# Drilling a round bottom hole



## Danny Ellis (May 18, 2010)

Hi, First post here.

I am trying to drill holes with a round bottom, picture a half spherical hole in a block. My first attempt was 1-3/8"w x 11/16"d

I don't have access to a lathe so I tried taking a spade bit and filing the point and corners off and rounding it up using a template and then sharpening the leading edges. I tested using a piece of Cherry and a block of graphite. I tried this in both a drill press and a hand drill, with each there was a lot of chatter and on the graphite it traveled off to the side. I also tried drilling a small pilot hole to guide it since there isn't a leading point and I still got a lot of chatter and the cut surface was very coarse and torn up. The hand held drill made a somewhat smoother surface, I'm guessing because it runs faster the my drill press can however it wandered off to the side while drilling and I do need reasonably smooth, even surface with-in the hole.

Can anyone here offer any suggestions on how to make it work?
Thanks
Danny


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## dodgeboy77 (Mar 18, 2009)

Welcome to the forum!

I like your idea of reworking a spade bit. One of the problems might be that the long shaft makes the business end unstable, then you get the chattering. Maybe cut the shank length down?

Or maybe a round nose router bit like one of these: http://www.mlcswoodworking.com/shopsite_sc/store/html/smarthtml/pages/bt_core.html Of course, I don;t see 1 3/8" listed, but maybe someone else makes one.

I wondered if this would work so I thought I'd try it myself. Unfortunately the biggest round nose I have is a 1/2" but I tried it on a piece of red oak with my drill press. It made a nice, smooth 1/2" dimple. Now whether it would work for you in a much bigger size, I don't know.

Bill


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## ACP (Jan 24, 2009)

I am glad you posted this Bill, because my immediate thought was a router bit in the DP too. I know that you aren't supposed to put drill bits in routers (obviously) and until this post never considered the router bit in the DP. Obviously Bill was successful in his attempt. Can anyone tell me why this would be unsafe if it is? DP speed is much slower than bit recommended speeds on smaller bits, but with the larger ones I can't think of an issue.


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## Danny Ellis (May 18, 2010)

Hi, thanks for your replies.

I tried a 1" core box bit that I borrowed from someone and I think my drill press turns too slow because it tore out and also burned the wood, It also slowed the motor down unless I used very light pressure which burned even more. I don't have a router to use. I'm not a woodworker, I'm a glass artist so I'm trying not to buy stuff I can't afford when I might not ever need it again. I think the shank on the spade bit I used is tapered but I'll check it out later and maybe cut it shorted and try that.

Thanks again
Danny


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## woodnthings (Jan 24, 2009)

*You need 2 bits*



Danny Ellis said:


> Hi, First post here.
> 
> I am trying to drill holes with a round bottom, picture a half spherical hole in a block. My first attempt was 1-3/8"w x 11/16"d
> 
> ...


Use a standard bit (unmodified) to start the hole to the shoulder depth you need, then switch to your rounded off bit to round the bottom. The rounded bit will wander if you try to use it just by itself as you found out. Of course this is only an unfounded theory, but worth a try. I don't know if your math on the depth will work out using 2 bits, but you could always drill a deeper hole than required then saw off the the work at the required depth....I donno? :thumbsup: bill

Grizzly has a 1 1/2" router bit: http://www.grizzly.com/products/C1065


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## GeorgeC (Jul 30, 2008)

"I am trying to drill holes with a round bottom, picture a half spherical hole in a block. My first attempt was 1-3/8"w x 11/16"d"

You say that this is what your first attempt turned out. Just what final size do you really want?

I have seen, and have some small ones, rasp bits for the drill that are essentially a ball. Depending upon how big you need and how big they make them, that would be a good solution.

G


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## Danny Ellis (May 18, 2010)

woodnthings said:


> Use a standard bit (unmodified) to start the hole to the shoulder depth you need, then switch to your rounded off bit to round the bottom.


The Cherry wood and graphite blocks I have are 1" to 1-1/2" thick so I can't do that but I can try to cut a hole in a second piece, clamp them together and use it as a guide block. That seems to me to be the almost the same thing.

-George C. The spade bit was an 1-3/8" bit. That is the size I'm looking for right now but if I can figure out how to make this work I'll try some other sizes. I realize that won't end up being an exact size but it doesn't need to be. I'm trying to make marble molds so the surface of the wood will get burned out and get bigger with use. When I tried the ones I've already made they were too rough.

I have a couple of smaller (5/8") rotary files. If I had bigger ones they may work. Another thing I tried was to take an eye bolt and cut the threaded end off then chuck that up, I then made a slurry of aluminum oxide powder and water and tried to grind my way in. Even at a slow speed, you can imagine the mess that made. Picture a blender turned on with the lid off.


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## Streamwinner (Nov 25, 2008)

How about a modified drum sander? I made a Welsh love spoon for my wife a few weeks ago and that's how I hollowed out the bowl of the spoon.


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## dodgeboy77 (Mar 18, 2009)

Danny,

I remembered I had a bigger round nose/core box bit in the 1/4" shank variety. It's old but a 1 1/4" diameter so I tried it on a piece of scrap cherry. It worked pretty well but started chattering as the hole got bigger. I think this may have been because I only had the stock clamped on one end. Anyway, it did a pretty nice job. I bored the dimple to a bit over 1" diameter. The surface was pretty smooth and didn't burn the cherry at all.

The pic is just from my cell phone so it's not high quality. Is this what you were looking to do?

Bill


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## woodnthings (Jan 24, 2009)

*Man, that's "cherry"!*

Nice work there Bill, :thumbsup: bill


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## Danny Ellis (May 18, 2010)

Hey Bill, That's pretty much what I'm trying to do but it sure didn't work out so nice when I tried it, maybe my wimpy table top press is just too slow and not enough HP to do the job.

I didn't make it to my studio today to try again, maybe tomorrow.

Thanks
Danny


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## dodgeboy77 (Mar 18, 2009)

Danny,

Make sure your bit is sharp enough. That one in the picture is older but still has a good edge on it. I wasn't spinning it very fast but I was using a full size drill press and it had no trouble from a power standpoint.


Bill


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## JBS (Dec 11, 2011)

I know I am late to the chat....but any idea about drilling larger diameter round bottom holes? Like 3" across?


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## BigJim (Sep 2, 2008)

JBS said:


> I know I am late to the chat....but any idea about drilling larger diameter round bottom holes? Like 3" across?


JBS, that thread is an old one but with a dimple that wide maybe you could chuck the wood up in a lathe and cut it like they make bowls.


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## woodnthings (Jan 24, 2009)

*not exactly "round"*

But not flat either. This one may work?
http://www.toolstoday.com/p-6033-3-piece-cove-raised-panel-door-making-set.aspx


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## jredburn (Feb 20, 2011)

Danny
What you want is piece of cake for a CNC machine. There have to be a bunch of shops in the Chicago area that will cut the holes for you.
I would be glad to do it for you but I am in SW Florida.
Regards
Joe


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## cabinetman (Jul 5, 2007)

JBS said:


> I know I am late to the chat....but any idea about drilling larger diameter round bottom holes? Like 3" across?


Mounting this cove cutter in the TS, and placing the stock over the cutter, and slowly raising the cutter, while rotating the stock will machine a wide round bottom hole.
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## cabinetman (Jul 5, 2007)

woodnthings said:


> But not flat either. This one may work?
> http://www.toolstoday.com/p-6033-3-piece-cove-raised-panel-door-making-set.aspx


I don't think it would work very well with a bearing on the bottom.










 







.


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## cabinetman (Jul 5, 2007)

cabinetman said:


> Mounting this cove cutter in the TS, and placing the stock over the cutter, and slowly raising the cutter, while rotating the stock will machine a wide round bottom hole.
> .



I was thinking about it and it could be done just as easily with a 7.25" carbide tipped sawblade on the TS. You would still have to configure a fixed point for rotating the stock around the blade.












 







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## jschaben (Apr 1, 2010)

Danny Ellis said:


> Hi, First post here.
> 
> I am trying to drill holes with a round bottom, picture a half spherical hole in a block. My first attempt was 1-3/8"w x 11/16"d
> 
> ...


You said "first attempt". Is that what you want to end up with?
Here is one that will give you 1 7/8" at a depth of 11/16" or 1 3/8" at a depth of 1/4".
http://www.ebay.com/itm/1pc-1-2-SH-...641601736?pt=Routers_Bits&hash=item20bee488c8

This one will give you 1 3/8 @ 7/16" depth.
http://www.ebay.com/itm/1pc-1-2-SH-...601594550?pt=Routers_Bits&hash=item1e68762eb6

You really need a bit with an 11/16 radius but I haven't been able to locate one. I haven't really looked at any of the hi $ sites though.:smile:


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## Midlandbob (Sep 5, 2011)

I have had good results with the regrinding of spade bits. Chattering --It is critical to get the two sides "identical" in contour, and to get the centre on centre and to sharpen with the slight negative grind opposite on both sides. Setting the grinder steady rest about 5-10 degrees negative and scratching the small arcs with a coin or washer at near correct profile befor starting to grind. I've used this to make tapered holes also.
Good luck.


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