Inertia will really depend on whether the OP’s bed moves or is stationary. Looks like it is stationary with the gantry moving along the bed.

If you are cutting right through you will want a sacrificial mdf topper, and you will start by machining that flat in situ If the bed is fixed a lower level of kitchen worktop will add mass and is fairly flat as a starting point.
For machining steel I would then add a temporary smaller sub-bed of Al perhaps 1/4″ and again machine flat in situ.

One of your problems is likely being able to find a piece of steel large enough yet with a good enough surface to use as the bed/table. Easy enough to get hold of hot rolled but really you would want a better surface than that.

I suppose it would be possible to mill it flat but that would only cover the 600mm x 400mm cutting area and leave a ridge around the edge.

One option would be to look out for a second hand cast iron surface plate 2ft x 3ft which would give the overall area needed, a flat surface and unlike stone would be easy to drill for a series of tapped holes to use for clamping work, vice etc. Also not affected by any coolant that could soak into say a kitchen worktop and cause it to swell.

To make a flat concrete base cast it upside down onto a piece of float glass bedded on sand and levelled with a good machinists level. (not  to get it horizontal but to get it flat corner to corner). Then bond say 10mm aluminium onto it that has been drilled and tapped for a matrix of holes. Then machine it flat with the cnc itself so actually the concrete doesn’t need to be that flat.

However if you are spending more than another hundred pounds on this it is going to be better to back track and convert a real milling machine.

Look out for a redundant catering griddle, usually gas. 16mm or 20mm thick steel,  a useful lump for all sorts of things – welding table Etc. often just dumped. Noel.

On 16 May 2024 at 10:41 Steve355 Said:

Shear Modulus of Rigidity, price, for 729×480 mm sheet

Mpsi.    Gpa     Price   Thickness

cast aluminium              3.85    26.56    £462     20mm

Cold rolled steel          10.95.     75.5        ?           ?

hot rolled steel                 ?          ?       £391     15mm

Interesting!

I can’t seem to find rigidity stats for hot rolled steel. It may not be stable or flat enough for purpose…. BUT this is a router, not a milling machine.

Stiffness of ‘steel’ is pretty much constant (Young’s modulus). Hot rolled will be more stable than cold rolled, but neither will be flat enough to use as a bed without machining (IMHO). If you machine cold rolled it will bend.

Properties like damping are important, too.

Generally its purpose will be cutting out shapes and engraving. Work will be held on a piece of sacrificial plywood.

In which case, forget metal and use a piece of thick MDF. You can use the machine to surface it.

If engraving you will want a flat surface, I have engraved on my CNC mill to a depth of 0.06mm so if you don’t have the work flat then the depth of your engraving won’t be consistant.

It can be got over by adding a sacrificial board or plate and milling that flat first, then fix the work to that but starting off with soming flat to start with is an advantage.

I did have a look at that brand of machine as don’t think my surface plate idea will be that good as you need the plate to allow the frame and rails to go below it. I also think a lot of the vibration etc is likely to be coming from the structure not just the bed.

Flimsy clamps as in your photo don’t help and with them just being at the edge your board is likely to flex in the middle. Better to drill some holes in that MDF and use several counterbored holes so it can be held down with screws at regular intervals. Additional holes right through the board to allow clamps in the tee slots to be used nearer the work to hold that down will also help rather than a lot of what I see on routers which is a bendy bit of flat bar and a wood screw into the MDF. Engraving can use two layers of tape and superglue to hold the parts but you say you want to cut the sheet in which case a more solid hold is likely to be needed.

While on the subject of MDF, the DIY shed bought stuff is not that dense, get hold of some MR MDF (Moisture resistant) as that is 50-70% denser

If you can “cast” the infill with the steel level and flat then best option would be to then flip it over and have the steel at the top. All threaded inserts will then be pulled against the steel rather than the epoxy. I suppose doing it while laid on the existing bed would be the flattest option

Set some threaded inserts in where it bolts to the frame below so they make contact with the frame rather than the infill which is unlikely to self level to any accuracy.

On 16 May 2024 at 22:37 Steve355 Said:

Ok, I will row back a bit on the “it’s a router” statement. That’s it primary purpose initially,  but I’m sure before long I’ll be wanting to do more ambitious things with it.

I was reading today about epoxy granite. It many ways that sounds ideal. But I think I have 1” of depth to play with.

I need to decide something!

The 1″ thickness is a limitation of that style of machine (where the bed has to span the length of the Y axis) and will ultimately limit the stiffness you can achieve. Stiffness goes up with Young’s modulus and the cube of thickness, but comes down with the cube of span for that configuration (hopefully your Y axis is the shorter travel).

I suspect that epoxy-granite has a lower Young’s modulus than aluminium (but don’t know of any definitive data – its properties will obviously be highly dependent on the raw materials and method of preparation).

Rather than trying to cast something, you may be able to obtain an off-cut of granite worktop in a suitable size. It would also be substantially flat. You may be able to drill it and glue in some threaded inserts for work holding – I’ve not tried it though. If you choose to laminate a layer of metal onto it, do both sides so that the structure remains symmetrical for better stability.

If you want to go the whole hog and cast a base, then consider reconfiguring the machine with twin Y axis drives so that everything can be mounted on top of a substantial base slab with (effectively) unlimited thickness.

I’d also echo Jason’s comments about the rest of the structure – every part of the machine needs to be stiff for the whole machine to be stiff.

From what I recall about epoxy granite one casts it into a box with threaded inserts from the bottom, then once cured pour a thin layer of low viscosity epoxy over the top and leave to set to provide a level surface.  I think that may have been covered in John Macnamara’s build threads, which appeared both here and I think on https://www.mycncuk.com/

If using a granite worktop offcut, beware that some of the compounds used contain silica and the dust can cause silicosis – apparently there is a mini epidemic of this amongst kitchen fitters and people machining the material both here and in the US.

Also I think both McNamara and someone who wrote on stiffening an X1 mill in MEW mentioned that straight concrete isn’t a good idea because it can warp once set as it cures.  Not an issue for buildings but not good for machines!