[HBR] HBR-13 Progress

[Mike Hanz]

But by *far* the most accurate method is to use a step drill, starting 
with a #0 or #1 center drill.  I use center drills often with a hand 
drill to start quick jobs too easy to put on the milling machine - it 
will provide a consistent center, even when all you have is two 
orthogonal scratch marks on the workpiece.  No center punch 
required...and in fact the punch can move the center a noticeable 
amount, so I almost never use one.  Irwin makes the best step drills, 
but there are Chinese copies that will get you through a few projects 
before they get too dull and require a cutting edge touchup with a 
grinding wheel.  Use kerosene or WD-40 as a cutting edge coolant in 
aluminum, especially in the soft 5052-H32 alloy favored for chassis 
material - it's gummy and needs a coolant even more than harder alloys.

            73,
  - Mike  KC4TOS
  www.aafradio.org

On 4/6/2014 8:55 PM, Walt Hutchens wrote:
> Whitebear1122 said:
>> Are you able to get accurate location for large bits like 3/8" or
>> 1/2"?  The small ones are good but I seem to get more error with the larger ones.
> With the big ones on a soft aluminum chassis results aren't going to
> be very good.  Depending on the precision that's needed I may use a
> reamer or a file to enlarge a 5/16" hole.  At 1/2" and up I have
> socket punches for most sizes and I drill a bit oversize for the punch
> screw and center it using the marks that are (usually) on the sides of
> the punch.