheels

After roughing out the basic shape I mounted the whole thing on a mandrel to allow finishing of the OD then shrunk on a steel tyre. I found the gas cooker on nine would expend the tyres by about 10 "thou".

After getting all the wheels heated up and the tyres shrunk on the next job was turning the outside tread profile. For this I decided to make a mandrel and do all the turning with the wheels mounted on it

Well after the easy start on the wheels I decided to make a start on the front truck frame. The frame had been designed as a 3 part iron casting. I wish now I had pushed the pattern maker to make it in one piece But at the time it was a more cost effective solution and Terry was a bit more comfortable making it in three pieces. Looking now at the job he made of the trailing truck pattern, a one piece masterpiece I don't know what he was worried about, still more of that in the future.

The truck frame was designed with four pockets and corresponding arms that fitted to the pockets. So as a start I need to mill out the two pockets per side frame. This was relatively easy just getting the thing set up to be as square as possible being the only time consuming part.

I used a cobalt roughing cutter to get through the surface hardness of the casting and I must say the cutters turned out to be a life saver where I could not get a tipped tool in to do the work. In retrospect this pocket idea was a compromise from the start there was a better way to design this but that is with the benefit of 20-20.

 

The next stage in the process was to machine the center bolster, for bolting the two side frames to it. I try whenever possible to do everything in as few settings as possible. This reduces the margin for errors but generally makes some of the machining a bit of a challenge.

To start with I clamped the casting onto the table and drilled the pivot clarence hole in it. This would also provide me with a suitable hole to clamp the casting down in future operations.

 

Once the hole had been drilled I then turned the casting over to take a skim off the top face to again provide a datum to work off.

Once the casting had been turned over i set it up square in the X and Y directions and machined the wings that bolt into the pockets. This had to be done at fairly steady feed rates due to the tool extension I used, to enable it to be machined in one setting. At this setting though i could get both the overall width and length set as well as the pocket width and length..

The last operation was drilling the holes for the bolts, I drilled tapping size first as i was going to match drill through into the side frames. Once the drilling was done i was ready to drill and bolt up the frames to the bolster.

So the next step was to temporarily clamp the side frames to the center drill though with a tapping drill and clamp the frames up.

 

Not the most elegant of solutions but it worked well. I fitted a bolt to each side then drilled the rest of the holes before drilling out the bolster holes to be a tight fit on the bolts.

As I said in the frame section the format of the page is going to change as it makes it a lot easier for me to code the page.

The full size frame had two cross members between the forward pedestals. As the frame had been cast in three pieces it was necessary to replicate these using mild steel bar.

The cross members stated off as two pieces of black bar which were milled to form a slight taper to the front side

Two shots of the milling operation and the taper being formed on the front side.

Once the bar had been machined to shape and also length the frame was set up on its side to drill the fixing holes for the blocks

Drilling through the frames to spot the blocks prior to tapping.

Although you cannot see it in the above snap the frame had pads cast on where the blocks would fit so with the pads machined to just proud of the frame it was easy to get the blocks the correct length. Just a slight tap fit into the frames which made it easy to spot through the frames to locate the holes for tapping in the blocks.

Countersinking the bolt holes for the cap head screw.

Once the frames were drilled through into the blocks, the frames were opened out to take the heads of the cap bolts as eventually the holes would be filed up to disguise the construction method. So we now had the frame in one piece and it was now time to sort out the pedestal liners or horn blocks as us Brit's call them. So first thing was to sit the truck on its back, this would enable all major machining operations to be done at one setting. To do this I had purchased a long series cobalt cutter of 16mm diameter. This had a flute length long enough to machine the faces of the horn blocks to their full depth.

Taking a cut of the bottom of the horn blocks

First job was to face the bottoms of the horn blocks this allowed all the other measurements to be offset from them. I then used a digital edge finder to center the casting and zeroed the digital read outs for the X and Y axis. As all the measurements would work away from these datum points

Cutting the faces

in the above shot you can see all the machining areas needed on each axle pedestal. I started by machining the outside and inside faces. The truck uses cannon box axle boxes so the outside face is the least important. The inside faces though had to be accurate.

Then using the datum of the center of the frame I moved out so the cutter was on the center of the axle then machined away form it on each side to get the two faces the correct width to take the axle box. This again was an important dimension to adhere to and I took only very light cuts towards the end to ensure the dimensions were correct.

The last job was to machine a step on the feet for the pedestal binder to sit in. Again you can see this step in the shot. shown below. You can also see the cut line on the face as I worked my way down with the cutter.

You can see the filler where the bolt holes were in the above shot.

The last job on the frame was to undercut the black bar blocks to represent the original shape of the casting.

You can just see the radius formed behind the cutter on the end blocks.

It was now time to turn the frame over to complete the machining of the top side but I will describe that later.

 

 

 

 

 

Home | 25NC | Traction Engine | Gallery | Contact | Links | Can you help? |

©2007 Northernsteam