Variables

[mikejohn]

I have never, to date, had to use variables in a cutting file, but have now come up against a problem.

14

This diagram shows a proposed jig for cutting what I believe are called 'sliding dovetails'. The long ones that, for instance, allows you to set a shelf into a bookcase side.
The white triangle is the base of the jig, set at the same angle as the dovetail bit.
The green shape allows a reference point (yellow circle) that can be the initial 0,0 point, and can also be accurately referenced to point B.
Now we have no problem moving the bit to the start point at 'cut start' and removing the red section.
However, I wish to remove precisely that much red from each board, whatever the thickness of the board (within reason).
Knowing the thickness of the board, this is not difficult.
However, this point changes with thicker or thinner boards.
So I have to change the position of x/z cut start for each different thickness board.
So I imagine I can have a variable which is var1 (-x), and var2(+z).
Can I write a file that allows the program to halt at the start, to allow me to enter the 2 variables?
Or do they need to be hard coded?
It is easy in Excel to make a table that produces the right variable for each thickness.
It would be nice to have a constant (thickness) whereby entering the new thickness the program would compute the new varx and var z in relation to the Constant.
The alternative is to produce a whole series of files for each thickness of timber, which is not that arduous, but I was hoping for a more elegant solution.

.............Mike

[richards]

Entering a value into a variable can be done like this:

' variables
' &thick
' &depth
INPUT "Enter Thickness" &thick
&depth = &thick / 2
MZ, -&depth

[waynelocke]

Your drawing shows a 90º cut on the shoulder while it should be the angle of the slope of the dovetail. You would need to cut it with a dovetail bit.

Why not register from the bed of the jig and make several files for different ranges of thickness, i.e. one file for material 5/8" to 1". This would compensate for individual variations in thickness and create dovetails in each range which are the same size. This would simplify cutting the sockets because you would only need one file for each range for mating sockets.

Wayne

[mrdovey]

Mike...

If I knew where the jig was, I think I'd look for a way to use the known angle and my zzero plate to calculate the thickness.


...Morris

[mrdovey]

Mike...

Digging through a batch of my old drawings I found this design for an adjustable sliding dovetail fixture and thought you might find it of interest.


15

[mikejohn]

Mike
Thanks. That looks like what I was hoping for.

Wayne
The solid red part is the cut out piece, which is angled, done with a dovetail bit (in the example 12.5º)

Morris
My original idea was a little block that fits on the top corner of the wood (the solid fill right angle) and measure that point.
But I would need to do this with every board.
If I simple measure the thickness, (and I like your Z plate idea), Then I know that position relative to the jig, by calculating before hand.
I will report back on my experiments, see if I get what I want.

Thanks

...............Mike

[beacon14]

Might it be easier to create a holding jig where the part is held UP against a reference surface, so you are always referencing off the top face of the part? Then the toolpath would be the same regardless of material thickness.

[mikejohn]

David
With the help of Mike, I am now an experienced ShopBot programmer

I am still working out the details, but what I have is a jig that is easy to x,y and z zero.
From this I can accurately cut the desired dovetail in the centre of the thickness of the board, providing I know it exact thickness.
The secret is in creating the 'tenon' part of the dovetail to fit the groove.
You simply enter the board thickness and the file does the rest.
When I have it working, I will publish my results here.
I'm dragging old Basic programming skills from the recesses of my memory (which at my age is not that easy!!), but there is excitement in the John household!


By John household, I don't mean an outhouse out the back, but a household with a surname like mine!

..............Mike

[charles_o]

Mike
I have assembled quite a number of cases over the years using sliding dovetails, and found that a well fit joint can be a bear to assemble due to the friction between the pieces. If you cut a slight matching taper on both the pin and the socket that locks into place as you slide the joint together, it creates both a strong joint and one that assembles with ease.

Do you think that this could also be accomplished within you program.

Charles

[mikejohn]

Charles
Is this what they call a tapered sliding dovetail?
If so, I am assuming its the thickness of the tail and pin that is tapered, not along its length.
Put it another way, the 'groove' is a little deep at the point of entry, but is perfect at the 'stop' end of the groove.
If I knew what angle of taper was needed, then ,yes, it could be accommodated.
And using my new found variables, could work by entering the board width as well as thickness.
If it's possible to give me some idea of the amount of taper, I will build it in.
Irregardless of the -14ºC (7ºF) temperatures, I intend to make the jig tomorrow.
The taper is in the software however, so I can get on with it.
I'm guessing that a perfect taper allows the joint to go together easily by hand until you can push no further, with maybe 8mm (1/3") still to go, which is tapped home with a mallet.

I will Google a bit, see what I find on tapered sliding dovetails.
............Mike

[mikejohn]

OK, I Googled!
I now believe this is a tapered sliding dovetail.


16

17

The shoulder and tail face (or is it pin face?) remain parallel along its length, the tail is the same depth throughout.
The width of the taper narrows along its length, as shown in red.
My intended method will allow this taper to be made, but not a taper that makes the shoulder and tail face tapering.
Can I taper just one side?
It will be necessary to tell the program whether the board is facing up or down, left or right.

.........Mike

[ron brown]

Tapering one side was common in older furniture. It can also be a superior method of assuring a part stays true to a certain plane.

The "Can you taper just one side?" May need review by the language use police. (Or should that be buy?)

Ron

[mikejohn]

Behave yourself,Ron. I've just had one thread suspended

[mrdovey]

Mike...

Your jig looks like a winner to me.

I can't imagine any reason why a very slight end-to-end taper couldn't be cut by positioning the bit at the low side and cutting toward the high side.

Do try this on a piece of scrap first, since the center of the taper cut will exhibit a slight concavity dependent on the diameter of the bit and the angle of taper. If the resulting quality is satisfactory, this should be a lot easier than putting shims under either jig or workpiece.

...Morris (not affiliated with language police!)

[charles_o]

Mike
Your second example is what I was refering to, allthough lass exagerated of an angel than what the picture shows. Morris' point of shiming would work, just be sure to shim the same edge on both sides of the workpiece. I guess you could do this on one side, but I would keep things symetrical I was doing it. I did do a repair on a wardrobe once that had joints like this.

18

[mikejohn]

Morris and Charles
Thanks for your input.
I will give full details when I have completed my experiments.
The mathematics are quite straight forward to create the angle in the file, with just variables of width and thickness input at the start of the file.
My ShopBot manual is at the workshop (4 freezing cold, ice covered miles away) where I am now off to. I will check what commands are available in shopbot files.
Then I will make the jig (which is not complicated). Then I will cut the files and report back.

Morris, I recognise the slight concavity, but it seems very small, even over 150mm (6"), and shouldn't be a problem.

Charles, I did consider a joint like above, but was thinking more of dovetailing both sides, but only tapering one side.

Need to get the dog sled out now, and be on my way.
In the words of Oats to Captain Scott "I'm going outside now, I may be some time!"

...............Mike