Building the Woodgears Box Joint jig

Over on another forum I was asked the following early in my build:

My biggest concern, I think, would be the human element introduced by the turning of the primary gear. In the video I watched, he's talking about advancing the jig by turning the primary gear x number of turns, or half a turn, etc., and that these turns (or parts thereof) can produce advancements that can be measured to the millimeter. It seems to me then that the entire accuracy of the jig is dependent on me accurately turning that gear the right amount. But there doesn't seem to be any foolproof way to mark when I've completed a full rotation of turning, is there? Isn't it counting on me to know "ok, the handle was right about here, so I'll turn it 360* and, yup, I think that handle is about in the same spot again"? His chart might say "1 full turn per cut" and I may attempt one full turn, but if my judgement of how far to turn it is off by a little bit, i.e. I turn it a bit too far or a bit not far enough, then can't that affect the spacing significantly?

I thought the answer there was interesting to the build process so I am re-posting it here also:

Hi,

The short answer to your question ("can't that affect the spacing significantly") is: No.

I couldn't answer you when you posed the question, but last night I finally sat down and worked my way through the math a bit, which is noted in my post above about cutting the gear. This morning I actually made my first test cut, and I'll post photos tonight. It was like magic. So I just sat down and finished working through the math, so that I can answer your question intelligently. :D

Here we go, put on your thinking caps... :rolleyes:
  • I'm cutting 1/4" finger joints. I've got a Freud box joint blade set in my saw, set to cut a nice square cut exactly 1/4" wide.
  • I have a threaded rod that moves the stock box, which is 16tpi, (threads per inch). So each full turn of the rod moves the box 1/16th of an inch.
  • I have a 12 tooth gear (the smaller one) mounted on the rod, which I am using.
  • I have a 48 tooth gear mounted in the gear block, which is meshing with the 12 tooth gear.
  • This big gear has a handle on it, which I use for turning the gear.
  • 48/12 = 4. So each full turn of the 48 tooth gear turns the 12 tooth gear 4 times.
  • Therefore, each full turn of the 48 tooth gear turns the threaded rod 4 times. 4 x 1/16th = 1/4.
  • Therefore, each full turn of the 48 tooth gear moves the stock one quarter inch.
  • Therefore, for making 1/4" finger joints, I turn the 48 tooth twice after each cut. That moves the stock 1/2" --> 1/4" for the bit just cut, and 1/4" for the next finger.

I hope you're still with me, the math has not been too dense so far! :huh:

So the question remains. I am just eyeballing this as I turn the gear. What if I'm off? What kind of error do I get?

First of all, the gear mounting block has a clamp in it. The 48-tooth gear is removable. (So you can put in different sized gears for different cuts.) So when I set up the jig to make my cut, I loosened the clamp, lifted up the gear, and then moved it and set it back down such that the handle on the gear was positioned right beside the secondary gear. This gives me a nice visual reference for stopping the gear each time I turn it.

But secondly, and more importantly, let's go back to math.

Suppose I do make a mistake. There are 48 teeth on this gear. Let's say I make a turn and am sloppy and am off by one or two teeth when I stop the gear, what happens? Remember, each full turn of the gear moves the stock box 1/4". We're dealing with small measurements here, so I think it is easier if we start talking about thousandths of an inch:
  • 1/4" is is 0.25 inches, or 250 thousandths of an inch.
  • 250 divided by 48 teeth is 5.2.
  • Therefore, each tooth in that 48 tooth gear, in this setup, is equivalent to moving the stock 5.2 thousandths of an inch.
  • So if I'm off by two teeth, that is 10.4 thousandths of an inch.
  • One thousandths of an inch is .001"
  • 10.4 thousandths is .0104" which is less than 1/64th of an inch (.0156") --- It's actually about two-thirds of a 64th... Ummm, that is a bit under 3/256th of an inch.
  • (I'm the one doing the math, and I find 3/256th to be a silly fraction!)

So, in my example, if I am off by turning the gear two teeth, it is only going to make a difference of less than a 64th of an inch. I don't think that is really going to make much difference. (And I have a hard time imagining being off by more than two teeth when turning the gear.)

One final point. Let's say I'm off by 2 teeth on one cut. This does NOT accumulate. Odds are the next cut I will be back on track, since I'm just positioning the gear. So if one cut is 1/64th too small, the next one will likely be 1/64th too big, and then after that it's back to normal.

How's that? Clear? Clear as Mud? ;)

Stay tuned, tonight I hope to post photos of my first ever test cut.
 
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Now that the jig is built, it is time to test it. I did not take pictures during my first cut, but I did during my second test cut. I took several shots, which I hope does a good job of explaning how to use the jig. As well, there are several videos on the woodgears website that show the jig in use.

To start with, I want 1/4" finger joints, so I put in a Freud Box Joint blade set. (That was the most expensive part of deciding to build this jig!) Blades like this are not mandatory, you can make box joints with a regular dado set, or with other blades. What you really want is a blade that leaves a flat-topped cut, and my dado does not.
I layed a piece of stock down, and set the blade height to be just a touch higher than the thickness of the stock. My goal is to have fingers that are just a bit proud of the joint, which can then be trimmed flush.
IMG_2821.jpg

Next, stand up the two pieces for the test cut. If I was building a box, I would have all four piece in at once. Snug up the pieces in the corner of the stock box, and turn the crank until the pieces are just touching the side of the blade.
IMG_2822.jpg

One optional step here is to loosen the knob that is holding your gear in the gear block, so that you can lift it up and turn it such that the knob (or some other visual reference mark) is lined up with the two drive gears on the threaded rod. This gives a nice visual reference for lining up the gear as you turn it.
IMG_2824.jpg

I am making 1/4" finger joints. So I need to turn the 48-tooth gear one full turn to move the stock box 1/4". Then carefully slide over the front piece of your stock, while leaving the back piece untouched. And then clamp them securely in place.
Honestly, this was one of the most difficult bits to get right, as the two pieces wanted to slide together. I would think it would become easier with practise, and also if I was using heaving hardwood stock.
IMG_2825.jpg

Turn on your saw and begin the cutting process. Push the jig with the stock through the blade and back. Then turn the gear the required number of turns and repeat until done. Again, I am making 1/4" finger joints, so I need to move the stock double that -- 1/2" -- each time. So I turn the large 48-tooth gear, which is meshed with the small 12-tooth drive gear, two full revolutions.
IMG_2826.jpg

And here is the result.
IMG_2827.jpg

As an aside, I do not like the thin finger on the right edge of these pieces. However, for utility shop projects, I probably would not care. As well, these were just random test pieces that I grabbed from the scrap pile. For ideal finger spacing, I would want to use stock that is sized to be a multiple of my dado width, which is 1/4".

The joint slipped together perfectly. It was like magic. No sanding, no chiseling, no fiddling. I just slipped it in, and tapped it together. Note that this photo is actually of the very first test joint that I cut with my jig.
IMG_2828.jpg IMG_2831.jpg

And that is pretty much the end. I think I'm going to really enjoy using this jig, and I can forsee a lot of use of it in future projects!
 
You did it now Art! Looks like i'LL BE SENDING Matthias some more money for his box joint plans. I've already built three of his tools/jigs/machines and consider them some of my best investments.
 
You did it now Art! Looks like i'LL BE SENDING Matthias some more money for his box joint plans. I've already built three of his tools/jigs/machines and consider them some of my best investments.

At the princely some of twelve bucks, I think it is a steal of a deal.

Maybe in the fall I'll do the pantorouter. That also looks like it would be a fun tool.
 
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