The sketch shows us that the end mill only cuts at the tip of the end teeth and not
across its full width. That though is dependent on the end teeth not being convex
and to ensure this it the case they are always sharpened very slightly concave.
Another factor that will affect the load on the machine is the state of the cutting
edge. Handling a brand new HSS cutter it will soon be realised that the cutting edges
are almost razor sharp and must be treated with caution. Typically holding them with
a piece of cloth when screwing them into a collet. Tipped tools though, even new
ones, have a less sharp edge compared to HSS end mills and therefore place more load
on the machine. Of considerably more importance however is the tipped tool's lack
of positive helix and rake, as was illustrated by Photograph 10, that will dramatically
increase the loading. For this reason their use should be limited to use with cast
iron where a hard spot on the initial surface of a casting could do serious damage
to a HSS tool. Ideally, the tool should only be used for surfacing using the end
of the tool with depth of cut being kept to a minimum, that is, just sufficient to
remove the surface skin.
If however, you find it necessary to surface a casting using the side of the cutter
it will be necessary to be patient and step the cutter down the face in very small
increments, say 2-3 mm maximum, and make multiple passes. Once the casting's skin
has been removed it will be OK to use your HSS cutters that will speed up operations
It should be noted that it is vital that the feed directions should always oppose
the direction of rotation rather than both moving in the same direction.
Some further brief comments worth bearing in mind!
ALWAYS fasten the two axis not being traversed for maximum rigidity when making a
cut. Also, the helix angle of an end mill can, if being used to cut an appreciable
depth, draw the saddle forward if not locked, deepening the cut as the workpiece
is traversed, either by the cross or vertical slide.
Releasing an axis can result in a very small movement of the workpiece and may cause
a machining mark if the cutter is rotating. Always ensure the cutter is away from
the workpiece if rotating when releasing a slide.
Having set up a part for machining check that the amount of traverse available will
enable the required machining to be completed fully before commencing the task.
This is a more difficult one. Slides in the horizontal mode are always pushed or
pulled along using their leadscrews. However, for the vertical slide this is not
always the case, in particular if carrying a large vice and or workpiece. In this
case the slide can fall under its own weight causing errors in reading the slides
dials. This will be especially so if it is a borderline case, sometimes being pushed,
sometimes falling. Ideally, the slide should be set stiff enough for this to not
happen. However, it may then be stiff to operate when fitted with a very light component.
You must be aware of this problem if working to tight tolerances. Ideally, you should
adjust the slide to cope with the weight of the assembly fitted onto it.
Fitting the vertical slide to the cross slide can sometimes very marginally distort
the cross slide causing it to stiffen up. You may therefore find it necessary to
change the cross slide adjustments between milling and turning.
The limited space between headstock and vertical slide make's it all too easy to
catch your knuckles on the end mill in the chuck whilst making adjustments to, or
measuring, the assembly on the slide. Always move the saddle as far from the headstock
as is possible and cover the cutter with a few layers of cloth.
Photograph 16 shows the items included elsewhere on the site and that have been made
using just a lathe and drilling machine. These show how versatile the format is and
having just a lathe and a drilling machine is no bar to an interesting hobby!