Lead screw or rack and pinion

[Sproj]

Hi

I am completely new to CNC machines. It has been a while now that I have gained interest in CNC machines and am now on the point where I want to buy one. Should I get a lead screw or rack and pinion machine?

The two machines I have in mind are almost priced the same and both have a table size of around 3 feet (1000mm). The lead screw machine has a working area of about 27.5" (730mm) on X and 31.8" on Y (810mm). Rack and pinion is about about 31.5" (800mm) (also X and Y) -- both machines' Z-axis is lead screw.

This will initially be for my woodworking hobby, but I am planning to turn my hobby into a small scale commercial workshop, so I need accuracy as well.

1. Will I experience screw whip over roughly 3 feet? From what I read is that it is 'safe' to use screws up to 4 feet?
2. Is there a noticeable difference in accuracy between these two?
3. What is the best size spindle to use in these machines? The lead screw has an option for a 1.5kw or 2.2kw water cooled spindle and rack and pinion comes with a 1.5kw air cooled spindle.
4. Will the speed of rack and pinion be useful with the spindle options?

It may be worth mentioning that the wood thickness that will be cut most of the times will most likely be around 1/2" or 3/4" (12 - 19mm) and 99% either plywood or pine (my first project in mind will be 21mm pine plywood - useless info, maybe?).

I will appreciate your input - even if it is directing me to another article that explains the above in a way that a newbie can understand.

Thanks!

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[ger21]

1) depends on the screw diameter, and RPM it'll be used at. My machine has 1/2" diameter screws 60" long. They start to whip around 500-600rpm, if the machine is at either end.
2) Both come in different grades of accuracy. High precision versions of either are very expensive. Screw driven machine will tend to have higher resolution, but that does not affect accuracy. As long as the machine is repeatable, you won't notice a difference.
3) I'd recommend a 2.2KV with an ER20 collet size. 220V versions are much better than 110V versions.
4) No matter how fast the machine is, you'll wish it was faster.

[Sproj]

Hi Gerry

Thanks for the response.

The screws are TR8x8(P2) ACME Lead Screw (around 0.3" [8mm]) and rack & pinion has MOD1 (copied from the website - not that I know what it means). Accuracy on both machines is around 0.0039". On the rack and pinion they state it with a no load %.

If I have to make my assumptions here, I would say that I need the rack & pinion frame with the spindle available on the lead screw machine (R&P only has 1.5kW air cooled spindles - well the option that is available to me).