Slot drills

I needed to cut a 1/2" slot the other day. I didn't have a 1/2" slo
drill, so I considered using a 12mm slot drill and after one pass
nudge over 0.7mm and take a second pass. Then I vaguely remembere reading somewhere that one must only use a slot drill in a slot. I this true? What happens if you don't? Does the slot drill break? Or cu badly? These things are expensive so I'm loathe to experiment!
Brenda
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anotherid wrote:

Not true.. I do a lot of peripheral milling with slot drills, or even drilling counterbores.
I think the original term came from years ago, when all slot drills were 2 flute, and because they have plenty of chip clearance are ideal for slots.
These days you will see 2, 3 and 4 flute described as slotdrills. If they are centre cutting, then they are slotdrills. Even some 4 flutes can centre cut nowadays.
Wayne...
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On Thu, 16 Nov 2006 17:04:48 -0600, anotherid

Let me try to explain about end mills and slot drills. Many people have their own description of these but I'm afraid no one is right any longer - even me !!! First off lets go back in history and see what we had. There were 4 flute end mills and 2 flute end mills. Some could plunge some couldn't. Confused? well you will be. The two flute ones were called slot drills in the UK and were ground with one cutting edge over centre so they can plunge cut vertically down and then traverse, hence the name. The 4 flute end mills usually had a hole in the centre for clearance when grinding and so can only cut on the sides of the tool. Trying to plunge with one of these will leave a central pip that will break the cutter when you try to traverse. Now when we got towards the end of the 20th century things started changing. We had better methods of producing cutters with fancy 5 and 6 axis CNC grinders. We started to see 3 flute cutters getting popular as a compromise between rapid metal removal and chip clearance. We also started to get the 3 and 4 flute cutters with one lip ground over centre like the two flute in the picture. This means that these can also plunge. To cap it all the manufacturers put paid to any standards by calling their cutters all sorts of different names. Where a 4 flute cutter was called an end mill it can now be centre cutting and be called a 4 flute slot drill. If you look in any cutter catalogue nowdays you will find that they call a certain type of cutter by one name on one page and another manufacturer calls it by a different name on another page! The manufacturers have moved the goalposts. The cutter descriptions we all grew up with in Chapman's and Moultreich <sp>; has gone out the window with the advent of grinding techniques. I took a 20mm solid carbide 4 flute end mill [ non centre cutting ] in for regrind last month and when it came back it was 4 flute over centre slot cutting cutter. What do I call this now? The main thing to look for when buying cutters is the number of flutes you require and whether they are centre cutting or not. I can see the time when the old 4 flute centre pipped type will become obsolete as most cutters are ground up from blank material nowdays. I had a load of cam track slots to do the other week at 7.2mm wide. I had a choice of doing two passes with an undersized cutter or having a cutter ground down to 7.2mm I decided to go with the special cutter and asked the local T&C grinders to supply and regrind an 8mm cutter down. Instead they just ground the new cutter straight onto a 8mm solid blank, 3 flute centre cutting. It was on the machine for all of 3 minutes. -- Regards,
John Stevenson Nottingham, England.
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John Stevenson wrote:

It is quite easy: Cutters that can dive (drill) and cutters that can only ramp and finally those that are bad in even ramping. The later ones are those that go onto a mandrel and do have a clear hole along their CL.
With CNC, ramping is easy, with a manual it isn't that much. Drilling with a mill is always not so good, as they tend to wiggle (depending on the quality of your spindle and the rigidness of the toolholder) and make a bigger hole.
Nick
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OK, chaps, my head really is spinning now! Guess I need to do a bit mor
reading. However ... I did a quick test tonight and cutting a slot wide and wider with my two-flute "slot drill" doesn't seem to cause problem. Mind you, I only cut it 0.7 mm wider :)
Brenda
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wrote:

I think that you slightly misinterpreted what you read originally:-
"one must only use a slot drill in a slot." doesn't mean that the only place to use a slot drill is in a slot. It means the only cutter to use in a slot is a slot drill. This isn't because of the centre cutting capability of slot drills, its because 4, 6 or higher tooth cutters will tend to cut wide when used in slots. This is because the flute that is cutting at the front of the slot tends to push the cutter sideways and if a tooth is cutting the side of the slot at the same time, it will cut too deep. Since slot drills, traditionally, only have two flutes, when a flute is at the front of the slot, there is nothing at the side of the slot to make the slot too wide. More modern three flute cutters seem to work quite well also.
Don't be distracted by the fact that the flutes are helical. All non-carbide/ Al2O3/CBN/etc. cutting tools are made from rubber and bend all over the place.
Hope I haven't made things worse :-)
Mark Rand RTFM
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I finally remembered where I read about slot drills. It's in Harol
Hall's book, Milling A Complete Course. He says (page 38) -
"If however a sufficiently wide slot drill is not available, then wide the slot using an end mill as slot drills are too fragile for cutting o one side only".
Slot drills have been previously defined as those with two cuttin edges.
Brenda
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Can not understand this. "Slot drills are too fragile for cutting on one side only". When cutting a keyway only one side is cutting

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Not when used as designed. To cut a 1/2" wide keyway on a manual machine , use a 1/2" slot drill. The cutting forces on the two sides balance out (with a 2-flute drill) and the result should be close enough for ordinary mortals to 1/2". For some reason, the Americans don't seem to believe it, though <G> You might need to cut it in two or three passes, but always make sure they're in the same direction.
Cheers Tim
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On Mon, 20 Nov 2006 20:29:43 +0000, Tim Leech

Since the making of Capricorn One they have got very touchy. Never mention slot drills and three phase in the same sentence......... -- Regards,
John Stevenson Nottingham, England.
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Tim Leech wrote:

OK Tim, you lost me with that one.
When cutting, other than plunging straight in, only one side can cut, as the other is on the opposite side of the tool shank.
I have no idea how the author of that book concluded as he has, that a slot drill is too fragile to cut on one side only. ?????
FWIW, I was taught that the slot drill definition never had anything to do with whether the end was center cutting or not, but rather was to do with that the cutter with two or three cutting edges does not get pushed to the side and cut the slot wider through the flexing of the tool due to cutting forces. An end mill would generally have at least two edges in contact with the work, one sweeping across the "front" of the cut, and one on the side that the tool flexed towards as a result of the cutting. Center cutting end mills (4,6 flute)are much more common today than they used to be, I think.
In general, though, I think that "slot Drill" is rapidly becoming a phrase that has about as much meaning as "engine lathe" in the lexicon. There are so many more available tools than there were when much of the terminology was applied in an accurate manner.
Cheers Trevor Jones
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wrote:

I think I've lost myself now <G> It will of course only be cutting on one edge, but the two flutes opposite one another will help to keep the slot to the correct width. With more flutes, the sideways force on the 'main' cutting edge may force another edge to remove material from the side of the slot, resulting in an oversize slot. It may even perhaps be possible to get an undersize slot by using very slow feed with an odd number of flutes?

I can't see any logic to that either.

Should have read your whole post before starting to type. It's a bit early for my brain to be fully operational <G>

I'm sure you're right, but I'll still go for a 2-flute slot drill to cut a keyway, because I know it'll do the job. Well enough for the real world I inhabit, anyway ;-)
Cheers Tim
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Tim Leech Wrote:

You think you guys are confused? What about a poor beginner like me I'd assumed that the Workshop Practice series were the last word, a least as far as the basics go?
Brenda
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anotherid wrote:

Naahhh!
There is no last word. Not until the human race goes extinct, anyway.
Get hold of some of the old issues of Model Engineer magazine and read of the means to an end that were the way our hobby was done.
We have it SO good!
The more you read, the more options you will see, for ways to get from one point to another.
I'm pretty happy that I don't have to use carbon steel tools unless I want to, too.
Cheers Trevor Jones
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Tim Leech wrote:

Just don't worry yourself about it.
If you want to peripheral mill with a 2 flute DO IT Just increase the spindle speed a bit.
Me personally. I rarely use 2 flute cutters these days, not really through choice, just 3 flute + are far more versatile to me, as I do more CNC milling than anything now, and slots are normally cut with a smaller cutter than the slot width.
Wayne....
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Wayne Weedon wrote:

Apparently, there is a large market for the two flute cutters in the CNC world for cutting light alloys. The speeds and feeds are pushed very high, and there is benefit to be had from the chip clearance available.
I watched a gantry mill carving sections of wing skins from solid Al. blocks (1m x 3m x 40cm thick). They said that they were running a .5 inch, two flute cutter, and that they had about 35 horsepower at the spindle. They had a couple guys with snow shovels clearing chips off the table behind the gantry.
Cheers Trevor Jones
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Trevor Jones wrote:

True in most respects. Though these days there are special high helix cutters for al-alloys. I can run 3 flute cutters are pretty high rates in al-alloys, but limited by my 8000 rpm spindle ;)

A local firm mills stringers from solid. Thet fill a 6 yard skip in a shift!
Wayne...
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On Mon, 20 Nov 2006 13:18:45 -0600, anotherid

Bloody hell, 42 years wasted.................................................
. -- Regards,
John Stevenson Nottingham, England.
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So is the trick if you don't have a slot mill, to drill a pilot with a conventional drill that is larger than the hole in the end mill, and the to use the end mill?
- Stewart
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clearance
to
the
Yes, or if you are cutting a slot, to zig-zag down rather than plunge directly.
AWEM
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