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    Default Re: Filling steel tubes with epoxy granite- machine before or after?

    Hello. I would like to share my view on the EG topic with regard to cnc design; I will also try to be as adequate as possible, although English is not my first language. We all know that EG is great in compression but not so great in traction or torsion. Both gantry beam and column mill design solution are primarily more subject to torsion than either to traction or compression. Especially in a gantry design, the vibration impact on the dimensional accuracy of a work piece lags far behind the Z axis overhang. I believe that having to deal with the Z overhang requires solutions that will also partly satisfy the requirements of a good vibration dampening system, and here I concur with Peteeng that the greater the mass the better. Also, as Pippin 88 pointed out, it is better to have an increased section of the beam than an increased wall thickness. Now, it is common knowledge that any Z axis that is off centre of a beam will impact the torsion angle of that beam, no matter how stiff the beam would be. So, a designer’s quest would be to find what angular deflection of the beam would be reasonably acceptable and at what distance from the center of the beam (overhang), so that the tool deflection caused by the beam’s torsion be less than a certain threshold. Let’s run some numbers, to make it more tangible: 1) steel, 1 m unsupported length beam, fixed ends; 2) square section 200mm/200mm/12,7mm to take Catahoul’s dimensions ; 3) cutting force= say 300 N; 4) Z axis weight = say 1000 N; 5) (horizontal) distance center beam to Z axis centroid = say 250 mm; 6) (vertical) distance center beam to the table= variable between 100 mm- 400 mm (300 mm the light under the beam) . Let’s calculate tool deflection from gantry torsion (A), tool deflection from gantry bending during cutting in the X axis (B) and Z axis displacement from gantry sag (C) for different Z axis overhangs. If the tip of the tool lowers to 101 mm from the centre of the beam (basically 1 mm bellow the bottom of the beam), than we will have: A= 0,00442 mm; B=0,00014 mm; C=0,00068 mm. If it lowers further to 200 mm from the center of the beam, we will have: A=0,00951 mm; B=0,00014 mm; C=0,00068 mm. When it lowers to 300 mm, we have: A=0,01555 mm; B=0,00014 mm; C=0,00068 mm. Finally, at 399 mm we have : A=0,02243 mm; B=0,00014 mm; C=0,00068 mm. Please bear in mind that A is the tip of the tool deflection from gantry’s angular deflection alone, and not from tool bending due to cutting forces . Therefore, apart from this deflection (A) there will be the well known sources of deflection from : tool holder runout, spindle runout, speeds&feeds cutting forces, milling bit runout, etc. Now , what we see is that max A is 0,022 mm and only the designer assesses if that is acceptable or not. For milling metals, most likely not acceptable over 0,01 mm, which basically would mean an overhang of max 220 mm from the center of the beam, a.i 120 mm bellow the bottom of the beam. It means that the user of the mill will either use risers of 180 mm in height, in order to mill plates, or the beam gets increased to: 300 mm/300 mm/8 mm (91 kg and 0,01 mm total deflection from combined gantry torsion and gantry bending during cut on X axis direction, at 450 mm from the center beam) . Now, out of 1 torsion and 2 bending forces, please take a look at the minimum value of torsion (A=0,00442 mm) as compared to the maximum bending (C=0,00068) and we see that the impact of torsion is about 6,5 times bigger than the impact of bending. At the other extreme, (A=0,02243 and C=0,00068), torsion impact is about 33 times bigger than the bending impact. Now let’s fill in the beam with EG; it means for the 200mm/200mm/12,7 mm an added weight of about 80 kg. The displacement from gantry sag C goes from 0,00068 mm to 0,00087 mm, basically peanuts. So adding EG is not a problem from a weight standpoint, but obviously the acceleration will suffer a bit. I wouldn’t worry though: first we calculate the inertia of the portal, ballscrews and stepper motors (assuming there will be steppers), altogether, as if there is no acceleration (the gantry+legs+railways +ballscrew+ Z axis with motor, should come in at about 300 kg with EG) and then we multiply it by the desired acceleration and see what torque would be able to deliver that. For that, assume 300 rpm is a speed where a stepper feels good, a.i where the trade-off between the drop in torque, increase in speed and the heat generated is in its right mind. We will have 300 rpm=31,4 rad/sec and this is the angular speed that will move the gantry , if the ballscrew pitch is 5 mm, then there will be a feed of 1500 mm/min (ok if you ask me). The acceleration time will be anything between 0,5 sec and 1 sec, therefore the angular acceleration in 1 sec is going to be Acc 1 sec = 31,4/1 = 31,4 rad/sec^2 and the acceleration in 0,5 sec is going to be Acc 0,5 = 31,4/0,5 = 62,8 rad/sec^2. Suppose the total inertia of the gantry would require 0,1 Nm to move the gantry at 0 acceleration; then the torque needed to bring the gantry to the desired speed would be anything between T= 0,1 x 31,4 = 3 Nm, and T=0,1 x 62,8 = 6, 28 Nm. Although I have not performed any calculations on the inertia of the portal , my guess is that a Nema 34 , 8 Nm stepper will do, plenty good. Now, we can see that filling EG in the beam doesn’t really screw things up either from a weight or from an acceleration point of view, assuming the conditions set out in the beginning of this calculus. The question is: does it have any measurable benefits as far as vibrations is concerned ? Well, it depends on measurements, something that a hobbyist has little means to do. Can we anticipate anything ? Maybe, but I doubt there is a recipe that can be carried over from one design to another. There are so many elements that can affect the overall dynamics of the system that only one minor element that changes will change response frequencies to various scenarios. The difficulty is that this is a trial and error thing, a.i you measure effects and then you work on possible causes. If the science were better at explaining what vibrations really are, we would have a different understanding of vibrations and probably a better way of tackling them. Frankly, I have never been a fan of imagining vibrations as oscillations in the strong force. Basically, more weight means more atomic metallic bonding, which means more strong force that should be able to dissipate the kinetic energy generated by vibrations, by either of the methods explained here as well. In conclusion, I wouldn’t worry trying to calculate to what extent some EG dampens vibrations and in what circumstances; I would try to include EG in my design anyway and I would try to calculate/assess where it screws up the design (acceleration, motor torque, etc) and how much that would bother me. If it would mess with my desirable parameter, I would include it in design. On the other hand, some reasonability in desires/expectations would not hurt anyone. I have seen some people trying to squeeze the life out of aluminium by reinforcing aluminium profiles with EG or steel plates, in a manner that made little sense to me (other than dissipating heat). Better steel than aluminium on steroids. The point is that heavy, agile and cheap don’t go well together. No affordable hobby cnc mill will do best in both steel and aluminium, at least that’s my understanding. Some choices have to be made from the outset and then it is advisable to stick to them. Sorry for the lengthy post.

    Last edited by steelmind; 04-07-2020 at 04:06 PM.


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    Default Re: Filling steel tubes with epoxy granite- machine before or after?

    Was rading a bit about damping and found this power point presentation about a manufacturing method and material called iCTech. Looks like a cast iron frame filled with something similar to epoxy/concrete and aggregate.

    This is not a scientific paper, but it claims much better damping and stiffness to weight rateio than regular cast iron. If this is true then filling steel beems with epoxy granite should help on both stiffness and damping also. Although if it is cost effective is another question

    What do you think?

    Filling steel tubes with epoxy granite- machine before or after?-ictech-pngFilling steel tubes with epoxy granite- machine before or after?-ictech2-png

    Attached Thumbnails Attached Thumbnails Filling steel tubes with epoxy granite- machine before or after?-ictech2-png   Filling steel tubes with epoxy granite- machine before or after?-ictech-png  
    Attached Files Attached Files


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    Default Re: Filling steel tubes with epoxy granite- machine before or after?

    Hi Bouyen - Increasing dampness is generally achieved when you mix materials. This is due to more internal interfaces, more internal friction and more mass. Many times the increase in stiffness is debatable and engineering calculations show that the stiffness increase is not much eg filling steel tubes with stuff other then more steel. Mass is a big factor (takes more energy to move more mass) and is the significant effect especially when you have it in a stiff outer materials such as steel or CI. This is because the load path is rarely through the middle of a structure but it is in the skin of the part so in the published case not much strain is in the internal filling. But there's lots of mass in the internal bit so its the significant factor. There's lots of debate about this subject here. Its quite easy to achieve the various improvements over traditional machines as these have sort of grown up over time vs the things we can now do with engineering analysis and optimal design methods. Its a matter of defining what machine static and dynamic stiffness you need and designing a machine to that spec.
    Please note that the article quotes the CI having a damping ratio of 0.1% and the epoxy granite as 2%. Many 1000's of high precision cnc machines over the last 20 years have been built from EG for this very reason. Its much better to make a machine entirely out of EG then a CI and EG mix. Many top machine builders now are using epoxy carbon fibre parts just like the aerospace use. Thats even a better step up in stiffness then EG. EG is about 35GPa CF epoxy 90GPa... and its just as damp maybe more so...Peter



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    Default Re: Filling steel tubes with epoxy granite- machine before or after?

    Quote Originally Posted by peteeng View Post
    Thks Pippin - good info. But they don't measure stiffness. All rubber like materials are very damp (visco-elastic) . But they are not stiff. So using this sort of flexible PU/filler as a structural material is no good as its not stiff. If you put it into a steel structure as I have said before the load path is through the steel (the stiffest path) not the fill therefore the dampness of the fill is moot. Make moulded EG or epoxy fiberglass parts, your well ahead.... Peter

    Plus all of the research uses large deflections to enable measurement of damping ratios. If we design a machine correctly the deflections hence strains are microscopic so the effects we want are not in the large deflection arena as we need to think in terms of micro frictional effects and the structures loadpath. Filling things with EG/foam/granules does not fulfill these micro level requirements. The only thing it does do is add mass and maybe support thin sections a bit better. but then they should not be thin anyway in this application....

    Pippin pick a single material (or a single composite epoxy fibreglass, epoxy CF, epoxy sand ) and work with it. Trying to use mixed media is unhelpful. Do the best you can in the chosen material, get it working then solve any particular issues that come up. KISS Simples
    Peter, agree for the fixed parts where section size and weight are not an issue. Large EG sections work well for the base and (non moving) columns.

    The question is for moving parts like the saddle. There is enough out there to show that combining materials is of benefit. Eg steel for stiffness and EG for damping. The question for the hobbyist is the best combination of performance, cost, and ease of implementation, and doing it only once. Not an easy task.

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    Default Re: Filling steel tubes with epoxy granite- machine before or after?

    Hi Pippin - I think that steel for stiffness and combined with EG for "dampness" is incorrect. I think the EG just adds mass (which is therefore damper) and its easier to make the steel really thick then add EG and get to the same place. You design a steel or aluminium saddle and I'll convert it to composite and I'll build it for you if you participate. We'll do an FEA, a modal and transient analysis just for the hell of it. Start a thread and off we go... Are you east coast or west? I'm at murwillumbah half way east coast...Peter



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    Default Re: Filling steel tubes with epoxy granite- machine before or after?

    Hi Pippin and others - A little exercise. I built a Dia100mmx6mm thick steel tube. 1500mm long. I then made another identical tube and filled it with EG E=30GPa density=2300kg/m3. I then made another tube Dia100mmx13mm thick which is same weight as the filled tube. I then supported them all at an end so they are cantilevers and ran a modal analysis. I'll round these to full hertz for simplicity:

    Dia100x6mm vibs at 42htz weighs 21kg
    Filled tube vibs at 33htz weighs 42kg
    Dia100x13mm vibs at 39htz weighs 42kg

    The open tubes will be about the same dampness so the difference is 42 to 39htz. Depending on the driving freq bandwidth will depend on if this is good or not. As cnc machines have ascending and descending operational speeds this delta is not enough. These are very close in terms of machine type driving vibs.

    The filled tube at 33htz should be damper but generally we want the structure freq to increase not decrease.

    Stiffness: I applied 300N to each cantilever and the 100x13mm deflected 0.49mm the filled tube deflected 0.67mm and the open tube deflected 0.83mm

    So for the DIY person the Dia100x13 is the pick, its stiffer and has similiar vibration modes to the filled and costs far less. I'm working on doing the damping analysis but its not straightforward with different materials in this software. I may have to do it in my GP FEA and that takes a bit to set up. For the DIYer this says use thick steel sections so you don't have thin walls panting and the corners are stiff to prevent sections lozenging...

    If you look at the stress plot you can see that the EG is not stressed very much at all. The loadpath is in the skins not the core. For the EG to be effective the material has to be cyclically strained which it does not when inside a steel container. I conclude that the extra mass is the benefit not the dampness of the EG so go heavier wall if necessary. In this particular case the delta is not very much... If you make the structure entirely out of EG thats different as the strain does then travel thru the EG. cheers Peter

    Attached Thumbnails Attached Thumbnails Filling steel tubes with epoxy granite- machine before or after?-deflection-jpg   Filling steel tubes with epoxy granite- machine before or after?-tubes-jpg   Filling steel tubes with epoxy granite- machine before or after?-stress-jpg  


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    Default Re: Filling steel tubes with epoxy granite- machine before or after?

    Hi Pippin - I keep coming around to laminated aluminium and plywood. The aluminium pretty much stops the moisture changes of the ply (I use lightweight ply 370kg/m3 which is balsa with outside timber veneer. Called Falcata ply from DMK Forest products) but normal good quality ply is fine. Maybe use F22 as its considerably stiffer then the falcata....Many aircraft floors that you used to walk on are aluminium balsa laminates. Al and timber are easy to cut using std power tools, routers etc. Timber is exceptional damp and comes in fixed size, no need for moulds. Sheet aluminium comes in std sizes so rough cut them to size, laminate with epoxy and you have a very stiff, very damp material... I think the issue with any new material is unless you can do the engineering maths you don't know how to size things, connect things or deal with it. So welded steel is the norm. Construction Al extrusion has made life easy but machines are not optimised, EG has low modulus and is relatively dense, you have to make moulds therefore you have to understand geometry and rigidity. I'll calculate a equivalent gantry size in Alply.... Peter



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    Default Re: Filling steel tubes with epoxy granite- machine before or after?

    Hi Pippin - I remember you are talking about a mill. Plywood no good for a mill... Peter



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    Default Re: Filling steel tubes with epoxy granite- machine before or after?

    Quote Originally Posted by peteeng View Post
    Hi Pippin and others - A little exercise. I built a Dia100mmx6mm thick steel tube. 1500mm long. I then made another identical tube and filled it with EG E=30GPa density=2300kg/m3. I then made another tube Dia100mmx13mm thick which is same weight as the filled tube. I then supported them all at an end so they are cantilevers and ran a modal analysis. I'll round these to full hertz for simplicity:

    Dia100x6mm vibs at 42htz weighs 21kg
    Filled tube vibs at 33htz weighs 42kg
    Dia100x13mm vibs at 39htz weighs 42kg

    The open tubes will be about the same dampness so the difference is 42 to 39htz. Depending on the driving freq bandwidth will depend on if this is good or not. As cnc machines have ascending and descending operational speeds this delta is not enough. These are very close in terms of machine type driving vibs.

    The filled tube at 33htz should be damper but generally we want the structure freq to increase not decrease.

    Stiffness: I applied 300N to each cantilever and the 100x13mm deflected 0.49mm the filled tube deflected 0.67mm and the open tube deflected 0.83mm

    So for the DIY person the Dia100x13 is the pick, its stiffer and has similiar vibration modes to the filled and costs far less. I'm working on doing the damping analysis but its not straightforward with different materials in this software. I may have to do it in my GP FEA and that takes a bit to set up. For the DIYer this says use thick steel sections so you don't have thin walls panting and the corners are stiff to prevent sections lozenging...

    If you look at the stress plot you can see that the EG is not stressed very much at all. The loadpath is in the skins not the core. For the EG to be effective the material has to be cyclically strained which it does not when inside a steel container. I conclude that the extra mass is the benefit not the dampness of the EG so go heavier wall if necessary. In this particular case the delta is not very much... If you make the structure entirely out of EG thats different as the strain does then travel thru the EG. cheers Peter
    33 to 42 hz is going to be easy to encounter right? If so, we need something to damp the vibrations. Steel alone will not be damp enough.

    I am not a fan of add mass (alone) to increase damping - it's not getting to the root of the problem. Increasing stiffness is always good, but reaches a limit due to size and weight (servo sizing etc) at some point. Decreasing weight is good - higher natural frequency and less inertia for servos to deal with. Damping is also needed to remove vibrations that do occur.

    Static stiffness is not hard to achieve. Dynamic performance is the trickier bit.

    Quote Originally Posted by peteeng View Post
    Hi Pippin - I remember you are talking about a mill. Plywood no good for a mill... Peter
    Yes, I'm looking at travels of 700x500x500mm. Dedicated metal mill. I'll do a design thread at some point. Just moved countries (back to Aus) so a bit busy. We used to holiday at Murwillumbah. I'm east coast also.

    I find carbon fiber interesting, however I have no experience with CF or FG and thus doubt it's a good way for me to go. It's not a simple first part to make...

    Vibration Damping Analysis of Lightweight Structures in Machine Tools: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5503333/

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    Default Re: Filling steel tubes with epoxy granite- machine before or after?

    Hi Pippin - Firstly its a design exercise then we can decide to make it or not. Collaboration is in at the moment and I've made all sorts of boat, aircraft, motor vehicle and industrial stuff so happy to help.... As I said also happy to make it...Can make aerospace quality parts in your garage using infusion these days. Step forward Pippin your well supported here...Peter



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    Default Re: Filling steel tubes with epoxy granite- machine before or after?

    Peteng, would MDF have similarly desirable properties like ply?

    I am considering a sandwich of 1/8”AL- ½” MDF - 1/8”AL for a router.


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    Default Re: Filling steel tubes with epoxy granite- machine before or after?

    Morning Koenbro and others - See attached calcs and data.

    1) MDF has a modulus of ~3500MPa, cheap F8 plywood is about 9000MPa so is twice as stiff. MDF is about half the cost of plywood so its a cost benefit ratio
    2) for bending loads your proposed [1/8"Al / 1/2"wood / 1/8"Al] is equivalent to a 5/8" (17mm) piece of aluminium. Its 19mm thick so its very worthwhile for that purpose. MDF and Ply are about the same density so no benefit in weight
    3) In plane the stiffness of the sandwich is 6mm so you gain stability if the loads are inplane but you don't gain stiffness
    4) I'd take advantage of the sandwich effect and use thicker timber. Being a lightweight seeker I'd use lightweight ply (370kg/m3) but again dearer then MDF
    5) On a completely engineering note I'd use MDF as its cheaper and will do the job. 3mm is quite thick for this sandwich skin so I'd go thicker MDF
    6) Definitely edge seal with epoxy or suitable resin to prevent moisture ingress at edges

    The alloy and temper of the aluminium is not critical its its modulus (70GPa) that your using. But as you probably will bolt things through the sandwich use a hard temper to resist crushing. I use 2mm a bit as its lighter and by using a thicker core can get same stiffness. But then if its really overall thick this may impact your structure size.

    You can glue the sandwich with epoxy, urethane or contact cement. Do not use foaming urethanes I have found them to grow a little over time...

    Cheers Peter

    since this thread is about damping, the sandwich you describe will be very damp as well. Timber is extremely damp and in combo with the Al is a great solution I think. Start a design thread will follow with interest.

    If you live in an area of high moisture variation I'd go with the ply as it will be more stable over time. The cost of the timber in this project will be a small % of the total project whether ply or mdf. With any machine use the best materials you can afford for a project. If the machine has a long life the cost delta will be insignificant vs the machine reliability and performance in a couple of years time. Unless one of the design tenants is "lowest cost" of everything then mdf is the answer. I suppose you could use MR (moisture resistant) mdf in that case...

    Attached Thumbnails Attached Thumbnails Filling steel tubes with epoxy granite- machine before or after?-mdf-jpg   Filling steel tubes with epoxy granite- machine before or after?-plywood-jpg   Filling steel tubes with epoxy granite- machine before or after?-stiffness-jpg  
    Last edited by peteeng; 04-13-2020 at 05:13 PM.


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    Default Re: Filling steel tubes with epoxy granite- machine before or after?

    {Vibration Damping Analysis of Lightweight Structures in Machine Tools: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5503333/[/QUOTE]

    Hi Pippin and others - I've read this article a couple of times. Its interesting because they have gone to a lot of trouble evaluating various things. The author however has a bias towards aluminium foam AFS (or the work sponsor has probably) . Even though the aim is to reduce weight the AFS is the heaviest solution, he concludes that this can be fixed with a redesign. The CF however is lighter stiffer and damper yet its relegated to second choice. That's not really the point for me, the work shows that CF (and by extension) fibre reinforced laminates are stiffer stronger, damper then metals and much easier to make then foamed aluminium. Interesting also in the specific stiffness chart that steel does better then cast iron. I'm not sure why E to the third is used, E/density is good enough for this comparison. So I encourage people to get familiar with composites as that's the way these things have gone and will continue to go. Aircraft and planes have gone from steel to aluminium to composites in my working career so should we. I helped build a 20mx3mx3m 5 axis router some 25 years ago. It was 100% carbon fibre as the company made composite planes so said why not a composite machine? Other high level machine builders are already building CF parts so should we... Peter



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    Default Re: Filling steel tubes with epoxy granite- machine before or after?

    Hey Pippin - I'm setting up some Tetrium and CF to be modulus and strength tested at a Uni so I'll have some good numbers very soon. Cheers Peter



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    Default Re: Filling steel tubes with epoxy granite- machine before or after?

    Hi All you EG people out there - I have been thinking about how to evaluate the dampness of a material at home vs in a lab. Last night I thought about the concept of co-efficient of restitution. Many years ago I made scooter wheels for a company. My wheels had good "pop" apparently. This is because I used hot casting resins and post cure to create a very resilient resin. The company tested competitive wheels buy dropping them and noting their rebound height on the brick wall at their factory. I was going to make a fixture so it was a bit more formal but I could not make wheels at the price they wanted so that project stopped. But we can use the same approach for the materials. By dropping ball bearing onto the surface of a material we can either see its rebound or count its bounces. If you have dropped a steel ball onto a steel surface they bounce very well. If you dropped it into plasticene it would coalesce. So now we can compare the rebound of EG AL etc against steel to rank the materials. Now I have to find a good size ball bearing. I have the rest... Peter

    Attached Thumbnails Attached Thumbnails Filling steel tubes with epoxy granite- machine before or after?-wheel-1-jpg   Filling steel tubes with epoxy granite- machine before or after?-mould-jpg  


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Filling steel tubes with epoxy granite- machine before or after?

Filling steel tubes with epoxy granite- machine before or after?

Filling steel tubes with epoxy granite- machine before or after?