Jump to content
I Forge Iron

Mortise and Tenon Construction

Recommended Posts

I was watching one of Chandler Dickinson’s videos about making a trivet from railroad spikes and to get the mortise/tenon to the right size, he heated the tenon and drive it through the mortise, rather than heating the mortise. It seems to me that heating up the mortise and driving the tenon/river through as a drift of sorts would be more effective. What is the proper thing to do here?

P.S. I think that making a thread just to ask this question is a bit silly, so could this become a thread dedicated to mortise and tenon construction? That would be nice. B91BC848-00BB-48D3-B990-640C363BC029.jpeg.9484511b3b26763edf8bdb8921f8396f.jpeg

Link to comment
Share on other sites

I don't know if heating the tenon is the "right/proper" way to do that or not. It's how I made my RR spike trivet, heated the tenon and drove it through then riveted the end. worked very well for me and the trivet was very stable. That's the way I do it for all riveted tenon's because that's the way I was taught by Master Blacksmith Ike Doss back in the '80s.


Link to comment
Share on other sites

I have always treated the tenon as a species of rivet.  So, you usually want the rivet to be hot and soft to easily pein over once it is through the tenon.  I have even inserted the tenon cold and then heated it with a torch while the work is held in the vice to have it good and soft for upsetting the head. 

"by hammer and hand all arts do stand."

Link to comment
Share on other sites

I think if you didn't want to peen the end of the tenon over and just wanted the interference fit then heating the mortise and driving the tenon in and letting it cool would probably be the way to go. However, If you are going to peen the end over then you might as well heat up the tenon, put it in place and peen it over in one heat. 

Link to comment
Share on other sites

Things shrink as they cool, but for small items it's not a whole lot.  As mentioned heating the hole piece will shrink it onto a cold tenon. Using a hot tenon will give a little bit of assembly room when cold.  If you then upset your "rivet" into the hole it will tighten things up again.

Link to comment
Share on other sites

I never heat the mortise. Shrinkage on the tenon is minimal so not a problem. By the time you are actually finalizing the head, the shaft in the hole is cool and your final blows upset the tenon/rivit shaft tight. Usually but not always I set my tenon's/rivits under 1/4" cold. When I do a hot tenon/rivit I use a torch for for my heat. I hold the torch parallel to the mortise and apply the heat, as best as possible, only to the tenon/rivit.

Link to comment
Share on other sites

Thanks for both the positive support and answers about tenons. Now I get that the tenon is heated because it will shrink when cool and make it easier to assemble, but heating the mortise could cause issues getting a good fit. I'm working on a scissor guillotine right now so I'll be sure to post my first go at mortise and tenon construction soon.

Link to comment
Share on other sites

Apologies for appearing to be patronising in this response, but I am just trying to go through the making process involved in producing this type of joint in a finished project, and avoid some of the pitfalls it can involve, a little forethought and planning can save a lot of time and mistakes.This is the method I use, others opinions may differ.

Tenons can be round,  square or rectangular depending on their purpose.

Consider where it is going to be used, and how best to manage it before starting. This will depend on its purpose in the structure it is to be used in.

Plan a logical way of making it for ease of forging process, accuracy in finished item, and then assembling for final riveting in situ.

EG if you punch and drift a bar for the mortice, then its overall length will alter, not much for a punched hole, more for a lit and drifted hole. For a tenon, you allow that the volume of metal in the tenon means the length to start the incising is much shorter than the required finished length, excess may be cut off, but it is wasted time and effort if you make them too long.

This is my take on making a simple arm to backplate item. I am going to use a square tenon as I don’t want the finished piece to be able to swivel.


These are two such items I used them on.



Mark out the piece where the mortice is to go, (use a centre punch to mark its location) You then have a choice to slit and drift, or punch to a size.(This gives the dimension for your tenon)

Take a heat and punch from one side most of the way through (may need more than one) and when there is an indication on the back of the piece, punch from that location to remove the slug/pellet Note when you do this, use a bolster or some suitable item to support the material around the area being punched through, otherwise the bar may bend and trap the punch making it difficult to withdraw. Then drift to final size if needed. Finally flatter or hammer straight and allow to cool.




Now make the tenon to fit into this hole. (You may or may not need to upset the end of the bar, depending on where the item is to be used as on a heel bar.)


Tools being used,   from left to right, Curved butcher, side set, set hammer.


Heat the end of the bar, and use the curved butcher to mark the shoulder all around for forming the tenon, the curved blade marks the corners of the bar. Ensure the blade is square to the bar's axis, and that the angle side is facing the short end of the bar where the tenon is to be formed, you can then turn the bar 90 degrees and continue marking all four sides, this leaves a crisp shoulder to work from




Repeat this, deepening the cut until you are approaching the finished size required for your tenon.

Now start forming the square tenon, use a good square edge on the anvil, using the side set start the tenon and either continue with this or if your hammer skills are good enough, forge the tenon to a near finished size equally from all four sides, using the existing hole in the backplate as a guide





If the anvil edge is not a clean square corner, use a square block/hardie but note a sharp edge in the corners at the base of the tenon is not desirable, and ensure there are no hot shuts or cracks in the metal. This can cause premature failing and fracture later.

You can then tidy up and finish forge the tenon and mating faces using the set hammer
until the tenon fits into the hole in the plate



The tenon should be central to all sides, ensure it fits and beds down nicely by monkeying/monkeeing it by using the backplate over a bolster,( swage block, hardie hole or in this case the pritchel hole) of an appropriate size to support the work adequately as it is being bedded in.


Allow the bar to cool slowly, if not it may be hard when you come to saw it off to the required length.

Assemble and mark out for rivetting, the rivetting allowance is approximately one and a half times the thickness of the tenon, and then saw off excess.



When the item is finished ready for fitting, reheat the tenon and using the leg vice or some other suitable method to hold it, rivet the item into place, use light rapid hammer blows working all around the tenon and keeping it straight to give an even head, and maintain its squareness, you should be able to do this without having to reheat the rivet to finish, as it cools it should tighten further.

The workpiece is then ready to be finished for whatever you need for its other working end, and you can mark it out using the tenon shoulder as a datum.

When the item is finished ready for fitting, reheat the tenon and using the leg vice or some other suitable method to hold it, rivet the item into place, use light rapid hammer blows working all around the tenon and keeping it straight to give an even head, and maintain its squareness, you should be able to do this without having to reheat the rivet to finish, as it cools it should tighten further.


Apologies for poor picture quality, and I hope you find this useful.




Link to comment
Share on other sites

Many do not realize this but the sockets used on air tools are actually softer than standard chrome sockets. They are made that way to reduce wear on the anvil of your air tools. 

I use a jig with angle iron and pieces of 1/2" square bar that you put sockets on for a bending fork, nice idea for a monkey tool i will use now. I got enough sockets. Would i be looked at funny if i used snap-on sockets for hot metal? 

Link to comment
Share on other sites

That extra fatness also provides more torque. Think of it like a fly wheel. Mac tools used to make a socket for Honda crank bolts that literally did have a fly wheel on them. They redesigned them and now are just about 3x the thickness of a regular socket. Honda crank bolts are extremely tight. My impact has 1,200# of break away torque and some times it struggled with them. 


Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...