Going great guns with my new mill, but questions arise which are not in the book.
Tubal Cain says to use slot drills rather than end mills for open ended slots, but doesn't say why. (So I've been using a three flute mini mill!) Another book the author "prefers" to use slot drills for open ended slots, but not why.
Your actual question is easy to answer but you have opened a can of worms here.
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 All the descriptions in Moletrich and Chapman are years out of date.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.
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?
They can cut from the open end but if doing an accurate slot they will cut oversize. 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 depending who make s it and what THEY decide to call it.
Two flute slot drills [ or end mills to the Yanks ] 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 time.