tools
I would like to know if it is possible to reduce a drill bit diameter (the body and not the shank). From times to times I need to work with dimensions that are not integer number or half number…. example: where I live I can only find drill bits 6mm or 6.5mm or 7mm….. however, in some cases I really need to have a dimension of 6.25mm.
Is this even possible to do, if yes how?
A knockout punch is typically capable of going through thicker metal - at the very least, it will do it faster. A quick search easily finds knock-out punches that can do 10 gauge steel, where most step bits don't seem like they'd do well beyond 16 gauge (though I can't find anything that really says the limit).
For thin metal though, the step drill bit is probably faster, if for no other reason than the setup/tear-down time of the punch (eg, after you punch the hole, you still have to unscrew the punch).
Knockout punch:
Step drill:
Here are some charts from Bolt Depot. Bolt Depot is a handy resource, and has lots of information about all different types of fasteners.
Pilot hole size:
Wood Screw Diameter:
****Major thread diameter** is measured on the outside of the threads.*
Best Answer
I would suggest that you look a little farther. Drill bits are available in many more sizes than you mentioned. One big supplier on-line, McMaster Carr shows available metric sizes for heat resistant cobalt steel jobbers drill bits in all of the following metric sizes:
If you can find access to a supplier that has drills on the numbered gauge scale or the lettered scale they cross reference to metric sizes as shown here: