What size drill bit?

[Ellen Benkin]

With the understanding that there are no dumb questions -- here's one

I am drilling holes in several pieces of mahogany to insert a 10-24 threaded rod. I've experimented and found that I can thread them through a 13/64 hole but it is very tight. The next size up 7/32 is too lose. I have borrowed a set of metric bits and will try them next. Any suggestions? I have a lot to do so I would be willing to purchase an odd size bit.

Thanks.

Ellen

[Dave Richards]

Are you sure it's 10-24 rod? If it is, it should have a major diameter of 0.190". A #11 drill would be 0.1910" dia., #10, 0.1935" dia. and 5mm, 0.1969" dia.

13/64" is 0.2031" and 12-24 would have a major diameter of 0.2160" which I would expect would have to be threaded in to get it through a 13/64" hole.

If you have #12 rod, a 5.5mm drill might be the ticket at 0.2165.

[Greg R Bradley]

A #3 is 0.01" larger than 13/64" but still 0.05" smaller than 7/32".

This doesn't quite add up as a tight clearance for 10/24 is #9 and a free fit is #7. Even the #7 is a tiny bit smaller than the 13-64" you say is too tight.

I added a nice chart that shows the progression of drill sizes including Number, Letter, Fractional, and Metric: 402151

[Jim Becker]

As an aside, I will add that a digital caliper is an invaluable tool in the shop, including for things like this. Most have the ability to switch between inch decimal and metric and some also have inch fractional. (mine does) While I originally bought mine in support of my CNC for measuring material thickness, I'm using it an incredible lot for all kinds of projects including determining clearance hole size for fasteners like this. The brand I have is from iGuaging and I'm very pleased with it for the $30 or so it cost. Don't buy a really cheap "plastic" one...the often miss a digit of precision and I found out the hard way they were not quite accurate, either.

[Ray Newman]

This might help: Tap and drill size chart -- https://www.imperialsupplies.com/pdf/I_DrillSizeDecimalEquivalent&TapDrillChart.pdf

[Bill Dufour]

This might help: Tap and drill size chart -- https://www.imperialsupplies.com/pdf/I_DrillSizeDecimalEquivalent&TapDrillChart.pdf

My chart says #9 for close fit #7 for loose fit.#9 make sense because that is the next size up from #10.

[John K Jordan]

This chart has "close" and "free" fit drill sizes for common machine screw sizes.
https://pages.mtu.edu/~suits/misc/tapsizes.html

Maybe describe in more detail what you need to do.
- How deep are the holes?
- In side grain such as through a board or in end grain?
- 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. I've used beeswax as lubricant. If fibers are suspected you might try dousing the inside of the hole with sanding sealer then redrilling after it's dry.

To slightly enlarge a hole in wood I've made "reamers" from a steel rod. 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. Mostly the insides of the wooden holes were burnished to a slightly larger size.

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.

JKJ



With the understanding that there are no dumb questions -- here's one

I am drilling holes in several pieces of mahogany to insert a 10-24 threaded rod. I've experimented and found that I can thread them through a 13/64 hole but it is very tight. The next size up 7/32 is too lose. I have borrowed a set of metric bits and will try them next. Any suggestions? I have a lot to do so I would be willing to purchase an odd size bit.

Thanks.

Ellen

[Ellen Benkin]

Thanks everyone. I did want the rod to slide without either binding or slop. It turns out that a 5mm bit works well.

Yes, Jim, accurate measuring would help but I use the measuring device so infrequently that the battery is always dead. And trial and error is so much more fun.

[Jim Becker]

The unit I mentioned has auto-off, so the battery life is substantial. It comes with a second battery in the package, too. :) You will likely use one more than you think!

[John K Jordan]

...measuring would help but I use the measuring device so infrequently that the battery is always dead.

My favorite calipers for the shop, especially at the lathe and when sizing for drilling. I like this so much I bought a second in case I accidentally set one down somewhere.
The batteries are guaranteed to last an eternity.

402202 402203
https://www.amazon.com/gp/product/B005FRGF1Q

The scale is 64ths and 100ths but the eye can get much closer when needed.

JKJ

[Greg R Bradley]

Thanks everyone. I did want the rod to slide without either binding or slop. It turns out that a 5mm bit works well.

Yes, Jim, accurate measuring would help but I use the measuring device so infrequently that the battery is always dead. And trial and error is so much more fun.
As I said in the beginning, something doesn't add up.

The 5mm working confirms that something is wrong. It is smaller than the the 13/64" you said was too tight. Now if you had calipers so you could measure the supposed 13/64" bit and determine its actual size .................

[Greg R Bradley]

My favorite calipers for the shop, especially at the lathe and when sizing for drilling. I like this so much I bought a second in case I accidentally set one down somewhere.
The batteries are guaranteed to last an eternity.

402202 402203
https://www.amazon.com/gp/product/B005FRGF1Q

The scale is 64ths and 100ths but the eye can get much closer when needed.

JKJ
That is a brilliant idea. Another example of where analog beats digital. OR, where metric beats imperial :eek:

[Dave Richards]

As I said in the beginning, something doesn't add up.

The 5mm working confirms that something is wrong. It is smaller than the the 13/64" you said was too tight. Now if you had calipers so you could measure the supposed 13/64" bit and determine its actual size .................

I suspect the "13/64 in." isn't really 13/64 in. probably a smaller one that ended up in the wrong hole in the drill index.

[bill tindall]

Every woodworker needs a set of number drills for just such occasions as this one. Surprisingly many woodworkers have never heard of number drills. Splurge and get a set of letter drills too.

[John K Jordan]

Every woodworker needs a set of number drills for just such occasions as this one. Surprisingly many woodworkers have never heard of number drills. Splurge and get a set of letter drills too.

This is the type of set I've had in my drilling and tapping tool cabinet for a bunch of years:
402236

Here's one:
https://www.amazon.com/dp/B00S1PZK2E

These sets typically have 115 bits with fractional (in 64ths), letter, and numbered bits.
So far I've always been able to find the bit I need for special clearance or for an odd-sized tap.
Get a good set and it will last a lifetime.

JKJ

[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:
$29.95 on Amazon.com


]CDN$ 100.06 (https://www.amazon.ca/iGaging-100-164-Fractional-Decimal-Combination/dp/B005FRGF1Q/ref=sr_1_1?ie=UTF8&qid=1548915674&sr=8-1&keywords=iGaging+6%22+Fractional+%26+Decimal+Inch+ Combination+Dial+Caliper) 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.