What size drill bit?

[Tom M King]

I was surprised to find that our Ace hardware carries the full range of replacement bits for it. The tiny ones come in sleeves of 3 or 5 -can't remember. One small toolbox drawer is almost full of replacement sleeves with spares, since the index only has one hole for each size.

There is a company selling on ebay with a lifetime warranty-Drill Hog. I had never heard of them before, but tried some of the middle sized left handed bits, and I haven't broken one yet.

[Greg R Bradley]

Any drill bit with a lifetime warranty is either massively overpriced for the quality or they plan to be unavailable for warranty or make it so difficult that they won't have to warranty any. This even includes Snap-On and other premium sellers. Any sellers on ebay will always be suspect.
Drill Hog, Drill America, etc. are overpriced cheap junk. Even sellers like Snap-On just sell the set for triple its value and then will actually warranty items.
For high quality, stick to Norseman, Viking, CTD, Cle-line, Triumph, Chicago Latrobe, Champion, etc for legitimate sellers. There is NO magic to drill bits. If there was, the hundred thousands of machine shops and manufacturers would have found them.
Try to avoid Cobalt for most uses. They are a bit more wear resistant but are brittle and have thicker cross section that make them work poorer in some materials.
The good companies like Norseman and Triumph have charts on their websites that explain which drill types work well on which materials.

[Keith Westfall]

I have and like the caliper that John showed and would love to get another...

Price:

[COLOR=#B12704 !important]$29.95[/COLOR] on Amazon.com

]CDN$ 100.06 on Amazon.ca

Nice, Amazon, really nice...

[Tom M King]

That's the most expensive iGaging tool I've ever seen. I was left all the machine tools by a friend who had retired from teaching Machining and Welding, and had bought all the school stuff when they closed the program. There are a bunch of dial calipers, including one Luger one.

[Ellen Benkin]

After all this advice I did as suggested and this makes me even more confused.

I bought the igauging digital caliper and this is what I found:
The threaded rod measures 4.76, the 5mm bit I borrowed from a friend measures 4.26 (and makes a hole that is almost perfect for the threaded rod) and a 5mm bit I bought from Lee Valley measures 4.99. I haven't tried the LV bit yet to find out if it will make a hole to work with the threaded rod.

So much for the scientific method.

[Tom M King]

Drill bits almost always make a hole slightly larger than their diameter, depending on the machine that's spinning the bit. Machinists drill a hole slightly smaller, and use a reamer to get it exact.

[John K Jordan]

...So much for the scientific method.

As Tom mentioned, your experience makes sense. Perhaps your scientific method wasn't quite scientific enough.

If you are drilling by hand or if there is any runout on a drill press/chuck or the bit flexes or chatters the hole will be larger than the bit. Drilling into wood can be worse since wood is non-homogeneous and the bit can try to follow the grain. Jobber length drill bits (a standard length) are more likely to wobble when drilling because of their length. For more accurate holes I use screw machine bits which are relatively short and stubby and much stiffer because of that. Also, when drilling in wood I like to start the hole with a machinist's center drill bit which is very short and can't flex. This helps the standard bit get started correctly in wood for a more precise hole. Note "more" precise, never perfect. Also, measuring across the cutting edges of a drill bit is tricky because of the spiral and the way the edge is machined. On many bits you can measure across the steel just beyond the flutes, keeping away from any numbers if they are stamped into the steel.

Although this is more of an engineering problem, I think the scientific method could still be used by devising an experiment: measure enough samples of the object (the threaded rod) to determine the dimensional variation, pick and measure a candidate drill bit all along the working length, measure the runout in the drilling machine with a precision rod in the chuck at the test length from the front of the chuck, drill several holes, measure and test each for fit, if the hole is too large or too small repeat the test with another size. Formulate a hypothesis concerning the size of the hole obtained with that bit and that particular drilling machine/method and that material. Test the hypothesis with other drill bits, drilling methods, and materials. Repeat all the tests with various speeds. For more complete data collection repeat at various temperatures, monitoring and recording the bit temperature for each hole. Enough of that and you will have a pretty good handle on what it takes to drill a hole! Maybe you can apply for a government grant to pay for it all.

JKJ

[Ellen Benkin]

Who would have thought my "simple" question would get so many technical responses. Thanks, everyone, but now that I know all the variables I'll stick with my "trial and error" approach to get the proper drill bit for the threaded rod so I can move on with my project.