Ten years or so ago, I had some riser blanks made up, but they got the holes slightly too large and the cutting required a lot of finishing, so ended up getting used for other things.
I had one 7/8 set left and figured I could sort something out for him. Hours involved would make them highly expensive so I suggested a reasonable price and perhaps a few trades down the track...
Did some sketches for him and he chose 'B'...
Marked out for the clamping bolts. Auto punched and centre punched then use a centre bit to keep everything spot on...
Squaring up in the drill vice. Use a piece of white card/paper behind so you can clearly see that the fine gap is the same width all the way up...
Using 5/16 UNF so appropriate drill bit is used all the way. Then an 8.5mm hole drilled half way. Here checked for depth. A handy tool that will be regularly found at swap meets...
Drilled and tapped. Countersunk allens need trimming and a counter sunk hole needed in the risers...
Here allens have been shaved on the lathe and counter sinking done...
Line up and fit nicely. It common when tapping female threads to get them slightly off square. I this happens, the can work the tap back and forwards to one side to square the thread up. I you are careful final thread will still be fine...
Now to mill out to 23.5mm. Using a drill bit to enlarge a hole by only a mm will damage the bit, so will use a milling cutter on the rotary table. If you don't have equipment like this, a die grinder or round file is the only answer.
Riser is centred on the rotary table with a machined up plug and clamped REAL tight...
Removing the plug is easier if it is threaded, so a bolt can be used to pull it out...
Checking with a a piece of round bar machined to 23.5mm...
Careful marking for cuts to create the top clamp...
Cut very gingerly with a 1mm angle grinder blade...
Risers need to be angled rearwards 30mm. Will do one angled cut and then weld the two pieces together. A right angled line is marked and then the angle worked as if a 'v' was to be cut out (right mark to red dotted line). Instead it will be one cut and then the bottom piece will be turned 180 degrees...
Cutting in the mitre saw...
Bottom of risers need to be drilled and tapped to take 1/2" BSW bolts (English bike). With short pieces such as these use the drill bit to get them square...
Drilled and tapped...
Set up for welding... note angled cuts so weld will fully penetrate...
Both welded on three sides. Front of risers need to be curved. This done and grooves cut to take weld...
Finished. Risers will be painted and new owner will do final finishing...
Begining and end...
This was a time consuming job and not at all economical for me to do, but took it partly as 1. an interesting exercise, 2. to help out a customer and 3. to show readers just what you can do with some knowledge and patience...
Hope you found this interesting a perhaps picked up a few pointers for your own workshop and the confidence to do some fancy fabbing.