GlossaryKind of term: Manufacturing termThread
What is a screw thread?
A thread is a helical profile on a shaft or in a hole, indicated on the drawing by type, size, pitch and tolerance class, such as M8x1-6H.
A thread is a helical ridge of constant profile on a cylindrical or conical surface: external on a shaft or bolt, internal in a hole. The drawing carries the designation: type and nominal diameter (M8 for ISO metric), the pitch where it departs from the coarse series (M8x1), the tolerance class (6H internal, 6g external) and the thread length or depth. Without a stated pitch, an ISO metric thread is coarse, for example 1 mm for M6 and 1.25 mm for M8. A left-hand thread carries LH in its designation.
How is a thread made?
Internal threads are made by cut tapping, form tapping or thread milling; external threads by turning, thread milling, rolling or whirling. Which method fits depends on size, pitch, material, through or blind, the clearance below the thread, quantity and machine. Form tapping needs a larger tap drill than cut tapping and a material ductile enough to be formed. Thread milling lets you adjust the pitch diameter through radius compensation and gets closer to the bottom of a blind hole than a tap with a long chamfer.
What does the standard say about a thread?
For ISO metric threads, ISO 261 and ISO 262 give sizes and pitches, ISO 965-1 the tolerances and classes, and ISO 6410-1 the simplified representation on drawings. Other types, such as pipe threads, trapezoidal threads or inch threads, follow standards and designations of their own. A tolerance class without a length or depth requirement leaves open how far the full thread has to run; the drawing or a company standard should state it.
How does a thread work in practice?
A blind thread in a tough material needs more care than a through thread in free-machining material: chip evacuation, chip breaking and the risk of a broken tap in a nearly finished part. How you carry that risk in the estimate is your shop's choice. Inspection is usually by thread gauge, go and no-go, plus a depth check for a blind hole. If the depth or the class is missing from the drawing, ask.
What is a thread often confused with?
A thread is not the tap drill hole: that is the bore in which the thread is made. Nor is a thread callout an operation: M6 says what the hole must become, tapping or thread milling is how you get there. And 6H is not an ISO 286 tolerance class like H7: threads have a tolerance system of their own, set in ISO 965-1.
How do you see a thread in Blake?
In Blake the thread callout sits with the part as a proposal in the drawing's own notation, which you confirm or correct. Drilling and tapping are included in the machining, per piece and per batch; the result is labelled as an estimate. More on estimating.
Frequently asked questions
What do I do when the thread depth is not on the drawing?
Ask. In a blind hole the depth decides the method, the drilling depth and the risk. If you cannot wait, record the depth you assumed, so it shows on the quote.
Tapping or thread milling?
Tapping is quick at small and medium sizes in free-machining material. Thread milling often suits large sizes, hard or tough materials, costly parts and blind holes with little room below. A broken thread mill is usually easier to remove than a broken tap.
What tap drill size does M6 need?
For cut tapping an M6 coarse thread, ⌀5 mm is the usual size. Form tapping needs a larger hole, around ⌀5.5 mm, depending on tap and material. Follow the tap maker's recommendation.