Building a EG or buying a milling machine

Hello everybody,

I am in the thinking of either buying a milling machine and then doing the cnc conversion or directly building a cnc expoxy milling machine. Before doing so, I read a certain number of articles and videos for the latter one and still finding myself having a lot of queries and mysteries for it. It is especially true for me as well as most machinists who didn't come across this types of material properties, mixing compound process and precision molding and constructioning. So, would any experienced ones can help to answer and will be greatly appreciated:

1)Some articles said epoxy granite material is zero in tensile strenght and excellent compression strenght. Is this true? Or, do the structure of this material need steel construction for reinforment?

2)What is coefficient of expansion of EG? Will it be same as steel, so by molding steel beam inside it, they will expand and shrink at the same rate. How about their length dimensions after a long period of time of curing/drying?

3)For lessening in price, I also want to use concrete(ie, a mixture of cement and sand etc.)instead of using EG. By putting steel I-beams inside. In the same building configurations for the two materials to compare, how much big differences between them in view of rigidity strengths and vibration damping?

4)I saw some people successfully buiding EG machine but of a size not big enough. What is the reason for it? Can it be built bigger, something like X-travel of 600mm and Y-travel of 400mm?

5)In order to have true flatness of EG surface for mounting rail guides, they are to be molded very flat with the help of a large pricely precise granite table or machined precisely afterward. Can I do it in a rough manner? After molding, then doing the shime process and using the paste for rail guide to fill the gaps between them and the EG surface. Will this paste stick equally well on EG as on steel?

6)There is also one advantage of building a EG than buying a milling machine in market, I will save a lot of transportation cost of a heavy machine because in building EG machine, some mixing ingredients of EG and steel construction materials may be bought locally.

Thanks in advance!

It is certainly possible to make the machine with 600 x 400 mm travels.
It will need to be about 2500-3500 kg in mass in EG/concrete.

X travel:
The problem is 400 mm travel in x vs 600 mm travel is 50% more.
=> 1.5 x 1.5 x 1.5 = 3.3 times less rigid on x.

Y travel:
200 mm vs 400 y, = 2 x 2 x 2 = 8 times less rigid on y.

3.3 x 8 = 27 times less rigid total.

Further to this.
The beam that supports the spindle, must also now be 27 times stiffer / more rigid.

Stiffness is column size cubed.
So a column 3 times bigger = with a section size 3 x bigger = 3 x 3 x 3 = 27 times more rigid.

Say the beam on an X3 type hobby mill == 150 mm x 150 mm, or 150 mm section size.
It now needs to be 3 x 150 mm = 450 mm in both x and y for the same relative rigidity.

As an example, the column on a very big Bridgeport Series 2 might be about 450 mm, and they are about 2000 kg in mass.
Cast iron is stiffer than concrete/epoxy granite, about 1.5 - 2 - 3 times, depending on how much steel You have in the column.

The real difficulty comes from the Y axis depth of 400 mm, which is why commercial old manual mills all had short travels in Y.
Or were massive, with == 400 mm columns (BP) or rams like the bigger Deckel FP series mills.

Otoh, a moving-table design is inherently stiff, and thus easier to make large.

No matter what, a 600x400 milling machine will cost more than 10.000€ in components and stuff, wholesale.
Because materials-and-stuff also goes up in price as a cube of the work area.

That's really a huge amount of material needed to be used!

BTW, if heavy duty steel frame to be used as well, the amount of them will be reduced drastically. As I visualised, at the moment, EG doesn't help much in providing sufficient strenght and rigidity of the machine, it does to help in damping the vibration. My hopefully feasible plan is:

>three pieces structures for the overhead bridge. Steel H-beam of 1/4" thick are used, with bolts or hole inserts pieces were welded on them first and poured them with EG.
>Base platform will do the same which composes of 4 to 5 pieces steel beams
>Looking like simple, but thinking that the tuning of squareness and flatness geometrical shape are the challenges. If not having a reference surface plate, a least to have a sufficient long straight and flat piece...

The H beam as mentioned is a bad idea.
It is not flat, and is not straight.
It also twists and is already twisted.

1/4 inch is far too thin, floppy, bendy.

Use cold rolled steel, which is pretty flat and straight, or any basic tool steel stock.
Use 20-30 mm thick sections.

Castings are 10-15 mm with major gussets in corners.
Without the gussets and webs, You need thicker.

I am using 20-30-40 mm thick tool steel, and 50 mm for all mount plates like the spindle mount plate, with a 90.02 mm hole for the spindle.

The latest version, version 5, is only about 50% heavier than the last one, but is about 50x more rigid.

Because the section sizes are bigger.
And, partly, because wall thicknesses are double and triple.

Ex.
First spindle mount similar to the ram on turret / Bridgeport type mills, was about 120x100 mm in size.
2 mm thick top and bottom. Flat structural steel. Poor.

Now, spindle mount is 240x200 mm, 20 mm thick tool steel all 4 sides.
With internally closed ribs, at both ends and middle, with 200x200x50 mm tool steel flats, bolted 4 ways.
Excellent resistance to twisting / rotational resistance.
This is about 50x more rigid.

little topic hijack, i have also some design questions and want to ask you some thinks hanermo, but cant send you private message because your inbox is exploding..

stef

Maybe now ...
I moved messages to another folder.

Quote:

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Originally Posted by stef110

little topic hijack, i have also some design questions and want to ask you some thinks hanermo, but cant send you private message because your inbox is exploding..

stef

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still not working, you can only have 20 message's in al folders combined i think.

Hello raychar1234
1. I made inserts without any reinforcement in between to keep the special epoxy granite properties you mentioned but the EG must have a certain thickness to hold up the forces without cracking.
2. I am not sure about the coefficient of EG, i don't like the Idea of mixing steel beams and EG. Ferroconcrete or concrete makes more sense, its cheeper and serves the purpose. EG will lose its function anyway inside a steel beam.
3. EG is very good in damping vibrations, i think thats the main and big advantage.
4. My CNC has 470 x 360 x 210 mm and the EG weighs already 380kg.
5. I made it in rough manner with tinfoil shimming, it worked temporary for me but i will machine the surfaces by opportunity. My reference surface was a natural polished granite plate from the stone cutter (+- 0,4mm accuracy in 650mm length for 80€)
6. Indeed, if you have the time to build it, you should do it, its also a big empirical value.

Thanks,

The choice of steel extrusions: U-, H- and rectangular hollow sections will depends on several aspects. For the moment arm of force aspect in Z axis during cutting, Traditional C-frame milling machine inevitably needs a large hollow round body as it is more rigid against twisting. For gantry bridge, the Y-frame or the base platform, using steel H-section is not so bad, as it will be supported on two far ends. In welding or screw assembly process in diy machine fabrication, U- and H-section beams have all edges/surfaces exposed which can be welded all along them or mounted with screws, but for the hollow ones, it can only be done on outside but difficult in inside. Also, the former two types are more widely used in building construction. In recycling plants, they may be got more easier as scraped pieces or at a low price....I am fresh in steel machining and building construction and mainly on aluminum material machining, now only want to build a milling machine of steel cutting capability just as some sorts of PM25, may be some more experience ones have the advices.....