A very interesting topic, really this is not as complicated as it seems, and there is a simple method to know which drill bit to use for making internal threads with a tap.
I remember very well that when I was studying in the technical training school, the teacher who was very skilled and had a lot of experience in the field of machinist taught us this topic in the most difficult way possible, but today I realize that the topic of Which drill bit to use for making threads, is one of the easiest things to calculate when it comes to the field of machinst.
Drill bit for metric threads
The following steps show how to use a drill bit for making threads:
- Differentiate between threads in millimeters and threads in inches.
- Know the diameter and pitch of the thread.
- Apply the following formula to determine the exact diameter of the drill bit that should be used:
B = D – P
Where:
B: Diameter of the drill bit to be used
D: Diameter of the tap or screw to be made
P: Pitch of the tap or screw to be made, both in millimeters and inches.
Let’s see an example with a thread in millimeters.
What drill bit to use for making a thread with a diameter of 10 mm and a 1.5 mm pitch?
The first step is already given, we are going to make a thread of the metric system, that is, we are going to work in millimeters.
The second step is also given, the diameter is 10 millimeters and the thread’s pitch is 1.5 millimeters.
Now, let’s move on to the third step:
B = D – P
Replacing values we have:
B = 10 – 1.5
B = 8.5 millimeters.
So, we should drill with an 8.5 mm diameter drill bit.
Drill for inch threads
Let’s see an example with a thread in inches.
What drill bit should I use to make a 3/8 in. thread with 16 threads per inch?
The step in a screw in inches is determined like this:
P = 1/N,
where,
P = step
and N = number of threads per inch
So to find the step of the proposed exercise, I must apply the previous formula
P = 1/16
Then B = D-P
B = 3/8-1/16 = 5/16
We must drill with a drill bit of 5/16 inch diameter to make a 3″/8 NC thread.
You may also be interested in: How to Make the Square Thread on Lathe Machine.
As you can see, the question “What drill bit should I use to make threads?” will no longer be a problem as with the information seen in this article, calculating the drill bit is child’s play.
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