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It was made to comb mail #6: Next step: good setup ensures accurate positioning

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Blog entry by Patternguy posted 02-22-2017 01:24 AM 799 reads 0 times favorited 0 comments Add to Favorites Watch
« Part 5: Putting draft where it's needed Part 6 of It was made to comb mail series Part 7: The pattern is assembled on the follow block »

Part 6

After draft is applied and edge radii are added, the rib pieces are ready to be fit together.

Spacer blocks are sanded to precise width with a draft angle that is opposite of the ribs. This type of draft is known as “back draft”. The spacers will maintain accurate dimensions between the ribs and to keep the vertical axis of each rib vertical, maintaining uniform draft on both sides.

Unfortunately, I do not have an image of the next step, but I’ll describe it and the next image may help clarify the procedure.

The next phase of construction requires the use of a “follow block”. A follow block can be described as “the underlying shape required to orient an object in 3 dimensional space.” In this case, the underlying shape is a trapezoid, where the bottom face is horizontal, sides are vertical, and the top is an angled plane that matches the bottom of the ribs.

Remember the sand mold…and the fact that the cope has to draw up, and the drag has to draw down?
Between these two directions is the horizontal plane. The parting line around a pattern may not be flat at the pattern, but the perimeter of the mold will be.

If a pattern is not flat, or cannot be laid down horizontally in a mold, a follow block is the way to achieve that orientation.

The follow block can be seen here, supporting the ribs. The board itself is parallel, but is fastened to angled battens.



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