View Full Version : slot mills end mills

Alistair Hosie
08-17-2005, 05:48 AM
I have a friend who used a slot mill in the lathe to bore a dead end hole in a piece of aluminum (spelled the good old American way) http://bbs.homeshopmachinist.net//biggrin.gif
Anyway he showed me how to spot the difference between end and spot mills.
I am none the wiser.Please someone talk me through the differences so that I finally get it at last Alistair

John Stevenson
08-17-2005, 06:15 AM
Let me try to expain 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. These are the ones on the right in JCHannum's picture.
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 Chapmans 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.

John S.

Taken from http://bbs.homeshopmachinist.net//Forum1/HTML/012329.html

[This message has been edited by John Stevenson (edited 08-17-2005).]

Alistair Hosie
08-17-2005, 06:29 AM
Thanks John now you can see whay an amateur like me gets confused http://bbs.homeshopmachinist.net//biggrin.gif I seriously think there will be many amateurs who like me get confused over this one, so this may be a good thread to continue if anyone else is also confused here is your chance to clear it up http://bbs.homeshopmachinist.net//biggrin.gifAlistair

Mike Gibson
08-17-2005, 09:22 AM
There are 2 flute straight sided double end mills shown in the Penn Tool Co. catalog on page 140. Slotting is what it is designed for.

Is this what is referred to as a slot drill?

Ian B
08-17-2005, 03:33 PM
Drilling is an operation where the tool is plunged into the work along its axis, and milling is (generally) where the cutter is moved sideways through the work.

Therefore, shouldn't these little beasties be called either "end drills" or "slot mills", irrespective of the number of flutes, over centre edges etc?

Often wondered this...


Alistair Hosie
08-17-2005, 03:44 PM
surely the point is that slot mills don't just drill they both drill and mill, as oposed to end mills which can't drill ??Alistair

08-18-2005, 03:11 AM
The collection of cutters one has assembled over a period of time since the seventies gives all sorts.
There are cutters that are all of the above and of course round nosed cutters with four edges. Some with screw threads and some with their own screw on collets which have two lugs in the top for driving I guess. some with flats for holders of various kinds and busted ones. Because I do not use above 1/2"cutters it has been easier to buy a new one when ever the price is right.
My trouble is sharpening the ones worth saving. I have started on a grinder but it is slow going. There must be a better way than all those ball handles on a quorn.
Any suggestions.

John Stevenson
08-18-2005, 05:41 AM
Yes but it's up for ridicule *AGAIN* http://bbs.homeshopmachinist.net//biggrin.gif http://bbs.homeshopmachinist.net//biggrin.gif

Posted this a couple of times before to total apathy, derision and hoots of maniacaly laughter but when I get world patents in a few months you twonks are going to laugh on the other side, http://bbs.homeshopmachinist.net//biggrin.gif http://bbs.homeshopmachinist.net//biggrin.gif http://bbs.homeshopmachinist.net//biggrin.gif

OK staged shot BUT two have been built in the UK and work.

You need four off the shelf parts.
A bench grinder
An import X - Y table
A Spin Indexer
and a brick.


Added to these 4 parts you need a piece of heavy angle or a proper machined adaptor to hold the spin indexer on the X - Y table and a base plate to bolt the X - Y table to.

The rest is self explanitory.
The brick goes under the grinder to get the hight up http://bbs.homeshopmachinist.net//biggrin.gif

The spin indexer and the X - Y table will give you all the moves needed for sharpening cutters.
The rest is up to your imagination.

Now laugh away.

Sir John.

[This message has been edited by John Stevenson (edited 08-18-2005).]

John C
08-18-2005, 06:43 AM
John S, Looks great but dummies like me need more detail: What angle do I present the mill to the wheel? Am I grinding the inside edge of the flute or the outside? How do I advance the mill along its spiral?

Sorry to be dim. John C

08-18-2005, 10:21 PM
One additional point worth mentioning in the 2-flute / 4-flute discussion is the effect of cutting forces in a slot. If you plan on cutting a slot the exact width of the cutter you want a 2-flute (or maybe 3) rather than a 4-flute. With a 4-flute, as the cutter feeds forward, the sidways deflection from the cutting tooth will deflect the left side tooth into the sidewall. The 2 or 3 flute cutters don't have a tooth at that position to dig into the sidewall. Thus, in this situation a 2 or 3 flute will indeed be a better slotting tool than the 4-flute. (And it took me 25 years to realize this.)


Alistair Hosie
08-19-2005, 05:21 PM
These little indexers are brilliant they have come down in price quiote a bit they can be bought for around £30 now I see some joker on ebay trying to sell a second hand one for buy it now of £70 I suppose some poor soul might bite.Alistair

10-12-2005, 03:29 AM
Sir John,
That looks as if it could be made to work. The angle of the grind should not be a problem as long as sufficient clearance to avoid rubbing is given. It takes so long to get a clearance for a patent these days that I can probably sort it out and grind a little cutter before you tie up the loose ends. love the brick. much more accurate than the roadside rock.
my little 5"shopmate might be up to the job but I think I will have to spend some hard earned and get a 6".
I have this ex-food blender motor and I think I can get sufficient speed with pulleys and I was on the lookout for a cup wheel. one of those white ones.
We will see .
thank you,

Norman Atkinson
10-12-2005, 04:23 AM
Sir John,
On another posting, i have just posted a very similar idea to yours.

Sorry, no attempt to steal your thunder.
Just great minds thinking alike.
Question- did this come from Tony Jeffree?
I mentioned this at a ME Exhibition to him.

Your loyal Page


John Stevenson
10-12-2005, 05:17 AM
Baron Norman,

No the idea for the T&C grinder came off the top of my head.
I did mention it to Tony as we often meet at shows.

Sir John

Your Old Dog
10-12-2005, 07:08 AM
Sir John, wouldn't you have to levitate that puppy horizontal so as to NOT get a concave grind on the face?

Thanks for the explanation on the cutters. I'm still a little confused but I can refer my inquires to Alistair for clarification!

Norman Atkinson
10-12-2005, 08:32 AM
This is something which is a matter of choice. My preference was for a cup wheel to be facing the 3 way vice. In my workshop, I milled the vice bit off. One way of getting rid of vices!

This was in preperation for a flat top in MS plate and thereafter, the choice of a tool holder is one of preference.
Whilst Sir John has opted for a 5C collet job, there is no reason why any other tool holder could be adapted. At the last show,
I showed BikePeter, one made out of wood.
Sort of out of my own head!
My project fell by default as I picked up a clarkson for £125- and then for £200 a whole crate of goodies.

I also looked at the X Axis and felt that a lever of sorts would improve on what is an idea just waiting to take off.

Like Sir John, I cannot see what the hang up is. Maybe, it is too simple.

My compliments, Sire,



Lew Hartswick
10-12-2005, 10:51 AM
I just completed a holder to sharpen Silver and
Deming drill bits on the fixture I previously
built to do endmill cutters (ends only) yesterday.
Was going to post a pix with this but can't see
how. In any event it worked pretty well. It does
generate a series of flats on the end rather than
a nice "spiral cone" ? (what shape is that?)

10-12-2005, 02:41 PM
John S -
I would love to be able to sharpen drills and cutters effectively!
I read that you have a cd for sale on ebay which describes tool and cutter grinding, would it be much work for you to add step by step instructions for your spin index setup and how it is used for different cutters? Perhaps a short video?
Or - I could get the stuff and experiment.

10-13-2005, 03:04 AM
A CD?? Tell me more!

On the original subject: When I asked about this a year or two ago, the best thing I learned (from Evan, IIRC) was that 3 flute end mills cut clean slots to size!!

2 & 4 flute bits almost always leave chatter "waves" along the sides of slots - at least in my poorly set-up projects - milling on the lathe.

Just something I thought was worth repeating. Thanks again Evan.


and where can I get Sir John's CD?!!?

10-13-2005, 03:06 AM
On Sharpening: How's the Tinker? Seems I remember end mills were one of its strong points.


Norman Atkinson
10-13-2005, 07:10 AM
I recall this debate developing into a similar one between those with a Quorn and those with Norman Tinker's original design.
That was in 1973! One must quietly take stock as we move further into the fourth decade since those days!

Forty Years On- which would make a second Alan Bennett play, we are still at it.
Since that time, many techniques have been introduced with new materials.
John S has suggested a cheap and seemingly progressive design which should satisfy the needs of most workers whilst new off the shelf tooling is readily available.
Today, we should look at prices. For those who seem to have studiously ignored them, I suggest that a sheet of paper and a calculator will reveal just how much kits for grinders cost and then add the additional costs of motors wheels, nuts, bolts and all the other nick nacks needed before the first tool is reground.
After that, and I suggest that I am still up in cost accountancy, what is the cost of labour to reach that point.

I have a Quorn, I have a Kennet and I have built a Stent and have another on the stocks. I have also a Clarkson with almost all the bells and whistles which cost less than the price of one kit.

Gentleman, this isn't an ad for John S's CD, it is a statement of sound commercial practice.

Or would you rather be a fish?