Sine bar errors

[PJs]

Thanks Marv and everybody! I've been lurking hear since Marv started it and Learned a lot. Thought I might chime in here with a relevant experience.

I had an experience last year where I picked up an across the pond variety of Angle blocks (45-45-90 & 30-60-90) for set up and granite work on my meager budget. The specs said, "back then" that they were good to ±20 arc second and for $30, even if they were ±50 arc seconds I would be thrilled. First thing I did was set them against a known good 123 block and checked for daylight...Oh My. Was able to put a 6 thou feeler gauge on the far end of most sides. I contacted them and they sent me another pair...and ask if I would change my initial review of them because they didn't respond to my contacting them. When I checked the new ones they were only slightly better on one angle and worse on the others. The 45 was the best but the 30-60-90 was something on the order of ~400 arc seconds. And worse was the two sets were different in size fairly significantly. The difficult thing was the Obliqueness of the triangles and trying with my meager tools to measure and calculate the real error.

Had lunch with my son a week after and he said to give them to him and he'd measure them on their Hi-End Keyance IM-620 visual measuring system good to <.5µm...and calibrated regularly. Here are the print outs from his test.



Basically these show that the 30-60-90 were out of spec by 25-41 times the spec, but the 45-45-90's were only out by 1-2.5 times. Obviously the 45's were Very usable for my meager shop but the lopsidedness of the 30-60-90 were pretty much paper weights (or maybe a chip scraper as Paul suggests)...unless I could use Marv's idea of compensation. It would mean needing to mark them permanently some how and having a way to use them with some method of compensation. Would love to hear your thoughts on this Marv.

Hemingway, there were several other things with this experience but interestingly they have now changed the specs on the website to show a tolerance of: "Accuracy +/-20 seconds, 0.0001" per inch. " These things are about 3" long. @¿@ Obviously they didn't get it Or Did, but were tired of me as some do get. They are still for sale if you want some. Doubt I will purchase their incremental angle block set though (same new spec) even if it's only $40 instead of hundreds for a good set of B&S.

Thanks again Marv for starting this great thread! ~PJ

[Paul Jones]

PJs,

Perhaps there is a way to lap the surface by clamping the 30-60-90 to the correct angle (kind of a catch 22 but can be done) and systematically remove the taper. There are ways to set up the lapping so the part (the 30-60-90 angle block) is lapped but not the reference surface.

I have bought several pairs of 1-2-3 blocks with specs of +/- 0.0002 or +/- 0.0003 on parallel surfaces and 0.0003" over-sized on all three parallel sides. The interesting thing is the manufacturer (three different companies) states they purposely make the 1-2-3 blocks slightly over-sized to allow for manual lapping to precise dimensions. This is not high on my priorities but could be done. By the way, of all pairs of 1-2-3 blocks I have, the solid ones without any holes are my most useful pair. Most of my set-ups just require a precise spacer without bolting on or through the block. I find the ones with holes collect the metal shavings and have to be cleaned and inspected for chips. It is so much easier to just use your hand to wipe of the solid blocks. So if you need a second pair, buy the solid kind.

I like the discussion here and have learned a lot from all of you.

Paul

[mklotz]

This is a bit off-topic but may be useful for current and future readers.

Most of you know that measuring a taper involves measuring the part diameter at two locations a known and precise distance apart. With a tapered part this can be a 'fidgety' task at best. But 1-2-3 blocks can make it easy.

Lay the blocks so the 1" dimension is vertical. With the tapered part held vertical between the two blocks lay your caliper's jaws flat on the horizontal 2-3 surface on the blocks and measure the part diameter.

Now arrange the blocks so the 2" dimension is vertical and the caliper jaws lie on the horizontal 1-3 surface. Take another reading and, voila, you have two diameters separated by exactly one inch.

[PJs]

Thanks Paul. I have considered lapping them by holding the long side (hypotenuse) as a reference and work on the 90, but feels difficult to adjust the obliqueness with any thing I have to accurately hold the correct angle and allow the slow whittling away till I get there. The other thing is my height gauge only measures to 3 places plus a floater half, and the long side of 30-60-90 is just slightly bigger than my 3" 10'th mic, which is my largest. It would be difficult to check to any accuracy as I go along and wouldn't want to bug my son. Need to crunch some numbers to see what I "could" hit with 3 decimals and how I might hold it to get there. I'd love to salvage them as the 30-60-90 is the one I really wanted but just couldn't justify sending them out or much more time on them.

Interestingly, both sets of these have one side that is ground perpendicular to the plane and the other two are ground in the run. Definitely Friday units while the QA people were out to lunch.

I only have one set of 123 blocks and believe it or not they are from the same people. When I got them I measured and swept them every way I could thing of and they are withing 2 tenths in all directions and parallel. They are oversize by 2 tenths but are definitely matched, but like you say it's not a priority...Straight/square/parallel is workable for my meager needs. I use them more on the granite than for setup but do use them occasionally for a set up for lapping perpendicular to a small granite I have. I do like your idea of a solid block though, as the threaded holes on these are worthless and cobby but for $20 I can live with that. I'll keep an eye out for them.

Me too on the learning and fun! ~PJ