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Pierced Joinery by Glen Jarreau

I recently received a scholarship from the Francis Whitaker Blacksmith Educational Foundation that allowed me to attend the Advanced Blacksmithing class taught by Clay Spencer at the John C. Cambell Folk School. Traditional joinery is a prerequisite for any project that is constructed in this class.
I designed and began construction of a mirror frame wall hanging. I found a bar splice joint in the March 1993 issue of The Blacksmith’s Journal that I wanted to incorporate in the design. The joinery technique I used for the frame corners required ½” x 1” rectangular holes to be pierced perpendicular through ½” by 1 ¼” top and bottom rails. These holes would accept side rails of similar dimensions reduced on the ends to ½” x 1” (Figure 1).
After completing a test piece, I discovered that the pierced bar was thinning and stretching at the ends of the slits located in the middle of the bar and the slits were not completely disappearing when the bar was drifted (Figure 2). These thinned spots would not have been a problem if the side rail tenons were shouldered on all four sides and the ends of the side rails were riveted on the outside to hide the thinned spots. The design I used has the side rail tenons shouldered on only two sides. This with the scroll work on the outside of the frame hides only a very small area of the pierced holes allowing discrepancies in the piercing to show on both sides.
The slitting chisel width had been calculated matching the area of the finished dimensions of the rectangular hole (.5” x 1”=.5 in²) and a comparable area for a round hole listed in Francis Whitaker’s The Blacksmith’s Cookbook. The closest equivalent area for a round hole is 13/16” which fell between the listed chisel edge length for a ¾” hole of 1.050” and a 7/8” hole of 1.225” in Table I on page 85. This worked out to a slitting chisel edge width of 1 1/8”. A slitting chisel was forged and ground at this width with the addition of a center point. The center point acts as a locator to find a center punched mark identifying the center of the area to be slit (Figure 3)
Thinking the slitting chisel was too wide, I reduced the chisel width to 1” in an attempt to eliminate the visible edges of the slit. A second test piece was made yielding essentially the same results, thinning at the slit ends and the slit ends slightly less visible.
A discussion in class with a fellow student yielded information about an old chisel design that cut the center slit with the edges of the chisel punching two round holes at the same time. Without taking the time to forge and shape a 2nd chisel as described, I decided to drill two 1/8” holes at the termination points of the slits before the slit was cut. The holes were drilled 1/16” inside the 1” marks to yield 1” outside to outside dimensions. This would allow clearance for the slitting chisel edges (after reducing the width again) to complete the slit between the drilled holes without cutting into the outer walls of the drilled holes. (Figure 4).
I reduced the edge width of the chisel again from 1” to 7/8” to ensure the chisel would not cut into the outer edges of the drilled holes. The bar was heated and the slit was cut between the drilled holes (Figure 5)
The bar was heated again and the slitting chisel was used to open the hole enough to allow a ½” x 1” drift to be inserted with the 1” sides of the drift parallel to the sides of the bar (Figure 6 and 7). The chisel was driven only far enough into the slot to drift the hole open without the chisel edges cutting into the outer walls of the drilled holes.

The ½” x 1” rectangular drift is centered and driven into the bar equally from both sides with the 1” sides parallel to the bar until the ½” edges just begin to touch inside the slit. The slit is supported over the hardy hole in this and all subsequent drifting operations with the edge and ends of the slits pulled or pushed to one corner and edge of the hardy hole as the drift is driven deeper into the bar. (Note: A vice, swage block, or some form of bolster can be used if the drift you are using will not fit in the hardy hole.) This operation opens the slit in the bar enough without too much distortion to begin the process of upsetting the bar. At all times the bar and the ever widening hole and shoulders around the hole should be maintained straight and parallel. The difficulties encountered in trying to upset a crooked bar is well worth the extra time in maintaining a straight bar and a centered hole.
The upsetting operation will transform the oval shaped hole, still parallel to the bar, to a round, and eventually to an oval shaped hole perpendicular to the bar. The drifted slit is reheated close to a welding heat and the heat localized to the slit area by maintaining a small diameter forge fire and periodically cooling the areas on either sides of the slit with water. This is important to create the upset at the slit area and to reduce upsetting and distorting the bar above or below this location. It is worth noting that a coal forge has a distinct advantage over a gas forge in concentrating the heat in a localized area for this type of operation. Three heats generally were required to transition the hole to a perpendicular position across the bar (Figures 8, 9, and 10).



Before drifting the hole perpendicular, ensure that rag from the slitting operation does not interfere with the proper alignment of the drift across the bar. Stop and file or chisel out the rag before proceeding any further. Work until the drift can be inserted 90° to the bar. The angle of the hole to the bar cannot easily be changed after the hole is drifted. Two out of ten holes that I had completed for the project did present some problems with the drift not aligning in the hole perpendicular to the bar
The ½” x 1” rectangular drift (Figure 7) can now be used across the bar to open the hole to the finished size. Reheat the hole and insert the drift, ensuring the drift is centered, 90° to the bar, and as before driven into the bar equally from both sides. The 1” sides will now be perpendicular to the bar. As before, the area around the hole is supported over the hardy hole with the edge and ends of the hole pulled or pushed to one corner and edge of the hardy hole as the drift is driven deeper into the bar (Figure 11).
Continue the drifting operation until the rectangular drift passes through the hole (Figure 12).
Drilling the ends of the slit greatly reduced the degree of thinning at the slit ends. The drilled holes disappeared when the hole was complete (Figure 13).
Normally a drift would be tapered at the hammer end to allow the drift to pass completely through the hole from one side unimpeded as the drift is hammered through the bar. The drift I forged was upset on the hammer end and dressed with a radius under the head (Figure 11, and 12). This was an option I had set up to blend the edges of the hole with the thinning areas, but fortunately it was not required. Because of the upset head, the drift had to be removed by hammering on the inserted end. This caused some minor upsetting and sharp corners to develop that required redressing the inserted end between holes (Figure 13). Forging a drift tapered on the hammered end reduces or eliminates the need to redress the inserted end of the drift. Overall length must be considered in a project like this. Each of the pierced holes reduced the bar length by ½” for each hole produced. This measurement had to be considered to prevent it affecting the final dimensions of the project.
The pierced holes required for this project were all satisfactory and after development of the required steps, predictable. It is important that the same steps and the order in which these steps are performed are adhered to produce pierced holes of equal and repeatable dimensions (Figure 14).