Just wondering if there is a good rule of thumb for calculating tenon length for counter bored holes. I want to join two pieces of 1/2 square at a right angle with a tenon filed flush with the outside of the leg. I've seen that 1 and 1/2 times the dimension of the tenon is good for forming a head but i want to keep the filing to a minimum obviously. Any input appreciated.

A few test samples by you will determine what your length needs are, due to - we do not know what size your counter bores are and depth of counter bores in your piece.

1/2 square bars with 1/4 tenon. Counter bored one size over about 1/8 inch. Just wondering if there is a good rule of thumb.

Yes, volume = volume  Volume of the hole + volume of the counterbored hole = Volume of the tenon Me. if I wanted to be fussy, I would set up a trial and figure it out in about the time it takes to boot up a computer and access this site and type out a question asking about it.  Not to mention that making more tenons before I started on the project would brush up my skills.

Stop on by and you can compare the angle on my counter bore with the one on yours and see if what I use will work for you---or were you assuming every counterbore was exactly the same no matter where in the word it was made or when or by who?  In my experience that would NOT be the case.

And a question: will these tenons be load bearing and take racking stresses?

Have a look at this thread which may help.

There are some pages from a pictorial book there which has some useful riveting information re countersunk / flush rivets.

https://www.iforgeiron.com/topic/47601-making-rivets/

Alan

on 3/4 bar I only needed a 1.5" tenon. However it is nearly impossible to forge a tenon that short because in order to do that I must start with only 1/4" of material......but when I bucher the mass to isolate the material and start the tenon it fish lips horribly bad and at that point you are going to get a tenon made up of metal folded over its self. So I have to use at least a half inch of material to prevent that unless I want to take the time to knock the corners way in.....but time is money.... so I end up with a tenon that is 3" long its just quicker to cut them off and then monkey tool the the tenon and move on. 

After a few years I realized no amount of math will get you perfect, not because I can't calculate math, just that forging moves things and you lose a little. So I use math to get me close and then a test piece......oh...... I have taken so many test pieces to the scrap yard, but....... i dont take very many pieces that fail to meet the specs.

Practical wisdom: sometimes "good enough" is BETTER than "perfect". For example if good enough wastes 5 cents of steel but perfect takes an extra hour to do...if you are running a business which is better?