When making a fuller (spring fuller or hammer die fuller) for making tenons, what size should the hole be to create what size tenon?

Let's say you want a 1/2 inch tenon to be exactly 1/2 inch in diameter. Do you make the hole in the fuller exactly 1/2 inch in diameter or a little larger, or a little smaller, and by how much?

It looks like no one wants to commit on this one so I'll say what I do but there sure could be better ways. I make my spring swedges by forging them, so if I was making a 1/2 inch swedge I would make the blank then heat it and forge it over a 1/2 inch cold rolled bar then after it was cooled I would grind the edges of the hole so that it would not cut into the tenon that I wanted to forge with it.
That said we know that the spring swedge will shrink some as it cools so it would be slighty under 1/2 inch.
If I made the tool by drilling rather than forging it I would put a card spacer inbetween the 2 halves and my hollow would be slightly under sized once again.
I think that most of the time having the tenon under size will make assembly go smoother. :)

I posted this on Farwest today in response to the same question:

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When I make a set of swages, I clamp the two pieces of stock in a vise and drill the size I want down the middle. I then take a die grinder and generously relieve the sides so it could well be 3/4" wide for a 1/2" hole.

I also usually mark a shoulder with a side set and then rough the tenon close to size before putting it in the swage. A piece of 1" square would be shouldered and roughed to 5/8 square or so by eye, then the swage applied to round up the tenon to the proper size. >>


Normally, you also want a generous radius at the transition of the tenon to the parent stock. 1/4" radius is not too much - the matching mortise is also radiused with a bob punch so the shoulder of the tenon seats properly. Too square a corner will create a stress riser and potentially cause a tenon to break off. Voice of experience - I did this twice on a set of andirons before I figured out what was happening.

I disagree about needing a 1/4" round transition. Using a monkey tool to upset the end of the parent stock and sharpen the shoulder gives a nice look and a tight fit. If done hot enough, there should not be an issue with cracking. Normalize or anneal the part if you plan on cold riveting. Hot riveting, particularly with a torch, makes assembly much easier.

I've had core drills recommended to me for making tennons in a production setting. However, catalogs do not tend to specify the center hole size. I picked one up recently and plan to try it out. Another option would be hollow mills: Genesee Manufacturing

These are more expensive but adjustable. Either option would give a nice sharp corner and could be set up to make each component to exact length. If made just oversize in length, you could still heat the ends and upset the shoulders to get the required length and better look.

I made my dies for forging the male tenons slightly oversize to allow for scale, shrinkage and metal loss due to finishing with a grinder when the tenon when it cools down. I make my punches to the exact size as the amount of loss due to scale is countered by contraction upon cooling.