In most cases the traditional two flute slot drill will be ctr cutting.

However you may find it hard going plunging a 12mm slot drill into say steel on a small machine, in cases like this it would be better to pilot drill each end of your slot say 10mm so that the slot drill will not have much work to do as it plunges.

An alternative is to ramp down into the cut that is slowly feed downwards as the tool moves sideways approx 2deg won't put much load onto the end of the cutter.

The 2-flurte design should also produce a slot closer to the nominal size than a 4-flute.

Despite above post you can also get ctr cutting 4-flurte end mills which can be plunged but I don't think yours have that facility. Also the end cutting edges of a 2-flute cutter should not meet in the middle one is longer than the other so it can cut.

Edited By JasonB on 16/03/2022 12:54:52

I wouldn't start from here. The cutters look to be poorly ground on the end, although both are theoretically capable of plunge milling. I buy cutters from professional tool suppliers. At least one can then be assured that they are properly ground. On a slot drill one edge is ground for the full radius for plunge cutting, the other edge is shorter:

Similarly 3 and 4 flute cutters intended for plunge cutting have one edge at full radius and the others are short. The only professional cutters that I have seen with two, or more, cutting edges at full radius are those with radiused corners, intended to provide an excellent surface finish on final facing cuts, like this:

Low power mills need all the help they can get, which is why I use professional cutters.

Andrew

RDG's description of these as Titanium Slot/End Mills raised my eyebrows! Pretty sure they're not made of Titanium, or are the special type of cutters used to machine Titanium.

Not a problem though, because they're almost certainly the usual HSS coated with Titanium Nitride for extra hardness. Might even be the same set as the ones I have (not bought from RDG) which work well.

I only plunge and slot with 2-flute cutters. You can get 3-flute cutters which are sort of dual-purpose. (I want to try them but keep forgetting to order any.)

The problem may be asking too much of the cutter and/or the machine. Or that the metal doesn't machine well: unknown scrap is often nasty stuff and ordinary mild-steel a little difficult. Beginners may take a while to find their machine's sweet spot. It's some combination of RPM, cutting area, and feed-rate. Too gentle and too aggressive operators are both bad: cutters are rapidly blunted by tentative rubbing, and the machine may not be rigid or powerful enough to take heavy cuts. I set the RPM according to the diameter of the cutter, and don't attempt to go much deeper than 20% of tool diameter. Feed rate is done by ear – I like the machine to sound noticeably loaded but not labouring.

Lots of coolant is a good idea when slotting and it's essential to remove swarf. Mincing swarf is another super-quick way of blunting tools, and blunt cutters are poo.

Dave

I was merely responding to the above comment about climb milling which is ambiguous – the outcome could have been a decent finish or a broken cutter/ruined workpiece. I've no qualms about climb milling on the CNC which has ballscrews and minimal backlash but very cautious on the manual mill.

Climb milling has a place – even with relatively small (as in ‘hobby&rsquo milling machines – a finish cut can provide a far better finish than conventional milling. Just need care – a light cut is essential.

A few pictures for interest, milling the edge of tooling plate, 25mm thick, with a (worn) 10mm carbide endmill. First, conventional milling and cutting dry:

Climb milling, cutting dry:

And conventional milling with a squirt of WD40:

Andrew