The Shop > Electronics & IC Programing
DRO questions
Bogstandard:
--- Quote ---unless your scale is fitted to the tool point there are a myriad of ways in which accuracy can be compromised.
--- End quote ---
Actually Steve, after using glass scales for the last couple of years, I can even tell when a tip requires replacing to bring me back on line to getting about 5/10ths accuracy.
I worked with vernier scales for a few years, and really struggled to keep my accuracy within a thou or two, they really are sometimes very inaccurate and obstinate. Whereas after using glass scales for so long now, hitting 1 thou accuracy could almost be done without even thinking about it, and that could be hit time after time after the machine has been allowed ten minutes to warm up.
It all depends what you are after, and in my case it was what my customers required. If I couldn't hit 5/10ths or better either way, they weren't interested.
That is all finished with now, but having got used to it, I would never go back to anything less.
With regards to the newer magnetic strip heads. I am under a personal opinion that if you are working with any sort of ferrous materials, especially cast irons and steels, you might be asking for trouble. The size of particles coming off either of these two materials can be absolutely minute, and could be attracted thru even tiny gaps where they could adhere and cause problems. As far as I am concerned, the jury is still out on those, and until they are proved to be unaffected, I would prefer to stick with the slightly longer glass scales.
But if anyone would like to donate a few of the latest Newall scales, I would certainly take them off their hands as a direct replacement for mine.
Bogs
John Hill:
I have not yet progressed to the point where I understand just exactly how DROs can be used for accurate work on a lathe.
Suppose for example I want to turn this bit of rusty steel shaft down to a diameter of 20mm. I mount the bit of steel in the lathe, touch up a suitable tool with my hone and mount it in the tool post.
How would I set the DRO, if I had one to indicate exactly 20mm when I have reduced the diameter to that? :scratch:
John Stevenson:
You take a cleanup pass and measure that, then set your DRO to that value.
John S.
Bogstandard:
First off John, you would take a light skim off the rusty old bar to get yourself to clean metal. Then take a micrometer reading off the bar.
Depending whether you work in imperial or metric, you press a button on the display to put you into the correct units, then press another button to zero up the display.
Also depending on how you originally set the display up, either amount off the diameter, or like most people and lathes, amount off radius (1mm off radius takes 2mm off the diameter).
You can then do what John S has said, input that figure into the display and cut until your display shows what you want to end up at.
Or do it the way I do, a**e about face.
Subtract the diameter you want to end up at, say 15mm, from the mic measurement, again say 19.5, leaving 4.5mm.
By taking your cutter display into half that figure (2.25mm) in easy stages, once you reach 2.25mm on the display, your bar should be exactly 15mm diameter.
That does sound rather complicated, but in fact, once you get used to it, everything becomes second nature and you can cut down to your 15mm size continually, time after time. In fact if you zero your tool when you reach the correct size, no matter what size bar you put in there, if you cut down until the display shows zero, you will be at 15mm diameter exactly.
There are different ways of doing things with these display boxes, each type having various features you can use, even some that will help you to turn tapers.
Depending on the type of scale that is fitted to the display, governs how accurate and repeatable the cutting action becomes. Using cheap vernier scales you would expect your accuracy to be within 0.002" (0.05mm), whereas with the more expensive glass scales, that figure can become 0.0002" (0.005mm)
John
kwackers:
--- Quote from: bogstandard on February 20, 2011, 06:08:07 PM ---It all depends what you are after, and in my case it was what my customers required. If I couldn't hit 5/10ths or better either way, they weren't interested.
--- End quote ---
Hi Bogs, your point above is what I mean. Most of us amateurs don't have customers, we also lack both decent machinery and the nounce to make do with the slightly iffy stuff that abounds.
(I've long observed that a good man can do fine work with poor tools, whilst a poor man will always benefit from good tools).
Fitting glass scales to an amateur machine and suddenly expecting 2/10ths accuracy is bound to lead to disappointment. Many machines don't allow for good placement of the scales meaning racking can be an issue and that's before you come to realise that they're not a substitute for machining skills.
There does seem to be an attitude on here that it's glass scales or nothing and that's simply not true. I survived years with cheap chinese scales and managed just fine. I think rather than be put off because people can't afford a full set of glass scales people should be encouraged to use what they can afford. Chinese scales make a lot of things so much easier even if you have to crane your neck to look at the built in display.
When I changed machines I switched to glass scales and there's no doubt they're an improvement but if I couldn't afford them then I'd have no hesitation using Chinese scales.
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