Drilling holes accurately on a marked centre?

[ian maybury]

This perhaps sounds like a very basic question, but I don't seem to have a 100% reliable solution. Thinking of two situations - freehand with a hand drill, and under a pedestal drill.

Marking a centre accurately is relatively easy, but getting the drill bit to start right on that centre isn't always that easy. I tend to use a normal centre punch to make a mark, and on standard metal cutting twist drills to then start with a pilot drill before moving on to a larger size.

Brad point drills are easier to get centred, in that the centre pin will drop into the punch mark.

The one issue that's always present is visually judging when the drill is centred on the mark. It's easy to get it wrong, especially under a pedestal drill when the workpiece is clamped down. It sometimes helps to let the workpiece float free so that a lightly lowered drill can centre itself in the punch mark - but it's not always possible to do this, and it's not 100% reliable.

Relatively harder growth rings in the timber can displace the drill too.

Thoughts anybody?

ian

[Lee Schierer]

Brad point bits will increase your accuracy tremendously and will also tend to wander less as they encounter growth rings.208472 There is also no need to use a center punch with a brad point bit.

[Bob Wingard]

One simple option is to first chuck up a pointed rod such as a TIG electrode or even a small bolt or setscrew with a point filed/ground on one end.

Place the point in your punch mark and clamp the work piece in place.

Swap the pointy rod for a center drill, and go to work.

It's a bit tedious, but it works and it is extremely accurate.

[John Coloccia]

As I've said to many, many people over the years, drilling an accurately placed, ROUND hole is one of the most difficult tasks to perform (for metal workers too). When it just has to perfect, Bob's technique is what I do. I have center finders for my router that I also use for this task.

[Wes Grass]

In metal? Piece of cake. Just load a spot drill, undersize drill, boring head, and a reamer (if you're too lazy to dial the boring head in) in the VMC and get 'er done. And if it's really got to be dead on for diameter, take it over to the Sunnen hone ...

;-)

[ian maybury]

Thanks guys - glad I'm not the only one that sees it as an issue. I'll have to try the pointed rod route.

A spot drill is also an option as Wes says: http://www.ebay.co.uk/itm/10MM-NC-SPOTTING-DRILL-90-DEGREE-COBALT-SPOT-DRILL-NEW-/130561469918?_trksid=p4340.m263&_trkparms=algo%3DDLSL%2BSIC%26its%3DI%26itu%3DUCI% 2BIA%2BUA%2BFICS%2BUFI%2BDDSIC%26otn%3D8%26pmod%3D 140567488398%2B140567488398%26po%3D%26ps%3D63%26cl kid%3D3050999583464781368

There's possibly a subtle difference in issues though - so far as I know the job of a spot drill is to cut an accurately located depression in metal that the following drill will accurately centre on. Which is fine, but a slow feed probably takes care of most of the wandering issue on wood - and there's still the issue of getting the spot drill accurately positioned.

The issue I find a problem is more to do with this - with getting the drill lined up accurately on a marked 'x' - which sounds more like a task for the fine pointer...

[Phil Thien]

I have a few scratch-awl shafts of various lengths I use for locating the workpiece on the table. I select a shaft length which corresponds roughly to the length of the drill bit I'll be substituting once I have the workpiece located.

[pat warner]

"Drilling holes accurately on a marked centre?
This perhaps sounds like a very basic question, but I don't seem to have a
100% reliable solution."
You're trying to do something that can't be done!
Don't punish your self.
Drilling on center by eyeballing the laser, punch mark, knife or scribe line is always an estimate.
In wood who cares?, + or - .010 - .015" is usually not a problem; that accuracy
is nearly always possible with the above indicators.
Yes drill sharpness and design play a role, especially with respect to material and
the drilling machine. But given the sharpest of tools you're always guessing at
the mark. I have had students, whilst next to the work on the drill press, with
reasonably good eyes (corrected & uncorrected) who miss the mark x a mile.
Many folks just don't have the eye/hand line-up skills to get close.
How then? A drill press with fences and stops can get you close but there's
more to it than that; it's understanding how to hold the work, how to index it, how to
make it behave whilst against your references. A workpiece, poorly prepared, will
register to nothing. Moreover, just where the hole resides can only be proven if
you know how to measure. Can you measure hole diameter (often a stated drill
diameter is not the same as the hole it makes), can you show the work is flat,
straight and uniform in thickness? And are your fences and stops straight, flat
and well secured? Drilling x hand drill or x a hand drilling fixture is far more
ambiguous.
Finally, appreciate that a work piece, whatever you're using to hold that work,
and the drilling tool itself, are subject to motion; a spontaneously slip or rotation
here or there. There are 12 modes of slip/slide, up/down, in/out, and 6 modes
of rotation. How many ways have you tried to isolate the work?
Drilling is as simple as it gets in the most complicated of ways (http://patwarner.com/images/holding_jig.jpg)!
**************************************

[Kent Chasson]

I saw a centering scope like this on craigslist recently but it was the same price as new. http://www.penntoolco.com/catalog/products/products.cfm?categoryID=6953

If I could find something like that for $100, I'd buy it.

[Tom Hargrove]

As others have said, a brad point bit makes things easier, but are still not fool proof. A trim carpenter friend taught me to use a nail set instead of an awl to set my mark. The square tip goes staight in and doesn't follow the grain like an awl or other "pointy" punch tends to do. I do this whether drilling freehand or in a drill press/pedastal drill. Its easy to start the drill from the mark in the wood.

[ian maybury]

I think maybe a couple of hours on a friend's lathe with some steel rod of various diameters might be the trick Phil - to as you use make up a set of pointers of varying lengths. I'd no idea a tool like that centring scope existed Kent.

I thought for a moment Pat you were putting me off trying to drill accurately, but luckily I followed your link. It's obviously a topic you've put quite some work into. My situation in comparison is that while I've done a fair amount of drilling in the context of machining metal parts (way back when) my drilling of wood has so far been in a fairly casual and carpentry/DIY context rather than fine woodworking. So it's only very recently that I've started to think about accuracy and getting clean cuts.

As ever I guess it's a horses for courses sort of deal. There's situations where a decent level of precision matters, and where not so much. Maybe most important of all is having a menu of practically useful techniques to hand to draw on. Those drilling booklets look interesting - it's almost like you heard me coming. I'll be e-mailing you in a few minutes....

Pardon my missing your post first time Tom, the multi level system of displaying posts can be a bit misleading. That sounds like a useful trick too - a bit like the way an old fashioned cut (rectangular section with a flat end) nail is less inclined to cause splitting.

[Trevor Walsh]

Do you have an example of the job needing the drilling? I find that a lot if times drilling a hole and using that hole as a reference mark can be the better option (faceplate or 4 jaw chuck in metalworking, threading a hole on center in the case of a knob or other piece of hardware). Transfer punches can be very helpful, and those setup center points for a DP situation. By using a point and floating the workpiece with centerpunch under the point to center, then clamping with the quill down you can be pretty accurate, using a fine pencil for layout and an optical punch would get you layout far greater than I think wood requires. For quick work this way .010" tolerance is easy, at least for me. Having a trammed drill press is I think also very important.

[David Larsen]

I friend taught me that if you are having a hard time getting a screw through tough material with a screwgun you run it in reverse and let the screw turning in reverse burn your way partially through the hole to give yourself some "play". After you are partially through it, put the drill in forward gear and drive it in. Use this example to drill your hole with a drill bit. Use a center punch to mark your center then reverse your drill to create an impression then forward to follow it through. (don't spin it enough to burn though if you want a pretty hole)

[Bob Wingard]

Running a drill bit in reverse can chip it.

[Kent A Bathurst]

My method: Mark with awl. Use bradpoint bit [or, for some applications, forstner].

Lower the DP to the workpiece - futz around as necessary to get the bit point in the awl mark. Hold the DP head in place against the workpiece as I tighten the quill lock. Clamp the workpiece in place, and move the fence into place. Release the quill lock, fire it up, make a hole.

Takes less time to do it than it took to explain it.

[Phil Thien]

One of the things I often do with my scratch awl (for metal, for wood I just use the brad-point bit) is I hold the punched workpiece to the bit, and then I use the press to push the awl and workpiece to the table. Once I have it pushed to the table, I clamp the workpiece.

It is a technique I learned from a machinist back in the day, and works pretty well.

[ian maybury]

I've done something similar Phil when drilling small holes in smaller pieces of wood - I've found that it sometimes pays (when using a standard twist drill that's small enough that the piece being drilled can be hand held) to let the piece float and the drill centre itself in the punch mark - before gripping it tightly to prevent it spinning once it's located to go on to drill the hole. (not a good idea on larger drill sizes or situations where the drill may grip enough to spin the workpiece out of your fingers)

I don't have a very specific job in mind Tom, I was thinking more in terms of triggering a general discussion on the topic that might bring out little hints and tips on what is a much subtler issue than we might at first think of it as. Which is what has happened - so thanks to you all.

That said I'm building a non standard filter cabinet for my Pentz dust system (just a knock about sort of job), and it was the vagaries of drilling and countersinking birch and cheaper (luan?) WBP ply that got me thinking - especially (a) drilling edge on into the laminations in the ply where I found the glue layers and variable hardness of the laminations can trigger some wander (and it's surprising how obvious to the eye a minor inaccuracy in the placement of a 5mm dia hole in the edge of a piece of 18mm ply can be - especially if it displaces the panel being screwed down away from an edge), (b) the way that it can be a bit of a PIA to get even a brad point (never mind a conventional twist drill) lined up over a mark and the workpiece clamped to the drill press table, and (c) that ply needs very sharp drills and carefully handling to get clean entries and exits. Countersinks can be quite prone to producing tearing, and to wandering off the hole centre too.

The quality and accuracy of set up of the drill press (if used) is clearly a big factor as has already been said too. One thing that's clear is that many of the cheaper Eastern made pedestal drills that are about are not ideal - chucks/spindles may not run true, tables can be a bit flimsy, quill travel is often short, and out of balance belts and pulleys can cause vibration.

One issue that can cause problems too is that it can be hard to get a smaller drill (needed to create say a pilot hole for a screw to drive into and to prevent splitting) centred in the bottom of a previously drilled clearance hole. (e.g. it's often necessary to drill a clearance hole through the piece being held down by the screw, while the mating piece has got to be drilled to a size that the screw will engage in)

I suppose a taper or stepped drill is the answer, but maybe there's another way like slipping a metal or plastic tubing sleeve over the smaller drill to locate it in the previously drilled larger hole.

To feed some info back:

A good German brad point like say the Famag or Colt spurred or non spurred type cuts cleanly into ply provided you are careful to feed it gently when starting and breaking out, and use a backer piece. http://www.fine-tools.com/bohr.html

I've never had much luck with this type of multiple edge countersink tool, they chatter badly when used with a hand drill: http://www.mscjlindustrial.co.uk/CKR-90500B/SEARCH:KEYWORD/product.html

This I've found is a very clean cutting and long lasting type - even if it's intended primarily for metal use. It does drift off the centre of a previously drilled hole very easily when hand drilling if it's not held dead square, or if the wood is not of uniform hardness: http://www.granlund.com/main.php?menuId=722

This Colt set works really well - it cuts cleanly, and being carbide should last well - and can be used with to simultaneously drill and countersink (with the brad point drills in the set fitted), or with a piece of rod to pilot it into a pre-drilled hole. It's very sharp, and so cuts cleanly, and the pilot means it stays pretty much on centre: http://www.axminster.co.uk/colt-drill-and-countersink-set-prod847326/

Engineering style centre drills or countersink drills I hear work pretty well, and stay on centre too: http://www.mcmaster.com/#countersink-drills/=e8rk8h

And finally.... my pedestal drill while it runs pretty true is far from being a top quality model.

On drill sharpening. Is there any system that makes it feasible to accurately sharpen both brad point and ordinary twist drills? Does anybody do it? Does the system done by WorkSharp do a good job?

ian