10-32 bolt clearance holes cut on our CNC router should be spec’d as a #9 drill size or 0.196”
10-32 bolt clearances holes made on a drill press or with a hand drill should be spec’d as a #7 drill size or 0.201”
Or asked a different way: is your team good enough, confident enough to use #9 by hand for 10-32 clearance holes?
Full disclosure, we’ve been using #7 for everything and it feels sloppy for some parts, so I’m thinking about transitioning to #9 for parts made by CNC that connect to other parts made by CNC.
I’m no machinist, but online searching would indicate “it depends”.
For a single hole, drilled with a precise bit of kit (CNC included), #9 is seems to be the way to go to get a nice close fit with little to no wiggle.
For a hole pattern, #7 seems to be recommended, as it’s less likely you’d get all the features exactly aligned and need that bit of slop to ensure things can be assembled.
Ultimately, it seems like the answer really just comes down to “how good is your team at putting holes in the right spot?”.
In the context of an FRC season… I feel the tradeoff I want to make is to accept slightly sloppier mechanism assembly, for gaining time back from not having to remake/redrill parts, or have folks be all frustrated with why some bolts don’t quite go through and start wailing on it with a hammer.
I’m certain there’s at least a few god-tier machinists on teams out there who are good enough that their teams don’t have to make that tradeoff. I’m not in that camp though.
We both drill and machine all 10-32 bolt holes to a much closer clearance that even #9. Our usual hole is 3/16" (i.e., 0.1875"), which gives a good tight fit. You can machine this easily using a 4mm bit in your CNC, since it’s just slightly over 4mm (4.7625mm, to be precise.) If you’re trying to make a tight fit for a 10-32 bolt, I’d ditch both #9 an #7 for 3/16".
Technically, #10 bolts are supposed to be 0.19 thread diameter. Practically, we’ve never had them be too large for a 3/16" hole. The difference is, after all, only 0.0025" so you get the tight fit you’re looking for without a problem. If you need a loose fit, then the answer is a 7/32" hole.
FRC designers tend to load the actual bolt bodies in shear, rather than holding loads through friction between the plies.
Even in FRC hole patterns, if the pattern is in a tube, you won’t develop enough tension to hold the joint with friction before crushing the tube, unless there’s an insert or filler of some kind.
We start with 3/16, with match drilling during assembly where things aren’t fitting.
The joints slip back and forth under high load.
if we keep it at near-zero play at assembly, we usually can run the mechanism through the end of the season okay.
This is not a professional bolted joint.
In professional context, a close or free fit is the move, because bolt joints should be resisting shear with friction between plies, and your assemblers need to move fast. None of the bolt threaded bodies should experience shear directly, if the bolts are tightened to proper tension.
If you do proper layout, center-punch carefully, and use a drill press, you can pretty easily locate a hole to within a couple thou by “hand” (no CNC tools).
This is one reason why rivets are often a better solution for final mechanisms (I still very much like bolts for things that need to come undone, though).
I agree “I” can… and some of our students too, but not all. Even still being off a few thou means a .1875 hole definitely won’t work when lining up holes in two pieces. So 0.196 or 0.201? What’s your vote?
Can you give an example of a case where this doesn’t work? Clamping parts together and matching the holes with a 9 bit works great. I consider it to be a basic skill for robot construction.
Student A makes a 1x1 with a bunch of holes. Student B makes another part with several holes, then we bring them together. We do this when we are doing a big parallel fabrication path. Some students are more skilled than others at punching and drilling. If you can clamp and drill great, but we are often making in parallel.
In any case its great to see several opinions about how this is best done, so I appreciate all the folks who shared here. Good food for thought!
To answer your question, we typically drill holes in the following manner:
CNC/Laser->3/16
Hand layup->.196"
Holes that are missaligned->.196" and wallered out to fit
Note: In my time in the aircraft industry, we typically only put the hole pattern in 1 part with a pilot hole in the mating part. Pilot holes are then cleco’d together and then the hole pattern is transfered from the 1st part to the second.
I recall a table in The Machinists Handbook (maybe?) relating number of holes to drill size. E.g. when the hole pattern has more holes you ought to use a looser fit to ensure things drop together appropriately. A part of my day job is doing tolerance stacks between mating components to tolerance these interfaces. In FRC…
95 uses .201 / #7 for everything. Most interfaces we use have at least three holes and fasteners, often many more, so using a looser size hole isn’t much of a detriment. Rivets and clecos tend to pull mating holes concentric instead of most bolts which will allow more misalignment. Since we assemble most parts with rivets we do not see detectable misalignments. An easy way to get some decent self-aligning behavior from bolts is to use countersunk bolts, the taper of the countersink centers one hole pattern on the other.
.196 for anodize parts and .201 for powder coating. I’ve always felt (true) .201 was a little rough for 3/16 rivets, but can be ideal depending on your tolerances. We tend to “web” probe each tube to evenly eat the +.005/+.01 tube tolerance on each side of the hole pattern. You can definitely have a bad day using .196 w/ poorly toleranced tubes and manufacturing.
I’ve never found it has caused us too many issues to have our holes slightly too large. Most of our hand made parts are drilled to 13/64th for #10 and 3/16th rivets and I don’t think it’s ever cost us a match. For sketched CAD holes that we laser cut it’s normally 0.2in and hole features are clearance holes at 0.201. On the router we normally drill either 3/16" or 5mm since we have collets in those sizes.
As far as I’m aware, I’ve never used or even seen a number-sized drill bit.
A #10 machine screw needs a 7/32" hole for a clearance fit, or (much less common) 3/16" followed by several passes with a de-burring tool for snug fit.
Just to confuse the issue further, there are also letter-sized drill bits.
You may have used them when doing tapping prep–e.g., 1/4-20 uses a 7 bit before the tap, I forget the sizes for 10 screws (10-24 and 10-32 done right use different sized drills). Take a look at a tap chart for all the sizes. Going slightly larger or smaller definitely affects thread life and how hard it is to get a screw in there.
