go to the page above this one – making round to flat adapters – concave


Smokebox saddle – round to flat adapter

This is interesting because there are at least four different ways of doing this.

1        workpiece on an angle plate on the milling table using a boring head

2        workpiece on an angle plate on a rotary table on the milling table, vertical head tilted, ball ended endmill

3      workpiece on box, vertical head horizontal fitted with boring head

4       workpiece on box, vertical head fitted with horizontal arbor


1        workpiece on angle plate on milling table using boring head

2        workpiece on angle plate on rotary table on milling table, vertical head tilted, ball ended endmill


3      workpiece on box, vertical head horizontal fitted with boring head


4       workpiece on box, vertical head fitted with horizontal arbor


This is useful where a long, curved surface needs to be cut. This shape is very commonly needed wherever a round shape has to be fixed to a flat surface. An example of this would be the support for a smokebox. This could be done using a lathe with a faceplate and with an angle plate. However this is much easier to set up on a milling table.

This can be cut with an endmill. If the endmill is not long enough to cut the full length of the workpiece then it is possible to tilt the vertical head. In either case a better finish would be achieved using a ball-ended cutter.

In the example instead of clamping the workpiece to an angle plate a vice is used.


Fig. 453 – milling the base for a smokebox ****

Another method of doing this was discussed earlier. This used a boring head as the cutting tool and the workpiece being held in a vice mounted on an angle plate. The method described here is probably able to remove metal faster.

Suppose the workpiece is exactly the right size but without the curve cut in it. Suppose it is solid though it might need to be milled out afterwards. It has to be mounted on an angle plate so it is square with the rotary table in two planes at right angles. It can be held on by drilling it and then tapping it to take bolts which will hold it in place.

The bottom of the workpiece is set slightly above the surface of the rotary table by placing it on a parallel.

The curve is going to be cut between the corners of the two edges near the center of the rotary table. The radius of the curve will be solely determined by the distance from the center of the rotary table to each of the corners of the workpiece.

The essential geometry is shown in fig.

Fig. 454 geometry for milling a round to flat adapter – 1008

The radius “r” is set be centering the rotary table and the spindle. The Y direction is locked. The table is moved “r” along the x direction.



At first sight it might seem that what was needed was a long endmill. But it does not follow that this would give a good surface due to the endmill bending. However if the vertical head is tilted over a short endmill could cut a surface limited only by the z movement of the milling machine.

But an endmill has (or should have) a sharp corner. A better solution is to use a ball ended mill which should give a smoother surface.

Alignment – smokebox problem

The trick here is to fit a spigot in the hole in the rotary table with a center in it. A pair of dividers is set to the required radius has one point in the center in the spigot. The other point should then touch the corner of both edges of the workpiece exactly.


rotary table horizontal – horizontal mode

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