How to perfectly size a hole (by tiny fractions of a mm)

[Luke Dupont]

So, in building my frame saws, I'm using bolts for the mounting pins and I need them to fit very snugly so that the blade doesn't turn in use.

I bought 8mm bolts to use for this. And they're too loose in holes drilled with an 8mm bit. I found a 7.5mm bit, but they're too tight (ie, don't go in at all / might split the wood if I really drove them in)

Now, I can order a 7.8mm bit or 7.9mm bit, and I'll try those out for convenience. But I feel that is the lazy way of solving the problem, and not a solution that will work for everyone if I were to give anyone advice on how to build one themselves. I prefer not to rely on buying super specialized tools all the time.

So, let's say that you have drill bits in the following sizes: 6mm (1/4), 8mm (5/16), etc, and you want to get a very snug fit around an 8mm (5/16) bolt that that is too loose for your drill bit. How would you increase the size of that 6mm hole by 1.8mm or so? A rattail file is going to take a while to do the job, I think, and is prone to introducing error and inaccuracies. It could work, but I was wondering if there are better solutions.

[Stu Gillard]

If your bolts are (actually) 8mm you could use a 5/16" bit (7.93mm).
This should be a snug fit.
Easy to test without buying extra bits.

[Alan Schwabacher]

Tapered pins work very well for this purpose. All you need is a very slight taper made with a file while the rod is chucked into a drillpress. Ream the hole with a similar rod with a lengthwise groove to cut the wood. If the taper is sufficiently slight, you may not need to ream.

[John K Jordan]

I don’t understand your design and if you’re working in wood, steel, or both, but when I want to size holes precisely I sometimes reach for a set of over/under reamers - I keep both metric and imperial sizes. I also have some adjustable reamers similar to these:
https://www.littlemachineshop.com/products/product_view.php?ProductID=5940&category=

If the design permits, for very snug bolts in either metal or wood that don’t need to slide in and out of the hole I drill and tap the hole. Again, I know nothing about the design and materials of your frame saws (or any frame saw), the type of bolts, or the function of a mounting pin so I’m just guessing.

[Jim Koepke]

If a hole is too big, you might be able to wrap a shaving around the bolt to make it tighter.

jtk

[Rafael Herrera]

You could try running the drill bit back and forth to enlarge the hole. However, if you're relying on a snug fit to prevent the bolts from moving, that may only be temporary. Try some other method to lock the pins.

[michael langman]

Luke, Can you buy roll pins where you are. They are usually oversize, andare spli to act like a spring to give a tight fit.

[Tom Bender]

A couple of approaches that might work depending on what you are going for.

1. saw a slot into the hole along the grain, fill the slot with epoxy and squeeze to the needed fit.

2. wax the bolt, wrap it in package tape to protect the wax then epoxy it into the hole. With luck you will be able to remove it. Practice on scrap.

3. if the bolt does not need to move, epoxy it in place

4. if the bolt must not slip, knurl it and epoxy in place

[Mark Hennebury]

Put the drill in a drill press and turn it on, hold a sharpening stone to flat to the side of the drill, move the stone up and down a few times and check. any kind of stone will work, water stone, diamond etc.

[Andrew Seemann]

Letter and number bits are also made, to add to your options:)

They had something on this in my 1950s copy of Machinery's Handbook. If you want a drill bit to make a slightly larger hole, grind the point a shade off center. This will cause one side dig in a little more and make a larger hole. You don't want to go too far though, or you risk breaking the bit.

[Jeff Wittrock]

I keep some old spade bits around for when I need to drill an odd size.
I just find one that is close and grid the width down to achieve the diameter needed. A reamer would be better but I never have the size I need on hand.

[Jim Koepke]

Part of the problem here is many of us are in the world of inches while Luke is in the world of metric.

In one of my drill indexes the bits are labeled in inches and are graduated by 1/64" in size. Imperial bits may not be available in Japan.

A tight fit can often be achieved with these or other bits on hand. It often requires the use of a dial caliper or micrometer to first check the bolt bolt or dowel for size. Often sized rod is used in preference to bolts. If threads are required, that is the reason for having a tap & die set.

jtk

[Alan Rutherford]

Drill the hole as close as you can get and still be undersized. Then turn a bolt into a drill bit by filing some teeth into it. You can do this fairly quickly with a triangular or flat file. Make 3 or 4 vertical grooves in the threads, making sure the appropriate side of the grooves is vertical or undercut if possible. Cut the head off and chuck it up, or just drive it with a socket wrench

[John C Cox]

If I needed a really tight fit and only had one hole to do, I would drill a hole slightly undersized with whatever drill bit I already had. Next, wrap sandpaper on a dowel and work the hole till the bolt just fits.

if I needed a bunch of holes, I'd use my calipers and measure the bolt, then buy the right drill bit

[Luke Dupont]

Thanks guys. Lots of great solutions. I did buy a 7.8mm bit (and a 7.9 which was okay but still not as snug as I wanted, though I think it's different with hard and soft wood. Haven't tried in hardwood just yet).
So between those two I have the perfect size now. But, I just wanted some solutions to offer anyone who might want to build one themselves, or in case I run across this problem again and don't want to buy yet another bit.


I keep some old spade bits around for when I need to drill an odd size.
I just find one that is close and grid the width down to achieve the diameter needed. A reamer would be better but I never have the size I need on hand.

Now that's a great idea! I didn't think about spade bits. They'd be a lot easier to grind to the correct diameter than any other kind of bit.

[Michael Schuch]

I picked up a Harbor Freight 115 piece fractional, letter and number drill bit set 25+ years ago. When ever I break a bit I replace it with a quality bit and always keep 10 packs of common sized small bits on hand. When I need an off size I have it. It looks like the Titanium Nitride coated set still goes for only $50. They aren't the highest end premium bits but thet get the job done and as I have replaced the broken ones over the years I have high quality bits for the sizes I use frequently.

[James Pallas]

It is worth it to buy a full set of bits, meaning letter and number sizes. These days you can by less expensive sets and you will have nearly all you need for a lifetime. Whether you work in wood or steel most inexpensive bits will cut a few holes. The ones you use a lot you can replace with higher quality when needed. The advantage is you will usually have what you need. The sets that go to 1/2” are enough. Woodworkers generally have spade bits, augers and such for bigger things.
Jim

[Jim Koepke]

Don't forget folks, Luke lives in Japan. They do not use SAE bits, number, fractional or letter bits. There might be some in Japan at higher prices. Metric bits are based on millimeters and tenths of a millimeter.

0.1mm is ~0.0039" or about the same jump between many of the SAE number and letter drill bits.

jtk

[Luke Dupont]

Don't forget folks, Luke lives in Japan. They do not use SAE bits, number, fractional or letter bits. There might be some in Japan at higher prices. Metric bits are based on millimeters and tenths of a millimeter.

0.1mm is ~0.0039" or about the same jump between many of the SAE number and letter drill bits.

jtk


Yeah, it seems that most fractional sizes stop at 6mm (about 1/4") here, at least in the hardware store. Bigger than that and you have 1mm or .5mm increments. I'll see what I can find online though!

[steven c newman]

One question....sometimes, a bolt's threads actually stand just a hair proud of the shank....to the point that IF the threads pass through, the shank will be a bit loose...

[Richard Coers]

A common bolt is not a precision fastener. The diameter over the threads is not the same as over the body. Use a shoulder bolt and you have a precision ground body that is larger than the threads. As mentioned, a drill bit is also not precision tooling.

[Tom M King]

AN bolt and bushing if fit is critical, Another good thing about AN bolts is you can get the length of unthreaded shaft you want, with short threads on the end.

[Jim Koepke]

As mentioned, a drill bit is also not precision tooling.

That depends on how much you are willing to pay for quality.

Precision holes are not usually drilled with hand drills, eggbeaters or a drill press with measurable runout.

Or maybe luck has been with me over the years when drilling in metal for an interference fit of various items.

jtk

[Richard Hutchings]

I think a tight fit that will last, must be tapered like the endpin on an acoustic guitar. A tapered reamer doesn't cost that much, even though it's kind of a specialty tool.

[steven c newman]

Would this bolt be the ones that hold the blade in a Bow/Frame saw? One end into a wooden handle ( bow saw/turning saw) or, have the head removed, slotted for the blade, and drilled for a pin...with the threaded end through the handle of the frame..and a wing nut/washer installed to tension the blade? To "turn that, you loosen the wing nuts, and then turn the blade the way you need it to go. A bolt too loose will cause the hole (wooden) to waller out, causing too much wear in the hole through the wood handle part of the frame.
474518
Frame saw.

[Richard Coers]

That depends on how much you are willing to pay for quality.

Precision holes are not usually drilled with hand drills, eggbeaters or a drill press with measurable runout.

Or maybe luck has been with me over the years when drilling in metal for an interference fit of various items.

jtk
I thought we were talking about drilling wood? Wood grain will quite often effect the direction of the drill.

[Jim Koepke]

I thought we were talking about drilling wood? Wood grain will quite often effect the direction of the drill.

Yes, we are. Brad point bits and forstner bits are less prone to wandering. A sharp "standard" drill bit can also be used without wandering in a drill press or lathe with a slow feed.

These were actually done on a lathe:

474540

These are a scratch awl & burnisher made for my grandson.

Bits used were ~0.003" smaller than the HSS rods inserted. That made for a very tight fit.

There are special auger bits made for dowel work to provide a tight fit. They are not common, but can occasionally be found.

There are also ways to reduce wandering by hand drills if that is all one has to use.

jtk