It's in the name, end mills are really designed for milling against the end face
of the tool, slot drills are for cutting slots.... :-))
End mills can't sink a hole as the centre of the tool end face is not a cutting
face, but a slot drill has an odd sized pair of cutting faces to do just that.
It's not quite as simple as that in practice, as both can be used for either
job, but that's where it all started.
On Wed, 10 Nov 2004 22:38:17 +0000 (UTC), Peter A Forbes
Your actual question is easy to answer but you have opened a can of worms
As regards centre cutting if you look at the end of a 4 flute cutter there
are two main types.
One type has a hole in the end and 4 equal teeth run radially from that to
the outside. The second type has one of the 4 teeth overlapping the centre
of the cutter and no hole.
The latter is termed centre cutting as it cuts the centre. This cutter is
able to plunge in and then move sideways to cut a slot.
The first type can't do this as when they plunge in they leave a pip equal
to the centre hole. Trying to move sideways will only break the cutter as
it's stuck on this pip. These cutter are made for working from the outside
of work inwards.
Now for the can of worms.
There are many names given to the types of cutters, 2,3,4,5,6 flute slot
drills and the same in endmills.
These vary by location in the world and also manufactures descriptions.
In the UK 2 flute cutters were always called slot drills and 4 flute
cutters had the centre hole and were called end mills.
In the US they used the term end mills on both and this query about what
could centre cut and what couldn't occurred.
Later on manufactures started making 4 flute cutters that could centre cut
but still called them end mills.
To further muddy the waters with the advent of 5,6,and 7 axis CNC grinding
you can now get any configuration you like. Manufactures haven't helped as
they call their products by conflicting names.
Just look in a current catalog and they list 4 flute end mills centre
cutting and 4 flute slot drills, Both the same animal.
It's no good reaching for a textbook to clear this up as no books have been
written since the advent of CNC grinders. All the descriptions in Moletrich
<sp> and Chapman are years out of date.
Even different manufactures call the same thing different names.
Things are slowly getting better in the Home shop stakes as the cutters we
use, up to 3/4" normally are all being done centre cutting now. The older
centre hole series is slowly being phased out as the new grinders don't
need this run off area.
I know I'm going to get flack over this description but that's the way
things are now. It doesn't matter one iota what you were taught, they have
moved the goal posts.
Take the throwaway FC3 cutters, most are 3 flute centre cutting. 10 years
ago no one had seen a three flute cutter let alone a throwaway. Look in the
books we had as apprentices for a 3 flute, no not there are they.
In the current J&L book 3 flutes all centre cutting are listed as slot
drills and end mills depending on manufacturer.
Now we know who to blame.
I while ago I needed to cut a load of 7.2mm wide slots. I could have used a
6 mm cutter and done two passes or had a cutter reground to 7.2, I decided
on the latter.
I called round to the T&C grinders to ask how he wanted to play this,
supply one them selves or me supply one and what size.
I was told no need, they took a piece of 8mm HSS blank stock out the draw,
put it in the CNC grinder, pressed a few buttons and 5 minutes later I had
a 4 flute centre cutting endmill / slotdrill, double ended.
That's how much the goal posts have moved.
That was all very lucid John, and highly educational - seriously.
Perhaps what originally should have happened was that the slot drill
should have been called a slot cutter? The description 'drill' implies
sinking rather than passing or traversing.
You said that your recently acquired 'made-on-the-spot' cutter was
double ended. I am having difficulty with this concept! How the hell
do you hold it??? This is totally outside my experience.
John Stephenson replied thus to my query about holding double-ended Slot
Drills/End Mills ..............
Thanks for that! All a matter of collets really, a subject in which I
am not particularly well versed. :(
I realise now that of course I have seen double ended tooling previously
- centre drills.
In passing; is there a standard work somewhere which defines all the
many types of collet system? I believe that this has come up on the NG
before, but I have never delved beyond my immediate needs.
Which is why I couldn't understand why an end mill is not recommended for
cutting an open ended slot -
Which answers another question for me - why make them non centre cutting in
the first place
I showed an old timer (with US and UK m/c shop experience) a copy of the J&L
flyer - "don't know what that is, but its not a slot drill its got three
flutes!" So that explains that too!
I'd forgotten about the CNC cutter grinders. A long time ago I worked for a
company with a couple of 7 axis Huffman's so that it could not only re-grind
cutters in house, but deal with some of "speciality" cutter requirements.
Have you noticed that if you mention 7 axis CNC to someone, they start
gyrating their hands to understand what the axis might be?
Thanks John, more answers than questions!
They can cut from the open end but if doing an accurate slot they will cut
More on this below.
They had to do this as if the T&C grinder was et to the depth of the tooth
by the time it had traversed to width of a full tooth it would have cut
into the opposite tooth.
The hole is just for run off clearance. The new CNC grinders can feed in
traverse and reduce depth at the last to stop this interference.
I'm afraid we have a problem we will have to argue over and live with for
many years until someone writes a new standard text book and everyone
accepts it - not going to happen.
Your example above is both a slot drill and an end mill - or it could be an
end mill and slot drill <g> depending who make s it and what THEY decide to
Two flute slot drills [ or end mills to the Yanks <g> ] are recommended for
cutting accurate slots. Three and Four flute cutters will cut over size
because of the extra teeth.
Take a 4 flute cutter working up a slot only the two leading teeth are
cutting at any time .
Looking from the top of the cutter with it going clockwise and the slot
vertical the tooth at 12 o'clock is cutting the top of the slot as the
tooth at 9 o'clock just starts cutting.
3 o'clock has just finished and 6 o'clock is in fresh air.
Because of the resistance to cutting the tooth at 12 o'clock pushes the
cutter over so the 9 o'clock tooth cuts deeper than it should.
This doesn't happen with a 2 flute as when a tooth is cutting it's mate has
finished so it can't cut deeper.
Three flute cutters can still do this but it's not so pronounced as you
don't have opposite teeth.
The biggest fault people make with cutting slots and keyways is to run up
and down the slot. The cutter will always cut larger on the return path as
it changes from conventional to climb mode.
With a 2 or 3 flute cutter, a rigid setup and only cutting one way and
returning out the cut you can get accurate slots.
Sorry? I have trouble with the rubbing ones head and stomach at the same
Slot drills are two flute and are centre cutting, use for 'plunging' into
closed slot. Drawbacks, two flutes tend to have deeper cut flutes and thus
less strength than multi flutes. Feed rate is normally given as feed per
tooth ie. 0.1/ tooth hence with a two flute feed is reduced.
End Mills tend to have three or more flutes but are not centre cutting dot
not try plunging into a closed slot otherwise the end mill will shatter.
Modern cutters can be multi flute end mills that are centre cutting, these
have the benefit of a plunging capability with the added benefit of a higher
Hope this helps rather than hinders.
George Thomas covered this in chapter 6 of his "The Model Engineer's Workshop
His basic argument is that where a cutter having more than two cutting teeth are
there will always be an out of balance thrust on the cutter which is why end
slots and have problems when leaving the end of a slot. His analysis seems good
The slot drill can drill downwards.
The end mill cannot (reliably!) because it has no cutting
surface in the axial centre.
Solution - drill a hole first using a twist drill and then
put your end mill into the resulting hole.
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