Squaring off the bottom of a blind hole

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I'm am so surprised at how often this comes up, and that the understanding of terminology has somehow still not crossed the pond in either direction. 😂

A slot drill is a center cutting endmill. That is all. We do not call them slot drills here. I've nver seen a 2 or 3 flute end mill that wasn't center cutting. I've never seen a non center cutting (true) endmill other than the ancient stuff acquired in drawers of retired machinists tool boxes. And those were only 1/2" and larger.

Of course, if you look at modern tooling you'll find stuff that isn't center cutting, inserted and not, which requires a ramp tool path to cut axially.
Be aware that center cutting endmills have a 1 - 2 degree relief ground in the flutes which
means that it leaves a convex , not flat, surface at the bottom of the hole.
This means a 1/4 " diameter hole may have a .001 crown at the bottom - not flat, but not noticeable either
That makes a fairly good seat for a ball if you are making a check valve. If I don't feel like grinding a single lip cutter for the job, I will use an end mill for this.
O.K. A picture 1000 words etc.etc.

As I have always known it :-
#1, 2 & 3) are a 2 flute, 3 flute and 4 flute (resp.) slot drills - they can be run back and forward in a blind slot (like a Pratt & Whitney keyway slot) and at the end of each run you can plunge down axis to your next depth.

#4 is a 4 flute end mill which cannot be plunged down axis - you can only mill "passing through" type slots. There is no relief on the hole in the middle - you can plunge only microns before problems set in - so you don't mill blind slots with this.

#5 is a hand sharpened flat bottomed drill c/w split point.

#6 is a HSS "Bullet Point Pilot Drill" touted for rapid drilling in wood but makes an excellent drill for counterbored holes - only contrary to normal practice you drill the counterbore first - and follow with the clearance drill on the pre-drilled pilot hole.
You even get these in high Cobalt for use on stainless.

Regards, Ken
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When cutting a groove, or into a corner, any end milling cutter tends to pull to one side as a reaction to the cutting forces. With a two flute cutter, at the point in its rotation where the flutes are lined up at right angles to the direction of feed, there is no lateral reaction force, and this means that it will cut a slot pretty much the nominal diameter of the cutter even with a heavy cut. That is why it is called a slot drill. Cutters with more than two flutes will be subject to lateral forces throughout the rotation and will therefore cut a slot accurate in neither width nor position (although they are generally good enough if used delicately).
As a follow up to what. is a most. informative discussion, may I ask how one- in the restive confines of the amateur's workshop ACTUALLY round the corners of their own worn mill and end mills?
I've researched what must be an inordinate amount of comment and some scant information of how it is done
As a non- engineer/ machinist--
Thank you
But a slot drill has one flute ( Cutting edge) longer than the opposite one

This is very un-clear!

This is a 16mm Presto brand slot drill. One cutting edge is 6.7mm long, the other is 8.2mm long. The longer edge passes through the centre of the cutter (i.e it's longer than half the diameter), so there's no "dead zone".


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As a follow up to what. is a most. informative discussion, may I ask how one- in the restive confines of the amateur's workshop ACTUALLY round the corners of their own worn mill and end mills?
I've researched what must be an inordinate amount of comment and some scant information of how it is done
As a non- engineer/ machinist--
Thank you
Being a metal butcher of the lowest order, when my end mills get a 'bit' worn, in other words just scratch the metal instead of cutting, I take a diamond file and just round off the blunt bit. Brutal, I know, un-scientific, I know, usually different amounts get taken off the cutting edges and different radii result, but hey what, for a butcher it suffices for a while until I put hand in pocket and get a new cutter.

The Emerald Isle
I have hand stoned the edges of end mills and flat bottom drills to get a radius when needed.
That is the easiest way that I have found to do it.
Us a hard stone. Use your radius gages, and blueing.

Now that I have seen a picture of the long sided cutting lip on the mills you are talking about, I do recall seeing them and using them. Seems to me Weldon made some in the past.
As I remember, they are an ; uneven cutting, unbalanced load when trying to do plunge cutting.
Another way to discribe it is ; they liked to walk around.
Another way to discribe it is ; they liked to walk around.
As Char les Lamont says and me in my earlier remarks, it is the slot drills that are accurate whilst the end mills suffer 'cutter flutter'

I think that merely pontificating or just chancing a guess, the the proof is in the actual precatice.

My opinions, of course.
I am not guessing! What happens with a drill when the the cutting edges are not equal?
It will drill a larger diameter hole, same with a endmill when plunging, as stated earlier.
If you gotem use em!
This is the experince that I remember having while using them. Out of quriosity, I have been looking for a vendor that still sells them and doing online inquires I am not finding any listings over here. I found old stock but nothing new.
My Guide and Comforter- in all things model engineering is in George Thomas's Model Engineer's Workshop Manual. I have, as I said earlier, a number of tool and cutter grinders- some bought,, some made. I can only suggest that set up a slot drill and perhaps grind it yourself and come back with your own findings.
Harking back to happier days, I have been known to bore the vertual limit( 13 inches) perfectly parallel and to size down to an 1/8th of an inch in Africal Blackwood using nothing more than a sort of D bit fashined in silver steel( your keysteel and tempered with sticking it red hot into an old potao.
Importantly, I am not alone in doing such things. I used the tailstock on a somewhat rickety old Myford lathe.

The shepherd's who were pipe makers of Northumbrian Small Pipes( Bag Pipes) had far less equipment and sharpened things- like my father did- on sandstone window sill.

Now I'm over 90, crippled partially blind- sorry!
I am a somewhat weary old man and was having a break in these unfortunate proceedings and was listening to Emma Johnson playing a Peter Eaton Bflat clarinet( My daughter has a better one!) and note tha the tome holes werre not merely drilled but 'undercut'. You don't buy under cutting tools in the shops!
I mentioned GHT earlier and in his 2nd book 'Workshops Techniques' he describes the making of what he called a Small Versatile Dividing head- with a one hole plate and TWO Acme threads but to get the holding down bolts done he actually made an inverted cutter out of carbon steel hardened and tempered. And as I am NOT an engineer, never was- made one. I was a retired bean counter who has now been retired than I never needed to work. So it can be done, thousands of other model engineers with similar moderate skills have done it-- but I'm not weary- I'm tired now. Sorry
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What happens with a drill when the the cutting edges are not equal?
It will drill a larger diameter hole
Yes with a 118° drillpoint - not so with a flat pointed stubby slot drill (depending on how rigid your machinery is).

I have used the unequal grind of a drill point to deliberately drill an oversize hole - a Ø7.1 from a Ø7.0 drill for use with a rotary hexagonal broach.

A word of caution it will also drill bell bottomed - the diameter gets bigger as you go deeper - surprisingly so if you overdo the unequal.

Regards, Ken
Its a well known trick to grind a drill slightly offcentre when drilling drawn Phosphor Bronze to prevent the material grabbing the drill - a very difficult material to drill even with extreme care and even then it does not always work - as below. Step drilling isn't the answer in this material either. Heat builds up extremely rapidly and can result in very firmly stuck drills or reamers. Lots of coolant and slow progress is about the only road to take

This one, a ground off centre one too, took hold so quick it snapped before I could even react.

I round off worn cutters (and not so worn if I want an unimportant radius) free hand on the off hand grinder. I use the corner of the wheel and offer the corner of each flute to it in turn drawing the cutter from each side of the wheel corner (which has a radius on it from use anyway) and holding it such to create the cutting angle. Only thing to watch out for is not to grind the front edge of the flute below which is hidden from view. There are quite a few such modified cutters sitting in a tin - a useful technique to acquire. You will only achieve that with practice though so give it a go - lets face it if the cutter isn't working well it's buggered anyway you can't make it worse by trying :)

Tug, Best way around "snatching" is to grind the drill to zero or negative rake.
You only have to do just a small flat on the cutting edge - not more than your feed rate will stop "snatching" dead in its tracks.

Unless you intend to keep the drill in this configuration, a small flat means less regrinding to get it back to normal.
Regards, Ken
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I read up the advert for the Acute Tool Grinding System which originates from Eccentric Engineering- an Australian firm but with a family member as an agent in the UK.
In Pommie Bastard' prices, the drawings are only £20 plus postage and whilst a completed tool and a kit are available, apart from a few Bristol clamps( or home made ones) can be constructed from little more than one's scrap box.
What is important is an excellent series of U Tube articles describing clearly the construction but it follows with how to grind a conventional lathe tool but, importantly here, how to completely grind a two flank end mill. It assumes that one has a double ended grinder with a CBN wheel on one end and a white aluminium oxide wheel at the other.
I have no financial interst in the company but would mention having bought the plans and a kit of parts.
To avoid further questions, it is not a tool which will 'do everything' but seemingly will perform adequately to saisfy the normal requirements of the home model maker.
As my sonn is also intersted, a completed tool has been ordered and perhaps it will arrive for Christmas.
As I have intimated before, I have an ancient Potts vertical slide with a division plate and a No2MT spindle with a Myford threaded nose. To stock up dwindling stocks of metal, I have taken delivered of what can only be described as a heap of scrap metal offcuts- and will have a go to see whether with a few nutsand bolts, a decent tool and cutter can be made.

It all should solve the problems of the impecunious as well.

Hi Ken - thanks for your thoughts but yes I am aware of reducing the cutting edge to negative rake when drilling 'brass' to prevent snatching and particular so when opening up previously drilled holes.
However there is a distinct difference from most brass that has that tendency to the drawn phoshor bronze referred to above. This material gets hot extremely quickly - even as said with a freshly ground drill and with a degree of offset. This doesn't snatch like brass but somehow rapidly closes on the drill or reamer in very quick order. I fear if one was to use a negative raked tool on this it would exacerbate the situation far more than resolve it.

Drawn bronze is the 'pink' or coppery colour not the yellow type. Once you have experienced a drill getting stuck in it you'll know ;)

Regards - Tug
I so agree with Ramon - normally a hole in a piece of metal expands i.e. a 0.25 diameter hole becomes bigger as the heat builds up. However in bronze it seems to contract because if you don't lubricate and suppress heat generation IT WILL GRAB and break your drill. I haven't found end mill plunging causing the same distress but I'm guessing that end mills "wander" around resulting in more clearance.

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