How accurately can you machine?

I am trying to find a 'standard' to which the home machinist, with a small lathe and/or mill, etc, can expect to produce work.

From experience, it seems that anything less than plus or minus a thou* on diameter or length is pushing things (ie it requires special care or an extra finishing op to do better than that). Is this reasonable?
*0.0025mm

And what would be just as reasonable in terms of angles?

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.

Thanks for your help

Regards, Tim

PS I'm sure I have seen a page of helpful data somewhere in the Machinery's Handbook, or similar, but I cannot find it. Can you?

I'm probably about half as good as I claim to be

Neil

So assuming we are only allowed basic home/hobby machines I assume similar will apply to measuring equipment and we won't have anything that measures better than a thou either

Most of the time I would say better is not needed, I tend to do 99% of my measuring with the digital callipers so not the most accurate measuring device despite reading to 0.0005.

If a part needs to fit another then I would tend to turn until I'm a couple of thou of the measured mating part and then offer the part up until it fits so thats feel not measured.

I rarely get out the *micrometer but will use that if I need to get to a good size that can't easily be test fitted, something like a crank pin for example and will usually polish that down to size wit emery on a stick. Pistons and bores may need lapping or honing and the piston would be fitted to teh bore not measured.

J

* Note I said THE mic, as I only have one that is 0-1" anything else gets the digital callipers and I do make quite a few larger size models. Tell a lie I do also have a digital 0-1" but don't like the feel of it.

I always try and do the job as accurate as is needed. I have friends who chase silly accuracy when the accuracy of a plough would do. As a retired toolmaker I chase a thou only when really required, most jobs I do a couple of thou is good enough and gives the speed to get on with the job. Greater care is taken if boring/turning for bearing fits ETC. If I am making nuts and bolts for a tractor or motorcycle then the thou or two is usually good enough.

I don't understand 'why' you need to know. As Jason pointed out you only need to make things fit. If you are building a one off model it doesn't matter how accurate other people make their work, as long as yours fit. Unless of course if your hobby is making things as accurately as possible. If so, that is an honourable pursuit.

Edited By norman valentine on 06/06/2016 15:39:16

I'm with Jason....a fitter turner....lol.

You can have the Rolls Royce of machines capable of maching to accurate finish , but if the tools for measuring are poor, then thats as good as you will get....

I tend to aim for finished sizes as per drawings, but in the absence of such, its a case of if it fits its right....

I once turned a pin for some guy up in Scotland, gave me dimensions to work to....I got to within a thou when finished...Never did get to know whether it fit or not...

Just to be pedantic, a thou 0.001" is 0.0254mm (or 0.01mm is about 4 tenths). If I'm on the lathe I'm disappointed if I get worse than 0.01 mm out from my target. But it all depends - sometimes you need a clearance hole - I've just turned up a Vesconite cutless bearing for the boat, which needs around 0.15mm clearance on the shaft, plus 0.10 minus 0.03, so not exactly difficult to achieve that.

The other point is that it helps to know a few dodges for when things go wrong - machining a bore for a ball race demands good tolerances, probably to under a thou in old money, but Loctite or even a few strategically placed centre punch marks can sometimes rescue a job that has gone south on the final cut (so I've heard.....).

Edited By Steamer1915 on 06/06/2016 16:47:16

I think the difference we have as hobbyists is having the time to work to tolerances that are closer than needed, while not doing enough work to develop the skills of a jobbing engineer. Like Jason, I generally work to what fit is required and what 'looks' acceptable. Sadly all my care comes to nothing once I put a coat of paint on top and ruin it.

Neil

Do people working in metric work to tighter numbers (note I didn't say accuracy) because the 'round figure' is a smaller dimension? We know that many early lathes didn't have dials as standard and the early Myford topslide screw was 12tpi with an 80 division dial (optional extra).
Not so long ago Modellers didn't have mics and might be used to working to fractions rather than decimal so when the drawing said 3/8 in did they convert it to decimal and work to a thou - I doubt it. A thou is after all a human invention that just appeals to our fondness for round figures.

If you have the skill and patience, it looks as if you may be able to approach 1um accuracy. I think you can safely rule me out on that score though.

Edited By Muzzer on 06/06/2016 17:09:09

When I did this for a living the production turners did not need to chase thou's as an allowance of .005" - .010" was invariably left on for grinding. The same applied to most milling. Making one off bits - as we do as hobbyists - to a more accurate size is more demanding and certainly more time consuming. This is where emery cloth, grinding paste and Loctite come to the rescue. In the final pursuit of precision and without having the benefit of grinding machines etc. I think that constantly aiming for machined super precision when using lathe or mill is for the most part, time wasting. Viva the bodge - long live the fudge!

Rik

As a raw beginner I'm happy enough when turning; if I want to be super precise inside a thou is OK using a good micrometer or dial gauge as appropriate. The key is good sharp tooling.

Where I come a ball of chalk is in tasks other than turning; cutting to length is definitely iffy, and milling in the lathe is a bit of a nightmare at the moment. It's mostly lack of technique plus some poor instrumentation, so I'm doing my best to rectify both.

I find the cheap digital calipers pretty poor; not a patch on my steam driven Moore and Wright micrometers.

Eug

Posted by Eugene on 06/06/2016 17:12:52ing my best to rectify both.

I find the cheap digital calipers pretty poor; not a patch on my steam driven Moore and Wright micrometers.

Yes but with calipers you can make the figures match to within 0.01mm / half a thou regardless of the whatever the real fit is Vernier is allegedly French for very near.

I went digital on almost all my measuring instruments some time ago. The mechanical ones (mainly M&W + Starrett) are still there in their well worn cases from a generation or two ago but are rarely used.

They set their firm leg caliper with the aid of a steel rule to 3/8" and machined the part until the feel of the jaws over the part was right

Is it only me that often turns things exactly 0.5mm wrong when using a metric micrometer. Pestilential things, use a digi caliper to get near then switch to the mike if that sort of accuracy is needed.

Edited By RichardN on 06/06/2016 18:50:23

As others depends what it is, the only way I can get 0.0005 accuracy on mill is to send it down several times, where 0.001 will start to become sloppy thats a thou. Its due to the flex of cutters, machine, vice, job etc.
Just so happens got to finish off a dovetail friction fit no slop which means 0.005mm off each face becomes sloppy, done by trial and error as Chinese Super Lux machine wont hold tolerances within 0.35mm whether one off job or production.

Lathes worn out only had it 5 years struggle to hold 0.06mm, so experience takes over from there on to get tolerance.

Careful how use calipers have to make sure its square by feeling for both flats on job and pulling in for external. Theres a 0.04mm discrepancy on my Mitutoyo coolant proof internal to external measuring maybe because dropped dozens of times. Also changes with temp as much as 0.06mm often around 0.03 within 20 mins, have to be careful. Other Mitutoyos not as bad had 6 and used daily without respect, its a dispensable tool.
Mitutoyo digi mic can have discrepancy of 0.001mm needing constant zero before use every time.

Has anyone on here any thoughts on the difference between these two methodologies for obtaining repeatability?

1- first cuts take relatively deeply- maybe 30thou/pass, then as you approach the final measurement make progressively lighter passes, 15thou, 5thou, then shave the last thou to suit.

2- take first cut at around 10thou, measure before and after so you know what that tool actually removes at a pass (which may vary due to sharpness, material, speed, time of day etc) then remove whatever amount is required allowing for a final pass or two at the 'known' depth of cut.

I assume 1 thou cuts with a dull cutter are just rubbing and polishing, so not guaranteed to be reliable and consistent?

I'm in the process of building the S10V & am working to kit dwg sizes; I am using digi vernier callipers having converted certain dimensions to 'thous' although I have the Stuarts book 'Building a vertical steam engine' by Andrew Smith, revised by Pengwern, where all dimensions are in metric & imperial, some of the kit dwgs have certain dimensions missing so this is where the book comes in to play... I digress, I am mainly using the digi vernier callipers, but recently have used my 'Mitutoyo' digi mic to check sizes in the crankshaft & webs, to check the hub of the flywheel I have used a pair of spring outside callipers then checked with the vernier between the nibs for 3/4" ... this brings into play the question of 'feel'...

I also have Starret / M & W analogue mics which I use on occasions so have plenty of ways to check sizes if needed, at present on this model I am working to + / - .001"- .002" ... .03 - .05 mm which I find is more than acceptable for my needs. So what is accuracy... however accurate you need it to be, to meet dimensional requirement which usually includes some form of + / - figure, & how accurate are your measuring tools, usually set up to some form of 'Standard' ... so, if all your equipment has no play whatsoever, all your gibs are set perfectly, you don't get any frequency vibration from your electrics, your bench / tables are 100% rock solid... & you want to work to a tenth of a thou', & your tools can measure that... then go ahead, no one is going to stop you, I have on the very rare odd occasion but must have hit a lucky day...

George