[peteeng]

Calm down guys - Sus can figure out what's best for himself. He's been messing around with the components and will get to the structural config soon. Peter

[suspension]

That's right, I think they've overstayed their welcome by a decade or more. If you offset the saddle onto a wider column you get a much stiffer machine, you can load heavier blocks onto the table, your frame is bulkier, your spindle can move faster than a saddle+table and your z travel is not compromised. All in all better stiffness better dynamic movement and the design doesn't change that much from a c frame. C frame is great for belt driven spindles I give it that.

For these types of spindles a lightweight fixed double column with a moving table would probably be best.

But wouldn't this require an additional ball-screw?
Also, why cant we make the stiffness same in C frame by having the column width match the width of the base for the whole height (will look ugly though but so does the bridge style ).
Z travel required for X can easily be offset with a taller column?

[suspension]

No I did not check, I mean how would you do so? I don't have the measuring equipment to come anywhere close to measuring this stuff accurately enough to be considered a check.
Craig

Back to ball screws!
Did you try checking the accuracy of the parts you make? For example using micrometer/etc?

[ardenum2]

Hi,

BS.

Craig

FEM disagrees with your opinion. You're an electronics guy, just your choice of iron casting a c frame proves you have zero idea about structural, as much as I have zero idea about EE. So why don't we stick to what's best for both of us.

[ardenum2]

But wouldn't this require an additional ball-screw?
Also, why cant we make the stiffness same in C frame by having the column width match the width of the base for the whole height (will look ugly though but so does the bridge style ).
Z travel required for X can easily be offset with a taller column?

for the double column yes it would require 2 ballscrews on the z but not with a single column. the stiffness disadvantage with c frames comes from the spindle assembly, one it gets thinner at the end, two the spindle is pretty far away from the column. The gantry that moves up and down is the key to the increased stiffness.

Having said that, for the type of spindle you want to use, a lightweight frame would probably be better, unless you want to change your spindle in the future to something that requires the additional stiffness. It all depends on where you want to end up.

[peteeng]

Hi Sus & others - I have mucked about with FE and structural configs for years. There is no one solution. It comes down to what you want to do with the machine that should drive the config not the other way round. All configs can be made the same stiffness, just have to have the right amount of material in the right places. The "correct" config comes down to application. Thats why commercial machine builders have different designs for different applications.... For a 550x350x350 machine the Bridgeport config makes sense (C frame) but the C frame design has been developed historically from how the ways work and how they are built into the saddle and column etc. Now we have linear guides other configs are possible and practical. I think the two starting points are A) do you want the job to move? Yes then you have a moving table and you have to sort the other two axes via a saddle or a moving column B) do you want the job to stay still? then you have several options....by answering that question your structural config journey begins. Peter

I looked at some small CNC mills and they have a Z of 400mm I think I'd go 350mm min.... for Z. Also they weight 1.8 tonne so lots of metal for stiffness.

https://www.machineryhouse.com.au/mini-cnc-milling

[joeavaerage]

Hi,

You're an electronics guy, just your choice of iron casting a c frame proves you have zero idea about structural

BS.

In case it has slipped your attention the vast majority of the worlds CNC machines are STILL made of cast iron....why?? Its the most cost effective, stiffest and well damped material
for machines, and has been proven over and over again for two hundred years. But along comes Ardenum....'No, you don't know what your talking about'

Craig

[ardenum2]

Hi,



BS.

In case it has slipped your attention the vast majority of the worlds CNC machines are STILL made of cast iron....why?? Its the most cost effective, stiffest and well damped material
for machines, and has been proven over and over again for two hundred years. But along comes Ardenum....'No, you don't know what your talking about'

Craig

you're missing the point, the issue is not cast iron but your choice to waste those wonderful casting resources on a c frame and then saying bs when I point out its flaws. If you had done any relevant amount of fem prior to that, you'd know what I'm talking about. Let's not continue this any further.

[ardenum2]

Hi Sus & others - I have mucked about with FE and structural configs for years. There is no one solution. It comes down to what you want to do with the machine that should drive the config not the other way round. All configs can be made the same stiffness, just have to have the right amount of material in the right places. The "correct" config comes down to application. Thats why commercial machine builders have different designs for different applications.... For a 550x350x350 machine the Bridgeport config makes sense (C frame) but the C frame design has been developed historically from how the ways work and how they are built into the saddle and column etc. Now we have linear guides other configs are possible and practical. I think the two starting points are A) do you want the job to move? Yes then you have a moving table and you have to sort the other two axes via a saddle or a moving column B) do you want the job to stay still? then you have several options....by answering that question your structural config journey begins. Peter

I looked at some small CNC mills and they have a Z of 400mm I think I'd go 350mm min.... for Z. Also they weight 1.8 tonne so lots of metal for stiffness.

https://www.machineryhouse.com.au/mini-cnc-milling

Thanks for bringing some sanity into this Pete.

[joeavaerage]

Hi,

your choice to waste those wonderful casting resources on a c frame and then saying bs when I point out its flaws.

Peeteng has just posted that any frame type can be made arbitrarily stiff, and thus any design can be made equal to any other. If there were a clear winner,
ie a design that used either less material or fewer or simpler parts that made a stiffer machine, that design would have dominated the market. There is no such clear winner,
ultimately all designs can be made as stiff as required.

If there is a specific need, say hugely massive parts, then a design where the part remains stationary while the spindle moves might appeal, or alternately when you have an extremely heavy spindle then
the spindle might be best stationary while the part moves relative to it. For a general purpose machine where there is no overriding constraint then any mixture of designs will be equally good.

Craig

[joeavaerage]

Hi,

If you had done any relevant amount of fem prior to that, you'd know what I'm talking about. Let's not continue this any further.

For your knowledge I did many hours, days, even weeks with FEM and I came to the conclusion that all designs could be made equal......so then it was a matter of choosing
the design that best suits your means, the materials you have at hand and your budget. I made my choice.......and it works exactly as I designed it to. It is neither better,
nor worse, than any other machine of the same stiffness. Who cares about its shape.... what really matters is how rigid it is relative to the materials used.

Craig

[peteeng]

Hi All - To discuss the C Frame design a little more. Linear guides were first introduced by THK in 1972. Thats very recently history. My young self designed my first CNC in 1980 I think and it used round rails. A Brief History Lesson on the THK Linear Guide | Technico Linear guides enabled us to move away from ways in machine parts and machine design. Prior to guides we were stuck with issues such as stick-slip and lots of friction which meant we HAD to design drives that went through the friction centre of the ways otherwise the moving parts did not move. We also had to use production materials that could be cast, machined and had good friction co-efficients hence the development of suitable cast irons. The C frame design is the best solution given those variables ( maybe the only solution) and the c design has carried on using guides because change is slow in the machine industry because the machine life cycles is long and machined product life cycles are long. But this has changed recently as well. Even the hobbyist has changed from machining to printing...

Now we have linear guides that do not have stick slip we can place the drive anywhere we like and it works. We also are not limited to CI as we can use materials that do not have to cast/machine and have low friction. Linear guides also improved speed and accuracy. So now we have many configurations that work. Soon we shall be printing machine parts, this is already happening and they won't be in CI. Maybe soon we will not use subtractive tech at all!! Personally I want my mechanics up out of the muck so the C frame, saddle etc is out and a fixed table is in with high set mechanics... Swarf control is big... If your going to make lots of aluminium parts your going to make tonnes of swarf which will get into low set mechanics guaranteed.... So Sus I suggest you forget about mechanics for a while and get your structure and envelope settled, then come back to the mechanics. Peter

[suspension]

I have already decided on a C frame due to additional cost of bridge style one. Also many of the DIY machines seem to use C frame. This decision is mostly due to cost.
As per Ball screws, I am biasing more towards the ebay guy. The cost of TBI screws are also very high. (I got a quote from TBI directly). I am thinking of a machine with travels of approx 600 X 400 X 300 (mm). These sizes are based on availability of unused (new - open box) screws from the ebay seller. I think I am happy with X and Y, but not sure about Z. Isn't 300 mm enough even if I had to setup multiple vices on top of each other? I can add 100 mm to Z for an additional 100 USD.

[NordicCnc]

300mm Z sounds okat but consider if you are going to use a Haimer probe. Then the additional 100mm will be a good thing. X and Y 600x400 is good, but you will need quite a huge column with Z-axis stick out to reach that work area. That is easier to achieve with a gantry design, either moving or fixed gantry design, with thr fixed gantry design being more rigid.

Skickat från min SM-A528B via Tapatalk

[peteeng]

Hi Sus - Z height comes down to what you are going to do with the machine. If your parts are flat then its Ok. If they are blocky type things then maybe not, depends on vice heights and tool lengths. Up to you. 600x400mm is a big table your column/base is going to be big to match it. Peter

so your headed to something like this? Mini Mill 2 | 40-Taper Mill | Vertical Mills – Haas CNC Machines

[ardenum2]

I have already decided on a C frame due to additional cost of bridge style one. Also many of the DIY machines seem to use C frame. This decision is mostly due to cost.

they use it because they don't know any better. Stiffness wise a c frame is enough for your spindle, but it sure ain't cheaper or easier to make, fabrications are a pain overall, you have a complicated saddle and a complicated z assembly fabrications. With a raising gantry the saddle becomes your gantry, and spindle holder becomes simple, no additional costs to the structure.

If you're sending everything to machining at a company which is very expensive, you should not settle on an obsolete design.

[joeavaerage]

Hi,

but you will need quite a huge column with Z-axis stick out to reach that work area. That is easier to achieve with a gantry design, either moving or fixed gantry design, with thr fixed gantry design being more rigid.

That is essentially correct. My first mini-mill had fairly small travels, 180mm x 180mm x 180mm, but the real constriction with regard to the size of the parts I could machine was the distance between the column and
the centre of the table. I decided when I built my new mill that I would make the column-to-table distance bigger to accommodate bigger work pieces. This I duly did, and the size of my new mill is such that I can accommodate
big workpieces even though the travels are modest 350mm x 350mm x 350mm. The downside is that the top of the C frame has to be longer and that counts against rigidity. Likewise the column must resist extra bending stress
and must also be bigger/heavier to get the required stiffness.

This is the essential trade-off that Ardemun and Peteeng have talked about. I argue that a C frame can be made as rigid as any other design....but it is certainly true that the individual parts of the C frame must be very big
to get the required stiffness.

Nordic has posted that a gantry design can be made more rigid, and probably for a given mass of material that may be correct, but a gantry design has to counter torsional stress, the greater with increasing Z axis
travel. Whatever you save (material wise) is matched by the increase in mass of the gantry. Ultimately both a gantry design and a C frame design can be made to be identical with respect to stiffness.

My particular philosophy in selecting the C frame was design and construction flexibility. I have three cast iron axis beds, all from the same pattern. I bolt them to a frame, and I chose steel for the frame on the basis of
'stiffest result for a given sum of money'. I would have preferred to cast the frame in two pieces, but the castings alone were likely in the region of $5000NZD-$6000NZD and my budget was near exhausted, ergo I used steel.
The advantage of the design I've adopted is that should I decide at some future date to cast the frame in iron, that I can unbolt the axis beds and transfer them to a new frame. Also you've seen that I deliberately left
the Z axis column somewhat longer that I could shift the Z axis to recoup lost Z axis travel at a later date. If I decide I require extra travel of the X axis I can unbolt it from atop the Y axis and replace it while the rest of the
machine remains the same.

It may well be that over a period of time my machine changes and a C frame would accommodate changes better than any other design I can think of. The cost of that flexibility (design/arrangement, not lack of stiffness!)
is that the individual components must be very rigid to achieve and overall rigid design. The 'beam' or 'extension' that creates the top of the 'C' is an important part of that consideration. The bigger the distance between
the column and the center of the table certainly is convenient for the placement of parts on the table but causes a major loss of rigidity UNLESS that 'beam' is designed to match the requirement of the overall machine.
Thus it is the Y axis travel that has a major bearing on the rigidity of a given machine. The Z axis travel can be made to be whatever is convenient. It is true the the longer the Z axis travel and therefore the column height
increases the bending stress in the column but is much easier to accommodate than increasing the 'beam'.

It is very seductive to say 'I want nnn X travel, mmm Y travel......etc, only to find that to achieve the required overall rigidity that the frame, of whatever design, becomes too big and expensive. Don't be fooled into
thinking 'I must have this or that' because it my well force you into a compromise in rigidity later on. In the case of a C frame machine the Y axis travel and the table-to-column distance are the critical parameters.

Craig

[joeavaerage]

HI,

you should not settle on an obsolete design.

More BS.......there are dozens of machines from all the major manufacturers that are C frame types or near analogues on sale right now, current designs. That includes the Hass MiniMill
that Peteeng just linked to. That is by definition not 'obsolete'.

Craig

[joeavaerage]

Hi,

The cost of TBI screws are also very high. (I got a quote from TBI directly).

I am interested. You say the quote direct was very high......would you care to guess how various AliExpress vendors are selling 'TBI' products as cheap as they are?
Is it, do you suppose that the quote given you was artifically inflated? Or could it be that the AliExpress vendors are selling knock-offs?

I don't know whether you have seen that there are vast numbers of AliExpress vendors selling what look like Hiwin linear rails and cars. They look the same, identical colors, the same part numbers
as genuine Hiwin but are not in fact Hiwin at all?. I wonder whether these vendors are doing something similar with TBI products.

Craig

[peteeng]

Hi Sus and others - The Mori M1 config is my choice at the moment for this type of mill . That's the one on the left in the twin image... The one on the right is second choice. Peter

1) mechanics are up out of the way
2) Z can be long but bearings are well positioned