Deep hole tapping becomes a different manufacturing problem when an M6 thread must run through 42 mm of 5082 aluminum. In this project, the drawing clearly called out M6 THRU. Our available tapping setup could reliably produce only 20 mm of usable full-form thread from one side. Instead of stopping for customer approval, we tapped from the side where the callout appeared and left the opposite entry unthreaded.
The parts could not be assembled as intended because the customer inserted the screw from that opposite side. The original requirement was a continuous 42 mm thread, although the customer was also willing to accept threads tapped separately from both ends with a discontinuity in the middle.
The customer chose to complete the additional tapping locally, and we issued a $100 credit. The drawing was not unclear. The problem was that we changed a through-thread requirement into a one-sided partial thread without obtaining approval.
Confidentiality note: Part images, geometry, quantity, and other identifying details have been omitted. This case study discusses only the material, thread specification, machining depth, and corrective action.
Project Requirement and Actual Result
The drawing specified:
- Material: 5082 aluminum
- Thread callout:
M6 THRU - Total threaded length: 42 mm
- Original requirement: one continuous M6 through thread across the full 42 mm length
What we delivered:
- Tapped from only the side shown by the drawing leader
- 20 mm of usable full-form thread
- No complete thread through the remaining length
- No thread entry on the side used for assembly
Even if the 20 mm section met the local thread dimensions, it did not satisfy the drawing or the assembly requirement. A dimensionally acceptable partial thread was still a functionally nonconforming feature.
Why Deep Hole Tapping Became Difficult at 42 mm
A 42 mm M6 thread has a length-to-diameter ratio of 7:1. Through hole tapping generally provides better chip evacuation than blind-hole tapping, but this depth still creates significant process risk.
Stable tapping depends on more than the workpiece material. The process must also account for:
- Tap flute and neck length
- Tool overhang and rigidity
- Chip evacuation through the hole
- Lubrication at the cutting zone
- Alignment between the tap and pilot hole
- Machine control of feed, speed, and torque
With the equipment and tap combination used for this project, 20 mm was the depth at which we could consistently produce usable full-form threads. That was a limit of this specific setup—not a universal limit for M6 threads.
The correct response was not to force the tap farther and accept a high breakage risk. Once we knew the available process could not meet the specified thread depth, production should have stopped until the customer approved another method.
What M6 THRU Means for Manufacturing
For this part, M6 THRU required an internal M6 thread through the complete 42 mm feature. It did not mean that only the pilot hole had to pass through the part, and it did not authorize a 20 mm partial thread from the side carrying the drawing callout.
A leader identifies the feature being specified. Its location on the drawing does not establish the functional thread entry side. With a compliant through thread, the customer should be able to use the feature from the required side.
This distinction matters for any deep threaded hole. When the available process cannot meet the drawing, the manufacturer must disclose the limitation and obtain approval for a defined alternative. In this case, the alternatives could have included:
- A process capable of producing the continuous 42 mm thread
- Threads tapped separately from both sides, with an accepted discontinuity
- A specified partial thread from the actual assembly side
- A clearance hole with a bolt and nut, if the design allowed it
Each option can be valid, but the manufacturer cannot choose among them without understanding the assembly.
Two-Sided Tapping Is Not a Continuous Through Thread
Tapping from both sides was a workable recovery option in this case, but it does not automatically create one continuous helical thread.
When a tap starts independently from each end, the two thread helices may not align where they meet. The center can contain a discontinuity or a short unthreaded section. If one screw, threaded rod, or gage must travel through the entire 42 mm length, this method is not equivalent to a continuous thread.
It can still work when separate screws enter from opposite ends and neither screw reaches the center transition. Our customer confirmed that this condition was acceptable, which made local tapping from the functional side a practical corrective action.
The important point is not whether the two thread segments are geometrically continuous. It is whether the approved result supports the required engagement length, load, and assembly method.
Corrective Action
After the issue was identified, the customer chose to tap the part locally from the side used for assembly. This created usable threads from both ends while allowing a discontinuity in the center.
Because the customer had to perform the additional work, we issued a $100 credit.
The credit addressed the immediate rework, but it did not replace a proper root-cause review. The issue could have been prevented during DFM review or detected before shipment by verifying thread entry and required engagement from both sides.
What We Changed in Our Engineering Review
This deep hole tapping case changed how we review long through-thread features before production. Our review now needs to establish:
- Whether the drawing requires a continuous through thread or a specified partial thread
- The functional entry side and required screw engagement
- Whether the available equipment and tooling can produce the full usable thread length
- Whether an extended-reach tap, a suitable through-hole tap, thread milling, or another process is feasible
- Whether threads from both ends are acceptable if they do not align in the middle
- How the approved thread condition will be inspected before shipment
Any manufacturing method that differs from the drawing must be documented and approved before machining begins.
For this project, a short message explaining that our setup could reliably produce only 20 mm of full-form thread from one side would have been enough. The customer could then have approved two-sided tapping before production, avoiding both the assembly problem and the local rework.
Final Takeaway
The visible problem was that we tapped from the wrong side. The deeper issue was that the drawing specified M6 THRU, while we produced only 20 mm of thread and did not obtain approval for the deviation.
Equipment and tooling limits are normal in deep hole tapping. The preventable risk is continuing production after recognizing that the planned process will not meet the drawing.
For long through threads, confirm the usable thread length, functional entry side, required continuity, acceptable alternatives, and inspection method before machining. That review is far less expensive than customer-side tapping, rework, or replacement parts.
If your design includes a long or deep threaded hole, send the drawing for a DFM review before production.