Tolerancing of features to a center line of a thread

[Jack.Smith]

Hi Everyone,

I am working on a Clevis for a materials testing machine and want to ensure that the center of the retention pin (9mm) and the slot (5mm) are centered to the M10 fixing attachment on the base. Could I please get some advice / help on the best way to do this?

I have attached an image.

Thanks

Jack

 

[drawoh]

Jack.Smith,

As a rule, the fixturable feature of a female thread is the minor diameter. Check your Machinery's Handbook for tolerances on this. Your next option is a threaded pin that screws into your M10[×]1.5 hole. I would assume that this picks up your pitch diameter.

Your primary datum feature would be the bottom flat face. Your secondary datum feature would be the tapped hole. Your tertiary datum would be either your [⌀]9mm hole, or your 5mm slot. You need the tertiary "clocking" feature.

How accurately do you need to locate stuff? Thread minor diameters are not super accurate features.

--
JHG

 

[Doug Hunter]

Hi Jack,

If you must have the thread as a datum reference for the control of the other features, then I agree with JHG.

My recommendation would be to make the hole and the slot the datums, and check the thread position back to them, either together or individually, along with the face and/or the OD. I think it's more reliable to check the position of a thread to good datum features than to check the features back to the thread.


Best regards,
Doug Hunter
Altarium Technical Consulting

https://www.altarium.us/

 

[jassco]

I would not use the tapped hole as datum feature. If your thread hole is a precision one, generally it is inspected with thread plug.

If I were you, I would use datum features below:

Primary: OD (59mm)

Secondary: Bottom surface

Tertiary: 5.00 slots

Your 9.00 hole should be two separate holes as 59mm is too deep to drill.

Best regards,

Alex

 

[Burunduk]

Since you say that the M10 threaded hole is the "fixing attachment," it is the best candidate to be selected as a locating datum feature. It probably should be a secondary datum feature, while the face perpendicular to it, against which the component is clamped by the screw, is proper as the primary, to give the part its main orientation.

Note that without adding a note such as "MINOR DIAMETER," the datum is defaulted to be derived from the pitch diameter of the thread. There are devices designed to engage with the pitch diameter that could be used in a fixture. But, if you reference the threaded hole as a datum feature at Maximum Material Boundary, that device could be similar to a go-gauge for M10 integrated with a device that would simulate the primary datum plane. That would give you some Datum Shift, and you should consider whether the extra variation it allows is functionally appropriate.

There is not necessarily a need for a tertiary clocking datum because a Simultaneous Requirement (assuming ASME Y14.5) can take care of the relative orientation between the slot and the 9 mm hole.