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ajoeiam

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Hello there
This is one of the BEST Explanations I've read in a long while on WHY a CENTRE DRILL will not lead to a very precise and accurate Drilled Hole.
Much appreciate the Post
Thanks again for the input
aRM
I would disagree with the absolute (will not) would prefer something like 'may not'.

Have drilled enough holes to know that this 'starter hole, if you will, is only one of more than 2 or 3 variables in the process.
 

ajoeiam

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I disagree - with everything


Alexander Pope and echoing Juvenal much earlier said:-

A little learning is a dangerous thing
My question of you, and them, would be : how much is enough?

(Yes I agree with the sentiment but quoted too often leads too many to not want to learn and that is very negative for the system and its longevity!)

(the answer that is most likely would be "You can't learn enough" which is also true)
 

goldstar31

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My question of you, and them, would be : how much is enough?

(Yes I agree with the sentiment but quoted too often leads too many to not want to learn and that is very negative for the system and its longevity!)

(the answer that is most likely would be "You can't learn enough" which is also true)
The more that one learns, the less one knows.

But on a far less important issue, it is obvious that Van Royen was never read or perhaps someone had an ear for such things as Van Gogh.
I made a Quorn to the design of a Professor of Engineering and who explained in detail of how the 4 and 6 facet grinding should be accomplished. Chaddock actually made the Dore Westbury mill and continued to make( not merely grind) the 1/10th inch diameter drills which he used in making a V6 engine to his own design but obviously to embody RR Practice! Again, he wrote in mModel Engineer' how to make piston rings and ball bearings.
For a party piece(?) he was on the Atomic Bomb project.
Happily, the Mark 3 has had its scope - simplified(. ho ho) but it does avois the terms in ballet to describe rounding off the cutting edges of lathe tools and the end mills - which are really flat bottomed twist drills( clears throat again).
What has to be recalled is that the Mark 1 suggests a vertical pillar of ONE INCH thread- per inch.
As mentioned at an earlier comment, I have a Potts one( as well) but Mr Potts also made a drill grinder to the Van Royen concept. It appears in Ian Bradley's The Grinding Machine.
I hope that I have written a little to avoid 'putting my foot in my mouth'
Best Wishes

Norman at 90++
 

tornitore45

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The OP relate to drilling a deep hole straight. The initial spot method and tool is immaterial once you accept that the first 1/2" MUST be BORED. If you are just starting a hole without boring then you are not concerned with accuracy of location roundness and straightness. At that point you can cut the centering divot with a chisel and be almost as good as with a spotting drill.
 

traction engine

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Well to each his own. Maybe it's the equipment but in the 50+ years I've been drilling holes, I've NEVER had a problem with straightness and I've never started a drilled hole by boring. However there may be a variation in methodologies.
You are correct, if the starter is off, the drill will follow. My equipment puts the center drill on center so my holes are straight. Fix the problem because poor equipment inevitably gives poor results.
 

terryd

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Wait a miniute. I am too old to learn! But it is always helpful to know about these spotting drills. I never knew about them till I lookt them up because of all your posts. I thimpfk I have never owned one of these drill bits. Maybe I 'll get a couple to test out. Even so, I am still incapable of learning.
Hi Richard,

Amazon sell a niceset of accurate spotting drill, I've been using mine now for quite some time with good results

TerryD
 

BaronJ

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Hi Guys,

One thing that doesn't seem to have been mentioned is that drills by their very nature, flex ! Even if you have a perfectly symmetrically ground drill, if you cause the drill to bend whilst drilling, It will drill a bent or oversize hole !

Having said that, one little trick worth knowing is if you want a slightly larger hole and you don't have the right size drill, grinding a drill with one cutting lip longer than the other will produce a bigger hole than the drills diameter ! For all the reasons previously mentioned.
 

mcostello

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That trick is supposed to work well on the harder bronzes which can be a bear to drill. Fortunately making a drill bit cut bigger is one of the raw talents I seem to have in spades. ;)
 

Ken I

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It's a trick you need to be careful with - it not only drills bigger but continues to drill bigger and bigger the deeper you go.
Years ago I was production drilling Ø7mm holes prior to a 7mm hexagon wobbly broach - we needed to drill Ø7.05 - on one occasion we overdid it and by the end of the 11mm deep hole the bore was so bell-mouthed that the hexagon vanished - the result an expensive warranty return from VW when the hex drives stripped on assembly.
I couldn't believe the Ø7 hole had bell-mouthed to Ø8 in just 11mm - of course the damn thing passed all the Go / No-Go gauges - you could only see the error if you sectioned it.
 
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BaronJ

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It's a trick you need to be careful with - it not only drills bigger but continues to drill bigger and bigger the deeper you go.
Years ago I was production drilling Ø7mm holes prior to a 7mm hexagon wobbly broach - we needed to drill Ø7.05 - on one occasion we overdid it and by the end of the 11mm deep hole the bore was so bell-mouthed that the hexagon vanished - the result an expensive warranty return from VW when the hex drives stripped on assembly.
I couldn't believe the Ø7 hole had bell-mouthed to Ø8 in just 11mm - of course the damn thing passed all the Go / No-Go gauges - you could only see the error if you sectioned it.
Hi Ken, Surely you mean a "Bell Bottomed" hole !
But I do take your point, a "D" bit would be the right way to go !
 

goldstar31

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Hello Sailor! Rude English ex[ression-- said he waving his wooden leg aloft:mad:

On a more even keel( !!!!!), I have just ordered a rahr large order stub drills, metricated ones in 0.1mm and tool bits to ' part brass rags' with all and sundry.
Boring though it is, Boeing is the thing.

Anyone thought of how Northumbrian Small( bagpipes were drilled? I made a set.
Any one for A and B flat clarinets?

Regards

Norman
 

Richard Carlstedt

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Well, it all depends on what accuracy level you want ?
Deep hole drilling has some considerations that rise above normal considerations
Unfortunately, it conflicts with many of the "standards" we have learned in our machining careers or hobby. So let me begin :
Lathe work- First know that that your tailstock , may not perfectly match the C/L of your spindle OR the writer who mentions he has perfect accuracy ? And this measurement must include parallelism of the spindle and tail-stock quills !
Mill work- How much spindle run-out do you have , both in bearings and "tool" holder readings?

I am not being a jerk by mentioning the above , but trying to point out that no two people will achieve great results because the subject of Deep Hole Drilling (DHD) boarders on Art Form and success is based on approach and machine( + tool) capabilities !

Here are some pointers , at the Die shop I worked in , we did DHD every day and to 25 to 40 x diameter. So what are normal perimeters you ask ?
For gundrilled work in a gundrill lathe you can achieve .0001" per inch drift or less
For gundrilled work in a mill , you can achieve .0015" per inch drift or less
For conventional drilling without prep, it is not unusual to see .010 to .020" per inch drift
The difference you see is because having the "work" piece turning will always produce more accurate holes as the lathe's axis redirects the drill bit far more than the drill bits axis does.
So if you want straighter holes, use your lathe.

The single, and most important factor to a straight hole is ...."Hole Prep"!
Center-drills and Starting drills do not cut it !
Small drills or step drilling do not cut it ! ( more later here)
To start the hole, use a Ball Endmill or bore the hole -finished size... to a depth of 2 x diameter
Few machinists know or realize that a Ball Endmill is one of the most precisely ground tooling they have and the nature of the ball form drives that cutter to a precise round hole.
Then having a hole with 2 diameters depth, the "flutes" of your drill bit will be automatically fully supported ( 360 deg) to give you the best chance for a accurate hole.
Now if your lathe is under powered, you can still do your step drills, but first having the hole prep done properly, when you use the full size drill, your load will be less, but the drill will have support from the prep area and produce a much straighter hole .
For example, a 1/4" drill bit is much stouter than a 1/16" drill and having the 1/16' drill first means greater chance of deflection, and if you want the bigger drill to follow the same path, you cannot expect superior results.

The only problem with Ball Endmills is limited size availability ( by 1/8 or 1/16"), but even so, use them whenever possible for hole starts. I never use a center-drill, unless it is on a shaft for tailstock support . For regular drilling I use a starter drill in the mill. And if the hole is a normal hole when in the Lathe , I use my drill bit, (yes ! ) but that should be another posting for that technique is 100 years old and not taught anymore -but give dead center holes.
Rich
 

Mike Henry

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Hi Rich,

What are your thoughts on using a spot drill to start holes that don't have sizes common to ball end mills?

Mike
 

TSutrina

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I have never needed to drill a very deep hole accurately. The century old method is to gun drill the hole. Modern gun drills have a hole for oil cooling and removing the chips, however; being centuries old the is an improvement done because we can and it speeds up the process. A gun drill can be DIY made far easier since the relief is a straight flute, ~ 1/4 of the cylinder. Gun Drilling & BTA Drilling: Ultimate Deep Hole Drilling Here is a youtube showing use and sharpening
 

Richard Carlstedt

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Hi Mike.
yes, a spotting drill or "starter drill " ( same drill) works very well and definitely use them if a ball endmill is not available. And get them in carbide if possible due to their rigidity . But when you do, try to go to 2 x diameter so you get the full support for the jobber drill that follows. I know that most machinists will only do it to get a conical starting surface, but make it deeper to stabilize the drill bit flutes that follow. (Assuming that the next drill is the size you want -if smaller, then you don't need the 2 x preparation)

Something that shop guys should do is look at the Helix Angle of their drill bit flutes.
Doing so means looking at the flute distance for it to make a 180 degree rotation. Lets take a 1/4" drill for example here
Hold a machinists scale parallel and against the drill with the "0" point-edge of the scale at the cut edge and see where the second flute crosses the scale . This may be anywhere from 3/8" to 1-1/2" depending on the helix angle . THIS Length is how deep the hole prep should be !
This length means the drill will be fully supported 360 degrees and is makes it more accurate in size and depth and reduces egg shaped or triangular holes ( which are caused by the drill rattling around in the hole) . The short cut answer I gave was 2 times diameter, but it really is related to the Helix length I mentioned--and which takes precedence over 2 x X for good work
Rich
 
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