Finial Tetra

This is a finial for Halloween fences made from PVC pipe. This particular model is the very popular traditional Victorian four-fin delta shape.

See below for detailed instructions. In particular, be sure to take note of the "Scaling to other pipe sizes" section, as you will need to size this to the pipe you are using.

Notes:

Layer height is not terribly important for visual detail, as the finials aren't usually examined up close. I tend to print the finials with 0.2 mm layers.

The v1 and v2 models included a built-in support under the base.  I've found that this support (inside the pipe opening in the base) tends to stick to the build plate as you remove the part, but if it doesn't, you can break it off manually after printing if you like, or even leave it on. It should still fit inside the PVC pipe. However, note that it will be in the way if you want to secure the finial with a long screw, so removing it may be desirable.

I've removed this support from the v3 model.  I've changed the profile under the finial to an (admittedly a bit shallow) cone shape rather than a dome, so the overhang angle doesn't change like it used to.  This change also increases the thickness of the plastic between the finial and the base.

What is this for?

Faux wrought iron Halloween fencing can be made out of PVC pipe (see the Instructables link at the start of this description), and is both decorative and functional (it discourages vandalism and keeps enthusiastic ToTs from traipsing straight through your yard and possibly tripping over hidden electrical wires, gravestones, and other decor).

However, the finials (decorative tops of the fence posts) are often a problem. You can order plastic finials online, but they come with square bases that don't work very well with the round PVC pipes. The usual solution is to soften the PVC with a heat gun enough to force the finial onto the end, squaring the circle of each post by essentially shaving off the curved sides. This is labor intensive and not terribly safe (aside from the heat itself, PVC is likely to give off toxic fumes when heated like this). And even then you aren't done; securing the finial usually requires drilling a hole for a screw for each post because they resist gluing.

I thought it might be better to design a finial that simply slides onto the end of the pipe, can be secured with a friction fit (or on the v3 model, via a screw through the built-in screw hole), and is made of plastic that may be more glue-compatible.

If you are starting right before Halloween, it is not terribly practical to try to individually print every finial for a major run of fence, but it's doable if you have enough lead time or only need a few.  I originally calculated that the full run of fence for my yard would need 81 of these finials printed at ½ inch scale (see below), and that would've taken almost two weeks of non-stop printing if done individually, plus a substantial amount of part removal and cooldown/heatup time.

A larger nozzle will also allow faster printing (on my newer printer – Anycubic Chiron -- with a 0.6 mm nozzle, I can get the time per finial at ½ inch down to below 2 hours each, so it'd only take a week or so to do all 81).   You can also reduce these times by putting multiple copies on the build plate, but then you risk losing several at once if there's a failure.  Another option is to print a few and make molds to cast them in something weather resistant (Great Stuff expanding foam, Silicone, etc.).

Scaling to other pipe sizes

You can use the finial with either schedule 40 (white) or schedule 80 (grey) PVC. It won't matter which of these standards you use. The outside diameters are identical; only the inside diameters differ. Since the finial fits over the pipe as a cap, we only care about the pipe's outer diameter. Grey is usually significantly cheaper, but it also comes with a short bell on one end, so you lose a few inches of the length. There's also black schedule 40, which could save you some painting, but it's more expensive.

This finial was designed to fit a 3/4 inch PVC pipe, which I chose as a compromise. If that's the size you need, you can probably use this as-is (maybe with tweaks to tolerance).  However, it's more common to use 1/2 inch PVC for the fence posts, and to use larger PVC for the ends of each section of fence. So you may need to scale the finial up or down from its default size.

The default finial is nearly six and a half inches tall. Due to cable management issues on my original Wanhao style print head, I would have needed to mod my Z axis (e.g. AzzA's Z extensions) before I could print that height. Scaling for a 1 inch pipe would be even worse; the finial would be more than 205 mm tall, well over my Maker Select's theoretical max build height of 180 mm. "We're gonna need a bigger printer." (cue Jaws theme) Deltas and larger Cartesian printers might fare better.

If you do need to scale the part for a different pipe size, some things to keep in mind are that 1) you are scaling to fit the model to the pipe's outer diameter (plus a bit for tolerance), 2) the name of the pipe bears little resemblance to any actual measurement you'll get off of it, and 3) you need to compensate from the "ideal" scale factor to keep the tolerance more or less constant.

For the pipe standards mentioned above, here are a few common scales:

• 3/4 inch pipe = The model's base has an Inner Diameter of 1.08 inches. This equals the pipe's standard outer diameter of 1.05 inches, plus a designed clearance around the pipe of 0.03 inches (a bit more than three quarters of a millimeter). That clearance seems sufficient for a relatively snug fit. Of course, printed parts have some dimensional variance depending on calibration and material, and the pipe also has some accepted variation, so Your Mileage May Vary.
• 1/2 inch pipe = 0.84 inches O.D. The exact scaling factor from 3/4 inch to 1/2 inch pipe is 80% (0.84 / 1.05). Probably the actual number you need to scale by will be a little larger; you are shrinking the tolerance as you scale down, so the finial may not fit if you go all the way down. 81% is a good starting point as it seems to fit snugly in my tests.
• 1 inch pipe = 1.315 inches O.D. The scaling factor from the default pipe size to this pipe is 125.24% (1.315 / 1.05). However, the actual number that you will need to scale by will probably be a bit smaller; you are enlarging the clearance between parts as you scale up, so the finial will be loose if you don't stop a bit short. (Going down to exactly 125% was still a bit loose in my latest test; you might want to work down from there.)

The easiest way to check the tolerance for yourself is to print the included test ring, scaled using the same factor you intend to use on the actual finial. You can also generate your own ring by printing only the first few millimeters of the base of the model, either by manipulating it in your slicer or by aborting the print after it gets tall enough.

Change history

2023-10-19: v3 file added.  This adds the built-in screw hole and removes the built-in support under the base (as it is probably unnecessary for this version).  The v1 and v2 models are deprecated.

2016-07-23: The v2 file includes tapered fins to facilitate casting, and both v1 and v2 have had a spindle-like support added to the pipe opening in the base to eliminate the need for slicer-generated supports (at least on the base).