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  1. #41
    Member peteeng's Avatar
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    Default Re: WillMill - HSM Benchtop Composite Mill

    Hi Will - When you mould something it has to have "draft" this allows the part to be removed from the mould. So th draft will make traps natural. Usually use 2-3degs on a mould but the more the better. Peter.



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    Default Re: WillMill - HSM Benchtop Composite Mill

    Hi Will - A little bit on infusion for you. Peter





  3. #43

    Default Re: WillMill - HSM Benchtop Composite Mill

    Hi Peter, thank you for the replies. I'm aware of draft angles, all the relevant surfaces will have draft angles on the final part. I've been researching again and reading some threads on the forum, I've learned a lot more and gained more confidence in the art of composites, although I still don't have any real world experience (that will come soon). Peter you're right I have reverted to saying strength instead of stiffness without even knowing it. Thank you for correcting me.
    Great video Peter, I saw it just a couple days ago haha. I like Easy Composites, they make som great content and they have a nice web shop. I currently have a few items in a shopping cart on their website.

    This is a good read for anyone learning about building machine parts with carbon fiber.
    https://www.cnczone.com/forums/diy-c...40246-cnc.html

    CompoTech makes a variety of components from carbon fiber and composites. They have also designed a carbon fiber machine and written a few whitepapers, read about it here:
    https://compotech.com/about-us/downloads/

    Design update:
    Gantry have been redesigned to a height of 180 mm.
    I'm currently working on a simple way to simulate carriages for linear rails - I might just have come up with something genius, I'll share it soon. When the linear rails responds realistically to the forces on the machine, we can do a full gantry simulation with the z-axis and get a better idea of the machines stiffness. From here we can decide whether or not the the height and z-axis travel can be increased.

    - William

    Attached Files Attached Files


  4. #44
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    Default Re: WillMill - HSM Benchtop Composite Mill

    stiffness measurements cnc mk3

    router stiffness measurement. built from extrusions 1.25N/um.. Peter



  5. #45
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    Default Re: WillMill - HSM Benchtop Composite Mill

    Hi Will and Others - I have been looking at the placement of bearings on square gantries. Seems the diagonal approach is the stiffest. This is 150x150x6mm sq section with 1000N at 250mm. Not by much but it is stiffer and it also has advantages with bearing & drive placement. Cheers Peter

    Attached Thumbnails Attached Thumbnails WillMill - HSM Benchtop Composite Mill-torque-1-jpg  
    Last edited by peteeng; 05-18-2020 at 06:54 AM.


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    Default Re: WillMill - HSM Benchtop Composite Mill

    Quote Originally Posted by peteeng View Post
    Hi William and others - I was watching a video of the Taiwan Machine show from 2015 and saw this, a CF gantry on a Hiwin machine. This particular gantry is a cantilever. I'm sure we shall see more of these around. Peter


    I do like air bearings and intend to make a machine with these soon.
    Carbon fiber has been quite common for laser machines parts for some years, light weight high speed low inertia gantry's carbon fiber is a perfect fit for machines like this

    Mactec54


  7. #47

    Default Re: WillMill - HSM Benchtop Composite Mill

    Thank you for sending a link to the thread on stiffness measurements Peter, that was insightful.

    Regarding the arrangement of the guideways Peter, could the increased stiffness potentially be due to the extra material? - and not the placement of guideways. Some bearings/cars are stiffer in certain directions, I guess this would be a scenario where one could benefit from certain placements of the guideways.

    *I will refer to the sliding block on linear guideways as a bearing.

    Regarding the machines design there is not much to update you guys on, I have however found a great way to simulate linear guideways. In order to simulate a bearing so that it deflects realistically it's easiest to get your hands on a data sheet for deflection values of specific loads for the type and model you intend to use. At the moment the choice of guideway and linear rail will be a 20 mm RG bearing from Hiwin. The RG series has roller bearings instead of ball bearings. I have chosen the RG series, because It seems like it will have less vibrations as it's in contact with a bigger area of the guideway compared to ball bearings. The RG-series from Hiwin also has other properties like increased rigidity. I have contacted Hiwin and distributors with no luck, I couldn't get my hands on a data sheet with deflection values for the RG series. Peter has sent me a document from Hiwin with some deflection values for different bearings in the EG series, so my target deflection values will be based on numbers from this document. I'm not sure if I'm allowed to share this document, as it says it's confidential with a red square around it. I'll ask Hiwin if I'm allowed to share it here before uploading, as I'd like to stay out of trouble.

    I can imagine four ways to simulate bearings for linear guideways.
    1. Full simulation with every part and element of the bearing with correctly assigned material properties.
    2. Constrained simulation of simplified bearing (use simulation tools like contacts and other constraints in the used FEA-package to achieve deflections according to the manufactures data)
    3. Simulation of an simplified bearing geometry that is modeled to behave according to the manufactures deflection values.
    4. Split a simplified bearing into two different bodies and edit their individual material properties in a way that it will deflect according to the manufactures data. (Peters idea)
    * Solution 2-4 could be mixed together

    Now these options give me a few different ways to achieve the desired results. I'm going to rule out option 1, as I'd like to keep things simple, and because it's unnecessarily complex. Option 2. however will depend on the FEA software available. In Fusion 360 a stiffness factor can be applied to different contacts. There's different contact types for different purposes... A contact tells the computer how two surfaces should interact. I made a "bond" test in Fusion 360's simulation workspace simulating a contact type with a stiffness factor of 0,01 and 100. I found that the stiffness factor only affects the "rough" type contact. This means the the rough contact is the only type of contact of which the stiffness can edited. I made a simple bearing the can be found in the PDF document below. The top surface of the guideway had a bonded contact to the bearing, and the sides of the rail had a rough contact to the beaning so that the deflection from the front load could be controlled by the stiffness factor. I found stiffness factor that had equal deflection for front and top loads, but I couldn't figure out a way to control the deflection for loads applied to the side of the bearing, so this solution was neglected. - By editing the geometry that is in contact with the top surface of the guideway, one might be able to find a solution that has equal deflection in all three directions.

    I have tested multiple designs and mixed different materials, stiffness factors for rough contacts while simultaneously editing the geometry in an attempt to make the deflection more controllable. In total there has been a total 368 iterations and even more simulations in an pursuit to come op with the simplest way to simulate bearings for guideways. The solution is 100% geometrical and deflects equally in all directions. The scale of the deflection can be controlled by simply editing the stiffness of the bearing material. The ratio between the deflection of each of the forces can also be controlled relatively easily (not all cars deflect equally in all directions). The beauty of having a geometrically controlled bearing deflection is the fact that you don't have to spend time applying different types of contacts and stiffness factors for each bearing in the simulation - This could be time consuming and have potential for human errors, as there would be a lot of contact's to control and stiffness factors to worry about. Without the use of contacts (all contacts bonded - by default) the bearing can simply be inserted into the assembly and will deflect the way it should.

    The solution, the finished bearing design, can be seen in the picture below. It consists of "springs" that basically are thin and scew walls that can deflect. I've put two pairs of these springs on the bearing and oriented them 90 degrees apart, so that the radial and lateral deflection can be somewhat controlled. The thinner the scew walls are, the more the bearing will deflect under a load from the top surface. The scale of the deflection can be adjusted by editing the stiffness of the material applied to the bearing being simulated. The dimension of the bearing that I have used are based on a 20mm RG bearing from Hiwin without the dust caps. The simulations of this design that can be seen below does not have the correct deflection values, they should be closer to 0.03 mm for the type of bearing that I'll be getting. More on this in next weeks update. The simulations in the picture below all use a force of 3000 N applied to the whole surface.

    The design will not reflect the deflections of the data sheet from Hiwin 100% but it's close. If you'd like to try and find a better solution than what I have found, I encourage you to do so and post the solution here. Simulate -> results -> edit stiffness factor/dimensions/material properties --> repeat.


    I'll upload the file next week along with other details. I'm working on a website that that I'd like the file to be uploaded to, that's why I'm not uploading it here.

    Attached Thumbnails Attached Thumbnails WillMill - HSM Benchtop Composite Mill-20mm-rg-rail-geometrical-spring-bearing-jpg  
    Attached Files Attached Files
    Last edited by Williamlii; 05-19-2020 at 10:26 AM.


  8. #48
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    Default Re: WillMill - HSM Benchtop Composite Mill

    Hi William - The extra material should decrease the stiffness not increase it. I think its because the local load is less as the bearings are further apart (1.4x apart if the tube is square) and the loads are applied to the section more symmetrically leading to less shear deflection. You can publish the hiwin data I have in other spots in the forum and the material was supplied not under a non disclosure agreement so I expect its public knowledge.

    In your carriage model can the car still slide? allowing it to slide is more important then getting the local stiffness correct. The sliding produces less stiffness (zero stiffness in the pitch dirn) and if so if the load is "up" how does it stay connected to the rail? or is this a spring connection? Regards Peter



  9. #49
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    Default Re: WillMill - HSM Benchtop Composite Mill

    Morning William and lurkers - William (and others of course) the axial stiffness of an element is FL/EA. See attached. So you can quickly calculate the required dummy material stiffness for the car. In this case I took the EG15SA car stiffness. It deflects 0.015mm at 2000N in the heave direction. So I made a 40x15x1mm thick pad at the car rail interface. The manual calc says the material has to be 222MPa to do this. Then I made an FE model and it predicted 0.012mm. This is because the FE model includes the poisson's ratio effect so it gets a bit stiffer as it stretches as it's contained. But you can back the E off or consider it close enough. You then make patches in the transverse dirn and your done. In this way you can define sliding surfaces as well and the bearing will behave nicely. Regards Peter

    I have attached the bearing data so you don't have to fret about lawyers turning up on your doorstep... time for you to reveal a mill model..

    Also remember that in the early rounds of the design process this condition is one of the very minor contributions to deflection, sliding is more contributory for instance. So you maybe over complicating things early on. Once you have honed down the design to near final then it maybe worth doing this to see how much it contributes. For instance you have bonded connections vs bolted and fasteners will be very compliant vs the bearings.

    Attached Thumbnails Attached Thumbnails WillMill - HSM Benchtop Composite Mill-block-jpg   WillMill - HSM Benchtop Composite Mill-stiffness-jpg  
    Attached Files Attached Files
    Last edited by peteeng; 05-19-2020 at 07:14 PM.


  10. #50
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    Default Re: WillMill - HSM Benchtop Composite Mill

    Hi Will - This is the stiffest mill I have found. https://www.mmsonline.com/articles/a...e-for-titanium its 350N/um.... Peter



  11. #51
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    Default Re: WillMill - HSM Benchtop Composite Mill

    Hi William and other persons of interest - Had more of a think about the diagonal bearing arrangement.

    1) When you place two bearings for instance on a cantilever you create an "analytical wrench' The wrench works about the centre of the bearings. When the bearings are on the front face of a gantry the centre of action is forward of the front face. This means the moment is eccentric to the gantry ie you have a global moment on the gantry and a local moment (wrench) on the gantry. The wrench contributes to lozenging which is the shear deflection.

    2) The shear centre of a member is the place at which if you place a moment the member does not lozenge, the member twists but does not change local shape. The shear centre on a square section (or any closed section) is its geometric centre or centroid. So by placing the bearings on the diagonal the wrench centre is close to the shear centre meaning the member is most effective at resisting torsion. So maybe this can be improved further by ensuring the bearing centre is at the shear centre of the member if possible. Cheers Peter

    Theory | C3.2 Shear Centre | Solid Mechanics II

    https://www.civilengineeringterms.co.../shear-center/



  12. #52

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    Hi Peter, you have definitely written some insightful very helpful information in your replies. I'm currently out sailing with some friends. I'll be sure to reply when we get home in the weekend :-)
    - William



  13. #53
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    Default Re: WillMill - HSM Benchtop Composite Mill

    Hi Lookers - There is a slight hickup in figuring out the action centre of the square rail bearings. If the rails are the round type that can't support a moment then the bearing geometric center is close enough. If you are using square rails that support a moment then the bearing center is unlikely to be at the geometric center. The action center will be dependent on the structure stiffness and the cantilever length. If you are using FE then you can look at the bearing reactions and figure out its action line. If not then the geometric center is your best guess. Cheers Peter



  14. #54
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    Default Re: WillMill - HSM Benchtop Composite Mill

    Here's some nice work -

    My aim would be to achieve the same quality machine at 1/3rd the weight.. Peter



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    Default Re: WillMill - HSM Benchtop Composite Mill

    Hi William - Enjoy sailing, I'm away for a week so won't hear much of me for a while. Peter



  16. #56

    Default Re: WillMill - HSM Benchtop Composite Mill

    Hi Peter - The simplified bearing I made is "bonded" to the linear rails. Good catch about the fact the they should able to slide in order to predict the deflection of the machine more accurately. I changed the contact type from bonding to sliding, and F360 gave me some weird and unexpected results. I'm sure I can figure out a workaround, but at the moment I feel like I have to get back to the machines design instead of going further down this rabbit hole. I believe that the current simplified bearing that I presented above will land me in the ballpark of realistic deflection of the linear bearings I intend to use.

    #49 Regarding the deflection in the heave direction of the simplified bearing I designed, the deflection is the same as when it was compressed (load from top to bottom). I'm amazed by what you can achieve with some quick calculations Peter, I always find those very interesting. But with the calculated dummy material I'm not sure how your bearing would behave when the load comes from different directions? - the goal is to make it deflect somewhat equally in all direction (top, front, right). - On a side note I'd like to thank you for posting the document on bearing stiffness here on the forum.

    I'm applying to become a mechanical engineer so that I too can make calculations like these and more!


    #50 The ssvf machine frame is very interesting Peter. What a machine! I'm genuinely impressed by the ingenuity of what some engineers can achieve. - I didn't know that hydrostatic bearings exited for guideways, what an amazing piece of technology.

    #51 Great explanation Peter, thank you for introducing me to the principle of shear center. This is very valuable information for someone like me.


    Thank you it was a great tour on the boat. The sailboat was made of glass fiber, so I couldn't stop thinking about building boats with all my new knowledge on composites (no experience working with them yet) haha.

    Cheers William



  17. #57
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    Default Re: WillMill - HSM Benchtop Composite Mill

    Hi Will - The dummy bearing has to have the "cushion" material on all sides not just the top. The model was quick to check the hand calcs. The model should also incorporate a feature to make the car captive to the rail. I'd use the actual rail model and a conforming car to suit.

    William when I graduated from Uni, FE was for the rocket scientists and us mech engs had to do everything by hand. You are in a period where all of these things are available on the desktop but you must understand the basics or the desktop software will lead you astray. You must do reality checks often... The first FE I did was a beam model of a wheelchair. It took me a week to calculate all the co-ordinates of the nodes and enter them into the software and it crunched several hrs to get an answer (Z80 8bit processor). Then once I got into serious FE work automeshers were very very expensive so I spent a few years building FE meshes manually. You learn a lot doing these things.

    Now for some time I have been interested in plywood metal laminates. So I did a comparison of a 100x100x6mm thick aluminium extrusion vs a 100x100x16mm thick MDF box. Outside the box is 4mm of al or 2mm of steel. The corner rad of the extrusion is 12mm so the "plating" is in between the fillets ie its 76mm wide by 1200mm long. The MDF handles the shear flow within the section and the plating handles the bending. Both the Al and steel plated sections deflect the same as the 100x100x6mm al section. This type of structure is easy to do and can incorporate extra features and the weights are similar. So I think the next machine I build may go this way. Peter

    The LHS image is the aluminium section... the pad at the front is the load patch. I have placed a 1000N (100kgf) load 200mm below this to torque the gantry. They both deflect 0.2mm

    Attached Thumbnails Attached Thumbnails WillMill - HSM Benchtop Composite Mill-timber-jpg   WillMill - HSM Benchtop Composite Mill-plating-jpg  
    Last edited by peteeng; 05-31-2020 at 02:45 AM.


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