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| BillTodd:
Thanks for the encouragement Pekka - it is appreciated :thumbup: I've fixed IN/OUT the problem with a flat DTI tip. It now repeats within a few microns. However :scratch: :scratch: :scratch: I spend a couple of hours checking and rechecking my taper setting using a sine bar. Finding a stack of objects exactly 3.5355" was a challenge but, in the end, two V blocks & a thrust washer measured to better than 1/2thou". Then I carefully set the top-slide (the picture below is of the general method used ) and re-cut the tapers - well I say "re-cut" but actually there was so little difference between the existing tapers and the new uber-accurate ones it was hardly worth touching them :( So, with my backache getting the better of me, I set about measuring the top taper in the indicator : Unsurprisingly; 2.000mm indicated on the DRO read 1.920mm on the DTI :bang: :bang: So, where is this error? I then stepped the DTI 100um at a time while noting the DRO reading (see PDF) to see it I could spot a jump or anything that could explain a taper (as shown by the difference plot) of nearly 0.1mm (>3thou") over just 2mm (~3/32") (that should be almost visible!) The only place it can be, is in the right-angle hole but that doesn't look too bad from what I can see so.... what am I missing??? Ideas welcome. (remember I'm testing this without the lower taper part and ball-slide so it should be at least as good as the DTI) |
| PekkaNF:
You are going to nail it big time! Every step gets you closer and closer. I even can't cut realiably very smal DOC. I'll try to fix last cut around 0.1 - 0.2 mm depth (even though I clock it someting like 0.123 mm indicated to read the desired final dimension), othervice my parts seem to come out worse geomertically! But it's lack of my skill - no doubt. I'm pretty damn sure your DRO and DTI reads on same axle, angular difference should be pretty large to cause error of that magnitude. Your graph show classical scaling error that could come from incorrect aplication of metric/imperial correction on engineering of DTI or settings of DRO. I worked sometimes with SONY magnatape system and there were quite a few parameters that could be keyed in or set wrong on DIL's. I would first check DTI against gaugeblocks to clear it. My bet would be that is the reliable one. Then I would assume DTI fairly reliable and use same measurement range and DTI reading against lathe slide to see how much deviation is on one single axis of movement. See where DRO measures the axle and double check that you read with DTI same movement that tape/sensor sees. Many DRO:s have many settings, not only to cobat linear erros, but also for periodical errors and sometimes error tables too, that are wonderful to have, but pain to calibrate. PekkaNF |
| BillTodd:
Hi Pekka, I've checked the DTI against the DRO directly and both correlate very well (to within the thickness of the needle). I have great confidence in the Sony DRO - I have checked it against standard length gauges etc. - It was a very expensive piece of kit when new ! I'm now thinking about increasing the bore of the right-angle hole to 8mm (as I have some 8mm bearing balls) that should compensate for the material removed from the tapers and give me the opportunity to correct any taper (or irregularity) in the bore. [edit] I threw caution to the wind and drilled the hole out to 6.5mm - worse errors and on close inspection there was a small step in the bore just where the ball would be running. Drastic action... Dismantled the body parts completely and drilled & reamed another right-angle hole at 90° to the previous one. Result - less error but still ~50um short ... Back-ache stopped play - retired to the pavilion ... Bill |
| DaveH:
Hi Bill, I really hesitated to post this, but it seem (to me) the error comes from the 45 deg taper. The way I'm looking at it the DTI reading is always smaller, so the ball isn't moving as much as it should. So if the taper angle was larger than 45 deg that would cause the ball not to move as much. Try a new 6.5 mm dia hole opposite the very first one, that should eliminate any slight difference to the accuracy of the main bore and the right angle hole not being exactly at 90 deg. Just a thought Bill :beer: DaveH |
| BillTodd:
--- Quote ---I really hesitated to post this, but it seem (to me) the error comes from the 45 deg taper. --- End quote --- Don't hesitate if you spot something ('cos I'm a blind as a bat when it come to some things) Anyway you're kinda right .... Cracked it :ddb: Or rather my friend Chris cracked it :beer: I was showing him the problem this evening and he came up with the brilliant suggestion that the ball might not be running down the centre of the cone . BINGO Indeed it will try to do everything in its power to avoid running down the highest spot Any error in the centring of my hole, or play between the ball and the hole, will allow the ball to run to one side of the taper and thus run on a parabola (a conic section) not on the true 45° face. The fix was simple: replace the ball with a small brass slide that presents a flat plane to the cone thus only following the highest part. Testing on the lathe now shows the right-angle DTI tracking to within a few microns (<8um at 2mm) . I'm back on track :) I now have to adjust the fit of the large thrust washer, so it goes a bit deeper into the body (again!) and I'll be ready to test it properly (again!) Thanks guys, :nrocks: Bill |
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