Well, this is not a very detailed HOW TO, but it does instruct, so maybe it fits here. First, let's talk about hoarding.
I'm not sure if hoarding is a recognized mental defect, but I tend to think it is. If not, it should be. I obviously suffer from the disease. Since I got my big Rikon, I have wanted to make a radius jig. Nahm uses one, it's pretty simple to make, no need to provide how-to details on this, because one look at the picture (yes, below) will show you how I made mine. But back to the hoarding.
See, I have had this nice piece of 1/2" (approximate dimension these days) plywood stored in my wood hoard area. I went looking around for something to use to make my jig, and immediately my eyes went to the ply. Oh no, I can't use that for a jig... it's too nice. Now this was not baltic birch, or quartersawn sapele ply, it was what was left of a standard BORG panel I bought for something a while ago. I continued to look around. Nothing came to mind or sight. I deliberated. I contemplated. I even thought about it a while. Went back in the house and refreshed my coffee. Came back. Cogitated more.
I would have to rip a 24" wide piece of ply to 19", leaving a cutoff of only 5". What could I do with a 5X36 piece of ply? Should I cut only 18" (my bandsaw table is 19") so I would have a nice nominal width to save? More reflections and ruminations... Okay, what the heck. Let's rip at 19".
Anybody else go through this agony before you decide to ruin a wonderful piece of easily replaceable plywood to make a jig?
Okay, about jigs. There have been several postings lately about jigs and fixtures (and I won't belabor the difference in a jig and fixture, we'll just agree to call them both jigs). See, the thing is, deep within the psyche of the woodworker is the innate desire to make something useful. The time spent designing and making a jig is, at least to me, some of the most quality time I spend in the shop. When I finish, I have made a tool. Something of enduring practicality. A time saver perhaps, a means for achieving amazingly accurate repeated parts/pieces. But without doubt a joy to make and joy to use, perhaps something my grandchildren may one day find and remember their gramps, happy as a clam making stuff out of precious junk.
Okay, here it is:
Like I said, nothing special. No difficult cuts, no fancy setup required to make it, just cobble it together from the precious hoard of wood you've been saving for just the right project. In use, there are two clamps holding the fixture (yes, this is really a fixture), I just popped one clamp on, strictly for photographic purposes. (You know, some safety guides have been removed...)
So, I just couldn't let this go without some math. I mean, how in the world do you figure out what the radius should be? If you can figger out the actual value of the radius (like when you're trying to match a pattern), you can easily replicate the cut.
Take a look:
The tape measure lying on the assembly table is measuring the LONG CHORD. The combination square is measuring the MID ORDINATE. With these two dimensions, you can easily calculate the radius,
Here's the formula: R = (LC x LC / 8 X MO) + (MO /2)
If the formula puts you off, here's what you do. Multiply the long chord by itself and divide that result by the mid ordinate multiplied by 8. Then add half the mid ordinate. The result is the radius of the arc.
In the example above, the LC is 20" and the MO is 5" (I'm working with whole numbers for simplicity. When we multiply 20 x 20 we get 400. The MO (5) multiplied by 8 is 40. 400 divided by 40 is 10, and half of the MO is 2.5, so the radius in the picture is 10 + 2.5, or 12.5.
I opened the access door to my bandsaw and with a magic marker, I wrote the equation on the inside of the door. You know, right there where you need to find it next time you need to calculate and cut a radius (arc/curve).
And that's your math lesson for today.
I'm not sure if hoarding is a recognized mental defect, but I tend to think it is. If not, it should be. I obviously suffer from the disease. Since I got my big Rikon, I have wanted to make a radius jig. Nahm uses one, it's pretty simple to make, no need to provide how-to details on this, because one look at the picture (yes, below) will show you how I made mine. But back to the hoarding.
See, I have had this nice piece of 1/2" (approximate dimension these days) plywood stored in my wood hoard area. I went looking around for something to use to make my jig, and immediately my eyes went to the ply. Oh no, I can't use that for a jig... it's too nice. Now this was not baltic birch, or quartersawn sapele ply, it was what was left of a standard BORG panel I bought for something a while ago. I continued to look around. Nothing came to mind or sight. I deliberated. I contemplated. I even thought about it a while. Went back in the house and refreshed my coffee. Came back. Cogitated more.
I would have to rip a 24" wide piece of ply to 19", leaving a cutoff of only 5". What could I do with a 5X36 piece of ply? Should I cut only 18" (my bandsaw table is 19") so I would have a nice nominal width to save? More reflections and ruminations... Okay, what the heck. Let's rip at 19".
Anybody else go through this agony before you decide to ruin a wonderful piece of easily replaceable plywood to make a jig?
Okay, about jigs. There have been several postings lately about jigs and fixtures (and I won't belabor the difference in a jig and fixture, we'll just agree to call them both jigs). See, the thing is, deep within the psyche of the woodworker is the innate desire to make something useful. The time spent designing and making a jig is, at least to me, some of the most quality time I spend in the shop. When I finish, I have made a tool. Something of enduring practicality. A time saver perhaps, a means for achieving amazingly accurate repeated parts/pieces. But without doubt a joy to make and joy to use, perhaps something my grandchildren may one day find and remember their gramps, happy as a clam making stuff out of precious junk.
Okay, here it is:
Like I said, nothing special. No difficult cuts, no fancy setup required to make it, just cobble it together from the precious hoard of wood you've been saving for just the right project. In use, there are two clamps holding the fixture (yes, this is really a fixture), I just popped one clamp on, strictly for photographic purposes. (You know, some safety guides have been removed...)
So, I just couldn't let this go without some math. I mean, how in the world do you figure out what the radius should be? If you can figger out the actual value of the radius (like when you're trying to match a pattern), you can easily replicate the cut.
Take a look:
The tape measure lying on the assembly table is measuring the LONG CHORD. The combination square is measuring the MID ORDINATE. With these two dimensions, you can easily calculate the radius,
Here's the formula: R = (LC x LC / 8 X MO) + (MO /2)
If the formula puts you off, here's what you do. Multiply the long chord by itself and divide that result by the mid ordinate multiplied by 8. Then add half the mid ordinate. The result is the radius of the arc.
In the example above, the LC is 20" and the MO is 5" (I'm working with whole numbers for simplicity. When we multiply 20 x 20 we get 400. The MO (5) multiplied by 8 is 40. 400 divided by 40 is 10, and half of the MO is 2.5, so the radius in the picture is 10 + 2.5, or 12.5.
I opened the access door to my bandsaw and with a magic marker, I wrote the equation on the inside of the door. You know, right there where you need to find it next time you need to calculate and cut a radius (arc/curve).
And that's your math lesson for today.
