Long travel drill guide

Question

I need to drill a 10mm hole 200mm through timber 50mm wide. If these dimensions sound odd it's for a pin through a right angle finger join to another piece of the same size.

I start centred 25mm from the three nearest vertical faces but even with a drill guide and some care it is very difficult to emerge centred in the opposite face.

If my drill press had 200mm of travel it would be just the ticket, but it doesn't.

Is there a usual solution for this type of positioning problem?

Here's the jig I eventually created from one 50mm length and two 300mm lengths of 50mm angle iron. Holes are 10mm diameter with centres 25mm from nearby faces. Cut, then drill, then assemble as illustrated clamped around a sample job. Tack weld, remove timber and strengthen welds. Do not weld the inside seam, it would foul on the job when using the jig.

Clamped flush onto the work-piece, this jig positions an auger bit and aligns it parallel to the 200mm tall vertical face. When the board is cupped, place it against the concave side for stable alignment. The guides are 100mm apart so a 190mm auger bit will drill about 80mm into the work-piece.

The jig can then be removed and the hole used to align the bit for drilling through with adequate precision.

For optimal results combine this jig with the technique described in the accepted answer. For cupped boards this will require a double ended version of the jig due to the need for opposite chirality .

Welcome to Woodworking.stackexchange! Are the 50 x 200mm boards solid or laminated assemblies? — Ashlar, Feb 27 '16 at 22:35
The boards are 2400x50x200 or 8' 2x8" solid pine CCA treated. — Peter Wone, Feb 28 '16 at 11:49
It's not the ideal answer unless you already have a brace but this is the kind of thing a swing brace and auger bit is ideal for — long, straight holes. See previous Question for a bit more detail, Which type of drill bit for which jobs?. — Graphus, Mar 01 '16 at 09:05
Graphus, I had to look up "swing brace" but I knew it when I saw it. I agree that it is a tool well adapted to the task but I fail to see how it would help with the alignment issue. — Peter Wone, Mar 01 '16 at 14:14

score 5 · Accepted Answer · answered Feb 28 '16 at 01:41

The usual solution to this problem (let me to restate it in inches - a 3/8" dia hole goes 8" through a 2" x 8" board) is to drill from both ends of the hole.

Drill approximately half way through from one end of the hole, then drill from the desired exit point back towards the other end of the hole. These two holes will most likely not meet exactly as you would hope, but will be a wee bit offset. When you drill the second hole the bit will seek and follow the first hole and you will wind up with a hole beginning and ending exactly where you want it.

The hole will accept the dowel which will be perfectly tight at the ends and may be unnoticeably loose in the center. The pin might have to bend slightly on its way through the hole put can readily be pushed into position with a minimum of force.

An advantage of this approach is that you will not have to worry about tear-out as the drill exits the other end of the hole.

I may accept this answer, but first I have some testing to do. I've designed a couple of jigs and if any of them work well I'll publish the design and drawings as an answer. One approach exploits my drill press to produce two 50mm blocks with perfectly aligned holes; I line them up with dowel then fasten them to two more pieces that hug the faces of the job. This gives me a drill guide 100mm deep. Since my auger has a reach of 190mm this should give me enough perfectly aligned depth to serve as a guide for the other half. If it doesn't I'll combine it with your answer. — Peter Wone, Feb 28 '16 at 23:39

score 2 · Answer 2 · answered Feb 27 '16 at 22:46

I can think of several approaches to try.

First is to drill with an long auger drill bit. A new sharp bit with the screw tip on an auger may drill straight through better. I do not know if you can get one to drill the whole 200mm, but if you get most of the way you can use a small long shaft standard drill bit to penetrate through to the other side and then enlargen the whole once it is through.

As an alternative, you can assemble the finger joint from separate pieces of wood, drill the holes in each and laminate them together with a dowel in the whole to properly align them.

A new auger of exactly the type you depict is what I'm using. The problem is not so much of drift due to variations in density and hardness as of alignment. — Peter Wone, Feb 28 '16 at 11:52

score 1 · Answer 3 · answered Feb 29 '16 at 05:21

I'll try, but I fear that I might need more pictures to go with the explanation...

Draw a square line all the way around your 50x200. Mark the middle of the edges.
Find a couple of scraps of plywood, any width and somewhere in the length of 300mm.
Draw a straight line (not even square -- just straight) across the middle of the 300mm dimension. Carry the line to the bottom edge with a little tick.
Align the line on the plywood with the line on the flat of the 50x200. By now, you should have the timber on edge, on the ground, with the ply pressed up against the flat of the timber. Clamp or brad nail the ply in place.
Clamp the other chunk of ply onto the other flat side of the timber. (Terrible artwork:)
Drill into the pencil mark, keeping the drill bit between the chunks of ply, and aligned with the line on the ply. Drill half way. Flip the timber and repeat.

If the auger bit isn't very long, try to set yourself up so that you at least get started with a couple of inches/50mm. This will align you for the rest of the drilling as well as possible.

Long travel drill guide

3 Answers3