It was made to comb mail #2: Making use of a layout

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Blog entry by Patternguy posted 02-19-2017 12:48 AM 819 reads 0 times favorited 0 comments Add to Favorites Watch
« Part 1: A mahogany foundry pattern Part 2 of It was made to comb mail series Part 3: the pieces become a sub assembly »

Part 2

A quick disclaimer here…I originally thought I’d just throw the pictures up and leave it at that. However, the more I visit this site, the more I realize the level of knowledge and talent of its inhabitants is pretty dam high.

I have to give the subject the attention and respect that it deserves. That’s going to take me some time.

On to the story…

Castings are made by pouring metal into a closed hollow mold made of sand. The hollow shape inside the mold is made by the pattern. Patterns are very accurate wooden replicas of the casting. But, they are more than replicas, patterns are a collection of precise shapes that are engineered to obey the rules of good foundry practice.

Good foundry practice for pattern making means the pattern will have the draft needed to mold, that the parting plane is properly located, and that parting lines are smooth and natural, with no abrupt changes of direction.

Draft is the amount of angle, or taper, on all vertical surfaces of a pattern. Draft can be added, or subtracted, from a pattern. Patterns must have some amount of draft, or they cannot be removed from a mold without damaging the mold.

The parting plane is the plane that splits a pattern into two. Pattern are not always split in half, nor is the parting necessarily flat.

Partings that are not flat, or planar, are called “offset partings”. Offset partings are usually non-planar, free form shapes that are defined by a parting line. Think of the edges of a potato chip.

Remember the sand mold? has to be opened up to remove the pattern. Molds are separated at the parting.

Ideally, partings are flat. Think of a sphere…its parting is a flat plane that will always pass through its center. Its parting line travels smoothly around the perimeter.

Remember the potato chip, it is definitely not flat. There is no plane that can split it either.
Where can the chip be parted? The answer is, at the parting line that runs around the perimeter of the chip.
All geometric shapes have a parting line somewhere. It is the lines, arcs, or splines that travel around the perimeter, separating each face of a shape into the up direction, or the down direction.

All this rambling about partings and lines is important to understand how draft was built into this pattern.

The pattern maker’s first step is to understand his blueprint well enough to virtually “build” the pattern in his head.

The next step is a full size layout with accurate views of critical features or dimensions of the pattern. Layouts are crucial to accurate pattern construction.

That is where I’ll start.

The pattern maker chose the side view of the fingered ribs for his initial construction layout. 8 ribs and 48 fingers will be assembled on top of the layout.

Wood stops have been glued to the layout. Their edges located on the layout lines that form 3 edges, or faces, of the layout.
Smaller wood stops for locating each finger on the rib are also glued to the layout. Rib and finger material will be planed to a precise thickness that accounts for draft that will be applied later in construction.

Each piece is fit onto the layout. This method of assembly is accurate, precise, and repeatable.

One thing I noticed in the shop is how easy it is to fall into a mindful zen feeling when one is focused on cutting, sanding, and fitting 56 different pieces of material together.

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