There is a box near weld that opens the weld options dialog. If you are using this but not able to enter that small of a number, go to Customize->Preferences and there is a "Spinners" category where you can change how many decimal places the dialog will accept.
If you do those things but still not getting your weld, there is a trick I sometimes use.
Make an instance of your object (hold shift and drag with the move tool and select "instance" when you've moved the instanced object far enough) with that instance selected in object mode, use the scale tool and make it really big or at least big enough that your weld will now work with the larger decimal value.
This technique can also be useful when working on things at an angle--just make an instance and keep it at 90 degrees.
Replies
If you do those things but still not getting your weld, there is a trick I sometimes use.
Make an instance of your object (hold shift and drag with the move tool and select "instance" when you've moved the instanced object far enough) with that instance selected in object mode, use the scale tool and make it really big or at least big enough that your weld will now work with the larger decimal value.
This technique can also be useful when working on things at an angle--just make an instance and keep it at 90 degrees.
Hope it helps.
Edit : Memory's first way is work, but it's 0,0001 instead 0.0001