Cost vs Accuracy when "modest" accuracy is required

[stevec]

I think this thread should have been posted to a different heading but I couldn't find a heading titled "nit picking".

Since I do not see a Mr. Green or a Wink icon, I presume that you meant this as not as a "Tongue in Cheek" statement but as a specific criticism. However I fail to see why the issues presented are not reasonable topics for discussion in the areas of:
-- the accuracy potential of a particular type of measuring instrument,
-- how much accuracy it might reasonably be expected to have,
-- Its maximum accuracy capability,
-- the cost for accuracy that could or should be expected,
-- the variablity of accuracy from one sort of tool to another from the same source,
-- what are reasonable cost expectaions for given levels of accuracy,
-- etc.

Perhaps you are highly experienced as a machinist with a vast array of measurement tools and expertise in the use of them all, but some of us acknowledge that we are learning. Asking questions such as in the original post seeks to tap the expertise of those who are our seniors in experience. I fail to see how my perplexity over the drastic difference in the accuracy of two products, from the same supplier, in the same price class, is nit picking.. Especially seeing as how the somewhat simpler and only slightly cheaper tool one is not even close to being in the same ball park of accuracy.

I offer my apologies.

[warmstrong1955]

Be nice to have a 'Consumer Reports' for machine tools don't-cha think?

Same test criteria to all sorts of brands....to really be able to compare, like CSR does with cars & blenders?

Oh well.....

[Mr Ron]

I am extremely careful with my precision measuring tools and I am not a machinist, just an amateur. I love fine tools. I have many fine tools from Starrett, Mitutoyo and others. I also have some tools from Harbor Freight; like a 6" digital caliper ($9.95) and a General 1" micrometer (around $10). When I take a reading with the el-cheapo tools, they read within .001" from the fine tools. I use these mostly and use the fine tools when utmost precision is required. .001" is the closest I work to if I'm lucky. The point is; you don't need the most precise tools if you are an amateur machinist.

[Harold_V]

The point is; you don't need the most precise tools if you are an amateur machinist.

Depends on one's objective. Here's the problem.

If you are trying for a given fit, you'll struggle if the measurements you take are not precise. One tends to work towards a dimension that isn't representative of the one desired, so error is introduced at the outset, with success coming more by chance than by design. It is for that reason that calipers are not recommended when readings are critical. Whether the tool can repeat the same reading or not isn't important, not so long as the reading is erroneous.

Harold

[dgoddard]

The last few posts are moving in the sort of direction that I think is useful.

A particular measurement issue that I have and would like to know how to resolve without spending more treasure than necessary is about measuring bores.

When trying to achieve accurate bores for press fits, such as oilite bronze bushings in pulleys, (primarily when making several), the only way I have found to achieve a good fit is I make up a "GettinClose-Go-NoGo" gage in 3 steps from a piece of cold rolled stock. The smallest step that I call the "GettinClose" step will be about 0.010" -0.020" undersize. This allows me to bore out the hole taking rougher cuts without taking too much time, and warns me when it it time to get precise and careful. It also leaves enough material so that I can get a feel for how much different the tool behaves on small cuts rather than the coarser "material removal rough cuts"

For instance I sometimes find that when taking 0.010 out of a bore (on the radius), that advancing the boring bar 0.010" it actually takes 0.010", but when advancing the boring bar by say 0.002" It takes less or more than that depending on how the last cut affected the surface of the bore. Exactly how the tool behaves in the material depends, for example on things like
-- deflection of the boring bar
-- rigidity of the machine
-- work hardening of the material surface by the last cutting pass.
-- tool sharpness
-- built up edge
-- etc.
Inconel 600 and some of the 300 series stainless steels can be a real pain in this respect. They can be cut to very accurate dimensions but if you only want to take off another thousandth or two and the previous cut was only a few thousandths you may be cutting in the work hardened zone and the behaviour of the tool and machine can be very different. And some here may remember that I have previously commented on some 1018 cold rolled that had hard spots in it that were a real pain. If one were taking only a few thousandths. This would happen because the tool would deflect as it passed over the hard spot. I have also experienced tool/machine/material combinations that were prone to produce "bell mouth" bores

All of the above relates to factors in getting a precise bore either on the lathe or mill but If I wanted an easy way to measure such bores then
-- what instruments
-- at what sort of prices
-- and what sort of precision
should I be able to expect with different instrument options?.

I am aware of and have some experience with snap gages, dial bore gages, dial calipers, and have even used inside/outside calipers with a micrometer.

Making a "GettinClose-Go-NoGo" gage in 3 steps. Is reasonably practical when making multiple bores, but it would be nice to have some sort of good way to measure bores when I am making onseys or twoseys.

Oh by the way, my"GettinClose-Go-NoGo" gages in 3 steps are usually made by
-- using a parting tool to make grooves between the zones,
-- turning the steps to within 0.001" to 0.002",
-- polishing them to size with emery cloth for a good finish,
-- measuring them with a micrometer and,
-- stamping the sizes on the end.

[SteveM]

The point is; you don't need the most precise tools if you are an amateur machinist.

I will disagree.

You can't make a press fit on a 1/2" shaft without precise tools. You're looking at a half-thou over basic (0.5005) and then you need to be within a tenth of two of that, otherwise, no press fit.

Oh by the way, my"GettinClose-Go-NoGo" gages in 3 steps are usually made by
-- using a parting tool to make grooves between the zones,
-- turning the steps to within 0.001" to 0.002",
-- polishing them to size with emery cloth for a good finish,
-- measuring them with a micrometer and,
-- stamping the sizes on the end.

Love that idea!

Steve

[Rick]

I am aware of and have some experience with snap gages, dial bore gages, dial calipers, and have even used inside/outside calipers with a micrometer.

What about telescoping gages and small hole gages, they are widely used with great success. I have also used them as a "get close" just set the size with a mic and goto it. They are not all that expensive for a set that will cover most all of your needs.
Yes using telescoping gages takes some getting use to but its pretty straight forward and with a little practice very accuate holes can be produced, small hole gages are just easy to use. Pretty much have just used these for all my boring work (home and business) for the past 35 years

http://www.mscdirect.com/browse/tn/?sea ... d=12107848
http://www.mscdirect.com/browse/tn/?sea ... d=12107845

[Harold_V]

What Rick said. If you can't use telescoping bore gauges, you're the problem, assuming you own a quality set.
In the hands of one with skill, they are VERY capable of providing readings within .0002", which is generally all one could hope for from machines using cutters instead of grinding wheels or stones.

Such gauges provide the very data that is required, both for roughing and finishing. Once a bore size is determined, the dial (or DRO) of the machine can be trusted to get the part near final size, at which time several light but identical cuts should be taken. That addresses issues like pressure of the cut, which will vary with depth.

There was mention of bell mouth. If that's happening with a boring bar, it's time to explore the reason. Even well extended, a hole should not size differently at the entry, as the pressure on the bar is identical, regardless of its position in the bore. A bell mouth is generally a sign of a bar with problems, but at the cutting tip. If the tool isn't properly ground, the bar will flex enough to cause it to cut by increasing pressure. That is the source of bell mouth. Get tool geometry right and keep the tool sharp and that condition should go away.

Even with work hardening prone material, light cuts should be possible without any work hardening. Stay away from negative rake carbide (which is generally not considered a finishing tool, anyway), and, if possible, stay away from carbide, period! Use sharp HSS, with a keenly honed edge. It will cut without any work hardening, so long as you operate within acceptable surface speed limits.

Yeah, it's slow. But it works. Isn't that the whole idea?

Harold

[dgoddard]

You can't make a press fit on a 1/2" shaft without precise tools. You're looking at a half-thou over basic (0.5005) and then you need to be within a tenth of two of that, otherwise, no press fit.

Actually that is not totally true. It is quite possible to do so for some situations.

My first encounter with this was when the lower front suspension ball joint on my 67 Volkswagen beetle came apart. The upper ball joint is in compression but the lower is in tension and the socket had worn thin in the last 150,000 miles and the ball pulled out. I managed to jack it back together and lash the upper and lower arms together with bailing wire enough to drive the 15 miles back home.

When I removed the suspension arm and forced the old joint out I was aghast to find the the roughly 1.5 inch diameter joint was 0.020" larger than the hole it was supposed to go into. The trick was that the O.D. of the joint had a straight knurl on it. I could see that the original which took some serious persuading to remove had thoroughly smooshed straight knurling on it but the bore in the trailing arm was quite smooth showing the knurl pattern only in a thin oxide layer that had formed. A bit more force was required to get the new one in but not too much.

Later I learned about how the technique works. The materials can be and need to be comparably hard provided neither is particularly soft and there is a workable amount of ductility, in the male piece. The tops of the ridges of the straight knurl are at a disadvantage relative to the smooth bore and while they go in they get flattened, but not without a fight. The hole is not damaged but there is a considerable radial force between the parts. At least it was enough of a fit to withstand 150,000 miles of suspension system pounding without ever loosening. If all you have is a diamond knurl, the trick can be used but I would recommend very clean parts and not too deep a knurl and some press fit Loctite or equivalent. However the straight knurl goes together better, will take up a bigger gap, and sustain larger torque, from what I understand of the process. I have used it a time or two when I had to salvage parts.

Yeah, it is a bit of a cheat, but a very practical one in some cases. and of course can be done at sloppier tolerances. So for situations where it works it can be a substantial cost saver.