While I'm working late into the night...and thinking...
as it's calm and quiet...
I thought I'd mention a few points about the machines
I've been working on.
I don't make these things because they're cheap. In fact, I
use the higher quality and more expensive parts for them.
Tiny little details make it all easier to assemble and be
considerably more reliable. Reliability matters when $$$
material is being cut for hours on end.
I make them because they make a lot of sense. In a lot of
different ways.
Also, because the more you do something, the better you get
at it. Even cheap MDF machines. maybe even *Especially cheap
MDF machines...
Most anything can be refined to very high levels. That's true with
an MDF machine that uses round rails.
One thing I've noticed...long ago...was that it's most often
Not the MDF that's the problem with certain designs. So, I
thought it was the round rails. What it can be is the combination
of the two. Solve that problem and it's a very strong and rigid
and reliable thing.
Sizes of the MDF are used within what limits they work best at...
from tons of trial & error. Same with the round rails.
A 3/4" Thomson round shaft is a very precise thing. It's also
amazingly affordable for the very impressive precision it's
made to.
At the max length for this BBX design of 24"...using 3/4" MDF
as the frame, IF...it's all carefully and tightly built with high
quality parts, it's very well within limits of doing Excellent
work. It's nowhere near as flexible as what it's often made
out to be. But it does matter a Huge amount "How" it's worked into
the design.
If that same exact case hardened precision shaft were welded within
a thick steel frame, suddenly, it's not even flexible enough to be
called flexible. Else many hydraulic equipment/machines worldwide
would have been failing so long ago as to not even exist today.
But...we can't weld it into an MDF frame. So the next best thing
is to support the ends as well as they can be within such a frame.
(Tensioned together, there's is no combination of screws, glue, nails,
bolts of any kind, or all together that will equal the stability
and strength of the tension rods for the MDF frame.)
Without the relatively thick 1/4" plates on the outsides of the
MDF frame, if the tension rod nuts are tightened too much, they can easily
dig in to the MDF. Even large fender washers will deform and sink in.
So we need something that can't sink in. It needs to be so strong that
the rod or nut will strip before any problematic deformation of the
MDF can occur.
Of course, we don't want to strip it, but when that is a concern, then
MDF suddenly isn't such a weak flimsy material. It can actually Not
matter if it's MDF. Which is good, because it's cheap N easy to get
and very easily worked.
When the Metal end rail bushings are used, The MDF is sandwiched between
and again, it's so strong the bolt could be stripped.
With the plastic bushings, the nut can be so tightened that the plastic
will easily deform...and there's where some of the flexibility of
these type machines comes from.
If someone pulls back and forth on a stop sign long enough, it'll give.
If it's embedded in concrete, the pole with shear off at some point.
If it's just in a hole in the dirt, the whole thing will fall over
much easier and quicker.
The MDF machine is much more often comparable with the sign post in
the dirt hole. A mud hole in many cases...
But, tiny little un-thought of details can work to make the MDF/round rail
machine so strong and stable that it's a sensible thing to build and use
...and rely on.
The shaft doesn't want to deflect. It's a thick solid steel spring at worst.
Very hard to bend it. If it's well supported and not overly long for
the design at a given size, it's good for the long term.
Flexibility adds up in several places before it even gets to the shafts.
Wood tool mounts are the beginning point. Nuts trying to dig in a tiny
bit at a time are the second point. There can be an easy 50 thousandths
of an inch right there. Very easily. Getting completely overlooked.
Remember, the machine is rocking back and forth and wants to come apart.
Like the stop sign, weak points give a little at a time. It's very
rarely the shafts that have anything to do with it. Unless they aren't
supported well enough, but even then, that force is transferred to the
nut that can dig in if possible. Then it's suddenly looser and flimsier
by the minute. The shafts are acting as levers.
But, if it's understood what is happening to cause such problems, and
if the problem area can be made to matter less, if at all, then the
simple design becomes possibly up to 10x stronger. Enough that excessive
pushing of the machine actually can flex the shaft. But in all reality, the
shaft couldn't go far enough to matter on it's own. It's extremely resistant
to being deflected.
In which case, the limits of the machine are clear. No harm done.
Well, gotta get back to work, but I wanted to make a point that is absolutely
is possible to build a solid and reliable machine with amazingly inexpensive
and easily sourced material. Reliable matters. Simplicity contributes
Greatly to reliability. This simple frame with withstand lots of hard work
and mistakes.
I doubt it can possibly get any simpler for what is gained.
Thinking out loud. Again...
John
Reply with QuoteOriginally Posted by doorknob
