Hi folks! Welcome to part seven of our fixture recap series today we're focused on thin
parts--what do I mean by thin parts? It means parts where you can't use a traditional vise
or clamping matter by squeezing the part together because it would flex so we've got to use things
like direct clamping down onto a subplate of some sort super glue and vacuum first up we made this
powder coated sign back in widget 168 classic example of using a vacuum work holding system a
couple of great rules of thumbs if you're new to vacuum work holding you get about 14 pounds per
square inch of work holding power if you multiply the surface area or the length times the width
for a simple rectangular shape times 14 that gives you an idea of about how much pressure is holding
down on it so as a quick example you have a six by six inch part that's 36 square inches times
14 pounds per square inch means you could think about having a 500 pound weight sitting on top
of your part holding it down just gives you an idea of thinking about your clamping pressure
when i think about vacuum work holding with a six by six inch part i know i've got pretty good
work holding power smaller than that you do have to think about your cam strategies how
aggressive you're cutting on and even the helix angle of your end mill so it can be done but be
careful on this project we used a sharpie in the spindle to lay out where the final part would be
that way we kept the gasket inside that profile while also maintaining the maximum surface area
inside the gasket because remember it's only the gasketed area that counts towards that
14 pounds per square foot and then we did another classic vacuum trick here which is putting a piece
of sandpaper between the vacuum chuck and your part and this does two things number one it lifts
up your part by the thickness of the sandpaper something like five to ten thousandths of an inch
which is really helpful if you're trying to do a profile pass around the outside of your part at
full depth without machining into your really nice vacuum chuck other thing it does is it provides
some lateral stability that sandpaper acts as an additional form of grip because vacuum is really
good at sucking things down but it doesn't have the kind of lateral stability that you wish it
might have and the sandpaper really helps with that that sign is almost five years old and it
still looks great in widget 138 we had to make a pair of these wheels we spent a good amount of
time in the first part of the video talking about the cam strategy how we were going to fixture them
program the toolpaths and in this video we also started addressing the use of cutter comp but when
we start the machining process around 16 minutes we're showing off a classic trick or hack when it
comes to vacuum work holding which is if you have to make multiple parts leverage the additional
surface area of having a larger piece of material to make those parts make sure you don't machine
all the way through or else you will immediately lose all your vacuum work holding but this is a
great trick when you need to make a small part in the next video widget 139 we talk about op
2 for those same wheels now our original plan was to machine this fixture which we did and use that
center hole as a port to hold down these parts with a vacuum it's a great workflow because the
vacuum pulls down on your part the fixture itself provides that rigidity that we just talked about
where sometimes vacuum doesn't have that lateral stability and this fixture would solve the problem
of needing to clock the part for op 2. ultimately it failed the vacuum just didn't work we didn't
have enough surface area here we instead realized that the final part itself had a tapped hole in
that center hub so we used that tap hole in that fixture you could have also used super glue but
it's still a great technique that absolutely can work and it's worth keeping in your toolbox here's
another great example just a photo of a part i saw once where they had a really cool custom opto
hybrid fixture so it's a vacuum fixture machined into this block of aluminum that has a number of
locating features to hold this pretty complicated taper angled apart for a final operation so you
can see the vacuum port coming on on the left side they've machined this honeycomb pattern to allow
the vacuum air to flow through they've got two different gasket sections and it's hard to tell
from this photo but you've got a tapered wall on this front face and some locating features
on this top side we machined these parts out of a ceramic material called macor back in widget 170.
one of the most difficult parts i've had to make maycor is not a fun material to machine
you can't actually cut it but it tends to chip and these parts were very small so we're making
use of that part density by machining a number of these in a larger sheet of the macor material
we were very deliberate in this video and spent a lot of time walking through
the fusion 360 toolpaths of how we were roughing out profiling and where we wanted to leave tabs
and where we wanted to leave stock to leave making sure that we were certain to not cut through the
floor of that material such that we would lose our vacuum but also come as close as we could to
producing the finished part in a reliable manner if at all possible when doing this type of work
your z height level can be very critical and it's really helpful to first deck your vacuum chuck
so that you know your z level is constant the plane that you're machining on and holding on is
perpendicular to your spindle because even a few thousands of an inch variation across a relatively
large area can cause you to lose your vacuum and then it's game over next up isn't a specific part
but rather the tatron neo this vacuum system absolutely blew our mind it uses a vacuum pump
with a semi sacrificial sheet called vacuum cart it's a directional material and what that means is
we could set a part on top of this vacuum card that also like the sandpaper gave us a little
a little bit of a buffer that way we weren't machining through the vacuum card and into the
table and it had this directionality to it which meant that if and when you either intentionally
or accidentally say machined a through feature or all the way through the periphery of the part you
didn't lose all of your vacuum you might lose some depending on how far you went through or how large
of an area you expose but the machine was aware of that it was monitoring the vacuum pressure and you
could adjust that threshold such that if you drop below a certain level the machine would actually
feed hold for you we made so many parts on this including a number of cobit emergency parts back
in spring of 2020 polycarbonate sneeze guards and so forth and it was an absolute game changer
that opened our eyes to just how amazing this work holding system is we could switch between
small parts large parts we have a saunders machine works pallet that we could drop onto it which we
just held with vacuum that gave us a mod vise or other traditional workholding technique to it
kudos to them this is an absolute phenomenal system next up in widget 260 we need to machine
some relatively large parts for johnny five and we're doing the classic technique of super glue
with the either painters tape or in this case the high temperature powder coat tape
the takeaway here is that you don't have to have complete tape coverage in this case we were just
putting down strips provided plenty of work holding rigidity for these sheets of material
sometimes actually makes it even easier to remove the part when you're done you can save some money
over time with the cost of tape and by the way we'll have a whole video dedicated to super glue
but if you are looking at using the super glue technique using this high temp powder coat tape
has two major benefits number one it's more temperature resistant if your part gets warm
number two it's much more coolant resistant and finally one of our oldest videos
from 2014 we're machining a large aluminum plate sometimes the best thing to do is just directly
clamp it down to your either machine table or in this case a fixture plate strap clamps work
great for that but don't hesitate to get creative add and move clamps as you need to to make sure
you maintain adequate clamping as you machine or work the part don't hesitate to complement
those clamps with something like super glue or even vacuum and that's actually our final note
coming back to vacuum one of the best things you can do to increase the process reliability
of vacuum especially on tricky parts is add a simple even single strap clamp or other
mechanical fastener of some kind we found that massively increases the reliability around vacuum
especially for those tricky to hold parts as always folks hope you learned something hope
you enjoyed we'll have all these videos listed out over on nyc cnc otherwise take care see you soon