# Newbie, router bit sizing for Dado question



## techbird (20 May 2020)

Firstly, apologies if the question has been addressed many times before.

As completely new to woodwork, I have a few DIY projects that I'd like to try and am buying some general equipment and tools (to sink or swim I guess). Having procured a palm router I am looking to buy some bits for Dado'ing and had a question about accuracy of channel width. 

My simple requirement, for example, to cut a 18mm Dado for slotting in 18mm mdf - on paper all logical. Having identified 18mm straight bit from a certain brand (rhyming with 'blend') I noticed comments such as 'Bought 9mm and 6mm straight cut bits, 9mm cuts a 10.4mm dado,6mm cuts a 7.1mm dado.' Is this normal, reads very odd?

Can you advise. Is there a brand which, for example, specs D=18mm which results in actual 18mm Dado; if not, how do people know the actual cut size if bits are incorrectly sized? Anyone explain?

Thank you.


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## marcros (20 May 2020)

The problem with 18mm is that you assume that 18mm ply or MDF is that size...

Better to use a smaller bit and make a second pass to get the exact size that you need.


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## Eric The Viking (20 May 2020)

["crossed in the post" with Marcros - quite agree with him!]


techbird":yrg0x1sf said:


> As completely new to woodwork, I have a few DIY projects that I'd like to try and am buying some general equipment and tools (to sink or swim I guess). Having procured a palm router I am looking to buy some bits for Dado'ing and had a question about accuracy of channel width.
> 
> My simple requirement, for example, to cut a 18mm Dado for slotting in 18mm mdf - on paper all logical. Having identified 18mm straight bit from a certain brand (rhyming with 'blend') I noticed comments such as 'Bought 9mm and 6mm straight cut bits, 9mm cuts a 10.4mm dado,6mm cuts a 7.1mm dado.' Is this normal, reads very odd?


you have to wonder if he's bent the shaft of his router!


> Can you advise. Is there a brand which, for example, specs D=18mm which results in actual 18mm Dado; if not, how do people know the actual cut size if bits are incorrectly sized? Anyone explain?
> 
> Thank you.



You'll get a lot of recommendations for Wealden, who are excellent. In my limited experience, Trend's professional stuff is good, but Wealden and Axminster are slightly better. There are lots of options, CMT and Titman also being excellent.

BUT... 18mm MDF is rarely exactly 18mm, so it's arguably better to get a smaller cutter and make two passes, to bring it to the exact width you need, and that will vary depending on what you are making. And 18mm might be a bit much for a palm router in one pass anyway, as they usually only go up to 8mm shank, tops and won't accept 1/2" cutters.

I'd note also in passing that many of Wealden's straight cutters have a third carbide edge, let into the end, so they plunge cut nicely, and the bottom of the slot will be a bit cleaner. I have a few, and they work jolly well. I suspect they may only be in 1/2" shank though.

Who makes the best cutters is a bit like debating sharpening techniques. Everybody has their own view. So I'm outa here and let battle commence 

Welcome to the forum, enjoy your woodworking!

E.

PS: Wealden sponsored the late Ron Fox, router expert, for many years, and have a number of his introductory videos on their site. He's not a twenty-first century presenter exactly, but the content is excellent. I also ought to mention my friend Steve Maskery, who has a range of training videos, including a few on router jigs and shop-made accessories, but I expect he'll be along himself in a minute...


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## Eric The Viking (20 May 2020)

PPS: Wealden have revamped their site a bit, so here's a direct link to Wealden's Ron Fox videos. 

Enjoy!


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## techbird (21 May 2020)

This is exceptionally helpful - thank you kindly.


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## Phil Pascoe (21 May 2020)

If people are cutting housings that inaccurately, there is something wrong with either their methods or their routers.
Don't forget also that in the real world it can be difficult to fit a dead 18mm board into a dead 18mm housing, especially if you need to fit more than one piece at a time.


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## porker (21 May 2020)

I would recommend building an exact width dado jig. 

There are a few with plans on the tube. I will see if i can dig out the plans for the one I made. The beauty of the jig is that it will match the material width perfectly because you use this to set it and you can use any bit as long as it is a bit smaller than the dado you are cutting. It's not difficult to construct.

I think the one I made is similar to The Wood Whisperer
Edit - I remember now, I didn't copy his as I went for the simplest. Mine references off the body of the router itself. Some use a template cutter and the bearing bit as the reference and some a guide bush. I wanted to use straight cutters with no bearing so used the router budy method.


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## Sgian Dubh (21 May 2020)

techbird":1s7dbq1p said:


> My simple requirement, for example, to cut a 18mm Dado for slotting in 18mm mdf - on paper all logical.


Sounds logical, but in practice, no. The board material does vary in thickness for a variety of reasons, the most common two being manufacturing discrepancies to very slight variation in thickness caused by changes in the material's moisture content which, in itself, is caused by variations in atmospheric relative humidity.

So a common solution is not attempting to get an exact match between a housing (across the grain) or groove (with the grain) with the board thickness in the first place. For example, working with board material that is nominally 18 mm thick, use a 12 mm router cutter to create the housing appropriately deep in, say a carcase side, and cut a tongue on the end of the shelf to match that housing. 

That doesn't mean making the housing/groove width to match the board thickness methodology is wrong, but it's not the only solution. Generally, I prefer the method I described in the previous paragraph ... it's stronger for a start. Slainte.


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## MikeG. (21 May 2020)

Just checking.........are you talking about housing or grooves on the face of a board, on one hand, or slots in the edge of a board on the other hand? It makes a very great deal of difference as to the type of cutter and to the set-up.


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## Phil Pascoe (21 May 2020)

As he's wanting to cut an 18mm "dado" in 18mm mdf, probably not the edge.


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## MikeG. (21 May 2020)

Ah.........ermmm...........yes, well.........oh, look over there, do you see that? Wow.......well, goodness, is that the time? I must be going....... :lol:


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