Acme Threaded Rod
| Upgrading from
Allthread to Acme threaded rod The smaller leadscrew tables in the 5 Machine plans were designed to use inexpensive off-the-shelf materials. The only exceptions are the belt and pulleys on the dual leadscrew tables; this was done because their use dramatically improves performance at a relatively low cost. Another upgrade, that is addressed on this page but is not part of the 5 Machine plans, is the use of varying sizes of Acme rod for the leadscrews. Information regarding Acme components is addressed in the 24 x 48 Rack and Pinion/ Leadscrew Plans. .................
The threads of Acme rod have a profile that is squared as compared to the more triangular threads of regular hardware store Allthread 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 inch long sections of 3/8 inch precision Acme, general purpose Acme, and regular Allthread. Their thread count per inch is 10 with two starts on the precision, 12 on the general purpose Acme, and 16 on the Allthread. |
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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 an inch for five rotations of the rod, rather than in ten rotations as it would were the rod single start like regular threaded rod. This five rod-turns per inch allows lower stepper speeds to move the table faster. Acme is more efficient than Allthread but it is still only 30 to 70% efficient; this depends on the threads per inch and type of nut. 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 rods’ size, the thread configuration, and the choice of leadnut. Three of the least expensive options, that are as close as possible to drop-in substitutions for the Allthread used in the plans, were tested on the 25 x 37 leadscrew table. Also tested was general purpose 1/2-10 Acme on a prototype that is similar to the 25 x 37 table. Two of the Acme sizes were 5/16-14 and 3/8-12, both of which are sold as general purpose rod rather than precision rod. Their surfaces are not as smooth and they are likely to arrive bent since they are not considered to be of high quality by the supplier. The final option tested was 3/8-10 two start precision rod. Unfortunately this also arrived somewhat bent, but it was obviously a superior product. .............................
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| The 5/16-14 Acme rod was
around $20
for 6 feet and standard Acme nuts were
available for $2.50 each. The nuts can be used similarly to the ones in the plans as leadnuts, and their backlash is comparable to the Allthread version. The table's speed about doubled; HobbyCNC's 305 oz.in. steppers could rapid at better than 60 ipm. The price for this upgrade was around $90 dollars; it included three six foot rods and a number of nuts to tension the rods in the skate bearings as well as to serve as leadnuts. No other components had to be changed since the rod is the same diameter as that specified in the plans. Again, the rod arrived bent and obviously was not considered a precision product by the supplier. The bends were almost impossible to remove, and they caused whipping which hurt performance. The suppliers do not sell precision leadnuts for this product, which indicates its quality. |
The low cost 3/8-12 Acme was
similar but arrived so bent that it was
difficult to judge its capabilities. The 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 are only a dollar more each, and no dimensions in the plans had to be altered since the bearings fit in the same homemade bearing blocks as the skate bearings. Right image: Same OD, different bore.  The pulleys require a larger bore; they can be purchased,
or they can
be bored-out with a drill to fit the larger rod, or the
rod 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 this rod as well, and they can be used as leadnuts and leadscrew tensioning nuts. This size of rod is available in varying grades and includes a precision version. Precision leadnuts are also available for this sized product. The upgrade price for 3/8-12 ranges from around $100 to over $200 depending on the quality of rods and leadnuts used. Only the cheapest version was tested because the thread count was still relatively high and 3/8-10 two start was going to be tested. With this lower cost 3/8 rod, the results were about the same as the low cost 5/16 Acme. There was improvement in the machine's performance as compared to Allthread but the 3/8 inch rod was heavier and the whipping, because of the bends, was more severe. Overall this upgrade was a disappointment. .........................
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| Half inch diameter 10 tpi Acme
leadscrews were also tested on a prototype that is similar to the
25 x 37
machine; their use
on the 25 x 37 would require reworking the dimensions in the plans
because the bearings have
to be enlarged to 1-1/8 inch OD. Also some clearances have to be
altered. The attachment to the stepper requires a rework as well; a pulley-belt rather than coupler system was used here. Pulleys with ½ inch bore are available so turning the ends of the rods for this upgrade is not mandatory. Stepper to leadscrew couplers for larger leadscrews are also available; they cost around $8 each from dumpstercnc.com and work well. Standard and precision Acme nuts are available for the product. Standard nuts have much more backlash than the original plan’s 5/16 inch Allthread rods and nuts. The anti-backlash nuts from DumpsterCNC initially caused significant drag but did eliminate the backlash. Rapids on the Y axis were 75 ipm with the standard nut. Over time the precision nuts wore-in and rapids approached those of the standard nuts; accuracy remained tight. The advantage of the larger Acme is its stiffness, which helps eliminate whipping, so it is useful on long axes. ........................
Note: Half inch 13 tpi Allthread rods are quite rough and have not worked well on the stepper tables. The product is used for the Y axis on this shop's 4 x 8 servo table and it works okay, but that table is not used for precision work. .......................
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The final Acme rod tested on the 25 x 37 machine was 3/8-10 two start precision rod, which gives five turns per inch of axis movement. It was hoped that this lower number of turns per inch would take better advantage of all of the different steppers' low speed torque, and it did perform well. This product arrived slightly bent but it was possible to remove most of the bowing. This too, like the 3/8-12 Acme was almost a drop-in substitution, but again, the pulleys and bearings had to have larger bores.  A
catch was, I could find no standard hex nuts for this rod; only
expensive precision nuts seemed to be available. This was resolved by re-threading one end of each rod with a 3/8-16 die and using standard Allthread nuts for the tensioning nuts against the bearings. Left image. This crude but effective cobble meant that neither a lathe nor special Acme clamps were needed for the upgrade. |
| 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 were used on the opposite ends to hold the rods into the bearings; the tension was adjusted on the ends with the hex nuts.  The precision leadnuts that were used were flanged;
their use required
reworking
the plan's leadscrew mounting system.Part of the flanges had to be removed for clearance on the X axis. Right image. No attempt was made to replace the Z axis' leadscrew with a precision version since the clearance is so tight on the Z axis, but others have done it as shown on the Gallery page. Rapids on the Y axis 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 Allthread; rapids on Y were near 180 ipm. The X axis was slower, but speeds were still well above the other Acme and Allthread. This upgrade, though adding around $250, did improve the machine's speed significantly. .....................
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. A 20 turn per inch leadscrew-stepper setup may be able to approach 1000 rpm. However, a 2 turn per inch leadscrew-stepper setup will be pushing its limits at 500 rpm. Again, based on the many other prototypes tested in this shop, 500 rpm has proven to be a safe general estimate for the stepper’s overall top speed, but there are many variables that can improve or diminish performance. .....................
Note: Precision Acme rods of 5/16 inch with precision leadnuts are available, but they are over $100 per six foot length. The two start 3/8-10 Acme is less than half that price and performs well. Parts were from mcmaster.com and dumpstercnc.com. Shipping overseas with mcmaster has been a problem; mscdirect.com also sells the components but their prices are higher. Use-enco.com sells 1/2-10 standard Acme at a low price. ....................
Acme rod will improve performance but it can approximately double the price of the mechanical components for the 25 x 37 machine. Again, the 5 Machine plans do not address the upgrading to Acme rod because cost was the primary consideration when the plans and prototypes were developed. Information regarding Acme is included in the Rack and Pinion/Leadscrew plan book. |