Sensible Slot Drill Size for Mini Mill

There are several factors that come into play such as feed rate, lubrication, cleaning away debris and others that will give good performance from any size bit...the number of flutes and rotational speed can also play a role...

Have a look at these suggested tips...

https://www.cnccookbook.com/10-tips-for-cnc-router-aluminum-cutting-success/

Ta, I'm not CNC but some useful info there.
Their suggestion of 3/16 would tend to point towards 4mm being a sensible minimum and prob' 2 flute :)
Del

If it were me, I just saw it off on my band saw and then make one face cut to the proper length.

That was just an example. I want to mill out some more complex shapes with corners that can't be simply sawn...
I was just trying to be sure that people understood... but I obviously failed
Del

Oh, OK I've got it now.

By the way, you said, "Imagine I want to cut 6" off a 10" length of flat 8mm Aluminium bar."

If you cut 4" off a 10" length of flat 8mm Aluminium bar, wouldn't you have less waste.

I just can't help myself sometimes.

I would still go with Lyn's suggestion and then apply the profiling as required to the cut end.

It's always safer to have a small mill tool cutting on one face only - assuming that you don't apply too much force on that face, than it is to use it as a cut off tool where it is working on multiple faces at once, this puts a lot of load on the tool and will often result in breakage.

I assume you can't use a slitting saw due to the length of the piece and, being a small mill, not having a horizontal shaft.

If I really need to cut a small slot then I use a slot drill with a larger shaft as in my pics below, you can see a 2mm, 2.5mm and 3mm all with 6mm shafts and a 2mm straight cutter, it's obvious which would be stronger.

Of course you could bury the smaller one further into the collet chuck to give greater rigidity, but it makes it very hard to see what you're doing.

I agree that for having minimum waste on a straight cut, and to vastly reduce breakage, a slitting saw is the way to go, also assuming the mill spindle can be made to turn at an appropriate speed. I have custom made extra-long arbors to reach down far enough.

Del: I've been meaning to ask whether your mill has provision for collets. if it does, use them whenever possible. When using end mills to cut sideways, they have a terrible tendency to gradually work out, making a deeper cut than intended. Also, using a collet will make it possible to use much larger diameter (and therefore more rigid) shafts.

Two carefully planned slitting saw cuts can make an inside corner.

Yeah, I#ve got a set of collets and I leave the collet on the mill.
I've got the pillar drill on the other end of the bench with a 3 jaw chuck so I have the best of both worlds, quick and dirty with the drill or more precision with the mill.
Del

Thanks Tony very useful answer
Del

The other problem with small cutters in aluminium is the tendency to clog with swarf. 2 flutes & plenty of coolant will help.

when using any under size machine such as a mini mill, the technique that i have found most valuable is to take several passes using the end of the end mill.

overloading the machine will create lots of vibration which will show on the surface of your cut and require a final light pass anyhow. too much load on a tool will also apply force on your x-y table which will show you how much slop you have in your machine. again, when i say "show you", i am referring to a poor machined face.

after you have used your machine, you will become aware of how large a cut you can take on a paticular type of metal. soft metals such as aluminium would require a 2-flute end mill so the chips will not clog it.. 2-flute also is used to bore holes too.

Yes ... exactly... and that's why I'm asking what a sensible compromise slot drill diameter is, for light cuts on a small machine when cutting out profiles... using several passes and light cuts.
I know not to force the machine. But the question is... what's a sensible minimum optimum slot drill diameter?
Plenty of great answers to questions I haven't asked
Del

1/8" to 3/16" seem robust. Carbide seem more forgiving for speed on something like stainless. below 1/8", the end mills are very weak and break easily. a 2-flute bit has less cross sectional area than a 3-flute bit and therefore weaker. maybe even consider 5 to even 8 flute bits for more strength.

this info is the same for those who use small taps, more flutes equate to more strength.

You also need to understand "Up cutting vs. Down cutting", small machines have a big problem with up cutting material.

Ta
Del

Thanks everyone.
I've ordered a 4mm 2 flute carbide slot drill for aluminium with 10mm flute length which will do nicely for my 8mm Ali' . Only £4
Del

You ordered only one? That almost guarantees breakage!

Use generous amounts of lube to reduce buildup of aluminum on the cutter.

Another possibility is to use a standard drill to make a series of holes, spaced a few thousandths (hundredths of a mm) more than the diameter of the drill, so all the slot drill has to do is to cut the webs between the holes.Then you can use a much larger diameter cutter to clean up the surface. With soft materials like aluminum and copper, the final light cut should be a climb-mill cut, to avoid re-attaching bits of swarf to the finished surface.

As someone else pointed out, climb-milling tends to push the workpiece in the same direction the leadscrew is moving, with a high tendency to break the bit if there is much play in the leadscrew-nut joint. For light work like this, you can probably push or pull on the table to minimize sudden movement.

WD-40 is really good lubricant for Aluminium. great for drilling, milling and lathe work.

Agreed!

Why Flute Count Matters - In The Loupe - Machinist Blog

Something I didn't mention.

An end mill, has typically four cutting teeth and generally can't plunge, i.e. drill a hole and only machines from the side.

A slot drill, has typically two cutting teeth and can be used like a drill, be gentle. As the name suggests these are typically used for milling slots. Examine the end of the cutter a blade must pass across the centre to plunge.

Standard length is the most rigid, but long series are available.

Sometimes the cutter has a screw thread on the end to suit a Clarkson type collet and this thread stops the cutter being dragged down, most collets can use either cutter type, but the threaded type is more expensive.

Briefly mentioned is climb cutting, where the action of the cutter drags the mill table in due to the backlash in its feed thread and can break cutters.

Tony

Del, I couldn't imagine you, " too lazy ".

LOL... well no, but I'm working on it
Del