Another grinding question.

I have a surface plate, 12 x 18 inches. It has some abuse including rusting and some dings.

I will have an 18 x 18 good surface plate coming in a week. Now I want to scrape the damaged plate in, however to get the rust pitting out will be a very laborious process, especially as I am weak of arm due to a muscle wasting disease. So anyone know where I could get it surface ground, say taking 5 thou off. That would help to reduce the amount of scraping I have to do.

I don't want a precision job, just a good 5 thou removing, anything else I can scrape!

Regards,

Andrew.

Edited By Andrew Tinsley on 08/05/2017 14:52:43

If you are to get it surface ground why are you going to scrape it?

If you are going to scrape it why can you not mill it?

regards Martin

Forgive me, but I was always under the impression that scraping would give a more uniform surface than grinding. I have scraped stuff in the past that had been surface ground and found some highs in the work. An old timer said this was because there were hard spots in the casting and these were revealed when the usual bluing was applied to the master surface plate and the plate to be scraped was then "rubbed" on said master plate.

Now I have no idea if such hard spots are common or if it was a bad casting. The same old timer was quite convinced that a surface ground plate was much inferior to one that had been scraped by a skilled worker.

If I have a good surface plate with no hard spots, then if surface grinding is as good as scraping, then fine! What are other peoples opinion on this? I could well do without the exertion of scraping, if surface grinding is as good!

This is the beauty of the beginners section, you can ask stuff and not look too much of a fool!

Oh! I just noticed that you suggest milling it instead of scraping. Well most mills that I have used do not give a very good surface finish and the surface will only be as good as the accuracy of the slideways. I can assure you that they are nowhere near good enough to give you a true surface for a face plate. Maybe the same comment applies to a surface grinder? I really do not know!

Comments please,

Andrew.

Edited By Andrew Tinsley on 08/05/2017 15:57:58

For my last remark I meant if you are goint to scrape it after, you could mill it instead of surface grinding. This may be easier and or cheaper. after all if you are scraping you are going to generate your own final surface and as you say you just want to get rid of the pits.

regards Martin

possibly a silly question, but why do you want to surface plates?

That used to be true but hasn't been so in manufacturing for many decades. Of the common processes, grinding is the most accurate, and is only improved on by other abrasive methods like lapping. (Not all grinding is precision grinding though.)

Scraping is a skilled manual technique. In the main it's gone out of favour because it is hideously slow and expensive. However, it is strongly associated with quality bed-ways, one argument being that the frosty effect holds oil. Being a suspicious Hector, I wonder if we believe scraping means quality as a kind of folk memory, not because it actually produces the best surface. Anyone able to put me right?

In the home workshop different rules apply. We are unlikely to have a milling machine or a grinder capable of producing the very best results. What we do have is time. I'd think it perfectly reasonable to clean up a surface plate with a mill and then scrape it the old fashioned way against a reference.

There are a few Precision Grinding services about. I've no idea what their charges are.

Dave

Hello Martin,

I am sorry that I misunderstood your final sentence. You may have a point about milling to get rid of the pits and then scraping to get a good surface. I am still unsure about the accuracy of surface grinding compared to a scraped finish, using a really good surface plate as a reference. My very experienced engineer of the old school (now long since dead), was quite adamant that scraping would produce a much more accurate surface than that obtained by surface grinding, I have no personal experience to say one way or the other! Maybe others can chime in and resolve this?

Hello Duncan! I am sorry that I did not read my post thoroughly enough to prevent such ideas to popping into your head! It just shows that you have to be careful even in the beginners section!

Just had a thought about milling to get rid of the rust depressions. The plate is 18" x 12" and I don't know of anyone who has a mill big enough to do the job at one setting. Not sure I am too keen on repositioning the plate to complete the job, using a smaller mill. But then what do I know about milling!

I am sure that someone will chime in and say that unless you are experienced at scraping, then you are likely to make a complete mess of the job. Well I have done a fair bit of scraping, enough to know the pitfalls anyway. I would not claim to be an expert by any stretch of the imagination. I have scraped much smaller jobs with success, but doing an 18" x 12" plate may well be another matter. Time will tell!

Andrew.

Where are you located?

David

Hello Dave,

Thanks for putting me right about precision surface grinding. It has been many decades since the old timer said scraping was more accurate, so that all ties up!

It was getting on for maybe 35 years ago when I started doing some model engineering. Looking at my notes (amazing that I still have them). I asked the very same question on a forum, about the very same pitted surface plate. I asked about precision grinding to rectify the pitted areas and was torn to pieces by people who said that the only real way to get accuracy was to scrape it! Personal circumstance changed and I have only recently come back to the fold so it looks as though things have changed over the intervening years, which I suppose is hardly surprising! At least this time, the job will get done either by precision grinding, or milling out the corroded area and scraping!

Andrew.

Hello David,

I just saw your post, I am located in Rutland, just about midway between Leicester and Peterborough.

Andrew.

Scraping V surface grinding really boils down to how good is good enough and how good a surface grinder do you have access to. Typically basic surfacing grinding flatness tolerances are in the region of ± 1 or 2 thou per foot. Which can be improved on with extra effort but it starts getting expensive fast once you go beyond a few simple passes. Considerable overlap with milling capabilities and its quite practical to produce milled surfaces as flat as basic ground ones. For relatively unsophisticated processes the big difference between milling and grinding is in surface finish.

This link gives a nice short overview of practical tolerances and surface finish for various processes :- **LINK** . Downloads a short pdf file with figures & text.

Scraping accuracy pretty much depends on how patient you are and how good your master(s) are. Allegedly any patient person willing to learn the technique can get to 50 millions of an inch per foot (0.000050, half a a tenth of a thou). If you really want to work at it an order of magnitude better is quite possible.

Clive.

And I thought DW had made a typo and was asking why you wanted two suface plates!

Not a myth that scraping produces a better bearing surface on cast iron than grinding. Low speed tribology is a complex subject but the oil retention effects of multiple scraping depressions are real. its been demonstrated with stick-slip measurement of the power needed to shift lathe and mill size slides from stationary up to normal travers speeds. Generally considered necessary that ground beds should be hardened to get a satisfactory life which further indicates lubrication issues. Interestingly many of the last generation manual "super-lathes" had hard tool steel inserts for the main bed guideways with scraped cast iron saddles running on them. Theory was the hard steel guides could be replaced when worn. In practice it was exceedingly rare for the machine to wear sufficiently for replacement to be needed. Allegedly adding matching hard steel insets to the saddles didn't work out too well and was never done.

Clive.

Were are the grinding companies that can grind 18" x 12" cast iron surface plates at a reasonable cost ?

Correct, should read more carefully before pressing 'add posting'

Looks as though I should go the scraping route, it should keep me off the street!

Andrew.

My 24"x24" surface plate has the appearance of being done on a Planer and does me fine. I wonder just what accuracy of work people try to do at home that requires anything more accurate than say the table on their mill. The only thing I can see is the satisfaction of knowing they have scraped the job.

Ummm, Clive? One or two thou per foot is when you tear down the grinder for a rebuild. Even one or two tenths per foot raises eyebrows over the condition of the grinder....

Pete

Hello Chris,

What is wrong about that? Each to his own, I say! Why even bother wasting ones time doing model engineering? Yes it is the satisfaction of doing the job, be it scraping or anything else.

Andrew.

P.S. I don't have a mill!

Well I have a J&S 540 which is 18 x 6 capacity and am in Nottingham if that is any help.

David