are postings from the Fireball CNC forum that have an influence on my
design decisions for the CNC router/
Re: Help with table flatness,
Posted by: "cnc739" email@example.com cnc739
Thu May 27, 2010 5:01 pm (PDT)
The very best way to handle it is to use layers.
length of the table (the V90 table) is at the limits of what a piece of
3/4" MDF should be. The machine is built as large as it can be without
problems due to MDF elements in it's construction. The smaller pieces
like the gantry will remain stable flat and rigid pretty much
indefinitely. But the table is really as long as it should be without
support. It works very well 99% of the time. It's really intended as an
expendable base to be built up and customized to a users needs. A
second layer should be used.
2 layers of MDF at the size of the
table are stable and rigid enough to remain so for a very, very long
time. It'd be hard to flex the 2 layers....where it's not too hard to
press down on a single layer and see it flex downward a bit. It's
generally rigid enough to spring right back, so it's rarely any problem.
The trick to it is getting those two layers together so they're both
flat to each other.
bolts in them isn't a best way to do it. Both can flex as the bolts are
tightened and the problem is still a bowed table.
Using T nuts to
bolt the two pieces together can bring them both into flatness very
easily. That 2 layer sandwich is strong enough to park a car on....but
not recommended while it's on the machine ---> ;)
T nuts have the
advantage of acting as washers...preventing any possible "pulling
through" the MDF....like a bolt/nut combination can. They also remain
in a fixed position...where the nut/bolt can and will shift as it's
The second layer is tightened down using those t-nuts on
a known flat surface. They pull themselves together into as near
perfect flatness as is possible. If 6-8 t-nuts are placed in the bottom
layer, and the second layer is attached....Flat and Stable. Good for
the long haul...:)
It's a good idea to arrange more t-nuts into that
second layer while it's all being done to use to bolt down an even
third layer. That third layer can then be easily adjusted with the
paper shimming I often mention and it's easily done. As the third layer
is often much smaller than the original bottom
layer, it's very easy
to do. I use several different third layers as fixtures for different
jobs/projects. The closer the work is the the bottom of the Z, and the
shorter a bit can be for a given job, the more rigid and stable
everything is. Even aluminum can be carefully milled with all is in
balance....(though not officially recommended...;)
Just be sure to
make layer #2 in such a way as the 2 layer sandwich can be easily
re-attached to the machine...access to the bolt-down holes. The second
layer doesn't need be the same size as the bottom layer.
I have found waxing or, like someone mentioned...latex paint, works
very well to seal everything.
a little care and thinking it through, the layers will solve 99.9999%
of table/flatness problems. 2 layers are very strong, rigid and flat.
Posted by: "cnc739"
Thu May 27, 2010 6:08 pm (PDT)
so often mention using t-nuts....I thought I'd mention how I use them.
I've seen people just drill a hole and try to hammer them in. Not good
You can get them at Lowes, Home Depot and a variety of on-line hardware
have 3-4-6 prongs. the 3-4 prongs work better than the 6, as we're
going into MDF and 6 is a little too many at the sizes we're needing.
The way to install these.(for use with 1/4-20 machine screws) is to
locate where the holes need to be.
(1) From the top.....drill, all the way through, a 1/8" hole. (This
1/8" hole acts as the pilot hole for all else to follow.)
Using a transfer punch, (even if you have to make up a quick homemade
one), will greatly improve exactly *where* a hole
located.... Cheap transfer punch set...(without doubt...Indispensable
for making anything that needs holes in the "right place")....
From the back....drill down into the MDF about an 1/8 of an inch using
a 7/8" spade type wood bit. This makes the base of the t-nut flush with
the MDF surface....and also provides a "bite" into the material. (The
1/8" pilot hole guides the spade bit perfectly.)
hammering these t-nuts into the surface is not good. The surface of MDF
is planished and hardened a bit. The T nut will fall out with a little
vibration. The teeth need to have something to bite into. 1/8 into the
surface of the MDF is about as ideal as it gets. If it's in
exact right place...a few drops of super glue can hold it even further
if need be...but rarely needs be...;)
(3) Next, drill all the
way through with a 5/16" regular old drill bit. Drill down on an
expendable scrap MDF surface and the bit will make a clean hole as it
goes through the first layer. Else...it'll surely "punch
through"....Clean, straight holes in the right place are always a
(4) Press the T nut in place, and using a short 1/4-20
bolt/screw with a washer..from the top....pull the t-nut into place.
The washer will act as protection to keep the bolt itself from tearing
into the material.
It takes a little bit more time to do it
right, but you have holes that can be used indefinitely to attach any
jigs/fixtures etc to the table. If one were to ever strip...just punch
it out using a gentle hammer tap with a long bolt screwed into it.
I hope that's useful. T-nuts are really and truly the very best way
I've found to repeatedly bolt things down. And even for permanent uses,
they're still the best option for use with MDF material. I think I
tried everything at one time or another....;)
Posted by: "Gaston Gagnon" firstname.lastname@example.org gaston_gagnon
Sat Jul 31, 2010 9:29 am (PDT)
mentioned before that the first line of defence against dust spreading
all over is to keep the spindle from blowing it up in the air in the
Look at Exhaust Deflector for an idea here: http://public.fotki.com/Gaston-Gagnon/fireball-cnc/dust-boot/
That makes it a lot easier on the vacuum to catch most of the fine dust and some of the larger chuck of flying stuff.
gizmo's could also be used favourably by those who gave in and built an
enclosure ;-) . For them, it could help avoid some of the dust to
recirculate back trough router's ventilation system. I use a Ridgid R2400 but the same could be used for other routers.
I also redesigned my dust boot to put the tool at the centre of the vacuum tube.
Look at Boot Version 2 here: http://public.fotki.com/Gaston-Gagnon/fireball-cnc/dust-boot/