Page 2 of 17 FirstFirst 123412 ... LastLast
  1. #11
    I Charlie,

    I wrote the original stiffness v5 sheet that you mentioned in the first post a few years ago, plus later posted a v7 which may be of interest.

    I've tried to replicate your results but cannot match them (don't know all the dimensions etc that you used). So I'll just make some general comments.

    The 13um in Z is bending due to the spindle weight applied to the centre of the gantry. This seems fine to me.

    The 30um in X sounds like you've only noted the 'tool deflection' number which is only the bit due to twisting. There will be a bit more overall deflection due to X bending to give you a final total deflection in X. This is starting to sound a bit high (30-40um is 0.03 - 0.04mm) for aluminium. However, this is at maximum extension so you can reduce this problem as you've stated by raising the job in Z.

    Biggest thing to point out though is that you haven't run the analysis on the gantry sides (plates) in the Y direction. Depending on your exact numbers this is likely to be similar or greater than the X deflection and because it is just bending (you can ignore Y torsion) and there is no work-around in terms of raising the job in Z etc. This is because all the tool forces in Y go through the ballnut and this is at a fixed height above the X axis bearings. This is why Jazz keeps pointing you to the raised X axis designs for aluminium since this offset between ballnut and X axis bearings is basically zero. You are then free to beef up the bed/raised X axis sides to react this force.

    I have a machine broadly similar to your posts so far and it will cut aluminium but I you can tell from the sound it makes and machine vibration that it is approaching the limit. If I was cutting aluminium more regularly then this machine would not do. For the occasional piece it is OK.

    Finally, feel free to experiment with the spreadsheet but do just use it to get a feel for general comparisons and where to place effort and material. As a like-for-like comparison of different gauge, sections, and dimensions it will give you direction, but don't expect to get the quoted deflections out as there too many simplifications. So long as you use it as just another way of investigating the design (along with reviewing the build logs and the invaluable subjective opinions of experienced builders) then you will be OK.

    Good luck with it and welcome.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  2. #12
    Thanks for the info. The deflection was calculated only for one 100x50 5mm box section as in my first design, now there are 2 in an L shape.
    This is meant to be a hobby machine so I honestly dont know how much aluminium will be cut on it but I do know I want to be able to cut so I will take the advice and modify the x sides to bring the y axis down.
    Has anyone ever built the gantry with the carriages in a triangle formation? I am thinking if the 2 bottom cariages were 150mm apart and the top 2 were 250mm apart (from outside dimension) I could keep the same footprint and machining area?
    Cheers, Charlie

  3. #13
    Quote Originally Posted by CharlieRam View Post
    Just noticed this bit, It wasnt meant to reach the table because I was hoping to offset it with adjusting the spindle height and I left 150mm between base of gantry and M/C bed so I could hopefully fabricate a 4th axis on the unused part of the mc bed.
    If I'm understanding you correctly then the Unused part is exactly that Unusable and putting a 4th axis there is pointless as you can't access it with spindle.? . . . . ( Edit: Ok Excuse that comment I'm not exactly firing on all cyclinders at the minute and realised you mean to put 4th axis head in that space and tail stock at other end running material down it's length.! . . . I was thinking material across the gantry.!!)

    As for rest of machine then like I say it will cut aluminium no problem and if it's only occasionally and your not wanting perfection on the finish then it will be fine. If your wanting to cut deeper and get high quality finish the machine won't be stiff enough.
    Personally I'd just go for it and if you find your cutting more aluminium than anything then build a machine just for this purpose because while a machine for cutting wood isn't 100% for Ali the same applies in reverse to some degree. Can't beat purpose built machines and Jacks of all trades always fall short in some department.

    The bed and workholding on any machine can be a pain and often under estimated how important it is. This is esp true on machines that are intended to cut multi materials.
    Problem comes from how you hold it down and the type of work and how you machine the material. If your cutting or drilling thru then having a nice aluminium bed isn't much use because it doesn't leave a nice warm fuzzy feeling when you chop into it.! So you need some spoil piece anyway and often this needs surfacing each time you use it if any decent accurecy is required.
    Now IME the best bed is a combination of both.? So nice stable material for Machine bed that is surfaced parallel to the cutter with either all of it or part covered with sacrificial spoil board. If bed is large enough like mine is then having half n half works well and saves swaping between and surfacing for jobs that need the accuracy of the base.
    Last edited by JAZZCNC; 19-07-2014 at 03:18 PM.

  4. #14
    Well ive started the redesign so no turning back now :-) and tbh I think I will mostly be machining metals more than plastics and wood so I may as well make it right. I have changed the z axis but that may well of mucked up my plans for the 4th axis as the spindle plate and rails come down now!
    I will try and get the plan finished and post it up so you can all tell me why it wont work again!
    Thanks for the help and advice
    Charlie

  5. #15
    Here goes, My 4th design with base, The base is made mainly with aluminium 2" x 2" 1/8th thickness with some 1 3/4 bar inserted at the bolting points (not added in yet) I will also have adjustable feet, do you think it will be strong enough or does it need beefing up.
    Also my redesigned gantry again! :-) there will be a 15mm plate across the back for the X stepper and pulleys to attach to but I thought I would post my plans before I start adding in all the details such as bolt holes etc.
    As before, feedback good or bad appreciated.

    Thinking more about the frame, I don't like it so maybe I will have to learn welding. My dad used to do it years ago and he's retired now so maybe I can get him onboard.
    Where is the best (read cheapest!) place to get cut to length steel including mitre cuts or would I be better ordering full lengths and investing in one of these:
    http://www.screwfix.com/p/evolution-...aw-240v/60839#
    Its 100 but says it will cut steel, ally, wood and whatever.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	idea 4.jpg 
Views:	301 
Size:	244.3 KB 
ID:	12849  
    Attached Files Attached Files
    Last edited by CharlieRam; 20-07-2014 at 10:52 PM. Reason: Steel frame maybe?

  6. #16
    Another question, Do all these plates need to be 20mm, If I make them 15mm will it affect the machining ability substantially? I'm on a cost/weight cutting exercise!
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	20mm thick.jpg 
Views:	290 
Size:	166.4 KB 
ID:	12856  

  7. #17
    Quote Originally Posted by CharlieRam View Post
    Another question, Do all these plates need to be 20mm, If I make them 15mm will it affect the machining ability substantially? I'm on a cost/weight cutting exercise!
    Well first I'd dump the gantry back plate as it adds nothing but weight with very little strength gain and unless perfectly machine and setup could cause you all sorts of trouble with bearings to binding up.!
    350mm rear plate and bearing plates etc could be 15mm but wouldn't change the front plate.

    Don't go chasing weight it will be your friend when cutting and if you need really high speeds with high velocity changes then put the correct motors on it and you won't have any trouble.

  8. #18
    Quote Originally Posted by JAZZCNC View Post
    Well first I'd dump the gantry back plate as it adds nothing but weight with very little strength gain and unless perfectly machine and setup could cause you all sorts of trouble with bearings to binding up.!
    350mm rear plate and bearing plates etc could be 15mm but wouldn't change the front plate.

    Don't go chasing weight it will be your friend when cutting and if you need really high speeds with high velocity changes then put the correct motors on it and you won't have any trouble.
    Actually that back plate is connected to the ball nut so without it my Y axis will go no where!

  9. #19
    Quote Originally Posted by JAZZCNC View Post
    Also that Z axis design is bad news. Again check out threads and look at Z axis designs that put the rails on the front plate and bearings on rear. Your design is a constant length lever that won't even reach the bed surface and will give the same amount of flex whether it's at the bottom of top of it's travel. With the other design the lever length is variable dependent on material thickness or distance from tool.
    Hi Jazz, is that Z axis any weaker the way I had it? I am struggling for room and didn't want to compensate by reducing my cutting area, I have gone back to Idea 3 but raised the rails on X. I would still like to use the fourth axis along X and this would allow that, giving me an effective work area of 150mm diameter by whatever length travel I decide on.
    My X rails are 810mm apart, carriages are 250mm apart from outer dimension, Would I suffer from binding if I just had a central ballscrew or would you still recomend one on either side?
    Thanks
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	idea 3.jpg 
Views:	252 
Size:	181.5 KB 
ID:	12875   Click image for larger version. 

Name:	idea 3 front.jpg 
Views:	196 
Size:	86.3 KB 
ID:	12876  

  10. The Following User Says Thank You to CharlieRam For This Useful Post:


  11. #20
    The normal convention for the Z axis is to have the rails on the front plate and the carriages/bearings on the back plate. This adds stiffness to the front plate which is where the axis most needs it. Also stick with 20mm plate if you can rather than 15mm. With regards to ballscrews two are way better than one, you undoubtedly would get racking with one central ballscrew at that width I would say.
    Neil...

    Build log...here

Page 2 of 17 FirstFirst 123412 ... LastLast

Similar Threads

  1. BUILD LOG: Steel frame cnc router design/build
    By CraftyGeek in forum DIY Router Build Logs
    Replies: 110
    Last Post: 06-05-2015, 10:00 PM
  2. BUILD LOG: Steel/Aluminium design/build, sorta smallish, cuts ali hopefully
    By mart154 in forum DIY Router Build Logs
    Replies: 27
    Last Post: 19-09-2013, 05:48 PM
  3. RFQ: Aluminium frame Machining
    By mZeles in forum Projects, Jobs & Requests
    Replies: 4
    Last Post: 27-12-2012, 08:17 PM
  4. Alu 40x40 frame - strength/design question
    By Rogue in forum Gantry/Router Machines & Building
    Replies: 0
    Last Post: 12-11-2011, 05:02 PM
  5. Design help etc required with DIY CNC Router Design / Build
    By MikeyC38 in forum Gantry/Router Machines & Building
    Replies: 12
    Last Post: 21-10-2011, 04:50 PM

Bookmarks

Posting Permissions

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