Large Aluminum Worm Gears Fabricated From Aluminum Strips

Homemade Aluminum Worm Wheels Project

This webpage describes how to fabricate large aluminum worm wheels by wrapping a strip of aluminum around a wood disk and cutting the gear teeth with an ordinary power drill. Long aluminum strips are inexpensive and available at most hardware stores, making fabrication of extremely large worm wheels and sectors relatively inexpensive. This webpage gives all instructions necessary to fabricate a jig to bend the strips and a machine to hob (cut) the gears using only an ordinary power drill.

The final results:  215 tooth (left photo) and 450 tooth (right and bottom photos) worm wheels that were hobbed using an ordinary power drill:



Most  homemade telescope mount components can be found at a local hardware store, with the exception of large worm wheels. Because the cost of large worm wheels far exceeded my hobby budget, I spent several years experimenting with other types of telescope drives. Eventually I decided to find an inexpensive way to fabricate large aluminum worm wheels, without expensive metal lathes or specialized equipment. My first tests used an ordinary power drill and nylon gear blanks cut from kitchen cutting boards. I had a lot of fun hobbing plastic gears, but the results were generally unsatisfactory. The main problems were uneven gear teeth (from the power drill moving during hobbing) and the final nylon gear being too soft (the worms sometimes slipped when under a load). I put gear hobbing on hold for several years until construction of my homemade GEM forced me return to the issue of finding an inexpensive method to produce large worm wheels. From the tests hobbing nylon gears, I determined that a machine was required to: (1.) hold the gear blank and cutting tool in the proper orientation, (2.) precisely advance the cutting tool into the gear blank, and (3.) increase the distance between the power drill and the cutting tool so that larger gear blanks would not rub against the power drill chuck. 

The next challenge was where to find large aluminum gear blanks at a reasonable cost. I discovered that if an inexpensive strip of aluminum was wrapped around a wood disk, it could be hobbed to a worm wheel. I experimented with different ideas to attach the aluminum to the wood disk and eventually found a simple and effective solution: insert the aluminum strip into routed channels between two plywood disks. This simple solution means the only requirements for producing large worm wheel blanks are: a router, a small hobby saw with a blade for cutting metal, a file, plywood, and a strip of aluminum. The procedure to produce these gear blanks is very easy and only requires a few hours.


The remainder of this webpage describes woodworking and metal cutting processes. Eye protection was mandatory during all processes described on this webpage. I also used hearing protection and work gloves. Gear hobbing and lapping is a very messy process, so its best done in a workshop or garage....not on the dining room table! Anyone interested in gear hobbing, who lacks the necessary experience, should find someone with the proper skills who can help. I assume no responsibility or liability for any injury or damages resulting from what you may do. Anything you do is at your own risk, so be sure you know what you are doing and accept all risks prior to beginning. I'm going to repeat the most important message: Eye protection was mandatory during all processes described on this webpage.

Dimensioning the Gear Blank 

The gear blank requires an outer circumference containing an integral number of gear teeth; this is necessary for the cutting tool alignment after each cutting pass. If the gear blank is not cut to the required dimensions, the cutting hob will misalign after one complete cutting pass (the gear cuts will not line up with the previous cuts). The first step is to decide how many teeth you want and what size teeth. I decided to hob 450 tooth worm wheels with M12 teeth. Using the M12 gear pitch (the distance between two adjacent and corresponding points on the cutting tool), I calculated the required gear circumference to contain 450 teeth. The next step was cutting the plywood disks that hold the aluminum strip.

Cutting the Plywood Gear Blank Disks

The plywood gear blank disks contain a groove that holds the 2mm thick aluminum strip. It is important to precisely cut this groove to the proper dimensions to hold an integral number of gear teeth. The easiest way to cut this groove is using a router with a circle cutting attachment. I set up my router to cut a circular groove with an outer radius containing an integral number of gear teeth (for 450 M12 teeth this worked out to be 125.30 mm). I used a digital caliper to exactly set my compass to 125.30 mm (below left photo) and scribed the groove radius on 1 cm thick plywood. Using a router with a 3 mm straight bit and a circle cutting attachment, I first cut the gear blank grooves (below center photo) and then cut out the plywood disks (leaving a 1 cm lip around the groove). The below right photo shows the finished plywood gear blank disks. Note that it is OK if the grove is wider than the aluminum strip thickness. This is because the aluminum strip will be cut to tightly fit into the groove and press outward against the outer groove edge (more about this in the Assembling the Gear Blank  section). I used a 2 mm thick aluminum strip inserted into a 3mm groove and it worked just fine.


The Aluminum Strip Bending Jig 

The next process is bending the aluminum strip to fit into the groove on the plywood gear blanks. I built a simple jig to bend the aluminum strip, consisting of a scrap of plywood containing two fixed rollers and one adjustable roller (below left photo). The separation between top and bottom rollers is adjustable, as indicated by the yellow arrow. I covered the aluminum strip in masking tape to prevent scratching the finish and gave it a first pass through the jig. After each pass, I decreased the separation between the adjustable and bottom rollers, increasing the bend curvature (below center photo). Eventually I bent the strip to the required curvature (below right photo). Note that there will be a small section of unbent aluminum on the ends, which is discarded.

Assembling the Gear Blank 

The next process is cutting the bent strip to length and assembling the worm wheel blank. I first cut the unbent aluminum from one end and smoothed the cut with a file. I kept the masking tape on the aluminum strip to protect the surface against scratching. Next I placed the aluminum into the groove on the plywood gear blank (below left photo) and marked where to cut the aluminum strip (blue arrow). I left about 0.5 cm of  extra aluminum and then slowly removed this excess with a file. It took 5-10 cycles of test fitting, filing down the aluminum, and rechecking the fit to get the aluminum cut to length. After the aluminum is filed to length, I used a rubber hammer to tap the aluminum strip into the groove. The aluminum strip should fit very tightly into the groove and require a hammer to set it in place. If too much aluminum is removed and the strip fits loosely against the outer groove edge, it will deform under pressure (hobbing). If the aluminum strip fits very tightly into the groove, it will press against the outer groove edge and form a perfect circle (as perfect as the routed circular groove). The below center photo shows the finished worm wheel blank. The below right photo shows the seam in the aluminum strip. The two ends must fit perfectly together, without any substantial gap; this is why you should leave an extra 0.5 cm of aluminum and carefully file the strip to the final length. Because the gear teeth will be cut into this tight seam, it does not affect the gear performance. I have produced worm wheels with the seam falling in between teeth and directly under teeth, and have not noticed any effect from the aluminum seam. After the aluminum strip is set into the plywood disk, the groove can be backfilled with an appropriate glue.

An alternative to the sandwiched plywood disk configuration is to glue the aluminum around a wood disk, giving a thinner gear blank. Note that although this configuration is possible, it is less favorable! I cut the wood disk to the desired outer diameter minus 2x the aluminum strip thickness. I bent the aluminum strip, test fitted it around the blank (below left photo), and cut the length as described in the previous section. Next I roughed up the backside of the aluminum with a file to improve the glue adhesion. I coated the wood disk with two component epoxy and tightened two large band clamps around the aluminum strip while the glue set. The below right photo shows several 215 tooth worm wheels after final hobbing. This configuration gives thinner gears that more closely resemble aluminum worm wheels, but they are considerably more difficult to fabricate. Variations in the glue thickness can cause uneven tooth depth, since portions of the aluminum strip can be farther from center. The sandwiched plywood disk configuration is much easier to fabricate and gives better worm wheels (closer to a perfect centered circle).   

The Gear Hobbing Machine 

The gear hobbing machine construction required only a few hours. The machine was constructed from 2x4 pine and cost less than $20 to build (the M12 tap cost about $4).  The machine performs three functions:
  1. It securely holds a power drill and cutting hob tangent to a rotating gear blank
  2. It has a mechanism that advances (presses) the gear blank into the cutting hob
  3. It holds the cutting hob away from the power drill, so that the large gear blanks don't contact the power drill chuck  
Below are four photos of the gear hobbing machine. The horizontal 2x4 securely holds the power drill and supports the cutting hob. The power drill sits on a small wood ledge (not shown). The cutting hob is a M12 metal tap inserted into a scrap of copper tube, secured by a set screw. The copper tube extends through a large ball bearing and is supported at one end by the power drill. The cutting hob is supported by a brass screw that is filed to a point; the hob has a small hole in the end face, which accepts the brass screw (plus a drop of oil). The two vertical 2x4s form a track to hold the sliding plywood platform. A small scrap of 2x4 is attached underneath the plywood and slides in the track formed between the two vertical 2x4s. Tightening the nut and lock washer on the M8 threaded rod pulls the sliding platform into the cutting hob. The gear blank screws onto the plywood platform and rotates on the castor wheels. I purposely made the track for the plywood platform as long as possible to improve stability and give the option of hobbing very large gear blanks. The bottom right photo shows the machine with a gear blank installed. 



Indexing The Blank

Before hobbing, the gear blank requires indexing. Indexing is just the process of cutting a first notch at each tooth location around the gear blank circumference. The index notches give the cutting hob something to grip onto and rotate the gear blank. I found a few websites for hobbing worm wheels where the indexing was manually cut using a slitting cutter or some sort of cutting wheel: the gear blank was manually turned and each indexing notch cut at the correct location. Unfortunately I do not have the equipment to manually index the worm wheel. Through trial and error, I found that the gear blank can be easily and quickly indexed using the hobbing machine. I wrapped a piece of masking tape around the gear blank and inserted a piece of M12 threaded rod into the gear hobbing machine; I removed the brass screw support and substituted a ball bearing to support the M12 threaded rod end. I adjusted the power drill direction so that the hob cut downward, pressing the blank against the castor wheels. Cutting upwards can give problems, because the hob can lift the gear blank off of the castor wheels.

The threaded rod gripped into the masking tape and turned the unindexed gear blank. After the first pass, the threaded rod began indexing the gear blank. Without the masking tape, the threaded rod couldn't grab onto the gear blank, so it would not smoothly turn or index. Below are two photos showing incorrectly and correctly indexed gear blanks, left and middle respectively. The left photo clearly shows how the first and second pass cuts misalign if the gear blank is cut to an improper circumference. The middle photo shows a correctly indexed 400 tooth gear blank after multiple indexing passes. The right photo shows the 400 tooth gear blank that was indexed with a threaded rod.


I eventually switched to using the M12 tap to index the gear blanks, since this gave a better index cut. The procedure was the same as described above. I began by installing the blank into the hobbing machine (below left photo) and then adjusted the height to center the hob (below right photo).


I wrapped the blank with 2-3 layers of masking tape, and slowly began indexing. After the first index pass, I stopped and checked the indexing (below left photo). The new indexing aligned perfectly with the first pass indexing, so I ran 4-5 additional index passes. The below center photo shows the 450 tooth worm wheel after 4-5 index passes. Note that the aluminum strip seam didn't cause any problems during indexing; the hob cut over the aluminum seam as if it wasn't there! The below right photo shows a 215 tooth worm wheel during indexing.


Gear Hobbing

I found that it was very important to start with a slow cutting speed and always in a direction where the hob cuts downward against the gear blank (so the blank doesn't lift upwards). I used lots of cutting fluid and advanced the hob slowly into the blank. As the teeth became deeper, it was possible to increase the cutting speed. I periodically stopped the drill, used an old dental pick to remove aluminum cuttings from the hob, and oiled the brass screw tip supporting the cutting hob. I hobbed until the threads were at full depth, then removed the blank and used the dental pick to remove any cutting debris that still clung to the gear blank. Placing a small aluminum tray under the hob caught most of the cutting tailings and cutting fluid, making clean up a lot easier. 

The below photos show a 450 tooth worm wheel about half way through the hobbing process:


The finished 450 tooth worm wheels (including friction clutches) are shown below. The design in the plywood disks was cut with a circle bore set and the holes were covered with red Plexiglass.

The below left photos show a 215 tooth worm wheel about halfway through the hobbing process. The below middle and right photos show the gear blank at the end of hobbing. 

The below photos show a finished 215 tooth gear blank before lapping.


Gear Lapping (Polishing)

After hobbing, the gear requires lapping (polishing) against the worm. I inserted the threaded rod to be used for the worm into the hobbing machine and polished the worm against the worm wheel. I didn't have any polishing
compound, so I made my own from a mixture of acid free hobby oil and fine brick dust. The homemade polishing compound worked just fine.
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