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Sectional View of thread

Hi,

So I have a large piece that I want to thread (the entire region in blue), but I am not sure how much space to leave at the bottom for retraction of the blade (region in red), see above image.

For example if I have a M55 x 2.0 thread, should the little box the in red be the size of one thread?

Meaning: Width = pitch , Height = (Major Diameter - Minor Diameter of thread).

Or can it be smaller while ensuring that the entire blue region is threaded?

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    $\begingroup$ I'm not quite clear about your drawing. Is the axis of the threading horizontal? In other words, are you tapping the blue region from right to left with the red at the bottom? I'm not sure I know the answer any which way, but I think the key phrase you'd need to google is "blind tapping". The tap manufacturer likely has a recommendation. $\endgroup$ – ericksonla Jul 19 '18 at 15:27
  • $\begingroup$ yes, it is threaded from right to left and the axis of the thread is horizontal. the hole is too big to be tapped, it has to be milled. I am not sure how much room to leave at the bottom(red box) of the hole to ensure that the male threading would not meet any resistance at the left most side of the female threading (blue highlight). I had asked a contractor and he said should be smaller than the size of one thread, but no specific numbers. I trying to find a specific number, not sure if you know. $\endgroup$ – Sandon Jul 19 '18 at 21:43
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You need clearance at the bottom of the threaded hole or shorten the depth of the thread. This is to allow for the machining process and clearance for the cutting tool. Otherwise your thread will not be a long as you specified(because the machinist needed clearance, or the machinist will break a tool or drag the tool at the bottom of the hole if they are not careful.

If I interpret your picture correctly, then I think 4mm(equal to 2 x thread pitch) would be sufficient/safe.

Ultimately, talk to the machine shop and ask what tool will be used to cut the thread. A single point tool or a thread-mill? That will show you how much clearance below the thread you need. sketch See below what a single point thread mill looks like and you will see how the tool requires clearance at the bottom due to geometry of the tool

single point thread mill

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