[JRoque]

Hello, I'm hoping this is the right forum to post this. If not, hold on to your keyboards while the moderator moves us.

After a few months off, I'm now wanting to return to my CNC project and continue improving the machine. The next stage is to replace the temporary MDF bed with a steel or aluminum one. My machine bed if 54" wide x 63" long. I'm leaning toward a .25" thick A36 steel slab mostly due to it's price. It comes out to about 260 pounds that on one hand will be a killer to mount but on the other will soak up vibration and give it a solid, flat cutting base.

What is everyone doing for their machine bed?

Thanks,
JR

[Ken_Shea]

JR,
I am having difficulty envisioning any thing that size and only 1/4" thick being any where near flat, what are you mostly cutting ?, MDF may not be that bad a choice if you are cutting thin metal or wood.

There are I believe different density's of MDF, perhaps a better grade and then face mill the entire surface flat.

EDIT:
If you are wanting to run coolant then MDF may be a problem

Ken

[JRoque]

Hello Ken,

that size and only 1/4" thick being any where near flat

That's also one of my concerns. I was assuming it would be close to flat and then grind any imperfections with the router itself.

Yes, MDF has worked fairly well so far but I wanted to move up with a more solid bed. I guess if there's "Medium Density Fiber", there should be "High Density Fiber" also but I haven't come across it. I want to cut everything and that's a problem. Cutting aluminum or wood hasn't been a problem for the most part except when MDF flexes downward during plunge. The biggest, and unexpected, problem has been milling PCBs. When the difference between a copper cut and not is 1 or 2 mils having a solid foundation is key. Of course, if that foundation is not perfectly flat that can also cause issues. I recently improved PCB milling by inserting a sheet of aluminum in between the base MDF and the sacrificial MDF under the PCB. So I figured if that worked that way, why not having the entire bed be made out of metal?

So, what do you have for a base on your machine? I'll be checking people's galleries and see what's out there.

JR

[ger21]

I don't normally work with steel, but I've built some bank desks that used 1/4" steel plate as a bullet resistant panel. I don't know if it was the same thing, but what we used was far from flat. I've also bought some 1/4" aluminum plate which was similarly bowed.

[Ken_Shea]

My mill is a commercial unit using some type of cast iron.

Can you increase the thickness of the MDF?

[JRoque]

Hi. The plate would be bolted down to the machine frame which might help flatten it. But point taken, I'll give MDF another shot. I'll start by adding more 80/20 cross beams and securing the bed to the frame a little better.

I'm all ears if you have any more suggestions but again, thanks for your help!

JR

[ger21]

If you're looking to eliminate the flex, build a good strong wood torsion box. Check out my thread. http://www.cnczone.com/forums/showth...?t=1356&page=1

[Geof]

A plain hollow core door is a very good torsion box. Get two 27 inches wide, cut to length and recap that end, glue together and surface with mdf. Very light very rigid.

[jeffs555]

Steel is much stiffer than MDF, but if you replace thick MDF with a thin piece of steel you may not gain anything. Deflection varies directly with the elastic modulus of the material. Steel has an elastic modulus maybe 60 times higher than MDF, so if you replace 1 inch thick MDF with 1 inch thick steel it would deflect 60 times less. The problem is that deflection varies with the cube of the thickness, so if you replace 1 inch thick MDF with 1/4 inch steel, you haven't gained anything (4 cubed = 64).

PS Those must be some monster circuit boards if you need a 54x63 bed to mill them on.

[jeffs555]

I agree with Gerry and Geof about the torsion box. However, for a fixed base, weight is not a problem like it is in a moving gantry. Mass is good in a base because it dampens vibrations. You could gain a lot of stiffness by just going to much thicker MDF, or using glue to laminate several thinner sheets of MDF. Like you said, you can use the machine to mill the surface flat, so it doesn't matter if the material is flat, as long as it is stiff and dimensionally stable. I haven't finished my machine, but I am planning to use a piece of a large old office table. It is 2 inch thick MDF with formica on the top and is extremely heavy and stiff. I hope the formica is thick enough that I can mill it flat without cutting through the formica.

[ger21]

I suggested the torsion box because he mentioned deflection. To add weight, use 3/4" skins, and then add a second layer of 3/4" MDF.

Jeff, the Formica on a countertop should be about .05 thick.

[jeffs555]

For milling pcb's many of the articles I read talk about using a spring loaded head with a teflon depth stop that slides along the board to get precise cutting depth. Here is one article. http://pacificsun.ca/~robert/Gerhard/mill.htm

[JRoque]

Great ideas thanks. I'll add 80/20 beams using this principle.

PS Those must be some monster circuit boards if you need a 54x63 bed to mill them on.

LOL. The largest PCB is 3" x 5" - long story. Hey, my wife could be reading this.... it's a well known fact that that large of a CNC machine taking up half the garage space is needed cut even small PCB.

JR

[greybeard]

My own project uses a bed of aluminium extrusions bolted down onto a piece of kitchen worktop. This is a wood particle board 39mm thick with a formica layer both sides. However I'm seriously considering concrete for my next. Has anyone done this, or have any ideas on its possibilities ?
On another thread, polycrete type products have been mentioned, but I just want to consider casting a simple slab with rail mounted onto moulded tracks.

[jeffs555]

The problem with concrete is that it is very strong in compression, but not strong at all in tension. In bridges and buildings it gets all the tensile strength from the rebar embedded in it. With the proper reinforcment it might make a workable base.

[greybeard]

Does it need that much tensile strength ? I was assuming that it would be supported,
either on brickwork piers, more concrete, or even cast onto the ground.

[Geof]

I think you could make a relatively lightweight frame using steel angle bolted together with your rails mounted on spacers a few inches above the frame. Place the entire frame in a shallow box, level it up carefully and then cast it into a self level topping compund such as this http://www.epoxysystems.com/1004.htm

[JRoque]

Ok cool, after we all get our bed/table answers, I'd like to post my next related question.

Say that I decide to use 80/20 extrusions and bolt a 3/4" MDF on top for the bed. What do you guys recommend to use for work holding? I was thinking of drilling holes and tapping threaded inserts in. I figured if I make or buy slotted holding clamps that are, say, 5" in length, I would place one of these inserts every 4.5". That's a lot of inserts! Is this how is done?

Thanks,
JR

[ger21]

I'd use T-nuts inserted from the back of the MDF. Threading inserts into MDF would be difficult, and a pita.

[jimbo]

what about t-slot track?