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Wheel cutting

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Niloch14/06/2009 20:58:19
371 forum posts
After several false starts 'am on the verge (pardon) of cutting the teeth on my first wheel.  One or two issues however, some experts suggest coating the edge of the blank with blue felt tip or similar as a kind of witness when the correct tooth profile has been achieved.
 
J.Malcolm Wild's book suggests that the tooth height for a 0.6 module should be 0.069 but having fed the cutter down this distance, measured accurately, with a DTI the tooth profile doesn't seem to be correct.  The outer edge of the blank still has plenty of blue showing and the teeth are by no means pointed.
 
Any suggestions please?
 
Equipment is Myford lathe & dividing head, genuine Myford vertical slide, Grip-tru chuck, a beautifully made JMW milling spindle with CI flywheel pulley,  Thornton HSS cutter.

Edited By Niloch on 14/06/2009 20:59:53

The Harper14/06/2009 22:21:15
18 forum posts
12 photos
Hi,

I think you will find you need to go in double the amount. If you think about it, you would take 0.069" off of the diameter of a bar if you move in 0.069" on the dial, but here you need to go 0.069" deep on the tooth only (radius). This would in effect reduce the diameter of you blank by 0.138".
 
I hope this makes sense, it is easy sitting here knowing what I mean, it is not so easy putting it in words.
 
Regarsd
Paul
 
The Harper14/06/2009 22:27:20
18 forum posts
12 photos
Me again,
 
Just another point, if you were to carry out this operation on a milling machine you would only have to move the dial 0.069, as that is the distance wthat the table would travel.
 
Welcome to the strange but thoroughly interesting world of engineering!
 
Regards (got that right this time!)
 
Paul
Niloch15/06/2009 21:25:34
371 forum posts
Thank you Paul for your contribution  -  I'm not convinced!!  I'm using the Myford as a mill, the wheel is held on a mandrel in the chuck and the dividing head is fitted to the outer end of the headstock spindle.
If I might take your second sentence in your first post, surely if you move the cross slide dial by 0.069 you will actually remove twice as much on a lateral cut from material held in the chuck.  I used to explain it to school children by saying: "Don't forget you taking 0.069 (for the sake of the discussion) of both long sides of the bar thereby reducing the diameter by 0.138.  I apologise if this is what you're trying to say.
Having made another pass all the way round this evening I think the best way is to ensure that all traces of blue disappear at which point the correct cycloidal profile will be obtained.
Maybe!!!!!
mgj15/06/2009 23:47:28
1017 forum posts
14 photos
The difference is that on many (most?) lathes the graduations are for diameter - so a movement of say .25" or mm, will result in a change in diameter of .25. The actual tool movement will be .125.
 
On the Myford the graduations refer to cross-slide movement - or a reduction in radius.  So we have to divide by 2 and set the .125 directly.
 
Correct cycloidal profile achieved - unless the addendum has been adjusted to allow a slight clearance and prevent bottoming which would jam both pinions. The dedendum +F,  will certainly be slightly overdeep - and it is the space, not the tooth that is being cut!
In which case you can expect a slightly  "blunt" tipped tooth.  Ivan Laws book Gears and Gear Cutting pp30/31certainly shows that, and the fact that a correctly cut gear engages only on the flanks of the teeth.
 
All these modules and DPs etc have all been carefully worked out, and it would be wisest I suspect to do it to drawing!. The reason for that is that your gears will not maintain constant velocity if the PCDs of the two pinions do not touch. By changing the depth of cut you will alter the PCD of each pinion slightly.
 
One of the advantages of involute gears!
 

Bizibilder20/06/2009 15:54:22
avatar
173 forum posts
8 photos
Hi Niloch
 
There are a couple of other easy (!) ways to get wheel cutting wrong.  From the above you seem to be OK as far as an infeed of 0.069" on the Myford handwheel.
Have you checked -  Firstly that your wheel blank is exactly the correct outside diameter? (yours may be too big) and secondly - have you set the dividing head to cut the correct number of teeth? You may be cutting too few (I, of course, have NEVER made either error  !!!!!!!!)
 
Seriously, I've cut about three dozen wheels using the "just leave a witness of blue" method with no problems at all (apart from operator error).
 
Hope this helps.
Bizibilder
 

Edited By Bizibilder on 20/06/2009 15:56:40

Niloch20/06/2009 19:19:08
371 forum posts
Thank you Bizibilder for your most constructive response/post.  My blank was not as accurate in diameter as it should have been being .0005 under the size stipulated.  I suspect my elderly Myford S7 (sight glass variety) needs an expert over-haul but I shouldn't be blaming the equipment.
 
The wheel I was trying to cut was of 60 teeth and as I'm sure you know that with the Myford dividing head, this means sixty revolutions of the detent and any of the hole circles can be used (unless you know different!!)
 
I also now know that the cross-slide jib wasn't tight enough and I'm pretty sure I inadvertently jogged the hand-wheel on one occasion  -  not helpful.
 
Luckily I was using practice blanks and by cracky the practise has involved a steep but, nevertheless, enjoyable learning curve.
 
I don't suppose you live anywhere near Hampshire do you !?!?!?!?
 
Thanks again, much appreciated
Bizibilder20/06/2009 19:33:58
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173 forum posts
8 photos
Niloch
I doubt if 5 tenths on diameter would be to blame.  As to the cross slide - set the cut and then lock everything! (except the feed of course).  The good thing about 60 tooth practice blanks is that they can be turnrd down to make smaller wheels - thus saving on expensive brass!
 
Bizibilder
p.s. Sorry - I live in East Anglia.
Dave D30/06/2009 15:14:35
1 forum posts
Hello Niloch
 
I was intrigued by your wheel cutting problems since I have never had any trouble using Thornton cutters.
I always use cutting information from the Thornton website.
 
http://www.clock-works.clara.net/cata/wnpc.htm
 
For a 60 tooth wheel with a 0.6 module cutter the pitch circle diameter is 60x0.6= 36mm
 
The blank diameter according to the above site
equals      module(Number of teeth+addendum allowance) and the addendum allowance for a wheel is 2.76
 
So Blank diameter= 0.6(60+2.76) =37.66mm
 
Therefore the addendum =37.66-36=1.66mm 
 
The dedendum is listed as 2xmodule =1.2mm
 So the tooth height must be 1.2+1.66=2.86mm or 0.113 inches.
 
I can't explain the  Wild figure of 0.69 inches since I do not have his book.
 
I hope that this helps.
 
 
 
 
 
 
 
 
 

pat02uk07/09/2009 13:38:13
1 forum posts
The Thornton literature (not available online) gives:
 
Outside diameter of blank = (N + 2.76) x M = (60 + 2.76) x 0.6 = 37.66mm
Root diameter = (N - 4) x M = (60 - 4) x 0.6 = 33.6mm
Therefore depth of cut = (37.66 - 33.6) / 2 = 2.03mm
 
OR
 
Depth of feed = 3.38 x M = 3.38 x 0.6 = 2.028mm
 
There are a number of standards one could take notice of, but using Thornton cutters I have found the above to be accurate.
 
Pat
John Haine01/10/2009 15:28:57
5563 forum posts
322 photos
My understanding is that on imperial lathes the cross slide calibration corresponds to the actual movement - feed in .01 inches indicated and the X-slide moves .01, but it takes .02 off the diameter.
 
On the metric lathes I have used, including my own S7 and a 254, on the other hand, the calibration corresponds to what the lathe takes off the diameter, so the slide moves only half what is indicated.  Personally I find this absolutely fine and logical, but I can see trouble coming when (and if ever) I use an imperial lathe!
 
John.
chris stephens01/10/2009 23:20:53
1049 forum posts
1 photos
Hi John,
What you say is the norm, so says the man at Myford.
The most confusing thing about a metric lathe is when screw cutting,   you have to remember to double the infeed.
chirs stephens 
mgj02/10/2009 17:55:07
1017 forum posts
14 photos
Or fit a DRO, and switch between radius and diameter at the touch of a button!
 
And metric and imperial.
chris stephens02/10/2009 18:12:35
1049 forum posts
1 photos
Hi Meyrick,
 Love to but I think variable speed comes first, when funds are limited. Earlier this year I decided that any new purchases had to come out of "profits".  Got DROs on the mill, though, and wonder how I ever managed without.
chris stephens
 

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