Scraping a magbase

So I decided to try my hand at scraping, and figured a good first project would be my cheap magbase, which seemed to rock like a rocking chair on two of the corners when placed on the surface plate.

As I started scraping, what to me looks like a pretty good pattern started appearing, but i cant seem to get rid of the rocking -- when i take one pass on the high corners, it simply starts rocking on the other two. I wonder if I'm making a beginner mistake of some sort, or if it's simply the nature of having an extremely small contact area on the magbase?

Although it rocks on two corners only one is high, it may be worth working on one high corner only.

Mike

Are you scraping away all the blue? I think you need to start breaking the blue up into smaller pieces each time. Do a rub on the plate and put up a picture.

Rich

If you have something that is rocking the bluing can be misleading. Imagine trying to blue a convex lens. As you move it it will roll and the blued area will be larger. A better strategy as Mike says would be either work from one corner or try to ascertain where the ridge or high spot is and remove it by rough scraping.

Regards.

David.

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It is probably better to think of the thing resting on three points, rather than rocking about two ...

MichaelG.

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Edit: Here’s some light reading for you:

https://archive.org/details/whitworthmeasur00whitgoog

[ ignore the title ... there is some excellent stuff about scraping ]

Edited By Michael Gilligan on 24/01/2020 09:14:42

I agree with the suggestions above, but also the small area (I assume 40-50mm square) means you will have to remove very small amounts of material.

Watch you don't end up with the classic 'dining table to coffee table conversion'.

Neil

You don't really need a scraped surface on a mag base. Assuming you are talking about the run of the mill dial indicator stand type with shallow V groove up the middle and thus standing on two narrow edge strips.

You might be better off to  tape a sheet of 800 grit wet rub paper on the surface plate and carefully rub the mag base around in a figure 8 pattern to get it flat. Then finish off with some 1200 grit same method.

Otherwise, be careful too that your rocking is not due to tiny burrs around the edges of the base surface. Dress with a small fine file or rubbing stone around the edges after scraping. And rub a fine stone over the scraped surface to knock down any burrs thrown up by scraping on the flat surface itself.

So you might as well go straight to the wet rub paper method really.

Edited By Hopper on 24/01/2020 10:44:37

Bought one of these from Amazon, probably not a good idea,

base was so not flat I put it on the milling machine. A review *** HERE *** which covers some of the other work I needed to do to make it useable.

John

Thank you, Michael. I had to revise a little French in the section referring to end and line standards. Erudite chaps in those days!

Bill

Here's my rub:

Although to me this looks somewhat decent, some of the corners seem to produce a hollow ringing sound when tapped, as if there was some rock to them. at this point, however, I'm starting to feel like this changes depending on the exact position on my (second hand) surface plate, and am starting to wonder if maybe I'm approaching the limits of what my plate can do.

Thank you, I'll certainly have a look through that when I have a moment, looks like quite the read

I did consider that, but I am rather nervous introducing abrasives to my first and only reference surface :D don't want to unknowingly rub a hollow in my plate, by doing something wrong.

John, mine cost about the same as yours, but also included a cheap dial indicator in that price -- no wonder the quality is not up to scratch. Shame, I don't have a mill to true it up on, so this is my only real option on fixing it up.

Have you run a 0.001"/0.0015" feeler gauge around the joint when it is on the surface plate ?

The feeler will enter at the gap & show you where you don't want to be acraping - it is easy to rock a part when bluing it & get a false sense of what is touching. My former fitting colleagues used to thump all four corners of (larger) parts when placed on the plate to feel if a corner was high & not resting on the plate, then work the diagonally opposite corners until the part sat flat in contact. Only then would they start to work on getting alignment & when close to alignment start working on getting a good bed. They always checked around the part as they progressed with a thou (0.001" feeler gauge ("tester" & when getting close would refer to progress on the part as being "tester tight".

Most feeler gauge sets don't include a thou gauge, with thou and a half being the smallest - Starrett sell thou feeler strip, but only in long rolls (used to be arounf £40 a pop).

Nigel B

Nigel, I dont even own a set of feeler gauges, much less a specialty 1 thou shim stock -- i wish, though :D

Thumping all four corners is what's giving me the idea that something may be wrong -- the base is not rocking visually, but thumping corners makes a ringing sound that makes me believe something is off.

Lost a posting from earlier ... but I asked how much it rocked if it was actually like a rocking chair! A feeler gauge under the corner would give you an idea of how much needs to be scraped. More than a about 0.05mm would need a lot of scraping.

If no mill, mount it in the 4 jaw chuck and skim?

I agree with Hopper here - try gently moving your mag base around (in a figure of 8) on a fine grit paper and look for bright spots on the edges where it may have been 'bumped'.

I had a problem with an old Myford vice recently that was a few thou off level when I checked it (because I couldn't get an even cut in my hand shaper). Using fine grit showed some high spots on one edge where it had perhaps been dropped. These were easily removed in a few minutes. I don't see any problems with using a fine grit paper on my surface plates, provided I don't move the work off the paper and clean up carefully afterwards....

I could of course have tried scraping this but I think (in my case) it would have been a lot more hassle to get to the same result with something that was very localised in nature - as opposed to removing small amounts of metal across a broader area - where scaping would have been a better approach...

Regards,

IanT

Originally it rocked quite a bit, after some scraping now, the rocking is not visually noticeable -- what trips me up is thumping the corners still makes it ring, as if it were rocking.

Unfortunately, no lathe either -- I have an old hand shaper, which is so worn I would not trust it to make it any better than factory finish, unfortunately. Until I have time to scrape that, hand tools it is for me :D

Thank you -- I may try to sand it after all then, although I was pleasantly surprised at how fast it was to remove material by scraping, at least in whatever pot metal the stand is made of.

Feeler gauge

I dont even own a set of feeler gauges

Must be the cheapest bit of precision measuring kit going - and readily available pretty well everywhere.

Nigel B