Squaring off the bottom of a blind hole

[clockworkcheval]

On off-center grinding drills: the old hands in my first machine shop would do it for intermediate sizes. For instance if you need a 10,3 mm diameter hole, and you don't have a 10,3 mm drill, you take a 10 mm diameter drill and grind it 0,15 mm off-center. I have never become good at replicating it with the same precision as the old hands.

 

[Charles Lamont]

If you heat drawn phosphor bronze up to dull red and let it cool it will alter the grain structure to something more similar to a cast bronze. I have found it much easier to drill after this treatment. It also improves its bearing properties. It is a while since I needed to do this and I can't remember if I quenched it or not. Heating to red is highly unscientific anyway, it probably anneals at a lower temperature, but I have never bothered to find out.

 

[Ken I]

Another problem I have experienced with bronze bar - is a result of its origins as a spun cast rod - it can have a corkscrewed central grain which leads the drill askew.
The deeper you drill the more it binds up on your drill. (The drill is trying to drill a corkscrewed hole.)
You can't see it but you can generally hear the screaming.
Annealing can greatly reduce this.
Similar problems with spun cast cast-iron.
Regards, Ken

 

[Ramon]

Charles
I have never given thought to 'heat treat' it first. That's a thought for the future so will try heating the end of a bar sometime to see if that improves matters. Thanks for the tip.

Ken
There is a distinct machining difference between drawn and cast bronze. I use the latter a lot and only use the drawn when needs matter.
Drawn bronze machines in all other aspects with relative ease but you do need very sharp tools. It is it's ability to generate heat so quickly that makes this different from the cast bronze which is lovely stuff to work with.

Regards - Tug

 

[Peter Twissell]

I suspect the issue with phosphor bronze is that it heats locally at the cutting edge, then cools rapidly, so a 1/4" hole at the cutting edge becomes less than 1/4" just behind the cut, grabbing the lands of the drill.

 

[Charles Lamont]

I suspect the issue with phosphor bronze is that it heats locally at the cutting edge, then cools rapidly, so a 1/4" hole at the cutting edge becomes less than 1/4" just behind the cut, grabbing the lands of the drill.

I think that might be the wrong end of a chicken and egg argument. As I understand it (which is not well) the bar contracts as a result of releasing compressive stress in the bar.

 

[L98fiero]

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

Aluminum bronze is the same, I've had to drill 3/16 hole 6" deep in the stuff and can confirm that 'dubbing' like Ken I described isn't going to work where grinding one lip longer does.

 

[Ramon]

I'm not sure what actually is the cause of this particular phenomenon with this material but I've been aware of it for many years. Just this morning I have experienced it using a 3.8 drill but not to the extent as the previous photo. It's as if the hole drilled is microscopically 'tapered' as the further in the drill the more difficult it is to withdraw. Heat is without doubt a major factor. The heat generated can be easily seen increasing as any oil/lubricant soon begins to smoke! A 4mm reamer however followed without issue

It is 'get roundable' but something to be aware of. When making the bushes for I/C conrods I always bore it to size - bore first, OD second.

I haven't machined aluminium bronze L98 but do know of it's difficult machining properties. That's a deep hole at the best of times. I think I may have a piece of something that may be similar under the bench - to all intents unworkable on my home kit.

Colphos of course is free cutting and a different matter.

Tug

 

[L98fiero]

It's as if the hole drilled is microscopically 'tapered' as the further in the drill the more difficult it is to withdraw. Heat is without doubt a major factor. The heat generated can be easily seen increasing as any oil/lubricant soon begins to smoke!

I'm not sure what the reason is but suspect that it's a combination of the tip of the drill expanding and the hole size decreasing because of thermal expansion, the hole would normally get larger but my experience is that if the part's significantly larger than the hole it decreases in size. If you're quick and careful sometimes you can get a little farther by getting more coolant in the hole but once the hole shrinks again the drill won't bring it back to size either, at least with extrusion cast aluminum bronze. I've not had much luck using oil for this.

 

[Ken I]

The problem with aluminium bronze (amongst other materials) is work hardening - if you don't have the power or rigidity to keep cutting "under" the hardened skin you are generating - especially drilling - then the cutting edge goes real quick, followed by the corners, then the flute lands and you end up trying to drill with what has become a tapered drill in short order.
The odd length lip grind does help. I wouldn't "dob" the edge on AllyBronze.
Regards, Ken

 

[goldstar31]

deleted

Nornan

 

[goldstar31]

Deleted

Norman

 

[Steamchick]

If you heat drawn phosphor bronze up to dull red and let it cool it will alter the grain structure to something more similar to a cast bronze. I have found it much easier to drill after this treatment. It also improves its bearing properties. It is a while since I needed to do this and I can't remember if I quenched it or not. Heating to red is highly unscientific anyway, it probably anneals at a lower temperature, but I have never bothered to find out.

I agree Charles. Copper and alloys change state at around (or just below) a "dull red heat" - NO quenching required. Actually, as the "colour" is directly related to temperature, especially tempering colours, it isn't that unscientific, just a "large tolerance" of judgment. And it works and is simple... Technology that has been around since... Hmmmm, maybe the Bronze age, isn't that unreliable so should still work today.

Ken 1, Cheval, Peter T, 'n all - Thanks for all the advice - I had never given it much thought myself, but have learned a lot from this discussion. I have experienced the "binding" when drilling bronze... so I'll try "home hand-ground" drills in future instead of "Factory new" precision-ground drills! Maybe my drill-sharpening imperfections are the same as the off-set ground drills? Gudonya!
K2

 

[teeleevs]

Thanks for all the explanations on this post, I picked up some chunks of brassy looking 2 inch bar, about 30 years ago, probably boat prop shaft, it will take front and sides off a parting blade.

 

[L98fiero]

Thanks for all the explanations on this post, I picked up some chunks of brassy looking 2 inch bar, about 30 years ago, probably boat prop shaft, it will take front and sides off a parting blade.

If it is propeller shaft it's probably C46400 naval bronze/Tobin bronze, machinability rating is 30% of free machining brass. Turn it slow, use a sharp tool with no more than 5% top rake and don't let it rub or it will work harden. One cutting edge longer than the other for deep holes and don't dub drill cutting edges, that's for free machining brass.

 

[Steamchick]

Fascinating.... But Gentlemen, one needs a good grounding in Jargon to appreciate this one. The use of different names is confusing me a bit...? But I am an amateur, so please correct me where I am wrong?
I understand a "slot drill" to be a type of milling cutter - not a drill at all - probably 2 flute, without any end cutting face? And I thought it has sharp corners for making a sharp-cornered bottom to the slot? Or is this a "Side cutter"? A "more normal" twist drill maximises the swarf-clearance space, and uses the self-centring of the cutting tapered-end (118 degrees or whatever) to ensure only end thrust on the drill - effectively supported from bending by the material around the hole. Drills are notoriously bendy so should never be used for any side thrust - Or so I was taught? But the "slot drill" or "side cutting Mill" (correct jargon please?) minimises this side relief, as the swarf clears away on the non-cutting side of the cutting motion. The centre metal of a slot-drill is as large as practical for stiffness to overcome the bending of side thrust, with minimal swarf relief slots. The end face is simply relieved as it effectively never cuts with the side-motion (perpendicular to the axis of rotation) from the traverse of the metal past the tool. I have some with helical side cutting edges and some with straight cutting edges. Please can to help me by explaining "which" one is used "when"?
End mills, as earlier explained by a few of you, can make flat-bottomed holes - as I (and many other amateurs?) will have used them, but we all will have learned that they can wander unless introduced into a previously drilled starter hole. And they do clog easily as the swarf relief is not as large as a twist drill. I understand that end-mills should be used to make blind-ended slots, where you need the end-milling face to cut fresh metal before the side motion can be used to make the slot. Please teach me the correct application - as "self-taught is worth naught!" There seems to be some differences of opinion in the thread, that conflict with some contributors' ideas? But here (this thread) we amateurs (like me) need a clear advice on "when to use what" from experts. And not having a Tool and Cutter grinder, some of us buy second-hand or cheaper tools for the odd-job as most of our models are "one-offs" anyway. (Most of my tools are "hand-me-downs" and second-hand). Where we have a choice of tool, or want to buy the "correct one for purpose", we need to understand what the correct tool should be.
Thanks for your advice, we know it is all well intended,
K2

 

[jack620]

I understand a "slot drill" to be a type of milling cutter - not a drill at all - probably 2 flute, without any end cutting face?

I posted a picture of a slot drill earlier. The box is labelled “slotdrill”. It was made in the UK, so the terminology is native to your country. Yes, it is a two-flute milling cutter. It DOES have an end cutting face, so it can be plunged into the workpiece.

Can we put this to bed now?

 

[Ramon]

K2, Hi

I began as an amateur in 1972 but trained and earnt a living by machining from 1980 so have seen both sides and understand your concern.

Today with the advent of CNC and absolutely incredible machining centres machining has moved on a pace since my day not only in the machines but in the tooling too.

However - fundementally you can only cut on a mill with two basic types of cutter - a face mill or vertical tooling such as an end mill. What they are made of and whether they feature inserts is secondary - the function is the same.

I can't speak for outside of the UK but there are (were) two basic types of 'vertical' cutter - a Slot Drill and an End Mill. Usually, though not always, the slot drill has two flutes and can cut on it's end face across the full diameter. As has already been explained one flute is ground to the centre point to enable this to happen. As designed, technically, it can be plunged into the metal as deep as the flutes allow and can then cut on the side to create a slot. If a cutter was to be used like this ie to it's full depth, then the slot would be roughed out in steps with a smaller one first - the size cutter being used to finish.

An End Mill - despite its terminology is not designed to cut on its end! The flutes - usually four or more have their centre reileved on the end face. Any attempt to cut on the end quickly fills this relief with swarf so preventing any further penetration into the workpiece. An end mill is designed to cut on it's side and bottom edge but not for plunging.

Since the advent of CNC there are many variations of cutters now with multiple flutes and multiple purposes. Many 'endmills' of varying flutes are now ground to enable plunge cutting. I guess the real culprits are people like myself who still use old terminology as opposed to say "use a three flute end cutting mill" etc.

So to sum up - you can, within the limits of it's length, clean up the corners of a predrilled blind hole with the same size end mill but can't deepen the hole by any degree.

You can do the same using a slot drill but this can be used to deepen the hole from solid.

Like many things there's a specific tool for the job but most times at home and sometimes in the working environment you have to compromise and get by using a tool not quite for it's intended purpose.

Hope that helps some

Tug

 

[Ramon]

Can we put this to bed now?

Why? If someone isn't sure why not help them. Sorry Jack but that seems a bit dismissive to me.

It's a true old cliche - no question is stupid if you don't know the answer even if you can't see it the first time.

Tug

 

[Steamchick]

Thanks Jack620, and Tug, a whole page of discussion was hidden by the software so I missed a lot.
I just found Ken1 's pictures and explanation that were "hidden text" by the thread software... - so that answers some of my queries. See attached:
So I now understand the "SLOT DRILL" to be an end mill that will drill because the cutting edges cross the centre of rotation. That means the "SIDE MILL" is the tool that I have without any end cutting faces. and "END MILL" is a tool that has some end milling faces, but no centre so cannot "DRILL" satisfactorily.
Is that correct?
On Drills, in a factory where I was a Design Engineer (so "Knew nowt abaht owt" according to the shop floor) they hand-ground all the drills in the "heavy" machine shop (drills over 1/2") and the centre was always ground to a sharp point - 2-sided - like a wood-working bit centre. I understood this was so they could accurately position the large drilling machine over the centre pop for starting the hole, and because the centre core metal of the drill was so large they were actually grinding a small tapered drill in the centre to clear that metal before the main cutting edge with the swarf relief slots. - Similar principle to a bullet ended drill? Has anyone else experienced these? (Sorry to detract from Milling).
I do understand the difference between drilling and milling, as drilling is working ONLY on the end face = axial forces only, whereas Milling is essentially repeated scraping of a surface with the side of a rotating tool - thus applying severe bending forces to the mill, if not supported at both ends. Therefore the strength of the tool to resist bending is paramount with Mills, whereas the swarf clearance path is a dominant factor in grinding away most of the core of drills.
I think Ramon summed-up my confusion with his explanation: "An End Mill - despite its terminology is not designed to cut on its end! "
I'll say no more and just read more of this discussion.
K2