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feedrate/motor tuning
« on: March 21, 2011, 08:19:45 PM »
Hi all,

I found the answer a while ago whilst browsing the forum but now cant for the life of me find the post.

Im up and running now and have gone through the motor tuning and calibration. I found the maximum smooth motor velocity for eaxch axis and then knocked a bit off for safety. Whilst I can get to high speeds, the motor sounds rough. At the moment the maximum I am able to get is about 800mm/min. I will be mainly cutting mdf, plywood, hdpe and some aluminium. The machine is small and uses a proxxon (like a dremel) so only small bits. Is this speed ok? I know the feedrate varies depending on the material, tool, etc but from looking at videos on youtibe mine seems quite slow and whilst a few test cuts have been sort of ok, I want to get the machine and software set right before I start going any further.

Thanks

Offline Hood

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Re: feedrate/motor tuning
« Reply #1 on: March 21, 2011, 08:25:39 PM »
I am more used to metal cutting machines but I would say a feedrate of 800mm/min is a bit slow for a wood cutting machine with a high speed spindle.
I think routers tend to have a larger pitch or a multistart thread so that the distance moved in one revolution of the screw is a lot greater than you would expect for a metal cutting machine, this in turn gives you greater feedrate possibilities.
Hood
Re: feedrate/motor tuning
« Reply #2 on: March 21, 2011, 08:39:00 PM »
Hi,

I should have posted more details now thninking about it.

It moves 1mm/revolution, the calibration set it to about 318 steps/rev. motors are nema 23 1.8 degree 200 steps/rev. running at 36V I can get to over 3000 in the motor tuning but it sounds ill to me, a sort of high pitched sound rather than the smooth sound I would expect. I did get it running a bit better on the motor tuning but then when I jogged the motor stalled. Im not sure what I should be aiming for, if Im a mil eoff then obviously I need to fix it but Im sort of stuck without knowing what to aim for.

Thanks

Offline Hood

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Re: feedrate/motor tuning
« Reply #3 on: March 22, 2011, 04:04:40 AM »
To me 1mm sounds a small pitch even for a metal cutting machine let alone a wood cutting machine. Assuming you have no gearing then your 3000mm/min means the stepper is doing 3000rpm and that to me sounds too high for any kind of stability. Small steppers will rotate faster than large steppers but how much I dont know but I usually hear a figure of 1000rpm banded about for steppers as a ball park figure.
 If you have the torque available it might be an idea to step up in gearing so for example the motor turns one rev and the screw turns three but I better leave these thoughts for people who are more used to steppers.
Hood
Re: feedrate/motor tuning
« Reply #4 on: March 22, 2011, 05:51:37 AM »
Hi,

No gearing, the main frame, lead screws, mountings etc was bought as a kit (new but 2nd hand) off ebay. Didnt come with instructions but I managed to find it and it is this one http://modularcnc.blogspot.com/search/label/CNC%20Mini%20Mill The third one down. According to that page, the lead screws are 1/4 x 20. I'll double check my distances when I get back home. Is using an imperial thread and me working in metric going to cause any issues?

Thanks

Offline Hood

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Re: feedrate/motor tuning
« Reply #5 on: March 22, 2011, 06:41:22 AM »
No probs with metric/imperial, I would say if metric will be your main units then set the default units to metric and work out your steps per unit as metric. That would be (200 x microstepping?) / (25.4/20) so say your microstepping is 10 then (200 x 10 )/(25.4/20) =1574.8031496 steps per mm

Hood
Re: feedrate/motor tuning
« Reply #6 on: March 23, 2011, 09:32:17 PM »
Hi,

Thanks for the info. The stepper drivers Im using can only do 1/2 step so from your formula this gives me 314.9606 steps per mm, so my measurements in the mach3 calibration were pretty close. So going from this, basically I need to either change lead screws or add some sort of gearing to give me 1:3 ?

My machine is very small,  the original reason for building one was for pcb milling but now its working I have many more uses. I plan to either build a new one or increase the size of this one, the problem is Im limited for funds at the moment so its going to be a bit at a time. At the moment Im thinking of changing mine into a moving gantry type rahter than the moving table which should give me a larger working area for the same footprint and I should hopefully be able to do it without taking the machine out of action for long as I would like to use it to make parts for the new one.

Going from that, would the best idea be to change lead screw pitch (I'll need new longer ones anyway) or do the gearing? the motors are way overpowered, I sort of bought larger ones rather than ending up being underpowered. The ones I have are these ones http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=260725764391&ssPageName=STRK:MEWNX:IT

Thanks

Calv

Offline Hood

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Re: feedrate/motor tuning
« Reply #7 on: March 24, 2011, 06:48:48 AM »
If the steppers are a lot bigger than needed then I would go the gearing route as it will allow more flexibility than changing the pitch. Having said that it will mean your screws are rotating a lot faster than they would be if the pitch was greater. Maybe a combination of both would be a good idea as you are getting new screws anyway.
Maybe see what the other router guys are using, think Gerry (ger21) is a big router user, so maybe he will jump in and say.

Hood