[John Charnock]

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

 

[John Charnock]

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!

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

 

[John Charnock]

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.

Thanks that makes perfect sense