Author Topic: Building a 7" rotary table  (Read 60677 times)

Offline Maryak

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Re: Building a 7" rotary table
« Reply #15 on: September 02, 2013, 12:58:51 AM »
Hi Rich,

Let helix angle = A

Tan A = Pitch/(EffectiveDiaxPI)

Tan A = 0.1/(0.686xPI)

A = 2.650

Best Regards
Bob
« Last Edit: September 02, 2013, 01:14:35 AM by Maryak »
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Offline SandCam

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Re: Building a 7" rotary table
« Reply #16 on: September 02, 2013, 02:36:51 PM »
Hi Rich and Bob,

No... I have not left this world yet :old:... just been busy on other things in life.
 
Bob... How are things doing down under? Hope you and yours are keeping well.


The helix angle you have calculated is certainly correct, however, it may not be clear to some viewers, whom may not be so well versed in trig, exactly how you arrived at it...

If I may be permitted to elaborate a little on your posted formula…

Let helix angle = A

A = arctangent (Tan A)

Tan A = Pitch/(Effective Dia x PI)

Tan A = 0.1/(0.686xPI)

Tan A = 0.0464

A = arctangent (0.0464)

A = 2.650

NOTE… on a calculator the arctangent key is usually marked as   tan-1

The Effective Dia is found from OD – (0.64 x pitch) for a Whitworth thread form.

In the case of the Whitworth thread form this is also the PITCH DIA.

Hope this clears up any possible confussion.


Which brings me to my next point…

Rich… I don't recall seeing one of these on your Portable Engine >:D :lolb:

The formula you used for the OD of your blank

OD = (Number of teeth x pitch) / Pi

This will give you the PITCH DIA… not the true OD
For Whitworth thread form… you need to add the thread depth to this to obtain the true OD

Thread depth for a Whitworth thread being (0.64 x pitch)… in your case this will be 0.064”.

If you use OD= ((number of teeth + 2) x pitch) / Pi
This will give you a very close approximation.

If you use the pitch dia for your OD... then you can only cut to a depth of 0.032” before you begin to damage the thread form by thinning it too much… you also run the risk of reducing the actual number of teeth by cutting beyond this depth.

Have you counted the actual number of teeth on your last finished worm wheel?

Hope this helps a bit and does not add to any confussion.

Keep happy. ;D

Best regards.

Sandy  :cheers:

« Last Edit: September 02, 2013, 05:53:18 PM by SandCam »

Offline mklotz

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Re: Building a 7" rotary table
« Reply #17 on: September 02, 2013, 05:02:01 PM »
Quote
Tan A = 0.0464º
A = arctangent (0.0464º)

A minor nit, Sandy.  The tangent is equal to a unitless, i.e. no degrees, number.  When you take the arctangent, the result is an angle, A, which you express with the degree symbol. 
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Offline SandCam

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Re: Building a 7" rotary table
« Reply #18 on: September 02, 2013, 05:51:07 PM »
OOPS!!!

You are quite correct Marv... my error... sorry... I will get back in the corner and write 100 lines.
Original post edited to remove error.

Best regards.

Sandy
« Last Edit: September 02, 2013, 05:54:19 PM by SandCam »

Offline Firebird

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Re: Building a 7" rotary table
« Reply #19 on: September 02, 2013, 09:47:59 PM »
Hi Bob Sandy & Marv

Erm tangent helix arctangent :shrug: tan A pitch  :headscratch:  :noidea: :noidea:  :help:

You all lost me after the first line  :embarassed:

But I now know its 2.65°  :cheers: :ThumbsUp: :whoohoo:

Now Sandy lets move on to the diametre of the blank I need.

If you use OD= ((number of teeth + 2) x pitch) / Pi
This will give you a very close approximation.

90 + 2  X  .1  ÷  3.1428  =  2.9273   :thinking:

Why do I have to add 2  :noidea:

I haven't counted the actual number of teeth, never thought to do that  :facepalm:

Thankyou all for patience.

Cheers

Rich   



Offline SandCam

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Re: Building a 7" rotary table
« Reply #20 on: September 02, 2013, 10:59:51 PM »
Hi Rich,

I thought that lot would get you going  :mischief: :naughty:

In your original calculation you had...

OD = 90 x 0.1 / pi which gave you 2.864"

This would be the Pitch dia of the gear wheel... for WHITWORTH threads this would be the diameter at half the thread height... therefore, you need to add the other half to both sides of the circle.

You do this, for WHITWORTH thread form by adding the thread depth to the PITCH dia... this takes care of both sides of the circle.

For WHITWORTH threads the thread depth is calculated as follows: -

Depth = 0.64 x pitch... = 0.64 x 0.1... = 0.064"

The true OD would thus be 2.864 + 0.064 = 2.928"

The OD for a spur gear is... 

OD = ((N+2) x pitch) / pi... where N is the number of teeth required.

So ((90 + 2) x 0.1) / pi ... = 9.2/pi... = 2.928"

So.. by adding 2 to the tooth count the calculation takes care of adding the tooth depth for you.

 You would still need to find, or calculate, the tooth depth though... so you know how deep to cut.

Strictly speaking, there is still a small error using either method since both of the above methods are for STRAIGHT cut spur gear teeth.

A worm wheel has helical cut teeth and therefore the PITCH of the teeth is increased, over that of a corresponding spur gear,  due to the angle... which in turn increases the OD... but for this excercise the above calcs will be adequate... unless you want to get into Cosines as well as all the others :censored:

I hope this all makes sense.

Keep happy.

Sandy. :cheers:
« Last Edit: September 02, 2013, 11:15:58 PM by SandCam »

Offline Maryak

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Re: Building a 7" rotary table
« Reply #21 on: September 03, 2013, 01:12:30 AM »
Hi Rich,

Bloody gears.........one more thing and that is the width of the wheel blank.

Axial width = (2.38 x Pitch) + 0.25

                 = (2.38 x 0.1")+0.25

                 = 0.488"

Hope this helps

Best Regards
Bob

PS Hi Sandy, The spring is on us in down underland after a very wet winter. And I have no objections to your expanding the formula. It's good somebody looks over my shoulder, preferably with a can of WD 40 at the ready to loosen up my gears.
« Last Edit: September 03, 2013, 02:03:16 AM by Maryak »
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Offline Pete49

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Re: Building a 7" rotary table
« Reply #22 on: September 03, 2013, 04:03:20 AM »
thanks for bringing this up again Rich. I saw this type being made recently and never gave it much thought but as you was making a bigger RT I was thinking (yep bad move :Lol:) I wonder if its as easy as it looks :headscratch: Anyhoo I found a piece of scrap ali and a 1/2" BSW tap and had at it ...wow it is as easy as it looks and I started on another round of dangerous thinking until Bob Marv and Sandy got involved and now my head is in a real spin. Damn those maths experts messing up my mind  :Jester: The main thing is you got me to try an idea and it worked so now its filed away and maybe used at a later date
Pete
I used to have a friend.....but the rope broke and he ran away :(....Good news everybody I have another friend...I used chain this time :)

Offline Maryak

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Re: Building a 7" rotary table
« Reply #23 on: September 03, 2013, 04:19:22 AM »
Hi Guys,

I took the liberty of running this through my CAD and worm spreadsheet which I have attached.




Oh and Pete in my case when it comes to maths:

Expert =  X,spurt................X, (the unknown factor) and spurt............a drip under pressure.

As Sandy alluded to; there are some minor differences between actual worm calculations and those using a tap as the hob. e.g. helix angles. In a practical sense the gashing angle does not have to be all that accurate just good enough and with the correct number of teeth to give the tap a reasonable start.

I hope it's of some use.

Best Regards
Bob
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Offline Pete49

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Re: Building a 7" rotary table
« Reply #24 on: September 03, 2013, 01:54:18 PM »
Just to clear up a slight error in my post ....my expression of expert was not as yours Bob more a complimentary version as in extremely clever. But you knew that....I hope :Doh:
Pete
I used to have a friend.....but the rope broke and he ran away :(....Good news everybody I have another friend...I used chain this time :)

Offline mklotz

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Re: Building a 7" rotary table
« Reply #25 on: September 03, 2013, 05:04:12 PM »
Just to clear up a slight error in my post ....my expression of expert was not as yours Bob more a complimentary version as in extremely clever. But you knew that....I hope
Pete

Thanks for making that clear, Pete.
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Offline Tennessee Whiskey

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Re: Building a 7" rotary table
« Reply #26 on: September 03, 2013, 08:58:12 PM »
Y'all have been around y'alls wives way too much. I had to reread it three times to even have a clue why Pete was apologizing. Glad y'all got that one worked out.

Whiskey

Offline Firebird

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Re: Building a 7" rotary table
« Reply #27 on: September 03, 2013, 09:16:20 PM »
Hi

Wonderful stuff gentlemen, thankyou very much  :praise2:

I got a 3/4" 10tpi spiral flute tap today, I'll get a photo posted soon. It may work without gashing the blank, I'll give it a go next time I get out in the shop.

Pete the xspurt tag I could wear easily  :lolb:

Bob, if I can cut a gear as good as your drawing I will be a happy man, and thanks for gear blank width formulae.

Sandy, cosines  :facepalm2: now why didn't I think of those

Cheers

Rich

Offline Maryak

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Re: Building a 7" rotary table
« Reply #28 on: September 03, 2013, 10:49:38 PM »

Bob, if I can cut a gear as good as your drawing I will be a happy man


Me too, the real world is far more demanding than cyberspace but it does give an idea of what things should look like.

Best Regards
Bob
Если вы у Тетушки были яйца, она была бы Дядюшкой

Offline SandCam

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Re: Building a 7" rotary table
« Reply #29 on: September 03, 2013, 11:33:34 PM »
Hi Guy's,

OK then... I have finished writing my 100 lines.

" the tangent is = to a unitless number...PROOF READ before posting" :hammerbash:

Thanks for picking that up Marv. :praise2: :NotWorthy:


Rich... You are welcome... we are always happy to help where we can.

I think you will find the spiral tap will work a lot better as it will maintain the drive to the blank more consistently... the straight flute tap can lose drive if the cutting tooth exits before the next one makes contact... resulting in a more intermittent drive.

Pete... I don't believe I would even qualify for the xspurt tag when it comes to maths... I like to think I know/remember just enough to get me into trouble... but thanks for the complement... and sorry for the head spin... that was not intended.

Bob... I hope you have a better spring than we had this year... ours was cold and wet until late May.

Keep Happy guy's.

Best regards.

Sandy. :cheers: