Cutting your own racks, while doable is a heck of lot of work for the
lengths you are talking about, even if you make it in short sections. If
your time is worth even a fraction of minimum wage you would be better
off using off the shelf rack and pinions.
Here is how I think I would go about gluing a stationary belt in place:
Fit the belt with clamps at each end that can pull it tight. Now fit a
temporary solidly mounted pulley on the gantry that engages with the
belt. Run the gantry back and forth to make sure there are no tight
spots and adjust the pulley as needed for the best engagement.
Remove the belt, apply a high viscosity urethane rubber adhesive such as
'tiger seal' or 'sikaflex' and refit. You probably want a helper when
doing this. The adhesive will push the belt away from the mounting
surface a bit but don't worry about that. Now run the gantry back and
forth a few times. The pulley will push the belt into place, squeezing
out any excess adhesive. Be warned this stuff can be messy so have
plenty of disposable gloves and paper towel to hand to deal with the
excess. Petroleum based solvents will remove it when it is wet. It's a
nightmare to remove once cured.
You'll probably have to move the gantry back and forth a few times as
the adhesive sets (say once an hour for a day). This should leave you
with pretty accurate belt alignment, even if the mounting surface isn't
perfectly aligned. Note I see that open ended belts tend to be
polyurethane which is difficult stuff to glue reliably. Even
polyurethane adhesives don't stick that well.
Thinking of alignment, a system like this could be made pretty tolerant
of alignment issues. The whole roller+pulley assembly could be spring
loaded to keep the two belts tightly clamped together. Spring loading
doesn't work well on racks because a spring that is strong enough to
prevent the pinion from pushing itself out of engagement will cause
excessive wear and noise. However due to the way the belts engage, this
belt system should be able to handle considerable pressure without undue
wear.
One issue I see with off the shelf belts is that none of them have a
symmetrical profile. T section looks symmetrical but if you check the
drawings it isn't. This is a big problem with trapezoidal sections as
they will either have a lot of backlash or not mesh at all, depending on
the profile. However I have some HTD here and I think it could be made
to work. In this case the round parts of the teeth would mesh, giving
you a 45 degree pressure angle. As long as enough preload is applied to
prevent the belts slipping out of mesh this would result in a very low
backlash setup. The only backlash would be the teeth distorting under
load. When they mesh there is a gap between the tips of the teeth and
the root of opposing belt. This could be a good thing in a dusty
environment. Dust and dirt should be squeezed into these gaps and
hopefully would then fall out, much as it does with racks. HTD8M30 belts
are easily available up to 2.8m long. After that they seem to only be
available in open ended which is not that easy to obtain.
Dammit, you now have me thinking about building another plasma cutter. I
already have too many projects on the go...
Les
On 23/04/2019 16:24, Leonardo Marsaglia wrote:
My idea for the "racks" is to make a fly cutter in the CNC lathe and then
use the same lathe to fly cut the teeths, mounting the aluminum rack on a
simple jig on the turret.
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