Horn blocks



The Hornblocks need to be machined to mate with the frames. Holding the raw casting can be difficult, so a jig similar to the one in the picture can be made that allows several of the initial machining operations to be carried out in one setup. The casting needs to be filed so that it sits flat without rocking before being bolted down to the jig. There is a raised location piece on the jig to give a more positive location. Castings can vary so this has to be checked before hand against all of the horns to make sure it's an appropriate size.

Holding the horns this way allows for the whole mating face and it's shoulder to be machined in one setup. The bottom of the horns can also be machined at this point if so desired. Make sure that the hornblock is a close fit in the frame.

Machining the other faces is relatively easy but it may be advisable to leave an additional 0.1mm or so on the side that mates with the frames. This allows that face to be finished exactly flush with the frame when it has been riveted on.

Riveting the horns.
A rivet snap can be made from Silver Steel using a ball nosed slot drill to form the shape of the rivet. There's no need to harden it.

Bolt the horns into place using screws in all of the holes except the one you're riveting. This should prevent it from moving.

The rivets are driven into countersunk holes on the frame. Take great care not to bruise around the area you're riveting. This is another reason for leaving a little extra material on the horn flange!

Filing the finished rivet flush is a tedious job, so a milling machine can be used to remove the bulk of it. Clamp the frame onto parallels and spot onto the top with a slot drill. You can touch down the stationary cutter near to each rivet and note the position so the cut can be stopped just short.

Machining the inside of the horns.
Make sure that any spare flange material from the horns is filed flush with the frame surface. The frames must mate perfectly with each other of the machining of the inside of the horns. Use the reamed E to locate the frames to each other using dowels if you did that when you machined the frames.

For those with a milling machine, this is one way to do the job.

Note:- Before you start, make sure that the head is clocked vertical to the table using a tramming method. Alternatively, machine a diameter onto a piece of brass held in the spindle, using a lathe tool clamped to the bed. The diameter is created using the movement of the knee on the type of mill pictured.

Clock the frames level and true to the Y-axis travel. Use as much support as you can. The setup pictured took a whole day to get to this stage.

A long series end mill is used to machine the inside of the horns. This can either be done using small cuts at the full depth, or roughing out to almost the finished size in small steps. In either case, it's best to use one cutter for roughing and a fresh one for finishing.

Getting the geometry as accurate as possible is the aim here. Ideally the centres of the three horns should be as close as possible to the drawing dimensions. The exact width of the slots is less important.

Finishing the horns.
The inside of the horns should be as parallel as possible so that the axlebox can move freely with a very small clearance. Slip gauges can be used as shown to make sure that the horn is parallel all of the way to the bottom. The last few millimeters on the horn shown were a little tight, and had to be eased with a needle file. A test piece could be machined as a gauge to perform the same function. Slip gauges come up on ebay for a huge range of prices so they aren't out of reach for the home workshop if you're prepared to bide your time and grab a bargain.

Hornblock Stays.
The plans don't indicate that there needs to be clearance to allow the rods that hold the springs to move without binding.There needs to be +/-1mm movement either side of the centre line of the holes as shown. This allows the axle to lift to the fullest extent without it touching.