PDA

View Full Version : Problem comprehending metric fasteners...

QSIMDO
12-18-2016, 02:46 PM
The four bolts holding down the column of my milling machine show inconsistencies in measurement.

I assume they are described as 16mm but when I measure them all four are different measuring 15.75, 15.77, 15.78 and 15.80mm across the threads.
Head diameters and heights also show discrepancies.
The only consistent measurement is length.

These are also not OE fasteners as I changed them out from a dependable vendor and I've noticed discrepancies with other sizes as well .

Were I to turn or mill something to a metric dimension I'd try to get it to the specified dimension.
Is metric really that cavalier?

dian
12-18-2016, 03:08 PM
bolts are rolled.

QSIMDO
12-18-2016, 03:12 PM
bolts are rolled.

And pie are squared...so does that mean anything between 15 and 17mm is 16mm?

Dave C
12-18-2016, 03:13 PM
I appreciate your pain. Your experience with metric threads is similar to what has kept me confused about them for decades. Unfortunately, as a young machinist I never had the time, need, or inclination to study the subject, so to this day I remain ignorant on the subject.

Decieved
12-18-2016, 03:20 PM
Here you can find your screw tolerance.

How much deviation on thread diameter are you used to see on your imperial screws?

Toolguy
12-18-2016, 03:31 PM
All bolts, inch and metric, are a few thousandths under the nominal diameter. When measuring the threads on a 1/4" bolt, for example, you will find it to be around .246 - .247 major diameter. The crest of the thread is usually not sharp, but has a small radius or flat at the top of the triangle. If it was full diameter and sharp, it would bind up in the mating thread and not screw together easily. It is simply a matter of clearance.

What matters is the diameter on the flanks or sides of the thread, somewhere between the major and minor diameter. This is measured with a thread mic or a regular mic and thread wires, etc.

EVguru
12-18-2016, 03:37 PM
The major diameter of a male thread is always less than the nominal diameter since the crests are specified with a flat.

The allowable minimum and maximum OD for an M16x2 is 15.682mm and 15.962 a difference of 0.28mm or 11 thou.

Your bolts show a variation of 0.05mm, which is about 2 thou.

The allowable minimum and maximum OD for a 5/8" by 11tpi thread is 0.6113" to 0.6234" a differnce of about 12 thou.

wierdscience
12-18-2016, 03:42 PM
Paul has it right,on a bolt that diameter either SAE or Metric the OD tolerances can be fairly wide.

BCRider
12-18-2016, 03:45 PM
You might have bought these bolts from a reputable dealer but that doesn't mean that dealers don't buy lower grades as well as better ones. From the variation you are noticing on the heads it would appear that these fasteners are certainly of a grade point. The cheaper grades in inch sizes would show much the same sort of tolerance issues. And to put it into perspective with the maximum difference being .05mm we are talking about a .002" worst case variation on a bolt which is up around 5/8" diameter. That's not what I'd call a big deal on lower cost fasteners.

Up this way metric is about a 50-50 sort of deal. So I see a fair amount of metric hardware and have also bought my share of it. I've had no problems with finding decent quality fasteners for either metric or SAE when I pay a similar price.

Highpower
12-18-2016, 03:56 PM
And pie are squared...so does that mean anything between 15 and 17mm is 16mm?

I find it easiest to just round up to the next whole MM. So a bolt that measures 16.724 would be a 17mm - not 16mm. What sucks is having to buy extra taps and dies when your set only comes with one 11mm and it's the wrong pitch for what you need. :rolleyes:

Edwin Dirnbeck
12-18-2016, 07:19 PM
STOP IT
Your bolts are WELL WITHIN TOLERANCE.End OF STORY. Edwin Dirnbeck

Doc Nickel
12-18-2016, 07:29 PM
The underlying cause it simply that nuts and bolts are not machined. It might be more accurate to say they're forged.

To make a bolt, a piece is snipped off a roll of wire, clamped in a vise, and successively bashed until a head is formed. The other end is then squeezed between two ribbed plates in an action a lot like rolling out a snake of Play-Doh, which presses threads into the shank.

The only part that could really be called "machining" is when the head is forced through a punch to form the flats.

And, this is all done at surprisingly high speeds, on machines that are more likely than not decades old, which have made many millions of bolts each.

None of it is precise, nor does it need to be.

Doc.

enl
12-18-2016, 08:04 PM
The underlying cause it simply that nuts and bolts are not machined. It might be more accurate to say they're forged.
.
.
.
None of it is precise, nor does it need to be.

Doc.

I would add "and if it does, the cost goes up markedly". Fasteners are available, off the shelf or custom, if needed, to much tighter tolerance than commodity graded parts. Even the cheapest box store graded parts are generally in the middle 20% or so of the tolerance range for grade and class, though. Even the counterfeit graded parts. It is quite a mature manufacturing process.

Of course, the cheapest of all might not meet any grade at all, and the counterfeits might fail on mechanical properties.

Willy
12-18-2016, 08:19 PM
While on the discussion of how bolts are made I thought some here would enjoy a look at a US facility that is well known for manufacturing high quality fasteners. These aren't cheap hardware store grade fasteners, although much of the processes of manufacturing them are similar.

Below is part one of a three part series on how ARP fasteners are made

LKeithR
12-18-2016, 11:01 PM
bolts are rolled.

Yeah, the difference between 15.8mm and 15.75mm is .05mm which equates to just a little less than .002". That's nothing. One of the things that beginners have trouble with is understanding that not everything has to be made to super tight tolerances. Lots of times "working right" is all that matters.

I've seen people agonize over the size of a drilled hole when they're putting a thread into it. Ninety-nine per cent of the time if the chart says to use "X" drill for a certain thread that's all you need. Drill your hole--with decent drill bit--run in the tap and you're done..

BCRider
12-18-2016, 11:26 PM
The underlying cause it simply that nuts and bolts are not machined. It might be more accurate to say they're forged.......

Cold forged in fact. What is roll form threading and cold setting the heads if it is not a cold forging process?

PStechPaul
12-18-2016, 11:40 PM
Interesting video. Amazing how fast these fasteners can be made, and the quality that can be obtained. I was surprised by the absence of safety glasses, (the narrator girl had hers on top of her head), and the use of fingers to handle many of the fasteners into and out of machines.

dave_r
12-19-2016, 12:27 AM
It's why you have 10 fingers. You can make 8 mistakes and still pick things up.

dian
12-19-2016, 01:25 AM
another thing about metric screws: please dont call them by the size wrench they need. first its wrong and second it can vary.

J Tiers
12-19-2016, 01:59 AM
another thing about metric screws: please dont call them by the size wrench they need. first its wrong and second it can vary.

Who would do that?

dian
12-19-2016, 02:34 AM

Sim
12-19-2016, 03:07 AM
When we are nitpicking:
You're = You are.

Lew Hartswick
12-19-2016, 07:56 AM
Who would do that?
<Originally Posted by dian http://bbs.homeshopmachinist.net/images/buttons/viewpost-right.png (http://bbs.homeshopmachinist.net/showthread.php?p=1086735#post1086735) another thing about metric screws: please dont call them by the size wrench they need. first its wrong and second it can vary.>

:-) Any Brit that is use to Whitworth hardware. :-)
...lew...

QSIMDO
12-19-2016, 02:55 PM
So anything X.5+ (or XX.5) will go to the next highest number?

BCRider
12-19-2016, 03:22 PM
So anything X.5+ (or XX.5) will go to the next highest number?

Yes. It's the same thing with SAE. A 3/8 bolt won't measure .375 across the crests of the thread. They are more something like .368'ish.

QSIMDO
12-19-2016, 03:54 PM
Yes. It's the same thing with SAE. A 3/8 bolt won't measure .375 across the crests of the thread. They are more something like .368'ish.

Cool, thank you.
All what we're accustomed to I 'spose...3/8 I can generally just look at and know it's 3/8.
Not without cheaters though.

TN Pat
12-19-2016, 05:45 PM
To be fair, some odd sizes come out of China. Some folks report mixtures of metric sizes and inch pitches, or the other way 'round!

But yeah. Round up when it comes to threads. In fact, above 6mm, it would be "uncommon" to find an odd-number nominal metric screw (M17, for example). They exist, of course, but are not common. You will see M2, M2.5, M3, M4, M5, and M6; from there, it's M8, M10, M12, M16, M20, M24, M28... so on. It actually corresponds pretty well with inch sizes, which makes it worse for those who are inclined towards imperialism.

Toolguy
12-19-2016, 05:49 PM
I have M7, M9, and M11 taps and dies among others.

EVguru
12-19-2016, 06:14 PM
'Odd' sizes may be missing in ISO (International Standards Organisation) Metric charts, but M7 & M9 for example are common in the JIS (Japanese Industrial System) that pre-dates ISO.

There are quite a few M7 threads on my beloved Morini motorcycles and it seems to be quite popular in Automotive applications. Certainly I have no difficulty in finding affordable taps, dies and even thread repair kits in those sizes.

The Artful Bodger
12-19-2016, 06:16 PM
To be fair, some odd sizes come out of China.

Are you kidding me or are you saying they are not to be found elsewhere?

British Morris (including MG) engines were made from 1923 to 1955 using metric threads with bolt heads and nuts to suite corresponding Whitworth spanners. The famous de Havilland Gipsy aero engines did something similar although I think Australian made Gypsy engines are something different.

Right now if I go to our workshop I can find a set of taps and dies that appear to be imperial size but are stamped with their metric size including several decimal places!

Jaakko Fagerlund
12-22-2016, 03:05 PM
So anything X.5+ (or XX.5) will go to the next highest number?
All bolts are under the nominal size, so if you measure a (metric) bolt to be 15.0 or above, it is most likely M16 thread that is either undersized or worn out.

Paul Alciatore
12-22-2016, 04:16 PM
If you are trying to UNDERSTAND threads, both English and metric, then there are some things that you must know.

First, although the thread size on bolts and nuts is given in terms of the OD (outside diameter) and the thread pitch, the OD is just a NOMINAL value that is used to determine the actual dimensions for both the screw thread (male) and the nut thread (female). The real value that determines how the male and female threads fit together is the PD (Pitch Diameter) which, in simple terms and for a Vee form thread, is the point where there is 50% space between the threads and 50% solid material in the thread. This PD MUST be larger for the female thread than for the male if they are going to fit. Otherwise, the threads will interfere and assembly will be impossible or at least very difficult. The values of the PD are determined from the nominal OD of the screw size using various formula and fudge factors (allowances).

The PDs are the real, primary determining factor when a male and female thread will fit. It is possible for the OD of a male screw to be completely out of spec in the negative direction but if the PD is far enough out of spec in the positive direction, then the thread simply will not fit.

Now consider, if the roots are allowed to have a fill, then a sharp Vee crest on the mating thread will interfere. So, the mating thread, both male and female, MUST have a matching reduction in the peak. This is the flat that is shown on the crests of all Vee thread specs. Both the root fill and the crest flat will have specs that make sure that the threads will fit, even if the male and female threads are chosen with a different class of fit.

As others have said, most commercially produced threads, even high precision ones, are made by a rolling process. This is simply the most economical process for this. It literally takes only a second or two and the flat dies can produce many thousands of fasteners between sharpening. When a thread is rolled, the blank will have a diameter around the PD of the finished thread. This is smaller than the nominal OD. The OD of the rolled thread is formed by material that is squeezed UP by the dies. Although this can be calculated to a fair degree of accuracy, there will be variations due to the material itself, the condition of the dies, the spacing of the dies, and probably other factors that I am not even aware of. So, there will be variations in the OD of threads produced.

Notice that this is in total opposition to cutting a thread, male or female, on your shop lathe or with a die or tap. When cutting a thread you start with a given OD or ID and cut away the material not needed. Nothing is squeezed or formed. So the original OD or ID is probably the final OD or ID in most cases. This makes the OD or ID a lot easier to control, but that is in our shops, not the world of mass produced fasteners. In reading the specs, you may notice that they are not written for ease of use in your shop. This is probably because there were no home shop people on the committees that constructed those specs. They came from the manufacturing world and were primarily concerned with commercial production and use of the fasteners.

All thread specs have allowances and tolerances that take into account the problems of both the user (all male and female threads must fit) and the manufacturer. In this process, the PD is a more important factor than the OD. The PD would need to have a tighter spec while the OD can vary by more, perhaps even a lot more if it is in the negative direction.

I hope this gives at least a start of an understanding of screw threads.

Peter S
12-23-2016, 02:37 AM
The four bolts holding down the column of my milling machine show inconsistencies in measurement.

I assume they are described as 16mm but when I measure them all four are different measuring 15.75, 15.77, 15.78 and 15.80mm across the threads.

Maybe they are not metric bolts at all? 5/8" (0.625") is 15.875mm....diameter is only part of the story, always check pitch/tpi too.

M16 x 2mm pitch is a very common size (that's 12.7 tpi). But there is also a M16 fine option, 1.5mm pitch (17 tpi)
UNC 5/8" x 11 tpi (2.3mm pitch).
UNF 5/8" x 18 tpi (1.4mm pitch).

Spin Doctor
12-23-2016, 06:11 AM
Believe me, once you've worked with metric fasteners for a while you can tell what are Metric/ISO and what is Imperial/SAE with the exception of the smaller sizes. M5x.8 and 10-32 are really close. Plus one look at the head is all you really need to do. Metric none of the lines and triangle crap. The grade is right on the head. 8.8, 10.9, 12.9, A4-80 (stainless). At one time I didn't like metric fasteners, now I prefer them. Stronger for a given diameter, plus you don't need a tap chart. Just subtract the pitch from the diameter and that's your size. Of course once you get into stuff like bearing lock nuts, then you really do need charts. They make inch and metric sizes based off of the same ODs or close enough. That's because the thread OD is based on the nominal bearing ID which for Deep Groove Ball Bearings, Cylindrical Roller Bearings, Spherical Roller Bearings, Needle Bearigs and Angular Contact Ball Bearings are Metric in their dimensions. Plus the number of the bearing means something relative to the bearing itself. Try that with Timkens. But I digress

Dave C
12-23-2016, 11:52 AM
This may help those who are metrically challenged, like myself.

Common fastener grades and property classes.

ASTM A325 Bolts meet ASTM A325 Type 1 standards for structural steel joints.

Grade C Nuts Strength exceeds Grade 5. Use with heat-treated medium-strength steel fasteners like ASTM A325 structural bolts.

Grade G Nuts Strength is similar to Grade 8. Use with ASTM A325 structural bolts.

Grade 2H Nuts Strength is similar to Grade 5. Use with ASTM A325 structural bolts.

Metric Class 4 Similar to Grade 2.

Metric Class 8.8 Similar to Grade 5.

Metric Class 10.9 Similar to Grade 8.

Metric Class 12.9 The highest metric class for strength, it exceeds Grade 8.

QSIMDO
12-23-2016, 12:57 PM
Thanks all, quite an education!

Juergenwt
12-23-2016, 01:09 PM
The four bolts holding down the column of my milling machine show inconsistencies in measurement.

I assume they are described as 16mm but when I measure them all four are different measuring 15.75, 15.77, 15.78 and 15.80mm across the threads.
Head diameters and heights also show discrepancies.
The only consistent measurement is length.

These are also not OE fasteners as I changed them out from a dependable vendor and I've noticed discrepancies with other sizes as well .

Were I to turn or mill something to a metric dimension I'd try to get it to the specified dimension.
Is metric really that cavalier?

As I said many times before:"95% of all Machinist, Mechanics, Tool&Die Makers, Engineers etc. etc. who grew up with imperial and who now claim to be able to work in both imperial and metric equally good - are just kidding themselves". Some people who are now doing their work in metric most of the time get pretty good at it. But for someone who on occasion uses metric
- they will never develop a "feel" for metric, just like a person from Europe will have a hard time adjusting to imperial in the US unless he/she gets fully immersed in the imperial system.

That is why we have these problems like we see in the above post. I can not count the times I sat with Engineers and others who stumbled over converted print dimensions like 1 3/8" converted to 34.925mm and than had problems when the actual measurement came out as 34.922mm. No "feel" for 0.003mm. Just be honest. Most people having grown up with imperial will first go and convert 0.003mm to inches before making a decision on whether the part is good or not good.
For my part - I worked in all metric for the first 25 years of my life. Than switched cold turkey to imperial for the nest 20 years and now back to a mixed system. Honestly - it took some time to start thinking in metric again. If I had a problem than I can just imagine how hard this is for imperial people.
One thing I still have a problem with is whether to write 5/2/2016 or 2/5/2016 - this after over 50 years in the US.