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Wheel Cuting thin tooth?

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Les Jones 108/01/2012 10:01:41
2292 forum posts
159 photos
Hi Peter,
First I agree with all of the comments discounting the size of the blank. The same also goes for the shape of the cutter. The problem is to do with the ANGLE of rotation of the blank between the teeth. Looking at the way the strap on which the indexing pin is mounted makes me think this could be a source of the problem. I assume that the nut at the bottom end of the strap is not undone during indexing. If it is was play in this hole could be the problem. I am assuming that you just flex the strap to index between holes.
The way the bottom of the strap is fixed to the bench rather than somewhere on the headstock casting could be the problem as any movement in the bench between where the lathe is bolted to it and the bottom strap fixing would cause errors. The only other thing I can think of is end float in the cutter shaft but you have probably already eliminated that.
 
Les.
blowlamp08/01/2012 10:16:44
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1885 forum posts
111 photos
 
The second wheel in your picture is duff at the 10:30 position.
 
The thing I notice in the third and last of your photographs is that the support arm of the indexing pin is anchored to the bench rather than the lathe.
 
If the lathe is able to move in relation to the bench - and hence the the arm - then a slight rotation of the headstock spindle will occur and give the error we are seeing.
 
Is the lathe firmly secured to the bench so that it can not creep across it?
 
 
 
Martin.
Billy Mills08/01/2012 12:37:49
377 forum posts
If the three wheel pictures are the latest cut and given that the second has a thin tooth then something is loose or the cutting procedure has changed for the second wheel. It would be good to mark the first notch when cutting the next wheel and the starting position for the index plate. With a cut wheel mounted how much can you turn the wheel- i.e. how much backlash in the pin/plate/mandrel/ wheel system? How much can you move the culler along the spindle axis?
 
It could be that the apparent rotation is in the first cut or the last cut . Not at some other position because that would produce two bad teeth, but it has not happened every time. So the evidence suggests that the plate is good but that some other gremlin sometimes moves stuff at the beginning or end.
 
Billy.
Peter Bell08/01/2012 13:28:18
399 forum posts
167 photos

It looks like the captions did not come through with the pictures, not sure what I did wrong? The middle wheel with the thin tooth is one of the originals, the top and bottom ones are new, cut last night.
 
The arm does indeed flex and is springy enough to allow it to be pulled back to index to the next hole and it locates with a firm clunk. Agree that the mounting could be better as it is only screwed to the wooden bench but just checked and the lathe is till firmy bolted down and the arm bracket is screwed tight. It would have been nice to report that something was loose!
 
Peter
Jens Eirik Skogstad08/01/2012 14:06:30
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400 forum posts
22 photos
I has same setup in my lathe each time i am making geat wheel or other work such as hole for cylinder head, turbine wheel and et cetera..
 
1. rule: No play in the lathe/millinghead!
 
Check all slides and bearing in the lathe/millinghead are without play with force of hand (move the sliding or bearing in all rights of way) when you are checking the parts in lathe are without play. Check play with dial gauge if neccesary.
 
Is there chattering/vibration of lathe/millinghead when you are making the gear wheel?
Is the blank wheel/indexing disc firmly secured? (Can not rotate with force by yours hands)
 

Edited By Jens Eirik Skogstad on 08/01/2012 14:08:29

Billy Mills08/01/2012 16:14:21
377 forum posts
Did you turn off the cutter drive while the cutter was in the last slot before retracting the cutter ? or always retract before turning off. If both then it could be that the cutter is moving the few thou when driven so most slots are cut with a rotating cutter approaching the slot. On the last slot if power is removed after the slot is cut the cutter could move over whilst slowing down and shave the last tooth.
 
It could be something like this because you have proven that the equipment and the operator can produce good wheels.
 
Billy.
Phil P08/01/2012 17:42:01
851 forum posts
206 photos
In the top and bottom scans, something looks a bit odd about the tooth profile at around the 1.00 o'clock position to me.
 
Or is it just distortion in the scan ?.
 
Phil
 
johnp1011/01/2012 14:55:40
25 forum posts
4 photos
Hello Mr Hall,
I was interested to read that you are building my skeleton clock and would be interested to know how you are getting on with it
May I invite you to contact me on [email protected]
I would like to hear you experience of this project .
Regards.
John Parslow .
Stub Mandrel12/01/2012 18:46:53
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4318 forum posts
291 photos
1 articles
I have been thinking. the holes in teh wheel are OK, and its direct dividing.
 
Somewhere I think it was mentioned that the last tooth was wrong. Perhaps it was really the first?

What if the first hole is selected in a different way to the second, third etc., meaning there is some backlash that disappears - perhaps over a series of cuts?
 
Neil
RobC13/01/2012 10:35:39
54 forum posts
3 photos
Peter,
 
One thing which has not been mentioned so far (I believe) is how you are achieving your depth of cut on each tooth.
Are you cutting to full depth in one pass, or are you taking multiple passes at different depths?
If you are taking a single pass, then we are all on track with the challenges and can safely ignore the rest of this post. But if you are taking multiple passes, there is a possibility of errors creeping in depending on your technique.
I have only ever cut two gears myself, one of which is sitting on the table beside me (I am building Peter Heimann's regulator) but I have noticed a few things with gear cutting.
If you are taking multiple cuts, you should cut a full 360degrees of teeth before altering the depth of cut for the second (or subsequent) passes. If you don't there is a possibility of different width teeth due to positional issues. (Not explained well, but you probably get the idea).
As to the others who mentioned the number of teeth requiring a correct sized blank; the blank size only determines the finished tooth width, which is obviously important from a meshing perspective. A correctly working indexing system will produce the designed number of cuts for any size of blank, but the teeth will only be the correct form for the appropriate blank size. Too large a blank and the teeth will be too wide, too small a blank and the cutting process will make them too short and thin.

Rob
 
Stub Mandrel13/01/2012 22:26:06
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4318 forum posts
291 photos
1 articles
On reflection, I think Blowlamp has the answer.
 
If the lathe creeps a tiny bit during each cut, then the final tooth will be slightly too far round, or not far enough, and teh error will be distributed more or less evenly across all teeth.
 
PLEASE take his advice and fit the detent to the bed of the lathe and try again.
 
Neil
Michael Gilligan14/01/2012 08:37:23
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23121 forum posts
1360 photos
This is an interesting hypothesis; but it does beg the question as to how Peter subsequently managed to cut good wheels with the same set-up.

One simple test for Peter [in the absence of a ShadowGraph projector]
Try holding the cutter against several of the cut spaces in your bad wheel.
... if it is a snug fit, then we still have a mystery.
... if it is loose, then creep may be the answer.

One thought:
I note that the indexing arm is rather long and slender ...
Assuming that [to remove backlash] you are pre-loading with the typical "weighted string around the chuck" ... do make sure that you are putting that indexing arm in Tension, not Compression.

MichaelG.
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