EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: cowasaki on December 01, 2018, 07:05:27 pm
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I'm going to start building a CNC 3 axis cutter/router/etc machine over the next few months. I'm happy enough with the mechanical side plus I've found a HEX image for the Arduino that turns it into a Gerber device and controls the three steppers.
What I'm not sure about and what I want to avoid spending too much on or buying the wrong stuff is the stepper motors/control boards etc
I've not used stepper motors before but I had 3 tiny motors and 3 control boards that came with my original Arduino years ago. I've messed about with them but clearly, they are just toys.
What I'm looking for assistance on is what size motors I'm really going to need. I'm seen a video of a CNC 3 axis machine about the size I want to build which used 3 Nema 17 motors and I've looked at these on Ebay but there are lots of different ones with different specs. I'm hoping to be using a 1/4" or 3/8" router as the main tool but will be building it capable of changing the head for other devices.
Any pearls of wisdom ?
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if you want to go with small motors then you need to minimize friction, i.e. linear rails (with ball-bearing blocks), and ball-screws for the drive.
To minimize force on the Z-axis some designs use a counter-weight (like in an elevator) for 'neutral bouyancy'.
the usual advice is to direct-couple steppers as they are good at turning at low rpm but the torque falls off at high rpm.
the usual problem with stepper CNCs is lost steps which ruin the part. if you plan on milling hard materials (aluminium, steel, etc) then think hard about servos with encoder feedback instead of steppers.
for servos gear/belt reduction is usually used as the motor likes to turn at a high rpm.
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My design is a direct drive. I don't mind getting motors rated above what I need if it makes the design more reliable but the problem I have it not knowing what force I realistically need.
I'm looking at a design running fully on bearings and from a video of a similar device it is very smooth. As I said the one I've seen was using 3 Nema 17 steppers but there seem to be so many different specs for it!
I suppose the best way would be to build it minus the motors and actually measure how much force is required but where is the fun in that!
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You don't say how big of a machine you're building or what materials you intend to cut, you don't say if it's belt, screw, or rack & pinion drive, what type of spindle you'll be using or any other details which greatly impacts the selection of axis motors.
With that said, a 1/4" tool, IMHO, sound pretty big for a machine with NEMA17 size drive motors, let alone 3/8" but it all depends on the available spindle power, depth of cut, type of material and so on.
For example, if you intend to build a machine using threaded rod from the hardware store the efficiency of that threaded rod is around 1/4 (or even less) compared to a ballscrew (of the same pitch). This means that your drive motor needs to put 4 times the amount of torque into the screw at any given speed in order to get the same amount of linear force. The thread pitch of that threaded rod will also be much less that what it is on a ballscrew meaning the motor not only has to put more torque into the screw but also turn a lot faster - and as it happens with step motors, the faster they turn the less torque they're able to deliver.
Then, if you go "the other way" ie belt drive - where one motor revolution might equal 30, 40, 50mm worth of travel you don't need the motor spin as fast but you do need 10 times as much torque compared to a 3, 4, or 5mm pitch screw drive in order to get the same amount of linear force.
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1/4 or 3/8 router bits generate a lot of forces in a lot of directions so start there in your design and generally wont be put in a conventional router under 750W. Think about that mass you then need to shove around (including your gantry or bed), change direction and push into your job. Nema 17's are not what you need IMO and will lose steps and fail, Nema 23's and fairly gutsy ones at that or even Nema 34's for preference. Doing this by trail and error gets expensive so at minimum find a complete design that works with the bits you want or do the numbers.
Look up some data sheets on 'Nema' steppers and there are very large variations in length, wind, step angle ......... evilbay is not a place to get truth in spec's ;) That said some dedicated CNC sellers do exist.
In evilbay terms you would be talking a 6040 spec or better for those bits. Spec list for what it is worth is on this listing and may help you get your head around so parts if you are determined to roll your own https://www.ebay.com.au/itm/New-3-Axis-Desktop-CNC-Router-Engraver-Milling-Drilling-Machine-Engraving-6040/232705187439?epid=12016920160&hash=item362e4f5e6f:g:ne0AAOSwi0xaNJXX:rk:3:pf:0 (https://www.ebay.com.au/itm/New-3-Axis-Desktop-CNC-Router-Engraver-Milling-Drilling-Machine-Engraving-6040/232705187439?epid=12016920160&hash=item362e4f5e6f:g:ne0AAOSwi0xaNJXX:rk:3:pf:0)
Good Luck :)
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I'm looking at building a frame approximately 1m x 1m from box section steel welded into a square and strengthened. X carriage will run on ball bearing races along smaller cross pieces. The y carriage will also run on bearing races with both using 750mm Antibacklash SFU1605 Ball Screw 750mm + BK/BF12 + 2 6.35x10mm Couplers Set. Z will be a similar setup but shorter. I'll be building it with a replacement carriage so I can fit a, probably 1/4", router; dremmel; laser engraving module or other tool. What I'll be cutting is likely aluminium, MDF & plastic.
The closest to my design I've found is by a bloke in america and his runs off 2 Nema 17 motors in the same configuration I was looking at ie direct drive. I couldn't work out the best way to do the Z axis but might just borrow his design for that!
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This is similar to what is on my recent acquisition work area is 1.0x0.6m and the bed is outside that at about 1.2x.8m. The sort of stepper kit it runs in this (motors are these with two on the X) https://www.ebay.com.au/itm/3-Axis-CNC-Kit-3-1Nm-Nema-24-Stepper-Motor-Driver-CNC-Mill-Router-Lathe-Robot-/171935443867?_trksid=p2385738.m4383.l4275.c10 (https://www.ebay.com.au/itm/3-Axis-CNC-Kit-3-1Nm-Nema-24-Stepper-Motor-Driver-CNC-Mill-Router-Lathe-Robot-/171935443867?_trksid=p2385738.m4383.l4275.c10)
Couple of Pics of it for an idea of the build strength down below. Spindle is 2.2kw btw.
Dremel's dislike side loads so forget that option for more than drill holes in PCB's accurately and safety with even laser engraving strengths is crucial on an exposed machine so buy some very good glasses or a curtain.
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This is similar to what is on my recent acquisition work area is 1.0x0.6m and the bed is outside that at about 1.2x.8m. The sort of stepper kit it runs in this (motors are these with two on the X) https://www.ebay.com.au/itm/3-Axis-CNC-Kit-3-1Nm-Nema-24-Stepper-Motor-Driver-CNC-Mill-Router-Lathe-Robot-/171935443867?_trksid=p2385738.m4383.l4275.c10 (https://www.ebay.com.au/itm/3-Axis-CNC-Kit-3-1Nm-Nema-24-Stepper-Motor-Driver-CNC-Mill-Router-Lathe-Robot-/171935443867?_trksid=p2385738.m4383.l4275.c10)
Couple of Pics of it for an idea of the build strength down below. Spindle is 2.2kw btw.
Dremel's dislike side loads so forget that option for more than drill holes in PCB's accurately and safety with even laser engraving strengths is crucial on an exposed machine so buy some very good glasses or a curtain.
Thanks. I actually saw the same advert and those three motors looked good!. I think 1m square might be my limit although a little larger might be nice. The 1/4" or 3/8" router will handle side loads whilst the dremel might be used for other tasks such as you mentioned. I have a PCB making kit from Mega Electronics a PA310 3 PCB Bubble Etch Tank with PCB shears, large UV light, through hole stuff, tinning stuff, PCB drill stand and all the rest of the gubbins but it might be nice to get the holes drilled for me. In fact I've heard of people using a CNC router to cut away the un-wanted copper off double sided boards. I might have a go with making some single sided boards with it but I've become lazy and quite like just ordering them from China. I did offer my full kit for sale on here a while ago but nobody seemed interested.
I'm probably going to order the metal side to size from my local metal supplier so most of the hard work is done and those ballscrew kits shouldn't be too much trouble. My Prusa i3 Mk3 3D printer should arrive in the next few days so I suppose I might be able to make some of the parts with that but I was hoping the majority would be steel.
These are the motors I have been looking at:
https://www.ebay.co.uk/itm/Free-ship-for-EU-4Axis-wantai-Nema34-Stepper-motor-1232oz-in-driver-DQ860MA-cnc/152686189657?hash=item238cce5c59:g:tMcAAOSwuiNZqMJD (https://www.ebay.co.uk/itm/Free-ship-for-EU-4Axis-wantai-Nema34-Stepper-motor-1232oz-in-driver-DQ860MA-cnc/152686189657?hash=item238cce5c59:g:tMcAAOSwuiNZqMJD)
or
https://www.ebay.co.uk/itm/DE-ship-wantai-CNC-Kit-4Axis-Nema23-Motor-425oz-in-dual-shaft-Driver-4-2A-Power/152481493148?hash=item23809af09c:g:7YIAAOSw-KFXedJL (https://www.ebay.co.uk/itm/DE-ship-wantai-CNC-Kit-4Axis-Nema23-Motor-425oz-in-dual-shaft-Driver-4-2A-Power/152481493148?hash=item23809af09c:g:7YIAAOSw-KFXedJL)
There is a big difference between the sizes of motors hence the question..... I'm going for an extra motor as I'm thinking of another addition!
These are the ballscrews:
https://www.ebay.co.uk/itm/Antibacklash-SFU1605-Ball-Screw-200-750mm-BK-BF12-2-6-35x10mm-Couplers-Set/162559827351?_trkparms=aid%3D555017%26algo%3DPL.CASSINI%26ao%3D1%26asc%3D55148%26meid%3D37d61fe8628c4bd8993d8b1eb0ba1472%26pid%3D100505%26rk%3D1%26rkt%3D1%26%26itm%3D162559827351&_trksid=p2045573.c100505.m3226 (https://www.ebay.co.uk/itm/Antibacklash-SFU1605-Ball-Screw-200-750mm-BK-BF12-2-6-35x10mm-Couplers-Set/162559827351?_trkparms=aid%3D555017%26algo%3DPL.CASSINI%26ao%3D1%26asc%3D55148%26meid%3D37d61fe8628c4bd8993d8b1eb0ba1472%26pid%3D100505%26rk%3D1%26rkt%3D1%26%26itm%3D162559827351&_trksid=p2045573.c100505.m3226)
I have not narrowed down my search by price etc yet as I'm still just looking for the parts for the design at the moment. Once I narrow the design down I'll know what I want and start looking for a good price as well.
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Hit me up via PM if you want measurements on bits but I am not a CNC designer by any means so joining https://www.cnczone.com/forums/ (https://www.cnczone.com/forums/) or certainly doing some reading would be in order too.
I still plan on using my modded 3020 Chinese cheapy for PCB's when needed but I have tended to etch boards in the past as the router never had an enclosure to keep the dust down in the past. As well as a wiring tidy up the big one is getting an enclosure and the baby will fit on the bed inside that enclosure so I may find myself changing to milling boards.
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Hit me up via PM if you want measurements on bits but I am not a CNC designer by any means so joining https://www.cnczone.com/forums/ (https://www.cnczone.com/forums/) or certainly doing some reading would be in order too.
I still plan on using my modded 3020 Chinese cheapy for PCB's when needed but I have tended to etch boards in the past as the router never had an enclosure to keep the dust down in the past. As well as a wiring tidy up the big one is getting an enclosure and the baby will fit on the bed inside that enclosure so I may find myself changing to milling boards.
Thanks, I'll take a look at the CNC forum.
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The NEMA34 kit is likely overkill for this. The NEMA23-kit looks about right though.
Many of these kits boasts a high holding torque only to counter that with very high inductance which limits the speed of the motor for a given supply voltage. Those type of motors are generally suitable for belt- or rack and pinion drive where the rotational speed of the motor is fairly low. For a 5mm / rev screw drive the motors need to turn faster so you want low inductance/high current motors and the NEMA23-kit you linked to looks quite alright.
I would have liked a 48V supply instead of the 36V one but that puts it pretty close to 50V max of the included drive so... Those motors, 80V rated drives and a 60V supply would have been a nice kit.
I've had some grief from chinese ballscrew bearing support blocks. Not saying they're all the same but be aware.
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The NEMA34 kit is likely overkill for this. The NEMA23-kit looks about right though.
Many of these kits boasts a high holding torque only to counter that with very high inductance which limits the speed of the motor for a given supply voltage. Those type of motors are generally suitable for belt- or rack and pinion drive where the rotational speed of the motor is fairly low. For a 5mm / rev screw drive the motors need to turn faster so you want low inductance/high current motors and the NEMA23-kit you linked to looks quite alright.
I would have liked a 48V supply instead of the 36V one but that puts it pretty close to 50V max of the included drive so... Those motors, 80V rated drives and a 60V supply would have been a nice kit.
I've had some grief from chinese ballscrew bearing support blocks. Not saying they're all the same but be aware.
TBH to Nema 23 was looking like my favourite choice. I'm not stuck in going for a particular supplier or kit just at the moment nor is the ballscrew kit a final decision. These are just examples of what I am looking at. I'll look at a higher voltage power supply.
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Avoid the steppers and stepper drivers and go for an integrated servo drive like the Clearpath servos. You can order the servos with step and direction inputs if you want. Cost is around $230/per servo in the NEMA23 size. Well worth it as you can tune them easily, they are pretty robust and you get much better performance over steppers.
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This one turned up in my youtube feed today. Nice video down to earth nuts and bolts design no BS and a shopping list of parts for you :-+
https://youtu.be/0G3tEE7HEDA