Large Homemade Aluminum Worm Gears

Homemade Aluminum Worm Wheels Project

This webpage describes the gear cutting (hobbing) machine used to produce the aluminum worm wheels for my homemade German Equatorial Mount (GEM). This machine used only an ordinary power drill and a threaded rod or metal tap cutting tool to hob large homemade worm wheels. The final results, 215 tooth worm wheels that were hobbed using an ordinary power drill and a M12 threaded rod (below left photo) or a M12 tap (below middle and right photos):



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 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 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.


Everything in the following webpage sections is what I have developed/learned through trial and error processes. I'm sure that there are probably more correct ways to do this, but these may require expensive metal machining equipment. The following is what I have developed with my homemade equipment and lots of trial and error.


The remainder of this webpage describes processes that involved cutting and shaping metal. 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.

Cutting The Gear Blank 

The gear blank requires cutting to a circumference that will contain an integral number of gear teeth. Using the cutting tool gear pitch (the distance between two adjacent and corresponding points on the cutting tool), I calculated the required circumference to hold the desired number of gear teeth. If the gear blank is not cut to the exact required dimensions, the cutting hob will misalign after one complete cutting pass (the gear cuts will not line up with the previous cuts, see the below photos). 

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. I also recognized that I have limited metal working skills, so this procedure would be destined to fail if I attempted it! 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 could grip into the masking tape and turn 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.  I also indexed several gear blanks using masking tape and a M12 tap cutting hob, but indexing with threaded rod worked the best (the M12 tap sometimes cut too aggressively into the masking tape and shredded it to the point that the masking tape could not help the hob turn the unindexed blank). Below are two photos showing incorrectly and correctly indexed gear blanks, left and middle respectively. The plywood is from disks placed on top and under the gear blank during indexing. 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 800 tooth gear blank after multiple indexing passes. The right photo shows the 800 tooth indexed gear blank after removal of the masking tape, ready for hobbing. Note that the last webpage section (March 2013 Gear Fabrication) shows photos of indexing with a M12 tap.


Gear Hobbing

Below are three photos showing the hobbing process using a M12 tap (left and middle) and a M12 rod containing slits cut with a metal file (right). Note that before taking the pictures of the M12 tap hobbing, I cleaned up the work area. The right photo shows how messy this process is, producing lots of metal cuttings and dripping cutting fluid. Both cutting hobs produced usable worm wheels, but the M12 tap definitely gave the best results. The M12 tap is hardened tool steel and it cut much quicker and cleaner than the homemade M12 threaded rod cutting tool. Note that the gear on the top of this webpage (top left photo) was cut using only the M12 threaded rod cutting tool.

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 dosen'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 and used an old dental pick to remove aluminum cuttings from the hob. When using a M12 threaded rod tap, I also occasionally touched up the cutting surface with a metal file. 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.
Note that the last webpage section (March 2013 Gear Fabrication) shows additional hobbing photos.


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 (this is something I used in my days as a historical reenactor for polishing musket barrels). The homemade polishing compound worked just fine (below photo).

March 2013 Gear Fabrication

The declination gear on my  German Equatorial Mount (GEM) recently was damaged and I fabricated a new 215 tooth worm wheel. I took some photos during gear fabrication to further illustrate my hobbing machine and procedure:

I wrapped masking tape around the blank and gave it a first slow index pass. After I verified that the first and second pass indexing aligned (below left photo), I gave the blank an additional 4 indexing passes (below middle photo). The below right photo shows the indexed blank (5 passes) after I removed the masking tape. The horizontal line is just masking tape adhesive.

The below left photo shows the blank about halfway through the hobbing process. The below middle and right photos show the finished gear blank. 

The below photos show the finished gear blank before lapping.


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