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24" plate master scraping question

small.planes

Aluminum
Joined
Sep 5, 2008
Location
Leics UK
I posted this over in the metrology forum, but it was suggested that I might get a better answer here to the scraping part of the question.

Some years ago I bought a surface grinder. With it came a 24" cast iron surface plate that some muppet had 'reconditioned' lightly with a flap disk!
It was free, I had a trailer, and I needed a welding plate that was 'flat' for some level of flat, so I took it.

I also have a 30x48 Cast iron table, which looks unmarked - original planer marks are on it, only minor scratches. I have no idea how flat it is, it also is use a lot as a bench (covered with 3/4" sheet of MDF + anti corrosion paper to protect it).

I have no intention of scraping the 30x48, although I would like to understand how flat it is at some point. It seems useful to have a large flat thing for as yet unknown reasons (it was £50 and is a useful 'bench')

So I am going to buy a 450x450 (18") plate, probably granite and set it on its own stand. Better suited to the general size of things, and not a bench.

However the 24x24 seems like an ideal flat thing to start on, except I have no 24x24 or larger known master.

Can I scrape a 24x24 flat using an 18x18 as the master, but moving it around? something like do 18x18, then move and cover the last 6" or similar.

Comments - even 'no you cannot' type welcomed. :)

Dave
 
Dave,

Yes you can.

I remember an article about an old guy (a pro) re-scraping a large jig borer
with just a 24" prism master. Richard King talked about that article in one
of his posts.

Paul
 
Can it be done? Yes.
Could I do it? Very doubtful.
Could you do it? Equally doubtful.

I did once scrape a long part with a shorter reference. It worked out great until I bought a longer reference then I found it was dished. It's like asking a first-year apprentice bricklayer to build a vaulted arch.
 
I've never done a large cast surface plate but would like to get one eventually and I'll need to be able to check it. My understanding is that you can use a smaller calibrated surface to print and check the whole plate in sections, but then you need to determine that it isn't twisted or bowed overall, and for that you can use a long straight edge, preferably at least as long as the plate, and use it to check the plate length ways, diagonally, and short ways. All of the individual prints have to be considered together and you need to understand how each print hinges and see if one side is printing heavier than the other. It's the same methodology you would use when using a smaller standard to print a bigger machine way.

When they calibrate big granite plates, they do it the same way. Use one method to determine overall linear flatness, then use another method to determine small area "local" flatness.
 
It's not the ideal way to start scraping, but it is do-able.

Overlaps put a premium on being able to "read" the prints, and on checking "hinging" a lot. I think it is still easy to get in trouble.

Your case is almost ideal, since you have a 2/3 vs 1/3 split on the overlap. Even so, I'd bet you get fooled on the overlap prints, and have to end up re-doing the scraping to correct a dip around the 18" mark.

Given the little I have done that way, I think I would suggest "roughing" all of the plate first, then going back to refine it. Getting 18 x 18 good, and trying to make the rest match I think will be misleading and get you in trouble. At least it has gotten ME in trouble.......

Interested to hear how you do.
 
I'm not sure why you don't just get a 24" x 24" granite and be done with it. It's just a little bigger and would save headaches working on the cast iron plate (although a bigger reference would be better still).

Heck, get fancy with your stand so you can somehow have a counterbalance pivot between the two so you can alternate which is in use at any time, but maintain the same footprint.
 
I seem to be in the camp of buy as big a an A grade granite plate as one can and use it to recondition smaller cast iron plates as they become available then use the cast iron for day to day work when it’s small enough. Seemed logical at the time. And I find iron plates easy (not quick) to rescrape.

Not sure if I’d want to rescrape a 30” x 48” iron plate even if I had access to a larger master plate. Lifting gack can be rigged up easily enough, and the scraping would just take time, but I’d imagine hinging something that heavy would be difficult. Maybe I’m wrong?

L7
 
24” granites are close to twice the price of 18” ones. Given most of the work I do is watch sized 18” seems big enough. I did consider a 12”, but I think that might end up being just a bit cramped.
I’m only considering scraping the 24” cast iron plate because I have it already, and it was free.
It’s been kicking around in the workshop for 10 odd years...

Dave
 
Dave,
How flat/good are you looking to get the cast plate. If you just want a reasonably flat surface to work from the answer is yes you can scrape it with a smaller master. But you will be doing a lot of back tracking (correcting errors you made) and it will be time consuming. In the end if you are careful, have some skills and patience you could have a reasonably flat surface. I say go for it. You will gain a new skill and appreciation for the art of machine tool scraping. good luck

Ron
 
I had a 12x18" cast iron plate Blanchard ground for $75 recently. The operator told me the plate had 0.025" of wear in places. Scraping that off would have taken a year. If I were going to scrape in a cast iron plate I would start by having it ground. That would probably get it within half a thou which is much more doable.

metalmagpie
 
I posted this over in the metrology forum, but it was suggested that I might get a better answer here to the scraping part of the question.

Some years ago I bought a surface grinder. With it came a 24" cast iron surface plate that some muppet had 'reconditioned' lightly with a flap disk!
It was free, I had a trailer, and I needed a welding plate that was 'flat' for some level of flat, so I took it.

I also have a 30x48 Cast iron table, which looks unmarked - original planer marks are on it, only minor scratches. I have no idea how flat it is, it also is use a lot as a bench (covered with 3/4" sheet of MDF + anti corrosion paper to protect it).

I have no intention of scraping the 30x48, although I would like to understand how flat it is at some point. It seems useful to have a large flat thing for as yet unknown reasons (it was £50 and is a useful 'bench')

So I am going to buy a 450x450 (18") plate, probably granite and set it on its own stand. Better suited to the general size of things, and not a bench.

However the 24x24 seems like an ideal flat thing to start on, except I have no 24x24 or larger known master.

Can I scrape a 24x24 flat using an 18x18 as the master, but moving it around? something like do 18x18, then move and cover the last 6" or similar.

Comments - even 'no you cannot' type welcomed. :)

Dave
Technically, a surface can be made flat without a larger master. There is a Japanese video out there with a couple of guys scraping very large plates, using small plates for local flatness, and using autocollimation for the overall geometry.
The problem with using a smaller master under a larger part is that the unsupported portion of the part will impact the print, and you will have no way of knowing what are local highs or gravity is just fooling you. A much better option is to use the smaller master on top of the part. You still have to deal with overlaps, but if you developed skills in step scraping and reading the prints on that, you are a bit closer to getting the part flat. But I can tell you flipping an 18x18 granite on top of your CI plate will be a workout. New 24"x36" plates can be had for under 500 bucks.
 
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24” granites are close to twice the price of 18” ones. Given most of the work I do is watch sized 18” seems big enough. I did consider a 12”, but I think that might end up being just a bit cramped.
I’m only considering scraping the 24” cast iron plate because I have it already, and it was free.
It’s been kicking around in the workshop for 10 odd years...

Dave
Not sure how it is in the UK but over here, if you're patient and watch the auction sites, granite plates are quite affordable. I bought a 24" x 36" grade A black plate for $50. I recently also bought a 4ft by 5ft starret pink grade A, lst calibrated a year ago, on a stand for $750. Both bought locally. While $750 is not chump change, the pink plate is something like $6K new.
 
Magpie, did you get you Blancharded plate hight temp stress relieved after grinding?
I guess your question implies that Blanchard grinding induces stress in a cast iron part. How does the process of Blanchard grinding compare to, say, common milling of cast iron which does not seem to induce appreciable stress in cast iron parts—-at least not in blocky rectangular parts as opposed to something very long and thin like a gib where some deformation due to machining might be likely especially if that was a gib that had a ton of retained stress related to 20 years service in machine.

But in machining many many of my stress-relieved straight edges and from customer reports, deformation due machining is not observed.

Denis

Added: where is “Cash”? He has a wealth of grinding knowledge. Haven’t seen him post in a year or more.
 
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Denis, my grinding experience is limited to 10”x24” surface grinding and not Blanchard so would defer to Cash and other's knowledge. I have seen long iron sections noodle up (not your product) and definitely seems more with surface grinding than milling. In general grinding will put more heat into a part than milling, even with an open fresh dressed wheel and coolant, and I would assume Blanchard in particular with its fast metal removal rate will heat up a part more. Experts, please tell me if I’m wrong.

I’m lazy, and prefer to do a job just once and not have a piece continue to move. Esp scraping that takes a fair amount of time. My question to Metalmagpie was partly curiosity, partly based on experience making 24” parallels and other long iron sections. Given how cheap it is for me locally to add a small item like his surface plate to a heat treat load at a friend’s place, I’d definitely do it after grinding 25thou off at Blanchard speed.

Edit (now if I just had access to a place locally to stress relieve a 6’ long chunk of cast iron…)
 
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Every cast iron gib I've surface-ground for turcite has bent regardless of the amount of coolant used soI'm betting that blancharding a plate will do too.

Where I can I grind both sides to pull them back, otherwise I hammer-straighten them then scrape to finish. Did this one last week.


bent gib.jpg

straight gib.jpg

turcitegib (21).jpg
 
One thing I found when scraping gibs was that they're very sensitive to temperature variations. If one face is even a little bit warmer than the other, the gib will bend by a thou or more. After an operation like surface grinding, they can need an hour or so, resting on or just above the surface table to settle down again.
 
Yes, gibs are “all surface and no meat” so they would be the poster child for heat and stress issues.

I can recall grinding hardened 7” long 1/2” thick by 1" wide thick tapered arm that had many large lightening holes in them—-a lots of surface area, little metal, and a lot of air if you will. I tried several strategies of wheel dressing and generous coolant but they curled up like crazy. The thing that worked was to take a fairly light cut, of course, but to traverse the part super fast. My guess was that the part was traversed before the heat had a chance to migrate more than a few thou from the cutting surface into the part. I’d wait a few minutes for the heat to even out and again make a fast cut. It worked very well. Certainly that instantaneous hot foil-like surface exerted some bending force on the part, but it was so thin that it had little tensile strength. Different story once the heat soaked in an eighth of an inch or so and raised that amount of metal 10 0r 20 degrees maybe. That may not be a true explanation of what was going on, but it made sense at the time and I still don't know a better explanation.

Denis
 








 
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