Slot drills are generally two (occasionally three or four) fluted cutters that are designed to drill straight down into the material.
This is possible because there is at least one tooth at the centre of the end face. They are so named for their use in cutting keyway slots. The words slot drill are usually assumed to mean a two fluted, flat bottomed end mill if no other information is given. Two fluted end mills are usually slot drills, three fluted sometimes are not, and four fluted usually are not.
I usually don’t think slot drill, just two flute and four flute end mills. As a rule, I consider two flute center cutting and four flute not center cutting. There must be exceptions, but when I pick one up in my shop I think in these terms.
Speed and feed. The RPM should be determined by the material being cut and diameter of the cutter. Thus, a cutter will run at the same speed independent of the number of teeth. The feed should be determined by the material being cut and material removal per tooth, thickness of a chip. Then, a four flute cutter should move, feed rate, twice as fast as a two flute. This is what Jo said. The last element, depth of cut, isn’t as clearly defined. The depth of cut is determined by the HP of the machine, cooling, and rigidity of the machine. In hobby machining we seldom do the calc, just adjust until it feels right. I made a spreadsheet of speed and feed for my common end mills and materials. I needed this for writing CNC code. Machinery’s Handbook goes into great depth on feed & speeds, while other sources may be easier to apply. I tend to drop back from their recommendations though. Machinery’s Handbook aims for maximum production and I think holding back a little gives longer tool life.
So, a four flute has a higher feed rate. I find a two flute has an advantage in aluminum. The two flute better removes chips and has more room to get coolant/lubricant into the cut than a four flute. I’m seldom bothered by the time it takes to machine aluminum, two flutes are fine. Steel is slower so a four flute can have a time saving advantage. But it all depends on the particular cut, sometimes one just feels right.
In industry we have seen the rise of a four flute (centre cutting) slot drill. These can provide better slot cutting than a two flute when they are new, but produce a poor slot when the edges become worn. Industry also has a tendancy to flood the work with coolant aiding chip removal.
If you think of the forces involved the two outside edges will be in balance but the front cutting edge will be causing a sideways force that will get worse as the edge becomes blunt. Leading to an over sized slot.
Three flutes are a compromise between two flutes and four flutes.
For profile cutting they have less teeth so the surface is not as good as if you had four flutes. For cutting a slot the forces are at 120 degrees so they won’t wander as far sideways as a four flute slot dril as they get blunt.
Regarding the comment about slot drills being too fragile for side milling, he does not mean they will be prone to breaking but the flutes on a 2-flute cutter remove a lot of the “meat” of the cutter it will be more prone to flex away from the side giving a tapered slot
I’ve used 2 flute mills to trim the edges of things, and to widen slots, and for everything else. That was all I had for more than a year. And I’ve never broken one of those ‘fragile’ slot drills. Well, that’s not true. I did break one – I broke a 1/16″ end mill, but that was when I was cutting a slot. And I learned not to push those itty bitty bits so hard! :) But that was the only one. And I can’t imagine it was that much more fragile than a 4 flue ‘end mill’.
I would not call them more fragile but you need to think about what you are asking each tooth to do: On a slot drill you have half as many teeth cutting on the side. This means for a given feed rate each tooth must do twice as much work on the slot drill as the end mill so the side forces will be greater.
Most of us are not interested in pushing our cutters to the absolute limit so are unlikely to break the cutters but I suspect we might be interested in finish. Having twice as many teeth on the end mill means that the finish when cutting on the side is much finer.
What are slot drills? Slot drills (often referred to as slot drill cutters, slot mills, or 2 flute end mills) are cutting tools that are typically two fluted, flat ended, and double cutting. They have two opposing cutting edges boasting reinforced end teeth for enhanced durability and less material chipping.
Slot Drills are similar to end drills, but as well as being used on the edges and faces of materials, it can also be used in the central part of the application such as creating blind holes. These tools form part of our Precision Milling Cutters. The material variants include carbide, powder metal and HSS slot drills.