Making your own jig for ellipse-based arcs. (From WOODWEB's Cabinetmaking Forum) March 5, 2003
I want to know how to plot and cut out an ellipse for a casing. Is there a workable router jig that will cut along the major/minor axis with just setting them up, or do you plot and mill to a line, like me? Or does everyone else have a CNC in their back pocket?
(From WOODWEB's Cabinetmaking Forum)
Use 2 nails and a piece of sting wrapped around the nails and run your pencil around the inside of the string. Experiment with the distance between the nails and the length of the string.
From the original questioner:
I know about that way, as well as with a square, and can set up for the axes on stock and then cut. What I am after is a cross slide jig that a router can follow and cut the inner and outer edges rt off the jig. Does such a thing exist?
It seems to me some years ago Fine Woodworking or American Woodworker (or ??) had a jig to do this. It had a base with two dadoes perpendicular to each other. In each dado slid a short block. The blocks were connected by an arm, with each block being a pivot point. This is from memory, so details are foggy. I also remember one of my students (former shop teacher) bringing me a "do-nothing" his dad had made based on the same principle.
The above post is on the right page. You dado out slots like a cross, 90 degrees to each other. You will have to make the jig big for larger radius. I made mine 2FT square. After you make the dado slots, then make some slider boards to slide in the grooves. Make this very snug and wax them. Then all you need is a board attached to your router. Measure down your two points, attach to center of your sliders and you are ready to go.
From contributor D:
The two posts above are on the right track. Doing it this way will give you true ellipses. If you don't want a true ellipse, then figure out your major and minor axis and with trammel points, draw out your end radius but make an entire circle, then find an arc using the trammel points to find the connecting radius. But it's a lot easier to have a CNC or do it in CAD. If you need precision, the string method can be tedious and a real guessing game. Also, you have to watch out that the string doesn't stretch. I prefer the CNC method. Now what about the detail profile, if any?
From the original questioner:
I know the easy way would be on a CNC, however... Our uses are fairly infrequent and I do have a horizontal spindle molder with an open end, which I think I can rig up a holding 3 point system to profile the face using the same casing knives as the legs which we currently run on this machine. I have seen Stegger's setup and I don't think it's out of the question to simulate that type of locating system. Just need to work out the drive system. I have done constant radius work on this with a form and roller setup with good results. Maybe a CNC is the right way, but I am a little slow to catch on. First I have to get better at the computer. Until that happens, I wouldn't know where to start. Just for yucks, would it be worth it to a CNC shop to make the occasional head casings, elliptical or radius segments in profile?
From contributor D:
We make the round and elliptical templates for door and window casings for several moulding companies as well as make the moulding. Pricing is calculated by the width and the arc segment length. And yes, it is worth it. We also do the fancy stuff like carved moulding, curved sections of baseboard and curved plywood.
We made a very simple jig using the cross dado method with two guide blocks and a trammel. It's a very simple setup, with a very simple calculation-no trial and error. We made a very large elliptical reception desk that required 5 different ellipses, and they were all on the money to 1/32 over 16 ft. We laid out the project on AutoCAD with known major and minor axes. You position the exact locations of the guide block centers and the router location from these two numbers and that's it.
Micro Fence makes a nice ellipse jig. I use it and like it.
The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).
Comment from contributor A:
The issue of ellipses is a good one. Because we do a lot of elliptical stairs, we've had to form our strategies based upon our needs and there are two points that I'd like to raise:
First, whether you can see it or not, the CNC programs I'm familiar with at some point have to convert ellipses to arcs or line segments (it makes sense to me as g-code needs simple geometry).
Second, there is no such thing as concentric ellipses, so any offets from the ellipse are only definable by scribing from the parent ellipse.
Therefore, I'm a firm believer in converting to arcs. Depending on how radical the shape of the ellipse is, a quadrant (of an ellipse) can be comprised of 2-4 arcs which are tangent to one another. These can be worked out in a simple CAD program. The results can be very good and then the geometry is easy to transcribe on the shop floor. And best of all, offsets are easy, because the offset arc segments remain concentric.
Comment from contributor B:
I have used the perpendicular method and it works great. But if you are making something intricate where you will need multiple templates be careful here. You cannot offset your template and maintain a true ellipse. This is where I have gotten into trouble. You may need to make multiple templates and this can be time consuming. Do yourself a favor - draw your ellipse, then do some math to approximate that ellipse and you will end up with radii that are easier to work with and no one will be able to tell that it is not an ellipse.
Comment from contributor C:
I will use the old (simple) method. Two small sprockets and a sealed bearing with I.D. = router shank diameter. Weld some bike chain to the sealed bearing, across the diameter. Now with adjustment of chain length and some way to mount the sprockets to the jig, just do the string, pencil and two nails trick. I haven't put enough thought into how to hold the jig to the workbench or to the stock. But that is a simple start. Tool setup: slide the router shank through the chained bearing, then chuck it up properly. Kind of the reverse use of a follower bearing on a router bit. Chain length can be adjusted small amounts with a third sprocket used to pull in some slack. I haven't made this jig, but some of you other thinkers may know how to finish it.
Comment from contributor D:
A small modification to the nails and string technique which we use at work: Instead of string, make a band by sticking two layers of glass reinforced packing tape sticky side to sticky side. This works much better than string because it has almost no stretch at all and allows us to mark out pretty accurate ellipses for stage and film sets up to forty feet across.
Comment from contributor J:
I've built a jig with the crossing dado idea but went one step farther. The crossing dados are made from extruded aluminum track. The sliding blanks are machined from aluminum to side without being raised up. Attached to these blanks is a post which carries a sealed ball bearing.
When the blanks are set above and to the right of the cross os tracks, they are then fastened to a pair of trammel rods which in turn fasten to the router base. Accuracy is very good as long as you realize the offset of the bit diameter. As in an elliptical casing of narrow width, the actual cut will vary by one half of the bit diameter. My jig is made on a sheet of flat 3/4 plywood and has massive range.