Introduction: CNC Dovetail Joints
Here is a project I developed with Þórarinn B.B. Gunnarsson at Ísafjörður FabLab, Iceland.
After watching CNCWoodworker's video on YouTube we decided to try and make our own dovetail joints.
I have made several tests on the FabLab's ShopBot CNC using a dovetail bit.
I tested different dimensions, different wood orientations to find the best way of making these dovetail joints.
They are traditionally used on drawers, to assemble the front and the sides parts. They are very strong and quite long to make by hand, but it took me about 10 minutes to make them on the CNC.
In this Instructable you will find different types of dovetails, including half-blind dovetails and through dovetails.
Step 1: Drawing the Dovetail Joint
I started by drawing a wood joint with five dovetails on VCarve Pro (ShopBot's software). This file is made for pieces of wood which are 20mm thick, 110mm long and 67mm wide.
Here is my drawing, which includes two toolpaths.
The first toolpath is made for taking off material with a 1/4 bit (I chose to use a down-cut bit for a cleaner result).
Step Down: 4mm per pass (3 passes)
Feed Rate: 45mm/sec
Spindle Speed: 14000 r.p.m
The second toolpath is designed for a 73° dovetail router bit. The one I used is about 14mm long and 14mm wide. The tool goes straight into the material 12mm deep, so that's why a first roughing with a straight bit is needed.
Step Down: 12mm (1pass)
Feed Rate: 30mm/sec
Spindle Speed: 12000 r.p.m
I didn't use any offset to make the joints fit, but they fit really well. Some are even a bit too lose, that depends of the type of wood you are using. I only used fir wood, which is quite soft. The joint will be tighter if you use beech wood, for example.
Step 2: Making the Half-blind Dovetails
First you need to fix the pieces of wood at the same height, one horizontal, one vertical. Shift the vertical piece laterally so the offset should be half a dovetail: here, the gap between two lines in the drawing is 22mm, so the offset should be 11mm (see picture).
Then upload the G-Code into the CNC and use a 1/4 down-cut bit to get a clean result. Remember to zero Z axis on top of the pieces of wood. In the file I uploaded, Y origin is 20mm away from the edge and X origin is on the bottom left corner.
Just like in the file I uploaded, remember to make two separate toolpaths, so you will be able to change the straight bit to a dovetail bit between the two millings.
Step 3: Making the Through Dovetails
A "through" dovetail means that the male part goes all the way through the female part, making it visible from the outside.
The process is the same as for the half-blind dovetail but the tool has to go all the way through the horizontal piece of wood.
Unlike the half-blind dovetails, the wood veins sould be vertically oriented on the male piece.
Be careful not to go too far into the horizontal piece of wood (on the pictures you can see some failures where the milling bit got too far). A female milling should measure half the thickness of a male milling.
Be also careful not to reduce the thickness of the male part. The tails must keep the same thickness so they wont go too far into the female part.
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Please be positive and constructive.
Nice work on the toolpaths, it does seem that the same issue that exists with a 3-axis CNC machine trying to cut dovetails still exists in that you are limited to the Z hight of the machine for the vertical piece of wood. Correct me if I'm wrong, but unless you build a fixture for the end of the table are you still limited to ~6" or so because of the max Z hight on a Shopbot PRS Alpha? I only ask because the dovetail cutting part isn't super difficult, but finding a solution to allow for longer parts (such as drawers) is the big issue.
I don't know the maximum Z height on the Shopbot but you're totally right, the best way to solve this issue is to make a jig at the end of the table. Then you can also use it for many other long pieces.