need help! choosing right kit

[irving2008]

i do not determine certain speed but the faster choice is better for me

rack inf:
20 mm wide and 20 mm thick 1500 mm long Module 1.5 Pressure Angle:20°

commonly the machine is used for woodworking

I plan to reduce the gantry weight using one steel box 100*100*3 welded to two pieces of 150*100*3 gantry side



"this probably won't be accurate enough for machining aluminum" Why?

mod1.5 = 1.5mm dia per tooth so with 28 tooth pinion 1 rev = 28*1.5*pi= 131mm and direct drive gives 131/200 = 0.66mm resolution per full step

for reasonable speed on a rig of that size 4000mm/min would be a good minimum with 400mm/s^2 acceleration
so (to make math easier) 3930mm/min = 30rpm of pinion so gearing 2:1 = 60rpm, 3:1=90rpm, 5:1=150rpm with resolution of .33mm, .22mm and .132mm/step respectively

Assuming a 80kg gantry and ignoring minimal friction in linear slides, the force needed to get 400mm/s^2 (0.4ms^2) acceleration is F =ma = 80 * 0.4 = 32N, which gives the torque on a 28tooth, M1.5 gear is (radius =62.5mm) = 32*.0625 = 2Nm, geared 2:1 =4Nm at motor, 3:1=6Nm, 5:1=10Nm. This ignores losses so add 10-15% for safety.

[Boyan Silyavski]

This gantry will bend and twist like crazy. Aluminum is out of question if gantry not reinforced. Irving told you also about the rack and pinion and aluminum.

The difference between mediocre machine and a good one is not $$$ but the amount of time you do a proper research and later make right decisions.

Better read some build logs here and things will begin to clear. Or you will regret later. I don't see how rack and pinion will be much cheaper than ball screw at the current China prices. And the rotating nut assembly is not so much different than the RP assembly money wise.


No offence but most of all - Z300mm, seriously? On a first machine, big machine and making decisions about motors without making a detailed drawing not having clear idea about what material you will like to cut mainly.

So why don't you just take a breath, read the forums, make your drawings, show them here, discuss them, correct them, etc. Start from the bed structure, gantry, Z, rails etc.

[JAZZCNC]

Listen to Silyavski he's given you priceless advice about not Rushing and taking time to choose correct components wisely.!!

If you are going with R&P then forget nema23 and Also forget running Nema34 motors no matter the size with only 60Vdc.
Nema34 motors require much more voltage to get any decent speed from them. This is why they are often run with high power AC drives. Even 75Vdc is on the lower side.

You need to do some careful thinking regards ratios and components you'll use and the feeds and resolution you'll need for the type of work your wanting to do because this relation ship goes all the way thru the component selection and is very easy to get wrong when using R & P.

In general you need an excess of power to compensate for the inefficiency of R & P so if you get the selection wrong and don't have enough spare then it means running the machine at much reduced rate to compensate. This is often why people building with R&P are dissapointed and more often than not this is because of poor Ratio/Motor/Drive/PSU selection.
With 80KG + You NEED at least nema34 motors around 8nm and these need to be run with High power drives if you want the best from them.

Guessing and rushing not doing your home work will result in poor performance and wasted money I guarantee you this.!!

Such a large machine for first time project is a recipe for wasting money and dissapointment so LISTEN CAREFULLY to what is being advised and SLOW DOWN.

[irving2008]

That's because vendor wants you to buy the PSU he has on his site, the recommendation is meaningless as he doesn't know intended use.

Single ballscrew on that width isn't going to work, the gantry will rack and bind if you cut towards the edges.

Inertia of 25mm screw is too high for a 2.8Nm motor, you'll be severely constrained on feed rates as you won't have the acceleration to change direction quick enough. Unless you go with fixed screws, driven ballnuts.

You may well have made a usable machine before but from your description this is in a different league and needs much more thought and design. We appreciate your supply difficulties but you're wasting your money by rushing to buy the wrong kit.