Author Topic: Building a 7" rotary table  (Read 60308 times)

Offline Lew Hartswick

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Re: Building a 7" rotary table
« Reply #30 on: September 04, 2013, 01:57:54 PM »
< Expert =  X,spurt................X, (the unknown factor) and spurt............a drip under pressure.>
 OR. Ex as in "former" or "has been" , which results in:
A "Has been, drip under pressure".  The way I heard it.
   ...lew...

Offline Firebird

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Re: Building a 7" rotary table
« Reply #31 on: September 07, 2013, 04:44:55 PM »
Hi

Some success and some failure today.

I made a mandril to hold the big gear that I intended to use as the table. It just fits in the lathe.



Unfortunately when I tried to machine it I found that the gear must be hardened. I trashed 2 TC tips and had to give up. I will have to source another piece from somewhere.

Then I set up to cut another worm wheel using the helical tap.





A video of the cutting


Not bad at all



It looks good and the helical tap cuts easily and turns the blank without gashing.

But I have a problem, its got 94 teeth  :facepalm2:

Where oh where have I gone wrong  :help:

I machined the blank to

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

Heeeeeeeeeeeeeeeeeeeeeeeelp

Cheers

Rich




Offline Don1966

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Re: Building a 7" rotary table
« Reply #32 on: September 07, 2013, 05:05:02 PM »
Rich in that formula is the .1 the circular pitch or the thread pitch. You need the circular pitch here.
Or you could use (constant .3183*TPI*number of teeth) * (2+ depth of teeth).
I may be wrong here since I am pretty new to all this.

Don
« Last Edit: September 07, 2013, 05:09:48 PM by Don1966 »

Offline Brian Rupnow

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Re: Building a 7" rotary table
« Reply #33 on: September 07, 2013, 11:10:38 PM »
Very interesting stuff indeed. Math aside, I had no idea that a gear of this type could be made this way. I had never actually thought about this type of thing being built in a small "home" machine shop. Thank you for the education.--Brian
« Last Edit: September 07, 2013, 11:45:51 PM by Brian Rupnow »

Offline SandCam

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Re: Building a 7" rotary table
« Reply #34 on: September 08, 2013, 12:05:42 AM »
Hi Rich,

You need to gash the blank or the tap will revert to cutting teeth at 0.1" pitch on the periphery... which would result in 92 teeth.
Add in any slip and you could well end up with more.

When you have gashed the blank... bring the blank up to the tap (with the lathe stopped)and make sure the closest tooth lines up with a gash then add about 0.01" feed depth... then turn on the lathe... allow the work to rotate at least 3 to 4 complete turns, to allow the cutter to even out the pitch differences, before increasing the feed depth by another 0.01"... repeat this until you get almost to final depth, say 0.060", then do your final pass with the final 0.004" cut.

Hi Brian,

Good to see you have found us over here... and who has been a busy boy with all those lovely new engines  :ThumbsUp:.


Keep happy.

Best regards.

Sandy.

PS... make sure you get the gash angle sloping in the correct direction or all sorts of strange things will happen :naughty:

Offline Pete49

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Re: Building a 7" rotary table
« Reply #35 on: September 08, 2013, 09:14:21 AM »
Oh no more stuff to learn. The only gashing I know of results in my blood loss.  :lolb: Can you explain further?
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 Noitoen

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Re: Building a 7" rotary table
« Reply #36 on: September 08, 2013, 01:10:48 PM »
 :noidea: Instead of "pushing" the blank sideways into the tap, if the cutting could be approached "I can't find the right words" like it was entering a hole "tangentially??" with the depth of cut already set, maybe it would work better. The tailstock centre should be thinner and the blank would move gradually to the left, until it reached the correct depth of cut, i.e. the centre of the tap.

I could draw a "crap-o-cad" to explain  ;)

Offline Maryak

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Re: Building a 7" rotary table
« Reply #37 on: September 08, 2013, 01:24:08 PM »
Hi Pete,

Gashing means exactly what you said only in this instance it's metal removal rather than skin.

e.g.

Take the gear blank on a mandrel and mount it centrally on the rotary table /dividing head which is mounted vertically on the mill table.

Now angle the rotary table away from the vertical to a close approximation of the helix angle and ensuring the angle is of the correct hand to match the worm, (tap).

Lastly fit a slitting saw whose thickness is less than the width of the worm at the bottom and make a cut which should be deeper than the mean depth of the worm thread but not to full depth.

Rotate the blank to the next tooth and so on until all the teeth have been pre-cut so to speak.

This guides the Hob/Tap and helps prevent slip. It also ensures that the pitch is not cut on the periphery of the blank. Lastly it ensures contact by more than one tooth of the hob/tap.

Hope this helps

Best Regards
Bob
« Last Edit: September 08, 2013, 01:30:58 PM by Maryak »
Если вы у Тетушки были яйца, она была бы Дядюшкой

Offline Don1966

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Re: Building a 7" rotary table
« Reply #38 on: September 08, 2013, 03:00:45 PM »
I am having trouble understanding how you can get more teeth on the gear when you have only a given circumference. If the calculation are correct, you can only place so many teeth on the gear. Since i am still learning here, can some one explain to me how you do that? Confused in Louisiana.

Don

Offline Firebird

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Re: Building a 7" rotary table
« Reply #39 on: September 08, 2013, 03:10:40 PM »
Hi

Thanks everyone, all good advise  :ThumbsUp:

The wife went to work then keep fit for a couple of hours this morning not wanting to miss this golden opportunity I made a dash to the workshop.

First job was to machine another blank to size then figure out how to hold it at the required angle. I have a cheapo tilting vise, its actually very good value for money, not a precision vise but quite solidly made. My 4" rotary table fits in the jaws ok. In the next shot I'm using a DTI to centre the work piece in the 4 jaw chuck.



Then using a digital angle guage set at zero.



The table is then tilted to 2.65°. The angle guage only works to 1 decimal point so I had to feel my way between 2.6° and 2.7° but it should be near enough.



I have used a 1/64 slitting saw, all my others are 3/64 and 1/8 upwards too big.

Lineing up the slitting saw on centre



And after cutting 90 teeth



I have cut to a depth of .040

A bit of video, slightly wobbly and out of focus.


At this point I had to pack in before the good lady came home. I hope to get the now gashed blank back in the lathe one night this week.

Cheers

Rich

Offline Firebird

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Re: Building a 7" rotary table
« Reply #40 on: September 08, 2013, 03:19:28 PM »
Hi Don

You would think it would work wouldn't you but strange things happen :noidea:

I think Sandy has explained the reason why. I'm new to this method of cutting a worm wheel as well so its all a bit experimental at the moment.

Cheers

Rich

Offline SandCam

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Re: Building a 7" rotary table
« Reply #41 on: September 08, 2013, 05:28:30 PM »
 ;D

Hi Guy's,

Good fun this, is it not!!! :naughty: :LittleDevil:

Pete,

I think BOB has answered you question as well as I could... only thing I would add would be that you gash the blank for the actual number of teeth you want on the final wormwheel... not the added +2 teeth value used for the calculation of the OD.

Rich,

Looks good now you have gashed it so hopefully this will solve the pitch/number of teeth problem.

Don,

Yes it can be a bit confusing... the pitch of the teeth on a gear is usually specified at a diameter approx 1/2 way down the tooth and is usually referred to as the PITCH DIA.
The actual tooth spacing is specified as the distance between 2 sequential teeth as measured at the PITCH DIA... this is the CIRCULAR PITCH.

To get 90 teeth at a given pitch (0.1" in Rich's case) the PITCH DIA  would need to be 90 x 0.1/pi which equals 2.864" dia, the circumference of which would be exactly divisible by 0.1"... however since this is at 1/2 tooth depth it is necessary to add on the extra for the other 1/2 tooth depth.
This is done by changing the calculation to ((Number of teeth +2) x pitch) / pi... which now gives the correct OD for the wheel... this, however, increases the circumference by 2 tooth pitches.

If you were to cut the gear using a standard gear cutter using a rotary table or dividing head then you would have no problem.
If you were cutting the gear using a hob, again no problem, providing that the workpiece was being DRIVEN at the correct rotational rate for the number of teeth required.

However, if you were trying to cut the gear with a hob using the free wheeling method... as in this case (Rich is just using a 10tpi tap as a hob) this now presents the problem of the circumference being to large and the cutting teeth will attempt to cut 0.1 pitch spacing as soon as it touches the blank... clearly this will now be the 92 teeth you calculated with.


Also, since the method can cause rotational speed variations on the blank, due to cutter drag or 2 or more teeth cutting at the same time... then the actual number of resulting teeth can be somewhat unpredictable... this would be more pronounced if using a straight fluted tap since it is possible that no teeth would be actually cutting (in the flutes of the tap) resulting in NO ROTATION of the blank.

In addition, you are also now cutting a helical tooth form which has the effect of increasing the circular pitch a little from that of a straight cut spur gear so the length of cut increases and the cutter thrust is not at right angles to the work.

Gashing the blank with the required number of teeth spaces, at the approx helix angle, encourages the tap/hob to follow the gashes, since there would be much less cutting force involved... and cutting tools tend to follow the path of least resistance... A bit like us humans :Lol:

It is a crude, but effective, method of cutting a wormwheel but without access to expensive DRIVEN HOBBING equipment it can be a usefull one... unless MORE ABSOLUTE PRECISION is required... in which case... buy a purpose made item.

I hope this helps a bit :LittleDevil:

Keep happy.

Best regards.

Sandy :cheers:

Offline Firebird

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Re: Building a 7" rotary table
« Reply #42 on: September 08, 2013, 06:18:35 PM »
Hi

Yes its all starting to make sense now Sandy, thanks for your patience (it takes a while for things to sink into this old brain  :old: :hammerbash:)

Thanks for input Bob.

Noitoen, yes please lets see your idea.


The timing belt gears that I mastered cutting have been a real asset. I can now cut a couple of gears from start to finish in less than an hour. Iv'e used them on all sorts of projects including power drives on my mill and lathe.

If I can ,master the worm and wheel (I see the worm and wheel as the most complicated part) then I can see myself knocking up more rotary tables and dividing heads for future projects
Cheers

Rich

Offline Noitoen

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Re: Building a 7" rotary table
« Reply #43 on: September 08, 2013, 06:56:03 PM »
Ok, let's see if you understand what I mean. If the blank, with the correct diameter is positioned like in the drawing, vertical centre aligned with the beginning of the tap and the distance to the horizontal centre at the final depth setting, by moving the blank to the left slowly, allowing the tap to cut all around, I would think that the teeth would be cut to the correct size.

Offline Firebird

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Re: Building a 7" rotary table
« Reply #44 on: September 08, 2013, 07:34:58 PM »
Hi

Good idea Noitoen, it might work.

Cheers

Rich