Acme Threaded Rod


Upgrading from All Thread to Acme Threaded Rod


The machines can use a variety of threaded rods for the leadscrews.
Acme components are addressed in the 10x9, 18x24, 25x25 and the 24x48 plans.
Acme is not addressed in the 13x13 plans because the machine was designed to be as inexpensive as possible. It uses hardware store All Thread.
 
All Thread and Acme Rod
Threaded rods used for leadscrews.
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 more efficient than general purpose Acme.
 
The above image shows one inch long sections of 3/8 inch precision Acme, general purpose Acme, and All Thread.

The thread count per inch of the above rods is 10 with two starts on the precision, 12 on the general purpose Acme, and 16 on the All Thread.
All are right hand thread, which is usually preferred because of price and availability.

Note that there is no need for left hand thread on the Solsylva machines.
 

Starts

The two start Acme rod has two threads laid parallel with each other. This is similar to a striped multi-color barber pole or candy cane.

Two start leadscrew
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.

Note that steppers lose power at higher speeds; therefore, leadscrews that permit the steppers to turn slower can give the axes better speed and power.

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.

See also the Axis Tuning page.
 
Acme is more efficient than All Thread, but it is still only 30 to 70% efficient. This depends on the threads per inch and the type of leadnut.
The 3/8-10 two start rods with plastic nuts are ~60%.
Ballscrews are ~90%, and belts are ~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.

The 18x24 was designed to use 3/8 inch two turn per inch Acme or similar, but not All Thread.
The 25x25 and the 24x48 machines were designed to permit a variety of leadscrew sizes from 5/16 to 1/2 inch, and 8 to 12mm.
The 10x9 uses 5/16 and 3/8 inch, and 8 to 10mm leadscrews.

These three machines were 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.
 
Note that general purpose Acme rods' thread sizes are standardized; a nut from one supplier will match a rod from another.
However, precision and multi-start Acme rods are not always standardized. A multi-start leadnut from one supplier may not match the multi-start rod from another.
Therefore, when choosing leadnuts and leadscrews, use the part numbers, not the thread designations, that are listed by the suppliers.

I have mismatched multi-start rods and nuts as warned against above. After a few hours of use, the anti-backlash nut permitted backlash.
This was resolved by increasing the tension of the nut's spring by placing a spacer between the spring and the flange. The nut then wore to the shape of the leadscrew, and the backlash diminished.

This cobble is mentioned because it is not always possible to find anti-backlash nuts to fit the available rods, or the correct rods are priced far higher than similar rods with the same thread designation.
 

Speeds

Generally, the steppers and low cost drives 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.

Higher power drives, such as Geckodrives, can drive the steppers over 1000 rpm.

The turns-per-inch of the leadscrew is divided into this 500 rpm to give the speed of the machine.
For example, 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 gantry 250 ipm, though it can reach 200 ipm.
 

Upgrades

The 5/16-14 Acme rod was a simple upgrade.
The bearings did not have to be changed since the rod is the same diameter as the 5/16-18 hardware store All Thread, which fits in 608 bearings.

The table's speed about doubled as compared to 5/16-18 All Thread. HobbyCNC's 305 oz.in. steppers could rapid at better than 60 ipm.

3/8 Inch Rods

The 3/8 inch Acme rods require 3/8 inch bore bearings. These 7/8 inch OD bearings fit in the same recesses as the 608 skate bearings that are used in the original machines.

Bearings with same OD and different ID
Same OD, different bore.
 
The pulleys also require larger bores.
The different sized pulleys 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 inexpensive 5/16-18 nuts.

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.
Pulley held with 5/16 and 3/8 inch nut
Pulley clamped between 5/16-18 All Thread
and 3/8 inch Acme nuts.

The 3/8 rod is stiffer and less prone to whipping as compared to the smaller diameter rod, and it is easier to find than 5/16 Acme. Its price tends to be lower as well.
 
The Acme 3/8-10 two start, and 3/8-8 four start precision rods were also tested. These give five and two turns per inch of axis movement.

These rods, like the 3/8-12 Acme are almost a drop-in substitutions, but again, the pulleys and bearings require larger bores.
Their speeds were consistent with the assumption of 500 rpm steppers.

Axes movement ranged from 100 to over 250 inches per minute depending on the steppers and drives.
 
Standard hex nuts for multi-start rods are difficult to find.

Collar Clamp
Collar clamp.

Collar clamps, also called shaft collars, are available. They add cost, but are easy to use. Above image.

Hose with hose clamps can also be used as collar clamps. The hose is twisted onto the rod and its clamp is tightened to secure it.
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.
Rethreaded end of Acme rod
Re-threaded rod.

Only one end of each rod can be re-threaded; the multi-start leadnut will not fit over the altered threads for final assembly.
 

1/2 Inch Rods

Half inch diameter 10 and 2 turn per inch Acme leadscrews were also used on a the 25x25 and 24x48 machines.

Speeds were comparable to the smaller rods with the same thread count.

The advantage of the larger diameter and lower turn per inch leadscrews is their resistance to whipping.

For 3 feet long axes, 3/8 inch rods work well. For axes approaching 6 feet, the 1/2 inch rods have served well.

These 1/2 inch diameter leadscrews require larger OD bearings, and the machine's bearing recesses have to be enlarged.

Note: 1/2-13 All Thread rods are quite rough, and do not work well as leadscrews.
 

Acme Leadnuts

Standard and anti-backlash Acme leadnuts are available for a variety of leadscrews.

Anti-backlash leadnuts will eliminate play, but these plastic leadnuts do flex under heavy cutting loads. They are rated in the range of 20 pounds of force, which is far below the rating of all the leadscrews.

The anti-backlash leadnuts from roton.com and dumpstercnc.com that were used were flanged.
Part of the larger flanges had to be trimmed for clearance as shown below. The Delrin behaves like many other plastics and can be cut with a fine tooth saw, or it can be sanded to fit.

Flange trimmed
Trimmed anti-backlash leadnut.
 
See also the Drive Systems page.