584,817 active members*
4,841 visitors online*
Register for free
Login
IndustryArena Forum > Mechanical Engineering > Linear and Rotary Motion > A and C axis for a 5 axis milling machine
Page 1 of 7 123
Results 1 to 20 of 137
  1. #1
    Join Date
    May 2018
    Posts
    28

    A and C axis for a 5 axis milling machine

    Dear all,
    I am designing my first 5 axis CNC milling machine mainly to work aluminium. For X, Y and Z axis I have chosen Nema 23 stepper motors that provide 1.9 Nm torque and a lead screw 10x2mm. In this way I have a precision of 0.01mm and a very high linear force. Now I am looking to have a similar precision around the A and C axis considering a working volume of 200×200×200 mm, but it looks very complicate. Using the same kind of stepper and a reduction done with T5 pulleys from 10 teeth to 60 teeth, at 100mm from the centre the movement is about 0.5mm pet step. So to have an acceptable person at least I need to use x8 microsteps, but doing so the torque goes down to about 20% of the full step one and even considering the increasing factor by 6 due to the pulleys, we arrive to about 2 Nm, that is just a force of 20 N at 100mm from the centre.
    I don't think it is enough for to mill aluminium. Can you give me some advice?

  2. #2
    Join Date
    May 2015
    Posts
    1422

    Re: A and C axis for a 5 axis milling machine

    I have a little stepper + belt drive 4th that came with my machine. It's rubbish. To make it work I need to run stupidly light cuts that are carefully planned to not be too far off centre, because there's nowhere near enough holding or drive torque to do big stuff out on the edges of even smaller work than you're talking about.

    Most guys who are having any joy with this seem to be running much higher reductions (50:1 or more), with worm gears (but backlash), harmonic drives (but dollars) or even hypercycloidal drives. Yeah, it means you're not going to be using your fourth as a lathe spindle - or doing particularly fast rapids on the rotaries. But at least it is usable within a sane price range.

    Go digging, there's a few threads on here about DIY 4th axis setups which talk about the pitfalls of various approaches.

  3. #3
    Join Date
    May 2018
    Posts
    28

    Re: A and C axis for a 5 axis milling machine

    Thanks Dharmic, for your feedback. I have looked for arminc drives but they look very expensive. So I think I can put to series of pulleys 12/60 each, so in total I can achieve a reduction of 1:25 that should be accettabile.

  4. #4
    Join Date
    May 2015
    Posts
    1422

    Re: A and C axis for a 5 axis milling machine

    If you’re patient and careful you could probably make a hypocycloid drive on a 3 axis machine

  5. #5
    Join Date
    Jun 2011
    Posts
    692

    Re: A and C axis for a 5 axis milling machine

    Quote Originally Posted by dharmic View Post
    If you’re patient and careful you could probably make a hypocycloid drive on a 3 axis machine
    Harmonic seems simpler.. Only 2 moving parts!

  6. #6
    Join Date
    May 2018
    Posts
    28

    Re: A and C axis for a 5 axis milling machine

    I don't have experience of harmonic, but I think they are quite difficult to machine because I imagine they have to be by steel and very thin too...

  7. #7
    Join Date
    Jun 2011
    Posts
    692

    Re: A and C axis for a 5 axis milling machine

    No reason they have to be steel. Could be plastic, of course they wouldn't be as rigid.. It wouldn't exactly be easy, especially on the inside gear..

  8. #8
    Join Date
    May 2015
    Posts
    1422

    Re: A and C axis for a 5 axis milling machine

    Guys have done 3D printed strain wave gearboxes (harmonic is a trade name for the patent holding company) before but I wouldn't want to load them up too high. Other than that, I seriously doubt the capacity of most home or small shop environments to machine that flex spline.

    Part count is about the same for them.

    Strain wave: wave generator, flex spline, circular spline.
    Hypocycloid: cam, output and housing.

    The strain wave rigs can get higher reductions because of the larger number of teeth and thus closer ratio. But the hypocycloids are infinitely easier to make at home and are a lot tougher. I have a two stage hypocycloid running about a 100:1 reduction sitting on my desk, with four parts instead of three (the output of stage 1 and input of stage 2 are made out of a single piece). The whole thing is about 10mm thick and you can make it with a 2.5 axis machine and some dowel rod.

  9. #9
    Join Date
    Mar 2017
    Posts
    926

    Re: A and C axis for a 5 axis milling machine

    I want to eventually add a 4th axis to my machine (also for aluminum) but it is slim pickings if you are on a budget and want a ready to run rotary axis.

    I have been searching and researching (on and off) for a while and I haven't found anything that is both good (or acceptable) and cheap (or even cheapish). I have pretty much decided that the only way to get something robust enough to mill aluminum is to go with a semi-homemade one.

    I am currently looking into using a pair of 8" used rotary tables (with 4 jaw lathe chucks) with a used gear reduction and an appropriately sized motor.

  10. #10
    Join Date
    Apr 2004
    Posts
    5728

    Re: A and C axis for a 5 axis milling machine

    I think the reason people have been recommending harmonic drives is that they have little or no backlash. Those used rotary tables with used gear reductions will have lots of it. Since the angular 4th and 5th axes used to rotate and tilt the spindle of a milling machine act at a distance, any small inaccuracy is magnified considerably, so the tip of the tool can be quite a ways from where it's supposed to be. You will still get results, they just might not be the ones intended...

    If you don't mind used equipment, Harmonic drives do come up on ebay now and then, and they aren't necessarily super expensive: https://www.ebay.com/p/Harmonic-Driv....c100009.m1982
    Andrew Werby
    Website

  11. #11
    Join Date
    Mar 2017
    Posts
    926

    Re: A and C axis for a 5 axis milling machine

    I'm not sure it's right to generalize about backlash on all rotary tables. It will be like most things CNC or milling machine related - I.e you get what you pay for.

    With that said, it probably isn't worth investing in a quality rotary table for this purpose and I'm sure there probably are issues with the gear reductions on cheap rotary tables. I was thinking that, as part of the diy process, I would bypass the cheap gear reduction and use my own belt / pulley reduction (and maybe a little backlash compensation in the software).

    I have also looked at used harmonic drives but I don't know enough about them yet. I have more reading to do before I understand how to shop for a decent one and how to convert one into a finished rotary axis.

    Does anybody ever use a direct drive rotary axis with a larger motor to avoid backlash issues?

  12. #12
    Join Date
    Jun 2010
    Posts
    4252

    Re: A and C axis for a 5 axis milling machine

    how to shop for a decent one and how to convert one into a finished rotary axis.
    Build Thread The Design and Construction of a 'Backlash-Free' Rotary Table

    Cheers
    Roger

  13. #13
    Join Date
    Mar 2017
    Posts
    926

    Re: A and C axis for a 5 axis milling machine

    Quote Originally Posted by RCaffin View Post
    how to shop for a decent one and how to convert one into a finished rotary axis.
    Build Thread The Design and Construction of a 'Backlash-Free' Rotary Table

    Cheers
    Roger
    That thread looks like it's about converting rotary tables to a CNC 4th axis and nothing to do with harmonic drives - the thing I said I had to research to know how to shop for a good one...

  14. #14
    Join Date
    Jun 2011
    Posts
    692

    Re: A and C axis for a 5 axis milling machine

    Quote Originally Posted by Goemon View Post
    That thread looks like it's about converting rotary tables to a CNC 4th axis and nothing to do with harmonic drives - the thing I said I had to research to know how to shop for a good one...
    From the 3rd paragraph:
    Let's remove any doubts right up front: this is NOT about hanging a stepper motor on the crankshaft of a manual rotary table.

    Second page starts going in depth into harmonic drive.

  15. #15
    Join Date
    Jun 2010
    Posts
    4252

    Re: A and C axis for a 5 axis milling machine

    That thread looks like it's about converting rotary tables to a CNC 4th axis and nothing to do with harmonic drives
    Well, first of, I wrote that series of postings, so I do know what they are about.

    I started with the option of hanging a stepper on a commercial RT, before outlining the problems with that approach (mainly huge backlash).
    Then I went into some detail about how I used a Harmonic Drive to make a good CNC RT. 2nd hand, off eBay, OK price.
    I ended up with a lengthy discussion of how I MEASURED the actual performance of the RT once built.

    Cheers
    Roger

  16. #16
    Join Date
    Mar 2017
    Posts
    926

    Re: A and C axis for a 5 axis milling machine

    Quote Originally Posted by RCaffin View Post
    That thread looks like it's about converting rotary tables to a CNC 4th axis and nothing to do with harmonic drives
    Well, first of, I wrote that series of postings, so I do know what they are about.

    I started with the option of hanging a stepper on a commercial RT, before outlining the problems with that approach (mainly huge backlash).
    Then I went into some detail about how I used a Harmonic Drive to make a good CNC RT. 2nd hand, off eBay, OK price.
    I ended up with a lengthy discussion of how I MEASURED the actual performance of the RT once built.

    Cheers
    Roger
    That explains it. I had only read the first page. It looks like you spent a lot of time to get your 4th axis right.

    By any chance, did you look at what method is used on the expensive rotary axis systems? For example, the Haas trunnion tables on ebay that sell for between $2500 and $18,000. I can see that they use servo drives but not how they connect to the rotary table or what type of gear reduction is used.

  17. #17
    Join Date
    Jun 2011
    Posts
    692

    Re: A and C axis for a 5 axis milling machine

    Quote Originally Posted by Goemon View Post

    By any chance, did you look at what method is used on the expensive rotary axis systems? For example, the Haas trunnion tables on ebay that sell for between $2500 and $18,000. I can see that they use servo drives but not how they connect to the rotary table or what type of gear reduction is used.
    I know I've seen them that have a high resolution encoder on the output shaft and have a drag brake that is engaged during simultaneous machining. And then just use that encoder to implement backlash compensation.
    I'm sure there are more elegant solutions as well.

  18. #18
    Join Date
    Jun 2010
    Posts
    4252

    Re: A and C axis for a 5 axis milling machine

    Small ones work very well with Harmonic Drives. Larger ones have to use worm and wheel to get the torque capacity, but there are quite a number of sophisticated (and of course expensive) ways to get the backlash out of a worm&wheel. I am not saying that any of them are as good as an HD.

    A lot of RTs are really only used as indexers, and as such can use both a drag brake and a solid brake/clamp to lock the position. Not all CAM packages and not all CNCs can actually handle full simultaneous 6 axis machining anyhow.

    Cheers
    Roger

  19. #19
    Join Date
    Aug 2009
    Posts
    119

    Re: A and C axis for a 5 axis milling machine

    This is of interest to me also. At some point I want to build a 10" rotary table to use for doing fine pearl inlay work on wooden turned objects with a cnc router. The cutting forces will be very low, I use .010" to .050" end mills, with repeated shallow passes, not at all in a hurry. But I need really good repeatability and very low backlash to be able to take those repeated passes and minimize the use of filler.. Being wood, I'd be happy to work to +/- .001", and could probably accept .005" if I have to, altho the closer the better. Cost is a real issue, this will not get much use and not generate much income.

    So I have been thinking to use a Nema 23 stepper driven harmonic drive with at least a 1:100 reduction, plus an 8" diam crossed roller bearing to support the table. That would give me 20,000 steps/rev, and at 10" diameter that's .0015" travel/step, using full steps. Is this a reasonable approach?

    I'm thinking to make the table of 1/2" or 3/4" aluminum jig plate, and maybe drilling holes or cutting pockets to reduce weight. Can I expect to have enuf torque to move the table as accurately as I hope to?

    Another thought was to use a 5 phase stepper which gives me 400 full steps/rev. Any reason not to do this?

    My hope is to find good used components on ebay, I've seen a lot of harmonic drive units from Thailand and Vietnam. Can one expect them to be in reasonable shape? Do they wear out easily?

  20. #20
    Join Date
    Jun 2010
    Posts
    4252

    Re: A and C axis for a 5 axis milling machine

    Hi dixdance

    Read the full thread I cited earlier in this thread.
    Note that a full Harmonic Drive already includes a large crossed roller bearing, which is enough. As for torque: enough to tear your arm off. The HD I bought was in good shape.

    Cheers
    Roger

Page 1 of 7 123

Similar Threads

  1. Replies: 0
    Last Post: 04-27-2016, 04:08 PM
  2. MRF 3 axis milling machine
    By wjnagel in forum Mechanical Calculations/Engineering Design
    Replies: 1
    Last Post: 02-13-2013, 10:49 PM
  3. DIY 3 axis CNC Milling Machine.
    By Eagle23 in forum Uncategorised MetalWorking Machines
    Replies: 3
    Last Post: 10-20-2009, 02:20 AM
  4. How many axis to DRO on a Milling machine?
    By hotponyshoes in forum Uncategorised MetalWorking Machines
    Replies: 5
    Last Post: 05-15-2007, 08:16 AM
  5. 5 axis milling machine
    By toneV8 in forum Uncategorised MetalWorking Machines
    Replies: 46
    Last Post: 11-29-2005, 11:11 AM

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
  •