Will a 9.5mm bit for 10mm bolt work?

I'll delete this thread afterwards but just want to put it out here.

I ordered what I thought were 10mm drill bits from eBay but when they arrived I found they were stamped 3/8" or 9.5mm. I tested drilling a hole and inserting the bolt a little ways in and it didn't really feel any harder than when I use an actual 10mm drill bit. 

Given that 0.5mm is 3 thousandths of an inch, can I use the 9.5mm bit still? Mostly making sure the clips will actually engage on the bolt in a slightly smaller hole. 

For context I am using Team Tough's 10mm wedge bolts and 10mm bit from their website.

Having made a similar mistake a couple years ago, about 1/3 bolts would not engage the clip and just spun gaining no torque. This was in spite of every attempt to make them work. Needless to say it was a headache and a lot of work to fix. You may have better luck depending on the rock type but it will be a bit of a gamble with each individual bolt in my experience. 

Please don't delete it. It's good info for others. 

I'm sure all the armchair experts will weigh in here with absolutes but the answer isn't as straightforward as people might think.

In my experience it will work okay (still not ideal) for a bolt or two in harder (relative term) rock before the bit narrows from wear along the sides of the bit. At this point you'll have to bash the shit out of the bolts to get them in the (undersized) hole. They won't seat properly. 

Contrast that with softer rock (again, relative term) where a 10mm bit will end up drilling a slightly oversized hole. I'm not talking SW desert type sandstone, where obviously wedge bolts are inappropriate.  Rather soft(ish) granite like what you might find near the top of granite formations along the Front Range, for example. I use this example because you're in CO; there are plenty of others out there. In these cases a slightly smaller bit might do the trick.

Another example is when hand drilling, especially (again) in soft rock where the holes tend to wallow.

To ensure your hole is of the right size, just bring along one proper 10mm bit. After the hole is drilled with a 3/8" bit, check the hole diameter by inserting the 10mm one to see if it fits. If not, simply use it to enlarge the hole.

The method above also works well for saving money on more expensive 10mm bits. Just drill a "pilot hole" with a 3/8" bit and enlarge with the metric one afterwards.

Because the rock you're drilling in is not a perfectly predictable medium, there are always nuances involved. And these cannot be expressed in instruction manuals, spreadsheets, etc.

nbrown wrote:

To ensure your hole is of the right size, just bring along one proper 10mm bit. After the hole is drilled with a 3/8" bit, check the hole diameter by inserting the 10mm one to see if it fits. If not, simply use it to enlarge the hole.

The method above also works well for saving money on more expensive 10mm bits. Just drill a "pilot hole" with a 3/8" bit and enlarge with the metric one afterwards.

This is the easy solution I was looking for. Thank you for pointing it out and relieving some of my disappointment with eBay.

nbrown wrote:

Another example is when hand drilling, especially (again) in soft rock where the holes tend to wallow.

To ensure your hole is of the right size, just bring along one proper 10mm bit. After the hole is drilled with a 3/8" bit, check the hole diameter by inserting the 10mm one to see if it fits. If not, simply use it to enlarge the hole.

The method above also works well for saving money on more expensive 10mm bits. Just drill a "pilot hole" with a 3/8" bit and enlarge with the metric one afterwards.

Because the rock you're drilling in is not a perfectly predictable medium, there are always nuances involved. And these cannot be expressed in instruction manuals, spreadsheets, etc.

Slightly enlarging a hole like that by hand drilling is really difficult and annoying, if not practically impossible. The bit just wants to bind in the hole. BTDT.

Gunkiemike wrote:

Slightly enlarging a hole like that by hand drilling is really difficult and annoying, if not practically impossible. The bit just wants to bind in the hole. BTDT.

Rotate the bit counterclockwise. Works for going from 1/4" to 3/8"

Going from 1\4 to 3\8 won't bind the drill very much but 9.5mm to 10mm is a nightmare.

rocknice2 wrote:

Going from 1\4 to 3\8 won't bind the drill very much but 9.5mm to 10mm is a nightmare.

weird, any idea why the smaller diameter change causes more binding?

Fail Falling wrote:

weird, any idea why the smaller diameter change causes more binding?

Well, going from zero to (any diameter) doesn't bind at all. Going into something a hair too tight binds like crazy. 9.5 -> 10 is closer to the latter on the spectrum of bindage.

Gunkiemike wrote:

Well, going from zero to (any diameter) doesn't bind at all. Going into something a hair too tight binds like crazy. 9.5 -> 10 is closer to the latter on the spectrum of bindage

The comparison doesn't work in this case as we're not comparing drilling from scratch to drilling out a pilot hole. 

Going from 1/4" to 3/8" binds and going from 9.5mm to 10mm binds. What about the process of binding when drilling out a predrilled hole is exacerbated by the difference between the pilot and finishing hole diameter being closer together. 

Edit: hit the posting limits: Thanks Greggor and Jake that all makes complete sense to me now. 

Unless you do a really light tap, the bit easily moves too deep into the hole for you to turn it by hand. The normal process buries the chisel tip just a few thou, and turning it chips the rock sideways. With only the outer edges of the bit biting rock, the hammer blows drive it too deep. 

Fail Falling wrote:

The comparison doesn't work in this case as we're not comparing drilling from scratch to drilling out a pilot hole. 

Going from 1/4" to 3/8" binds and going from 9.5mm to 10mm binds. What about the process of binding when drilling out a predrilled hole is exacerbated by the difference between the pilot and finishing hole diameter being closer together. 

I’ve seen this happen (on concrete, not rock, but the principle is the same).  The 1/8” difference is enough to get a decent bounce and get some of the angled head of the bit to take out the edge of the outer circumference of the existing hole, which enlarges it.  Going from 3/8 to 10 mm is so close in size that far less of the angled portion of the head can bounce and instead the sides of the tip of the bit which are parallel to the existing hole will sink in just barely and bind.  The same happens with wood when only enlarging a hole a nominal amount.  Not enough material for the cutting surface of the bit to bite into, and thus remove- if you can picture it.

Gunkiemike wrote:

Slightly enlarging a hole like that by hand drilling is really difficult and annoying, if not practically impossible. The bit just wants to bind in the hole. BTDT.

I was referring to enlarging with a power drill. In most cases you probably wouldn't need to with a hand drill for reasons already stated. Given how few bolts (relatively) I can place with a hand drill compared to a power drill, saving wear on a bit is essentially a moot point; it would equal only a few bucks per outing. In short, if the rock is very hard I'd use a 10 mm bit. If it's not, my experience (at least a thousand, if not more, placed by hand) is that a 3/8 bit will likely create an ~ 10mm size hole.

Carry on keyboard warriors...