A Cheap Huge Worm Gear

[Stub Mandrel]

14 hours ago, Vroobel said:

My friend ask me months ago if I could use this solution, but I couldn't imagine it at work with 450mm wheel, so didn't undertake the idea. Should I regret? 

Je ne regrette rien. Ear defenders may be required.

 

 

[Gina]

She was good.

[Stub Mandrel]

1 hour ago, Gina said:

She was good.

I would rather hear K-pop. ?

[Vroobel]

There is nothing new in the topic of hobbing my wheel, but people interested in a wider description of the preparation, making the machine and the process of hobbing the wheel can feel invited to read an extensive article on my blog. I apologise the blog is still in Polish - I have no time to move the platform to dual-language one and prefer making the machines than struggling with software. Please use a translation of the page, that should bi fine enough, I hope.

Link to the blog:

How to make a large worm gear cheaply?

[Rusted]

I'd like to resurrect this thread to further congratulate the clever chap, Vroobel, who made his own wormwheels.

 

Regarding the number of teeth on a given blank: The teeth have some depth.

This puts the bottoms of the teeth at different radius to the tops. [Which is at the blank's smooth outer diameter.]

So you need to calculate a smaller diameter of wormwheel blank if the number of teeth is important to you.

 

I have often advocated the use of accurate, but mass produced, perforated strip material for dividing a blank.

Roofers have lots of different perforated strip for reinforcing and triangulating roof constructions.

These perforated strips could be wrapped around a large diameter blank on the same shaft as the intended wormwheel. 

The larger the dividing blank, the more accurate the smaller wormwheel will be due to the ratio between their diameters.

The perforated strip is cheap enough to make a huge dividing wheel fixed onto cheap plywood or MDF.

 

The wormwheel blank then needs to be gashed at the correct angle to match the tap's helix.

This could be done using a home made "fly" cutter. A pointed tool passing through and clamped into a driven shaft.

Or use a small metal cutting, slitting saw which are probably available on eBay.

Both types of gashing tool can easily be driven with a pillar drill or electric motor.

The dividing wheel gives you the correct number of teeth if this is important to you.

The thread cutting tap gives you the correct profile for the teeth and will faithfully follow the previously made gashes.

 

None of this takes away from the skill and imagination shown by our OP, Vroobel.

Very impressive work indeed! Thanks for sharing your artistry.  Seriously clever stuff! 

[Rusted]

On 11/11/2018 at 23:38, Vroobel said:

There is nothing new in the topic of hobbing my wheel, but people interested in a wider description of the preparation, making the machine and the process of hobbing the wheel can feel invited to read an extensive article on my blog. I apologise the blog is still in Polish - I have no time to move the platform to dual-language one and prefer making the machines than struggling with software. Please use a translation of the page, that should bi fine enough, I hope.

Link to the blog:

How to make a large worm gear cheaply?

Anyone can highlight, then right click on your text and select Google Translate in the drop down box. 

[Vroobel]

Thanks, Rusted - and other guys, of course - for nice words.

The size of wheel was calculated exactly for 888 teeth, so I was surprised having 896...

On 14/11/2018 at 18:37, Rusted said:

... The perforated strip is cheap enough to make a huge dividing wheel fixed onto cheap plywood or MDF... 

I understand the idea. I never thought about the perforated strip as pattern... 

I will try to consider it, but I should prepare another wheel with the strap around having diameter bigger or smaller than final wheel, and their ratio must be same like the ratio of 3/8" BSW thread (which I use) and the perforated strap. Am I right? 

The perfectly simple solution I could reach using strap having exactly 16 holes per inch that I could put on my heel, but I didn't find it on ebay (or I can't proply ask). If you know source of that, please let me know.

OK, I have to think about the idea before I start working with the second wheel. 

[Peter Drew]

If your system is producing too many teeth, try a slightly smaller blank wheel.  Good luck!    ?

[Vroobel]

Thanks, but it isn't proper solution. This 450mm size should give about 890 teeth, as equations say. I need exactly the 450mm, so I can't make it smaller. 

Actually my Raspberry Pi can deal with 8 teeth more or less, so it's not a problem. 

[Vroobel]

Hello again! 

I'm pleased to inform that the second wheel - also 450mm - has 897 teeth, that is 1 tooth more than the previous one!

[JamesF]

Something about free hobbing has been nagging away in the back of my mind since I first read this thread and it's taken until now for it to actually come into focus.  I've no idea if it's correct or not, but I'll post it to see what others think...

If teeth are desired at a certain spacing around the wheel, each pair of teeth will make an angle of 360 / number of teeth with the centre of the wheel.  The distance between the teeth is being measured around the circumference of the wheel.

Using a hobbing tool of the same pitch as the desired spacing, if multiple cutting teeth are engaged with the workpiece at the same time (which presumably must be the case for hobbing to work in the first place), the angle between adjacent cutting teeth and the centre of the workpiece will not be the same because the teeth are in a straight line.  The angle will in fact be slightly smaller than desired, formed by a triangle constructed between the teeth and the centre of the workpiece.  So the teeth will be a little closer together than calculated.  Presumably they'll be at the correct spacing to allow the worm to work properly as that's basically what the hobbing tool is, but maybe a sufficient error to mean that more teeth are cut than desired?

James

[Vroobel]

It sounds logical. I have to draw it and calculate. Thanks James!

[Stub Mandrel]

11 hours ago, JamesF said:

Using a hobbing tool of the same pitch as the desired spacing, if multiple cutting teeth are engaged with the workpiece at the same time (which presumably must be the case for hobbing to work in the first place), the angle between adjacent cutting teeth and the centre of the workpiece will not be the same because the teeth are in a straight line.  The angle will in fact be slightly smaller than desired, formed by a triangle constructed between the teeth and the centre of the workpiece.  So the teeth will be a little closer together than calculated.  Presumably they'll be at the correct spacing to allow the worm to work properly as that's basically what the hobbing tool is, but maybe a sufficient error to mean that more teeth are cut than desired?

Actually you have stumbled upon the truth! You need all those different angles to get properly formed teeth.

The hob itself has a straight-sided 'rack' form.

The teeth of a properly formed gear have an 'involute' profile i.e. they have curved sides.

The process James describes is exactly how that curved profile is generated from the rack when hobbing gears.

In fact there are three main ways to cut involute gears:

1 Use the correct profile cutter from a selection of about 9 that approximates the correct curved involute shape, indexing around after each cut to give the full set of teeth. You can approximate an involute cutter using round 'buttons' to cut eth sides of the cutter. These gears were cut this way:

2 Use a rack-form spiral hob (either free hobbing or using a geared drive to ensure the correct number of teeth) which is moved across the gear at the correct angle.

The gear for this rotary table was free-hobbed using a cutter made from the same bit of silver steel as the driving worm. The blank was gashed to help ensure that 60 teeth were generated! The two hobs are 10 and 20 tpi which suit 1" and 2" free-hobbed 60-tooth gears.

3 Use a rack-form non-spiral hob in the same way as a profile cutter. Hob also takes extra cuts from the teeth either side of the central one at each pass, according to James description. Instead of a curved profile each tooth typically has a few flat facets but in practice this is usually good enough to provide a usable gear. I made all these gears using this third method:

 

[Vroobel]

It's time to share of a final effects of my work with new drives.

But first of all, best wishes of Happy New Year for Everyone ! ?

All the effort and doubt at same time finaly gave me a great satisfaction yesterday. I completed last details before taking the telescope outside, to my garden and (shortly saying) aimed laser towards the Betelgeuse. I was really positively surprised checking every quarter within 2h and seeing the laser beam still touching the Betelgeuse! I used 6mm eyepiece as well and the star was stil in a center of its field. After the test I'm sure that the gears, drives and software are working superb! ?

You can't see the drives in the picture above, but I'll make next video on YT soon. 

Now I have to focus on supplying the telescope with bigger, 5-7" display (presently I use 4x20 LCD), run end stop switches and rewrite the software, tidy up some ideas and deploy a GoTo feature. 

[Vroobel]

Hi all,

I'd like to share with you how I've made new drives of the telescope. A picture below shows both the Az and Alt one, but you are welcome to visit my blog, where I describe the process with details. As the site is still in Polish, please use the Google Translate.

Az axis: https://astrovroobel.wordpress.com/2018/11/12/nowy-naped-w-osi-az/

Alt Axis: https://astrovroobel.wordpress.com/2019/01/06/nowy-naped-w-osi-alt/