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  1. #1
    Hey Guys

    I have been researching cnc machines for about 2 years now. Last year I took the plunge and built a MDF Router with Ali angles, skate bearings and threaded rod leadscrews.

    The plan was to build the machine and learn about the process without spending too much money, over the past year I have spent most of my time upgrading and tweeking it.

    I had always planned to make a better machine as my knowledge improves.

    After making new parts for itself for the last few months I have reached the point where i want to start from scratch. I feel I now have the confidence to go out and spend some money and get some decent linears and ballscrews to make something more beefy and accurate.

    So I have been mocking up some ideas etc and looking at alot of the other build logs and I really liked the design of Pavlo's Steel Framed machine, so much so that the design I have pretty much settled on is just a scaled up version of it.

    There are a few reasons this design appealed to me:
    It seems like it is strong.
    Steel is pretty cheap
    I have access to a mig welder
    My current machine is not up to cuttilng Ali so fancy Ali Gantry sides and Z Axis are out. All the Ali pieces are all rectangular so Im hoping that I can order them in to size and only be left with the drilling.

    Here are some pics of what I have come up with so far.

    Click image for larger version. 

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    X Rails are 1500mm 20dia supported. Y is 800mm 16dia supported. Z is 300mm 16dia supported.

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    Outer section is 100x50 Steel Box. Bed supports are 40x40. 20mm x 5mm pitch ballscrew will be in the center of the frame.

    Click image for larger version. 

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    Gantry is 50x50 Steel box. The strut at the bottom with the ballnut is some more of the 100x50. 16mm x 5mm pitch Y Ballscrew will run between the 2 horizontal 50x50 Y Rail supports.

    Click image for larger version. 

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    Z Assembly is 300 x 150 x 20mm Ali plate with a 16mm x 5mm pitch Z Ballscrew. Approx 190mm Z travel.

    I would really appreciate some feedback from you guys, nothing has been ordered yet so its not to late to change anything if it looks like I have missed anything out.

    There are a few constraints that I would like to stick to:
    I will be re using my existing electronics and spindle to keep the costs down. Kress 1050 spindle and 3NM NEMA23 Steppers.
    I want the cutting area to be 2' x 4' so I can buy a 8' x 4' sheet and get them to get it cut into 4 at the store and drop it straight on the machine.
    I may do some bits of Ail work on it, but the main use will be for acrylic and wood.

    I do have a few things that I am unsure of:

    I am not sure if my steppers will be up to the task of moving the heavy gantry around, I would really like to be able to take some pretty fast and aggresive cuts on wood and acrylic, I'm thinking quick direction changes in the X direction might be a problem?

    Will I have problems with the X and Y leadscrews whiping?

    Anyway I look forward to hearing some feedback.

    Thanks for taking the time to read

    Barry

  2. #2
    Quote Originally Posted by Stickio View Post
    I am not sure if my steppers will be up to the task of moving the heavy gantry around, I would really like to be able to take some pretty fast and aggresive cuts on wood and acrylic, I'm thinking quick direction changes in the X direction might be a problem?
    I'm using the same steppers for my machine which is bigger so probably heavier and they work fine. I used 10mm pitch, which helps as 1/4 the K.E. in the screw and on the X-axis rotated the ballnuts which helps even more...If you want high feedrates then get 10mm pitch screws and ideally timing belt/pulleys not direct drive to get the optimal ratio. I'm running the motors on 75V which is necessary to get high enough torque at high speed.

    Quote Originally Posted by Stickio View Post
    Will I have problems with the X and Y leadscrews whiping?
    Depends how fast is 'pretty fast'! Y will be fine. If you use 10mm pitch screws then the RPM of the screw is of course halved, so you can go twice as fast before whipping.

    X is 1500mm with angular contact on one end and single bearing on the other, 5mm pitch:
    (approximately)
    16mm diameter critical speed is 5000mm/min
    20mm diameter 6000mm/min
    25mm diameter 7700mm/min

    Double those if you use 10mm pitch. Those feedrates are achievable with the 3nm motors .. I got much more.

    On that size machine I would *definately* go for two screws on the X-axis to prevent racking especially as you want to take 'agressive cuts'.

    Your Z axis linear bearing blocks look too close together for that travel.

  3. #3
    Thanks Johnathan

    My current power supply is 40V so it looks like upgrading those should help out alot with regards to the steppers being able to cope.

    I suppose I should of been a bit more specific with my pretty fast comment :) my current machine rapids at 10000mm/min and I am pretty happy with that so if i go for 20mm dia at 10mm pitch on the X and 16mm dia also at 10mm pitch on the X and Z I should be able to achive those speeds.

    At first I will be direct driving the screws as it will be alot easier to mount the steppers. Once the machine is built I will be able to make brackets and motor mounts for a timing belt/pullys with the machine.

    I did consider 2 screws on the X and I do agree with you, the only reason I went with the single was so I didnt have to buy another driver, stepper and ballscrew, the cost is mounting up as it is, if there is any way to get arround using 2 screws would be a big help. I have seen some pretty good ideas on ways to help prevent racking which I may try out, one that sticks in my mind is a system using pullys and cable similar to those used on sliding patio doors I will see if I can find the link. Failing that I guess I will have to let some more moths out my wallet and buy the extras.

  4. #4
    a very good friend said to me once measure twice cut once, do it right first time.

    their are people here that will cut/machine items at a cost for you.

    save money and do it right first time

    James:naughty::naughty:

  5. #5
    Yes, more than 40V will be much better - so long as your drivers are rated for more?

    You can't get 10mm pitch 20mm diameter screws from eBay (or at least that was the case next time I checked) so it looks like you'll have to use 16mm pitch. I wouldn't go for 25mm as the moment of inertia of the screw will be significantly greater which will affect the acceleration.

    Remember that a 10mm pitch screw on the Z-axis will backdrive very easily, so you will almost certainly have to prop up the axis before switching off. I used 5mm pitch on the Z-axis, but with a pulley and gravity does cause the axis to drop when I switch off.

    If you don't want go spend a lot on another stepper/driver (fair enough) then why not link the two ballscrews with a pulley on each and a timing belt. Ideally 3 or 4 pulleys with the motor in between so that belt stretch is even... easer than the cable method I think.

  6. #6
    Ugg just checked my drivers and they are only rated at 50V, looks like im going to have to start replacing electronics anyway.

    I am starting to have a change of heart, the way I see it at the moment is that I have a machine that is working ok at the moment so I am not in a huge rush to get this one up and running so as James has pointed out lets do it right the 1st time. I will tweak the design to include twin X screws that way I can order the steel in and get on with making the frame while I save up for other bits.

    Where did you get your 75V power supply and drivers from was it zapp?

    Didnt think about the Z backdrive, maybe some sort of solenoid brake that retracts in the powerd up state and locking the z when controller is switched off. If I dont do something like that I know for sure I will forget and end up ruining something :)

    For those who have not seen it before here is the pully system I was talking about.
    Click image for larger version. 

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    At least with replacing the drivers and power supply I wouldnt have any excuses not to make a scratch built cnc mill I've been wanting to build, cant have those bits sitting there gathering dust :P

  7. #7
    Quote Originally Posted by Stickio View Post
    Where did you get your 75V power supply and drivers from was it zapp?
    Drivers were from Zapp - their service is excellent, but you can now get them a fair bit cheaper on eBay. I made (or rather assembled, there's nothing to it really) the power supply from a 500VA toroidal transformer on eBay, bridge rectifier and a couple of capacitors. Google it and I'm sure you'll find the general idea...

    Quote Originally Posted by Stickio View Post
    Didnt think about the Z backdrive, maybe some sort of solenoid brake that retracts in the powerd up state and locking the z when controller is switched off. If I dont do something like that I know for sure I will forget and end up ruining something :)
    Could do, I just quite literally stick a spanner in the works if required!

    I reckon the pulley diagram you posted wouldn't work very well because the acute angle between the crossing wires is very small. Too tired to calculate it properly now, but I'm pretty sure you'd want the same sort of thing but with one pulley on each corner of the frame, like on a drawing board ruler...

    Quote Originally Posted by luke11cnc View Post
    their are people here that will cut/machine items at a cost for you.
    That's me...
    Last edited by Jonathan; 17-08-2011 at 08:55 PM.

  8. #8
    Hi Barry

    I dont think you would have any problems with racking if you have a single drive on the x axis. Your gantry is identical to mine and I get no problems even on aggressive cuts.

    I dont see any advantage of a 10mm pitch on the z screw. x and y speed are important but how often do you need to plunge into the workpiece at high speed? With a 5mm pitch you will not need a brake (or a spanner).

    It looks good to me, but then I'm biased :)

  9. #9
    It's good to have high speed Z if you're drilling PCBs, but that's about the only thing I can think of.

    Quote Originally Posted by pavlo View Post
    With a 5mm pitch you will not need a brake (or a spanner).
    My Z-axis, with 2.2kW spindle, has a 5mm pitch and does drop ... it obviously depends on the weight of the axis.

  10. #10
    Thanks for the input guys.

    Time for an update,

    I have decided to stick with the single screw on the x for now, this is mainly due to the additional cost. Also it will be easy to add an additional screw at a later date if I'm not happy with the results.

    All screws will be direct drive to the steppers.

    I will be using 16mm screws all round, 10mm pitch on the X and Y and 5mm on the Z.

    I will use my existing drivers and power supply @ 40V.

    Z Assembly is going to be changed a bit to reduce flex, I will be increasing the distance between the bearings and extending the front plate holding the router. The stepper and ball screw will also be mounted there (I will post up a new drawing soon).

    I know that alot of these things go against what has already been discussed but at the moment funds wont stretch far enough to make all the changes now. Besides most if not all the fun for me in building these things is working on upgrades and improvements.

    Upgrades as funds are available will include (probably in the following order);

    Drivers and Power Supply @ 75V.

    Additional screw on the X axis if required.

    Spinning Ballnuts on X axis.

    Timing belt and pully to drive X and Y possibly Z.

    Water cooled spindle.

    4th Axis.

    This is going to be an ongoing project, at 1st I will have to run lower feed rates and smaller pass depths untill upgrades get made.

    I should be able to order all the steel in a few weeks, then hopefully the following month will be the linears and ballscrews.

    The best I can find the cut Alli plate (without going to a machine shop) for is +-2mm. The size doesnt worry as much as if the things are going to be square/parallel. Johnathan how do you feel about making the Z Assembly, Router mount and 2 ballnut holders for me .

    After that is all together I should be able to machine some ali myself if I stick to the center of the bed.

    Again thanks all for the help guys

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