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View Full Version : A table of disasters



Michael G
28th Nov 2016, 07:07 PM
I debated a bit with myself whether to post this (and if everything had gone to plan I probably would have not bothered), but after others have talked up my previous efforts, I felt it only fair to document my messes too.

This project was inspired in part by Tom Lipton (Ox tool co) who has a similar Al block in his mill vice. The advantage is that it can be drilled and tapped wherever necessary to hold items, and given a quick skim to flatten it out if it becomes to uneven.

This false table is for my rotary table. The R/T itself is 8" diameter with 4 T slots. At times I find it a little small and the T slots are usually in an awkward place. The false table is from 16mm Al and 250mm in diameter with 3/8" UNC holes on a 1 1/8" grid spacing (so I can use my existing clamp kit). First disaster right there. I wanted to be able to bolt my 1-2-3 blocks straight on but I did not check the hole centres properly, and the spacings in one direction are different from the other. Result is that as another project I will be making up some 1-2-3 blocks with matching spacing.

Things started off reasonably well. I secured the plate to the R/T and using a 1/4" cutter, cut out a disc. Lots of swarf though...
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I thought myself very clever too with the method I used to chamfer the edges, and used the rota-broaches from Pete F to counterbore. So far, so good.
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I used the DRO on the mill to give me my grid spacing because I wanted it to be square and neat but tried to be clever - I'd set up a hole and then rotate the table 4 times before moving to the next, as it was easier to rotate the table than move in X-Y. The results were perplexing...
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I assumed that I'd made an error in rotating (although in hindsight, can't see how that would do that) so decided to step off in X and Y with a slot drill to remove if possible the pilot hole. I stopped after starting the second row as the holes were still misaligned and perhaps even worse. I did a check, measuring over a rule and discovered that the Y axis was fine but there was an error in the X - a 300mm rule was 294.9 according to the DRO.
Back to the old fashioned way and marking out with a height gauge on the rear of the disc and the drilling through. It worked and I had a nice uniform grid but the alignment with the previous holes was terrible.
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The plan was to plug weld up and refinish. The small pilot holes were just enlarged with a counter sink and that was sufficient prep for welding. I made some copper pins up to preserve the holes and prevent melting away of the Al.
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The larger holes I had to fill to make a surface so I could grind back and get a centred countersink, then position the copper pins to allow me to fill them.
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Without the countersink prep, the results are not good, with they are much better. The countersink allows access for the welding electrode, so even though there is a horizontal lack of fusion, the metal around should be solid
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Some holes took a couple goes but I got there (although there are still some minor defects). Ideally I should have gone deeper and even welded in an Al pin before redrilling but I ran the risk of running into other holes if I got to much larger.
I eventually got a result
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As for the DRO, it looks like the problem was due to a small piece of steel swarf lodged on the read head for the magnetic scale. I've cleaned it out and it seems to be back to normal, although I will try to fit a wiper there so it does not happen again.
However, I have been forceably reminded to-

Check my assumptions before starting work
Check my DRO occasionally, especially before starting critical work
Make pilot holes as light as possible in case they are in the wrong place
When an error appears, stop then and think about it rather than try to fix it on the run.
Just because a specialist piece of kit like a tapping head does not get used often, it does not mean it should go


Michael

sossity
30th Nov 2016, 12:15 PM
I was pricing up magnetic DRO against glass scale DRO and thedrostore told me they have had a lot of feedback about magnetic DROs giving errors. I just assumed this was sales talk, to get me to go with their glass system, and at the time I checked the internet as best I could and could find no negative feedback about magnetic readers but found nothing. Now I'm wondering if they aren't as good after all. I went with magnetic but haven't fitted it yet.

Michael G
30th Nov 2016, 03:06 PM
I think the key question is why are they giving errors - mine started going funny because of some swarf trapped between the reader and the strip. The glass scales are better protected and would not do that. I think the mag scales are as good as but installation perhaps needs to be done with a bit more thought to prevent situations like this happening.

Michael

snapatap
30th Nov 2016, 06:38 PM
We use magnetic scales on the z axis of a couple of our lathes at work. We have never had any trouble with them. the older one is about 10 years old. They are very cost effective when you need longer lengths, the ones at work are 3 & 4 meters long.

Even the pros make mistakes sometimes, your lucky you don't have a customers screaming for their part.

Michael G
30th Nov 2016, 08:38 PM
Even the pros make mistakes sometimes, your lucky you don't have a customers screaming for their part.

They do, and I don't mind the odd innocent mistake or even a minor careless error but most of the mistakes in that build could have been avoided with a little thought.
I've learnt from the experience so there is a positive.

Michael