# cutting 3/4" plywood



## towtalsa (Sep 12, 2012)

Hi. Im having a problem with choosing proper bit. Im cutting curved gables/forms in 3/4" plywood. Ive been using 1/4" upcutting spiral carbide bit with a 2 hp hand held router, brass guide kit and plywood jig. It's been working very well, fast clean cutting however I have now broken my 4th bit. At $28 a piece Im not happy. They seem very brittle and unforgiving.


I have a lot of cuts to make so I don't have the time to pre cut with jig saw and trim after. Also my edges can be rough for they will be covered. 

Should I switch to a straight bit or go up to a 1/2" spiral($90) ? 

Thanks. Great forum


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## TRBaker (Jul 5, 2012)

Smit, welcome to the forum. If you are making this cut in one pass, then you definitely need to step up to 1/2" bits. I don't see the need for the spiral bit if that is the case and the straight bits are less expensive. Be aware that it will be hard on your router too, so take it easy. Good luck.


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## WayneMahler (May 17, 2012)

Smit
Welcome aboard. I use down cut spirals for plywood. And do the cuts in steps. If you wish to use a straight cutter, first start with a down cut, it shears the top layer of ply and leaves very little if any chip out. You can follow up with a straight bit if you wish. I just do my cuts in steps, haven't broke a bit yet. Hope this helps.


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## bobj3 (Jan 17, 2006)

hi

I use the 1/2" bit below it will cut 3/4" plywood like cutting butter in one pass..great for plastic too..

$14.50 the normal price..carbide bit...with free shipping..

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## mgmine (Jan 16, 2012)

Smit you lost me when you said that you didn't have time to cut with a jig saw and trim after. If the edges can be rough then why would you trim afterwards? A jig saw with the proper blade will cut a curve in a half the time. You could even stack the wood and cut them with a reciprocating saw or for that matter depending on the curve a circular saw.


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## towtalsa (Sep 12, 2012)

Hi, Seems like I am close with the bit. I have tried doing multiple passes and still manages to break a bit. I will try the 1/2" spiral and even a striaght bit and compare. 

I have used jig saws and routers to shape plywood. With a proper jig, the router is much faster, easier and will create less waste for I can pin my template to a full sheet and interlock my cuts. 

Thanks for your advice guys. I will update


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## Chris Curl (Feb 13, 2012)

bob, where do you get that bit? mlcs? the bit smit is referring to is also a spiral, but his is $90, and yours is $14.50 ... ?


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## bobj3 (Jan 17, 2006)

Hi Chris

here's the spot to get them from,I don't see them listed right now but Fred is a member of the forum and maybe he will jump in and will tell you when he is going to list them again,,it's the best bit I have like that.. 10 stars out of 10 stars..

69-075 Onsrud Double Edge Solid Carbide Upcut Chipbreaker Finisher Router Bit | eBay


69-075 Onsrud Double Edge Solid Carbide Upcut Chipbreaker... (320849990137)
Member id onsrudcutter2010 | Feedback Score Of 2010 | 100.0%
Sale date: 02/18/12


$14.50 


57-281 Onsrud Solid Carbide Double Edge Downcut Spiral Wo... (320887079328)
Member id onsrudcutter2010 | Feedback Score Of 2010 | 100.0%
Quantity: 2
Sale date: 04/21/12


$14.00


69-084 Onsrud Double Edge Solid Carbide Downcut Chipbreak... (320849990257)
Member id onsrudcutter2010 | Feedback Score Of 2010 | 100.0%
Sale date: 02/18/12


$15.50


69-083 Onsrud Double Edge Solid Carbide Chipbreaker Finis... (320849990186)
Member id onsrudcutter2010 | Feedback Score Of 2010 | 100.0%
Sale date: 02/18/12


$12.50

onsrudcutter2010 | eBay



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Chris Curl said:


> bob, where do you get that bit? mlcs? the bit smit is referring to is also a spiral, but his is $90, and yours is $14.50 ... ?


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## JOAT (Apr 9, 2010)

mgmine said:


> Smit you lost me when you said that you didn't have time to cut with a jig saw and trim after. If the edges can be rough then why would you trim afterwards? A jig saw with the proper blade will cut a curve in a half the time. You could even stack the wood and cut them with a reciprocating saw or for that matter depending on the curve a circular saw.


Agree. Sounds like he might be trying to rout too much too fast too. All my routers are 1/4" shank, but the bit I normally us is 1/2" flush trim, and I don't always trim close with a sabre saw first, so have to go thru a lot of wood at times, but I just take it slow and no prob. All my routing is with flush trim bits and templates. Altho I don't use much 3/4" plywood, mostly 1/2".


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## Harrison67 (May 30, 2012)

bobj3 said:


> hi
> 
> I use the 1/2" bit below it will cut 3/4" plywood like cutting butter in one pass..great for plastic too..
> 
> ...



That was one of the greatest designs in tool geometry, when it was first introduced. I'm talking about the roughing endmill designs for metalworking.

It's amazing, that woodworking is just getting around to the metal trades cutting tool geometry; after all these years.


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## towtalsa (Sep 12, 2012)

The bits are amazing. I went with the half inch shank and it eats through plywood. The plywood I am using is good both sides and it is hard. I don't know for sure the type for I got it from a film set that used it for rolling the cameras on. Picked it up cheep.

However, I think the reason for breaking the bits might be partially caused by the brass guide that I was using. There is only 1/16 ( or less ) between the guide and the bit, so Im thinking, with the hard and inconsistent material of plywood, and imperfect steadiness in hand, that if the bit should grab a little and flex, it will jamb into the guide and bind causing break. 
I had a little bind happen with the 1/2" spiral and it chewed into the brass guide pretty good but thankfully the bit didn't snap. This is when I considered the clearances on guides. Before, I didn't really think about flex in the bit for I am new to the guide system. Mostly used the bearing system. 

Could this be possible? Thoughts?

Smit


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## bobj3 (Jan 17, 2006)

Use a 3/4" guide or bigger,then you have the room inside of the guide...

+++



towtalsa said:


> The bits are amazing. I went with the half inch shank and it eats through plywood. The plywood I am using is good both sides and it is hard. I don't know for sure the type for I got it from a film set that used it for rolling the cameras on. Picked it up cheep.
> 
> However, I think the reason for breaking the bits might be partially caused by the brass guide that I was using. There is only 1/16 ( or less ) between the guide and the bit, so Im thinking, with the hard and inconsistent material of plywood, and imperfect steadiness in hand, that if the bit should grab a little and flex, it will jamb into the guide and bind causing break.
> I had a little bind happen with the 1/2" spiral and it chewed into the brass guide pretty good but thankfully the bit didn't snap. This is when I considered the clearances on guides. Before, I didn't really think about flex in the bit for I am new to the guide system. Mostly used the bearing system.
> ...


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