New Machine Build How much rigidity do I need - Page 2


Page 2 of 2 FirstFirst 12
Results 13 to 24 of 24

Thread: How much rigidity do I need

  1. #13
    Activation process peteeng's Avatar
    Join Date
    Jul 2018
    Posts
    297
    Downloads
    0
    Uploads
    0

    Default Re: How much rigidity do I need

    Thats a great article -Thanks Millnut.

    Henry why don't you buy a small mill as a starter? You then can make parts, learn about machining and get on your way. In US should be about $1000 USD? You can CNC convert it in time and do good things? Your heading down a path that is very difficult for someone in your situation and state of knowledge.

    Hysteresis - Materials have internal friction called hysteresis. If you get a paperclip and bend it backwards and forwards a few times then feel the middle you will notice its hot. This is the heat due to friction of the steels grains rubbing together just like you do to keep your hands warm. Steel has small grains very well packed together and this slippage is quite small resulting in a low hysteresis material. Also the grains all have the same elastic properties so they move together in harmony sort of. Cast iron has large grains and also has graphite particles within its grain boundaries. This allows the grains to slide past each other quite a bit more than steel so they have more internal movement and friction then steel has. The iron and the graphite being different modulii also means they move differently to each other further contributing to internal disharmony and increased damping. Epoxy concrete and fibre composites rely on this effect to be damp as well. Their different materials move differently internally so as to create internal resistances to motion and therefore have high hysteresis.

    One strategy used by aircraft against vibration (particularly flutter look it up) is to use metal laminates. Airbus make their fuselages out of "GLARE" an aluminium laminate. So Henry, consider creating a bolted metal laminate mill. a) the laminates being dismantlable are lighter for you to lug around b) they are damp. So your 25mm steel could be 4x1/4" steel or Al - in-plane they are the same stiffness as 1" solid, in bending if you bolt it correctly it will be the same in flexure stiffness as well. Being 1/4" thick you can get a local sheet metal place to bend it for you to create bolting flanges vs trying to bend 1" etc etc. You can also tailor the thickness in different areas easily vs having to mill them thinner. Look up a laser cutter/bender near you its a cheap process if you get the right company. More like 3D printing approach in layers even use 1/8" ??Peter

    Also do not think that a bolted up structure is not as good as a welded structure. Welds and bolts/rivets are just different concepts to get the job done. There are heaps of old bridges still standing that are riveted and bolted together. Aircraft are bolted together. If you welded them they would fall out of the sky due to fatigue. So think it though you maybe able to build your dream machine after all.

    Attached Thumbnails Attached Thumbnails How much rigidity do I need-mill-jpg  
    Last edited by peteeng; 04-08-2019 at 06:50 AM.


  2. #14
    Activation process peteeng's Avatar
    Join Date
    Jul 2018
    Posts
    297
    Downloads
    0
    Uploads
    0

    Default Re: How much rigidity do I need

    Hi Henry - I've just read the thesis linked prior. Excellent case study and summary of how a machine should be designed. The machine in the study uses viscous damping via layered tubes to dampen vibration. I mentioned that you could use laminated metal to build your machine. If you place a thin rubber sheet between your layers you have an excellent damp structure. Perhaps the start of a new technology for machine building and your just the person to get it off the ground. If you are not breaking down the structure for transport then epoxying them together or contact cement is the way to go. I was going to look at this for Maximus. 2mm zinc anneal is the cheapest material to cut via laser. So a ZnA laminate is the process I've been looking at. Peter

    Last edited by peteeng; 04-08-2019 at 05:00 PM.


  3. #15
    Activation process peteeng's Avatar
    Join Date
    Jul 2018
    Posts
    297
    Downloads
    0
    Uploads
    0

    Default Re: How much rigidity do I need

    https://drive.google.com/file/d/1O1N...ew?usp=sharing
    Does it look like it will work and do you spot any glaring design errors? Thanks a lot.[/QUOTE]

    OK why do you have two beams for the gantry? One beam the size of the two beams will be much stiffer. Every component must take up the maximum space made available for it to maximise stiffness. And the thick top and bottom of these for the rail connection, how do you intend to achieve this? Peter



  4. #16
    Activation process peteeng's Avatar
    Join Date
    Jul 2018
    Posts
    297
    Downloads
    0
    Uploads
    0

    Default Re: How much rigidity do I need

    Hi Henri - I mucked about with laminates and learnt a bit. The images attached are of a 20mm thick solid and 20mm thick laminate. The laminate has similar vib frequencies and shapes. But I expect that the viscoelastic nature of the interlaced material will dampen vibs quicker compared to a solid. I'll try to find the damping info for the materials and run some dynamic solvers to see if they are damper. I created them the same thickness as for a flat plate like this the modes are all bending so the thickness needs to be the same to keep it apples for apples. This model has the 1mm interlayers as epoxy. I played with aluminium, plastic and other stuff as the interlayers. There's lots of variation so lots of learning in this area if you get into it. Peter

    Attached Thumbnails Attached Thumbnails How much rigidity do I need-capture-899-jpg   How much rigidity do I need-capture-886-jpg  


  5. #17
    Member
    Join Date
    Aug 2004
    Location
    spain
    Posts
    690
    Downloads
    0
    Uploads
    0

    Default Re: How much rigidity do I need

    The pdf posted is excellent - read it some years ago.

    One takeaway - for milling steel.
    Milling machines need 20-30 Nm/um deflection // rigidity iirc.
    And better machines have 70 Nm/um and up.

    Look at a small 2 HP Bridgeport.
    If you push the edge of the table, 2 fingers, == 10-20 kgf = 150 Nm (middle), it will twist 0.04 mm, plus or minus something.
    So 200 Nm twists 40 microns, at the edge, or 5 Nm/um at edge ==> around 5x less at spindle, == 25 Nm/um rigidity.

    Likewise, a modern 40 HP HAAS VMC running, slap the spindle, the workpiece will have a visible divot (many microns, === 10-20 um maybe depending on the tool).
    ==> maybe around 40-50-70 Nm/um rigidity /my guesstimate.
    A japanese Mori Seiki or Okuma would typically have 50% more mass, and about double the rigidity of a HAAS, and double the price, per work envelope.

    The bigger machines have a big advantage in that the heavy tables and machine frames absorb vibrations.
    So they are more rigid and also more forgiving/efficient.

    Lathes since 80 years ago and modern machine tools (VMC etc) are all pretty similar.
    The rated load is == 50x the breaking load.
    Machine tools are only loaded at max 2% of their "strength" as in breaking load.
    The 1200 kgf max push force == 40 HP power equates to about 2400 kg mass on the table // VF3 / similar.
    Using 40-50 mm linear guides == 70-90 metric tons theoretical load.



  6. #18
    Activation process peteeng's Avatar
    Join Date
    Jul 2018
    Posts
    297
    Downloads
    0
    Uploads
    0

    Default Re: How much rigidity do I need

    Hello Hanermo - So if we use 25Nm/um as a design target are you saying that if I place a torque wrench on my machine tool end and exert 25Nm it should deflect 25um? 25Nm is 2.55kgf/m so over say 10mm its 255kgf. ? ie if I push the tool with 250kgf it deflects 0.001mm roughly Peter



  7. #19
    Member
    Join Date
    Aug 2004
    Location
    spain
    Posts
    690
    Downloads
    0
    Uploads
    0

    Default Re: How much rigidity do I need

    If you push the spindle nose with 25 Nm, it should deflect 1um, for 25 Nm/um rigidity.
    25 N force is 2.5 kgf, or 2.5 kg of push force.

    Force at the tool end no, at the spindle.
    The toolholder will flex a bit, so will the collet a bit more and the tool a lot.

    A toolholder with collet, gage pin inserted, torqued properly, is ideal.
    Push-pull toolholder, not the gage pin.
    Or push-pull spindle if possible.

    A dti clamped to table, indicate spindle nose, will show spindle nose deflection.
    It is likely to be 10x-50x more than You ever expected on (best) hobby stuff.

    There are youtube videos showing x3 mills, and iirc skyfire cnc machines, dti readings under load with hand forces.
    About 0.1 mm deflection or more under 20 kgf load (200N).
    Terrible, 5x worse than a bridgeport.

    Comparison.
    My 12x chicom lathe deflects about 0.015-0.02 mm, 10-20 um, under 40-50 kgf load by hand, 12 cm out from spindle nose.
    12 cm out is a big deal.
    Rigidity is length power 3.
    As I pull the 10" chuck really hard.
    It deflects about 10 um fairly easily, maybe 5 kgf, and then gets exponentially more stiff.

    The bearings have low preload, imo, and this is why it deflects easily at first.
    Probably 70 mm inner D bearings, == 2500 kgf rated load.
    Chester craftsman lathe, new from chester uk circa 2005.
    Heavy, light-industrial type lathe.

    Lathe (and vmc spindle) bearings are a very delicate topic.
    So, since it works extremely well, and I need the lathe, I have not had the courage + spare $$ on-hand, + potential downtime, to adjust the spindle bearing preload.
    The primary locknut needs about 0.05 mm of angular movement.

    Quote Originally Posted by peteeng View Post
    Hello Hanermo - So if we use 25Nm/um as a design target are you saying that if I place a torque wrench on my machine tool end and exert 25Nm it should deflect 25um? 25Nm is 2.55kgf/m so over say 10mm its 255kgf. ? ie if I push the tool with 250kgf it deflects 0.001mm roughly Peter




  8. #20
    Member
    Join Date
    Aug 2004
    Location
    spain
    Posts
    690
    Downloads
    0
    Uploads
    0

    Default Re: How much rigidity do I need

    I used Nm incorrectly, when I meant N as in force.
    10 N == 1 kgf, or 1 kg of push or pull.
    The f indicates force.

    My examples are meant to illustrate, qualify, add credence, and act as facts others may test and comment on.
    Thus where I make errors, others can easily see and point them out.

    Likewise, I try to add real-world values, so all here can see if these posts pass the smell test and are in line with generally accepted data / videos /etc.
    Likewise, I try to add some industrial data and stuff from what I learned and saw, within limits.

    Precise customer data and industrial data and pics must not be posted, unless it was posted/permitted by the customer/manufacturer to my personal knowledge.
    So I don´t post pics, sorry.



  9. #21
    Activation process peteeng's Avatar
    Join Date
    Jul 2018
    Posts
    297
    Downloads
    0
    Uploads
    0

    Default Re: How much rigidity do I need

    Hi Hanermo - What I and others in the forum are trying to establish is a reasonable design deflection spec for a machine. We have the tools to design machines if we have the specs. So from your writing above and please correct as necessary...

    Rough Process: place a tool in spindle or router and snug up. Use dial gauge at collet area as close to tool as possible. Push or pull here at 25kgf, what deflection will I get? also put spindle at mid span of gantry and full down Z, which should be worse case structural configuration.

    Hobby machine 25kgf deflects >0.1mm plywood, plastics and foam
    Good hobby machine 25kgf <0.1mm plywood, plastics, foam and aluminium
    industrial machine 25kgf <0.01mm or 0.001mm? metals
    precision machine 25kgf <0.001mm or 0.0001mm metals

    Thanks Peter

    I pick 25kgf as its easier to actually do. Nothing to do with actual tool force yet. The machine should have a linear response so if we have a tool load of 250kgf then we have 10x the noted deflection... Peter



  10. #22
    Member deadlykitten's Avatar
    Join Date
    Jun 2015
    Location
    Antarctica
    Posts
    2676
    Downloads
    0
    Uploads
    0

    Default Re: How much rigidity do I need

    hello, i am not so much into building machines, but maybe this helps : https://www.makino.com/resources/con...ing-moment/392

    so far i did not had time to apply & check what that video suggests, but, in short, it seems that BT cones are the most rigid for the "teoretical heavy specs", while other types are more rigid for the real, most common " cutting specs ", but, above this range, they start to deflect pretty fast; in other words, use the machine in a 'normal' manner, regardless of what shank type is there / kindly

    Last edited by deadlykitten; 04-12-2019 at 04:35 AM.
    Ladyhawke - My Delirium, https://www.youtube.com/watch?v=X_bFO1SNRZg


  11. #23
    Activation process peteeng's Avatar
    Join Date
    Jul 2018
    Posts
    297
    Downloads
    0
    Uploads
    0

    Default Re: How much rigidity do I need

    Hi Henry - Hows the project going? Looking back at your numbers the FEA model is 150lbf/0.001" this is 262N/um. Reading the thesis and reviewing Hanermo s input this is off the dial in terms of stiffness for machines so if the FE is true then you have a remarkable machine. 10-25N/um is a typical mill machine and a precision machine is 70N/um so get it going.... Peter



  12. #24
    Activation process peteeng's Avatar
    Join Date
    Jul 2018
    Posts
    297
    Downloads
    0
    Uploads
    0

    Default Re: How much rigidity do I need

    Hi Henry - 150lbf/0.001" is not 262N/um sorry my maths was wrong. Its 22N/um which is still a very stiff machine. Any news on the machine? Peter



Page 2 of 2 FirstFirst 12

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  


About CNCzone.com

    We are the largest and most active discussion forum for manufacturing industry. The site is 100% free to join and use, so join today!

Follow us on


Our Brands

How much rigidity do I need

How much rigidity do I need