[Haydn]

I can run my diy machine at 13,500mm/min or 531"/min without stalling and with plenty of motor torque left. Can't go any quicker as I can only get this with 45k pulse frequency in mach with fixed 10x microstepping. 10mm pitch ball screws directly driven by steppers from homeshopcnc and gecko201 drives. Don't need to go any quicker as it would only wreck the ball nuts/screws. Typically runs at 3000-6000mm/min with wood unless its a small part. Rapids usually set to 8000mm/min.

[ame="http://www.youtube.com/watch?v=3_qdZB9iGhU"]YouTube - CNC Stepper Motor Demonstration[/ame]

[1041448]

Hi all.
I'm building a CNC wood router machine , but i'm a little stuck with the mechanical things . I am trying to get the acme threaded lead screw and the stepper motors , but i dont know the specifications, which should i get first?? how to choose it's specification???
what should the speed of motion on each axis Be??
My machine is a 3 axis wood router , i'll be drilling MDF wood.
Y-axis : 4 feet long ,25 lbs horizantal movement
X-axis : 2.3 feet ,14 lbs horizantal
Z-axis : .6 feet ,11 lbs vertical

Help please ,thanks.

[marting]

Hi to all,
I'm not sure if anyone has ever come across this sort of problem but here we go.. I have built my own cnc router about 3 years ago, as most of you guys did on this forum and have a small setback that
has been bothering me for a while, so I was wondering if anyone on the forum has ever come across this one. My cnc
router runs perfectly fine in all aspects when running at a cutting speed below 28mm/sec. As soon as I take it up to
say 50 or 60mm/sec, my cutting path gets messed up.
My current configuration / setup is as follows:
1 - PC Pentium4 core II 2
2 - Servo drivers from Rutex R990 type
3 - running software Mach3 at max clock speed on OS WinXP
4 - encoders on servos 500ppr
5 - cnc structure mild steel
6 - driver motors - servos rated at 6000rpm at 60V and supply voltage 58V DC (As seen here
ElectroCraft Electric Brush Type DC Servo Motor )
7 - gantry driven by belt and pulley at a step down ratio of 4:1
8 - Using ball screws 10 travel per rev and gantry sliding on brass bushings
9 - working area 750mm x 550mm x 250mm


Now the problem is that when running at "higher cutting speed" the cutting path seems to initially loose orientation
but then comes back to the right path as can be seen in the attached photos showing "V" gap between cutting path. According to my calculations I should
be running at least 200mm/sec.

I have thought the following but could not figure out what could be wrong:
1- There doesn't seem to be any lagging between the gantry and the signal from the driver cards cause the gantry
starts going back a short distance clearly indicating it's way out of line.
2- I have tried a higher encoder resolution - no improvement
3- running mach3 at a lower clock speed - no improvement
4- checked servo actual running voltage - OK
5- Changed servo bearing the whole gantry movement to a more powerful one (type
http://www.kelinginc.net/KL34-170-90.pdf ) but running at voltage of 58V DC.

That's as far I have gone but to no avail and the only against me is the weight of the gantry which is about 20Kg
but nothing the motors cant handle.

Any comments please.
Thanks

[stewi]

Martin,
at 6,000 rpm of your motor and 500 ppr from your encoder (that is 1,000 of track A and B) you are getting 100 kHz pulses to your servo. I don't know your Rutex servo driver, but I would check for max frequency of encoder input.

[jfong]

Martin

50mm/sec (118in/min) on brass bushings??? How smooth does your gantry slide on the bushings?? Looks to me it can be following error if the servo can't move your gantry smooth enough due to bearing friction. Then finally catching up to position. Just a guess....

[Kenny Duval]

Fast enough to damage and break things.

[Jon.N.CNC]

If it's slow just cut deeper, how fast a machine goes is irelevant without a doc. 20m/Min @ .5mm doc is the same as 10m/min @ 1mm

So let's do this properly, My diy build will cut 3500mm/min @18 mm doc in plywood @ 150mm/s/s. Rapids @ 6m/min

[pk7639]

Old video demonstrating the effect of just changing the ballscrew pitch on a small router of mine.

[Jon.N.CNC]

Old video demonstrating the effect of just changing the ballscrew pitch on a small router of mine.

Have you noticed any loss in positioning accuracy?

[pk7639]

Not on that machine.
If you assume that the half steps of a micro stepped drive are reasonably precise (the microsteps tend to be non linear) then it has a resolution of 0.05mm. There's more flex in that little machine than that..

[Tkamsker]

Hi Our closed Loop vmc Mill Can Do 86 mm / sek using closed Loop Steppers in x and z axis y axis is a Little slower ,.. Using Open Loop it is around 50mm / sek the Mill has an 2,2 kW vfd spindle and iso30 spindle


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[Bob La Londe]

I do not currently have a completely ground up home built CNC.

I had a MaxNC 5 that except for the frame and dovetails was home built. My own screws, bearings, bearing keepers, spindle, after market control etc. It did 80ipm rapids and cut a 65ipm when I retired it.

My Taig never was able to reliably run faster than about 65ipm if adjusted to have low backlash, table slop, and head nod. That was with aftermarket controls and motors.

My X4 Speedmaster cuts at 80-110ipm 90% of the time and all axis rapid at 150.

I haven't powered up my new X5 Speedmasters yet, but with the reduced friction and stiction of going from dovetails to linear rails I expect they will be faster.

The Hurco KMB1 will cut at rapid speed if the speeds and feed and cutter will support it. I have a completely retrofit control system on it currently using DuGong 160 servo controllers. I have all axis set to 200IPM, but with only a 3600 RPM spindle it will rarely support that much feed.

All of that being said: For the type of machining I do acceleration is more important than feed rate. I do a lot of very short move 3D sculpting in aluminum. Who cares if the calculators says you can make the cut at 80ipm if the machine never reaches that feed rate anyway. This is where you start to get trade offs. High acceleration will cause your machine to fault almost before it starts. With stepper systems you may see the machine stall or chatter. With servo systems your servo driver throws a fault output to the control. On a stepper driven system (except some closed lop systems which act more like servos) you just lose position and your part is quality suffers from not quite right to totally destroyed. On a properly setup servo system the machine just stops when the following error is exceeded.

What can you do? Reduce the acceleration rate is one thing. On a servo system you can increase the allowed following error. If your controls can handle it you can increase voltage to the servos too. On a stepper system you might be able to increase voltage or current to your steppers, but most small steppers systems are already setup at their max. From my experience usually higher than their max for longevity. You are back to reducing acceleration. Sometimes acceleration also affects the top feed rates. It winds up being a balancing act in those cases.

What do my machines do?

The MaxNC accelerated (after I fixed it from its abominable initial setup) at a solid 20iss.

The Taig when adjusted to cut good tolerances was about 15iss.

My little Chinese router (forgot that one) seems like it will accelerate at 30iss, but on long jobs its position drifts on all axis with Z expectedly being the worst.

The X4 Speedmaster does a solid 25iss all day long on all axis (if the Z weight is balanced) and it just has big steppers. Currently its doing 90% of my paying work. Even so I'll often program 80-110 IPM and not see the display reach half that. I actually tested upto 45iss on X&Y and and 40 on Z after changing gas springs and adding weight to the head. I figure 25iss rock solid is good enough for now. 25iss is the setting in the stock config file.

The X5s remain to be seen, but I am hoping for 30-35 out of them since they also have more powerful spindles. It would be nice to produce at feed rates the 24K spindles can support.

My Hurco again is the odd machine out. Back when I ran GeckoDrives I had to keep turning the acceleration rate down as they were used. Over the course of a year they would drop from 25iss rock solid to 12iss and flakey. I'd replace the drives and it would be great again. I went to the DuGongs and it was like a whole different machine. The initial following error was huge so I did all kinds of crazy tests. Acceleration at 100iss. Rapids at 450ipm (X&Y) and higher. It was stupid. Scary too. When you see over a 1000 pounds of table being slung around like that its pretty wild. I cranked down the following error (I think I have it set at 50 with 10K pulse per inch) and I get a solid 30iss and 200ipm on all axis after much testing at higher accelerations and feed rates. I could run it faster, but there really is no need with just 3600 RPM out of that spindle. I keep playing with the idea of putting a high speed companion on it.

Anyway, my point is that while feed rate is important to material removal rates, so is acceleration. In some types of work a machine with good acceleration and modest top feed rate can outproduce a machine with wild feed rates and more modest acceleration.

[CitizenOfDreams]

My toy desktop machine maxes out at 800mm per minute (30ipm for you non-metric old school machinists). Mostly because it uses M8 all-thread instead of real ballscrews.

[CitizenOfDreams]

Update: I'm in the process of changing the all-thread to T8 trapezoidal thread screws. I did the X axis so far and achieved 9000mm/min (350ipm). Wow, what a difference!

[A_Camera]

Update: I'm in the process of changing the all-thread to T8 trapezoidal thread screws. I did the X axis so far and achieved 9000mm/min (350ipm). Wow, what a difference!

I understand the feeling... In my opinion quite scary, especially when you see it the first time.

My version 1 had 12mm threaded rods with 1.75mm pitch and I had a maximum speed of 1,500mm/min now with 1605 ball screws I can get 10,000mm/min, but hold it at 8,000mm/min which is very fast (more than enough) for me.

[linkreincarnate]

what are your specs for that machine?

[asuratman]

My version 1 had 12mm threaded rods with 1.75mm pitch and I had a maximum speed of 1,500mm/min now with 1605 ball screws I can get 10,000mm/min, but hold it at 8,000mm/min which is very fast (more than enough) for me.

What do you cut with that speed ... ?

[CitizenOfDreams]

What do you cut with that speed ... ?

Air, would be my guess. If your part has a lot of rapid moves (like text engraving), having fast screws helps a lot.

[A_Camera]

What do you cut with that speed ... ?

I don't cut anything at that speed. That is the MAXIMUM speed for rapids on each axis. Of course, it allows me to cut as fast as necessary also, and some times it is indeed necessary to cut fast, even if I have never cut anything at 8,000mm/min yet, it is good to know that whatever is calculated as feed rate can be set and done.

Otherwise I cut aluminum (dry), PCB and plastics, mostly delrin and acrylic.

Another thing I forgot to mention is that I am planning to add a 3D printer head to my machine and occasionally swap the spindle motor out with the 3D printer head. In 3D printing it is also important to have high speed, perhaps more than in CNC machining.

[A_Camera]

Air, would be my guess. If your part has a lot of rapid moves (like text engraving), having fast screws helps a lot.

Exactly. On the other hand, high acceleration is probably more important during cutting than high speed. If you have a lot of short moves the machine is never up at top speed, so acceleration gives more benefit than top speed. That's also a reason why I backed off from 10,000mm/min to 8,000mm/min. No point in higher speed on my small machine unless I can considerably increase acceleration also.