1/2 INCH UNC TAP PROBLEMS

[niall Y]

I've just purchased a length of 1/2 inch UNC studding, to make a mandrel for use on my wood lathe. I also purchased some nuts and washers from another company, and owing to the miracle of standardisation, they all fitted. I also bought some taps so that I can fix an aluminium boss at the end of the mandrel for engaging with the live-centre of my lathe.

When I checked one of the nuts with the tap, it would not fit. A quick check of the invoices showed that everything was 1/2 inch 13 tpi. Then on to check the components which all seemed good. I again tried to introduce the tapered tap into the nut but I could not get it to engage. I eventually managed to start off the nut ( a spare one ) on the back end of the tap before going on to re-cut the thread. Trying the nut on the studding proved that it still fitted , though was slightly more wobbly than the others. It will, however be OK for its intended purpose

So I have to ask the question - which of these components is the faulty one? The threaded rod has a diameter of 12.6 mm and the tap a diameter of 12,82 mm. I doesn't seem a lot but it's enough for it not to fit in, what I assume to be, a standard nut.

 

[J-G]

From the (limited) information provided it seems as though all the dimensions are within the specified tolerance. Though why you are quoting metric dimensions for Imperial threads causes some concern and the OD is by no means the most important dimension.

Depending upon the tolerance class (1A,2A,3A), the OD of the Studding should be within the range 0.4822" to 0.4985" ( 12.24788 - 12.6619mm) and it clearly is at 12.6mm. (0.496063")

The Tap - at 12.82mm = 0.504724" - may or may not be within tolerance. The OD of internal threads is only specified as a minimum which in this case is 0.5" = 12.7mm. To check further you would need to measure the 'Effective' or 'pitch' diameter. This should be in the range 0.45" - 0.4597 (class 1B) (11.43 - 11.67638mm).

Assuming that the thread form on the tap is 'perfect' then, since the addendum should be 0.025", the effective dia. would be 0.454724" (11.55mm) - again, within tolerance.

If the nuts have been manufactured at the lower limit of class 1B they could be nearly 5 thou smaller than the Tap which would certainly account for the fact that the tap didn't just pass straight though.

 

[RichElfin]

As far as I'm aware a 1/2" tap should be 12.7 mm so it's probably the tap.
Maybe do a couple of test hole in some steel it may knock the burrs of that could be the result of manufacture.

 

[J-G]

As far as I'm aware a 1/2" tap should be 12.7 mm so it's probably the tap.
Maybe do a couple of test hole in some steel it may knock the burrs of that could be the result of manufacture.

It seems that you didn't understand any of my explanation regarding 'tolerance' in manufacture. or in the 'Standard' that specifies the minimum OD of an internal ½" UNC thread to be 0.5" (12.7mm) but does not specify a maximum.

'Burrs' ??? Even the lowest quality 'cut' thread carbon steel taps are unlikely to have 'burrs' and all HSS taps will be 'ground' rather than 'cut'.

 

[niall Y]

From the (limited) information provided it seems as though all the dimensions are within the specified tolerance. Though why you are quoting metric dimensions for Imperial threads causes some concern and the OD is by no means the most important dimension.

Depending upon the tolerance class (1A,2A,3A), the OD of the Studding should be within the range 0.4822" to 0.4985" ( 12.24788 - 12.6619mm) and it clearly is at 12.6mm. (0.496063")

The Tap - at 12.82mm = 0.504724" - may or may not be within tolerance. The OD of internal threads is only specified as a minimum which in this case is 0.5" = 12.7mm. To check further you would need to measure the 'Effective' or 'pitch' diameter. This should be in the range 0.45" - 0.4597 (class 1B) (11.43 - 11.67638mm).

Assuming that the thread form on the tap is 'perfect' then, since the addendum should be 0.025", the effective dia. would be 0.454724" (11.55mm) - again, within tolerance.

If the nuts have been manufactured at the lower limit of class 1B they could be nearly 5 thou smaller than the Tap which would certainly account for the fact that the tap didn't just pass straight though.

Hi J-G, thanks for your clear explanation of what is going on here - it is much appreciated. And, yes I did use metric measurements, but those were the calipers I had to hand in the workshop
Niall

 

[stuart little]

From my days of working in inspection at a well known copier manufacturer, maybe the tap is a 'pre-plate' one - used when the component needs to be plated.

 

[J-G]

From my days of working in inspection at a well known copier manufacturer, maybe the tap is a 'pre-plate' one - used when the component needs to be plated.

It quite a while since I had any connection with 'plating' ~1960 !! but I do recall that the thickness was couched in terms of 1/10ths of a thou.

This is much smaller than the allowable tolerance even in class 3B and certainly well under the 5 thou estimated difference mentoned.

 

[Fergie 307]

A good question would be where did you get the tap, and what is it made from. Buy HSS taps from a reputable supplier like Tracey Tools and you will have no issues. Some cheap ones are very approximate, in my experience at least. Likewise your threaded rod. Good stuff will be rolled, and probably pretty accurate. Some cheap stuff may be cut or not very well formed.

 

[niall Y]

A good question would be where did you get the tap, and what is it made from. Buy HSS taps from a reputable supplier like Tracey Tools and you will have no issues. Some cheap ones are very approximate, in my experience at least. Likewise your threaded rod. Good stuff will be rolled, and probably pretty accurate. Some cheap stuff may be cut or not very well formed.

HI,there
You make some valid points here. The studding and bolts are both A2 stainless steel and look to be pretty clean and well made. The three tap set is carbon steel and not HSS. I would assume that these are reasonably serviceable and accurate - though you never can tell until you try them out. However, from my understanding of J-G's post, they do fit within the standard specifications
UNC, for me is not a thread type I would normally use, but since I could not find 1/2 studding in Whitworth ( for which I already have the taps ) , I had to plump for UNC.
and buy some new ones. Carbon steel taps are a lot cheaper than HSS , though there shouldn't be any reason why they should not be serviceable. After all HSS has not always been an option.
Niall

 

[J-G]

A number of points here Niall,

1 - A quick 'google' soon finds plenty of suppliers of ½" Whit. 'Allthread' in both mild steel & stainless.

2 - Why - in the absence of ½" Whit. (and the need to buy new taps) - did you not switch to M12 ?

3 - There is little 'wrong' with Carbon Steel taps other than the fact that they are more prone to wear than HSS and are unlikely to be capable of cutting threads in 'tougher' material (En8, En 24...). If of current manufacture, they are also likely to be 'ground' rather than 'cut'. - Whether they are 'cheaper' or not is moot - if HSS are twice the capital cost but last 5 times as long then it could be argued that HSS are cheaper.

4 - 'Allthread' or 'Studding' is now most likely to be 'rolled' rather than 'cut' - for a number of reasons - it's easier to maintain a 'straight' long length and it requires a smaller bar size (cheaper) and has no 'swarf' to be disposed of.

 

[niall Y]

A number of points here Niall,

1 - A quick 'google' soon finds plenty of suppliers of ½" Whit. 'Allthread' in both mild steel & stainless.
2 - Why - in the absence of ½" Whit. (and the need to buy new taps) - did you not switch to M12 ?
3 - There is little 'wrong' with Carbon Steel taps other than the fact that they are more prone to wear than HSS and are unlikely to be capable of cutting threads in 'tougher' material (En8, En 24...). If of current manufacture, they are also likely to be 'ground' rather than 'cut'.
4 - 'Allthread' or 'Studding' is now most likely to be 'rolled' rather than 'cut' - for a number of reasons - it's easier to maintain a 'straight' long length and it requires a smaller bar size (cheaper) and has no 'swarf' to be disposed of.

Hi J-G,
A few months back when I first started looking , there was a complete dearth of 1/2 inch threaded studding in the lengths I needed, except -quite understandably- from America. When I looked a few weeks ago I spotted this 1/2 inch UNC one that I promptly bought.
You ask quite reasonably, - why not an M12 one? The answer being that I already have an M12 Mandrel.
With the whistles I am making I am chasing small differences in size. that I'm hoping will make a big difference to the sound. Hence, enlarging the bore, which in turn should give me a thinner wall thickness, resulting in a better sound. That is the theory anyway, and I will see if I can get an improvement with my next whistles

 

[Fergie 307]

HI,there
You make some valid points here. The studding and bolts are both A2 stainless steel and look to be pretty clean and well made. The three tap set is carbon steel and not HSS. I would assume that these are reasonably serviceable and accurate - though you never can tell until you try them out. However, from my understanding of J-G's post, they do fit within the standard specifications
UNC, for me is not a thread type I would normally use, but since I could not find 1/2 studding in Whitworth ( for which I already have the taps ) , I had to plump for UNC.
and buy some new ones. Carbon steel taps are a lot cheaper than HSS , though there shouldn't be any reason why they should not be serviceable. After all HSS has not always been an option.
Niall

I entirely agree there is no earthly reason why a carbon tap should not be accurate if properly machined. The problem I have encountered in the past has been that many carbon taps I have come across, often in the boxed set for a tenner category, have not been well made. I can certainly appreciate your concern for cost, especially if you are only likely to ever use it for that one job. Too late now I know but it may be worth getting to know anyone locally who restores old bikes and cars, if they work on English stuff they will have all sorts, and will also know where to get nuts and bolts, which is not always easy. They may not be happy to lend the tools, but will probably be happy to do the thread for you, beer being the preferred method of payment in my experience.

 

[Fergie 307]

Sorry don't know what went wrong there !
I entirely agree there is no earthly reason why a carbon tap should not be accurate if properly machined. The problem I have encountered in the past has been that many carbon taps I have come across, often in the boxed set for a tenner category, have not been well made. I can certainly appreciate your concern for cost, especially if you are only likely to ever use it for that one job. Too late now I know but it may be worth getting to know anyone locally who restores old bikes and cars, if they work on English stuff they will have all sorts, and will also know where to get nuts and bolts, which is not always easy. They may not be happy to lend the tools, but will probably be happy to do the thread for you, beer being the preferred method of payment in my experience.

 

[J-G]

You ask quite reasonably, - why not an M12 one? The answer being that I already have an M12 Mandrel.

Ah . . . . Context is all !!

 

[John Hall]

What make of Tap is it?….I only use HSS Presto, Dormer, Goliath etc, which you can usually find cheaply on EBay…I’ve managed to acquire a good selection of metric and imperial sets over the years…
I know this is probably a one off buy, for a one off job for you, but buying cheap ones, usually ends up as false economy.
You can be certain the reputable makes will be the correct size…whereas the chinesium ones can be way out. And I would not advise cutting threads on stainless or high tensile steels, using carbon steel taps, they’re ok for thread cleaning, and softer metals..
If you lived near Manchester, I’d pop round with a set…

 

[clogs]

John hall,
perfect answer.......
I would never go near even the softer ST/Steels with a carbon anything....
For jobs that involve more than a few threaded holes in ST/Steel I always buy a new tap....
Stainless is awful stuff to machine/work esp by hand ....
I bought a metric tap 32mm dia in carbon for a one off job in ally.....
a decent european or US single tap was almost £80 on offer...mine came from China at 20!!!!!

out of interest I have just found a place that sell rolled brass threaded rod.....
again if u lived near me u could just pop round and get the job sorted....
not even for beer money as I dont drink.....lol....