Dear All,
I need to cut a half oval out of Med-ex for over an entry way. The cut will be approximately seven and a half feet long and approximately two feet tall. If it were a straight half circle i would just put a router on an arm, but I am not sure how to approach it. I would appreciate any ideas.
Best,
John
Replies
I would probably lay it out with a string and cut it by eye with a jig saw, for that application. You have two axis, a major and minor. Your major would be 7' 6" and the minor 24". I have a couple of pictures of the string method, but it is just for explanation. I wouldn't use a shoe string. To find the placement points for the two pins, you measure down from the top of the minor axis, one half the distance of the major axis. Tie a non-stretch string between the three points.
Your measurements are fairly wide compared to height. You may find that a true ellipse is too low on the ends. Many old New England fans were rather squat. Flatter on top with broad arcs in the sides. This can be achieved by superimposing a larger ellipse over a smaller one. You may also draw semicircles at the ends and blend them in with the shape of the ellipse top. If you look at old buildings, you see quite a variation in the shapes. Some stand out because they are so awful and unbalanced. Others exemplify the era with their proportions. In all cases, the hand and eye of the builder makes for an individual signature.
Dear Hammer,
You caught me as well. An ellipse is what I am shooting for. I agree with your idea on how to draw it, but I don't have the skill to execute it as you have suggested. At least not to the required specification which is "perfect". I am really looking for a way to mechanize this and to take the human element out. I know all about the "personal" touch, but my clientele won't have it on this job. Best,John
John
The attached jpg is from the site this link will take you to, with a description of an elliptical layout that will allow you to use your router attached to a straight guide to cut the curves. JL
http://www.toolsofthetrade.net/articles/showarticle.asp?articleID=2283&position=0&type=article
Another approach is to go to a Kinkos type place and have them draw this in CAD and then print it full size on a wide format printer. It's a simple procedure and shouldn't cost much.
You only need a quarter of the design printed and you use it and then flip it for the other half.
I do many layouts this way.
Edited 2/19/2007 11:49 pm ET by corners
Corners
I think he wants to be able to cut out the shape using a router and guide setup.
The partial CAD layout will not give him all the radii origin points that he needs. There are three required to do what he asked in the initial post and 4 to layout the complete ellipse.
String methods are fine for making an approximate perimeter line (albeit a close approximation), but it takes a geometrical construction method (using math and compasses) to trace a "mathematically" accurate ellipse.
I think he is not comfortable cutting freehand to a line...and I also think that he wants to give his client 100% satisfaction, as per the client's request and not as he interprets and imposes that interpretation on the client. JL
The "personal touch" wasn't meant to designate any lack of craftsmanship. It's more about finding a pleasing shape for the architecture and space limitations. A good jig saw and blade can split a hair. You'd have that cut out, installed and the check in hand before you could figure out where to start making a router jig, that large and that accurate.When faced with this type of work, I draw out a full size template on large rolls of paper. Fold it in half so you can cut it out evenly balanced. Go try the paper template in place. You might be surprised how it actually looks. After that, you can cut out the work piece anyway you see fit. I can tell you, elliptical router jigs aren't easy to make or use and there are a lot of places to screw up.Beat it to fit / Paint it to match
hammer 1
You do not need a jig to cut the ellipse. When it is drawn out with all the origin points of the radii on the major and minor axes, the cuts are regular curves and each one a segment of the circumference of a circle. A router attached to a piece of 1/4 inch plywood is as complicated as the equipment is going to get, and the resulting ellipse will be perfect. JL
"the cuts are regular curves and each one a segment of the circumference of a circle."
jeanlou
Methinks you have this reversed. A true ellipse does not have any part of it that is a true arc or part of a circle. It is a constantly changing radius. The string method (two pins a string and a pencil)will give you a true ellipse where using fixed radii will give you a close enough approximation to look good. If you take any CAD program and draw an ellipse using the draw ellipse command, and then ask the radius any place along it, you will get an error message telling you it is not an arc or circle, therefore cannot be dimensioned as a radius. The CAD echos everything that is in the old mechanical drawing books from before CAD existed.
Using fixed radii however is certainly easier to do and will give pleasing results if proportioned correctly. Fixed radii is the way I would go if using a router as especially if only making one.
RichThe Professional Termite
trialnut
I have a reaction to drawing an ellipse with a string. There are too many variables -
the stretch in the string - the need to keep a constant unchanging pressure on the string - the angle that the pencil makes with the surface being drawn on - all this being controlled by a person holding a pencil against a string.
The goemetry of an ellipse is directly reflected in the JPG that I posted earlier in this discussion. The complete ellipse is made up of 4 regular curves, each a part of a circle, established by the radii on the two axes of the ellipse. This is the classic way to lay out an ellipse, and more importantly, the result can be cut with a router attached to a fixed arm. The final product is reproducable by people with a wide range of woodworking competence.
Using a string leaves me unstrung! :-) JL
jeanlou-The technique you have described, using circular arcs, produces a visually close approximation to an ellipse, but it is not a true ellipse. It is an old drafting trick to indicate an ellipse when the only available tools are straightedges and compasses.A trammel can be used to draw a true ellipse. The technique is described athttp://home.frognet.net/~ejcov/ellipse.htmlThis technique can be applied to a router jig to guide the router in an accurate elliptical path. Commercial versions are available from several sources, one of which is Woodhavenhttp://www.woodhaven.com/detail.aspx?ID=2097An excellent description of a shop-built jig for elliptical routing is contained in "Router Magic", by Bill Hyltonhttp://tinyurl.com/2dx5nt
Donald
Thanks for the info. At least it is not a string method!
For work in the field my very close approximation will have to do, but for shop produced shapes I would pass on your info to anybody asking, with pleasure. JL
jeanlou
I agree that a string and pencil leave room for error. I use CAD for that reason. The "smoke grinder" jigs do draw a true ellipse with the accuracy being determined by the amount of slop in the jig. However, the point I was making was not about the accuracy of the method. The point is that the trammel method does not draw a true ellipse. It draws an oval. As someone else brought out an ellipse is defined as; a closed plane curve generated by a point moving in such a way that the sums of its distances from two fixed points (the foci) is a constant. With the trammel method the constant is the radii of each trammel rather than the sums of distances.
I know this is nit picking and when it is all done, if one is careful, it is hard to tell what method was employed. It is just easier to make a pleasing to the eye curve (IMHO) by using a method that draws a true ellipse. I also find it faster than trying to find that right blend of two radii.
Not trying to be anal here, just offering a clarification to what seemed to be a confusing issue in the whole thread. There is nothing wrong with using an oval for this application, or using trams to git-r-done.
RichThe Professional Termite
trial-"The point is that the trammel method does not draw a true ellipse. It draws an oval."I don't understand why you say the trammel method (Trammel of Archimedes to math fans) does not draw a true ellipse. You've given one definition of an ellipse. But if you start with the fundamental definition: (x/a)*2 + (y/b)*2 = 1 (where * indicates exponentiation)you can see that the trammel method (as I described previously; not jeanlou's two-arc approximation) does indeed describe an ellipse.Enter "ellipse trammel" into a search engine, and you will see many sources that derive the trammel method from first principles. Here's one http://www.du.edu/~jcalvert/math/ellipse.htm
Donald
The trammel method I was refering to in my last post was the one in the jpg attachment in this post 34583.6
The one you are refering to (the smoke grinder type that runs in dovetail ways) does make a true ellipse as I mentioned before. This type basically emulates the string method.
That is one of the reasons I said there seems to be some confusion here is there are two different methods both being called the same thing.
Bottom line is, if "it" just swings an arc of a set radius it does not make a true ellipse, as your math formula proves out. I believe we are on the same page.<g>
Rich
The Professional Termite
Edited 2/20/2007 7:32 pm ET by trialnut
After I have the shape printed out full size I glue this paper to a sheet of 1/4 inch MDF and cut along the line through the paper and MDF on the bandsaw. Then I fair the shape by sanding. This template is then used to guide a router with a bearing guided bit to cut the final workpiece.
I have used this technique for many shapes including elongated ovals, hearts, and serpentine curves. I'm going to cut some harp shaped chair splats this way as well.
It's really worthwhile learning CAD.
Rich and Donald<!----><!----><!---->
I appreciate your points and have stored them in my favorites for future reference. I hope that you will not, however, be too upset if I tell you that I will never use a trammel jig to router an ellipse on the job site. I prefer an inexact almost right method that I know will pass muster and that I can control in a way that gives me a guaranteed result. I did an elliptically shaped project at <!----><!----><!---->Pearson<!----> <!---->International<!----> <!---->Airport<!----><!----> last year. It was about 40 feet along the major axis and about 20 feet on the minor axis. It is a retail outlet built in stainless, glass, structural steel, millwork, case goods and a stone floor. It was all coordinated using my site cut templates, imperfect perhaps, but accurately duplicated and distributed among all the different trades. The location opened without a hitch. The architects and engineers found the ellipse pleasing and I was able to lay out and cut it alone, without another body to help. JL<!----><!---->
PS Describe to me how you would cut the template for the work I mention above.
jeanlou-You have my sincere admiration for successfully accomplishing such a large project. My comments have focused on the difference between an exact expression of an ellipse and an approximation. The method you previously described is a very pleasing approximation.If I were tasked with making templates for such a large project (and I really had to do it, not just talk about it) I would most decidedly leap to your two-arc method, but I hope I would check with the customer to see if that was satisfactory. Surely it would be; if it wasn't, then I'd calculate a bunch of x and y points with a spread sheet and hand plot them on a suitable substrate and cut close and inch up on the line as others have suggested previously. But then my templates would be only an approximation, wouldn't they?
Donald
One must always be 100% up front with clients. There is also an expression in French that translates "One must not be more Catholic than the Pope".
A carpenter / cabinet maker can rig up something to cut an arc with very long radii. The trammel jig is another matter to use efficiently when dimensions become industrial in size.
The original post was about something a little smaller, but if the person is uncomfortable cutting to a line with a jig saw then I do not think he will be comfortable using a trammel jig either.
I do appreciate the information and I am very interested in the method that you shared with me. I was serious when I said I filed it in my favorites. Thank you for that. JL
jeanlou
I am not upset at all. As I said in an earlier post, if I was making just one and using a router, I would likely use the same method you showed in post 34583.6. If it were something I was going to make a template for then I would first generate it in CAD using the ellipse command. If doing something like your airport job where it is just a tab more than my 42" rollfeed plotter could handle, I would use your method. Before I had CAD I used to use the two radii method on my drawings but would use the string method to create templates. Both methods have their place and applications. Both have their good and bad points. Neither is right or wrong. My only point to start with was to clarify the differance between an ellipse and an oval. Guess I musta stirred things up a wee bit by doing that. ;o) That airport job must have been a fun one.
RichThe Professional Termite
Rich
The job was no fun at all but it was a challenge and once the bugs were smoothed out, it was rewarding. (and I do not mean financially rewarding)
As far as the difference between an oval and ellipse, I always knew that one. We just don't build the ellipses in the same way. An oval is a no brainer - 2 half circles joined by straight lines. Thanks for your input. JL
Rich,
So, how do you set up the tramel? I have seen somewhere a method of positioning the focii for the string method, but is there a relationship between this and the setup of the smoke grinder?
Dave
PS. just curious -no practical requirement yet.
Jeanlou, I know you can approximate an ellipse with three centers. That might be fine for J. but it will require a little hand work to smooth out the transition between the arcs. You have to start and stop in the right place with the router. I made an ellipse jig once upon a time. Similar to the drawing jig linked below. The first attempt was just blocks in a groove. One of the blocks wanted to pop out. Next one used a T-slot. The tolerance had to be just right. You need infinite adjustment on setting the block positions but the attachment also has to be free to pivot as the blocks slide. After a bit of whittlin' and a pile of scraps, I had one that worked OK. It was best with an extra set of hands. Of course, it only cuts ellipses and you need a smooth even feed to keep from making any edge goofs. I found a manufactured jig but it is a little expensive for one job. http://www.woodweb.com/knowledge_base_images/zp/a_jig_for_drawing_or_cutting_ellipses_1.gifhttp://www.trendmachinery.co.uk/ellipsejigs/Beat it to fit / Paint it to match
hammer 1
"an ellipse with three centers" You must mean 4 centers. Three centers were to produce the wall frame that had no bottom.
Thanks for the links, JL
John
Do you mean an oval or an ellipse? An oval is the shape of a hockey rink or running track...two half circles joined by straight lines. Half an oval is one half circle with two straight legs. JL
Edited 2/19/2007 10:12 pm ET by jeanlou
Dear Jean,
You are correct, I meant an elispe.Thanks!John
Good suggestions for laying out. My advice has to do with the actual cutting....I would do this on a cheap piece of plywood first, maybe some mdo or something hanging around the shop with no voids. Draw my lines, cut it out with a jigsaw, do any fine tuning with a belt sander etc. Get it perfect. Use this ply as a pattern for a patternmakers/flush trim bit to cut final piece (after jigsawing close to line). It will cost a little more, but I like to make my mistakes in the cheaper, lower stakes material.
good luck.
Find someone with a small cnc operation. Have the operator program in the ellipse and play with it till you're satisfied then have the machine cut it out.
The guy I use for my cnc work would charge at MOST $150 for this (based on $75/hour).
No human error of string method and not expensive compared to the jig systems I've seen.
Dear ALL,
I want to thank you for all of the suggestions. I must apologize for communicating as effectively as I should. As many pointed out, I do in fact, need to cut half of an ellipse. Right now, I am leaning towards the CNC idea, but I will need to find someone in the area (CT). Thanks Again!John
All,
Just to put in my .02$ worth, here's yet another way to construct an ellipse, using only a rule or measuring tape, and a straightedge:
Draw a rectangle that encloses the area of the ellipse to be drawn (length=major axis, width=minor axis).
Divide the rectangle into quadrants. (Center lines, or axes). (Or you can just lay out one quadrant.)
On the perimeter of the rectangle, divide the length of each side into the same number (not the same size) of parts. The more divisions, the greater accuracy. On the poster's example of 24" by 36" (size of the quadrant) I'd probably divide each side by 12 (2" divisions on the short side, 3" divisions on the long one)
Starting at one end of the long center line, draw a straight line to the first of the division points you laid out along the adjacent (parallel) long side.
Moving to the first division on the short side, connect that point with straightedge to the 2d point on the long side. Continue till the last point of the short side is connected to the end of the short axis/centerline. This is 1/4 of the job, and can be used to make a quarter pattern or the process can be continued on all four quadrants to complete the layout.
The intersections of the lines just drawn describe a series of points along the desired ellipse. Connect them and cut out, or use to make a template.
This doesn't take as long to do as it might seem, especially if you use a quarter pattern and flip it to make up the whole thing. Faster for me than driving nails (too close together/too far apart) and looping string (too long/too short/too stretchy).
I've made one of those oval tracing machines (smoke grinder, whammydiddle), after reading a description of the one in the Dominy shop at Winterthur. It works well, but a lot of work to make, and limited by the length of its beam (I have a couple) as to the size of the ellipse it can draw...
Ray Pine
For those still curious there are a number of animated ellipse drawing ways here http://www.jimloy.com/geometry/ellipse.htmI got the book "Making Mechanical Marvels from Wood" a number of years ago. It is full of things from 18th century Mechanical Engineer's bag of tricks. When I retire and retreat to the shed, I think I'll putter away making machines that do nothing.
Or Here
Edited 2/28/2007 8:59 pm ET by byhammerandhand
Edited 4/3/2007 4:59 pm ET by byhammerandhand
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