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  1. #1
    Hi I'm looking for a suitable ball-screw to fit a Nema 34 8mm shaft for a simple direct drive.

    also what size lead screw is commonly used in a 8x4 gantry router.

    thanks for your time

  2. #2
    You can buy couplers with 8mm one end and 10mm the other (10mm common side for a 1605/10 ballscrew end.

    Or some places offer custom machining (e.g.BST Automation) for the ballscrew.

    8x4 is quite big so rack and pinion often used on the long axis. I think some people have used 2005/10 ballscrews but canít be sure so wait to see what other have used.

    Depends on what you are using it for and what speed you need and therefore which pitch.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

  3. #3
    JAZZCNC's Avatar
    Lives in wakefield, United Kingdom. Last Activity: 11 Hours Ago Forum Superstar, has done so much to help others, they deserve a medal. Has been a member for 8-9 years. Has a total post count of 7,350. Received thanks 1,262 times, giving thanks to others 80 times.
    Quote Originally Posted by Fozz84 View Post
    Hi I'm looking for a suitable ball-screw to fit a Nema 34 8mm shaft for a simple direct drive.

    also what size lead screw is commonly used in a 8x4 gantry router.

    thanks for your time
    Ok, No disrespect meant when I say this but you clearly don't know what you are doing otherwise you wouldn't be asking these questions. So I'm going to make a suggestion that you STOP buying and do some research.

    Building an 8x4 requires a completely different approach to a 4x4 or smaller, the components you use and the motors sizes, voltages, etc are all different. What works well on a 4x4 will be rubbish on an 8x4 or larger machine. Like the Motor you are asking about, I'll take a guess and say with 99% certainty you have bought the wrong motor or drive, probably both.!
    Yes on a 8x4 or a 4x4 you would use a Nema 34 motor but that's about where the similarities end, the Spec, and voltages you will run them at will be very different.

    Likewise, the system you use to drive the gantry needs to be chosen very carefully. Like RouterCnc mentions Rack n pinion is often used because using ball screws is more involved but it can be done provided you chose the correct size and pitch. 25mm is the minimum diameter you would use and 10mm is minimum pitch for an 8 x 4 machine. But this depends on other factors like motors and the feeds you are looking to achieve.

    The problem with ball screws at this length is they whip at high feeds so you need to control this in some way. The best way is to choose a pitch higher than you need and then apply a ratio using timing belts. This allows you to lower the screw speed and still get the feeds you require.

    So like I say building a large machine is very different to a smaller machine and sizing motors, drives, voltage, screws, etc becomes very important and it's a fine line between success and failure.!! . . . . This why so many people fail or are very disappointed at the result when they build a large machine.
    -use common sense, if you lack it, there is no software to help that.

  4. #4
    I havn't bought anything the reason i asked was that 8mm seemed a bit small for a large machine. i can twist steel with a impact wrench of that thickness.

    ok so it looks like its back to making the rack and pinion design.

    need to find a engineering company locally to cut the side plates and swing arms. (once i have the design finalised)

    hopefully you can make out the idea for the pressure adjust on the rack and pinion. it is early stages but closer than most.

    Click image for larger version. 

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    the more i look into it, the more complex it is. not impossible though

  5. #5
    JAZZCNC's Avatar
    Lives in wakefield, United Kingdom. Last Activity: 11 Hours Ago Forum Superstar, has done so much to help others, they deserve a medal. Has been a member for 8-9 years. Has a total post count of 7,350. Received thanks 1,262 times, giving thanks to others 80 times.
    Quote Originally Posted by Fozz84 View Post
    I havn't bought anything the reason i asked was that 8mm seemed a bit small for a large machine. i can twist steel with a impact wrench of that thickness.

    ok so it looks like its back to making the rack and pinion design.

    need to find a engineering company locally to cut the side plates and swing arms. (once i have the design finalised)

    hopefully you can make out the idea for the pressure adjust on the rack and pinion. it is early stages but closer than most.

    Click image for larger version. 

Name:	Capture.PNG 
Views:	37 
Size:	504.9 KB 
ID:	28211

    the more i look into it, the more complex it is. not impossible though
    You will need to size the motor correctly for this setup because you will have a lot of friction with that system. Also pay attention to the number of teeth engaged in the motor pulley, how you have it drawn there you won't have enough teeth engaged. This will lead to greater torque per tooth and potential tooth jumping and excessive belt wear.

    If you want anything like decent power and speed then you'll need Mains powered stepper system, 70-80Vdc drives won't cut the mustard with this setup and size of machine. But if you do go with low voltage drives then make sure the motors have the lowest inductance you can find.
    -use common sense, if you lack it, there is no software to help that.

  6. #6
    Totally agree with the low tooth contact area on the top pulley.

    Would another idler wheel below the drive pulley be sufficient.... ... Say 180 degree + contact area.

    Also I'm not familiar with the stepper drive power system you mentioned. I'm looking AT Chinese Hss86 closed loop DRIVES. To drive a nema 35.
    If its a way to power them, is there a handy video available. ☺

    Bear in mind the machine will be cutting ply, plastic sheet, wood and maybe a bit of aluminium.
    Last edited by Fozz84; 28-05-2020 at 11:58 AM. Reason: Names of parts

  7. #7
    JAZZCNC's Avatar
    Lives in wakefield, United Kingdom. Last Activity: 11 Hours Ago Forum Superstar, has done so much to help others, they deserve a medal. Has been a member for 8-9 years. Has a total post count of 7,350. Received thanks 1,262 times, giving thanks to others 80 times.
    Quote Originally Posted by Fozz84 View Post
    Totally agree with the low tooth contact area on the top pulley.

    Would another idler wheel below the drive pulley be sufficient.... ... Say 180 degree + contact area.
    Yes that would work better.

    A
    Quote Originally Posted by Fozz84 View Post
    lso I'm not familiar with the stepper drive power system you mentioned. I'm looking AT Chinese Hss86 closed loop DRIVES. To drive a nema 35.
    If its a way to power them, is there a handy video available. ☺

    Bear in mind the machine will be cutting ply, plastic sheet, wood and maybe a bit of aluminium.
    Well you have a mixture of materials there that require different feed rates, ranging from less than 1000mm/min (Alu) to +9,000mm/min to cut correctly. So it becomes even more important you choose the drive system correctly.

    Those drives will be limiting to the speed because they only allow a max of 100Vdc or 70Vac and with the required safety margin on the PSU of 10% you'll run them at around 90Vdc or 60Vac. So you'll be lucky if you get much more than 1000-1200Rpm before they saturate and stall, that's without any loads or friction.
    In reality you'll have a usable RPM of around 700-800.
    Now this depends on several variables and friction is a big one. So your twin pinion system is loading the friction up even higher, Rack n pinion is also very inefficient so a lot of power is wasted just overcoming this. These things along with other power robbing parts of the machine like Gantry mass, rail alignment, bearings etc all come into play to how well the motors perform.

    I've seen it happen many, many times that people building 8x4 machines mismatch the motors and end up disappointed with performance or worse still suffering with stalling motors in the middle of jobs because R&P binds a little or crap gets in between teeth and the motors are on the edge regards torque so stall and screw up the job. Even worse is that they where cutting too slow with poor finish quality and excess tool wear any way because the machine is crippled by the motors/drives/voltage.

    This is often why you don't see many 8x4 or 10 x 5 Diy machines finished and why you see so many on ebay etc selling part finished jobs or frames, gantrys, etc

    It's a very very fine line between getting it right and getting it very wrong, your at the point of getting it very wrong.!

    It's more expensive doing it correctly but it's a one time deal and actually cheaper in long run because you won't be wasting material, breaking tools and the cycle times will be shorter plus you get better finish and longer tool wear.
    If your doing this to make money with the machine then the first piece of expensive plastic it screws up on will cost you more than the difference to do it correctly first time.!! . . And that's with out factoring in the time you lose cutting the job again and sorting the machine or worse still lowering feeds to compensate so increasing the cycle time.!

    But I could be talking bullshit so feel free to ignore my advise because I'm sure your thinking " Well I've seen others using them.!" which is true, but they probably won't tell the truth or are happy to cut at a much reduced feed rate and don't care about tool wear or finish quality, which is crazy for a few £100 more on a machine costing £1000,s and with the potential to make 10s of £1000's.
    -use common sense, if you lack it, there is no software to help that.

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