reduction vs direct connection

[miljnor]

I've noticed reading through the forums that some CNC mill/drills are connected directly to the ballscrew and some use gear reduction. Normally i would assum this is do to the motors torque/rpm is greater or lesser on some of these machines. but the motors in some of the pictures look to be identical from machine to machine..

so my question is what advantages do you gain either way?

I am currently doing my first project as a mill/drill for Machining alluminum. will be using an old rutland/airgas mill/drill in good condition converting to square coulumn (like motimos?? awsome job by the way!)

I am making my mechanical drawings currently and reduction vs non makes a huge diffence in the bracketry.

[Al_The_Man]

There is an inertia matching factor between load and motor and in order to calculate this you have to know the machine mass and rate of acceleration you need. If you go with a direct drive, obviously a larger motor and drive is required, this utilizes the maximum feed rate that can be obtained with this motor. If you go with reduction, a smaller amp and motor can be used, the trade off is that max feed is reduced and inherently backlash could possibly be introduced. This is a couple of things to consider.
Al

[WoodSnarfer]

One reason could be found in the lead screw. For example, an 8 TPI screw will transmit twice the torque (but half the speed) of a 4 TPI screw.

Also, just becuase the motors look the same, the armature windings could be different (different torque sensitivity), and/or they could be run at different voltages.

Steppers tend to be direct connected, since they produce more torque at the lowest RPMs possible. Servos have a pretty flat torque curve over their operating range, but most seem to have a 'sweet spot' between 2500 and 3000 RPM (thus usually requiring gearing to keep the lead screw RPMs at a safe & reasonable level).

Maybe the best bet is to figure out your force requirements at the cutting table, and work backwards...this should give you a spec by which you can compare steppers vs. servos (and what type of power transmission you should use).

[Torsten]

Comparing your options

The good things about direct drive:
1) No Power loses due to transmissions.
High efficency of performance.
2) Mountings are cheaper to make build buy.

The bad:
1) Torque curve of motor has to match the machine requirements.
Expensive to make corrections if gear ratios need to be changed.
May require different motors drivers or Leadscrews to alter.
2) position of motor can not be changed it may be above table hight.
3) Handwheels may be harder to mount if needed.

The good things about Pulleys:
1) Easy and cheap to make corrections to ratios by altering pulleys,
to fine tune Torque curve of motors to the application.
This part is most important for a one of a kind type of machine with
many unknown propertys.
2) Alignment problems can be solved by pulleys.
3) Freedom of component Positioning including hand wheels.
4) Damping properties reduce shatter vibrations.

The bad:
1) Mounting and Pulleys will be more expensive to build / buy.
2) A slight loss of Power due to transmition.
3) Small Additional source of backlash and flex.

Good Luck

[miljnor]

working out the force and revers engineering sounds good, but , its been a long time since I did any real math (computers ) any formulas you could suggest.. like a 3/8 carbide endmill cutting aluminum? Don't know the feeds you can get on a table like these but at work we run the 3/8 EM flat out (10000rpm or 7500rpm depending on machine) at around 45 to 55 Ipm. Granted this little machin probably won't be able to take that but I don't know since its my first project.

Also a sorce for pully's would be great.

[WoodSnarfer]

Arvid wrote a nice paper describing this...look here:

http://www.cnczone.com/forums/showthread.php?t=8129

For pulleys:

https://sdp-si.com/index.asp

I'm not a machinist, but I would guess that you don't need much force on a tool like that (if your cut depth isn't insane)...so I'd be looking at direct-coupled steppers.

[miljnor]

thanks that helps now I can calculate the HP required for the cutters I want and do the math myself...

Does anyone have a starting point for me, and I have decided to go the more pricey rout and buy servos for the closed loop accuracy..

by starting point i mean what they ve used successfully and their mill drills for metal cutting