[joeavaerage]

Hi,

You will achieve very good repeatability, better repeatability then using C7 screws V300 = 0.050mm so what do you intend to do with this machine? Peter

Questionable.

Note how the C0 through C6 have both an accuracy over 300mm but also an accuracy over 2.PI, ie one revolution. This is called cyclic error. A C5 screw has a max cyclic error
of 8um. Thus my C5 5mm pitch screw could have a cyclic error of 8um over one revolution or 5mm. That does not sound so great does it?

A C7 screw has 50um error over 300mm which does not sound too bad....right? But the cyclic error is UNCONTROLLED. I have seen measurements on C7 screws which exhibit
35um over one revolution. So how does 0.035mm error over 5mm sound....none too flash I'll be bound.

If you could make a good mill from C7 screws then everyone would do so, well I guess the Chinese do, but all the mainstream manufacturers use C5 at the very least, C3 commonly and C1
for the top end machines.

You are right, it depends on what you want the machine to do, but to have any chance of making engineering parts then C5 or better is required.

I can almost hear everyone saying ....'well how about linear scales', and yes they can help, and even help immensely. If you use a C7 screw and it has large cyclic error the linear scale
feedback loop must correct that error within every revolution of the screw, which could be tens of revs per second. In short the rapid error fluctuations are likely to overwhelm the controller.
No, if you are going to the expense of linear scales, then all the more reason to use C5 screws.

If you could make a good machine with cheap parts then you would not even see the expensive ballscrews ever made....but they are made and sold....and that tells you they are required.

Craig

[peteeng]

Hi Craig - My point is that the machine has to be made from commensurate parts and commensurate technology to achieve what the Maker wants or needs. Currently we do not know what Lars wants to make, if he's making bread boards then allthread will do. If he's making microchips then that's another story. Accuracy is an exponential $$$ cost journey as you describe. Peter

[joeavaerage]

Hi peteeng,
i agree that the machine needs be built to use the precision C5 or C3 confer, but there is just no way you can make a C7 screw behave like its 10 times more
expensive cousin. That cyclic error is uncontrolled means that for my purposes C7 are just not up to it.

I figure it this way, you might say that a hobbyist effort is unlikely to produce a rigid machine that is sufficient to exploit the accuracy of a C5 or C3 screw. The reverse applies also,
namely that no matter how good you build your hobby machine, even were it to be perfect, if you do not use C5 or C3 the best you can ever hope for is the accuracy conferred by a C7 screw.

I come back to the fact I aspire to have a machine of C5 accuracy, and that requires C5 or C3 screws. Whether the rest of the build is worthy of such screws is another question.

Craig

[peteeng]

Hi Craig - I did not infer to use a C7 it was meant as a funny, it did have a funny face at the comment. A good machine no matter what its accuracy is commensurate in its parts and build relative to its application. But maybe Lars is making bread boards?? Peter

[joeavaerage]

Hi peteeng,
true, but given the 'night and day' difference in cost between ground and rolled screws I would presume that any one whom wants a C5 screw has a reason.

The more common path is that someone wants C5 accuracy but when they discover the cost of ground screws are forced into C7's as a budgetary necessity.

Craig

[Fatamorgana]

Hello and again thanks for the valuable feedbacks regarding my project.

Im working on getting the CAD files ready so I can attach it here in this thread by this day or tomorrow.

In brief words the project is somewhat different from other projects I see on this forum, one of the main backbone Im building this machine on is that I will do the machining of the lands/steel inserts for different axis myself, I have an 1800kg old russian mill of type "stankoimport 6r80sh" that I will rely on, one of the goals on the road here are to grow skills of machining for my own interests aswell. Im not dependent on having a 0,001 tolerance at the end of the road here, It will be as good as it gets, Im not planning on using this in a production manner for external customers etc. this is only for own use. I also think that learning curve of this type of machine is steep and I will see other needs after some years of use, and then I maybe want to take it a "step further" and build an even better machine. With that beeing said I have a quotation on 2000$ from a local company who can take my y/z axes and machine them within HIWIN 20uM parallism tolerances. (also flatness)

Im also gonna make the bases out of UHPC concrete, Im having a mail correspondance with moertelshop and have got that at a reasonable price. Im also used to working with concrete and feels most competent with that vs EG etc.

Only problem now is that Im stagnating cause Im waiting on the right ballscrews so I can adjust the CAD and build around the screws that are available. As mentioned earlier I have some 32mm screws that I can get for around 1400$ these are second hand. So that where Im at right now. (I already got the 750w servos)

Lars

[peteeng]

Hi Lars - If you have a large mill I'd suggest you consider building a steel or aluminum plate mill. Will be a more direct method vs making moulds and casting. steel is cheaper (stiffness/$) then UHPC and you sidestep various issues of casting/inserts/machining etc. Which concrete are you considering? Peter

[Fatamorgana]

Hi Lars - If you have a large mill I'd suggest you consider building a steel or aluminum plate mill. Will be a more direct method vs making moulds and casting. steel is cheaper (stiffness/$) then UHPC and you sidestep various issues of casting/inserts/machining etc. Which concrete are you considering? Peter

nanodur tegno uhpc cement from germany.

https://www.moertelshop.eu/buy-nanodur-cement-cheaply_

What kind of steel frame or beam would make a good base for a c-frame you think? Im trying to keep out of welding because of all the things that heat will affect every aspect


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[catahoula]

craig-
no, the "DGC5" means double flange cutting, ground, C5 accuracy. the R in OFUR means right hand thread. one can see in the finish that they are ground. i'm sure they're not as good as nsk/thk/rexroth, but they seem very good for the price (including custom machining)

https://www.technico.com/pdf/Blog/TB...ew(EN)1901.pdf

[joeavaerage]

Hi,

As mentioned earlier I have some 32mm screws that I can get for around 1400$ these are second hand. So that where Im at right now. (I already got the 750w servos)

I would say get them. I use 32mm 5mm pitch screws with 750W servos and the machine is faster than my nerves will stand, so I de-tune it to slow it down.

Strictly speaking if you are not concerned with accuracies better than 0.001 inch then C7 screws would be enough. If you want C5's then unless you want to pay a fortune you get what you can
and $1400 for a set of C5 screws is pretty damned good.

Craig

[peteeng]

Hi Lars - Totally agree about the welding. Avoid at all costs. For the last 4 years plus I have been evaluating materials. Initially I thought that an epoxy granite type material would be suitable. My aim was to make a material that is equivalent to aluminium in stiffness, can be machined in a router and can be cold cast. I've tried various ceramic additives, the usuals and I have been testing in flexure not compressive test as the civil engs do. This is because machine elements are in flexure not direct compression.

Going back further I have been involved in composite yacht and automotive structures for over 30 years and regularly make carbon fibre parts at E70 E80... My attempts at the cast material have shown that in flexure the high compressive stiffness concretes do not achieve high flexure stiffness. In fact they are very low flexural stiffness. So your Techno UHPC will be around 30GPa maybe 40GPa whereas steel is 200GPa. So you have to use a lot of concrete to get a stiff structure and that's exactly what they do. If your making large machines say 2m or 3m plus long then this works for you. For a small 500mm machine it works against you. The geometry is not big enough to compensate for the low stiffness. If your interested read the Milli thread here. It works through a lot of these issues. The short story is I have gone off the cold cast idea unless it was proposed to make lots of machines then making moulds make sense. If I needed the E70 plus then carbon fibre is the only material I have tested that achieves it in tension, compression and flexure.

If you are to build a single machine then a thick plate machine with the parts edge screwed makes sense. To this effect I have made 3 routers using plywood to test this theory out. They easily cut aluminium. I have yet to try cutting steel. On that point you want to cut steel so you need to investigate spindles. This is a big area to cover, but you need to resolve it early as the real estate needed for a low speed high torque spindle for steel is substantial. If you choose to use pulleys then the head design will take time. Sort that out now vs down track. It's a key hurdle to jump.

Here's Frankie a 16mm plywood bolted together router. If made from 12mm or 16mm aluminium plate it would be a very good mill. There is a steel plate bolted column machine in the forum. I'll try to find it. The usual comeback for aluminium or steel plate is dampness. (making broad statements warning) Machine dampness is a second order issue. If the machine is rigid enough then its dampness is not important. A bolted structure is very damp compared to a welded structure. Depending on what references you read they could be as damp as cast iron which is sort of the benchmark for commercial machine builds in terms of dampness. Machine damping is another 4 year journey and my conclusion is that you cannot solve dynamic machine stiffness issues using materials, they are not damp enough. (If materials were damp enough then we would not have shock absorbers on cars we would let the material do the work to absorb the vibrations) This has to be solved in the software and by using the operational sweetspots. Look up tap testing of cncs to learn more.

So I'll look up the plate machine for you. Since you have a mill you can machine the critical edges flat and square and edge drill for the threads. Using plate allows you to optimise thickness and position of the elements, create internal webs easily and avoid heat treat. Plus it can be pulled apart and changed if needed in the future. All benefits to a hobby/Maker/DIY machine builder who wants to learn about machining... Peter

on the D32 screw purchase, they are close enough and if the $$$ are right then get them

[joeavaerage]

Hi,
i disagree with peteeng, steel is the best 'bang for your buck' material there is. Welding is so versatile, although you have to stress relieve afterwards....but so what? Steel
has a flexural modulus of 205GPa whereas even the very best of UHPC's are about 50GPa, and aluminum about 70GPa. When it comes to the cost of material to obtain
a structure of a certain rigidity steel comes in at the very top and by a big margin.

Most hobbyists believe Thermal Stress Relief (TSR) is an impossible task which is total BS. It is a well known and practiced industrial process. I priced it here in New Zealand and it costs
$4.60USD/kg. Catahoula just posted recently that he got all his steelwork TRS'd for $1/kg. The point is that it can be done. If you exclude welding as a possible technique then you forgoing
a marvelously versatile and cheap technique. In short you are making the design task harder and more expensive because you refuse to consider a welded steel structure.

Craig

[peteeng]

Hi Lars - Here's some images of plate builds. I wouldn't use construction extrusions. It would be all al or steel plate... Peter

[joeavaerage]

Hi,
depending on where you are in the world you might consider cast iron.

Cast iron has a flexural modulus of about 110GPa, a little over half that of steel, but has very nice vibration damping characteristics. Depending on your skill as a designer
and the skill of your pattern maker and foundryman some very nice and intricate shapes can be made.

I had three cast iron axis beds cast for me, 700mm x 250mm x 130mm and 115kg each. It was not cheap, and that was well and truly surpassed by the vibratory stress relief and machining,
but they have made superb axis beds, accurate, rigid and well damped. After three years I've almost recovered from the shock and horror of parting with that much money (nearly $10,000NZD)
and every time I machine a part I can take satisfaction that my machine is doing as I designed to to do. Its very hard to beat that sort of satisfaction!

Craig

[Fatamorgana]

After these valuable feedbacks Im now considering steel, my initial design was indeed based on steel, I only found this attached basic drawings of the y axis. I left this plans as everyone seemed to be building the machines on EG, UHPC etc. So - weld - stress relieve - machine is the right order? On the positive side Its much easier for me to construction this in steel as I can machine everything on my mill.

Can this be rigid enough for a y-axis?


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[Fatamorgana]

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[Fatamorgana]

This is based on some materials I have available


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[peteeng]

Hi Lars - 1) do you have access to thermal stress relief? If yes then all good. 2) The base is a bit complex can be simpler 3) As discussed prior you are starting at the wrong place. Start at the spindle. You will run out of real estate doing it this way at the spindle. If you keep going with the design just roughly you will see. To answer your question , at first glance yes but it could be simpler and better. And we need to know the size of the bits to give a better assessment. Peter

You have your machine in the middle of the stand. It will vibrate up and down. It needs support directly under the machine. You have your rails mounted on hollow tubes, these will vibrate as well. Rails need to be on deep webs. If you look at commercial machines you will see this. The buttresses can vibrate as well due to their free edges (if you have FEA or simulation run a modal analysis and you will see which bits vibrate) ... Since you have a cap plate, have webs inside the "rails" this removes most of the free edges... Your journey has some way to go, keep at it... Peter

edit- Hi lars - every bit of a machine must fulfil a function. Take the buttresses (or outside webs) They are there to stop the rails from moving sideways, I assume. But what if the car is between the webs? Its a different stiffness to if the car is at the web so its not a great solution for a mill which is trying to keep the rails in the same place everywhere. I've analyses 100's of machines and have learnt that there's a lot happening at the rails and its the rail foundation that is really important in making a stiff machine. Your design maybe globally stiff but its lacks consistent local stiffness. Vertically it can hammock (as there is nothing under the rail) and horizontal I've just explained. So keep at it... its here in the design phase and using forum feedback that you turn an average design into a great design...

[joeavaerage]

Hi,
Christ!!!!.....what are you planning on loading onto it?. The table underneath looks like it could support tons and tons!

Use that material in the important bits like the Y axis, Z axis etc. Would it matter if it were sitting on a wooden table? It's not the table that makes
the machine, but the axes and how they are held orthogonal to each other.

Otherwise the Y axis looks OK. Usually the height of the ballscrew/ballnut is higher than the linear rails and cars so an axis requires a channel down the middle to fit the ballscrew while
the rails are perched on the tops of the channel. You can see that in the way I had my axis beds cast. I assure you the height difference between the top and bottom of the channel was calculated
to be exactly the difference in profile height between the rails and the ballscrew, and was machined to be closer than 0.01mm.

So - weld - stress relieve - machine is the right order?

Yes, very much so. Those big Russian mills are very solid bits of gear and you'll get great use out of it in making this machine. It seems right to choose materials for which you have the means to machine.

At my place of work ten years plus ago there was an Indian made Bridgeport clone. I used it extensively to make my first mini-mill. I got some huge cast iron elevator counterweights
from the scrap yard. They were about 800mm x190mm x 120mm and about 150kg each. I used the mill to make axis beds, not entirely dissimilar to my new build machine I pictured above.
As it turns out the cast iron was only so-so, it doesn't need to be good iron to be heavy! So, when I decided to build a new machine I'd already knew I was going to have bits cast for me where I could guarantee
the iron quality. It was the right choice, I've got exactly what I paid for, and they will last me for twenty years or more.

Craig

[joeavaerage]

Hi,
where in the world are you?

If in Europe, and possibly also the US there are companies that sell continuously cast rectangular sections typically in 3m lengths. I really wanted to get one length of 250mm x 150mm from the German
manufacturer, but the freight to get it to New Zealand just killed it. It was cheaper in the event for me to get my axis beds cast the right size and shape than buy a big rectangular section and machine
what I wanted out of it. Imagine paying tens of dollars per kg and then turning 50% or more into chips! Still, it would have been feasible had not the freight been so steep.

Craig