Postings from

Fireball CNC Forum

These are postings from the Fireball CNC forum that have an influence on my design decisions for the CNC router/

Re: Help with table flatness, please.
Posted by: "cnc739"   cnc739
Thu May 27, 2010 5:01 pm (PDT)

The very best way to handle it is to use layers.
Here's why....

The 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.
Putting 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 a bolt/nut combination can. They also remain in a fixed position...where the nut/bolt can and will shift as it's tightened.
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.
With 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.


Using T Nuts
Posted by: "cnc739"     cnc739
Thu May 27, 2010 6:08 pm (PDT)

I 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 for MDF.

You can get them at Lowes, Home Depot and a variety of on-line hardware suppliers.
Some 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
gets located.... Cheap transfer punch set...(without doubt...Indispensable for making anything that needs holes in the "right place")....

(2) 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.)

Simply 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 the 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.'ll surely "punch through"....Clean, straight holes in the right place are always a joy...:)

(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.

So, 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....;)


Dust revisited
Posted by: "Gaston Gagnon"   gaston_gagnon
Sat Jul 31, 2010 9:29 am (PDT)

I 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 first

Look at Exhaust Deflector for an idea here:

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.

This 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: