How close does a thread pitch need to be?

[Chris DentonParticipant @chrisdenton53037]

[Chris DentonParticipant @chrisdenton53037]

Afternoon, I'm trying to make a imperial thread on a metric lathe.

Measuring using a DTI on the cross slide against a faced piece of aluminium in the chuck, the pitch is 0.0695mm out over seven turns. Would this be ok for a thread that is only going to be 10mm long anyway?

Hope you understand what I'm on about!

[Brian WoodParticipant @brianwood45127]

Hello Chris,

In practical terms the thread you will make is 10 microns in error per thread. For a short attachment of 10 turns I don't suppose it will be significant as the accumulative error will be 100 microns which in old money is 4 thou. It might get tight at that, push it any further and it will try to lock up.

A dodge I have used is a little fine valve lapping paste to make such a mismatch fit; much will depend on your application and whether you want to 'deface' the components. The purists will no doubt go into melt down with such heresy, only you know if that isn't acceptable

Regards

Brian

[Anonymous]

It depends on the tpi of the thread, which isn't specified – Andrew

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Brian Wood on 28/11/2014 17:12:47:

A dodge I have used is a little fine valve lapping paste to make such a mismatch fit; much will depend on your application and whether you want to 'deface' the components. The purists will no doubt go into melt down with such heresy, only you know if that isn't acceptable

.

I once watched as a Trials Engineer adopted the same technique when the thread he had tapped in a fixture didn't quite accept the QA man's very expensive Gauge. The paste was applied to the Gauge thread and it was used to "finish" the female thread [in Aluminium Alloy].

Anyone who understands lapping will realise the likely result; but the QA man either did not, or he turned a blind eye … I shall never know.

MichaelG.

[Chris DentonParticipant @chrisdenton53037]

Thanks, it's 16 tpi.

[Neil WyattModerator @neilwyatt]

> The paste was applied to the Gauge thread and it was used to "finish" the female thread [in Aluminium Alloy].

I bet there was a significant increase in the number of accepted threads after that

Neil

[jason udallParticipant @jasonudall57142]

Shudder. …using a screw gauge as away of finishing the component. …shakes head…

[Clive FosterParticipant @clivefoster55965]

Easy solution is to cut the thread a touch deeper introducing enough backlash to accommodate the error between the male and female threads. Most shop or DIY shed bought nuts and bolts have more clearance than you will need anyway so you won't be doing anything radical.

Clive

[Brian WoodParticipant @brianwood45127]

Hello again Chris,

Now that you have told us the pitch involved some maths becomes possible.

The error you state, in percentage terms is only 0.63%; I really wouldn't think that matters very much.

Regards

Brian

[Nick HulmeParticipant @nickhulme30114]

If you are using changewheels to do the job you just need the N-ThreadP software and a 127 tooth gear.

[Nick HulmeParticipant @nickhulme30114]

The reverse should be possible.

[Michael GilliganParticipant @michaelgilligan61133]

Probably way off-topic [so, what's new ?]; but …

those with an interest in thread accuracy might like to read this, from 1896.

MichaelG.

[Author]

Posts