How accurately can you machine?

[Paul LousickParticipant @paullousick59116]

"Some years ago we had a big project (a very special wheelchair) that required several people for it's implementation. One of our members, a professed AutoCad expert, decided to do the drawings for us. He produced dimensions accurate to 0.0001" (there was no way we could work to that level)"

Another mis-understood word, along the lines of precision and accurate is "professional".

A professional is a member of a profession or any person who earns their living from a specified professional activity but this does not always mean that they are an expert in what they are doing. The professed AutoCad expert above may know how to create drawings in Autocad but could have no practical experience or qualifications in engineering which appears to be the case with his dimensions. CAD is only a tool for making drawings.

The designer/draftsman (sorry draftperson to be non gender specific) should dimension a component to achieve an acceptable fit or finish for a particular part. The machinist is expected to achieve that which is shown on the drawing and specifying a tighter fit or surface finish than that which is required could be costly.

Paul

[HopperParticipant @hopper]

Big difference between a professional and one who professes, but.

[Anthony KnightsParticipant @anthonyknights16741]

Many years ago, an old engineer told me "If you can't make it accurate, make it adjustable."

[Dinosaur EngineerParticipant @dinosaurengineer]

Posted by Michael Walters on 07/06/2016 13:58:57:

I would have to dispute that about contact measuring, i worked for a precision manufacturer and we found that in practice, the contact measuring machines were the final word on everything, because non contact will only measure what the image projects and any burrs or particles in the way of the image can alter the reading quite drastically. What they say can be accurate to on paper is not what you necessarily get in reality. You're never going to be able to beat the fact that a probe that is physically present on a given surface will always be more reliable than a probe which is not physically present on the surface and is reading an image of the surface.

Having said that, if you're working on things which arent too fussy you can save yourself alot of time by non contact measuring. Thats the alure of it anyway.

As an inspector i was told to take the shadow graph readings with a pinch of salt because it's a shadow that you're measuring.

"There are non-contact sensors, which have been available for many years, that can measure down to nano meters !" – Dinosaur Engineer

Michael W

 

Edited By Michael Walters on 07/06/2016 14:08:40

 

Edited By Dinosaur Engineer on 08/06/2016 10:55:01

Edited By Dinosaur Engineer on 08/06/2016 10:55:44

[Anonymous]

Calibration of high accuracy gauge blocks is done using laser interferometry with uncertainties in the tens of nanometres. Rather odd given that it's a non-contact measurement.

Andrew

[Neil LickfoldParticipant @neillickfold44316]

This is probably the worlds most accurate hand held micrometer,

http://ecatalog.mitutoyo.com/MDH-Micrometer-High-Accuracy-Sub-Micron-Digimatic-Micrometer-C1816.aspx

This is the new series of measuring technologies that will make the mechanical stuff obsolete like digital calipers have taken over from vernier calipers, but maybe more so. Interferometry I think is the next level of accurate measuring. Not priced for the hobby market or the small engineering companies yet, but prices will come down I am sure.

http://www.keyence.com/products/measure/micrometer/

Neil

[Neil WyattModerator @neilwyatt]

Posted by Andrew Johnston on 08/06/2016 11:12:39:

Calibration of high accuracy gauge blocks is done using laser interferometry with uncertainties in the tens of nanometres. Rather odd given that it's a non-contact measurement.

Andrew

I suspect we are talking about the difference between using them in a lab and using them in a workshop environment.

Neil

[JasonBModerator @jasonb]

Posted by Michael Walters on 07/06/2016 13:58:57:

I would have to dispute that about contact measuring, i worked for a precision manufacturer and we found that in practice, the contact measuring machines were the final word on everything, because non contact will only measure what the image projects and any burrs or particles in the way of the image can alter the reading quite drastically.

What happens if your probe happens to also make contact with one of these burrs or particles rather than a clean surface?

[SillyOldDufferModerator @sillyoldduffer]

Tim asked:

So, it would help if you could let me know how expert you think you are, and how accurate would your output be, remembering that you would be working with a home or hobby machine, and not with stuff worth more than a three-bed semi in Alderley Edge.

• I'm a member of the amateur bodge team who is trying to improve
• I usually go for a good fit rather than absolute accuracy or precision.
• Only the most expensive of my digital calipers came with a documented claim, it's +/- 0.02mm
• The DRO on my mill sometimes jumps 0.02 or even 0.03mm
• My 4 digital calipers all produce similar results up to 30mm. Over longer lengths they start to disagree, worst case about 0.06mm
• None of my measuring equipment has ever been properly calibrated
• I'm not as careful about removing dirt as I could be
• I have precision parallels but no gauge blocks
• Depending on time of year, the temperature in my workshop varies between 4 and 28C. I never compensate for that.
• The unskilled readings I take vary noticeably with hand pressure and angle. I get better repeatability from my micrometer but the results still wobble about a bit.
• When I make to a specific size against some sort of fixed gauge, there will be cursing and quite a few rejects.
• I don't do much with angles apart from using an inexpensive rotary table with a vernier dial. I successfully made a short taper for a drill chuck once, I'm sure more by luck than judgement.

It would be interesting to test just how effective people really are at taking measurements at one of the exhibitions. People could show up with their favourite measuring stick, be ushered alone into a tent containing various objects, and asked to submit their results under a psuedonym. The winner would be the guy with the lowest error rate. All we need is the NPL to provide some test objects and MEW to provide a gigantic cash prize…

Cheers,

Dave

[Tim StevensParticipant @timstevens64731]

An interesting debate, everyone – and it stayed mainly on topic. One or two answers:

The reason for the question was this – I suggested the use of an old roller-bearing race as a spacer in a machining job, and I was told that surely it was imperative to check the bearing carefully as it might not be parallel (or of a given dimension). My answer was that I thought all rolling bearings are made to high dimensional standards – higher than you can get with a home lathe or mill, in any event, and so you could have confidence that no problems would arise on that score. I said that the ordinary (non-time-served) operator could normally work to a thou as a routine minimum, but anything better than half that took time and special treatment. Those of you who answered the question with rigour confirmed this.

It might help to know that the non-contact methods include the use of interference patterns (Newton's Rings is a classic example) and what you are counting is wavelengths of light (or UV, etc). This requires a decently flat surface, of course, and you could reasonably expect any 'burrs or particles' to be sorted before entering the lab.

Thanks again

Regards, Tim

Edited By Tim Stevens on 08/06/2016 16:41:03

[Neil LickfoldParticipant @neillickfold44316]

Since I made a 0.5mm pitch cross slide screw and nut with the 0 to 100 dial from the 100 thou pitched screw, I can easily turn diameters to 0.005mm in diameter for small parts under 30mm diameter. The advantage is that the amount on the mic is the same as the cross slide dial. On lengths I can easily get to less than 0.01mm for lengths. To get lengths less than 0.005mm does take a lot more effort that is for sure and then knowing the work piece temperature can be important. For the head inserts for engines making the lengths is easy to 0.01mm and the diameters to within 0.005 is easy for me. Pistons are the hardest part that require making something to Ø 0.001mm is very difficult but can be done. But the pistons are made to fit a specific liner and not just made to a specific size to a tolerance of 0.001mm which is a very different thing. Precision milling to better than 0.01mm does take quite a concerted effort that is for sure and most lower end mills I have seen for the home workshop are not capable of better than 0.02mm precision work.

Neil

[Neil WyattModerator @neilwyatt]

I think many of us work to the precision of the measuring tools we have.

A contemporary example:

Since the advent of the ubiquitous 'wixey-type' angle gauge many folks now seem to work to 0.1 degree, whereas before you could estimate down to about 0.5 degrees with a protractor or work to much closer tolerances with a sine bar or dividing head. these little beasties have provided a quick and convenient way of working to a degree of accuracy somewhere in the middle with lots less bother and yet good enough for many purposes.

Neil

[Martin KyteParticipant @martinkyte99762]

Well I once produced a 240mm parabolic surface to better than 50nm by hand (not machining I know).

regards Martin

[MWParticipant @mw27036]

I wouldn't dispute that the high tech industry is working towards non contact measuring with good reason to, it's just in reality, the actual operators of these super high tech optical/laser measuring devices have found that it isn't as convenient as the PR image suggests, there can be alot of variation between readings depending on your lighting rig and setting up a program to accurately measure with effective repeatability takes quite a long time. Having said that, once it's set up, it's done and you can just speed through all the measuring that would normally take a very long time.

It's a given fact that pretty much all day to day measuring tasks are still done with manual tools because, for whats needed, it's still effective. A computer needs instruction to carry out it's magic whereas i can just measure what i like without having a need to define parameters. I know we'd be up the creek without them and i'm pretty much beholden to hand tools and many industries are still finding it hard to let go of them.

Michael W

Edited By Michael Walters on 09/06/2016 15:33:31

[Charles hirstParticipant @charleshirst60928]

Richard, you ask for thoughts on the two methodologies.

Some while back ME (or was it MEW) had a lengthy and very detailed article by somebody who had investigated tipped tools. His conclusion, arising from talks with tool manufacturers, was that these are designed for production and thus need to be worked hard – very fast with deep cuts and fast feeds, all of which are carefully calculated. Of course, it requires absolute certainty of where you will end up. Model engineers don't work like this, first Our machines cannot perform to the limits of production CNC machines, second, if you mic'd up an item and you knew that is was 2.6mm oversize on diameter, who would, with absolute confidence, wind in to take a final cut of 1.3mm in the knowledge that you would end up well within a thou of the required size? Not many.

We tend to approach the final measurement and thus the final cut is likely to be very small, much too small for a tipped tool. If you look at a tipped tool it is quite blunt compared with HSS tooling. Thus there will always be room for HSS tooling which can make very fine cuts.

Well, that is one view. On the other hand I went to a lecture at a model engineering exhibition where the speaker was extolling the virtues of being able to take very fine cuts with tipped tooling. He showed his results and the finish was very good.

Also, JB Tooling sell very shiny (they appear to be chromium they are so shiny) tips which are very pointed and very sharp. Jenny says that they are designed for aluminium, but in her experience they are excellent on steel, with the proviso that they are ONLY brought into contact with moving metal, otherwise the tip is likely to chip-off. I have used these and Jenny is right.

If I lived my life at my lathe, then I would get somewhere near, take a final measurement, and move in for the kill with a decent final cut (not less than 0.3mm). As it is, I will continue to get within 0.1mm and make this my final cut and I would use either a tipped tool (preferably one of the shiny ones, but a gold coloured tip would do), or a sharp HSS tool.

Charles

[Neil WyattModerator @neilwyatt]

Posted by Martin Kyte on 09/06/2016 14:02:26:

Well I once produced a 240mm parabolic surface to better than 50nm by hand (not machining I know).

regards Martin

And I bet you tested it by a non-contact method as well

Neil

[Neil WyattModerator @neilwyatt]

Good points, Charles. I have discovered for myself that the big issue with carbide is being willing to 'take the hit' of wrecking a number of inserts, because if you do you will discover how to get the best out of them with your machine. Unfortunately, unlike HSS a mistake it tends to be terminal, rather than meaning a quick trip to the grindstone. Also trouble usually means you need to speed up or increase the cut – which is not what you expect.

Neil

[Steve WithnellParticipant @stevewithnell34426]

Just because carbide tips are liable to chipping, doesn't mean you can't sharpen the edges and reduce the radius to allow lighter cuts to be taken. The tips are made for production use, but for home use you can play around with them as you are not on the clock in any sense.

So buying cheap tips on eBay and experimenting with a grinder can produce useful results. My grinding wheels c from RDG were as cheap as chips, so again, no great cost involved.

Steve

[Anonymous]

I don't find carbide inserts prone to chipping. This steel conrod was machined from rectangular using a round insert at 800rpm, so a pretty severe interrupted cut, and the insert was fine:

The only time I've chipped inserts is by running the insert into something stationary when moving the saddle, like the chuck.

Andrew

[MuzzerParticipant @muzzer]

Machining hardened steel ballnuts complete with mounting holes is a pretty good "interrupted cut" test. I used std general purpose (but "proper&quot Korly inserts and after 30 mins or so I'd removed a sizable flange at the cost of a few edges (ie 2 inserts, IIRC). Of course, there are grades of insert available that are specifically designed for prolonged machining of hardened steel with interrupted cuts but it's hardly worth the hassle of getting a few in specially for one job, given the cost of more general purpose inserts which work well enough to get the job done.

I now know enough about the myriad grades, coatings, chipbreakers and shapes of inserts available to recognise that the ones we see offered through the usual hobby channels are a very limited subset of what is available. On the other hand, I'm some way short of being able to offer an expert article to navigate through the subject myself. It would be great to see such an article but before offering to do so, I hope any would-be authors take the time to do some meaningful research into the matter, ideally speaking from professional experience rather than enthusiasm or (limited) opinion.

Murray

[Author]

Posts