Squaring off the bottom of a blind hole

[davidyat]

When drilling a blind hole, you are obviously going to end up with the bottom having a 118 degree "funnel" at the bottom. I'm looking at suggestions to help me "square off" the bottom of the hole. I've tried using an end mill and my boring head but I'm not comfortable with either. Any suggestions would be appreciated.
Thanks,
Grasshopper

 

[Ramon]

Hello David ,

It depends of course on how deep and what diameter the hole is but there are several options. The first and easiest is to 'flat bottom' the drill but that does require grinding the existing one or modifying another of the same diameter. The end is ground square then a 5-10* cutting angle is applied to each flute - much like a slot drill. If you just tweak the corners of the flutes it will travel down the predrilled hole without scoring. It's just used to take out the bottom - not for drilling the hole any depth as it will not cut on the centre

If it's much shallower then a slot drill of similar diameter will have the same effect but be aware the bottom of the hole will not be truly flat. A 'D' bit could be made from drill rod or if there's enough clearance it can simply be taken out with a boring tool.

If it's a deep hole then the Flat Bottom Drill or the D Bit are your two main choices

Hope hat helps - Ramon (Tug)

 

[Ken I]

Agree with Ramon - When I was told to do this as an apprentice - I thought my artisan was joking and checked with the foreman - but I then had to grind it back again after the job was done. (Was a dirty big Ø30mm diameter MT4 drill on a large radial arm drill - a job of 5 such holes IIRC.)
In my own workshop I hate doing this - so if I grind a drill flat bottomed - then it stays that way and gets kept in a box of flat bottomed drills.
If necessary, I buy another drill.
Otherwise I will chop off the end off a drill with worn lands - being a tad undersize is not going to be a problem as its only ever going to clean out a drill point.
Try to stick to common sizes - in my case metric - half millimeter steps - so I have most half-mil steps up to 13mm diameter.
A flat bottomed Ø3.8 hole better have a damn good reason or its going to end up Ø4.

Using an end mill is not a good idea as it doesn't cut to the center - use a 2 or 3 flute slot drill.

Regards, Ken

 

[davidyat]

Thanks. I like the idea of flat bottoming a drill bit. I have a lot of used bits and this idea sounds just like what I was looking for.
Grasshopper

 

[teeleevs]

I usually find I have a broken drill the right size in my box of such, a touch on the grinder with just a little cutting edge is all that is needed, also when sinking socket head bolts I usually have one already to size in a box.
Keep up the good work. Ted

 

[L98fiero]

Using an end mill is not a good idea as it doesn't cut to the center - use a 2 or 3 flute slot drill.

FWIW, there are center cutting end mills, McMaster-Carr lists them up to 11 flute. McMaster-Carr
but even a 4 flute isn't good at drilling more than about a 1/4 diameter deep and then using some care so a 4 flute is marginal at cleaning up the bottom of a drilled hole, even when drilled with a 135° drill point, at least that's my experience. The problem is chip clearance as only one flute is center cutting.

 

[Howder1951]

Has anyone tried an end mill or modified drill in a boring head? It appears sound in theory and could probably bemused as a desperate last resort or a one off need. Just asking.

 

[goldstar31]

Has anyone tried an end mill or modified drill in a boring head? It appears sound in theory and could probably bemused as a desperate last resort or a one off need. Just asking.

Many MANY years ago it was suggested in Model Engineer when a broken drill with one 'flank' had been shattered. There was NO tool and cutter grinder for the average little man to make. It was fitted to a small boring head as a single lip cutting tool. I tried it, it worked.
Today, with cheap double ended grinders and tool and cutter grinders in abundance and the introduction of cheap(ish) insert carbides- I simply doubt the logic. The world was full of Slocombe broken tips on centre drills and endless( in all ways) drills. All oof this scrap hss tooling took a second life as boring tool mends.
Today, hummmmm? I think that those people who are forced to attep such expedients should probably find another less expensive hobby.
However back to those people who who want a 'flat bottom'( no pun intended) Leonard Sparey Did write vthe Amateur's Lathe( 1948 in which apart from his excellent references to external and infernal engines mostly of his design, he demonstrated how to modify an ordinary drill for this purpose. Later George Thomas described amongst many things how to make a proper tool to successfully bore holes Better than what was available to us ordinary people in his excellent and seemingly unread Model Engineer's Worksop Manual and his Workshop Techniques.
My opinions but shared by many more expert hobbyists

Norman

 

[grahamgollar]

I would agree with the comments from L98fiero i.e. any end mill cutting in a blind hole would struggle with swarf clearance. This is particularly the case with centre cutting mills where the cutting end would quickly clog and overheat.
Aside from the current topic I have found that in general use the centre cutting mills are very good but, when cutting ductile steels, they produce some wickedly fine and sharp chips to be carefully avoided by your pinkies!

 

[Richard Carlstedt]

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
Rich

 

[goldstar31]

Well, yes and no. The amateur with a better than average tool and cutter grinder cannot round the extreme tipof end mills and and slot drills.

I've been following the var ious designs and their performances. I've just given a Clarkson away. No instructions were ever published until the radius turning attachment was finally introduced for the Mark2.

Te Quorn group have prattled on about the Mar k1 and Mark2 Quorns and I'm sitting with a Quornand a Kennet and a Chinese Deckel clone- Same answer!

Sorry and all that

Norman

 

[Ramon]

Unless held by something extremely rigid a centre cutting cutter, 2 or 3 or more flute is unlikely to cut a similar sized hole as the cutter itself unless the centre of said hole is previously relieved (drilled). Any slightest give either in the holding of the cutter, the spindle or the machine ways themselves will lead to the cutter moving off centre. Even then it is still frought with variation on diameter unless done by steps (in diameter) and even then not an 'accurate' hole as in reaming or boring will probably result

I suggested the slot drill as a 'means' to take out the corners of a previously drilled hole to 'flat bottom' but that does depend on having a corresponding size SD and certainly only on shallow holes - not really an ideal solution to the OP's question

I was shown the flat bottom drill technique circa 1975 as a means to overcome not having a counterbore for caphead screws. I have used it many times but something tells me it had been around for a lot longer before that than the time that has passed since

I do use slot drills and endmills on occasion to spot face or to open up before boring but would never do so to achieve an accurate diameter hole.

Tug

 

[Charles Lamont]

Well, yes and no. The amateur with a better than average tool and cutter grinder cannot round the extreme tipof end mills and and slot drills.

I've been following the var ious designs and their performances. I've just given a Clarkson away. No instructions were ever published until the radius turning attachment was finally introduced for the Mark2.

Te Quorn group have prattled on about the Mar k1 and Mark2 Quorns and I'm sitting with a Quornand a Kennet and a Chinese Deckel clone- Same answer!

Sorry and all that

Norman

Norm, I don't understand. Grinding a radius on milling cutter tips with a Quorn is perfectly possible, but Chaddock explains in his Quorn 'bible' that it is tricky.

 

[goldstar31]

If I recall Chaddock describes the procedure with a reference to-- and I quote a position in ballet dancing'
Obviously the Quorn group of which I am a member ( Ye Gods) have expressed concerns. It seems, and I have the Mk3 plans etc but hae n't tried, gone to fart greater lengths to correct the problem. How Chaddock could round the tips of his home made milling cutters of 1/10th of an inch in diameter- but ironically wasn't able to re-sharpen them. I have a Mark1 with all the gadgets.
Tom Walshaw writing as a retired lecturer in engineering and as Tubal Cain was aware of the problem and his solution was to 'stone; the extreme tips'.
Clearly, the treatment of milling cutters SHOULD follow what we do with lathe tools and round that little sharp end. Well, I have a Kennet designed by? but sold by Model Engineering Services and owned by Ivan Law-the boiler man and possibly Arnold Throp of Dore Engineering and author of Vertical Milling in the Home Workshop. No mention of 'rounding off'. In fact Arnold Throp was questioned why at a SME demonstration why he was'nt sharpening tools on his Quorn. Yes, It was I! The Quorn simply took too long to set up. He's made one- incidentally!
I have a Stent- oddly a fabricated one. Is there a gadget to do what is claimed possible. Perhaps think again- it doesn't happen.
So, again in the Home Workshop, what does? Possibly a modification to the replacement of the Kennet by the Worden Mk3 does. I'm still on with mine. Now the Mk3 Quorn comes in at over. £600 for the kit-- and not all the gubbins to make the thing and even then it is not complete because it needs a ER32 collet set with a 25mm shaft and the 2 preceding models are planed for 1".
In addition, I have a Chinese Deckel clone-- without the pivot point to 'round off. So I amd other withthat model will have-- to GUESS.
As you will see, I was being utterly practical and kept theory and guessing and -err fiction for another post

Cheers

N

 

[Chiptosser]

Slot Drill, Thats a new one to me.
From the definition of a slot drill, what I was able to find, sounds like something to use in wood.
I have only been in this hobby cense 1972 ! Must be a regional phrase or trade specific?

I have mainly used drill bits squared off to suit the need of a flat bottom hole as big as 3".
For small flat bottom holes, I usually like to use the mill for rigidity.
For larger pieces I will use the drill press because of its large table. I will use up to .500 diameter modified endmills, because of chuck capacity, and flat bottom drills.

Now remenber, you will want to run these flat bottom tools at a slower speed! They are more likly to cause chatter. You have more surface contact and because of the geometry.

You can dub the outer edges of the drill or endmill, so that the edges do not cut or dig in to the side of the hole.

I have used end mills to do this also. Now, granted, I would not us a endmill with an inch of flutes sticking out, unless you obsolutly had to. I would cut the end of a mill of to a short stub and let the shank guide throught the hole. Two flute , or four, depends on the depth diameter ect.

I have used Deckel grinders to make nice D shaped tools and ground radius's on the corners of drill bits, if you have that need, these I only use in the mill.

I grind most one off tools for this pupose (drills, mills) by hand, on the pedestal grinder and touch up on a diamond wheel or on the surface grinder with a indexer if needed.

A nice tool grinder over here in the USA, is a Darex tool cutter. They are becoming very inexpensive.
They have a air spindle for the tool collet , uses 5c collets. very user friendly.

Those of you who have not used, operated, CNC equipment have not been introduced to some of the newer end mills, Yes, there are many center cutting endmills availiable. Just make what you need, experiment!

 

[goldstar31]

A slot drill is a two cutting edged bit that surprisingly-- cuts slots.

A mill drill in the same task will probably cut oversize due to 'cutter flutter'

Next?

 

[Chiptosser]

So, you are calling a two fluted endmill, a slot drill?

 

[Ramon]

Slot Drill, Thats a new one to me.
From the definition of a slot drill, what I was able to find, sounds like something to use in wood.
I have only been in this hobby cense 1972 ! Must be a regional phrase or trade specific?

Me too 'CT' - first thing I learnt though was the difference between a slot drill to an endmill - as Goldstar says the clue is in the name but like so many attributions possibly only on this side of the pond

and as already said - taking the corners of a FB drill helps prevent potential scoring.

I'm sure you would agree though access to work funded equipment is a long way off to whats available to most in a home developed workshop.

PS Yes two flute = slot drill.

Tug

 

[Chiptosser]

Mill drill???

 

[goldstar31]

But a slot drill has one flute ( Cutting edge) longer than the opposite one