On Mon, 29 Dec 2003 13:17:15 -0600, Pete & sheri
wrote:

I am making a drilling/tapping block and I need to drill very straight
holes for 8 sizes of taps. I will be ok with the 1/4" and up, an maybe
even with the #10 tap. But I am looking for ideas on making the holes
for the #6 and #8 taps really straight. Any ideas? The holes will be
1" deep. I will make my prototype from mild steel, but later, I want to
make some from annealed 4140.
Actually, this tool will be used for both drilling and tapping, to
keep the tap drill straight and then to keep the tap straight. It
worked out quite well to just use the clearance drill of one size to be
the tap drill size of the next one, with a couple of minor adjustments.
So, I can cover both tap drills and clearance drills for 8 sizes (#6,
#8, #10, 1/4, 5/16. 3/8 and 1/2) with only 9 holes.

I would appreciate any ideas on making those small holes straight,
Pete Stanaitis


Some thoughts

The axis of the drill/workpiece rotation must be concentric with the
first point at which the drill starts to cut. A twist drill has litle
sideways stiffness so it initially starts a hole concentric with this
initial cut point and then tries to drift to cutting concentric with
the rotation axis.

Standard jobber drills working without an initial centre punch
dot are pretty poor and may start to cut at almost any point along the
edge of the chisel tip. A centre punch dot provides a strong initial
centering force but, unless the dot is truly symmetrical and precisely
aligned with the rotation axis there will still be a residual starting
error.

For centre punch dot starting a good strategy is to chuck a short
piece of hardened drill rod with a 90deg conical point and press this,
slowly rotating, into the workpiece to form a depression at least as
wide as the drill chisel tip. This gives a well centered symmetrical
start but may not be easy with a pretty hard workpiece. A split point
drill is helpful because of its short effective chisel point length.

An alternative is to start the hole with a very short stiff drill
with the smallest practicable chisel point. NC spotting drills are
ideal for this method. The spotting drill can be much larger (and
stiffer) than the final hole size because it is only used to provide a
start for the final hole.

If it's possible to chuck the workpiece in a lathe rather than
using a drill press, the start of the hole can be single point bored
to near full diameter and this will guarantee concentricity with the
rotation axis.

With twist drills, once entered the workpiece, the straightness
is mainly determined by the symmetry of the cutting edges and the
uniformity of the feed force. The main effect of slighly assymetric
cutting edges is to generate a sideways force. This will cause the
drill to cut a little oversize but, if the feed force is constant and
the workpiece material is uniform, the sideways force will also be
uniform so the hole will still be straight. In practice both are a bit
variable and the variabilty initiates sideways force variations.
These start asymmetries and the drill will tend to follow the
direction of any initially small induced asymmetry.

To minimise this problem every effort needs to be made to
minimise the unwanted sideways forces. Things that come to mind:-

Well formed and symmetrical drill point
Sharp cutting edges
Split point drill
Easily machinable workpiece material
Light and uniform infeed - preferably by power feed

Let us know how you get on and whether any of the hints
actually worked!

Jim