Help With Alignment Pin Hole Drilling

[Bob Korves]

The time you save in getting an accurate hole with a boring bar is incredible.

Using an ordinary boring head to bore holes is a fussy and slow business if you are trying to hit your diameter dead nuts on. It is maddeningly easy to overshoot your desired size. Most of the reason is because of spring in the boring bar. I prefer to bore to slightly under size to establish the location for the hole, then ream to final dimension. If it is a shallow or blind hole, the reamer must be one without a taper on it's end for centering the hole and starting the cut. It must be the type that is cylindrical for the full length and with angled cutters at the end to open the hole.

 

[ewkearns]

Best practice in aluminum for straight holes is:

1) drill to remove metal, (through holes are your friend)

2) bore to clean up drill marks to locate hole - leave at least .004, or more for finish, if holes persist in being oversize, increasing feed and/or decreasing hole size can help, as well,

either,

3a) finish bore to size (1,000,000,000 jig borers can't be wrong)

or

3b) ream to size using a floating helical reamer, using plenty of lubrication, but don't remove under power.

If you do this and still get oversiz holes, it is time to talk about:

Rigidity, worn ways, tool holding issues, adjustment of tool slides, or errors in indexing.....

 

[Bob Korves]

3a) finish bore to size (1,000,000,000 jig borers can't be wrong)

I don't see that many jig borers (or jig grinders) around in our hobby shops... They will certainly do the job very nicely if you are lucky enough to have one in your shop. Ordinary boring heads love to blow past the intended size, regardless of being careful...

3b) ream to size using a floating helical reamer, using plenty of lubrication, but don't remove under power.

Totally agree, and reamers are something that mere hobbyists can get their hands on for a reasonable cost and taking up little shop space.

Your sequence of operations is right on for any real chance of making a hole to the desired size and in the correct location.

Where are these 1 billion jig borers hiding? They must be cheap and everywhere, but I sure don't see much that I could afford to buy or find room for in my shop...

 

[ewkearns]

I've never had much trouble hitting the mark with a boring head, but RIGIDITY is the word.... Many of these jig borers are quite cheap! They are ideal for the home machinist, as it seems they have lost favor to the CNC folks. Ever the multitasker, I opted for a Kearney & Trecker Model "D," with micrometer standards and dial indicators. For those unfamiliar, see Page 10 of this document for an example. I bought a 9 foot radial drill and the milling machine for $6,000... sold the drill for $6,000, so I guess I have $0.00 in the mill!