Thursday, May 28, 2015

Figuring the sides profile of my saw till

Its all about ellipses...

Ellipse are fun to make, so simple once you understand the principles involved, a major and a minor axis are all the measurements you need to make one.

There are numerous ways to make one, including some fancy jig or as simple as a string between two push pins, but really its all about the same two axis, major and minor.

In my case all I need is a quarter of an ellipse.

Desired side profile. You can see that it is made of two 1/4 ellipses of different sizes.
Drawn slightly crooked, but you get the idea.

So all I have to do, is to figured out my axis to fit on my piece. I also have to keep in consideration the height of my piece and where my two rails will land on the profile. Purely for aesthetic reasons.

That should be easy, so off we go gather cardboard, push pins and some strings...

I got corrugated cardboard, and some more of my mint flavoured dental floss. Heh, don't laugh, that stuff is cheap, we both go for clean up twice a year, and they give us that stuff every time :-)
Now, I will admit that corrugated cardboard and dental floss is probably not the best stuff to try, but that's what I got, so...

After some experimenting, this is what I figured out my dimensions would be.

My side pieces are 32 in long, take off 5 in for the bottom drawers and another 5 or so for the bottom of the larger ellipse to start, that give me 22 in to play with.
My smaller top ellipse is roughly 9 in from the top and 3-3/4 in in from the outside edge
At that point I draw another 1 in to separated both and that give me my starting point for the bigger one. Now the fun part start. You may want to get a bottle of Aspirin, and a calculator to follow along...Or if you are really old like me, dust off your slide rule :-)

Bear with me its really not that complicated, it just seems like it. :-)

Long axis 9 in square = 81
Minor axis 3-3/4 in square = 14.0625
Subtract both 81-14,0625 = 66.9375
The root square of 67 (lets keep it simple) is 8-3/16 in long
That is probably an area where metric would be easier as in, it would gave you whole numbers, but, sorry, I am a staunch Imperial kind of guy! QE II was my Commander in Chief after all :-)

Now where is my bottle of Aspirins?

That dimension we just got (8-3/16 in ) is the radius of our major axis for drawing our 9 in ellipse
Draw a line 16-3/8 (twice the length of our 8-3/16) and put a push pin on each end
At the center, draw a perpendicular line 3-3/4 in long and now the real fun start . We need to figured out how long a string we need to hit that 3-3/4 in when the string is taut.

It takes some hit and miss trial to get the string length right

So here's how you do it:
Holding one pin with your free hand down into that corrugated cardboard, because it keep getting loose in no time and screwing up your careful measurements, you hold the other one with your knee, while balancing that bottle of Aspirin in your other hand and pulling the string taut with that same hand. All the while, do not forget to stick out your tongue out of one corner of your mouth and twist it slightly (trust me that help)

Now try to trace that %#@& ellipse without the string riding under the plastic body of the pin, hitting the steel shaft, because that's not where you took your careful string measurements from, remember? NO? take another Aspirin, you need it!

Once you somehow, by the grace of God, managed to draw what look like an passing Ellipse, all you have to do is cut out one quarter of it. Oh, did I mentioned that you did not had to draw the whole thing but only a quarter? If not, blame the Aspirins

Repeat the same procedure for the larger one with the following dimensions
Major radius 13 in and minor 5 in .
169-25= 144, square root = 12 (yes, I purposely picked numbers to gave me whole numbers)

See, there is nothing to it. In no time you would be drawing ellipse with your eyes closed, and your hands tied behind your back, pencil in mouth...promise!

Small ellipse done, making the larger one.
Did I mentioned that dental floss knots are kinda slippery??
Carefully cutting your @#% corrugated cardboard quarter Ellipse, 
you can now try them on for size

Phew, thank God for Aspirins, I now have my two quarter ellipses and I can finally visualize my shapes and finalize (??) my desired profile. Oh, yeah, trust me, I am not cutting any more #$^@ ellipse, these would have to do :-)

The profile is of course off by 5 in since it is sitting on top of the top shelf.

My wife made a remark about my frown line on my forehead, when present it usually indicate that my blood pressure is up, so she asks me if I forgot my meds this AM? No, I did took them, but I suppose they were no match for ellipses :-)

I think I will finish my joinery on the side pieces (Dovetails, open mortises (2), rabbets) before cutting the shape. Just to make it easier to hold the pieces while working on them.
Then it's off to my bandsaw in the garage to cut my profiles then some clean up and should be ready for assembly.

Once assembled, the next part would be to make the beads ship lapped boards for the back. Could have use plywood I suppose, but it is a rather large exposed surface, rather make it look nice :-)

Bob, satisfied with whatever small progress I made on the saw till, and made a small dent in my other projects punch list...Now looking for a cold beer, I need one, oh yeah! :-)


  1. Maybe you should have drawn them freehand? It would have eliminated the math which would make me reach for the aspirin.

  2. LOL, it really wasn't that bad, the frustrating part was using corrugated cardboard and waxed dental floss. The math part is easy (with a calculator). I wanted to try customs ellipse as opposed to free hand, Im really happy with the results. The big plane till had free hand curves, the ellipse profile on this till look better.

    Bob, who didn't really used Aspirins in the making of these ellipse

  3. That profile ended up looking really good.
    I have seen various jigs that can be used for making ellipses. But I guess that you would either have to like making jigs or have a strangely high need for drawing them in order to justify making one.
    I would definitely also go for the dental floss way if I had to make one. I believe that would be faster and easier.

  4. Thanks Jonas, I'm really happy with how my profile work out in the end. I was worth the trouble :-) It sure beat trying to make fair curve by hand, and it is repeatable.
    If you want to use dental floss, try to get some non-waxed kind. Making tight knots in waxed floss is a real exercise in frustration, trust me!! :-)
    Next time I will also try something better than corrugated cardboard, not sure what, I simply used what I had available to me at the time, but the pins slipped out way too easily and the string would ride on the metal shaft, throwing out my careful measurements out the window. Well actually, something else flew out the window, if I was on a ship at sea it would have been, Cardboard Ellipse overboard! :-)