New cnc router build looming, advice needed

[Mad Professor]

Good day all.

At the end of 2010, I started designing and building my 1st CNC router, see here for post: http://www.mycncuk.com/forums/diy-ma...gn-advice.html
Back then I only wanted a small desktop unit for making PCB's and doing work with plasic.

But as always I was finding that I needed to do more and more stuff with it, so at the start of this year I upgraded the Z-Axis, see here for post: http://www.mycncuk.com/forums/diy-ma...gn-advice.html

And once again, I am finding that I now need to cut bigger parts, and need to cut aluminium.

So I am now looking at building a new DIY CNC Router.

Here is the basic design of what I am looking at doing.

***** X-Axis *****
2x Ballscrew RM1610-1000mm
2x SBR25-1000mm Rails
4x SBR25LUU Bearing Blocks
1x 752 Stepper Driver
1x M60STH88-3008DF Stepper Motor

***** Y-Axis *****
1x Ballscrew RM1605-600
2X SBR20-600mm Rails
4x SBR20UU Bearing Blocks
1x 752 Stepper Driver
1x M60STH88-3008DF Stepper Motor
2x 24 Tooth HTD5-15 Pulleys
1x 54 Tooth HTD5-15 Belt

***** Z-Axis *****
1x Ballscrew RM1605-
2x SBR20-400mm Rails
4x SBR20UU Bearing Blocks
1x 752 Stepper Driver
1x M60STH88-3008DF Stepper Motor
2x 24 Tooth HTD5-15 Pulleys
1x 59 Tooth HTD5-15 Belt

***** Extras *****
Ethernet SmoothStepper (Rev 1.0)
42Volt PowerSupply (Due to be upgraded)
2.2Kw Water Cooled Spindle & VFD

------------------
I am looking to make the main base frame from 80x80mm steel box section.

This is still very much work in progress, so any advice or input would be grate.

Thanks for your time.

Best Regards.

[Jonathan]

I'd put the spindle mounts closer together. This will get some adjustment of the spindle height, so if you need it a little further down you just slacken the mounts and slide it.

[irving2008]

raise the sides higher and move them apart so that the gantry sits between the x-rails and the x-rail bearings are mid-point to the y-axis. Then you'll have virtually no gantry sides and a much more rigid structure

[Mad Professor]

Thanks for your input so far.

I have a question regarding my X-Axis, I am looking to use two 1610 ballscrews with a 2:1 Ratio from the stepper motor, but I am undecided if I should use one stepper motor and three drive belts, or buy an extra stepper motor and driver, so that I have two motors and drivers for the X-Axis.

[irving2008]

For maximum performance you should go with 2 steppers. The inertia of two ballscrews and all the pulleys is significant and with one motor you could have missed step issues if your acceleration is set too high.. then again you have to weigh that up against the risks of the two motor solution getting out of sync.. In reality the deciding factor is ease of construction as a 3Nm motor is plenty for either option...


One motor option:
lets say you wanted acceleration of 1.5m/s^2 and a traverse speed of 6m/min = 0.1m/sec and your gantry weighs in at 25kg say, then

F = ma = 25 * 1.5 = 37.5N, torque to accelerate load = F * L/2pi where L is lead in metres = 37.5 *.01/2pi = 0.06Nm but its geared down 2:1 so torque at motor is .06*2 = .12Nm

Torque to accelerate screws/pulleys = J (inertia) * a (angular acceleration), where J is the sum of the inertia of the screws, the interia of the pulleys and losses such as friction, rotor inertia, etc.

Inertia of screw = D^4 * length * 770 (for steel screw ) = .016^4 * 1 * 770 = 5.0e-5kg m2
Inertia of pulley = D^4 * 3 (for 15mm aluminum pulley) = 1.56e-5kg m2 for 30T 5mm pulley and 9.7e-7kg m2 for 15T pulley
For that specified motor the rotor inertia is 840gcm2 = 8.4e-5kg m2
So for a drive system with 2 screws, a 30T pulley on the motor and 15T pulleys on the screws the total inertia = (2 * 5.0e-5 + 2 * 9.7e-7) * 4 + 1.56e-5 + 8.4e-5 = 5.0e-4, say 5.5e-4kg m2 adding in 10% losses etc. The * 4 is because the screws are geared 2:1 so the reflected inertia scales up by the square of the ratio

The angular acceleration = linear acceleration / (2pi * L) = 1.5/(2pi * .01) = 23.8rad/sec2

So torque to accelerate drive system = 23.8 * 5.5e-4 = .013Nm

Total torque required = .12 + .013 = .133, say 0.15Nm for safety.

At 6m/min the screws are spinnning at 600rpm, motor is spinning at 300rpm = 1000steps/sec

From the torque curve for that motor, torque at 1000 steps/sec = 1.5Nm so plenty of margin. Microstepping at 4:1 (torque at 38%) or 5:1 is probably lowest safe level, but 8:1 might be OK.

Doing the same for the 2 motor option:
Force to accelerate load = 37.5N as before but shared between 2 motors = 18.75N each, so torque = .03Nm and since this is direct drive thats the motor torque needed

Torque to accelerate screws/pulleys = J (inertia) * a (angular acceleration), where J is the sum of the inertia of the screws, the interia of the pulleys and losses such as friction, rotor inertia, etc.

Inertia of screw = D^4 * length * 770 (for steel screw ) = .016^4 * 1 * 770 = 5.0e-5kg m2 as before but now this is the only inertia to consider as there are no pulleys etc

Total inertia = 5.0e-5 + 8.4e-5 = 1.34e-4kg m2

So torque to accelerate drive system = 23.8 * 1.34e-4 = .0032Nm

Total torque required = .03 + .003 = .033, say 0.05Nm for safety.

At 6m/min the screws are spinnning at 600rpm, motor is spinning at 600rpm = 2000steps/sec

From the torque curve for that motor, torque at 2000 steps/sec = 1.4Nm so plenty of margin. Microstepping at 8:1 (torque at 20%) or 10:1 is probably lowest safe level, but 16:1 might be OK.

Conclusion
In other words on anything around the 1 - 2m axis size the only real decider between 1 motor or 2 is ease of construction. I would also say that the larger motor with 840g cm2 rotor inertia is the biggest inertial component and a smaller 2Nm motor would probably perform as well if not better though there might be limitations on the level of usable microstepping.

For larger sizes (length, ballscrew dia) the 2 motor option may be better BUT on longer screw lengths whip becomes an issue so there is a different trade off.

[JAZZCNC]

Ok has requested.

First 60mm box section will be more than enough so I'd save some money here. I'd also simplify material list and use it for the bed supports.

The gantry looks fine to me and wouldn't take Irvings suggestion of lifting/widening putting the gantry between the rails has the alignment and adjustment would be a night mare. Plus the bearings would be under less than Ideal load on there sides.

Would take Jons advice and move the spindle mounts.

The single motor or twin is mostly a personal thing and dependent on other factors like what your cutting and speeds required.
Has you know Personally I prefer the single and belt setup has I don't like having the gantry run out of sync causing racking potential.
Missing steps from high acceleration hisn't an issue because often to enable the high acceleration you have to sacrifice velocity, put it this way I don't have any and my gantrys quite heavy.
Infact missing steps becomes more of an issue with slaved motors because often folks try to run too fast and the margin between doing so and not is close. Plus motors can sneakly drop steps here and there at higher speeds has the torque drops away resulting in slowly running out of position one one side or the other. On short to medium jobs this can go unnoticed and ounce each screw is separately homed (if correctly aligned) the error gets cancelled out. . . But on very long jobs it does show in positional error and if too bad can even lead to stalling motors has the error builds resulting in racking the gantry.!! . . Theres nothing more annoying than stalling 10hrs into a 11hr job.!!

Obviously if motors are tuned at a safe level then it's not an issue at all. With single motor/belt setup positional error will still happen if over tuned but the racking and stalling won't so the job will finish fine.!. . . It'll just be wrong in some way.!!!
The single motor will also allow slightly higher feeds has you can tune it closer to the limit and doesn't need quite has big a safety margin.!

Like I say it's personal thing but both work fine if setup correctly it's just the single setup tolerates more error without doing damage.!

I notice on the Spec you have 1605 on Y but 1610 on X.? Whys that do you plan to gear 1610 2:1 to bring both into similair speed ranges.? Thou it doesn't matter each Axis working at different speeds Personally I like to keep the X & Y axis balanced speed wise then one hisn't slowing the other down or limiting the potential feed rates. ( IE you have Y capable of 5mtr/min and X 10mtr/min then set a cutting feed rate of 6mtr/min the X Axis and feed rate will be restricted to the speed of the slowest axis when doing combined moves, this will increase cycle times quite a bit.)