Hi again,

the mortising technique that you use depends on the type of mortise bit that you are using. Some mortising bits can cut sideways, while others will only have an end cut.
Although bits that cut sideways can cut a mortise by moving back and forwards you will find that this will create certain problems with the mortising machine that you will be using.
Because although this works with a router bit on a router there will be certain differences between the bit you be using on your machine and a router.
The first difference will be that the router will be turning the bit at above 20,000 rpm, whereas the combination worker will be only turning at 3000 or up to 6000 rpm.
The high-speed is necessary to cut sideways smoothly and not get chatter. At 6000 rpm the sideway cut will create vibrations on the end of the bit and eventually variations in the width of the mortise.
The next difference will be the length of bit. The length of the bit determines the maximum depth of your mortise. Mortising bits are usually far longer than the standard router bit so end vibrations when cutting material sideways will increased because of the leverage effect. This usually makes it much more liable to vibration when cutting sideways with the tip of the bit.
For this reason, even though I use often mortising bits that can cut sideways like a router bit, I would always drill overlapping holes into the mortise before cleaning up the sides with a lateral sideways movement.
The router type mortising bit will deliver a cleaner interface surface to the mortise than a simple drilling bit, so it is preferable to use a router type mortising bit, but if the mortises are over a couple of centimetres deep. I would certainly still drill perpendicular holes into piece before moving sideways.