[MCImes]

Hello,
For those that thread whirl regularly, I have a question. Do you calculate the helix angle (or ‘lead angle’) based off the Major Diameter alone, or the Mean diameter between the Major and Minor.

For example, say you have a bone screw with:
6mm major
4mm minor
3mm pitch
Single lead

Using only the Major (6mm), I get 9.04*
Using Mean (5mm) I get 10.82*

Which do you use? Based on my experience, I most often use Mean Diameter, but sometimes the Major Diameter Only lead angle works better (like when a thread has a very small crest radius created by a full form insert) .

Anyone else have thoughts or experience with this?

[PixMan]

Split the difference, or thereabouts. It's really a tiny difference and swinging the head around you aren't going to see increments on the scale of anything tighter than 1º, many only show every 2º. The side clearance angles ground on the insert form should easily compensate for the remainder.

I forgot to mention: Any good maker of inserts (they are all custom made) will give you the helix angle to set on the attachment for a given insert. This is critical to maintain the exact form of the thread you are milling. As far as I know They make a calculation of the mean helix angle, grind the top of the cutting edge perpendicular to that angle, then grind the form of the thread onto the blank.

[cogsman1]

You do want to use the "Mean" to calculate the helix angle to set the head to. I always use an indicator to set it perfectly. FYI, any amount of error you have at setting the angle will effect not only the form but the amount of tolerance you will have between the Major and the Minor dias..

[asn]

Easy Whirling app from Apple app store will help you calculate the parameters.

[asn]

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