Type of steel for internal structure of eg frame?


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    Member DrPillCosby's Avatar
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    Default Type of steel for internal structure of eg frame?

    Hello, is there a consensus of what type of steel or metal that should be used when designing an eg cnc? I see a lot of projects that use steel that is precision ground and embedded in the epoxy for the hard points for linear rails, ball screws etc. I'm new and curious and I am trying to gather as much info as possible in the chance I'm every ready to attempt such a huge project. -thank you

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    Default Re: Type of steel for internal structure of eg frame?

    Hi,
    your choice will largely be affected by what you can find locally cheaply.

    While steels can a have a wide range of strength properties they vary very little in stiffness. Thus a high strength high alloy steel may have a Youngs Modulus of 230GPa,
    and a mild steel may be 205GPa.....so despite very widely differing strength properties they are remarkably close in terms of stiffness.

    I would suggest a readiliy available medium tensile mild steel. This would allows you to cut, drill and otherwise process into the required shapes, have a moderately high thread strength
    and will be nearly as stiff as some 'you-beaut' hardened and tempered high alloy steel, costing twice as much and a pain to work.

    Craig



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    Default Re: Type of steel for internal structure of eg frame?

    Choice partly depends on degree of machining after casting.

    Using precision ground stock usually means they are trying to avoid machining after making the EG casting.
    Hard to align and keep aligned to the necessary tolerances in a wood / non precision mold. So this may be a waste of time.

    Hot rolled stock is a better choice than cold rolled as the material has less stress. Stress will relieve when machined, potentially resulting in deformation.
    (Or you can get your stock stress relieved before EG casting. Cannot use thermal stress relief after epoxy).



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    [QUOTE=pippin88;2524704]Choice partly depends on degree of machining after casting.

    Using precision ground stock usually means they are trying to avoid machining after making the EG casting.
    Hard to align and keep aligned to the necessary tolerances in a wood / non precision mold. So this may be a waste of time.


    Thank you that makes sense, is there the need for heat treating the steel before the casting? This isn't something I'm chomping at the bit to start but would like to learn as much as I can and purchase a used manual vertical mill first to cut my teeth on first.



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    Default Re: Type of steel for internal structure of eg frame?

    H,,
    stress relieving steel in most important if you weld together various parts to make a structure. A structure requires a sizeable oven and is costly. It used to cost $6NZD/kg (about $4USD) and I suspect with
    LPG prices having gone up so would the stress relieving rate.

    If you use normal hot rolled steel, but just in chunks and/or bolted together then stress relieving prior to casting would be nice but not really necessary, its welding that really introduces the stress
    into a structure.

    Craig



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    Default Re: Type of steel for internal structure of eg frame?

    Hi,
    many whom undertake an epoxy granite design seem to think that its cheaper and simpler than steel or cast iron......it's not.

    Firstly epoxy granite is about as stiff as a limp noodle. Even with great care and expense with fillers etc the best you'll get is 50GPa-70GPa.
    Compare that to cast iron 110GPa and steel 205GPa. That means you have to use SO MUCH more of epoxy granite to be a stiff as cast iron/steel.

    Everyone loves the damping properties of epoxy granite......but who bloody cares how damped it is if its not stiff? 'Stiffness=Accuracy and Surface Finish'. If a machine is not
    stiff it will never produce parts worth a damn no matter how well damped it is.

    You need to decide what parts you want to make and in what materials, to what level of accuracy and surface finish. That will determine what the rigidity that is required.
    THEN start investigating among the different techniques HOW you are going to produce a machine of the required stiffness. Epoxy granite is one of the more expensive techniques.

    Everyone is put off by the expense of stress relieving (welded steel structures) and finish machining for cast iron/steel machines....and its true it is expensive, but what a result!!!!
    All the good epoxy granite builds featured on this site have had finish machining (be it grinding, milling, lapping or scraping) on the linear rail mounts etc, and so occasion the same
    sort of cost as you would on a steel/iron structure. If you imagine you can cast epoxy granite in a manner that DOES NOT require finish machining for accuracy then you are dreaming.

    Craig



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    Default Re: Type of steel for internal structure of eg frame?

    Hi DrPill - Firstly you have to determine what you want to achieve. This is 1) size of job you want to do 2) accuracy of the job required... Then once you understand this you can start deciding what material to make the machine from and what technology to use. I suspect without a doubt that a welded steel machine is your cheapest option. But this has the proviso that it must be stress relieved and finished machined if the result is to be close to a commercial machines accuracy. If your accuracy requirement is low then you can muddle thru without stress relief (maybe, there are lots of sad stories here about jammed cars, imprecise machining etc etc when you don't TSR). This is also coupled with the machine has to be rigid enough for your job. Steel gives you the most rigid conventional material.... People say steel is not damp but if the machine is designed and executed correctly steel will be fine. There are lots of examples of welded steel mills out there. EG will be the least rigid material so you need lots of it... In executing your design you have to make each step successful. If you do something like make it in EG in the
    "hope" it will work out, the universe will gang up on you and you will have to take remedial action. This action will always compromise your machine in some way.

    There are well documented and clear and correct ways to make machines. There are no shortcuts. Pick one and stick to it. If you want to invent things and experiment that's fine. Keep us informed as we may learn something. If you want to make a machine that's spot on stick to the conventional paths... cost everything before you do it. Your "cheap" machine will not be cheap if that's the intent. The cheapest machine is to seek a used machine that will do your job..

    To answer your initial question - You have to use a stress relived grade of steel. Cold rolled stuff is out so use hot produced stuff or have it TSR your self. Metal moves more then most people believe when machined, welded or heated. When you cure your massive epoxy cast it will grow 3x more then the steel so will trap stress inside it when cooled. There are many things to think thru... all are covered in threads here if you look or ask.

    Again plan to have your critical areas finish machined. If you make a large EG machine how will you get it to a machinist for this? Solve all of your logistics issues first (transport of large parts, stress relief, post machining) ... if you look at the timelines on this sort of build you may be making swarf in 1-2 years maybe seeking a second hand machine is a good plan. But I like making stuff so that's why I do it, maybe your the same... Peter



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    Default Re: Type of steel for internal structure of eg frame?

    Hi,
    peteeng has raised some very important questions and choices to be made about the whole design/build task.
    In particular:

    The cheapest machine is to seek a used machine that will do your job..
    This is so very true. I've built a new mill ( about a year old now) using cast iron beds that I had cast and then machined, a laser cut 32mm medium tensile steel frame, 750W Delta servos on each
    axis driving 32mm diameter 5mm pitch BNFN (double nut) C5 ballscrews. It does what I want really well......but was the cost justified?.......a most resounding NO. The total cost of what I've built (so far)
    is in excess of $30,000NZD, and I could have bought a much bigger more powerful machine second hand.

    This comes to the question about what you want. I've always maintained that 'the value of a hobby is measured by what you have to learn in the pursuit of it'. CNC has been a great hobby for me,
    I've had to learn entirely new concepts and extend engineering and physics concepts I've not touched since University days forty years ago. All in all I would say its the best hobby (may in truth be
    called an obsession) I've ever had. So as to the question about whether what I have done was worth it I say Yes, I've had the value from it, but it does not ,make any sense from an economic point of view.

    Buying something like a second hand drill mill might be a low(ish) cost entry to the hobby while you ponder and make your choice about the money you might spend building your own machine.

    Craig



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    Default Re: Type of steel for internal structure of eg frame?

    Hi DrPill - From my last 2 years of looking at various materials and processes I've come to this conclusion. The cheapest stiffest material is laser cut steel. I initially used stainless steel because mild steel painted was more expensive so I started playing with laser cut galvanised steel, no need to paint and cheap. But its only in 3mm thick max. So I started laminating steel and this I think is a great answer. Laminated steel laser cut and folded to spec, laminated with epoxy and bolted together. If you look up constrained layer viscous damping you will see several machines and attempts at this type of thing for machine building, The laminated metal is very damp, bolted connection are damp yet the material and geometry are very stiff. By laminating you can have any thickness you want where you want... By the way laminated metal is used extensively in aircraft. It is damp and is fatigue resistant. Look up GLARE most airbus fuselages are made of it...

    You still need to finish machine critical features but if you consider this during design you can make things geometrically sympathetic for machining then bolt it together and adjust. There maybe a good laser cutting sheet metal shop near you... Peter



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Type of steel for internal structure of eg frame?

Type of steel for internal structure of eg frame?