Bolt Holes

I am designing a drive train and I want to use 1/4-20 bolts for some connections. I am not sure how big the holes should be. I was also wondering if there is a way to know what hole size to use for different size bolts.

A hole size chart is indispensable.


Just Google “through hole size chart” and you will come up with many great charts like the one from this link…

Madison beat me to it, guess my old fingers don’t fly fast enough!

If you’re using Inventor (and probably SolidWorks too, but I wouldn’t swear to it), you can define clearance holes for various bolt sizes with the hole tool.

Also, just in case you did not know this, the first number in a two-number bolt number is the diameter of the screw, while the second number is the threads per inch.
i.e. 10-32= #10 diameter (this is where the chart comes in handy) and 32 threads per inch.

You can also just take a drill index and stick bolts in it until you find the right hole size.

I use a droid app to tell me such things.

This thing is bookmarked on just about every browser/computer I’ve ever used.

I wouldn’t consider that useful advice.

Both SAE and Metric are located here:

Print this and put one copy by your CAD workstation, one by your mechanical toolbox and one by your electrical toolbox, or wherever you will be drilling holes for tapping or clearance.

A little bit depends on what you are using the bolt for & your resources.
For strength or location critical you want to use small clearances. For something that is going to be disassembled regularly or has a bunch of bolts that have to line up, you want larger clearances. Hand drilling or CNC machine? Drilling to a drawing or a drawing or templating from the part?

1/32" over is a good rule of thumb. 1/16" is pretty sloppy. 1/4 drill will allow the bolt to be inserted.

Charts are good too.

Just be careful blindly using a clearance hole chart; the close and loose fits are based on assumed fabrication/position tolerances. The reason a hole is slightly oversized is that you are lining up multiple holes in 2 or more different parts, and the holes which are 2.000 apart on one part may be 2.005 on the other part. Picture taking 2 concentric circles and moving them apart - the open area where you can fit a bolt gets smaller. If you are drilling all of your holes in a pattern on a CNC mill in one setup, you’re probably OK; if you’re scribing and using a drill press with multiple setups, you’ll have quite a bit more tolerance.

However, even with all that, if you are just building 1 or 2 (or even 4 or 5) assemblies, then you can start with the standard clearance hole and match drill / ream anything that doesn’t line up enough to get a bolt in if everything else lines up for your needs.

There are several iphone apps that cover this and many more of those need to know mechanical questions.

I wouldn’t consider that useful advice.[/quote]

Why not? If you find a bolt that you want to use, for whatever reason, and you don’t know what size it is (let’s say you dug it out of a bin of unsorted machine screws of various practically indistinguishable sizes), testing it in the holes in a drill index or a drill gauge could easily tell you what hole size you need.

Of course, a chart or a rule of thumb like FrankJ’s is always better, especially when you know exactly what you’re doing, and I did not intend to present this as replacement for the precise way. But for a quick and unimportant fix, I see nothing wrong with the above method.

What does that tell you? The hole in a drill guide are oversized quite a bit, so it’s easy to put the bits in and out. Probably if you put a 1/4 inch bolt in the hole for a 1/4 inch drill bit, it would fit in really loose because the hole is larger. If you then used the 1/4 inch bit to drill your holes as you suggest, you wouldn’t be able to assemble the parts because of tolerances. You may not even get 1 in since the bit wears down.

Ah, I understand the ire now. I knew it wasn’t a GOOD method, but I was suggesting it as a LAZY method. I’m sorry I have encouraged laziness in the FIRST community, and will recant my previous two posts on this thread.

(Though if I’m tired and frustrated around week four of build season, I may choose to use a drill gauge on a bolt. I’m sorry, I just can’t stop myself. ;))

There is a “better” lazy method. Compare the screw to the drill bit shank, right next to each other. If the shank matches the screw size, then you should be all set. If the hole is too small, find the next drill size larger (or a non-working student or mentor with a rat-tail file or similar device) and make the hole large enough for the bolt.

Can’t tell how many times I’ve used that to find the drill bit that best fits small screws used to attach servo stuff to R/C airplanes…

Calipers work too, just use the flat parts, not the tapered portion which could slip in between threads and give an inaccurate measurement.

There are other reasons to drill holes larger than necessary for the bolt in question. A 14-20 bolt will easily fit in a 1/4 in. hole, but if the hole is in aluminum, once you have tightened the bolt, you have deformed the hole so that the bolt will be difficult to remove and reinsert the next time.

In different applications, hole clearance varies. For precision machines, specified clearance is less than for architectural use, for instance.

Use of a good, appropriate chart is suggested.

Dr. Bob