Lathes and Accuracy

As something of a lurker in this group and maybe an old timer who should be ignored I am often surprised by the constant search for accuracy in terms of numbers.

A simple lathe such as the Portass S which I have alongside my Myford ML7 is a case in point. I use it a lot making things for fishing boats and the local bus operator. No complaints from the traffic inspector. (please ignore this Greg)

I do have some digital calipers but I can never be sure which button to press or the battery has run out. I do use a vernier caliper from time to time by way of a magnifying glass.

For the most part I use firmer calipers, the type you tap.

I expect what I am trying to say is that achieving the right fit, be it running, press or other is a matter of feel or something like that. For the most part this works.

Donald South Uist

Reply to
Donald
Loading thread data ...

When the thing you want your bit to fit is on the other side of the world, working to numbers and tolerances is arguably a better alternative.

Reply to
Wally

Donald, after my impetuous youth had run out I realised that you ignored the "old timers" at your peril. Many times I have wished I had listened more and blithered less, still do on occaision. Can't blither too much today unfortunately (fpr me) as it's the wife's birthday so I'm on "fussing" duty as soon as she gets up (annual lie in).

Trouble is as Wally has said, in todays world of mass production the numbers do matter, too expensive to measure each part individually so make one part (in it's thousands) to the numbers and the matching part to it's own numbers and the two should fit everything being equal and both sets of measuring equipment being accurate. Of course, I completely agree with you if we have the time and the two parts to measure ourselves then the numbers (within reason) are totally irrelevant. At the other end of the scale where real accuracy (of fit) is important we ignore the cost and adopt your approach, so it obviously works.

One thing I noticed when I started to try and achieve reasonable running fits (small ic engines) was that measuring technique and indeed feel was vital. I quickly mastered the measurement of outside diameters so could rely on my "numbers" for that being reasonable; but inside diameters was an entirely different matter. I struggled for years to get repeatable numbers leave alone accurate ones, particulary from smaller diameters. Eventually, I remembered the lessons of years ago and started to put the numbers to one side and work the last few tenths with the "feel" between the bore gauges/calipers and the mic. Difficult to get new guys to understand this though, having spent =A3=A3=A3=A3's on the most accurate kit available they can't understand why a 0.5000000" shaft wont fit in a 0.5000001" hole.

Best regards

Keith

Reply to
jontom_1uk

This is a subject that is ver' interesting. It's all hobby stuff for me, I'm unlikely to be making stuff to fit anything but what is to hand.

I must confess to having 3 pairs of digital calipers (the cheep cheep type!), and almost never using any of them. But the box of firmer joint ones gets used! It is surprising, at least to me, how accurate I can get with them, even having not mastered the 'tapping'. What >do< you tap them on?? The side of my head (or other bit of wood)? The vice? The Polish people passing by?

not needed to use the inside ones yet, are they going to be tough to master?

I would appreciate advice on this measuring lark, it's somewhat forigen. When you work with 'puters a lot, the numbers mean something else, and become meaningless. Short of space? Add another terabyte, it's cheep. Short of power? Add another server, who cares? At home, when short of machining power, one needs to find another way to make the part!

Ed

j> > As something of a lurker in this group and maybe an old timer who should

Reply to
zedbert

I think that is a function of the digital age, where calipers, dial calipers etc are thrown away as not being accurate enough, where in fact they are just as accurate if used properly and with understanding of their limitations than the latest digital calipers.

Same as old lathes and milling machines, they can still turn out a useful job or three as long as you understand them and are conversant with their foibles.

The guys who used Bridgeports without DRO's etc etc when they were first in popular use (early 1960's?) have much more of my respect as machinists than the guys who are lost if their DRO blows up! :-))

Peter

-- Peter & Rita Forbes Email: snipped-for-privacy@easynet.co.uk Web:

formatting link

Reply to
Peter A Forbes

I would agree, but in today's economic climate with skilled labour in this country being costed at over a pound a minute in most cases, it's a no brainer to the bean counters that they have to provide anything which will save time. Especially now the same labour is available at a fraction of that abroad.

Greg

Reply to
Greg

Reply to
Charles Ping

Donald

The point that you've highlighted is that precision and accuracy are different. The pursuit of pointless precision annoys me - inaccurate components just don't work.

Charles

Reply to
Charles Ping

When it comes to making something to fit something then it is often worth thinking in terms of "making to gauge" rather than "making to measure" but the twist is to use the digital callipers as the gauging tool.

1) Put the callipers to part 'A' 2) Press the zero button 3) Put the callipers to part 'B' (and thus measure the difference) 4) Apply the appropriate putting-on-tool or taking-off-tool to either part 'A' part 'B' depending which is easier/ more appropriate/ etc.
Reply to
Mike H

Measure a part with a digital vernier and then measure it with a mic and then wonder why there is a difference.Even worse on internal measurements.The cheap digitals that are everywhere are even worse.I`ve yet to find one that will repeat zero five out of five. Mark

formatting link

Reply to
mark

Repeatable and precise OD measurement is not too difficult but ID measurement is a different matter.

The problem is that accurate measurement is only possible if the measurement tips are located on the true diameter (not slightly off to one side) and accurately square to the bore axis.

Minor axis and diameter offset errors are inevitable but the amount to which they affect the measurement accuracy is very dependent on the shape of the measuring tips. The knife edges of modern digital calipers are close to the worst possible geometry and, unless you are remarkably well experienced, the accuracy and repeatability of inside measurements made with these are far worse than the corresponding outside measurement!

A big improvement is possible if the radius of the measuring tips, at least roughly, match the ID of the bore. If the measuring tips are in the form of a split ball just less than the bore diameter, both axis and diameter offset errors completely disappear. Micrometer measurement of the OD of these tips then gives accurate and repeatable results.

Telescopic gauges with appropriately radiused contact points give better repeatability than calipers with their flexible pointy ends but the very best tools are the telescopic small hole gauges where the business end is a split true sphere.

Jim

Reply to
pentagrid

Provided the buggers send the right drawing and don't refuse to pay even though you made the part to drawing!

Reply to
Steve

Re: Donald Guest 9th July 2006

Donald I have recently obtained a Portass model S lathe and am lookin for an owners manual or copy of it. Can you please help me?

Regards

Mike Lockyer

e-mail: snipped-for-privacy@mlockyer9.wanadoo.co.u

-- michael@mlockye

----------------------------------------------------------------------- michael@mlockyer's Profile:

formatting link
this thread:
formatting link

Reply to
michaelmlockyer.2tuzwn

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.