# Making a Worm Wheel



## drmayf (May 10, 2018)

Hello,  drmayf  here.  I am an  old  geezer  who has recently decided to take up radio astronomy. So I am converting a 7 foot sat tv dish to have an equatorial mount so I can stay locked onto objects for longer receiving periods instead of drift scanning.  That means that the primary right ascension axis, the one that points to the  north star, has to turn at a rate equal to the earths spin rate, but in opposite direction, to maintain object lock on.  the spin rate is really slow, lol. But, a worm drive supplemented with some extra rearing can do the trick. I had wanted to hob a 360 tooth worm wheel using my old 1948 worn out lathe to do this. Sadly, neither it nor I am up to the tasks of making that wheel. There are many You Tube videos showing how to do this using a blank wheel and a tap as the hob and lathe or milling machine to do the cutting.  It looks dirt simple but there are dimensions that I know nothing about which are thread pitch circle and worm wheel diametrical pitch etc.  Most likely what I just said was gibberish, lol. The size I want is the worm wheel to have 360 teeth and the worm will have 16 threads per inch.  A further reduction of 4:1 via spur gears will reduce that to the correct anti spin rate if the initial drive is running at 1 rpm, like a clock motor.  I had previously tried to figure out the worm wheel diameter but turned out what i figured was simple the pitch diameter of the wheel.  That was 7.1619 inches diameter.  I think it needs to be a smidge larger in diameter though and how much I do not know. So, my question to the membership is , "Is there any out there who might like to attempt to make this worm wheel for me using metal I provide or pay for?"  So I would entertain offers to make this wheel for me with me paying for time and materials as required.  Brass or steel would be good, but aluminum would also be ok I think.  I am gong to seek ET phoning home, lol.

for   those  who  have  interest  in  just seeing how this is done, then google diy worm wheel  using the image filter.  and go to youtube and search for the same. Pretty slick. 

drmayf at mayfco dot com


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## markba633csi (May 10, 2018)

Couldn't you just buy the wheel and worm?  Boston Gear comes to mind
Mark


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## Eddyde (May 11, 2018)

A member here Petros, covered this a little while back:
https://www.hobby-machinist.com/threads/quick-dirty-worm-wheel-making.66248/#post-566337
Perhaps he can help.


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## Ray C (May 11, 2018)

drmayf said:


> Hello,  drmayf  here.  I am an  old  geezer  who has recently decided to take up radio astronomy. So I am converting a 7 foot sat tv dish to have an equatorial mount so I can stay locked onto objects for longer receiving periods instead of drift scanning.  That means that the primary right ascension axis, the one that points to the  north star, has to turn at a rate equal to the earths spin rate, but in opposite direction, to maintain object lock on.  the spin rate is really slow, lol. But, a worm drive supplemented with some extra rearing can do the trick. I had wanted to hob a 360 tooth worm wheel using my old 1948 worn out lathe to do this. Sadly, neither it nor I am up to the tasks of making that wheel. There are many You Tube videos showing how to do this using a blank wheel and a tap as the hob and lathe or milling machine to do the cutting.  It looks dirt simple but there are dimensions that I know nothing about which are thread pitch circle and worm wheel diametrical pitch etc.  Most likely what I just said was gibberish, lol. The size I want is the worm wheel to have 360 teeth and the worm will have 16 threads per inch.  A further reduction of 4:1 via spur gears will reduce that to the correct anti spin rate if the initial drive is running at 1 rpm, like a clock motor.  I had previously tried to figure out the worm wheel diameter but turned out what i figured was simple the pitch diameter of the wheel.  That was 7.1619 inches diameter.  I think it needs to be a smidge larger in diameter though and how much I do not know. So, my question to the membership is , "Is there any out there who might like to attempt to make this worm wheel for me using metal I provide or pay for?"  So I would entertain offers to make this wheel for me with me paying for time and materials as required.  Brass or steel would be good, but aluminum would also be ok I think.  I am gong to seek ET phoning home, lol.
> 
> for   those  who  have  interest  in  just seeing how this is done, then google diy worm wheel  using the image filter.  and go to youtube and search for the same. Pretty slick.
> 
> drmayf at mayfco dot com



Sounds like a fun little project.   Is there any chance you can provide links to images of this arrangement?  A worm wheel with 360 teeth takes a long time to make.  Quite possibly, depending on the torque needed, you can achieve the same rate of operation by manipulating other (simpler/easier) gears in the chain.   Also, since this is radio-astronomy (not optical), can you get away with a stepper motor and controller?  I should think the antenna could sustain tiny amounts of periodic misalignment -and since it would be a known periodicity, it could be filtered out with a FFT/DFT.

Ray


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## RandyM (May 11, 2018)

Boston Gear from what I can find does not make a 360 tooth gear.

However, I did find Stock Drive Products maybe offer what you are looking for. They have three to choose from and all have different ratios. Not sure the ratio you are going to need. Good luck, sounds like a fun project.

https://shop.sdp-si.com/catalog/?cid=p270&filter=a6:2:360.0&sort=undefined&view=table

Also, you could use multiple worm gear arrangements just as a suggestion. You know, One worm reduction feeding another. I think you have a lot of options on this one.


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## drmayf (May 11, 2018)

Eddyde said:


> A member here Petros, covered this a little while back:
> https://www.hobby-machinist.com/threads/quick-dirty-worm-wheel-making.66248/#post-566337
> Perhaps he can help.


Thank you! I will certainly look at the link and contact if appropriate to my needs!
drmayf


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## drmayf (May 11, 2018)

Ray C said:


> Sounds like a fun little project.   Is there any chance you can provide links to images of this arrangement?  A worm wheel with 360 teeth takes a long time to make.  Quite possibly, depending on the torque needed, you can achieve the same rate of operation by manipulating other (simpler/easier) gears in the chain.   Also, since this is radio-astronomy (not optical), can you get away with a stepper motor and controller?  I should think the antenna could sustain tiny amounts of periodic misalignment -and since it would be a known periodicity, it could be filtered out with a FFT/DFT.
> 
> Ray


I  believe  it  is  going  to  be  a   fun  project.  I am  currently making the changes to the satellite dish mount to make it full equatorial. One has to wonder if jitter may account for the inability to see the correct signals, ie being able to pick the fly  specs out of the pepper.  Steppers cause jitter and because this dish is 7 feet in diameter with wind loads being applied, the drive system has to be robust as well.  I bet I have looked at a 1000 or more diy systems and most are individual. Not many remotely the same.  As to the math part of the system, I am capable of that and most of that is and can be done with the display and analysis software available today. My plan is to use basic RTL - SDR radio systems with a LNA, LNB and LNC amplifiers and down converters to gather the signals and process them.  I have a dedicated win 7 pro OS powered computer dedicated tot eh project for noting but analysis when I get it going.  I have not shared with many, my ideas on what to look for in the radio signals. I wont be looking for suspected leakages, but for signals beamed out by ETI deliberately. I am still bemused by  my notion that this signal would be primitive, something like us trying to contact indigenous  natives in the amazon rain forest, who have never even seen anyone other than their tribe and trying to send them c band radio signals as contact medium.   We earthlings, of course are the natives, lol. Being retired, I need inexpensive as well as robust, lol meaning that commercial products are costly. A commercial out the door size to accommodate the dish can be as expensive as 13000 bucks or more. If alt-az mounts are used then full computer control is needed as well to drive the dish in two directions at once. An equatorial mount with the right ascension axis fixed and pointed at Polaris means that that is the only axis that has to be driven. Declination can be fixed at the object's parameters.  I was and am committed to cheap, lol.  Otherwise I cannot do it.  I have watched a lot of youtube vids of people making worm wheels with a lathe, drill press or milling machine and a simple tap. Most are made reasonably quickly. I did price one commercially made for my application and that simple worm wheel was 1295 dollars.   Out of my league, lol. And that was only the worm wheel, not including the worm. 

I do appreciate your interest and hope I can one day report that I have success, lol.

many thanks for listening!
drmayf


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## drmayf (May 11, 2018)

RandyM said:


> Boston Gear from what I can find does not make a 360 tooth gear.
> 
> However, I did find Stock Drive Products maybe offer what you are looking for. They have three to choose from and all have different ratios. Not sure the ratio you are going to need. Good luck, sounds like a fun project.
> 
> ...





markba633csi said:


> Couldn't you just buy the wheel and worm?  Boston Gear comes to mind
> Mark





RandyM said:


> Boston Gear from what I can find does not make a 360 tooth gear.
> 
> However, I did find Stock Drive Products maybe offer what you are looking for. They have three to choose from and all have different ratios. Not sure the ratio you are going to need. Good luck, sounds like a fun project.
> 
> ...



That was a great catch for the SDP products.  The worm wheel is however only 3/16 inches thick and the teeth are very tiny. And it is even at the desired tooth count, 165 bucks and I think that does not include the worm.  This thing has to live outside in the wind and weather and does need some robustness in order to function reliably.  And even though I have  made changes to the sat dish mount by installing pillow block bearings at the ends of the RA axis (points to Polaris), to reduce the drive loads, there is always wind and some binding due to heat and thermal loads  and just plain old inertia.  In order to focus on a star system beam width seems to be important for good reception and signal strength. I am a noob at this but having a coherent signal seems best to me. And fighting the set up seems counter productive so I am headed for robustness. Was the way I built my Sunbeam race car, lol, World's fastest Sunbeam now. 

I am gussing that the amount of gear reductions needed to drive a telescope of any sort is not fully appreciated. He are some numbers.: Stars are tracked using sidereal time and that is a day that is 23 hours, 56 minutes and I believe a few seconds.  A sidereal day is 1436.068 minutes long.  That is a single rpm of 0.00069635 minutes to counter the earth's spin.  that is the rate of the RA axis rotation.  With a worm and wheel of 360 teeth and a worm with 16 teeth per inch a single revolution of the worm moves the wheel only one degree.  But since the 360 degrees turns in 1436.068 minutes that means that the worm needs to rotate at only 1 degree or tooth at a 4:1 speed reduction. I'm my set up I would have a 60 tooth drive wheel on the worm with a small spur gear of 15 teeth driving that 60 tooth spur gear.  But that input is only 1 rpm! Any kind of motor that is robust is going to turn faster than that and most clocks use synchronous AC motors.  And most clock motors with that kind of output are very small.  I do have a Baldor DC motor with controller for 0 - 90 volts and it is big enough to run the system but even it has a built in gear box at 3:1 and its output is still 345 rpm, not 1. So there are more gears involved .  To be able to use that I need 5 sets of 12 to 1 spur gear reductions and then running at 173 rpm from the Baldor to get the required RA axis drive rate.  To me, none of this is trivial, lol. Especially for an old geezer like me, lol.

Many thanks for the information and I am off hunting the solution still! I appreciate any and all comments and suggestions on how to better do this! 

drmayf


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## markba633csi (May 11, 2018)

OK second suggestion: Buy 1 or 2 commercially available right angle enclosed worm gear reducers at surplus. Then 1 or 2 sets of additional spurgears to reach the final ratio desired. Total cost maybe less than 300 bucks if you find some deals.  But my feeling is the final reduction should be a fairly hefty unit in an oil bath which might best be purchased complete and ready to go, with accurately made steel gears.  Unless you have a milling machine and a dividing head....
Mark


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## hanermo2 (May 11, 2018)

Make a big thick worm and wheel in steel, preload, use a servo motor.
Done


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## RandyM (May 11, 2018)

OK, we weren't aware of all of your design constraints in the beginning. I am assuming you are trying to make this because the commercial stuff is outside your budget. I don't think anyone was trivializing your project, just trying to help. Here is what I found with another search, but with what you have just said, is outside your budget. I have a feeling there is a reason these things have the prices they are. The third one on the list looks like the most cost effective. I am using a Google search "360 Tooth Worm Gear".

http://www.mathis-instruments.com/Products/Gears.html

Here are some more.

http://www.aeroquest-machining.com/pricing.php

https://www.ebay.com/itm/Ed-Byers-360-Teeth-15-Drive-Gear-Worm-11-16-Thick-2-Bore-Hole-/361794136860

Good luck with your project.


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## markba633csi (May 11, 2018)

How much does this dish weigh by the way?


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## Eddyde (May 11, 2018)

A DC, PWM servo motor will turn at 1 rpm. The Baldor you have may be abel to run as a servo, all you would need is a PWM driver, it's worth looking into. You would be able to make very fine adjustments and with a feedback loop it would stay exactly at speed.


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## Ray C (May 11, 2018)

Eddyde and others are heading in the same direction as my brain is taking me...   Specifically; this is not a trivial problem.

If the OP is concerned about the discrete focus misalignment if a stepper motor is used then, we're talking about a non-trivial level of precision needed for the overall mechanism.   Assuming the gears are perfect, even the motor would need to be speed controlled with sensors and some form or controller or PID.

Standard design problem here:   The RPM needed for this is approximately 0.00069.  Significant digits out to the 5th place.   Assuming a drive source of 7 or 70 RPM,  you'll need 100,000 or 10,000 times reduction.   You can design this as 1:10 x 1:100 x 1:100 or, 1:100 x 1:100.  Depending on what path you go, the gear tooth count will be finite and from this, the specifications on the design can be roughed-out in terms of parts per million (or some other scaling factor).  Once that is known, the finite specs can be determined once the size of the parts are chosen.   This is the kind of thing that slight temperature variances during operation will change tracking accuracy.

One thing I will say, is that with 100,000 times reduction, the final gears will need to be beefy.  An ant on a treadmill at the input will be able to pull an oak tree out of the ground.

We need to know the desired level of tracking accuracy or, we're going to be shooting in the dark and crippled with analysis/paralysis.

Ray


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## drmayf (May 11, 2018)

Ray C said:


> Eddyde and others are heading in the same direction as my brain is taking me...   Specifically; this is not a trivial problem.
> 
> If the OP is concerned about the discrete focus misalignment if a stepper motor is used then, we're talking about a non-trivial level of precision needed for the overall mechanism.   Assuming the gears are perfect, even the motor would need to be speed controlled with sensors and some form or controller or PID.
> 
> ...


Guys,  no  need  to go deeper into this.  I  had  requested a quote from anyone to build a simple worm wheel using a tap as the hob for the wheel.  I included the descriptions of the gearing needed to get to the speed requirements of the drive to show only that it was non trivial issue. I do not expect anyone to design a drive system. My request was to build a simple worm wheel with  360 teeth using a 3/4 -1 thread tap as the cutting tool. I saw it done a number of times on YouTube and they seem to work just fine. I had hoped that there was at least one adventuresome machinist in the membership who would take on the task on making a wheel for me. 

As I said, no need to go any further on this. I have several other ways to achieve my goals!.  I thank all of ya'll for assisting and providing information to me and to others.  This group has helped me tremendously in the past when I was getting my old 1948 Clausing Atlas lathe back among the living albeit it is worn completely out.  

Again, many thanks and see ya'll later. 

drmayf

ps, regards accuracy, yeah one of the 600 foot diameter dishes has an accuracy requirement of 1 millisecond of arc at some mind bending stellar distance. Closer is easier.  Me I am sticking to under 20 light years because having an instellar conversation would take too long for anything further. 

dm


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## whitmore (May 12, 2018)

drmayf said:


> I had wanted to hob a 360 tooth worm wheel using my old 1948 worn out lathe to do this. Sadly, neither it nor I am up to the tasks of making that wheel.



Well, aside from the obvious comment, that my pet worm won't appreciate teeth on his wheel toy,
it might be interesting to consider using an odd-ratio (cheap) worm drive (like in lots of surplus
gearmotors), in conjunction with a good rotation resolver.  Instead of dead reckoning, you can use any old
ratio of drive, and take feedback from the shaft encoder to get it into the right position...

Not sure how useful that option is, all the 'encoder' items I see are high price and mediocre
resolution.


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## British Steel (May 12, 2018)

Looking at the reduction and accuracy needed, have you looked at harmonic drives? One could be a ready-made solution for the final reduction gearing and they're appearing on the surplus market...

When I looked after fast-moving 60-foot dishes, they used worm and wheel reduction in Az and El from DC servo motors (about 100 HP each...) but the servo feedback was partly derived from the signal at the focus - if I remember right, there was a phased set of probes that could determine whether the received signal was off-axis. That worked pretty well for the fine error once the dish's position and traverse rates were supplied by a PDP11-73, even for low-orbit vehicles that might go horizon to horizon in under a minute.

Dave H. (the other one)


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## Ray C (May 12, 2018)

British Steel said:


> if I remember right, there was a phased set of probes that could determine whether the received signal was off-axis. That worked pretty well for the fine error once the dish's position and traverse rates were supplied by a* PDP11-73*, even for low-orbit vehicles that might go horizon to horizon in under a minute.
> 
> Dave H. (the other one)



PDP11-73  OMG.  I've been placed into the "Way-back" machine...    Como saved my life a few times.

Ray


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## ch2co (May 12, 2018)

dm
Whatever you decide to do on this project, I for one would love to see what you do and how you do/did it. I almost pursued a project like this
a decade or so ago, and finally decided to just stick with optical telescopes. Keep us informed and good luck!

CHuck the grumpy old guy


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