Comments sought on new build: A CNC Router for RC Gliders and Planes

Not done anything on this since the last update but have been specifying a Nema34 and 240V driver for the router along with the pulleys etc needed. Just bought for work an Axminster horizontal band saw http://www.axminster.co.uk/axminster...utting-bandsaw which seems to work OK.

Attachment 13667

Gradually adding tools for making all sorts of stuff. I've got my small ARC welder and Pillar drill at work too so getting quite a nice little workshop together. only thing that would be better was if it was at my house instead of work but I do have access any time I like.

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Originally Posted by njhussey

Gradually adding tools for making all sorts of stuff.

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Tool collector or tool user ? that's the question. :joker:

That's the same saw as I have - they've just changed the colour scheme. Once I got the blade adjusted properly (i.e. not prone to fall off at awkward moments) it cuts well - nice and square.

Have you bought the motor & pulleys already? If not then did you calculate the required system or take an educated guess? If you post the parameters (mass of axis, length and diameter of ballscrews, desired feedrate etc) then I can verify...

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Originally Posted by EddyCurrent

Tool collector or tool user ? that's the question. :joker:

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Using them for work... :wink: making spacer rings for some of our products, making jigs and fixtures for pressing in and out bearings etc.

Attachment 13668

Soon hope to be using them on the router (and mill conversion) but it keeps getting put back as more and more "one offs" keep coming in that I need to modify/make....not complaining as I'm being paid to play with the tools which makes a change from sitting at a desk all day long!!

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Originally Posted by Jonathan

That's the same saw as I have - they've just changed the colour scheme. Once I got the blade adjusted properly (i.e. not prone to fall off at awkward moments) it cuts well - nice and square.

Have you bought the motor & pulleys already? If not then did you calculate the required system or take an educated guess? If you post the parameters (mass of axis, length and diameter of ballscrews, desired feedrate etc) then I can verify...

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Hi Janathan, I've only done a couple of small test cuts on some 25mm rod to test it worked...will be checking the blade and giving everything a once over before it's used in earnest.

I'll have to dig out the drawing of the gantry (estimated) to work out the mass, ballscrews are 1610's length is 1300mm, feedrate I've not calculated (or did but can't find the calculations on the spread sheet) but it will be cuttng balsa wood, plastic and aluminium.

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Originally Posted by njhussey

I'll have to dig out the drawing of the gantry (estimated) to work out the mass, ballscrews are 1610's length is 1300mm, feedrate I've not calculated (or did but can't find the calculations on the spread sheet) but it will be cuttng balsa wood, plastic and aluminium.

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Just to be clear that's two ballscrews at 1300mm long, not one? By feedrate I meant what feedrate do you want, not expect, so for cutting those materials I'd say around 8-10m/min would be suitable. The mass doesn't have to be that accurate - within 10% is plenty good enough.

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Originally Posted by Jonathan

Just to be clear that's two ballscrews at 1300mm long, not one? By feedrate I meant what feedrate do you want, not expect, so for cutting those materials I'd say around 8-10m/min would be suitable. The mass doesn't have to be that accurate - within 10% is plenty good enough.

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Yes twin ballscrews 1300mm long! Going to drive it with one Nema 34 8.7Nm stepper (or 4.5 if I can get away with it) and a 2M2280N High voltage digital microstepping driver and a long belt :encouragement: 55kg weight springs to mind but I'm at work and the details are on my laptop at home.

Based on:

• 2 of RM1610-1300mm
• 55kg actuated mass

Assume:

• 10m/min feedrate
• 2m/s^2 acceleration
• 20T pulleys for 1:1 ratio, 30:N pulleys for 30:N ratio.
• Cutting force of 25N
• Not pre-loading ballscrews

Variables to select:

• Motor size
• Drive ratio

For the common 3Nm Nema 24 motors the corner speed is about 1000rpm, so lets start with 1:1 ratio and see if the torque calculated is low enough to use two of those motors. Using the script (posted here, and attached), the required torque from each motor is 0.47Nm. Up to their corner speed stepper motors output about 2/3rds of their holding torque, so 2Nm up to 1000rpm for the 3Nm motor, so since you only need 0.47Nm this system will definitely work up to 10m/min (1000rpm*10mm) plus a bit more since the torque will only drop below 0.47Nm at a fair bit more than 1000rpm, so you have a good factor of safety. You could even use 30:20T ratio and get 15m/min feedrate since the torque for these parameters is 0.78Nm, though you would need to used fixed-fixed bearings on the ballscrews to get the required critical speed.

One interesting thing to note is that if you use 30T pulleys instead of 20T, the torque required increases from 0.47Nm to 0.70Nm, so don't use bigger pulleys than necessary.

Lets try one motor driving both ballscrews:

Assume:

• 10m/min feedrate
• 2m/s^2 acceleration
• 20T pulleys for 1:1 ratio, 30:N pulleys for 30:N ratio.
  
  • One 20T pulley on each ballscrew
  • Two belts (length roughly 1/2 distance between ballscrews) with two 20T pulleys attached to motor.
  
  
• Cutting force of 25N
• Not pre-loading ballscrews

Since the system is symmetrical, we can model it as a single ballscrew with the length doubled and twice the pulley inertia. The calculated torque is 0.75Nm (or 1.09Nm with 30T pulleys). That's still within the ratings of the 3Nm motor... so no need for a Nema 34 motor (though it would clearly work, just be more expensive). If you want higher resolution, you could use 16:20 ratio (16T on motor) and the torque required is 0.55Nm.

My script also now calculates the ballscrew (nut, shaft & bearing) stiffness and the torsional stiffness of the belts. For example if we assume 100mm center distance, then for two motors the linear error due to belt compliance is 5.6um, compared to the error due to ballscrew stiffness of 0.7um. That's assuming worst case scenario - so machine operating with maximum acceleration and cutting force. If you use one motor, say the belts have about 700mm center distance then the belt stiffness goes down by a factor of 7, so it's now about the same as the torsional stiffness of the ballscrew. That sounds like a big difference, but you have to consider the stiffness of the whole system as clearly if the gantry is not terribly strong that will make a bigger difference than the belt stiffness (or lack of).

I've attached the scripts with your parameters entered for X - change the file extension from .txt to .m. The script runs in MATLAB, or GNU-octave which is free/open source and can be downloaded here. I encourage you to experiment with the numbers to get a feel for what parameters affect the torque requirements the most. You could put your numbers in for X and Z...

Attachment 13676Attachment 13677

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Originally Posted by Jonathan

Since the system is symmetrical, we can model it as a single ballscrew with the length doubled and twice the pulley inertia. The calculated torque is 0.75Nm (or 1.09Nm with 30T pulleys). That's still within the ratings of the 3Nm motor... so no need for a Nema 34 motor (though it would clearly work, just be more expensive). If you want higher resolution, you could use 16:20 ratio (16T on motor) and the torque required is 0.55Nm.

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No sorry lovely calculations and all that but I can tell you from experience that 1 x 3Nm Nema 23 motor connected to 2 x screws with timing belts won't handle what will become 60Kg+ gantry and be reliable.!
Yes it will move it no problem and work but when the cutting gets hard or the speed and direction changes are fast it will struggle and the chances of lost steps makes it unreliable IME.
Either twin 23's or Single 34 is the way to go for this size/length and weight IMO.

I calculated that I only needed one Nema 24 3Nm stepper for the twin X axis ballscrews but on thinking about it and listening to Dean I'll be going with a single Nema 34 on the X axis.....just to err on the safe side...after all it's not my money now so I'll be getting a 240V driver to go with the Nema 34 as well...:applause:

I reckon they're both right :) Jonathan's calcs are ok if you are not microstepping cos then you have to derate the torque significantly not to lose steps, so Dean's answer is pragmatic in the likelihood you will be microstepping.

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Originally Posted by irving2008

I reckon they're both right :) Jonathan's calcs are ok if you are not microstepping cos then you have to derate the torque significantly not to lose steps, so Dean's answer is pragmatic in the likelihood you will be microstepping.

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Microstepping reduces the so called 'incremental torque' - i.e. the torque available to move from one microstep to the next and hold a microstepped position - the torque when the motor is moving continuously (i.e. the operating point I calculated) is the same regardless of microstepping. Also above low speeds stepper drivers switch to full stepping.

Personally I would still use two motors, but for other practical reasons - not due to the torque rating.

I've just about reached the stage on my machine where I have to decide one motor/twin belts or two motors. Sounds like the received wisdom is to go two motors. So, that means buying one more motor, another driver - but the old analogue ones I have at present are obsolete, so that will mean two new digital drivers, and, what's worse, ditching LinuxCNC and having to use Windows/Mach3 to be able to home two motors, and, probably, an external motion controller. Ho hum...

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Originally Posted by Neale

I've just about reached the stage on my machine where I have to decide one motor/twin belts or two motors. Sounds like the received wisdom is to go two motors. So, that means buying one more motor, another driver - but the old analogue ones I have at present are obsolete, so that will mean two new digital drivers, and, what's worse, ditching LinuxCNC and having to use Windows/Mach3 to be able to home two motors, and, probably, an external motion controller. Ho hum...

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I'm in a similar situation regarding the single vs two steppers, I can either buy another Nema 23 3Nm stepper and driver (but mine are the AM882 so I'd have to source from China or ebay) or a Nema 34 and 240V driver...decided on the Nema 34 for a bit of safety margin...

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Originally Posted by njhussey

I calculated that I only needed one Nema 24 3Nm stepper for the twin X axis ballscrews but on thinking about it and listening to Dean I'll be going with a single Nema 34 on the X axis.....just to err on the safe side...after all it's not my money now so I'll be getting a 240V driver to go with the Nema 34 as well...:applause:

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Ok well feel I've got to comment here about theses calculations or calculators. While I'm not saying the calculations are wrong what I am saying is that they are miss leading and not giving the full picture so sending people wrong. I've helped several people who have gone off them and been dissapointed with performance.!!

They don't and can't take into account all the factors that make up a working machine. They don't account for miss alignment or poor materials and how machine is designed and built or the affect this as on friction etc.
They don't account for the affects of resonance. They don't account for Cheap drives running on PC with poor pulses from a ropey parallel port.
They don't account for the DIY affect and the numerous things that can affect how machine performs.

All these things come into play and have a big affect on the overall performance and the calculators can never know these variables so IMO they are only to be used as a very rough guide.

This particular case is a prime example because while the Calculators says 1 x 3Nm Nema23 turning 2 x 1300mm screws connected with belts should all be hunkydory I can tell you with confidence you won't get anything like the performance that as been suggested. Yes it will move them but at much slower feed rate and acceleration than the calculator suggests to give a stable working machine.
So Neil believe me your Not Erring on the side of caution your doing what's is needed to get the level of performance your looking for.!!

Neale:

The single or Twin motor argumant falls down to ONE real differance.! . . How stable and accurate do you want your machine.?
If you wan't very stable with virtually no chance of screws losing sync even with cheap electronics then you need single motor/belts.

If you want to use twin screws then while they do work well they only do so provided certain things are correct.?
They are much more sensitive to motor tuning and need careful attention not to push too hard.
IME They work best and get best performance from them with Modern digital drives and External motion control cards which can provide nice clean pulses compared to ropey parallel port.
IMO it's crucial they have some form of stall detect or Error signal on the drives so if one motor stalls or drive fails the system shuts down. I won't build a twin screw machine without this option and Those that have built using twins motors and had this happen at speed will tell you how scary the big potential for damage.

I build more machines using Twin motors than single motors because it's easier but will ONLY do so provided I can build like the above. I Only build using Motion control cards and Digital drives and still leave a comfortable margin on tuning for best reliabilty and safety.
I never have any issues with twin screws building this way but it comes with a cost. Single motor and belts while more messy and involved is cheaper and more forgiving with lesser electronics.
I never truely trust twin screws not to stall or twist up the machine in some way if pushing hard for exteneded periods and I'm always mindfull while using and setting up. Where has I have complete trust in single motor setup and it never enters my head in use no matter how hard I push, no matter how long I push it for.!!

Both work.!!

M = Misalignment
Q = Materials quality
D = Design and build
F = Friction etc.
R = Effects of resonance.
C = Cheap drives running on PC
P = Ropey parallel port.
Y = DIY effect
O = other miscelaneous effects

For each of the above assign a value between 0 and 1; 1 = 'perfect' or negligible, 0 = 'total shite'

JazzFactor = M*Q*D*F*R*C*P*Y*O

(Note: if any value=0 then JazzFactor=0 hence idea=total shite so don't even bother.


Actual calculation = theoretical calculation * JazzFactor

Example:

theoretical torque = 0.75Nm

JazzFactor = (0.9 * 0.95 * 0.9 * 0.99 * 0.9 * 0.95 * 0.9 * 0.85 * 0.85)

actual torque = 0.75 * 0.42

actual torque = 0.32 Nm :joker: :joker: :joker:

Well I dusted off the frame this afternoon and took the last of the epoxy off. It's actually set really hard so I could have used it after all....but better safe than sorry I'm going to re - do it from scratch. 10 mins with a hammer and chisel (screwdriver) and and angle grinder with flapper disc and it's back to bare metal. Going to cut mdf sides this time and seal the gaps/edges/joins with silicone sealant. Got to do a bit more proper work now this afternoon on the lathe but hope to get the moat done tomorrow.

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Video showing the hardness of the set Epoxy...

http://youtu.be/cwzTb2aPNnU

Only managed 1/2 hour on this today but cut the MDF for the moat. Tomorrow I'll glue it together and seal it, thinking of sticking it to the steel with silicone sealant which will also act as a sealant to stop (hopefully) any leaks as I don't want a repeat of last time where despite my best efforts (obviously not good enough) I had leaks at some joins.

If I can get it together and sealed then I might go for a pour so it starts to set over the weekend. Whatever happens I'll not be rushing this....lesson has been learnt from last time :)

Attachment 14133

Neil,
I trust you read silyavski's thread about his epoxy method ?

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Originally Posted by EddyCurrent

Neil,
I trust you read silyavski's thread about his epoxy method ?

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With the metal strips?

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Originally Posted by njhussey

Only managed 1/2 hour on this today but cut the MDF for the moat. Tomorrow I'll glue it together and seal it, thinking of sticking it to the steel with silicone sealant which will also act as a sealant to stop (hopefully) any leaks as I don't want a repeat of last time where despite my best efforts (obviously not good enough) I had leaks at some joins.

If I can get it together and sealed then I might go for a pour so it starts to set over the weekend. Whatever happens I'll not be rushing this....lesson has been learnt from last time :)

http://www.mycncuk.com/attachment.ph...id=14133&stc=1

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Here was my effort flashing the heat gun across gets rid of a small bubbles: ..Clive

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Originally Posted by njhussey

With the metal strips?

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Well maybe not, this is the thread, also read a few pages earlier; http://www.mycncuk.com/threads/6619-...3914#post63914

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Originally Posted by EddyCurrent

Well maybe not, this is the thread, also read a few pages earlier; http://www.mycncuk.com/threads/6619-...3914#post63914

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Yeah I've read it....my pour went OK apart from a couple of leaks which I cured and me not having the correct ratio. I got no shrinkage and the epoxy was level all round (measured with a straight edge and level) so I'm going to go as I did before. Mine is only 1300mm long so no where near the length of Boyan's which I think is where alot of his problems came from.

Finished the moat today, another day of only 1/2 hour to work on the machine. Still ill not be rushing!!!

http://tapatalk.imageshack.com/v2/14...81d921eb9b.jpg

Built it with tabs so that I can lift it off and replace it easily after running a bead of sealant under/round it to seal the gaps. Started gluing the mdf with superglue but ended up using no-nails...will put some oil on the MDF first so the epoxy doesnt stick (as much) to it.

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Originally Posted by njhussey

...will put some oil on the MDF first so the epoxy doesnt stick (as much) to it.

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Would parcel tape work instead of oil ?

Taking no chances this time eh !

Yes, i was going to use that...Trouble is there's some oil already on it so the parcel tape won't stick...

Nope not taking any chances...going to triple check everything!!! ;)

Neil I used the Wests System and found it easier to weigh out the two parts with a digital scale rather than by volume and mixing with a flat paddle (strip of ply etc) if this helps. Good luck. ..Clive

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Originally Posted by Clive S

Neil I used the Wests System and found it easier to weigh out the two parts with a digital scale rather than by volume and mixing with a flat paddle (strip of ply etc) if this helps. Good luck. ..Clive

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Yes the reactive resins is done by weight, twice as much epoxy to harder I seem to remember....will write it down in big numbers so I can't get it wrong!!! I've got some proper mixing sticks this time...

Do you have access to a vacuum pump? IF so, you can put the well mixed resin in an over sized container and degas that way (a sealed 50 gallon drum can act as a chamber if the pump can handle the volume). That gets rid of pretty much all gas in the resin which will make for a much better pour and no worry over bubbles. The issue is getting access or having a vacuum pump that can handle the volume.

Michael

Or a drum that can take a vacuum .. Clive

Just how important is this degassing anyway? I stirred my resin vigorously, let it settle for a few minutes, then poured. I didn't even use the "pour through a small hole" technique. There were some bubbles in the resin as poured, but I paddled a bit of lolly stick back and forth to encourage the resin to find its own level, and also to remove bubbles. The few that were left were very easy to deal with once the resin was hard. It's not something that I'm going to spend much time worrying about when I come to epoxy the gantry rail mountings. For reference, that was using 500g resin/250g hardener on a similar size machine.

Ive not got access to a vacuum pump but I'm not too concerned about it as before I did the two pot mixing and pouring through a hole from about 12" and there were very few bubbles. Those that did form were dealt with by a piece of welding wire (didn't have a cocktail stick at work) which as Neale said also helped it find its level.

Cleaned the rails with thinners and used a tack rag to get all rubbish off then put silicone sealant down and put the oiled moat back on. Now waiting for the sealant to go off overnight so I can trim it back inside and then tomorrow am I'll do the epoxy mix and pour. That way it'll have 10 days over Xmas to set!

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Well......epoxy pour is done....triple weighed everything and double checked...

Leveled the frame on the floor...

http://tapatalk.imageshack.com/v2/14...37bf1ec5c1.jpg

Weighed the epoxy out...

http://tapatalk.imageshack.com/v2/14...381f42e41a.jpg

Weighed the harder out...

http://tapatalk.imageshack.com/v2/14...1b60bef689.jpg

Mixed it in a cut down 2 litre milk carton first...

http://tapatalk.imageshack.com/v2/14...bbe084b775.jpg

Then transfered to a flatter container with a 7mm (ish) hole drilled in a bottom corner with tape over...poured it from a couple of feet high to mix it as it was transferring

http://tapatalk.imageshack.com/v2/14...6d845e0ad7.jpg

Stirred it some more then poured it in the moat with fingers crossed for no leaks....

http://tapatalk.imageshack.com/v2/14...3daa8de143.jpg

http://tapatalk.imageshack.com/v2/14...fad18b05f0.jpg

There are a few bubbles but a quick swizz with a lollipop stick and they went. There are a few microscropic bubbles but I'm not too worried about them...

http://tapatalk.imageshack.com/v2/14...d58320d95f.jpg

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There are 3 leaks, one from a corner joint on the bridge, and 2 where there must be a gap under the wood. Happy with that as they were stopped with some paper towel and parcel tape.

You could see the epoxy going across the bridge where it was leveling itself off so it obviously works!

Clive, I used a flat stirring stick this time [emoji6]

Nearly all bubbles have gone, leaks have been stopped so now I can stop crawling round on the floor...

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Neil Looking good as you know I just flashed the surface with the hot air gun and the small bubbles just disappeared like magic. Happy Xmas. ..Clive

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Originally Posted by Clive S

Neil Looking good as you know I just flashed the surface with the hot air gun and the small bubbles just disappeared like magic. Happy Xmas. ..Clive

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Clive, I was going to use my blowtorch but there were only a couple of bubbles so i popped them with the lollipop stick, there were so few microscopic ones that I didn't bother....thosse seem to have floated up to the surface and popped over lunchtime.

Merry Christmas and a Happy New Year to you Clive.

Neil

Blowtorch as in naked flame!! Hot air gun no naked flame. Happy new year as well. ..Clive

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Originally Posted by Clive S

Neil

Blowtorch as in naked flame!! Hot air gun no naked flame. Happy new year as well. ..Clive

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Sorry yes hot air gun.....I still call them blowtorches :stupid:

Hooray ! it's done, the build will speed up now that hurdle is out of the way, looks good and a plus for Reactive Resins