Choosing the right rails for rigitdity !
Finally I am gonna embark on the CNC building journey been a dream for so long. I feel I should just start the project. I don't have any fancy tools just a B&D jigsaw / a bosch hand drill / Makita RT0700c trim router. I am an EE and been designing CAD for a really good while now.
Disclaimer : Don't hate me for the crude drawings and handwriting my PC is down so I am going analog. (typing on a really antic laptop)
I have gone through numerous threads/articles over the years and did some research on the building a CNC a lot of you here have inspired me
For a lack of CAD I am gonna use this design as a reference for you Building your own CNC router/milling machine - All.
I feel there are a few flaws in the design. I am no mechanical engineer so I have arrived to the conclusion just using my common sense.
1. He uses HGH25CA really huge Linear block. I feel that I can get by with using 2 of HGH15CA instead of HGH25CA or HGH20CA which seems to be the popular choice in the forum. Am I wrong ?
2. He uses only one Block per Axis. Even though HIWIN blocks are rigid as chuck I feel there might be some twisting action on the blocks. So I plan on using 2 blocks now and in the future 3(after I save up more money). This will increase the number of point of contacts and make it more rigid. Am I right to assume that ? Or would the difference not really matter. I love them accuracy and precision.
3. Belt drive and stepper is my other concern. I want to do the same I would be going for a NEMA 24 25kgcm or Nema 23 28kgcm or Nema 34 30+kgcm motor. Would the belt drive be rigid enough for the same ? I have been thinking of using HTD3M timing pulley and belt for the arrangement. I like the belt drive because it keeps the stepper in and no poking out. If that is no good I would be going for a chinesium rigid coupling.Which would be better in your expert opinion.?
4. He uses lead screw. I am gonna be going for tobaki ball screw[Local Distributor claims that they used to manufacture HIWIN products now they have released their own. Does anyone know ? He asked me to get TOBAKI branded Linear blocks and rails ] 1605 screws and Nuts. Would keep the stepper torque requirements down I suppose and smooth operation. Everyone uses that so I am pretty confident in that.
I am building this machine to mostly machine wood/acrylic and aluminium and would love to do steel as well[I know the machine should be really rigid and the steppers must be beefy so I have NEMA 34 in the list]. I don't mind waiting a long time and taking really slow passes to machine steel(I am a dreamer).
More Z travel is something I would like to have around 200-300mm would be lovely and that's why I haven't finished up the Y axis plate heights. Increasing the height would make the Z axis not so rigid (just using common sense feel free to correct me if I am wrong) but I would to build a mini 4th axis (I like to dream a lot )
In the crude image below would the distance between the HGH15CA blocks greatly affect the rigidity if so what is the minimum distance one should have to attain good rigidity ?I have seen many members on the forum who just put blocks close to each other(0mm distance). is that alright ? [I am not an mechanical engineering so I don't think I can calculate those so please kindly help me out]
And really rough Y axis plate Height would be 450mm from the bottom that would give around 250mm distance between 0 level on the machine to bottom of the Z axis plate. [More or less 250mm of travel] Would Y axis Plate height also influence rigidity ? Is that why most CNCs have a really less travel since it is all suspended and not fixed gantry.
Thank you for taking the time to read everything. Really Appreciate it
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Last edited by asniex; 02-16-2017 at 02:42 PM.
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