scraping technique

[Douglas JohnstonParticipant @douglasjohnston98463]

There is some confusion in my mind about the best technique for scraping a flat surface. I have in the past put the blue dye on a surface plate and then carefully moved the object to be tested over the surface plate. On lifting the item off the plate I scrape the spots which have picked up the blue dye.

Other people seem to put the blue dye on the surface to be checked, leaving the surface plate clean, then move the item over the clean surface plate. On lifting the item the scraping takes place on the areas where the dye has been removed.

I suppose both techniques amount to the same thing but is there any reason to use one technique over the other?

Doug

[Douglas JohnstonParticipant @douglasjohnston98463]

[mechman48Participant @mechman48]

As an apprentice I was taught to blue up the surface plate with a roller or dabbing with a cloth pad as the plate would be the master reference; apply the object to that, give a light pressure circular / back & forth movement, dependant on the size of the object & scrape off the high spots revealedon the object. I still occasionally do a bit just to refresh my grey matter, & for the fun of it.. sad eh . there are a plethora of videos on YouTube relating to scraping techniques, have a browse.

George.

[Kiwi BlokeParticipant @kiwibloke62605]

Well one good reason for not blueing the workpiece is that, at the end of a scraping pass, you would end up with a mess of scraping swarf mixed in with blue, which you would have to clean off, before removing the burrs. You'd also have to clean the plate. Doesn't sound like a pleasant process. Another reason is that it would not be easy to achieve a well-controlled blue film thickness, particularly in awkward places, such as dovetails.

[Pete RimmerParticipant @peterimmer30576]

If you blue the part you have to clean and blue it every time. Blue the plate all you have to do is clean the part every time, and run the roller over the plate to re-distribute the blue.

Also, if you blue the plate you rub the part gently to transfer the blue onto the touching points. the part should not touch the plate, only the blue. If you blue the part you're then using the plate to scrape blue off the high spots. Unnecessary wear on the expensive plate.

[Douglas JohnstonParticipant @douglasjohnston98463]

Looks like I have been doing things the right way then. I just wondered if there was any special reason why bluing the part rather than the surface plate is advocated at all by some people.

Doug

[Pete RimmerParticipant @peterimmer30576]

Few good reasons to do it the other way IMO. Scraping tapered gibs in-place for contact perhaps.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Pete Rimmer on 17/09/2020 12:37:19:

If you blue the part you have to clean and blue it every time. Blue the plate all you have to do is clean the part every time, and run the roller over the plate to re-distribute the blue.

Also, if you blue the plate you rub the part gently to transfer the blue onto the touching points. the part should not touch the plate, only the blue. If you blue the part you're then using the plate to scrape blue off the high spots. Unnecessary wear on the expensive plate.

.

Nice concise explanation, Pete

Noted … and greatly appreciated

MichaelG.

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