Re: Subject: not stiff enough!

Hi Richard et al

>I built this cncrouter:
>http://www.spellingbusiness.com/robot.jpg

Seems like great minds think alike - you didn't perhaps see mine first did
you :-) If not, have a look at my MDF CNC machine at the weblink below. Your
workbench seems to be to the same industrial standard as mine BTW :-)

>X is 1 1/4" shafts, 60".
>Y is 1" shafts, ~40"
>Z is 1/2" shafts, 18"

This setup is very similar to mine, except that you don't seem to have any
end support for your shafts or leadscrews.

>The problem I'm having is that it's not very stiff. I realized there would
>be some flex in the unsupported shafting when I built it, but I didn't
>realize it would be as bad as it is.

The shaft ends on mine have 60mm square by 15mm thick aluminium mounting
blocks - bolted at the corners to the MDF - only slight end float of the
shafting is visible, which is reduced by a couple of big split rings - I can
__see__ no lateral flex (but it must be there presumably :-) End results of
cut timber - ply/balsa/mdf/pcb etc are measurably ok (for timber that is -
I'm not looking for 1 micron flatness, or orthogonality to 2 milli radians
:-)

>I have 3/4" for Z, which would take care of some of the stiffness problem,
>but I can grab the top of the top of Z and move it by hand and the 1" shafts
>will flex visibly.

See above.

>1) use the 3/4" shafting for Z
>2) move the Y shafts further appart to reduce leverage

snip snip...

Leave it all alone, you've probably spent many happy hours lining up the
axes so that they don't bind :-)

Just lock the ends of the shafts with mounting blocks, and fit a similar
setup with thrust bearings to the ends of your leadscrews to stop any end
float (you ___can't___ depend on the end float on your stepper, and if you
have also fitted flexible couplings, keep them (although if you had no
thrust bearings fitted, this is another source of oval circles :-)

On the 1/16" router bits, I regularly use 1.5mm down to 0.8mm router bits on
my system - (a Bosch POF 500a dismountable router) with excellent results -
___BUT___ remember that the bit end is flailing about at (say) 30KRPM, so
that 1mm bit is probably nearer to 1.3 mm in diameter until it is let into
the timber. If you have an option to start in slow, and ramp up into the
cut, then you will get a much better cut. Do multipass also - on 3mm birch
ply, I do 2 passes (I can do it in 1 at 1.5mm, but it ends up a wee bit
rough).

The bearings on my X & Y are plain old bronze (3/4"), the Z uses linear
bearings (1/2").

Working area is 20" * 19" (it's also all currently for sale if anyone is
interested - I've started a new one, and don't have the space!)

Cheers

Neil

PS Failing all this, you can always try dissolving viagra tablets in
alcohol, and painting it all over your __machine__ for added stiffness
(nearly said tool there =:-)

Neil D. Gillies
SeaGull Technologies
11 River View
DALGETY BAY
Fife
Scotland
KY11 9YE

Tel: +44 (0)1383-823489
FAX: +44 (0)870-0543622
Email: neil@...
WEB: http://www.sea-gull.demon.co.uk