CNC Linear Drive Mechanics

A linear drive is the mechanics required to transfer from rotation of the motor to  straight line motion, as used in all the slides of a CNC Router, there are many choices, screw, rack and pinion, timing belt,

Lead Screw:

A lead screw is basically a long screw made with a continuous thread. In a CNC machine this lead screw is attached to a motor, either directly by a coupling or by timing belt with reduction. And the drive nut, which is attached to the table or slide as the fixed motor spins the screw, will drive the table to to specified position, it is important to have minimum clearance between screw and nut, excessive clearance will result in chatter and inaccurate cutting.

Resolution / Accuracy:

Whether you select servo or stepper motors, they will both have a number or count associated to one revolution of the motor shaft - this number or count is called resolution. For example, a stepper motor with 1.8 degrees, with no Micro stepping, will have to step or index 200 times per turn. The resolution can be increased, by using a Micro stepping controller, this will allow one single step to be increased by 8, or 1600 times per turn. However, with a servo motor resolution is determined by the encoder. An alternative way to increase Resolution is to select a finer pitch lead screw; however, with the increased finer pitch, a slower maximum speed will incur.

Lead Screw Diameters:

The lead screw diameter is measured from the tip of the threads - Example: 3/8-16 this thread diameter would equal 3/8"

Lead Screw Pitch:

Pitch is used to determine the thread spacing of the lead screw and matching nut. Pitch is the distance the nut travels along the screw when the drive nut is turned. One turn example: 1/4-20 is a 1/4" in diameter and has a pitch of .05", if the nut turns 20 times it will move one inch 20 x 0.05" =1.0".

Lead Screw and Nut Options:

In low cost lead screw applications, a drive nut will often be made from plastic or bronze; making the drive nut from these materials will give a low friction solution, at the same time no lubrication is needed.

UN Thread:

A UN thread (Unified Thread) is the standard thread shape that is used on general nuts and bolts that you find on most items attached with nuts and bolt.

Acme Screws:

Acme screw threads can be made much more coarse than UN threads. They are square cut threads and can hold much more load than a v style or UN thread; they are common components that are generally found in industrial machines, they are readily available.

Ball Screws:

A Ball Screw uses ball bearings in a recalculating housing. Ball screws use hardened ball bearings and the screw is hardened steel, and either come in a lower cost, rolled screw or precision ground screw. Most high- end milling machines will use ball screws - they have high efficiency and can be very accurate. They will, however, be the most costly to purchase, and, as there is no ball retainers in the drive nuts, they will have to be handled and assembled with care.

Multiple Start Threads:

Most general threads will be a single start, multiple start threads are used to increase the linear travel distance, per one turn,
and at the same time,  using multiple starts will also increase the thread contact area, you can easily identify the amount of starts
by looking at the end of the screw, a single start will have one entry point, a two start, will have two, and so on, manufacturers of screws
can use many starts, but in general 2 to 5 starts are more common in CNC machines, for example a 1/2-10 single start
will move a slide 0.100" per one turn, a 1/2-10  two start will move a slide 0.200" per 1 turn, you also must match the nut to the
screw, for example a 1/2-10 two start screw must be matched to a 1/2-10 two start nut.

Allthread:

Allthread screws or threaded rod will come fully threaded and are threaded with UN threads. This is the standard thread found on most machine screws and fasteners. Allthread is very common and can be purchased at most hardware stores and can be used as a very low cost, lead screw, the only issues with all-thread is inconsistant thread pitch and it only is normally available in fine threads

Anti-backlash Nut:

An anti-backlash nut will have zero clearance between the screw and nut, it is manufactured to allow preload to be applied to the screw. An anti-backlash nut is very useful in a CNC machine, as it allows very precise transmission from a rotating screw to linear motion. When cutting in any machine, holding the work as rigid as possible will make for smoother, chatter-free cutting. Anti-backlash nuts can be purchased in UN, Acme and Ball screws.

Supported Lead Screws:

In longer length slides screw, end supporting is important. A supported screw will have a bearing mounted to the lead screw on the opposite end of the motor, this bearing support will help eliminate a condition called lead screw "whip"; at higher, lead-screw speeds an un-supported screw will become unbalanced. To mount a bearing correctly, the threads will have to be machined off to a clean diameter, in order to fit the inside of the bearing; and the bearing will have to be mounted to the machine

Rack and Pinion

In longer length cnc machines, a rack and pinion  can work well, one drawback with a R/P is that carefull alignment is more time consuming and some reduction from the motor to pinion is usually needed for closer machine cutting resolution, a second main drawback is backlash, a properly designed anti-backlash pinion, works well, I see alot of incorrect setups,

where the pinion is preloaded into the rack,

Timing Belt

In longer length cnc machines timing belt drives are great, main advantage easy alignment as with R/P some reduction from the motor to the drive pulley is usually needed for closer machine cutting resolution, a second main advantage is backlash, there is no backlash in a timing belt pulley to belt,  timing belt Drive

Linear Motor

Absolutely primo best possible drive, found in most high end CNC Machine centers, one minor drawback they are extremely expensive,

A roller chain is made up of solid links, a solid link will never enter and exit a sprocket smoothly, without some slack in the chain, and if you tighten the chain , you get cogging, so you get one of two things, extreme backlash or cogging, both will reflect directly into the cut part, a no win situation

see movie

Here's an animation to show this cogging effect
see http://www.rockcliffmachine.com/sprocket.swf.html

Here's a builder that appears to be experiancing this effect, he increased to a 20T which was smart to do, as it minimizes the cogging, but unfortunately still not good
http://www.cnczone.com/forums/diy-cn...ish_bumpy.html

Wire / Cable Drive   

A wire drive can be used succesfully, if engineered properly, some pen plotters and printers used wire, good for lite load applications