Thread: How to run a straight trench along a long dowel?

Originally Posted by Skew ChiDAMN!!

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The more embarassing part is that my next step is to drill another 2m dowel with 1/8" holes at 12.5mm intervals for most of it's length.
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I had to drill some 5mm holes 25 mm apart down several 15 mm battens; so I made a simple jig for the drill press. [After I stuffed up a batten free handing!]

Jig consisted of two battens (from the above batten) screwed to a piece of 3mm MDF so that the batten I wanted to drill would slide smoothly inside. Plus two holes exactly 25 mm apart drilled on the centre line of the MDF. Clamp jig onto drill press so that the first hole aligned with the same 5 mm bit in the drill press. And a 50 mm piece of 5 mm dowel aligner, sanded marginally undersize and slightly sharpened on one end in a pencil sharpener.

Drilling Jig.jpg


Technique is very simple:

• Mark position of first hole near end of batten,
• Slide that batten into jig until hole position mark centred under drill hole in jig,
• Drill first hole,
• Advance batten through jig until hole just drilled is visible through second hole in jig,
• Insert alignment dowel,
• Drill second hole,
• Advance batten and repeat until all holes drilled.

After I had drilled a 2400 mm batten, I found that I had an accumulative error of about 14 mm. This suggests that the two holes in my jig were actually 24.85 mm apart rather than 25 mm. Not material in my case! [The error a mere of 0.15 mm accumulates!]

That's pretty much exactly what I'd planned to do!

Acceptable margin of error for this'll be +/-0.5mm per hole, so plenty of wiggle room so long as I keep an eye out for additive errors creeping in.

Some clever responses in all these.

Originally Posted by Skew ChiDAMN!!

That's pretty much exactly what I'd planned to do!

Acceptable margin of error for this'll be +/-0.5mm per hole, so plenty of wiggle room so long as I keep an eye out for additive errors creeping in.

One tenant in surveying is "work from the whole to the part." In other words - make sure the overall measurement is precise and accurate at the required 2400 mm. Then break the task into sections to control accuracy and precision and limit the effects of any errors.

There are also other potential errors - the "fit" of the pin and in the wear of the "registration" pin & hole that will change over time with use. Any slop will create a potential linear and angular error which on simple probability could be over or under.

Use a similar setup & technique to Graeme's, but break up into say 12 bays of 200 mm to limit the accumulation of errors. Mark the 12 bays of 200 mm on the dowel and add one more hole to the 3 mm MDF "template" at 200 mm. Drill all the 200 mm spaced holes first then use them as the "first" registration hole for each bay. Make a dummy run of one bay on some scrap to check accuracy.

As you are using dowel, make sure you don't introduce angular error - a twist / spiral into the line of holes buy using an additional "guide pin" and use the guide pin & registration pin as "boning rods." Clamping a secondary fence to align the guide pin would help.

Simply swaping the 3 mm MDF to say 12 or 19 mm MDF (or hardwood even better) will improve alignment and registration accuracy.

If you want to improve accuracy even more, for say tasks like cribbage board holes make a template with all holes per bay marked accurately then drilled in the template.

Originally Posted by Mobyturns

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Use a similar setup & technique to Graeme's, but break up into say 12 bays of 200 mm to limit the accumulation of errors. Mark the 12 bays of 200 mm on the dowel and add one more hole to the 3 mm MDF "template" at 200 mm. Drill all the 200 mm spaced holes first then use them as the "first" registration hole for each bay. Make a dummy run of one bay on some scrap to check accuracy.
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Nah; not necessary.

I just redefined the target hole spacing as 24.85 mm between centres. It was spot on!

Originally Posted by GraemeCook

Nah; not necessary.

I just redefined the target hole spacing as 24.85 mm between centres. It was spot on!

24.85 mm +- 0.005 between centres.

Originally Posted by Mobyturns

24.85 mm +- 0.005 between centres.

With my precision level it was 24.85 +- 0.0005, most definitely.

A drill press vice set to a slide fit, mark the hole centres on the top of the jaws and start drilling.

Originally Posted by rustynail

A drill press vice set to a slide fit, mark the hole centres on the top of the jaws and start drilling.

You must have far better eyesight and steadier hands than me, Rusty.