The next question That I have is about stepper motors and drivers. What is the advantages to having separate drivers for the stepper motors to having all drivers on one board?
I think I will buy the four separate drivers but wanted to check before sending a order.
Now I will buy nema23 motors but what holding torque should I buy 3.1Nm. or will 2.2Nm work just as well, with the RM1605 ball screws I have. Now I know That it would be better "faster" to drive it with belts and pulleys to change the ratio but at this time it will have to be direct drive. I can change it at a later date if I don't like it.
I will be cutting Balsa and ply 98% of the time but I will require the CNC to cut 3mm aluminium and some hard wood now and again.
I have been reading builds on the forum but it seems that most talk about speed but why? I would be happy as long as it does a good accurate job, if it takes a bit longer to cut I don't mind.
Last edited by Bush Flyer; 30-05-2013 at 05:10 PM.
Even if you don't end up needing the high speed available, the fact the machine is capable of it is advantageous since the machine will be more reliable at lower speeds - e.g you can pretty much guarantee if you've chosen motors/drivers which will move the machine at 10m/min they will never stall or have problems at 5m/min.
In princible there's nothing wrong with having several drivers integrated into one board, so long as they're reliable and suitably rated. The problem is in reality none of them will output enough current/voltage for what you need, so trying to get one to work will not be reliable. Just forget them and please don't post more pictures of TB6560 drivers ;)
You can try Irving's motor calculation spreadsheet (just search that on the forum) to work out which motors would be best. I'd advise using the common 3Nm motors with a 75V power supply. The difference in price between this and other options isn't that great, and it's the best performance you can get without spending a lot more money. I think CNC4You is currently the cheapest place to get the motors and eBay/aliexpress for the drivers.
The Following User Says Thank You to Jonathan For This Useful Post:
I have just done a member search on ebay but aliexpress have not been trading in the last 12 month
But I will take your advice and order 3.1Nm stepper and four separate drivers.
Jonathan has pretty much said it all but just to add that when I first started I said exactly the same thing "don't care if it cuts slow, just want it accurate."
I have since learnt (and stil learning) that cutting slow is counter productive to accuracy and if you take two small a cut then the cutting edge will skate over and work harden the surface making it harder for the following cutting edge to make the next cut, this then causes the tool to deflect and accuracy goes out the window.
It seems odd but there becomes a point when the cutting force increases as the chip thickness decreases. This is because if the radial depth of cut is small it produces a large shear angle so the cutter sees a large area to cut.
Accuracy is more dependant on the correct material removal rate so you need speed and power.
I seems you have to learn how to build a good machine and then learn how to use it
Jonathan also mentioned aiming for a higher design speed to give headroom on the constant cutting speed but this head room is not wasted as you can have fast rapids when moving between cuts (subject to mass/inertia limitations of course) cut at 5m/min move at 10m/min
Have you been to or do you go to your local Pain management centre? If you don't then it might well be something worth looking into. They will be able to help you in all sorts of ways, not with just being able to do a bit of work but pretty much every aspect of life.
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Similarly, if the G-code is well written then the proportion of rapid moves will be quite small (clearly there are exceptions). Suppose the rapid moves comprise 10% of the total time taken for the job. Here, doubling the rapid speed will only take 5% off the overall job time.
Clearly there are exceptions - if the rapid speed is limiting how fast you can cut, then having more speed (so long as the acceleration is sufficient to use it) will decrease the machining time noticeably. An example of this is surfacing an MDF bed, where cutting at 10m/min would be perfectly reasonable (I do it)...but then how often do you need to surface the bed?
Often it's a compromise between maximising speed and acelleration, since increasing either requires more power and the motors only deliver a finite amount of power. With pulleys you can adjust the ratio to optimise for better speed or acceleration, but in the end you still need motors with enough power - hence my suggestion to get the 3.1Nm motors as their power output is plenty and they're better priced than virtually all other realistic options.
Last edited by Jonathan; 31-05-2013 at 07:33 PM.
Jonathan thanks for the update in finding that site, I have now placed a order for drivers and have also bought the 3.1Nm stepper motors that you advised, That was a good saving in cost. Thanks.
Last edited by Bush Flyer; 03-06-2013 at 09:17 AM.
Been a while since I did any work to my CNC router as I was in Hospital again for a few days and the wife has band me from the garage. But I did get some wiring done, but have a lot more to do, I'm waiting on a E stop switch and more wire.
So just some photo's where I stand at this time. I hope to get going again in the next couple of weeks.
Last edited by Bush Flyer; 13-07-2013 at 12:31 PM.
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