How accurately can you machine?

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.

Nope, happens to me too & I have taken to checking the mike with a digital caliper to make sure I'm not 0.5mm out. Frequently get asked by operators at work to check a Metric mike reading for this - they also have digital calipers & don't think to use these as a quick check on the mike for confirmation.

Don't seem to have the same problem with Imperial mikes & have a poundshop large button calculator double-sided taped to a cabinet above the lathe for instant conversions when the 0-1" is to hand & Metric dimension required. The Polish 0-1" mike was a subscription "gift" with ME or MEW quite a few years ago - pity such useful "gifts" are not offered these days. I have Metric mikes to 100mm & Imperial to 6", so can measure most things I can fit in the lathe. These are not "officially" calibrated & most are used (some of the Metric are "new old stock" DDR made, with standards) but a check with a slips set from work suggest they are all pretty close - though should an accurate measurement be called for I would be wanting to check more carefully. Like others above have said, most times use a digital caliper (Aldi / Lidl) while roughing out & mike to finish.

I try to get within +/- 0.01mm on diameters at home (Super 7 with mainly HSS tooling) whether the part really needs to be that accurate or not - more to try to get into an "acccurate" habit so that there is (maybe) more chance of getting an accurate part first time when it does matter. Same goes for surface finish - practice getting it good so that good is the norm. Frequently don't succeed on either count but, hopefully, regular practice will make for more reliable results !

Milling / shaping probably get within +/- 0.03mm - I seem to be able to get reliably closer than this a work on an XYZ turret mill with DRO & new cutters, so probably a combination of less rigid machinery, no DROs & less use of new cutters at home gives the worse result (FB2 clone mill & Boxford shaper).

Have not yet tried the Ingar RT612 surface grinder in anger, but used the same make/model at the last employment & could get as close to a desired size as I could measure - just took time & elbow grease !

Richard your method is exactly the opposite to that taught to me by journeymen turners all those 1000 years ago when I was training. The one guy I learned the most from about machining in general was an ex British Army armourer. There was nothing he couldn't make, and all work done to a very high standard. The method I was taught was to plan your cuts 30 thou deep for all including finish cut. If the depth of cut was not a perfect multiple of 30 thou you could take a couple passes a little less than 30 or a little more than 30 but all cuts should be in that sort of depth. The main thing was to take that final full depth cut under load and with a known good tool / feeds / speeds setting, established in earlier passes, for good finish. He was always saying "don't take skin cuts!" loudly, meaning don't try to get to final size with .002 - .005"  cuts - you can get surprises that way, and ruin the work. 

Now this training work was on a good big Colchester Student that was rigid and had some HP on the spindle. Since then when using any big lathe I still follow the 30 thou/cut advice and normally get good results right on size. However on my home shop South Bend I reduce the 30 thou cut to 15 thou cuts but again the last cut is full depth and to size.

On Chinese hobby lathes, Atlas, Myford etc small light lathes this heavy cut to final size strategy does not always work due to lack of rigidity of the lathe design. If the lathe is not rigid enough you may get "surprises" but then you may get them with light cuts also.

Shaving or rubbing cuts with flat ended tools at shallow angles to the work can do OK but I have had these ops go pear shaped if workpiece materials don't co-operate.

Just my $0.02 worth. Your mileage may vary. JD

Edited By Jeff Dayman on 06/06/2016 19:48:35

I can't answer the OPs question as I'll get excoriated for not having typical home workshop equipment or measurement gear. However, I would say that it is the wrong question. The tolerance on a part should be set by its function and usage not by what the operator can, or can't, achieve. That means I work from tenths to inches depending on the requirements, not based on what I can achieve if pushed.

I'm not even going to attempt to tell you how I made the hole in an el cheapo stainless steel bowl this evening so that it fits upside down on the bird feeder in an attempt to defeat the squirrels. But it didn't involve anything precision.

Andrew

I agree there Andrew, parts made to a spec. for the purpose required. Force fit, running fit, sliding fit. Machining parts individually is OK as a one off. In the terms of business, parts need to fit all items so tolerances must be kept and adhered to. Imagine the car industry if tolerances were not kept.

Surface finish, type of materiel and sizes are up to the individual. It is quite easy to maintain sizes to less than 1/2 thou. which is adequate for most purposes. The design of parts is another factor in where only part of a shaft is machined to a high tolerance and the rest left a rough finish ie. bearing seats. Most people would machine a shaft to size all the way along its length when that is not required, only where the bearing is seated matters.

Clive, Andrew did you put a bearing in the bowl so that it spins. A fishing line swivel will give hours of fun watching squirrels fly off the edge of the bowl.

If they are anything like the squirrels round here they will work out how to jump past the bowl in less than a week

Every measurement has an error. It will be the measured value plus or minus a certain tolerance. Even a dro will have an error tolerance of at least one final digit, probably more

. If one needs better tolerances then one can measure by one's measuring kit, one needs to be surface grinding, honing or lappin

g.

I don't have measuring kit sufficiently accurate - it may be precice, but not accurate! - to get to the limits that some seem to aspire to. Reproducibility might be a better aim, for many, than absolute accuracy.

Many don't even know the difference between accuracy and precision. Do you?

Yes.

That's a precise, and accurate, answer.

Andrew

I put up an aluminium disc and hung it from a swivel and the Squirrel would jump on and it would dip and he would hang on by his finger tips until the speed of rotation built up and he flew off at high speed.

The latest is a feed box that they can just get their head in and as he does so a pointed bolt pierces the back of his head, Originally used in New Zealand to cull Possums.

Clive

I did my apprentiship at a high precision printing press manufacturer, and we worked to fine tolerances, on machinery that could deliver them, and with the appropriate measuring equipment and gauges. This has remained with me and I still try and work to such tolerances even though my equipment struggles to match what we had back in the day. Now I build working steam engines I have found that it is a disadvantage to be too precise on some jobs as expansion and a dirty environment means that precision joints can seize up. One needs to leave space in the joint for soot to fall through. I think the key to success is knowing when precision is needed and when it isn't.

Chris Gunn

> Many don't even know the difference between accuracy and precision. Do you?

Anyone who's an obsessive reader of this forum ought to!

Neil

My tractor-pulling brother uses the precision tools - the angle grinder, the welder and a 10 pound hammer to work to the snoot o' the bunnet down to the ba' hair accuracy and it seems to work well enough for the 100 yard pull at fearsome horsepowers.

I can make injector cones to spot on 1/2 thou accuracy on an old antiquated basic lathe.

Cheers,

julian

My accuracy is still ongoing. Sometimes it's pretty good, very satisfying. At other times it's almost unbelievably bad.

Tool flex is definitely an issue, but so is a lack of care taken at critical moments

I have found that if I tighten my toolpost up too much it makes the cross slide "sticky", only minutely, but definitely another issue when doing very fine work.

Duncan; I'm trying to use micrometers more and more, found the same issue as you had, so I picked up a couple of used ones.

I find that the thimble size varies quite a bit, as does the exact placement of the 0.5/1mm lines. My used Tessa micrometer has a large thimble, and the 0.5/1mm lines are slanted, so you can see them! Brilliant, and works every time.

My lathe dials are graduated in one thou increments. Micrometers likewise. So no dramas working to a thou if needed.

But mostly I find it is the nut that holds the cross slide handle that introduces most of the errors.

Plus or minus a post code unless you are paying serious money.

The position of the thimble on a micrometer is usually adjustable and can make the reading of the mic a little more certain.

Mike

Edited By Michael Poole on 07/06/2016 11:23:51

My metric micrometer is a Moore and Wright non-electronic digital, no problem with reading the lines but I do have other mics with too much clearance between the barrel and thimble resulting in the problems described.

With regard to accurate sizing and tight tolerances, these are the result of a need for interchangeability of mass produced parts or parts from different locations that need to fit together without further work when they are brought together for the first time. For most of us that means tapped holes need to be good enough for purchased bolts. Some holes need to suit purchased dowels etc. but often matching the second part to the first part is perfect whatever the size.

Martin

I always like to pay more for measuring equipment if i have to because measurement in the most basic sense its your primary reference point.However i do have a range of large moore and wright micrometers that i picked up at a bootsale for less than £10 each and have found them to be perfectly serviceable. I think the mitutoyo digi caliper has to be the most overly used instrument when it comes to measurement for me. Thats a sin i'll admit to.

I once made a go 1.000 and no go 1.002 gauge as a turning exercise.

I dont even want to get started on the whole "accuracy and precision" diatribe again, it went on and on and on and on, like maggie thatcher til we all fell over backwards foaming at the mouth. It's just off the cards i'm afraid. 

Michael W

Edited By Michael Walters on 07/06/2016 12:30:05