How can a standard sized drill hole be accurately enlarges?

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Forum topic by stefang posted 11-19-2015 08:43 PM 2041 views 0 times favorited 32 replies Add to Favorites Watch
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17040 posts in 4544 days

11-19-2015 08:43 PM

I have a hole that has to be 10.3mm, but that size bit is not available, at least to me. I have actually drilled the hole already at 10mm and I tried to enlarge it to 10.3 with a rattail file, but I think it is a bit to large now. Is there some way that a hole can be enlarged accurately? I will probably have to plug the hole and re-drill it for a perfect fit. The hole is in the center of a wooden gear which will be running on a 10mm axel and it has to be very accurate to run smoothly.

-- Mike, an American living in Norway.

32 replies so far

View Tom Howbridge's profile

Tom Howbridge

60 posts in 2592 days

#1 posted 11-19-2015 08:50 PM

View Kazooman's profile


1540 posts in 3162 days

#2 posted 11-19-2015 08:51 PM

Do you own or have access to any English measurement drill bits? It happens that 10.3 mm is just about exactly 13/32”.

View Dutchy's profile


4172 posts in 3378 days

#3 posted 11-19-2015 09:04 PM


That isn’t that easy.

Some options;
Do it with your router and a 6 or 8 mm router bit in combination with a template guide

Buy a HSS metal drill 10,5 or 11 mm in a local shop, put it in your hand drill machine and turn the drill around to the wheel of your grinder till the drill is 10,3. Probably you don,t have to drill a deap hole

Hope this helps.


View HokieKen's profile


19055 posts in 2348 days

#4 posted 11-19-2015 09:53 PM

Kazooman’s got the best answer. 13/32 is a dead ringer if you can get one.

The next best option is an adjustable reamer. You can drill the 10mm hole and then sneak up on the size by opening the reamer up a little at a time.

If you don’t want to wait for a reamer, try this. Drill the 10 mm hole. Now chuck a 7/16 or 1/2” wire brush in your drill press. Run it down through the hole and back out QUICKLY. Check the hole size. Rinse and repeat. The brush will take a little off each time but you should be able to sneak up on it well enough to hit your target.

Good Luck!

-- I collect hobbies. There is no sense in limiting yourself (Don W) - - - - - - - - Kenny in SW VA

View Dale J Struhar Sr's profile

Dale J Struhar Sr

522 posts in 4340 days

#5 posted 11-19-2015 09:55 PM

13/32 drill size US = 10.3 mm Is your axel wood? why can’t you make the axel fit the hole you have?

-- Dale, Ohio

View stefang's profile


17040 posts in 4544 days

#6 posted 11-19-2015 11:34 PM

Thanks to all who have commented here.

Tom Perfect but I can’t wait that long for delivery.

Kazooman Same delivery time problem.

Jan Both suggestions are good and I can do that right away. Double thanks.

Dale The gear runs on a brass 10mm axel and the extra 0.3mm is so it can spin freely on the axel without any significant wobble.

-- Mike, an American living in Norway.

View Jim Jakosh's profile

Jim Jakosh

26629 posts in 4315 days

#7 posted 11-20-2015 02:14 AM

Hi Mike, You could take a 11mm drill and spin it backwards over a belt sander until it get to .406” (11.3mm).

13/32 is just the right drill for that hole so you can make you one from a metric.

But it sounds like you are drilling wood so the best way to do that uniformly is to drill the 10mm hole. Then take a small wood dowel , saw a slit in the end about 1/2” long and then cut strip of about 100 grit sand paper to wrap around the dowel and trim the length until it is snug in the 10 mm hole and lap it out to 10.3mm. It won’t take much to remove .012”. (By the way, that seems like a lot of clearance for an 10mm shaft)
We used to do that in the tool room to lap out hardened steel blocks for the proper dowel fit when we did not have a regular lap with compound on it. I have used this method many times for a proper fit in wood, too.

cheers, Jim

-- Jim Jakosh.....Practical Wood Products...........Learn something new every day!! Variety is the Spice of Life!!

View hunter71's profile


3558 posts in 4396 days

#8 posted 11-20-2015 02:36 AM

I lived almost all my life in the US and have always hoped they would go METRIC. It’s so much easier. I keep both in my shop for those times “just a bit” more are needed.

-- A childs smile is payment enough.

View stefang's profile


17040 posts in 4544 days

#9 posted 11-20-2015 11:02 AM

Thanks much Jim, the dowel idea seems even simpler for my purposes and sounds like a very handy trick. I do agree that the 0.3mm seems like a lot, but that is what is called for in the plan. It might not be so dumb though as it might compensate for any slight closure of the hole due to humidity, which we have a lot of here in the summer. Just a guess.

Doug I agree that the metric system is much easier to use, but I can also see the enormous difficulty of shifting from the imperial system in the US. The metric system has helped me to avoid a lot mistakes, though I do manage to make quite a few anyway.

-- Mike, an American living in Norway.

View kiefer's profile


5852 posts in 3877 days

#10 posted 11-20-2015 04:01 PM

View MrRon's profile


6130 posts in 4453 days

#11 posted 11-20-2015 05:17 PM

Drills always tend to drill oversize. A 10 mm bit should work OK, but the 13/32 bit is the way to go. The sandpaper on the dowel is another good way. If you have to plug the hole, mix up some JB Weld or other epoxy and coat the inside of the hole. After it hardens, run the 10 MM bit through again. The problem here is getting the gear centered. If you have a lathe, you can mount it and trim the hole to size with small turning tools.

View stefang's profile


17040 posts in 4544 days

#12 posted 11-20-2015 10:29 PM

Klaus No, I don’t have a step drill and I don’t have time to order one.

Ron Thanks for the tip, and Yes, you are right, drill bits do make a slightly oversize hole and you are also right about the difficulty of re-centering, but I have a plan for that. I draw a circle the same circumference as the gear with a compass, then I drive small brad in the mark made by the compass leaving it just proud of the surface. I line up the gear with the circle and give it a little tap onto the brad to mark my center. I do have a lathe and that’s a good idea about turning the hole to size. maybe the best idea yet.

-- Mike, an American living in Norway.

View kaerlighedsbamsen's profile


1373 posts in 2923 days

#13 posted 11-20-2015 10:49 PM

Jim Jakosh´s method works but tends to make holes with that are larger at the surface.

A trick from my days as a tool maker – make your own reamer:
Take a M10 bolt (preferably a allen type as they tend to be extra hard), cut off the head and the threads. Hammer ont wo sides of the bolt untill it measures 10,3 mm. Take a hacksaw and cut longitudal slits in both sides of the thickest parts of the bolt making sure it stays perfectly straight. Clean off burrs but do not round over.
Use like this: Drill 10mm hole in drillpress with gear clamped to the table. Change to reamer and ream the hole making sure nothing moves. Should produce a clean, burnished hole at the exact diameter and with exact level sides.

Let us know how it ends!

-- "Do or Do not. There is no try." - Yoda

View stefang's profile


17040 posts in 4544 days

#14 posted 11-21-2015 12:21 AM

KB Yet another great idea. Thank you very much. I have a lot of very good options now. I will let you know what I did and how it came out after it’s finished.

-- Mike, an American living in Norway.

View helluvawreck's profile


32122 posts in 4076 days

#15 posted 11-21-2015 12:49 AM

Mike, you should be able to get a 10.3 mm reamer. Here’s an expandable reamer on Ebay that you can adjust.

You should be able to find a 10.3mm on Ebay for pretty cheap. When I had my machine shop I used to buy stuff for my shop all the time on Ebay.

helluvawreck aka Charles

-- helluvawreck aka Charles,

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