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  1. #341
    Join Date
    Oct 2009
    Posts
    733

    Re: David A's New 2x3 Bench Top CNC

    Hey Outlawtaz, thank you.

    I do not know enough about belts to give you an educated answer.
    However, I did talk to an engineer with the Belt Corporation of America some
    time ego and he told me that most belts have the majority of stretch within the
    first 10 hours of use. Then it tapers down and stretches little for a long time
    until material degradation starts and then it picks up again until the belt breaks.

    One of the main reasons I wanted to explore the power nut system is that I think that it is
    much less effected by belt stretching and belt backlash than other conventional systems.

    Let me give you an example:

    I am going to compare a standard pinion/rack drive system which most people
    use here and which is basically an industry standard. All numbers are rounded.

    Let's say there is a 0.01" of belt 'looseness' in the rack and pinion drive.
    The big pulley (60T) has a pitch diameter of 2.256" which equates to a circumference of 7.1".

    0.01" relative to 7.1" would be a 0.00141 ratio (0.01 ÷ 7.1).

    So, 0.01" of belt backlash produces a backlash at the big pulley at a factor of 0.00141 of circumference.

    The pressed-in steel pinion has a pitch diameter of 1" which equates to a 3.142" circumference.

    3.142" circumference at a factor of 0.00141 equals to a backlash of 0.0044" at the rack.


    Let's say there is a 0.01" of belt 'looseness' in the power nut drive.
    The nut pulley (40T) has a pitch diameter of 1.5" which equates to a circumference of 4.7".

    0.01" relative to 4.7" would be a 0.002 ratio (0.01 ÷ 4.7).

    So, 0.01" of belt backlash produces a backlash at the nut pulley at a factor of 0.002 of nut lead.

    The 1204 ball nut has a 4mm lead which equates to 0.158".

    0.158" of lead at a factor of 0.002 equals to a backlash of 0.0003" at the screw.


    I just hope I am not wrong with my math and, if I am not, a rack and pinion drive seems to have
    at least ten times more backlash than a power nut drive for a given belt looseness.

    Another possible advantage would be the pulley ratio. A 1:3 reduction (20T to 60T) means that
    the small stepper pulley just provides the necessary minimum amount of tooth engagement and
    the belt has to be fairly tout to avoid belt jumping.

    For the power nut, there really is no need for a gearing (38T to 40T) and the belt can be run
    looser to minimize friction and wear without belt jumping.

    Somehow, your questions always seem to require elaborate and lengthy answers.

  2. #342
    Join Date
    May 2018
    Posts
    69

    Re: David A's New 2x3 Bench Top CNC

    Quote Originally Posted by davida1234 View Post

    Somehow, your questions always seem to require elaborate and lengthy answers.
    Yea, I'm pretty good at asking questions like that... I spent close to 10 years working in Quality Assurance/Control.

    As for your math, I went over it a few times my head and it appears sound. Of course, unless I had an example of each sitting on the test bench in front of me, I wouldn't be 100% sure.

    The reason I asked is because thinking back, the backlash was paradoxical on the machines I worked on. The rotary axes all used a belt driven worm gear and planetary setup. Interestingly, the smaller the overall rotary, the more belt stretch and adjustments that had to be done to the belt. Conversely, the bigger and heavier the rotary axes, the more I had to play with the adjustments in the physical gear meshing. Unfortunately, I really have no idea where the forces amounts would be at the break even point (meaning equal adjustment on the gears and the belt).

    Then again, I'm not even sure my observations are even germane to your machine... I just drive alot for my job and so I sit there and ponder stuff that I find interesting...

  3. #343
    Join Date
    Oct 2009
    Posts
    733

    Re: David A's New 2x3 Bench Top CNC

    Hah, bored you are. Keep coming with your QA/QC questions. They are all somewhat germane.

  4. #344
    Join Date
    Oct 2004
    Posts
    94
    Quote Originally Posted by davida1234 View Post
    Hah, bored you are. Keep coming with your QA/QC questions. They are all somewhat germane.
    I don't know what the Germans have to do with this.

  5. #345
    Join Date
    Jan 2005
    Posts
    15362

    Re: David A's New 2x3 Bench Top CNC

    Quote Originally Posted by Arjay View Post
    I don't know what the Germans have to do with this.
    That's not what the word means
    Mactec54

  6. #346
    Join Date
    May 2018
    Posts
    69

    Re: David A's New 2x3 Bench Top CNC

    Quote Originally Posted by Arjay View Post
    I don't know what the Germans have to do with this.

  7. #347
    Join Date
    Aug 2018
    Posts
    621

    Wink Re: David A's New 2x3 Bench Top CNC

    Quote Originally Posted by Arjay View Post
    I don't know what the Germans have to do with this.
    Funny guy gotta love those $2 words.

  8. #348
    Join Date
    Oct 2009
    Posts
    733

    Re: David A's New 2x3 Bench Top CNC

    I am sure the method shown here is old news for many
    members. Please let me know if there are better ones.

    TRAMMING:

    Holes for linear trucks are slightly elongated on
    upper and lower plates of X-carriage for tramming:
    Attachment 415472

    Attachment 415474

  9. #349
    Join Date
    Oct 2009
    Posts
    733

    Re: David A's New 2x3 Bench Top CNC

    SIDE TO SIDE:

    Place something straight on
    the table.Here, a 1515 will do:
    Attachment 415476

    Place one dial on one end of the
    straight and note the reading. Start
    sliding the dial towards the other end:
    Attachment 415478

    Compare with the reading at the other end:
    Click image for larger version. 

Name:	5 IMG_7734.jpg 
Views:	0 
Size:	414.8 KB 
ID:	415480

  10. #350
    Join Date
    Oct 2009
    Posts
    733

    Re: David A's New 2x3 Bench Top CNC

    Adjust the straight with paper underneath
    so that both ends read the same on the dial:
    Attachment 415482

    Place both dials on the straight and see if the axis is
    off. Side to side was right on the money in this case:
    Click image for larger version. 

Name:	7 IMG_7739.jpg 
Views:	0 
Size:	400.5 KB 
ID:	415484

  11. #351
    Join Date
    Oct 2009
    Posts
    733

    Re: David A's New 2x3 Bench Top CNC

    FRONT TO BACK:

    Do the same with the straight
    in the other direction. This time
    the Z-axis was off as the weight
    of the slide has skewed the axis
    during installation:
    Attachment 415486

    By pulling the plates towards/away from linear blocks:
    Attachment 415488

    Attachment 415490

    Tram can be adjusted quite accurately:
    Click image for larger version. 

Name:	11 IMG_7741.jpg 
Views:	0 
Size:	376.7 KB 
ID:	415492

  12. #352
    Join Date
    Mar 2018
    Posts
    68

    Re: David A's New 2x3 Bench Top CNC

    Do you relly only on friction to keep tram or you have some kind of exentric shoulder screw?

  13. #353
    Join Date
    Oct 2009
    Posts
    733

    Re: David A's New 2x3 Bench Top CNC

    Excentric shoulder screw where?

    Just at the front portion of the X-carriage,
    where the Z-axis is bolted to, are 48 bolts
    to keep tram. And another 16 at the back.

    It's really not something you do often like
    a belt tensioner.

  14. #354
    Join Date
    Mar 2018
    Posts
    68

    Re: David A's New 2x3 Bench Top CNC

    Not complaining at all, just asking, this buld is fantastic, i guess that 64 bolts will be more than enough to prevent any slippage!

  15. #355
    Join Date
    Oct 2009
    Posts
    733

    Re: David A's New 2x3 Bench Top CNC

    No problem, just keep asking.

    Personally don't know of any
    excentrics for tram on a DIY.

  16. #356
    Join Date
    Oct 2009
    Posts
    733

    Re: David A's New 2x3 Bench Top CNC

    .
    X-AXIS POWER NUT DRIVE:

    X-axis power nut is installed.
    Break-in for about 10 min.
    Starting with one inch per minute.

    Very slight jerkiness, probably
    due to too short of a break-in.

    More videos tomorrow:
    https://youtu.be/PiEyv528mxA

  17. #357
    Join Date
    May 2018
    Posts
    69

    Re: David A's New 2x3 Bench Top CNC

    Ok, more random thoughts from my 150 mile drive home tonight...

    First, now that I see the belt size, I retract my questions about stretch or adjustment. I guess I didn't have a good perspective from your pics before and thought you were only using one of those 1/4 inch belts.

    Second, unless I missed it before, have you already accounted for any twist that could be present in your ways on the X and Y axes?

  18. #358
    Join Date
    Oct 2009
    Posts
    733

    Re: David A's New 2x3 Bench Top CNC

    Outlawtaz, where do you think twist could
    occur as I don't have any traditional ways per se?

    The only place I could imagine it occurring would
    be the ball screw itself. And I don't think that is
    going to happen at loads we have when cutting.

  19. #359
    Join Date
    May 2018
    Posts
    69

    Re: David A's New 2x3 Bench Top CNC

    Let me start by saying that I typically WAY over engineer my stuff, so take it with a grain of salt...

    Twist would be along the linear rails from one end to the other in both X and Y. And not along one rail but the surface of the pair of rails in relation to each other. It only occurred to me when you trammed in the spindle and you used a piece of paper to level your straight edge. Typically, paper is 0.004-0.005" thick (I always used 0.005 as a reference and a "round" number). It appears that the piece of 1515 was around a foot long so this could possibly induce a difference of 0.015" over the length of your machine. Even if you skimmed your base plate, it would just make a curved surface that would come out if you made parts that filled the entire machine. I hope this is making sense... It makes sense in my head but I can't always type my thoughts exactly...

    What I was always taught was to level the table / base first and remove any twist. Then, if I was concerned about X and Y being perpendicular, I would throw a granite angle on it to adjust one or the other. Next, came verifying Z to the table / base (if needed) again using the granite. Tramming the spindle would be the last step.

    Again, this is probably overkill. Also, I just happen to have all the precision tooling for this since I averaged doing this a couple times a month over 6 years when I was a field tech.

    I'm just impressed with your design and I think with a little playing around, you could have fairly precise machine with good repeatability.

  20. #360
    Join Date
    May 2005
    Posts
    1662

    Re: David A's New 2x3 Bench Top CNC

    Break-in for about 10 min.
    Starting with one inch per minute.
    We need more speed
    In particular to see the effect of the rotating mass. I think inertia increases by something like the square of diameter increase ?
    I don't know the exact formula and as they say the proof is in the pudding.
    This design is as light as possible with 2 flanged ball nuts, I hope this works as good as it looks.
    Anyone who says "It only goes together one way" has no imagination.

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