Standard for Internal Thread / Tapping Length

whtan02

Involved In Discussions
#1
Hello Everyones,

Need help.

Do we have a standard for internal tapping hole length, for example if the drawing only mention M2.5, then what is spec & tolerance for the thread length?

Thanks for the help.
 
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H

Hodgepodge

#2
Unless the tapped hole is a thru hole, the depth must be specified. If it isn't, then contact the design engineer or customer for clarification.
 

whtan02

Involved In Discussions
#3
Unless the tapped hole is a thru hole, the depth must be specified. If it isn't, then contact the design engineer or customer for clarification.
Thanks for your reply, Hodgepodge.

It's a thru hole. But if the taper / chamfer at the starting of the thread is bigger, then it will reduce the thread length, right? So in this case, should we specify a min. thread length requirement? If so, then what & how is the standard here?

Thanks a lots.
 
#4
Any chamfer(s) or lead in(s) (both sides) should also be dimensioned, to prevent reduction of the overall amount of threading. A minimum full thread may be specified, but that might not be as helpful as specifying the chamfer angle/depth.
 
Last edited:

whtan02

Involved In Discussions
#5
Any chamfer(s) or lead in(s) (both sides) should also be dimensioned, to prevent reduction of the overall amount of threading. A minimum full thread amy be specified, but that might not be as helpful as specifying the chamfer angle/depth.
Thanks for all the replies thus far.

Anyones can advise on the standard for min. thread engagement length, how do we specify such a spec.?

Thanks again!
 
H

Hodgepodge

#6
The requirement would be "thru". It is either thru or it isn't. This does not need a specification. The chamfer has a size and is measured separately. If the chamfer was too large, then the chamfer would be out of tolerance, not the thread.
 

Jim Wynne

Staff member
Admin
#7
Thanks for all the replies thus far.

Anyones can advise on the standard for min. thread engagement length, how do we specify such a spec.?

Thanks again!
It would be helpful to know why this is a concern. In addition, it's difficult to know what you mean by " taper/chamfer." If you're referring to an intentional countersink, its diameter, depth and angle should be specified. If you're referring to the natural small chamfer that results sometimes from drill entry, that's a different story.

The length of full functional thread engagement depends on the needs of the application. It's generally understood that a "no go" thread plug will enter the first few threads. I've attached a paragraph on the subject from an old Machinery's Handbook that I photocopied a few years ago when I was having a discussion on the subject with a customer.

Tell us more about your application and concerns.
 

Attachments

whtan02

Involved In Discussions
#8
It would be helpful to know why this is a concern. In addition, it's difficult to know what you mean by " taper/chamfer." If you're referring to an intentional countersink, its diameter, depth and angle should be specified. If you're referring to the natural small chamfer that results sometimes from drill entry, that's a different story.

The length of full functional thread engagement depends on the needs of the application. It's generally understood that a "no go" thread plug will enter the first few threads. I've attached a paragraph on the subject from an old Machinery's Handbook that I photocopied a few years ago when I was having a discussion on the subject with a customer.

Tell us more about your application and concerns.
I'm refering to the natural small chamfer that results from drill entry.
The tapping hole is on the casing of the DC motor used for installation on the customer's unit. There are 2 thread holes.
The issue here is we receive one incoming lot with bigger chamfer (also bigger top diameter) from previous lots. Checking with 'Go' & 'No Go' gage result is still Good. Question here is are we good to release this lot to production? Is it enough just by checking thread size by go no-go gage. Immediately what comes to me is the concern on the thread length or the engagement length, as bigger chamfer naturally will reduce the thread length.
As we have the thread length control in the Unified thread (inno. of thread per inch), so shouldn't we have a standard as well for the min. thread length for Metric thread?

Since the product drawing print does not specify control dimension/tolerance on the chamfer size, nor the top diameter size, just M2.5 & pitch 0.45.

Maybe we can judge by tightening torque & loosening torque, but I'm not sure it matters because I think loosening torque will be depending on the amount of the input of tightening torque, regardless to the thread length.
 
#9
Usually, when tapping a hole for a thread, the drill doesn't leave much, if any chamfer. The tap leaves a natural part chamfer, from the helix lead-in. Since this also can cause a sharp edge, which may become distorted and jam up the mating threaded part (screw, bolt etc), it is usual to chamfer the lead in.

It doesn't do away with the sharp edge completely, but it does provide a lead-in for the mating thread. This will provide for a more precise thread engagement and cross-threading is less likely. (see, I was awake when at engineering school!:lol:)

It's good practice, therefore, to call out a chamfer, even if it is only to "break edge" and not a fully dimensioned call out.
 
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