Milling - first cuts

So I've just got my first mill up and running sojust had a go at some actual machining. I've bought myself a set of cutters from rdg tools and thet are labelled as slot drills and end Mills. To me the only real difference Sean's to be in the number of flutes (2 or 4) Are either of these suitable for 'plunge' cutting? I'm trying to machine a slot that doesn't break through the side of the work and I'm not having much luck but I'm doing much better when feeding in from the side.

Slot drills (2 flutes) are for cutting slots.

End mills (4 flutes) are for milling the sides of blocks of metal etc.

You can plunge a good 2-flute slot drill down into the metal to start a slot.

You can't plunge a 4-flute end mill straight into the job as it has a non-cutting area in the middle where the cutting edges almost meet but not quite. But if you drill a hole in the job first with an ordinary drill bit, it's possible.

But a 2 flute slot drill is designed for slot cutting because four flutes can do weird stuff on the corners and when breaking though the edge of the metal out into space because of the geometry and uneven forces.

If the cutting edges on a two flute slot drill don't meet correctly in the middle, you may have to drill a hole for it to go down into too.

Practice on some scrap first.

Edited By Hopper on 16/03/2022 12:50:39

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

The slot drills should be, the end mills may or may not.

Look at the ends, if the cutting edges extend right across the face, then they should be capable of plunge cutting. If the cutting edges leave a gap in the centre ( end mills) then they won't plunge cut. Many of the small end mills 3 and 4 flute ) I have do centre cut and will plunge.

One trick I have learned when milling a slot like the one you describe, is to plunge cut ( or drill with a suitable size drill bit) to your target depth at each end. Then position the cutter in one of the holes to a reasonable cutting depth, then mill the slot across to the other hole using several passes.

I find it easier to get the end position of the slots accurate this way than trying to hit the same end point for each pass of the mill on the handwheels. A DRO would probably help.

Sorry the others got there while I was typing.

Edited By Peter Cook 6 on 16/03/2022 12:57:20

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

Something that was lost on me as a newbie was the direction of the milling is key, have a read about climb milling, its makes a heck of a difference to the outcome for me.

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

Not sure what you mean here - climb milling can be a recipe for disaster!

Thanks for this. I've definately been ticking a number of the issues highlited.

Scesp steel of unknown origin

Being tentative with the cutting

Climb cutting

Not working out the correct speed

Don't see the relevance when cutting slots.

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.

Apologies, poor English on my part. I was cautioning against climb milling. I was completely unaware of the concept until I’d destroyed a couple of cutters and parts.

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.

And just to make life complicated... climb cutting can sometimes be better, with less vibration and a better surface finish.

This is true even for small mills. I have a Sieg SX1L, which is about as lightweight as you can get, and often climb cut with it. Be cautious at first if you try this, and make sure your gibs are adjusted properly, but not too tight or you won't be able to feel what's going on.

edit: NDIY beat me to it

Edited By Andy Stopford on 16/03/2022 21:19:53

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

Just to clarify:

Climb milling removes more metal per watt and produces a better finish but it requires a well-adjusted powerful machine in good condition to take deep cuts. Most hobby mills aren't rigid enough to take deep climb cuts and they also hammer the drive and motor.

Conventional milling stresses the machine far less but tends to leave a poorer finish. So I remove most of the metal by milling conventionally and then finish off with a shallow climb cut, not deep enough to cause any vibration.

Dave