incorrect feed rate

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incorrect feed rate

Postby blarkscnc » Fri Dec 21, 2018 1:59 pm

Hi guys, I've just built a new machine, an openbuilds cbeam with g540. I have treid searching for an answer for some time and I just can't seem to find exactly what I'm after. My question is about the feed rate, I have a stepcraft 2 and have successfully cut alloy at approx 1200mm/min, now when I try the same operation with the cbaem I have to drop the feed override down to 30% which ended up being roughly 360mm/min I know this can't be right as it seems far to low. Cutter is 2 mm single flute spindle is the stepcraft version of the kress or rebranded kress and have it at around 5 on the dial. For reference min is 1= 4000 to max at 6 = 25000 rpm although I'm a little dubious at these figures and speeds but no way to test it really. I did try different speeds and after several broken cutters the above feed gave the best result. I have made a new profile and loaded the mach xml and then changed from there. At a bit of a loss as to what to do, it seems the feed rate is roughly a third of what it should be, in fact 1200 divided by 360 is 3.3 recurring.
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Re: incorrect feed rate

Postby ger21 » Fri Dec 21, 2018 3:18 pm

Is it moving the correct distance?
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Re: incorrect feed rate

Postby cncdrive » Fri Dec 21, 2018 5:59 pm

If your programmed "F" feedrate is correct then your steps per unit setup is incorrect.
Try to set a known point on the axis, move a known distance and measure it, it will be incorrect... you should setup the "steps per units" value properly for the axes.
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Re: incorrect feed rate

Postby blarkscnc » Fri Dec 21, 2018 9:27 pm

Thanks for the replies guys, I did do a calibrate and seems to be moving the right distance. The operation that I run was one that I had previously ran before on the stepcraft with no problem. A question, how far out would the steps be for it to equate to being a 3rd off in the feed rate?
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Re: incorrect feed rate

Postby ger21 » Fri Dec 21, 2018 9:33 pm

33%
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Re: incorrect feed rate

Postby blarkscnc » Fri Dec 21, 2018 9:55 pm

Ok so if the steps are say 10 units off then this would equate to how much loss in feed rate? Sorry if these seem like dumb questions but its really the first time I've had to setup uccnc like this, the stepcraft already had a profile included. The reason I ask about the steps as I seemed to get different results when I first did the calibration where from memory it was up around 258, then when I did a cut it wasn't accurate, so I changed it. I just did another calibrate and at 248 its moving 10 units and traveling 10mm. Do I need to "reset" the profile and start again maybe?
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Re: incorrect feed rate

Postby blarkscnc » Fri Dec 21, 2018 11:22 pm

So I've just gone through the whole setup from a fresh profile and ended up at the exact same spot. Feed rate in fusion is at 1200mm/min, Fact in uccnc reads 1200, cutter breaks, reduce feed override down to 30% works fine. Units are calibrated at 250 and travels 10mm per 10 units so I am truly unsure as to what is wrong.
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Re: incorrect feed rate

Postby ger21 » Sat Dec 22, 2018 2:59 am

Are you actually measuring the feedrate, or assuming it's wrong because the cutter is breaking?
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Re: incorrect feed rate

Postby blarkscnc » Sat Dec 22, 2018 3:08 am

I'm not quite sure on how to measure feed rate other than seeing what the readout says in uccnc. But if I go by the this it is saying 1200 and cutter breakage continues until I get down to 30% (360). So either the readout is wrong or the actual feedrate of the machine is wrong but uccnc is reading it as 1200. But I know that at that feed of 360mm/min cutting alloy was a disaster.
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Re: incorrect feed rate

Postby spumco » Sat Dec 22, 2018 3:11 am

Quick sanity check is to program a move along your longest axis and time it. Calculate a feedrate that should take a round number (10 seconds maybe) and use a stop watch. This will let you know if you're in the ballpark.

So command a 120mm move at 1200mm/min. Should take 6 seconds.

Don't feel bad - I've had to do this myself from time to time.
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