Andy,
It looks quite well made, so don't trash it yet.
It does seem that the ER chuck is using the flat face as a register, whereas your three jaw chuck is using a spigot ( for want of a better word) for a register.
Just to make it a bit clearer there is a spigot on your lathe nose spindle that locates into a recess on your 3 jaw chuck. The spigot and the recess are a very good fit so that when you tighten the chuck it registers between the spigot and the recess - this makes and keeps everything concentric. The flat face allows the item being screwed on, something to be tightened against.
Now there is a recess in your ER chuck but it seems to me that it does not register with the spigot on your lathe nose spindle.
DaveH
It looks quite well made, so don't trash it yet.
It does seem that the ER chuck is using the flat face as a register, whereas your three jaw chuck is using a spigot ( for want of a better word) for a register.
Just to make it a bit clearer there is a spigot on your lathe nose spindle that locates into a recess on your 3 jaw chuck. The spigot and the recess are a very good fit so that when you tighten the chuck it registers between the spigot and the recess - this makes and keeps everything concentric. The flat face allows the item being screwed on, something to be tightened against.
Now there is a recess in your ER chuck but it seems to me that it does not register with the spigot on your lathe nose spindle.
DaveH