Acme Threaded RodUpgrading from All Thread to Acme threaded rodInformation regarding Acme components is addressed in the 25 x 25 Dual Leadscrew Plans and the 24 x 48 Rack/Leadscrew Plans..................
The threads of Acme rod have a profile that is squared as compared
to the more triangular threads of regular hardware store All Thread
rod.This square shape makes Acme more suitable for carrying loads. The surface of precision Acme is smoother, and it is more efficient than general purpose Acme. The image below shows one inch long sections of 3/8 inch precision Acme, general purpose Acme, and regular All Thread. Their thread count per inch is 10 with two starts on the precision, 12 on the general purpose Acme, and 16 on the All Thread.  The two start has two threads laid parallel with each other; think of a striped multicolor barber pole. With this configuration, a nut on the rod will move twice as far with each rod rotation as it would on a single start threaded rod. This permits slower stepper speeds to move the axis faster. The starts are also referred to as "Speed Ratio" by suppliers. A two start rod would move the axis twice as fast as a single start rod when turning at the same rpm, so it is designated as having a speed ratio of 2:1. ............
Acme is more efficient than All Thread, but it is still only 30 to
70% efficient; this depends on the threads per inch and type of
leadnut.The 3/8-10 two start rods with plastic nuts are near 60%. Ballscrews are near 90%, and belts are near 95% efficient. Acme rod will add from $50 to hundreds of dollars to the price of these machines. The price varies with the rod's size, the thread configuration, and the choice of leadnut. ............
Options tested on the rack/leadscrew and 25 x 25 machines include 5/16-14, 3/8-12 general purpose Acme, and 3/8-10 two start precision Acme, as well as 1/2-10 and 1/2-10 five start Acme. These two tables were designed to permit a variety of leadscrew sizes from 5/16 to 1/2 inch. They were also designed to permit simple upgrading from one rod size to another after construction is finished. This way inexpensive leadscrews can be used initially, and they can be upgraded later if/when desired. Speeds Generally, the steppers and drives used in these plans can be expected to perform well up to 500 rpm. Faster stepper speeds are possible, but it is better to under promise and over deliver. The turns-per-inch of the leadscrew is divided into this 500 rpm to give the speed of the machine. Therefore, a 20 turn per inch leadscrew can be expected to move the axis at 500/20 = 25 inches per minute. Similarly a 5 turn per inch leadscrew will be able to move the axis at 500/5 = 100 inches per minute. The fewer turns per inch of the leadscrew, the less above 500 rpm the stepper can be expected to turn. Therefore, a 2 tpi leadscrew is unlikely to move the machine 250 ipm, though it can reach 200 ipm. ............
Details about the upgrades follow.The 5/16-14 Acme rod was around $20 for 6 feet, and standard Acme nuts were available for $2.50 each. The bearings did not have to be changed since the rod is the same diameter as the hardware store All Thread. The table's speed about doubled; HobbyCNC's 305 oz.in. steppers could rapid at better than 60 ipm. ............
The 3/8-12 Acme rod could almost be treated as a drop-in substitution
when the
bearings were up-sized from skate bearings to 3/8 inch bore 7/8
inch OD (Outside Diameter) bearings. These bearings fit in the same
homemade bearing flanges as the skate
bearings. Right image: Same OD, different bore. The pulleys also require larger bores; they can be
purchased, or they can be bored-out with a drill to fit the larger
rod, or the rods can be turned down to fit the pulleys.The turned down section can be re-threaded for 5/16-18 nuts. Left image. Standard Acme hex nuts are available for the 3/8-12 rod, and they can be used as leadnuts and tensioning nuts. With this lower cost 3/8-12 inch rod, the results were nearly the same as the low cost 5/16-14 inch Acme. This follows since the thread count is similar. ............
The Acme 3/8-10 two
start precision rod was tested; it gives five turns per inch of axis
movement.This product, like the 3/8-12 Acme is almost a drop-in substitution, but again, the pulleys and bearings require larger bores. Standard hex nuts for multi-start rod are difficult to find. Collar clamps are available; they add cost but are easy to use. 
A crude but inexpensive option is re-threading one end of each rod
with a standard 3/8-16 die, and using standard All Thread nuts for the
tensioning nuts against the bearings. Left image.Only one end of each rod can be re-threaded; the leadnut will not fit over the altered threads for final assembly. Hose and hose clamps can be used as collar clamps on the opposite ends to hold the rods into the bearings; the tension is adjusted on the ends with the hex nuts. ............
 The anti-backlash leadnuts from dumpstercnc.com that were used were flanged;
part
of the larger flanges had to be trimmed for clearance. Right image.Rapids on the gantry axis with the 3/8-10 two start were in excess of 100 ipm with all of the steppers, from the HobbyCNC 200s to the Xylotex 425s. The 425s did very well as compared to their use with the 5/16-18 All Thread; rapids on the gantry were near 170 ipm. ............
Half inch diameter 10 tpi Acme leadscrews were also tested on a the
rack/leadscrew and 25 x 25 machines; speeds were comparable to smaller rods with the same thread count. Standard and anti-backlash Acme nuts are available for 1/2 inch leadscrews. Standard nuts have much more backlash than the nuts on the smaller rods; anti-backlash nuts will eliminate the play. The advantage of the larger Acme rod is its stiffness; this helps eliminate whipping, so it is useful on long axes. Note: 1/2-13 All Thread rods are quite rough and do not work well on the machines. ............
Information regarding Acme is included in the plans for the 25 x 25 and Rack/Leadscrew machines. |