Mortise and Tenon

written by Ted McNett? in the July / August 2006 issue of the HAMMER & TONG

I hope that you were able to accomplish rivet making over the previous months.

Once you are able to continuously make rivets it is time to join two pieces of metal together. You should practice this skill with scrap material before you use it on the project. To accomplish this you will need to make a center punch and a round punch.

Punches

We will use coil springs for our source of tool steel. You want to talk to your auto repair shop about saving you old coil springs, especially from McPherson? struts because they are about V-i inch diameter. Most others are 5/8 inch or larger. If this is the only size you can get that is fine.

Mark Sperry recommends using short punches and a punch holder so you do not waste good material for something to hold onto. The punches that we made in the class are about 4 inches long. Start with a 3-inch blank of the coil spring.

To cut the coil springs you may heat it in the forge, straighten it, and hot cut it or use a cutoff blade in an angle grinder, heat it and straighten the blank. I do not recommend using oxy-acetylene because you have to re-cut the "burnt" end off of the blank to make a useable punch.

HINT: When forging high carbon steel stop hammering when the steel reaches a red color. If you hammer on high carbon steel when it is too cold you may crack or break the piece. If you burn the steel you need to start over.

Take the round coil spring blank and heat it to a high yellow heat. Make sure you rotate the piece in the fire periodically to get a good even heat throughout the thickness of the blank. Once you have a thorough even heat hammer flats along the length of the blank. The flats will prevent the punch from rolling away when you set it down.

Center Punch and Round Punch

Once the blank has been "squared," taper one and a half inches of an end down to about 1/8 inch diameter. Remember to taper it square, than octagonal, than sixteen sides, until it is round. For the center punch dress the end to a 60 degree sharp point. Dress the end of the round punch flat with a file.

After you have forged the piece to shape you need to normalize it.

Find two pieces of material that are a % inch thick. Measure in one-inch from the end of each bar and centered. Center punch a mark.

Punch Plate

A punch plate is used to back the piece of steel that the hole is being punched through. It helps prevent distortion of the steel. It can be made of a piece of scrap about % inch thick and 2 or 3 inches wide and long as available. Drill a hole about 1/64 inch larger than the punch.

Scribed lines help with alignment for punching.

Heat the center punched piece of steel to yellow, quickly place it on the punch plate using your hammer hand to hold the tongs. With your tong hand, pick up the punch and place on center punch mark, hold piece with punch. Now drop the tongs, pick up the hammer and hit the punch twice. Flip over the piece of steel, align the punch mark with the scribed lines and punch through from what was the backside into the hole. The small piece should drop into the punch plate hole. Now that you have punched a hole you may repeat this process on the second piece of steel. Once both pieces have been punched you are ready to rivet them together.

Rivet Preparation

Select two of the rivets that you have made since the last article. The rivet shank needs to be dressed to the proper length. The theory says that a rivet should be as long as the material joined plus one and a half to two times the diameter of the rivet shank. For our test piece here is the math: ¼ + ¼ + (1.5 X 3/16) = ½ + 5/16 = 13/16 inches long. Remember that this is blacksmithing and not machining, round up to ensure enough material for the rivet head.

Cut the rivets to the appropriate length. File any burrs off of the end of the rivet shank and test fit the piece.

Riveting

Heat the rivet to a yellow heat. Be very careful not to lose the rivet in the fire. Once the rivet is up to heat quickly remove the rivet from the fire, place through the holes in both pieces of metal, set the rivet head down on anvil face, gently and quickly strike the head with the flat of your hammer or the pein on a ball pein. (NOTE: historically ball pein hammers are not traditional) Pein the head to the desired shape. If the rivet cools continue to hammer into shape until done, rose head or round head. If you heat up the entire work piece in the forge the piece around the rivet will distort. Oxy-acetylene is a great invention for this, but not traditional until the 1900's.

Repeat this test process with scrap pieces until you are happy with the end result. You may choose to decorate your rivet head with a center punch or chisel marks.

Mortise and Tenon Joinery

Mortise and tenon joinery is attaching two pieces of steel together with a hole in one piece, the mortise, and a rivet shank forged on the end of the other piece, the tenon. This process is very similar to what you have practiced but the rivet shank piece extends 90 degrees from the mortised piece. This method is used for the corner joint of the shelf bracket.

Punching the Mortise

Mortising requires a square punch. This prevents the piece from rotating in the hole. Repeat the steps fro making the round punch but leave it square, about 1/8 inch at the point. Once the punch is made repeat the steps for punching the round hole. You may choose to make a square punch plate for this or use the pritchel hole on your anvil.

The Tenon

The tenon involves necking in the piece of steel to a rivet shank. There are several tools needed for this, refer to the Necking In section of the previous article. The shoulder should be at least ¼ inch deep if possible. On the one-inch bar for the shelf bracket you may end up with a rectangular tenon. (Just make sure to punch a matching mortise). This is fine and will help keep the piece secure. If you choose to you may make the tenon ¼ inch square. This requires a little more hammering though.

Once the tenon is made test fit the tenon into the mortise, file if necessary. (Hint: File the corners off of the tenon if it does not fit and retry.) Once the pieces fit snuggly together mark the tenon to the proper length, ¼ inch plus 1 ½ times the thickness (1/4 inch) not the width of the rectangle. The tenon length is 5/8 inches.

To rivet the tenon, heat up the tenon to a yellow heat. Place the piece in the leg vise with the tenon up, place the mortise over the tenon, and gently and quickly strike the head with the flat of your hammer or the pein on a ball pein. Pein the head to the desired shape. If the bracket will be screwed to a wall the rivet head must be flush with the back of the mortised piece. Counter sinking or chamfering this hole can accomplish this; the rivet head will fill up this area, file flush.

The final product should form a 90-degree angle; adjust as necessary. Again, practice with scrap material until you are continually happy with the end result. Now you can make the scrolls and pieces for the wall bracket. In the next issue I will discuss collars. Please bring this project to the guild meetings to share your learning experience.

-- RebStaup - 11 Feb 2008