domingo, 15 de enero de 2017

Pocket holes without special drillbits!



Pocket holes seem to be something really mysterious in Europe!  Wait, what is a pocket hole? Well, this image from Woodmagazine.com is worth 1000 words:


 As you can see it is an easy way to join two boards. You just need to drill a counterbore at a certain angle to fit the screw head, plus a pilot hole to drive the rest of the screw. The thing is, drilling those holes precisely at the required angle is a bit challenging.

I've always had a hard time finding jigs in stores simply because pocket holes are not a thing in Europe. Whenever I've seen one at a store, it had a ridiculous price. 

  
Small kreg jig

But anyway, why buy it when you can make it? It should be simple, it is just a thing to drill at 15º.  The challenge is that you need two diameters, the counterbore and the pilot hole. These commercial jigs usually include a special drillbit with a smaller diameter end that takes care of the pilot hole. That is because regular drillbits of such a small diameter usually are not long enough and the drill chuck ends up interfering with the board.
 


And additionally you will need special flat head screws to prevent the wood from splitting and those require a special square driver.



Well. I think that is enough. This was supposed to be simple. Suddenly you need a special drillbit, special screws and a special driver. No thanks :D 
There is a much simpler solution if you are not going to need massive amounts of pocket holes every day: Drill each hole from each side. This solves all the issues at the same time. Now you can use your regular drillbits and you can decide how long you want the pilot hole to make sure that area is strong enough and won't split with a countersink head.


The jig in this is a bit more complicated but it should not be a challenge, check the video for detailed instructions!






You can download free templates for 16mm and 30mm boards in this link

If you prefer to buy the jig, you can use the following links depending on your country:



martes, 15 de noviembre de 2016

Set the tension of your bandsaw blade with a guitar tuner!




Tuning a guitar is simple - You just turn the tuning keys until you get the right note. Tuning a bandsaw should not be any more difficult, right?
There's just one small problem: you have no idea how it should sound :D What you know instead, is the value of desired pressure in the blade to get the best performance. 
OK...so what? Well, Here's how you can use some simple math to determine the proper tension of a bandsaw blade using nothing but your ears (okay, and maybea guitar tuner).

Luckily for you, there is a simple equation that establishes a relationship between the frequency of a stationary wave in a vibrating string (or blade in our case) and the tension applied to it. That equation is called: the string vibration equation (easy to guess at this point)



MATH! Don't panic. It basically says that the frequency of the sound you will hear, depends on:

  • T: Tension (YAY!!)
  • L: Free length
  • μ: Linear density of the string (or blade) : mass per unit length

Tension is self explanatory: In the case of a guitar, you modify the tension of a string with the tuning keys and get the free string note. 
 
Free length is simply the distance between the ends of the string. This is the portion of the string that is allowed to vibrate. You can produce different notes by modifying the free length - pressing the string against different frets on the fretboard.

The linear density is what makes the strings sound different from eachother while keeping tension under certain levels - You don't want the neck of your guitar broken or bent .
Thicker strings (and thus, heavier) will produce lower notes and viceversa.

Now let's forget about frets and apply the theory to the bandsaw:
  • The linear density is just the weight of the blade divided by its length.
  • The free length will just be equal to the distance between wheels.
  • The tension is usually given by the manufacturer. You can also find recommendations for most kinds of blades online otherwise. If you find a pressure value, you just have to multiply that pressure by the cross section area of the blade to turn it into a force.
 Applied to my case, step by step:

Linear density: I just took a (not so precise) scale and measured the weight of the whole bandsaw blade. Turns out it's just about 22grams (0,77 ounces) Regarding the length, I just trust the blade manufacturer and use 1425millimeters (or around 56 inches)

Free length: pretty straightforward. I used a tape measure to get the distance between centers of the wheels. I did this when the blade was already a bit tight so it doesn't change much during the rest of the tensioning. The free length in my case is 396mm (or 15 5/8 inches)

Tension: I found 14500psi would be a good value for my blade. That is around 100MPa. Then I measured the cross section thickness and length in the blade throat (between the teeth), which is the weakest point of the blade. After all, the teeth are barely under any tension unless they are cutting.
 I made a quick CAD model and analysis to see how the blade is stressed under tension too, because this is how engineers have fun :) 
 
After measuring in several throats, I concluded the section was 4.65mm x 0.3mm. So, 100MPa x 4,65mm x 0.3mm provides a tension of 139.5N ≈ 140 Newtons (or ≈31,5 pounds)
 
 
Now we just plug in the numbers and complete the equation:

Frequency f=120 Hertz

So, you have to tighten the bandsaw blade until it vibrates at the frequency you got. Pretty simple. Just take your smartphone and download a free guitar tuner app (I used DaTuner lite if you are interested) or anything that can read frequencies through the microphone. Adjust the tension until you get it right. And that's it. You are done!




Well, just a second. There is something else you need to consider: harmonics. Basically, when you pull a string, it doesn't vibrate only in one way, but in several ways at the same time.  These are called vibrational modes or overtones. This graphic from Wikipedia shows the idea quite well. As the wavelength reduces by 1/2, 1/3, 1/4... the frequency is multiplied by 2, 3, 4, etc. 


So your string is probably vibrating with all these waveforms at the same time. But you don't need to know all of them, and their amplitude (That's called Fourier analysis). You just need to know the fundamental one, which is the first one. 

Why am I telling you this? Because usually your guitar tuner will be clever enough to find the fundamental one, but depending on how you pull the string, you might get it confused and have it showing a harmonic instead of the fundamental . For instance, my frequency is 120Hz, but sometimes the tuner reads 240Hz (2nd harmonic) and then it stabilizes back on 120Hz. Nevertheless, if at some point I found a 60Hz reading, that means that 60Hz was the actual fundamental frequency of the blade, since the fundamental is always the lowest one. Keep that in mind! On the other hand, If my tuner hadn't  stabilized on 120Hz I would have thought 240Hz was the fundamental frequency, and thus, that my bandsaw blade was much tighter than needed. 

That's all :D



The sources of this small study have been mainly Wikipedia and Matthias Wandel's approach (more precise than mine) linked below:


Wikipedia useful info:

Also check out Bill's site because he is nice and he helped me write this article :)



 

jueves, 25 de agosto de 2016

Hey there!!

This site is under construction so, thanks for coming! but you might want to check it again in a little while.

In the meantime, here is a video you can watch ;)