# Router Bit diameter vs material thickness



## John Charnock (May 2, 2017)

Hi 

What is the rule of thumb for selecting router cutting diameter vs material thickness? 

To put this into context, assume this question relates to a router table set up and I am trimming my job against a template and my stock material is 18mm plywood. Would I use a 12.5mm Dia or a larger Dia bit? 

Regards

John


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## kp91 (Sep 10, 2004)

John,

I don't know if I am 100% sure of the setup you are describing, but I can answer a couple of different ways. If you are using a template guide bushing (not a bearing) to follow your pattern, then you can use any size bit, just make passes that are a little less than the diameter of the bit deep.

If you are using a bearing guided bit, (either a flush trim bit or a pattern bit) use the biggest bit that your pattern will allow. A 1/2 Inch diameter bit is always going to be more rigid than a 1/4 inch bit, but if the details in your pattern are too fine for that size bit, then you will need to use a smaller bit. I have a 1" pattern bit, but it doesn't see much use. 

You can make life easier on the router bit by removing as much of the material either with a bandsaw, larger diameter router bit, etc. so that the smaller bit doesn't have that big of a load on it when cutting thicker stock.

Hope this explanation isn't too confusing!


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## Stick486 (Jan 4, 2013)

what Doug said...


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## Cherryville Chuck (Sep 28, 2010)

I'll 3rd that. Routers are great for finish trimming but poor at removing waste. Your router and your bits will last longer if you drill, jigsaw, or band saw as much waste as is practical out or away before you use the router.


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## John Charnock (May 2, 2017)

kp91 said:


> John,
> 
> I don't know if I am 100% sure of the setup you are describing, but I can answer a couple of different ways. If you are using a template guide bushing (not a bearing) to follow your pattern, then you can use any size bit, just make passes that are a little less than the diameter of the bit deep.
> 
> ...



This is where I struggle and one of the reasons I came to the forum to seek the guidance of the "experts 

Unfortunately, I don't know the difference between flush & pattern. Yes, I am using a bearing guided bit. 

With the job that I have in mind, I don't have any constraints on the pattern, it has no sharp corners or curves. 

Assume my pattern is 6mm plywood and my stock is 2 * 18mm MDF 36mm in total and I plan to use a bottom bearing bit. 

I would obviously need a bit length of more than 36mm. So any diameter 12.50 - 20mm would be OK? 

Would that be spiral or straight?

Thanks John


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## kp91 (Sep 10, 2004)

Flush trim bit = bearing on the bottom

Pattern bit = bearing on the shank

MDF has no grain pattern, so a spiral carbide or a straight flush trim bit will cut OK you are going to end up with "fuzzing" one one surface, but it should sand to a nice, sharp edge.

I would personally make my template a little thicker, I use 3/8 material whenever possible to give the bearing as big a surface to ride on.


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## John Charnock (May 2, 2017)

kp91 said:


> Flush trim bit = bearing on the bottom
> 
> Pattern bit = bearing on the shank
> 
> ...


Thanks that makes perfect sense


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