Traverse feed when hobbing of helical gears?

If one is hobbing helical gears, and the direction of traverse
is parallel with the cutting teeth of the hob, and the blank
is set over at the required helix angle, and the rotational
speed of the blank is adjusted by the cosine of the helix
angle, then is it necessary to have an automated traverse
feed at a specified rate, or can one do it at manual traverse
as with spur gears?
Reply to
Orator For Decency
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With true helical gears, the traverse is done in synchronization with work rotation. With manual traverse for each tooth, the shape of the tooth will be flat on the top rather than following the curve of the work piece. That being said, for mild helical angles, it will probably work, sort of. They will wear fast, though. Better to have them hobbed on a hobbing machine.
Mark
Reply to
M
Perhaps you misunderstood - a hobbing machine is in use but does not (yet) have automated traverse, just rotational coupling of the blank to the hob.
Spur gears are produced OK by this method, so, based upon the travrse being parallel to the action of the hob, why shouldn't helical be also OK, subject to the rotation of the blank being adjusted by the cosine of the helix angle?
M wrote:
Reply to
Orator For Decency
If you are talking about the mods Giles Parkes has done to the CES hobber then yes it will need an automated feed.
Giles knows this and has it all in hand to fit. Another member of the URME board has written him the software to do all the calculations.
-- Regards,
John Stevenson Nottingham, England.
Visit the new Model Engineering adverts page at:-
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Reply to
John Stevenson
Hi,
I've never cut a gear so you should not take this as gospel but the following is what pertains AIUI :
If you have the spindle set over wrt the table travel by the helix angle of the hob then all you need for cutting spur gears is that the rotation of the blank is the same as the spindle rotation speed divided by the number of teeth. This has the effect that as the hob tooth "moves down" by one tooth space so does the blank and so the teeth are cut cleanly and to size.
In the case of helicals, AIUI the setup is the same with the only difference that you additionally must angle the blank by the gear's helix angle wrt the table travel direction. The same relationship in respect of the rotation speeds applies : AFAIK there is no sin or cos term in the rotational gearing it's still just 1 / (number of teeth) as for spur gears.
John Stevenson's website had good details but the link didn't work when I tried it a moment ago.
Hth,
Reply to
Boo
Yes, I misunderstood your method. I thought you had mis-used the term "hob". If I understand what you are asking, then the feed is an integral part of producing the helix angle, and it would not be possible to generate it by hand. I believe a worm and worm gear would be different. There are a number of groups out there that can answer gear questions, and the members are much more experienced than I am. Here is a link:
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(You have to wade through the spam to get to the ham)
Mark
"Orator For Decency" wrote ...
Reply to
M
That appears to match my own analysis except in one respect. As you say, "This has the effect that as the hob tooth "moves down" by one tooth space so does the blank and so the teeth are cut cleanly and to size."
A movement of the blank at right angles to the line of cutting and traverse of the hob teeth, is no longer co-axial to the axle of the blank - the cosine factor comes into play (or, rather 1/cos) so that the blank rotation is transformed into the movement by one tooths space
"Boo wrote:
Reply to
Orator For Decency
Just rethinking that through over lunch - yes - I'm sure that's correct And I'm unanimous in that! ("Are you being served?"?)
For the blank to rotate one tooth's distance at right angles to the hob's teeth, the hob must be rotating at 1/cos times the RPM used for spur gears.
Orator For Decency wrote:
Reply to
Orator For Decency
Whereas that would be generally true, I posit that it isn't true for the SPECIAL CASE where the hob is traversing through the blank in parallel with the line of action of the cutting teeth. ie, set the hob over by its own helix angle (not to be confused with the helix angle that you're trying to cut) and with the blank set over at the helix angle that you _ARE_ trying to cut, then manual feed, as for spur gears should be possible.
A picture paints a thousand words, but I ain't got one!
Which reminds me that a new S.I. Unit has been introduced, the "milli-Helen" = it is the amount of feminine pulchritudinity that is neccessary to be able to launch one ship.
M wrote:
Reply to
Orator For Decency
Pardon me for labouring this but I want to be right (And if I'm also proved to be right, so much the better, but that's completely unimportant, as always!).
1. The hob is set over at its own helix angle so that it traverses in a direction parallel to the cut of the teeth. We agree on this - this is the essence of being able to cut normal spur gears with manual feed.
2. The blank must rotate one tooth's distance according to the way the hob's teeth are cutting. We agree on this.
distance is at right angles to the direction of traverse.
3. BUT BUT BUT the axis of the blank is set over at its own helix angle and therefore to achieve the rotational speed of (2), the blank's rotational speed must be increased by 1/cos. We DON'T agree on this, but it seems to me from a vector analysis of speeds that it must be so.
4. For spur gears, the blank must be geared down by 1/N from the hob. We agree on this.
5. From (3) and (4), therefore, the blank must be geared down by 1/(N * cos(Blank's Helix Angle)), and because all cosines are less than 1, this will mean a slight increase in rotational speed of the blank. I don't think that we agree on this (Yet?!).
So, resolving vectorially the movement of the teeth at right angles to the direction of traverse we'll end up with (1/(N * cos)) * cos = 1/N
6. However, if we could put ourselves in the position of the teeth (gaps between the teeth?) being cut, our appreciation of what is happening to us is the same irespective of whether a spur gear or a helicla is being cut and that therefore manual feed is possible, (With, of course, the proviso that manual irregular feed will result in a difference of quality across the face of the teeth)
Boo wrote:
Reply to
Orator For Decency

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