Re: Epoxy Granite Portal Machine
Steppers and servos both are capable of 0.001“ and way finer but first off, you're trading speed for resolution in some cases (the pitch of your ballscrews). Second, that won't be the problem in my opinion. It'll be backlash and slop and flex in the entire system which probably can't total more than like 0.0005 for you to hit your goal. So not only do the bearings and ball nuts and mounts all have to have less than that amount of backlash, but your system, across the largest spanned member, has to also flex less than that under load.
I'm taking some liberties in the math here but just trying to make the point that the motors are probably the easiest part to get "accurate".
That said, you're making this because you want to and many people have made great machines on their own! Just be realistic about what it'll do.
Sent from my SM-G960U using Tapatalk
Re: Epoxy Granite Portal Machine
Quote:
I'm taking some liberties in the math here but just trying to make the point that the motors are probably the easiest part to get "accurate".
Yup. In theory, steppers - when under no load to speak of - can be just a hair more accurate than servos of equal resolution, because servos have following error. However, in practice; a) you wouldn't have a servo with equal resolution; you'd have an encoder on it with finer resolution than the stepper; b) following error can be as little as +/- one encoder count with a well tuned servo; and c) in any case we are talking about errors on the same order as the quantization error of the control system - so it's not something to really worry about.
Where servos and closed loop stepper systems have an advantage is dealing with momentary mechanical overload - open loop steppers can get knocked from one step detent to the next; thus "loosing a step"; and throwing off any moves that come afterwords. Closed loop systems will detect this and make a correction - you might loose a little bit of surface finish (although, that might be a non-issue, if as is likely, the overload was during a roughing operation) but you won't ruin the part.
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That said, you're making this because you want to and many people have made great machines on their own! Just be realistic about what it'll do.
Yes; if your expectations are reasonable - or straight up low - you can be pleasantly surprised when your creation outperforms them. However, if you have unrealistically high expectations, you are guaranteed to be disappointed when it fails to live to up the "pipe dreams"...
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Do steppers and servos have the same mounting in case I wanted to swap later on? (I honestly cant seem to find conclusive data.)
Most steppers of the sort that you'd use for a CNC machine are made in NEMA standard frame sizes - 17, 23, 34, or 42, typically. Some servo motors are also available in those frame sizes; however there is much more variety in servo motor frame sizes than there is in steppers.
However, it's typically not a major problem to make some adapter plates; should one need them.
I have no personal experience with this company's products, but they have been recommended by other people on this forum... and they do have servos in NEMA frame sizes.
DMM | AC SERVO DRIVE | AC SERVO MOTOR | ROTARY ENCODER
:)
Re: Epoxy Granite Portal Machine
Thank you for your responses.
Does anybody know of a good spindle servo? (1.8 Kw, 8000 RPM) Is direct drive necessary if I want to do rigid tapping?
Re: Epoxy Granite Portal Machine
Thank you for your responses.
Does anybody know of a good spindle servo? (1.8 Kw, 8000 RPM) Is direct drive necessary for rigid tapping?
- - - Updated - - -
Thank you for your responses.
Does anybody know of a good spindle servo? (1.8 Kw, 8000 RPM) Is direct drive necessary for rigid tapping?
- - - Updated - - -
Oops. DIdnt mean to double post. Sorry.
Re: Epoxy Granite Portal Machine
Quote:
Does anybody know of a good spindle servo? (1.8 Kw, 8000 RPM) Is direct drive necessary if I want to do rigid tapping?
No, it's not required; but it can't be a friction-belt drive (like a V-belt) - they will slip a little; putting a large amount of force on the tap. Gear drive works if the backlash is low enough and you can setup the control to handle it; but timing belt drive works best if it's not directly coupled. It helps if the pulley/gear ratio is an integer multiple; but depending on the control may not be a strict requirement.
Re: Epoxy Granite Portal Machine
Quote:
Originally Posted by
Richmaple
Steppers and servos both are capable of 0.001“ and way finer but first off, you're trading speed for resolution in some cases (the pitch of your ballscrews). Second, that won't be the problem in my opinion. It'll be backlash and slop and flex in the entire system which probably can't total more than like 0.0005 for you to hit your goal. So not only do the bearings and ball nuts and mounts all have to have less than that amount of backlash, but your system, across the largest spanned member, has to also flex less than that under load.
I'm taking some liberties in the math here but just trying to make the point that the motors are probably the easiest part to get "accurate".
That said, you're making this because you want to and many people have made great machines on their own! Just be realistic about what it'll do.
Sent from my SM-G960U using Tapatalk
Hmmm.
On a mill I've used some decent AC bearings. Double nuts shimmed as tight as they can go, jaw couplings, a fair bit of work.
I can't get lower than 0.03mm (0.0012").
Some expensive linear rails on it then maybe I'll half that but still.
Re: Epoxy Granite Portal Machine
Quote:
Originally Posted by
ando600
Thank you for your responses.
Does anybody know of a good spindle servo? (1.8 Kw, 8000 RPM) Is direct drive necessary if I want to do rigid tapping?
Most I've seen is about 6000rpm (think was Mitsubishi or something). That's like about £1000 plus. Then you need a drive to match.
Generally ones less than £1000 go to 3000rpm.
Likely have to overdrive one with a timing belt.
Re: Epoxy Granite Portal Machine
Quote:
Originally Posted by
dazp1976
Most I've seen is about 6000rpm (think was Mitsubishi or something). That's like about £1000 plus. Then you need a drive to match.
Generally ones less than £1000 go to 3000rpm.
Likely have to overdrive one with a timing belt.
If by stepping up gear 4/1 to reach 12000rpm on spindle, with using the timing belt, will there be too much noise? But if changing to v-belt? will there be slipping problem and rigid tapping not be possible?
Re: Epoxy Granite Portal Machine
Quote:
Originally Posted by
ando600
Hi all,
I have been reading cnczone for a long time, finally time to post. :)
I am in the process of designing an epoxy granite portal CNC. I will be using it for personal projects and research. (R&D, robotics applications)
I currently have 2 700mm thk ballscrews with mounts, 1 set of 40mmx900mm linear rails with 4 blocks, one set of 35mmx750mm linear rails and a thk kr46x250mm linear actuator. I also have most of the electronics. (PC, ESS, power supply, g540) I have a bosch 2hp router and a SuperPID controller.
I have several questions.
1. I am considering using a granite surface plate as the base for my machine, and then building epoxy granite supports and gantry. Is this wise? What is the best way to construct the gantry? Also, what machine mass would be necessary? I was estimating 400-600lbs.
2. I am considering using servos instead of steppers. I was thinking of getting the MB3 BOB from cncroom, or the c80 BOB, and then the DMM DYN2 AC 400W Servo Kit. (Or maybe the 750W. I need to do the calculations for my ballscrews.) What is the practical difference between good steppers with a g540, ess and servos? My goal is accurate to 0.001 with decent feeds and speeds.
3. I realized that my bosch router is not going to be suitable for my workload. I might want to do rigid tapping at some point. I am considering a jianken atc spindle or possibly a bt30 atc belt spindle with a 1.8kw servo drive. Would the lower rpm on the belt/servo drive impact my capabilities? I am planning to machine aluminum, steel and occasionally titanium.
I have a limited budget. I bought most of these parts a couple of years ago, and I am in no rush to get this done.
I have access to a machine shop and a waterjet. I have a pretty good feel for manual machining, and I have access to knowledgeable people.
Any guidance is appreciated! This is my first CNC build.
Thanks
Just similar as you, I prepared precision parts for several years which were bought from ebay which include 4 pieces THK KR33 and 1 piece NSK linear actuators with some small size linear rails. No extra ballscrews are needed. Easy servo stepper motors were bought also. Now, my router is under the building and was finished around 60%. The most critical and getting-headache portion is the precision alignments of those precision parts on the metal frame. As well, the required measuring tools like granite parallel and right angle triangular blocks etc are bought.
Re: Epoxy Granite Portal Machine
If by stepping up gear 4/1 to reach 12000rpm on spindle, with using the timing belt, will there be too much noise?
That depends on the quality and size of the bearings, their alignment, and the mass of the spindle carriage. There will be some noise imho.
But if changing to v-belt? will there be slipping problem and rigid tapping not be possible?
Not a lot of slippage with the right V-belts, but enough that rigid tapping would not be realistic. But trying to do rigid tapping with a 12kRPM spindle is an unlikely prospect anyhow. I suggest that 120 RPM might be fast for rigid tapping, but doubtless someone will claim higher speeds (at very high cost though).
Cheers
Roger
Re: Epoxy Granite Portal Machine
what concern is the noise that comes from the hitting of gear tooth of fast rotating small gear with timing belt tooth, ..don't know ..has not yet enough experience?1
Re: Epoxy Granite Portal Machine
There might be some noise from the toothed belt if you are using one of the original 'timing belt' designs (with backlash), but the modern designs such as GT2 & GT3 are pretty much silent.
Cheers
Roger