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Making slots is a very common task. How it is done depends on some key properties of the slot.
does the slot go from one side of the workpiece through to the other side?
how wide is the slot?
how deep is the slot?
how long is the slot?
Is the slot accessible from the end of the slot?
Is there more than one slot, if so, are they parallel?
slots inside holes, ie, keyways, splines
There are many ways of making slots slots:
horizontal milling machine
vertical milling machine
slotting head on a vertical milling machine
lathe fitted with a vertical slide
Making slots where the slot goes from one side of the workpiece through to the other side
If a milling machine is available then the overwhelming preferred solution to this would be a horizontal milling machine. For most slots a slitting saw or a side and face cutter fitted on the horizontal arbor would be able to cut a slot of almost any depth, any length, any width and with an excellent finish.
With just one slot so long as the cutter can cut the full depth of the slot it does not matter what the diameter of the cutter is.
fig cutting a single slot in the horizontal mode
It is also possible to mill more than one slot at a time. It is possible to cut any number of slots at once. But in practice, since the depth of the slots relative to each is important it means the diameter of the cutters relative to each other is important.
fig cutting two slots in the horizontal mode
It is possible to use a slitting saw on a stub arbor fitted to the vertical head to cut a slot but holding the workpiece becomes more complicated. It the job can be done using a vice to hold the workpiece on a horizontal mill then if the vice is held on an angle plate then a slot can be cut using a slitting saw in the vertical head.
fig cutting a single slot in the vertical mode
Apart from the inconvenience, the other difference is that in the horizontal mode the length of cut is the movement of the table in the x direction, on the vertical mode the length of cut is the movement in the y direction, which is, of course, much less.
It will also be noticed that, with more than one cutter, the width apart of the cutters can be much greater in the horizontal mode than in the vertical mode because the length of the horizontal arbor is so much greater than that of the stub arbor. Part of this is because the horizontal arbor can be supported at the far end.
Making slots where the ends are closed
If is possible to cut a slot that is closed one or both ends with a slitting saw but usually where the end is closed it is usually necessary for the end to be vertical.
If the ends of the slot are closed but the material is very thin it is possible to cut narrow slots using a slitting saw. In this case the ends will be very close to square if the cutter is allowed to right into the slot. (With thin material it is easier if the material is supported underneath, right next to the slot, and the cutter cuts downwards into the material.)
The simplest solution is to use a slot drill. This will cut downwards without any problems. An endmill can be used but its ability to cut straight down is very limited.
Either way it is seldom possible to make the full depth of cut straight away, it is usually necessary to make a series of passes, lowering the cutter a bit more on each pass. The depth in one pass for a slot drill might be quite large, say, 10mm, if the material is soft, such as aluminium, and the mill is powerful enough. Whereas will an endmill it will be limited by the depth of the center of the endmill, say, 1 to 2mm.
Using endmills or slot drills the end of the slot will be rounded. If the slot goes through the material then square corner can be filed. If the slot does not go through it might be possible to cut more of the ends out by using a smaller diameter endmill or slot drill.
As the cutting diameter of the cutter gets smaller the tendency for it to bend or break increases dramatically. When the cutter is less than about 6mm this becomes a serious problem. The commonest solution is to reduce the amount being cut.
It will be noticed that if the load is too great the cut will not be straight.
With a very narrow cutter the tip is more prone to wander when the cut is very shallow. Even so if the cuts are small enough, it can be done.
Making slots inside holes
Neither of the above methods will work inside a hole. Single slots inside holes are used as keyways, multiple slots can be used to make splines. Both are used are methods of hold a part on a round shaft. A single slot as a keyways is usually adequate for doing this. A spline has the great advantage that the part can slide along the shaft. This is often used in gear boxes.
In both cases it is usually important that the fit is good. If this is not the case then it might be possible to use a file to cut the slot. Otherwise the choices are some form of slotting tool or a broach.
making a slot using a slotting tool
making a slot using a broach