# cutting small circle with circle centre outside



## toadhall (Dec 4, 2009)

Hi
i need to cut a small semi circle (radius 4") into a straight edge of a sheet of MDF so the cut appears as a large circular divet along the edge. The centre of the circle will actually just sit outside of the sheet, hovering in thin air (so it is actually less than a semicircle, more of third of a cricle).

I have no idea how to use my router to do this if indeed it is possible. 

I have also considered using a cookycutter drill bit attached to my drill. But i dont think this would work as I am cutting less than half a circle and so it would not hold steady.

Any solutions?
thanks
t


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## harrysin (Jan 15, 2007)

Why not rout, with a circle jig, an eight inch hole then saw the MDF leaving exactly the wanted cut-out.


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## gregW (Mar 14, 2007)

toadhall said:


> Hi
> i need to cut a small semi circle (radius 4") into a straight edge of a sheet of MDF so the cut appears as a large circular divet along the edge. The centre of the circle will actually just sit outside of the sheet, hovering in thin air (so it is actually less than a semicircle, more of third of a cricle).
> 
> I have no idea how to use my router to do this if indeed it is possible.
> ...



If I understand what you are trying to do correctly I would make a circle template and attach a fence to register against the edge of the sheet.

and then use a guide bushing in my router to follow the template and cut the recess.


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## bobj3 (Jan 17, 2006)

I think I would go with Harry's and Greg's post,,then you can make two parts for your project but use the jig that Greg posted . to make two of them at one time,just clamp/stick the template pattern in place then rip in on the table saw to size..


Use the cir.tool to make the template ( most routers come with a edge guide ,just flip it over and use it for the cir.jig.) and then use it for the cir.hole pocket hole.
Use your brass guides to clean out the hole or a dado router bit.

bit below
http://www.mlcswoodworking.com/shopsite_sc/store/html/smarthtml/pages/bt_dado.html
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## harrysin (Jan 15, 2007)

I really don't see why a template is required, drill a hole to suit the circle cutting jig, in my case 1/4", rout the circle then rip on the table saw after which re-saw the "waste" one to make it identical with the first one. It wouldn't take much longer than it takes to type this post..


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## gregW (Mar 14, 2007)

harrysin said:


> I really don't see why a template is required, drill a hole to suit the circle cutting jig, in my case 1/4", rout the circle then rip on the table saw after which re-saw the "waste" one to make it identical with the first one. It wouldn't take much longer than it takes to type this post..



I guess from the original post you need to make some assumptions 

He did write “i need to cut *a* small semi circle (radius 4")” which led me to assume only one cutout was required? Even if he does need to make two symmetrical cutouts he said that they only needed to be about a third of circle so I think that he would be required to make two rip cuts and there would be wasted material using the method you described – at least a 2-2/3” wide by (?) length strip.

The jig would be simple to construct and easy to handle, and it would work on a full sized sheet or a thin strip cut from the sheet. 

I usually make templates out of ¼” thick mdf, which is very cheap and easy to handle. I could make a 8” diameter circle in a template sized piece using either my drill press or a router and circle jig on my workbench very quickly and then you only need to attach a piece of scrap for a fence and you’re set to go.

Sometimes people seem to get hung up on having to make a jig when in reality a couple of dollars in material and just a few minutes of time could end up saving them a considerable amount of time in the long run and give them the professional looking results they’re after…even if you throw it out afterward it served its purpose of getting the job done quickly and efficiently


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## jlord (Nov 16, 2009)

Use a 4" holesaw. Use a piece of scrap same thickness for support on waste side.


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## Dr.Zook (Sep 10, 2004)

Hello toadhall, and welcome to the RouterForums. Glad to have you join us.


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## bobj3 (Jan 17, 2006)

Hi James

I'm with you,,, my hole saw set goes to 6 1/2" but I said to use the router to make Harry happy  LOL, he loves to cut holes out with his shop made cir.jig.. the hole saw makes quick work of it..1,2,3 and it's done..

===



jlord said:


> Use a 4" holesaw. Use a piece of scrap same thickness for support on waste side.


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## gregW (Mar 14, 2007)

bobj3 said:


> Hi James
> 
> I'm with you,,, my hole saw set goes to 6 1/2" but I said to use the router to make Harry happy  LOL, he loves to cut holes out with his shop made cir.jig.. the hole saw makes quick work of it..1,2,3 and it's done..
> 
> ===


a 8" diameter hole saw would be required to do the job if he needs the cutout to have a 4" radius ...


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## Lemuzz (Jul 25, 2008)

gregW said:


> I guess from the original post you need to make some assumptions
> 
> He did write “i need to cut *a* small semi circle (radius 4")” which led me to assume only one cutout was required? Even if he does need to make two symmetrical cutouts he said that they only needed to be about a third of circle so I think that he would be required to make two rip cuts and there would be wasted material using the method you described – at least a 2-2/3” wide by (?) length strip.
> 
> ...


I couldn't agree more. Use the KISS principal:agree:


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## jlord (Nov 16, 2009)

gregW said:


> a 8" diameter hole saw would be required to do the job if he needs the cutout to have a 4" radius ...


Your right, forgot about that. Then I would just use a trammel set at 4" from the pivot point to the far side of the bit.


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## harrysin (Jan 15, 2007)

The reason why I chose routing was because "toadhall" joined a ROUTING forum so I feel justified in making that assumption!as well as the fact that he said this: "I have no idea how to use my router to do this if indeed it is possible". If however "cut" is the operative word, then what could be simpler than a jig saw!


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