Large Scale Drawing Compass

I was needing to make some large circles for a project and my compass was too small. I ended up using some sticks and tape but I was knowing that I would be needing a large compass again.

There are 2 versions presented here, one had printed elements and steel arms while the other is made from steel. I have included all files and drawings to make both.

The printed one weighs about 3 pounds and is capable of creating circles of about 86 inches in diameter

The all steel version is about 9 pounds and will draw circles of diameter of about 134 inches... over 11 feet!!!

The arms are manually adjusted , the pointers and drawing implements are kept perpendicular to the work, and the arms are clamped in place so that the circle remains uniform.

As with most of my projects, I work on the fly so mistakes were made but corrections have been included.

I liked the look of the parallel arm style turn of the century designs so that is where I drew inspiration.

I admit that a beam compass would work quite nicely here but where is the dramatic reveal in that...

Plus I really just really wanted to make this.

During the mock-up, I used my printer to make parts that would allow assembly without drilling or welding.

The ultimate final version is mostly steel with some unique elements.

One final note is I chose to print some tools and templates make this drawing tool.

I like to have an actual item in hand so I tend to build things on the fly, so this was a little different for me.

I started out by making some CAD models of how I wanted things to look.

The arms are essentially a parallelogram and as such I made the arm pivot points to be 60 degrees apart.

Since this is to draw large circles, the points do not meet, this was deliberate since it would be massively awkward to draw tiny circles with this.

The upper body profile which I call the head kind of has a fierce cat like appearance with unicorn horn in the top.

That aside, I started with function and the form emerged.

There are two tool holders which allow this the be adapted to use multiple implements from scribes to pens and pencils.

I also created some tubing pivot ends so that drilling accurately into the square tubing is not a problem, 2 each of these are needed for each section of tubing..

You can use standard available cut lengths for steel thin wall square tubing in 3/4 inch and 1/2 inch sizes. All 4 tubes need to be the same length.

I used 1/4-20 bolts and nuts for all the pivots and tool holders.

The head clamp is 5/16 bolt and hand knob from Spaenaur 135-038.

It all assembles together in a couple of minutes after printing is done.

I wanted to use shoulder bolts but none of the proper size were available locally and on line they are about $6 each, so I had to make my own.

These are 2.5 inch bolts that are cut down to only have a quarter of an inch of thread. A nut is threaded all the way down for spacing and thread chasing after the cut.

The cut ends are filed and de-burred.

I couldn't find thumb screws that I liked to I drew and printed some. The bolt is a 1/4-20 long thread 1.73 inch long standard.

These are press fit into the printed thumb screw.

I began by installing a1.5 inch long 5/16 bolt through the middle part of the main body. this is threaded onto a washer then a knob for clamping the body together.

I wanted to test fit all the parts so I assembled the arm ends as shown and held them in place with acorn nuts.

The tool holder ends need to have a 1/4 20 nut installed into them, this is done using a short bolt to clamp them in place.

the steel tubing arms are simply press fit onto the the arm end pivots as shown.

The tool holders are then press fit in place as well.These are a tight fit and very difficult to remove.

You may notice that some of the pictures show different pivot end lengths, this is simply a mistake in geometry which is corrected in the files, the assembly procedure does not change.

Note that if this is printed hollow, the accuracy will be severely impacted since the part will wobble and lack clamping strength.

This one is made from steel. The parts are brazed together where solid sections are needed. I did this since brazing is a little more accessible than welding and all the points being joined are flat to one another..

Since I was making this big, I might as well go really big...

The parallel arms are 2 different thicknesses, I did this for aesthetics, the larger size is 3/4 inch square tube and the smaller is 1/2 inch square tube.

The height of the sections is 880mm or approximately 35 inches. (the length of the material that I had on hand)

The solid faces are 1/8 inch thick steel diamond plate.

The business end has 2 tool holders, I did this in order to have a more universal tool. As before, the holders can accommodate scribes, pencils and markers of various sizes and types.

These are also diamond plate parts, but folded using heat then brazed where needed.

I printed templates from the earlier drawings, I like the templates over paper patterns since they allow for tighter tolerances. If printing the drawings, simply glue them to the steel and cut the pieces.

Once the rough cuts are done it is a good idea to try to make the sections a close to mirror images as possible.

The rough shaping is done with a flap disk and angle grinder while the finishing is done with a hand file.

cut 4 sections of thin wall square tubing to equal lengths.

Then you realize that you have forgotten to take into account the length difference created by the pivot point. Now shorten the 1/2 inch tubing sections by 6.2mm

Flatten the ends then deburr the inside as well as the outside of the cross sections.

I created end caps for the tubing that will allow for proper marking and centre punching for drilling the pivots into the tubing.

While the caps have 4 holes, only the two opposite need to be marked for drilling. Be sure to only drill the sides that are the same

I decided to cap the ends of the tubing, I used bits of metal then polished and cleaned the surfaces.

All the fasteners used for construction are bare, zinc plated bolts and nuts release toxins when burned with a torch.

Use the template of drawing to mark the 5 drill holes. Drill one side the hole the two halves together and drill only the outer 4 holes all the way through.

The side with 5 holes needs to be slightly countersunk for the bolts to fit properly. The centre hole is drilled at 3/8 inch for clearances.

I was able to find 7 shoulder bolts locally that were 3/4 inch long. these have 10/24 thread. The side with only 4 holes needs to have there drilled and tapped for this thread.

I needed to braze a 5/16 bolt in the centre of the 4 hole half, alignment was done using the other half.

five 1/2 inch nuts are prepared as spacers these are aligned along the top portion of the back half, 2 additional nuts are used to help with clamping.

After the head has cooled, I cleaned and polished it.

once the cut is finished, the holes are pilot drilled using an 1/8 inch bit.

The edges are finished using a belt sander and hand files.

The pieces need to be scored for easy bending then the bent seams are brazed. This was done with a thin cut off wheel on an angle grinder

Heat was applied using my jewellers torch.

The long folds were done first then the upper arms were twisted to the necessary angles. This was all done by hand.

Once the pieces are formed, the screw mounts need to be attached, these are simply a nut that is brazed onto the bent opening at the front of the holder.

Now make another...

Clean and polish!

Assemble this one like the last. For the smaller tubing, use several washers. I found that it took 3 thin washers per side at all points.

You can use a 1/2 inch bolt as a holder on the top for more control.

It is a little awkward to use this at first.

I used a large self centre punch for the pivot and a felt marker.

You are done!!!