jeff Layman wrote:

On 16/08/2020 20:01, williamwright wrote:
On 14/08/2020 13:36, Jeff Layman wrote:

The mass of the chuck inhibits the percussive action. The bit slips. It
just doesn't work.

Not sure about that.

As far as I could see, the chuck was rotating and I could hear and feel
the hammer action. Out of interest, I weighed the chuck - it was 390g.
Add around 30g for the 6mm bit and it comes to 420g. The biggest SDS bit
I have is 22mm in diameter (200 - 250mm long?); it weighs 340g. I can't
see that the 90g would make enough difference to affect the
percussive/rotating action. Bigger SDS bits (25/30mm x 450mm) would
weigh far in excess of the chuck weight, but they would work.


Have you noticed, though, how the hammer action is less effective with a
heavy bit? I used to have to drill through brick and stone walls very
often, and I found that if the material was hard and the hole was quite
wide it was quicker to drill as far as possible with a short bit, then
change to a longer one when necessary. The reduction in drilling
efficiency when changing from, say a 12mm x 150mm bit to a 12mm x 400mm.

I have no doubt you are right, although I've no experience of short vs
long drill bits. It makes sense in terms of kinetic energy. The short
drill will reach a higher hammer speed as it has less mass to
accelerate, and as kinetic energy is proportional to the square of
velocity, the shorter bit will have more destructive energy than the
longer bit.

Also, I don't think the way a normal chuck holds the bit doesn't convey
the percussive force very well, presumably due to slipping slightly. You
know how a friction fixing that is seemingly very tight can be adjusted
a fraction of a mm by use of a hammer? A normal chuck is only a friction
fixing, longitudinally.

I suppose the rotation would be affected if the bit slipped in the
chuck, but the bit would be touching the end of the chuck where it fits
in the drill body and the hammering would not be any different. Also,
some of the large diameter bits have smaller diameter shanks which fit
in the chuck, and the larger-diameter shoulder rests on the outer end of
the chuck, so again hammering would not be affected.

In the absence of any real understanding of the physics involved I am
pretty sure intuitively that the second metal-to-metal junction will
alter the dynamics of the hammer blow and make it much less effective.
You can see this happening when you place something between something
you are hammering and the hammer. I'm sure it's a real effect, though
the maths is totally beyond me. I think sort of bouncing comes into it.



--

Roger Hayter