Here shown assembled with a 60mm diameter, 0.8mm thick hss slitting saw.
This article does not include all of the dimensions, more just pointers and approach.
This particular mandrel was made for 16mm bore slitting saws. You can buy these readily and they are one of the best sizes for a small model engineers milling machine or lathe.
The first task was to machine the main shaft down to 12mm diameter for a length of 40mm. This made it fit nicely into a 12mm collet for use on the milling machine or into a fairly standard chuck.
Once I cut this part off I then started the washer, drilled and countersunk it to take a 6mm countersunk caphead.
The washer was turned around, faced and then a female step 1mm deep and 16mm diameter machined. This allows the washer to go over the step and so accommodate slitting saws of less than 1mm thick.
When you tighten and unlock the saw it is worth wrapping the saw blade in a cloth so that you can grip it tight.
You can make this mandrel from nearly whatever round steel you have lying around and it is not hard to adapt this to various saw sizes. Simple to make and I think you get a better result than the cheap £15 (2014) mandrels that are at all of the tool shops.
In the main image you can see some teeth missing from the solid carbide slitting saw – more on that here.