# Drilling small hole over large hole or vice-versa



## Niki (20 May 2007)

Good day

I have a cheap DP and when I want to change the table height I have to unlock the table and while turning it left/right pull it up or push it down.

I have no problems till I want to drill a large hole over small hole or vice-versa. In this case you must maintain the table at the same position but, if one drill is long and the other is short, you must re-adjust the table height and it is very difficult to return to the same position so the large hole will be drilled at the center of the small hole or, the small hole over the large hole center.

So, that's what I did

Regards
niki

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## Woodmagnet (20 May 2007)

Nice one Niki, that's one for me to use. Cheers.


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## beejay (20 May 2007)

I was taught alwaysdrill the large hole first then the small. Also when drilling largish holes one surely would use a forstner, spade or spur bit which will always leave a centre point as reference for the next bit.
This seems like solution to a problem that doesn't exist,,, or am i missing something here? :? 
beejay


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## Niki (20 May 2007)

Thank you Kevin

Beejay
I'll give you the case that I had to use it (that is actually on the pics),

I'm cutting "wheels" with the hole saw to be used as "Knobs" for jigs so I cut a few of them in each diameter but, at the time of cutting, I don't know for which jig I'll use them so I leave the center hole as it is (6mm) and when I need to use them, I decide on the small hole diameter and the large hole diameter.

I'm talking about a case that the small hole is already drilled and I want to drill larger hole over it but at the same center.

I can cut the knobs individually for every jig but I prefer to cut a few in a few diameters in one shot (and one mess) and have them ready for use.

niki


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## engineer one (20 May 2007)

interesting idea as usual nik, and i am confused that others should be taught to drill the bigger hole first, maybe that's a woodworking plan.
certainly when i was trained in metal, we always drilled the small hole first, if for no other reason than you could always ensure a more accurate hole.

maybe i'm missing something too.

what does impress me is the way you consider a problem and then solve it in the simplest most practical way.

paul :wink:


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## motownmartin (20 May 2007)

engineer one":23772zqd said:


> interesting idea as usual nik, and i am confused that others should be taught to drill the bigger hole first, maybe that's a woodworking plan.
> certainly when i was trained in metal, we always drilled the small hole first, if for no other reason than you could always ensure a more accurate hole.
> 
> maybe i'm missing something too.
> ...


The other way is to do it on a Lathe, that is if you have one.


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## Scrit (20 May 2007)

engineer one":56wdnv6b said:


> ...... I am confused that others should be taught to drill the bigger hole first, maybe that's a woodworking plan.


I'm in complete agreement with beejay - it's because of the way woodworking drills work and the nature of the material, Paul. A traditional auger bit has a screw point:






which needs to "bite" into the timber in order to centre the bit. Without that material the bit will tend to wander off and in a hand brace it will be impossible to centre at all. Similarly a brad point bit or spade bit:











have points designed to stabilise them as they make the cut. With all of these bits if there is no material in the centre when to drill through there will be a tendency for the drill bit to wander off centre because the cutting action across, with and at an angle to the grain results in constantly changing forces on the bit (unlike metals which are pretty consistent in comparison) - even if you mount the bit in a 1/2 tonne drill like my recessor (see avatar)......... The only exceptions to this are when drilling using "centreless" or "rim guided" tools such as a Forstner bit - which is what Niki is using in his example. I, too was taught big hole first, small one second with the sole exception being if you had a Forstner bit for the big hole. 

To emphasise the point when "drilling" on a CNC router the only way to obtain consistent results if you are producing counterbored holes with router bits is to machine the big one first, then the little one - even in MDF - whilst in plastics it is much more consistent to work the other way around as the v-point of a standard twist bit:






can self-centre on the already-drilled pilot hole.

Scrit


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## Steve Maskery (20 May 2007)

Paul, that's because twist drills cut differently that wood-boring bits, like a twin-lip and spur.

In metal, the material is dug out by the sharp edges of the tip, essentially working from the centre out.

But with a TL&S, Forstner or spade bit, the edges of the hole are cut before the bulk of the material is cleaned out. This relies on the centre-point of the bit having something sound on which to locate. If you drill a small hole first, you remove the centre and the bit has noting to locate on and can wander, or worse still, kick as the edge makes contact with the workpiece.

Cheers
Steve
Edit - Ah the joys of crossing posts.


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## engineer one (20 May 2007)

so now i know even more, and you have confirmed that it is more specific for wood/ non metallic materials which is what i thought.

as usual scrit a full explaination.

paul :wink:


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## engineer one (20 May 2007)

thanks steve, nice to know more than one person understands the differences.

now i know why it took me so long to drill the bench holes last week for the dogs. i used a 12volt dewalt drill and 19mm spade bit. next time will use a mains drill :lol: :roll: 

paul :wink:


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## Scrit (20 May 2007)

Steve Maskery":d43lu6nu said:


> Edit - Ah the joys of crossing posts.


Ain't life just a ssshhhh-you-know-waht :wink:

Paul, there's actually nothing wrong with Niki's approach so long as the larger drill is a Forstner bit

Scrit


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## Niki (20 May 2007)

Thank you so much for the discussion, I also learnt some new things.

I would like to say again that I'm using this method because I like to prepare a bunch of knobs in different diameters and later use them for each jig.

Sometimes, I have to enlarge the small hole from the 6 mm of the "hole saw" drill to 8mm or 10mm and than, to drill a larger hole for the bolt head, washer or T-nut and it looks to me that that is the only way to do it (on my Drill Press).

Another point is that, even if I drill first the large hole but the small hole drill has different length, I shall have to change the table height and that's the main point of the try-square, to go back to the same center point.

Of course if you have the more expensive Drill Stand that you change the height by cranking, you don't need this method...

Regards
niki


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