Perhaps your scientific method wasn't quite scientific enough. The good companies like Norseman and Triumph have charts on their websites that explain which drill types work well on which materials.Īs Tom mentioned, your experience makes sense. They are a bit more wear resistant but are brittle and have thicker cross section that make them work poorer in some materials. If there was, the hundred thousands of machine shops and manufacturers would have found them. Even sellers like Snap-On just sell the set for triple its value and then will actually warranty items.įor high quality, stick to Norseman, Viking, CTD, Cle-line, Triumph, Chicago Latrobe, Champion, etc for legitimate sellers. Any sellers on ebay will always be suspect.ĭrill Hog, Drill America, etc. This even includes Snap-On and other premium sellers. Any suggestions? I have a lot to do so I would be willing to purchase an odd size bit.Īny 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. I have borrowed a set of metric bits and will try them next. I've experimented and found that I can thread them through a 13/64 hole but it is very tight. I am drilling holes in several pieces of mahogany to insert a 10-24 threaded rod. With the understanding that there are no dumb questions - here's one If the threaded rod needs to slide freely without slop for some mechanical reason, I might consider drilling a larger hole and gluing in a bushing, perhaps a piece of tubing if I could find one that fit or make a bushing by drilling brass, aluminum, or HDPE or Delrin or something. Mostly the insides of the wooden holes were burnished to a slightly larger size. I slightly flattened and widened a section near the end of a rod a bit with a hammer, filed a few grooves to help cut then spun the rod along the hole. To slightly enlarge a hole in wood I've made 'reamers' from a steel rod. If fibers are suspected you might try dousing the inside of the hole with sanding sealer then redrilling after it's dry. Do you just need to insert the threaded rod or do you need for it to be able to slide freely but without slop? If the 13/64 is too tight in side grain, could it be that fibers on the inside are making the fit too tight? Maybe the drill bit is not sharp enough. In side grain such as through a board or in end grain? Maybe describe in more detail what you need to do. WHERE YOU HAVE THE ROOM always drill at least a 1/4' deeper than the thread depth you need, that's where you will most likely break the tap, at the bottom.This chart has 'close' and 'free' fit drill sizes for common machine screw sizes. My experience/opinion: if you have the choice use the larger size drill, you will get a satisfactory thread and are much less likely to break the tap. They are usually not a standard size fraction, they may be a few thousand's of an inch +/- as stated in the other posts. That's where the letter drills come into play. It will tap easier and you'll be less likely to break the tap, in fact if you have a home drill press the same still holds true.Now, I say that without being familiar with engine manufacturing specs, perhaps things like turbo holes, main bearing/crankshaft holes, etc. For our purposes, auto industry, and the fact that most probably we will be hand tapping the hole it is sufficient to go with the 75% thread fit. The tap /drill size may vary from chart to chart, this is based on the percentage of thread desired, for instance an aircraft industry % maybe higher(tighter fit) than,say a boiler manufacturer. Based on 30+ years as a tool and die maker/machinist = a 'standard' clearance hole is + 1/16 (.060') up to 1/2' and over 1/2' it is +1/8(.125').

Drill bit hole size chart