UNO Port Thread Oversize

[Themanualguy]

Gday Gents

I’ve done a search but all replies were machine tapping, not related to the method I’m using.

Customer has knocked back some cylinder head caps with 3/4” UNF ( UNO Port ) threads that I made recently due to a sloppy fit in the threads.

I tapped the thread by hand using a brand new Sutton 3/4” UNF Gun Tap, in my lathe using a centre to guide and keep the tap square.

I’ve inserted a new M36 threaded boss and tig welded it in and re-drilled and re-tapped the port threads but the same thing has happened.

Customer requested I try their taps.
Decent condition HSS Sutton Hand taps. Result is no better.

I’ve done some measuring of the various taps and sample fittings using a thread mic for pitch diameter.

Sutton Gun Tap 18.02mm pitch diameter
Sutton Hand Tap 18.06mm
Standard 3/4” UNF pitch diameter according to engineers black book 18.01mm
Sample fitting 17.84mm

I’ve made a simple 4140 thread “gauge” using a 16 TPI full form Iscar insert and threaded until the overall OD measured 19.05mm, but the resulting pitch diameter is 17.92mm.

I’ve used a dial indicator to measure the side to side “play” of the various male threaded items in the holes.

Generally all have 0.3mm radial play measured right next to the mouth of the hole, with indicator tip square and central to the tap, thread gauge and hex head fitting.

From the on set it’s easy to say their fittings are sloppy but I’ve been doing UNO tapped ports for them for years without an issue. I will ask if perhaps the supplier has changed but I don’t want to accuse them of buying junk fittings.
They have requested I screwcut them using an internal threading bar which I will do for this job, but I don’t want this to set a precedence for the future that a UNF Tap won’t produce a thread with enough precision to be used as a hydraulic port.

Can anyone shed any light on;
- why are none of these taps cutting to size.
- any tips to achieve higher precision tapped holes for hydraulic ports
- any tips regarding making your own thread gauges using full form threading inserts.

Material is plain carbon steel.
Using threading oil as lube.
Using standard bar type tap wrenches
Manual lathe
Hand tapping, reversing tap every 4-5 turns to break at any chips.
I don’t have CNC so threading Milling is not an option.
Part is 13” x 13” x 3” thick hydraulic head plate so screw cutting a 3/4” UNF thread in the bore is not an easy feat using my large lathe.
Thanks Gents

 

[Hopefuldave]

4 or 5 turns is too many, in my experience I reverse when hand tapping every turn for a half-turn with hand taps - and if the tap guide is in an offset tailstock it'll cut oversize, have you checked the alignment?

Another thought - the 19.05 diameter for a 3/4 thread you used to make your plug gauge is the max allowable, go by the pitch diameter over wires, the thread crest flat isn't the crucial diameter?

The fitting pitch diameter may be out of spec though, at 17,84 (0.7024 Vs 0.7094, 7 thou"/ 0.18mm undersize, just a smidge undersize for a class 2) so that MAY be the problem... Useful calculator at
Unified Screw Threads and Tolerances (Inch)

What class of fit?
Class 3 male should have a minimum pitch diameter 0f 0.7056 -17.922mm (so you're a tenth under, not bad for a home-brew thread gauge!), female max of .7143 = 18.143mm - the female is harder to measure, admittedly, but if your gauge is fitting without slop it's pointing at the undersize fittings they're using - IF they're supposed to be class 3 fit.

Dave H. (the other one)

 

[john.k]

Are the taps a tight fit in the finished and cleaned hole?......If so I would bet they have either Indian or Chinese fitting which are either made to absolute minimum spec,or undersized out of spec.....Fitting are considered to be (by the third world) a zero stress component that any workshop set up in a cow shed can make ,and onsell to a middleman.....Ive had tractor connection parts from India where the threads were so loose a 1" UNF nut stripped on a pivot stud..........There are so many struggling hydraulic shops now that stock nothing but junk third world product.

 

[gregormarwick]

A new calibrated w/ cert 3/4" go/no-go plug gauge is under £100 over here. Presumably you can get the same for similar cost over there. Worth it for the peace of mind?

Alternatively, the min. P.D. of an internal 3/4-16UN is 18.019mm, max is 18.184. If you're making gauges you need to use those sizes, not the P.D. of an external.

Also, don't use a full profile external insert to make a gauge if you're making a pitch diameter gauge. Use a partial profile insert with a sharp point so that there is no chance of binding on the minor, you can also truncate the OD of the gauge a bit too to make sure there is no binding on the major either.

If you make a plug gauge with a pitch diameter of say 18.2mm and it goes in, then you know there is a problem with your threads.

 

[MattiJ]

Are the taps a tight fit in the finished and cleaned hole?......If so I would bet they have either Indian or Chinese fitting which are either made to absolute minimum spec,or undersized out of spec.....Fitting are considered to be (by the third world) a zero stress component that any workshop set up in a cow shed can make ,and onsell to a middleman.....Ive had tractor connection parts from India where the threads were so loose a 1" UNF nut stripped on a pivot stud..........There are so many struggling hydraulic shops now that stock nothing but junk third world product.

Also not unheard that cheapo fittings come with wrong thread flank angle. UNF/UNC with 55 degree angle and BSPT with 60 degree angle

 

[Themanualguy]

Thanks for the replies gents,

Tailstock alignment is OK
All taps after tapping are a loose fit, so I’ll try getting the tap square, removing the tailstock and tapping with backing off every half a turn and see if it makes any difference.

Very interesting info about the pitch diameters etc, thanks a lot for the help lads. I’ll report back later with how I go tapping another hole....

 

[Cole2534]

Never heard them called UNO before, only ORB.

I'll be damned, learned something new today.

 

[Themanualguy]

Never heard them called UNO before, only ORB.

I'll be damned, learned something new today.

Could be an Australian trade name or standard

 

[Themanualguy]

Ok so;

Re-tapped a hole under the best conditions possible.
Backing off half a turn
Using threading oil
No sideways force on the tap
Straight and round drilled hole.

Then I made a new male thread gauge, exactly on 18.02mm pitch diameter.

The new gauge which is exactly on the bottom size tolerance for a 2B thread, is a neat fit in the tapped hole, and is a neat fit in basically all the other tapped holes I’ve made for testing etc.

Long story short it is their fittings that are 0.2mm under the bottom tolerance for 2B fittings.

This fitting was a sample of a thread that is on a transducer that goes inside this cylinder, so I am off to the customers shop now with my gauge and thread mic to measure this transducer thread.