A good quality surface grind will probably be flat enough for most practical purposes. A scraped finish will look nicer, be flatter and a better surface for bases to glide over but unless you fancy the challenge it could save a lot of work to not go further than the grind. The surface of my plate is not scraped but doesn’t look surface ground either, it seems devoid of any type of machining marks.

Mike

My 36"x24" Windley Grade B (workshop) surface plate doesn't look like it's been ground or scraped:

There are a series of horizontal lines about 5/16" apart, almost as if it's been planed. Unless you're rebuilding machine tools part by part I'm not sure how a surface plate helps with machine tool setup.

Andrew

There is a sequence for lapping a surface plate absolutely flat, but involves lapping two others together and using the result to lap the final one.

Very much an iterative process, obtaining an improvement at every stage.

So neither quick nor easy.!

Full marks for aiming for the best that you can get with the facilities available. Go for it.

A machine tool needs to more accurate / precise than the work that it is required to produce. Tolerances and clearances, may be minute, but they build up into quite sizeable figures. 0.0005 on a shaft and 0.0005 on the bush in which it runs will give 0.001 clearance. Do that on the location of the bush in the Headstock, and on the Headstock / Bed locations and things start to look less than precise!

Howard

Scraping in a 18" plate is a lot fo work it's huge surface area. Granite plates by comparison are quick to lap in and reportedly wear more slowly. Certainly they can suffer clumsy handling without taking damage more than an iron plate can.

I recondition surface plates from time to time I tink I have seven in my workshop right now. Small iron plates I scrape, granite plates I lap with diamonds. Invariably they are out of certification, some by many years and I can tell you this – I never yet saw a used plate that wasn't out of specification for the last ticket that was on it and only one that came close.

Remember this when buying any used plate – you absolutely cannot rely on the 'grade' sticker and should completely disregard it as part of any selling spiel. Grade A, grade B it doesn't matter – they almost all need re-working.

I know there is a desire to see nice shiny surfaces but I wonder how unserviceable your surface plates actually are. Have a go at removing the corrosion chemically (Arc Euro do stuff for this) and then check one plate against the other. Once they are cleaned up I would be surprised if they are in any way unfit for service.

Incidentally does anyone use a wax on their surface plates to reduce friction and protect from corrosion?

regards Martin

Don't let me put you off Phil, but it takes a long time to remove metal by scraping as needed to restore a badly worn machine. For example, lathe beds tend to wear badly close to the chuck where the saddle spends most of its time make it necessary to remove a lot of unworn metal to restore flatness. Unlikely to pay off unless scraping is to be your main hobby! There's also an alternative technique for putting worn surfaces back with goo,

Scraping as a way of producing an original flat surface is rarely done today. Instead laboratory flat surfaces are produced by grinding, or perhaps a plane. The machines are laser guided and capable of producing highly accurate surfaces with minimal labour. Surface plates, starting at about £60 are cheaper than the tables needed to support them, and I feel there's not much point in a home workshop going the full Monty on a plate unless the same effort is put into supporting it correctly! Home workshops are full of dinged, out of tolerance, bent surface plates, well out of tolerance by professional standards but good enough for ordinary use. Float glass plate was once recommended as a cheap substitute.

Scraping has two main purposes today, one of them dishonest! Scraping being associated with quality, scrape marks are often added to create the illusion a piece of equipment is better than it actually is. (Same was done in the past,) More usefully, scraping is often added to provide a key to hold oil and improve lubrication. Neither of these two forms of scraping improve flatness.

Where scraping is useful for repair work is on worn plain bearings, especially if they've suffered a lube failure. Plenty of machines have roller bearings, which are just replaced.

There's always a balance between getting on with the job by flashing cash and saving money by DIY. Though I see the attraction I've decided scraping isn't worth my time. Would a renovator comment please; how much scraping is done during a typical rebuild? I get the impression time-consuming bulk flattening is usually outsourced to a professional grinding company, so the renovator can concentrate on the the machine's other needs. I may be wrong!

Dave