Scraping Something Which Has Moved?

L

loply

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Hi folks,

I have bought a new straight edge, approx 28 inches long, which has a beautiful finish on it - looks like 30ppi or so and very well scraped. At first I was really pleased.

Unfortunately it appears to have moved a bit though - as it's only bluing in the central 25% or so, and the blue fades out gradually.

I can't measure how far out it is with an indicator (not really showing any consistent reading) so I'm thinking it's not out by much.

My question is - do I need to go all the way back to a wide(ish) scraper and long strokes, or should I start off with a technique suited to the PPI the surface already has (ie short strokes and a blade which cuts a thin scrape)?

I'm just wondering what would be most expedient really. I'd hate to take the thing all the way back down to a few PPI then have to build up such a good finish again.

Cheers,
Rich
 
I have already stoned it gently and verified no burs are present, by the way, plus let it acclimatise to the shop (left it sitting on the surface plate for a few days).

Also checked it's temperature with an IR gauge, same all over.
 
If the straightedge was bought new and not second hand, I would get back with the supplier and see if they will replace it for you. If it's second hand, I would get the straight edge verified for straightness, unless your surface plate has been verified as being flat to "A" standard, not "B" standard. Your straight edge if new was more than likely scraped to a "A" or "AA" quality surface plate. As I said, if you indeed bought it new from a supplier, I would arrange getting it replaced, or at least verified for straightness. Let the supplier fix it if it's actually out at their expense.
 
is it cast iron or steel,if it is steel it will change all the time, it has to be cast iron
 
If you insist on fixing the straight edge yourself. First thing I would do is make sure your surface plate is flat. Two ways you can do this, first is send it out and have it calibrated, which would be the best way. Or second, take the straight edge in question and check by taking impressions in an "X pattern on the surface plate, then along each edge about 2" in, and last, about every 6" spacing in both x and y direction across the plate. A lot of markings to make, but it's the only way of doing it yourself. The results should show the same impressions on the straight in all directions. This will also indicate that you straight edge is really out of wack. If you get different impressions, send the plate out for reconditioning.
 
The metal the straightedge is made of may have settled since it was scraped. That happens quite often. Still, it should come to you ready to use for precision work. Talk to the maker.

I agree with Ken. You need to have a known straight straightedge, or even better a known flat surface plate to use as a master reference. Close is not close enough.

I need to get my surface plate calibrated or replaced, as it was bought well used, so I can have a known reliable reference surface. If you do not have a reliable reference, everything is merely guessing.
 
Hi guys,

Straight edge is second hand and quite old. Cast iron.

Surface plate I reckon is flat, I haven't had it checked but it's pretty new and came with a certificate and diagaram of it's flatness - imported no doubt so not certified to official standards - but my machinists level & 2um dial indicator can't find faults with it.

As I say the main question is whether it would be quicker to "go back" to rough scraping or whether I should start off at finish scraping technique, even though it's not bearing everywhere (but because it's locally flat & many PPI)?
 
Ok, the straight edge is second hand and old. Good! We know the direction you need to go in. And I think you do too.
Proceed in scraping just the middle area up about 6". Just do a heavy scraping over the area. Stone lightly, take an impression. Add more to the previously scraped area if the impressions start to go outward from the center of the straight edge outward to the ends. Once you start getting markings toward the ends, then you start working on getting your PPI's where you want them. To get 30 PPI is going to take a few days to accomplish. For us H-M, I would stop at around 15-20 PPI's. That's where I stop on my straight edges that I use. I'm not scraping in jig borers or precision grinders and such. Just old worn out machine tools.
 
Send it to Bebop, I had him finish my straight edge it was a rough casting that I milled and started scraping.... money well spent.
 
firstly find out by how much it is bowed
0.001 or 0.01 makes a big difference....
feelers will tell
 
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