building MK2 with rack and pinion

[JAZZCNC]

Wmh Transmissions are good for R&P http://www.wmh-trans.co.uk/

Ratio will 100% be needed but how much will depend on pinion sizes and motor speed etc. Typical minimum is 3:1 ratio but not uncommon for 5:1 or higher if large pinion is used. To decide on the ratio required you'll first need to determine what kind of resolution and performance your looking to achieve.
This will guide you to what Size pinion and ratio will be required. Also playing big part in the equation is motor speed and torque.

Along with this you have to decide on how you achive the ratio. Gearboxs or Pulleys.? Both have there advantages and disadvantages.!
Gearboxs are good because they easy to impiment and very clean setup, thou they can be expensive. They give high torque and are very strong robust units with low maintenance requirements and long life.
Some disadvantages are that they can have higher backlash than pulleys and are less efficient so require larger motor torque for same speed has pulleys would allow using smaller motor torque.
They can have other negative affects on the system in that because they are direct mount the resonance which can affect motors is higher so your drives need to be good ones which can deal with higher resonances on the system.

Pulleys are good because they are cheaper and relatively high efficiency lowering motor requirements compared to GB. They lessen resonance on the motors and tend to run smoother than GB.
Disadvantages are that it's difficult to achive high ratio because pulley sizes quickly get large. They are also more difficult to impiment taking more work to achieve and setup. They also need protecting from debris etc and require more maintenance than GB.

Regards the ratio, pinion and motor speed/torque and resolution required then it's critical they are matched carefully.
Large pinion means low resolution which means you'll need higher ratio to bring resolution back up. Higher ratio means higher motor RPM required to reach required speed.
Higher speed if using steppers means lower torque because torque drops as Rpm rise.

It's a careful balancing act between speed vs Motor torque and resolution. Ideally you want to be aiming to have the ratio/pinon size setup so that motor RPM falls in a safe zone of the motors Speed/torque curve while cutting at the upper end of what you think you'll be needing.
Rapid speed isn't so important and if you aim to size for cutting there will be some left.

The next problem with R&P is the fact it's so inefficient compared to ballscrews it's much harder on steppers so you'll need larger steppers. This brings it's own set of problems.?
Large steppers tend to be slower than smaller steppers so motor RPM affects ratios and pinion size etc. Worse than this is the fact to get any decent speeds from large steppers means using high power drives which cost lot more money.!

So for this reason often you'll see Servos used on R&P machines. These bring there own set of conditons and expenses into play which I won't get into now.

But hopefully you'll be seeing that R&P needs careful attention and it's much easier to get wrong than using ballscrews.