using one motor to move the gantry , not 2 of them

[smeagle]

I saw mechmate cnc with 2 stepper motor on each side with 1:3 reduction to the pinion and the motors are of 4.9 Nm or 700 Ozin so I decided to usea a 12 Nm or 1700 Ozin motor to move the gantry put on one side and transmitet to the other side from the end of the gear of 1:2 or 1:2.5 reduction by an axel long like Y axis from a side to another and powerd by belts .... what do you think about this , it will reduce costs and the gantry will be powerd from both sides by only one powerfull motor ....

[bradm]

Welcome, Smeagle! You need to be careful that the shaft ("axel" = "axle") doesn't twist. If it does, your gantry will track differently moving forward than it does moving backwards, and you'll have many problems. Even if you somehow manage to mount the motor in the middle, you'll have a backlash accuracy problem when you reverse directions. It's surprisingly difficult to avoid this twist in a long shaft, and I think that's why you don't see this design around - it doesn't work.

See this pdf from an engineering class. (http://www.freestudy.co.uk/c103/t3.pdf)

[danilom]

When I was at start of my build many said to go that way.Use one motor they said, because many home made machines other than cnc are designed that way to lower the cost. I even seen some big cnc machines with two ballscrews on sides and one motor in middle turning them with two belts. But the cost of a motor and driver, simplicity of design with two motor gantry and roughness of larger stepper motors, their big inductance are the facts that prevailed not to step away from original plans.

Sometimes you must put every fact and price on paper just to see does some changes pay out, in this situation my opinion is that it doesn't. Just look at the price of belts and bearings and a 2 meter axle and time do design custom transmission and adapt it for mounting to gantry, also 1700 ozin motors can not run that fast as smaller ones (for the price of a 12Nm motor I think you could buy sometimes even 3 smaller ones ~600 ozin).

[smeagle]

I think there is an ideea to use 2 smaller motors let's say 1200 oz (8,5 Nm ) instead of one , I will make some recalculation and I think you are right ...

[smeagle]

and you say that a 3 Nm - 600 oz it will spindle faster ? then tje 1700 oz ? it will need a litle power , I'm afraid not to loose steps , and for the fastest moves I calculate about maximum 600 rot/min from the steppers . and the reduction 1:2 or 1:2.5 ....

[danilom]

I think that I have not seen anyone on this forum using stepper motors that have more than 700-800 ozin of power. Using a gear reduction like in Oriental motors 3.6:1 and 7.2:1 and belt reduction 3:1 or 1:4 you theoretically gain that times more torque but because of torque depends on speed you loose some after 300 or so RPM. Look here for motor choice

http://www.mechmate.com/forums/showthread.php?t=1114

[smeagle]

yes , my idee is not to use gerad motors , is to use only belt redction 1:2 maxim 1:2.5 and have enough power motor so i'll have torq and enough speed from the motor of 8,5 Nm

[MetalHead]

The direct drive motors 640oz will drive the system just fine. The reason everyone is using gear boxes is resolution not speed. If your doing 2D work and basic 3D work the direct drive 640oz motors are fine. You need the reduction 3:1 , 4:1 , 7.2:1 to get more detail in your cuts. Research this in the forum and you will see belt or gear reduction in about detail not power.

[smeagle]

yes you are right , all I need is to calculate and make about 0.1 mm per step and for wood milling and plasma cut is enought ...

[lumberjack_jeff]

If I were dead set on using only one stepper (which admittedly would save a nontrivial amount of money... driver, stepper, rack and pinion ) I would try out a parallel-beam cable apparatus.

http://www.cnczone.com/forums/diy-cnc_router_table_machines/51485-make_your_gantry_rock_solid-4.html

The cable would have to be very tight, so wear on the pulleys would probably be a maintenance consideration.

[smeagle]

thanks for the ideea , but know I decided to put to smaller motors each side ...

[chopper]

I know you decided on a two motor system but,...I think one thing might not have been thought of here is when you use larger motors you also have more rotating mass or inertia so when your machine changes directions it will have a longer ramp up and down time, this may not be a big deal to some but it will cause an accumulation of time that is lost in running the programs over time it will add up, also if you are going to use this for a plasma cutter it becomes more important, the longer raping times will change the cut quality which will be undesirable, I think the smaller the motor the better ( as long as it is of proper size to move the machine) on my machine I use 450 ozin motors with 4 to 1 belt drives which gives me aprox 1800 ozin of torque..since the reduction multiplies the torque which allows me fairly fast ramp up and down times and plenty of power and the smaller motors are cheaper so it ends up being an all around win win if you will...
//chopper