# Thread: need help! choosing right kit

1. Gantry image

2. Originally Posted by msacnc
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.

3. 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.

4. 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.
Last edited by JAZZCNC; 16-09-2014 at 04:27 PM.

i read many many threads over 2 year from cnczone and other sites and i build a prototype of cnc using wood and it work fine for first build and i build the second one (cheap) since 1 years 2000m*1000 using wood and home made component such as linear rails and screws steppers from old big printer, home made power supply and so on but the machine is Vibrating like crazy when i test X gantry (rigidity problem)finally i stopped.

the big problem is in my county is the difficulty to buy cnc component from uk,usa and china (very very costly...shipping) , and also there is no Aluminum profiles here just thin Aluminum ,wood and steel.
someone bring some standard cnc components from china (like stepper kits and china gear i mentioned above)To sell them then i think to build a big steel cnc (better than wood) using these components(there is no alternatives to choose) he bring a few types of linear rails, ball screws, R&P and electronics and the problem is "i need to choose quickly before he sell them" 3 or 4 from each type no other chance (this is why i not yet begin to construct machine bed structure and gantry)

he bring :
(ROUND RAIL) all size and nuts
(BALL SCREW) 1605,1610,2505,2510,2525 (2m length)
RACK 20*20 2m length
PINION 28 TEETH
HELICAL RACK 30*30 S45c 1.5m length
HELICAL PINION 24 TEETH
gear rack 1.25mod 1:5
power supply 48V and 60V
steppers
JK57HS82-3004-02 (NM 2.2 - 311 oZ 3A)
JK57HS112-3004 (NM 3 - 425 oZ 3A 6.8mh)
JK86HS78-5504 (NM 4.6 - 651 oZ 5.6A 3.4mh)
JK86HS115-4208 (NM 8.7- 1232 oZ 6A)
chinese spindle 2.2kw air
chinese spindle 3kw water

i have already 4 driver microstep 80V 7.8A

6. i need help to select from these components

i appropriate you if you give me link of any steel cnc log

if with these components no chance to build big cnc suggest max dimension i can buid

i go to R&P because i read its suitable for large machines

i mainly use this machine for woodwoking

i need high accuracy finishing parts 2D 2.5D

7. Well you really have got a problem with this selection because the motors and PSU voltage don't match together well.

R&P needs the JK86HS78-5504 (NM 4.6 - 651 oZ 5.6A 3.4mh) or the JK86HS115-4208 (NM 8.7- 1232 oZ 6A) and with only 60Vdc you don't have enough voltage to get good performance from these.

The JK57HS112-3004 (NM 3 - 425 oZ 3A 6.8mh) are not suitable for R&P and are better suited to 1610 ballscrews but again with only 60V and high inductance the performance will be lower. They really need 70+Vdc to get good performance.

My advise would be don't buy any of them has they are not matched nice but if I must choose then I'd say go with 3Nm and 1610 and build to this size.

If you can push the Voltage upto 70-74Vdc and use toroidal transformer PSU then you could get away with JK86HS78-5504 (NM 4.6 - 651 oZ 5.6A 3.4mh) and 28T R&P with 5:1 ratio.
This would give pitch equal to 22mm (Mod1.25 28T DP = 35 x pi / 5=22 ) which would give decent resolution for wood and good speed so you could tune motors down so they run slower where torque is higher and still maintain good feed rates.

Really the only motors you can use are the JK57HS112-3004 (NM 3 - 425 oZ 3A 6.8mh) and JK86HS78-5504 (NM 4.6 - 651 oZ 5.6A 3.4mh) and both these need more voltage, esp the 4Nm 34's.
The JK57HS112-3004 (NM 3 - 425 oZ 3A 6.8mh) with 70Vdc would perform best.
Last edited by JAZZCNC; 16-09-2014 at 10:44 PM.

8. Hi there,

In My opinion the best way to estimate weight is the following:

First 1. Use Sketchup to build your model.
Step 2. Any components in your build must be converted into groups or components otherwise you are f****d in sketchup.

So for instance if you make a 100x100x3x 1500 solid in sketchup turn it into a group or component as soon as is made.

Step 3. if you right click a group or component and go into entity info depending on your unit setup (which should be mm for this type of work) you will see the exact cubic mm of the component.

Step 4. Go here: http://www.aqua-calc.com/calculate/volume-to-weight

If you dont see the cubic mm option click show more units and will be there.

Search for steel or aluminum or whatever and calculate.

Have a good night.

Vass
Last edited by HipoPapi; 16-09-2014 at 11:16 PM.

9. thank you very much JAZZCNC for your help

i think i will go to ballscrew as you suggest
one 2510 for X (2m long) but what about wrapping..
one 1610 for Y (1.2 m)
one 1605 for Z (0.5 m)

according to eq. from cncrouterpart suitable V = sqrt(mh)*32 which mean that Nema34 with 3.4mh required 59v for best performance which is close to 60v PS is this correct

i know that Nema23 425oz is better specially for speed but as you mentioned it requires at least 72V which is not available

can i use central 2525 ball screw with Nema34 for X and Y axis

10. Originally Posted by msacnc

can i use central 2525 ball screw with Nema34 for X and Y axis
Dont do that. Especially at that size gantry and that Z travel.

At 300mm z travel you have to make sure when you design it, that the long ball screws are more or less on the height where the cut happens. One design consideration most people overlook.
That is good for you because you will have maximum actual travel 2000mm long - ball nut legth

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