Hob Helix Angle

One for John Stevenson I suspect.

I cannot reconcile the helix angle engraved on a 0.7Mod Hob with my calulations.

The hob in question is 25mm O/D (ex ArcEuro) and it states a helix angle of 1° 46'.

If I use the PC Circumference as the base of the triangle (as I think I ought) () and assume that the dedendum is 1.4*MOD (for mod

Reply to
JG
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from JG

It appears not :))

This morning I spoke with ArcEuro but found that no-one there had any idea what I was talking about so I can only assume that my figures are correct and the informaion on the hob is in error.

JG

Reply to
JG

Not had time to go thru the figures but we are talking about a difference of 1 minute 35 seconds ?

Fuck me British Rail can't even keep to the same time zone and post code don't let it worry you....................

Reply to
John Stevenson

from John Stevenson

I quite agree John but I would like to know where I'm going wrong - I'm assuming that it is me going wrong - I can hardly believe that the Chinese are in error.

After talking to ArcEuro I sent them an e-mail with a little more detail and then one to Giles Parkes with similar info.

I've just put the phone down since Giles called me after getting my e-mail and a phone call from Ketan asking if he could throw any light on the issue :))

The one thing that Giles thought was wrong was the calculation of the PCD from the OD but he conceeded that my logic was sound and when I pointed out that using his logic created a larger error he went a little quiet . . . . . he is now going to talk to Ivan Law so we'll see what he comes up with.

What a hornets nest we can stir up !

JG

Reply to
JG

In response to your email which I received today, - I consulted Giles; as well as John as we "Arc" did not have an answer.

purchased and used the hobs have all come back with positive comments. I understand that this fails to answer your question. If you feel that the information on the hob is wrong, simply return it and I will refund you the money, provided it is un-used and in its original packing.

Again from a commercial view, I agree with your comment: "I can hardly believe that the Chinese are in error", because the factory which makes this product is a premium quality blue chip operation with long standing engineering history...and before you ask, I can't ask them the question. So, I am more than happy to sell this product without question, even if your findings are proven to be correct, as I have full faith in the working quality of this factories product even if what is stated on the hob may be an error according to you, and I make these comments with the greatest of respect for your observations.

The choice is yours :) Thanks, Ketan.

Reply to
Ketan Swali

Giles just called to say that he had a healthy discussion with you, and that it is probably a printing error - 46, instead or 45 (rounded). This would make sense as the person doing the printing is different from the person who would have done the writing, and to a Chinese, 5 or 6 is just a picture, and sometimes mistaken. I apologise for any inconvenience this may cause, but I stand by the working quality and results of this factories hobs, so I am happy to suggest that you use it with confidence, should you wish to do so.

Thanks, Ketan.

Reply to
Ketan Swali

from Ketan Swali

Thanks for your input Ketan.

As you will no doubt already realize I never had any doubt about the quality of the hob and will certainly purchase more as and when I need them. I simply wanted to be sure in my own mind that my calculations were correct since I am in the process of modifying the CES Hobber to make it easier to set the correct helix angle - and ultimately the correct angles for producing left and right hand helical gears.

It was interesting to discover that the 20DP Giles has (from your other Chinese factory) also appeared to have the wrong helix angle engraved on it but using my logic to calculate the PCD from the OD, I arrived at the same figure as the Chinese so we all seem to have learned some useful lessons today.

JG

Reply to
JG

Surely the PCD of the hob should be OD -2*(dedendm + clearance)

clearance will be between 0.1*module and 0.3*module depending on specification.

Assume a mid point of 0.2*module for clearance.

That would give PCD= PI*(25-2*(1.4*.7 + .2*.7))== 71.5

With 2.199 perpendicular, that gives an angle of 1° 45' 41''.

Half a second is acceptable as a rounding error!

regards Mark Rand RTFM

Reply to
Mark Rand

I meant half a minute!!! (actually a third of one)

Mark Rand RTFM

Reply to
Mark Rand

Try 19 seconds

Seriously though what home shop equipment can get to seconds of a degree. Even with a vernier scale you need a lot of room to get this in. Looking at a Gleason bevel gear cutting machine today and the scale took up about 8" on a large diameter casting and even that could only get to 5 second divisions with the vernier

Reply to
John Stevenson

from Mark Rand

That could well be the case if there was extra clearance. The calculation of the dedendum at 1.4*MOD takes account of the clearance. Adding .2*MOD for clearance would make the dedendum 1.6*MOD

Adding .2*MOD for clearance would make the dedendum 1.6*MOD and the total tooth depth would be 2.6*MOD -- 1.82mm (for 0.7MOD) -- and the root dia. 21.36mm rather than the calculated and measured root dia. of 21.64mm.

As you correctly point out in your second posting 1/3 minute would be acceptable.

You will see from Ketan's post that we now think that it could well be a typing error by the engraving operator who would be working from a hand written Chinese original.

The important point as far as I am concerned is that I am now confident in my calculations. Whether I can set my hobbing machine to this accuracy is another matter entirely !!

JG

Reply to
JG

from John Stevenson contains these words:

I fully agree John but the question was never about setting the angle, it was purely to get my head around the method of calculating what the helix angle should be.

As it happens I won't be using a scale, I'll be taking two measurements at 100mm centers and the difference will be 100 times the tangent of the angle -- similar to a Sine Bar but with a fixed Base rather than a fixed Hypotenuse -- and I'm sure I can measure 0.05mm with a dial indicator.

Whether that will be maintained after tightening the clamps is another matter but if you don't aim for perfection you will never achieve it!

JG

Reply to
JG

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