Tapping drill size for small diameter thread

[magpens]

We often hear the following rule for calculating a tap drill size to match an existing threaded device:

"thread diameter minus thread pitch"

For example, for a M10 x 1mm threaded device, the thread pitch is 1 mm, and so the required tap drill size would be 10mm - 1mm = 9mm.

I am wondering if this rule is reliable for small diameters, in particular at 6 mm ( M6 ) and below (the size range for some nib feeds).

I have been working with tapping a hole for a stated M4.2 x 0.35mm threaded device and I have the tap of this stated size.

Having some difficulty with the threaded device not fitting the hole threaded using the above rule.

Based on the rule, I calculate the tap drill as follows: Tap drill size = 4.2 - 0.35 = 3.85 mm, or 0.152" which matches a #24 drill bit.

The tapping process goes just fine, but the resulting tapped hole is too tight for the intended threaded device to fit.
Because the threaded device is brass, I do not want to risk forcing the insertion for fear the threaded device would be destroyed.

There is the possibility that the specs for the threaded device are incorrect.
There is also the possibility that the tap being used is not the correct one, although the marking on the tap is correct numerically.
(I do not have the ability to check the threads on the tap, being as fine as they are.)
There is also the possibility, I suppose, that the threads are burred so that the device cannot smoothly thread in. . Have tried several times without success.

One train of thought would be a "trial and error" method, trying drills of successively larger sizes.
I have done this and I do not like where this is leading, which is to a tap drill size that is getting very close to the OD of the tap that I am using.

I believe that the "rule" quoted at the beginning of this post is a kind of "rule of thumb".
So I am questioning its validity for small diameter threads.

I would appreciate comments/suggestions from anyone "in the know". . Thank you.

 

[FGarbrecht]

Don't know the specific answer to this question (reliability of the rule of thumb for small holes), but I have had similar problems tapping a section for Bock#5 triples. At this point, I just use the standard rule of thumb for tap drill size, tap the hole as usual manually, then run the tap in and out of the hole under power on the metal lathe a few times to open things up a bit more. Sometimes I also have to use a thread chaser (home-made) on the Bock housing, but can usually get things to fit at this point.
Also don't forget that if you are using a tapered bottom tap you need to tap past the tapered area to get a clean uniform diameter hole that will be big enough.

 

[mredburn]

Have you measured your drill bit to verify the correct diameter. I find dril bits to be undersized from the stated dimensions

 

[budnder]

Here's the math:

Tap drill size calculators

Tap drill size calculators for unified national imperal and metric threads

theoreticalmachinist.com

Note that there is an allowance you can increase for example, when using metal vs. plastic, which would result in a slightly larger bit in your case.

 

[More4dan]

The rule is correct in some respects but results in a 100% thread case. Most of the time it is recommended to have a 75% thread engagement maximum and a 55% minimum. So I would use 75% of the pitch in your calculations. It you don't have the exact drill bit, make sure it results in at least 50% thread engagement.

Here is a great calculator for drill bit selection. You can manually enter the diameter, pitch, and % and it calculates the diameter and closest drill bit.
You can also enter your nearest drill bit and it will calculate the thread %.

Tap drill size calculators

Tap drill size calculators for unified national imperal and metric threads

theoreticalmachinist.com

For a 4.2mm x 0.35
#23 gives 63.4%
5/32" gives 50.1%

#24 yields a 74.6% thread which should have worked. I would try the two larger bits in a piece of scrap to see which works best.

I would also measure the OD of your tap and compare to the OD of your threads.

Danny


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[More4dan]

I've been checking a bit and the metric pitch actually has the thread engagement of 77% built in to the size designation. So a 10mm x 1.0 using a 9mm drill bit gives a 77% thread not a 100% as I wrongly pointed out.

Danny


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[howsitwork]

The rule is a basic guide but not exact. As Danny says you can get away with much less % thread engagement. To explain that - with BA threads you can work quite comfortably with 60% engagement and they will not fail you.

To possibly solve your problem above try , after tapping running the tap in and out a few time to ensure you debur. Then run your tapping drill BY HAND back into the hole. This will remove the crests of your formed threads as some metal is extruded as the tap cuts ( it flows to form the shape of the thread) . You then run the tap through again recutting the thread and the resulting threaded hole,will be slightly larger than it was . Try this out on some spare brass first rather than risk ruining a pen. It might give you the clearance you need .

The above tips about drill sizes and taper vs bottoming tap are all valid to so check them as well but this " fix" might help at a push.

 

[Curly]

If it is a plastic blank you are tapping try, after cutting the threads, putting it in the freezer for a little while, then run the tap into it manually. The cold might shrink it just enough for the tap to cut a little more creating a looser fit. I haven't tried it but it is a simple cheap experiment.

 

[More4dan]

You might even try lightly sanding the threads ID and rerunning the tap to create some clearance.


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