Scraping a Cast Iron Surface Plate in a Hot no AC Shop

Richard King · Jun 22, 2015

I thought I would help Douglas (dsymes ) out here and restart his thread as his was closed. He is new and didn't understand the rules of putting the Subject in the Title.
http://www.practicalmachinist.com/v...ing-scraping-inspection/some-like-hot-305527/

Here is his question to me.

dsymes --Plastic --Join Date
Aug 2010 ---Location -- WA, USA

In his recent thread, "Teaching a Class in Oregon", Richard mentioned "working 10 hour days in the maintenance shop in the heat (90 to 107, so far)". While my first reaction was to wipe my brow in sympathy, it also occurred to me that the granite surface plates, straight edges, iron surface plates (Richards photos show one such plate being scraped), and other work being scraped were cycling from moderate temperatures at night and the early morning to quite warm in the afternoon.

So... I'm hoping that Richard and others can comment on what effect this kind of temperature change during the course of a day has on the percision of (1) a typical grade "A" or "AA" granite surface plate's surface, (2) iron reference tools such as a three or four foot iron straight edge, and (3) precision scraped surfaces, such as being able to achieve 0.0002/foot flattness and 40ppi.

Douglas

Richard King · Jun 22, 2015

First of all when I have the Rebuilding / Scraping Seminars we do them in a convenient place or inside a shop the contracting company uses. I have to use what is supplied.

As far as the granite table at Mcnary dam I checked it and it showed an overall accuracy of .0003" and I think it was a 48" x 72". From the chart below it says it's a AA grade plate. Granite Surface Plates - Pyramid Granite & Metals

I was not sure when it was last calibrated but the plate was covered and they said it came over from the powerhouse where it was kept. The owner and scraper-hand of the cast plate was Dave Fesler one of the maintenance machinists at another dam who drove down to Mcnary and the surface plate was his personal project. It looked like a Taft Pierce design. Well ribbed for stability.

As Dee and RC have said temperature is something a rebuilder must always keep a handle on. I often tell the story of a Van Norman Crank Grinder I rebuilt in a one man shop down in Iowa several years ago. We were rebuilding the machine inside the owners small shop. I was using my HK-72 Ribbed Camelback straight-edge that I was storing on a table next to a wall and I was scraping the bed. It was getting late in the evening and he had the garage door open and the mosquitos were driving us nuts. I said lets go and in the morning I should have it in a few passes.

In the morning we started around 8:00. and the first thing I did was wipe off everything and re-blued the bed and checked the hinge or how flat the bed was and the Straight-edge pivoted high on the ends a lot.

I doubted my tests I had made the night before and figured I had missed some dirt of had a burr...so I cleaned, stoned and re-blued and was again high on the ends. I set the SE down on the work bench and stood back trying to think of what happened?? Then I figured it out...there was a window behind the SE and the summer sun was shining in on my SE. We then taped some newspaper over the window and left it alone for 4 hours and came back after lunch and checked it again and it was back to normal.

Several years later (2010) when i was teaching machine building in Taiwan for PMC. We would take tours and visit new machine builders to doing process engineering advice. One trip was to one of the many CNC Slant Bed Lathe builders. They had good size plant and had around the inside walls they had work stations or a production line so to speak. The lathes were on 3 points stands and if you can imagine the building set up as a clock at 8:00 they assembled the pre-ground beds on the stands; 9:00 they leveled /aligned the bed, head-stock, scraped the Turcited saddle, turret slide, tail stock to the machine; 10:00 they installed the ball screws; 11:00 they wired the machine, test ran it; 12:00 they took final test cuts and accuracy tests, etc.

They told us they were having a major problem with the accuracy test station as the machine passed sometimes and other's it was off a mile in the inspection and when they took it back to the scraping area, everything was fine. They said it was good some days and other days it was bad. I looked around the shop and looked up in the ceiling..and their it was a big line of windows and that day the sun was shining directly on to the back side of the machine and the radiant heat warmed up the back side and not the front side. They didn't believe me and I asked them if they had a laser heat gun. They used it and sure enough the back side was hotter then the front side. The cloudy days they didn't have problems or the night shift, just the sunny days.

I have seen where hot air heaters blowing on machines or AC units blowing on machines changed accuracies. I wrap my SE's with rags and use cotton gloves so conduction heat won't affect things.

All of this is explained in The Moore Book, Foundations of Mechanical Accuracy page 118
I also saw some info on Surface plates in the Connelly book page 41. Will add more in a bit. Rich

Richard King · Jun 22, 2015

Getting back to the question.

One needs to think about the use of the plate when it is finished and what accuracies you expect it to hold.

The ideal temp would be 68 deg's for a laboratory but we were in a work shop. The plate was probably 50 years old and the iron was aged. So it was pretty stable. The heat warmed the plate the same all over as the heat surrounded it. It wasn't hotter on one side or the other. The plate had 3 points or legs under it. Dave drilled and tapped the ends for the eyebolts and we hit the plate several times with a plastic dead blow hammer to wring it while we scraped it. It like a instant stress relieve and vibrates like a tuning fork or bell.

Yes it was hot, close to or over 100 degree's F on the week we did the class up in Hermiston OR as Stu called it High dessert. We did the class in a huge building with no AC. We started the day at 7:30 and it was around 80 degree's and by 1:00 it was HOT, close to 100 and I think Tuesday it was 107, we had a big fan but it was still hot.

Dave first tested his plate Wed. Morning and it was high on the ends but we did not know exactly how low the center was. Set a surface gage on the CI plate and swung the indicator from the high spot to the low spot then we then used my 18" Camel Back SE that we pre-tested it on the granite plate and touched it up one or 2 scrapes as it got a ding on it from the plane ride inside the tool box. Testing the plate we could slide in a feeler gage under it and I believe. we figured it was .002 with both tests and it was low in the middle.

Then used a technique I call "blind" scrape. Using the yellow high-lighter and stepped scraped it in sections until we estimated it was close to flat. Scraping the plate sections to gradually. Each pass or section was about 2" wide and we had 5 sections outside edge to the middle. We write on the plate side 5 times, 4 times, 3 times, 2 times, 1 time.On each side of the low spot middle. We used the estimate that he was scraping off about .0002" each scrape or 10 times before we started to blue it up and scrape the blue spots.

The picture shows the results. Getting back to the question. A scraper hand needs to always think about temperature as I wrote before. A question for everyone is before 1960 how many shops had AC? Not many and precision machines were made and rebuilt. So worrying about temperature is important but it is not going to kill a machinist who is working .0002/12" on machine tools.

If he was going to be scraping a super precision machine like a jig bore then I would suggest he scrapes the plate and machine in a temperature controlled room. In the Moore book he explains this in detail.

As Dave scraped the surface plate the blue was consistent as it got flatter and flatter hinting the temp variation was not changing the plate as he scraped. When he got more points I had him "shine" up the plate by rubbing it in another spot on the plate and it hinged and showed the blue the same way as the other spot. The granite did not change for us, to what we could measure.We did the best we could do with the place we had and the equipment we were using.

The ideal temp would be 68 deg's for a laboratory but we were in a work shop. The CI surface plate was probably 50 years old and the iron was aged. So it was pretty stable.

I have scraped some large Pratt & Whitney and Fosdick Jig bored in shops that were not temp controlled and did not have issues. These massive castings change so little it was in measurable when we tested it.

I can share one story about scraping a machine in Malden Missouri for Federal Mogel where we had issues with humidity more then temp. We were scraping the ways of a Cylindrical Grinder and the B&S 36" straight edge would grow with the humidity change. The temp in the mornings when we started scraping the machine was 80 F and low humidity and the straightedge tested perfect on their granite plate. By 1:00 PM the humidity was 98% but the temp was still 80 deg's F and to my surprise the SE changed after a few days of pulling out our hair we figured it was the humidity. We were lucky we had another SE the same size and we scraped it for Afternoon scraping and used the other we scraped and used in the morning.

They eventually enclosed their maintenance shop and put in AC and the next time I went back about a year later, the SE was solved.

AC or scraping and testing machines is best done at 68 F, but one can scrape machines at different temps as long as the machine or SE is warmed or cooled all over and not one side as explained before. In the winter when it is below zero up here in MN and we need to scrape a machine in someone else's shop we take our SE's and test equipment over 1 day before the job so it adjusts to that shops temp as the trip over there the equipment changes in the cold weather. That's my answer and story I am sure others can add more.
Rich

rjs44032 · Jun 22, 2015

Richard King said:
Getting back to the question.

One needs to think about the use of the plate when it is finished and what accuracies you expect it to hold.

The ideal temp would be 68 deg's for a laboratory but we were in a work shop. The plate was probably 50 years old and the iron was aged. So it was pretty stable. The heat warmed the plate the same all over as the heat surrounded it. It wasn't hotter on one side or the other. The plate had 3 points or legs under it. Dave drilled and tapped the ends for the eyebolts and we hit the plate several times with a plastic dead blow hammer to wring it while we scraped it. It like a instant stress relieve and vibrates like a tuning fork or bell.

Yes it was hot, close to or over 100 degree's F on the week we did the class up in Hermiston OR as Stu called it High dessert. We did the class in a huge building with no AC. We started the day at 7:30 and it was around 80 degree's and by 1:00 it was HOT, close to 100 and I think Tuesday it was 107, we had a big fan but it was still hot.

Dave first tested his plate Wed. Morning and it was high on the ends but we did not know exactly how low the center was. Set a surface gage on the CI plate and swung the indicator from the high spot to the low spot then we then used my 18" Camel Back SE that we pre-tested it on the granite plate and touched it up one or 2 scrapes as it got a ding on it from the plane ride inside the tool box. Testing the plate we could slide in a feeler gage under it and I believe. we figured it was .002 with both tests and it was low in the middle.

Then used a technique I call "blind" scrape. Using the yellow high-lighter and stepped scraped it in sections until we estimated it was close to flat. Scraping the plate sections to gradually. Each pass or section was about 2" wide and we had 5 sections outside edge to the middle. We write on the plate side 5 times, 4 times, 3 times, 2 times, 1 time.On each side of the low spot middle. We used the estimate that he was scraping off about .0002" each scrape or 10 times before we started to blue it up and scrape the blue spots.

The picture shows the results. Getting back to the question. A scraper hand needs to always think about temperature as I wrote before. A question for everyone is before 1960 how many shops had AC? Not many and precision machines were made and rebuilt. So worrying about temperature is important but it is not going to kill a machinist who is working .0002/12" on machine tools.

If he was going to be scraping a super precision machine like a jig bore then I would suggest he scrapes the plate and machine in a temperature controlled room. In the Moore book he explains this in detail.

As Dave scraped the surface plate the blue was consistent as it got flatter and flatter hinting the temp variation was not changing the plate as he scraped. When he got more points I had him "shine" up the plate by rubbing it in another spot on the plate and it hinged and showed the blue the same way as the other spot. The granite did not change for us, to what we could measure.We did the best we could do with the place we had and the equipment we were using.

The ideal temp would be 68 deg's for a laboratory but we were in a work shop. The CI surface plate was probably 50 years old and the iron was aged. So it was pretty stable.

I have scraped some large Pratt & Whitney and Fosdick Jig bored in shops that were not temp controlled and did not have issues. These massive castings change so little it was in measurable when we tested it.

I can share one story about scraping a machine in Malden Missouri for Federal Mogel where we had issues with humidity more then temp. We were scraping the ways of a Cylindrical Grinder and the B&S 36" straight edge would grow with the humidity change. The temp in the mornings when we started scraping the machine was 80 F and low humidity and the straightedge tested perfect on their granite plate. By 1:00 PM the humidity was 98% but the temp was still 80 deg's F and to my surprise the SE changed after a few days of pulling out our hair we figured it was the humidity. We were lucky we had another SE the same size and we scraped it for Afternoon scraping and used the other we scraped and used in the morning.

They eventually enclosed their maintenance shop and put in AC and the next time I went back about a year later, the SE was solved.

AC or scraping and testing machines is best done at 68 F, but one can scrape machines at different temps as long as the machine or SE is warmed or cooled all over and not one side as explained before. In the winter when it is below zero up here in MN and we need to scrape a machine in someone else's shop we take our SE's and test equipment over 1 day before the job so it adjusts to that shops temp as the trip over there the equipment changes in the cold weather. That's my answer and story I am sure others can add more.
Rich

Richard,

I worked for several years in an environmentally controlled room cutting radii on natural and synthetic diamond materials. I can say from experience that a change of just 2-3 degrees would affect radius size at the diamond as much as a micron 0.00004". This was problematic while trying to hold waviness and size tolerances to 0.25 micron or 0.00001" We worked with air bearing spindles and the best equipment and environment available at the time. And these were small parts most under 0.5"

My hat's off to you guys who can scrape these huge bearing surfaces in extreme shop conditions. Makes what we did look smaller than it was. Thanks for sharing your experiences. I hope to join one of your scraping classes someday when you're near NE Ohio area.

Best Regards,
Bob

Dresden · Jun 22, 2015

There was an interesting comment on a closed thread about scraping under a hot tin roof; the comment was that scraping was comparing surfaces and this is true at times.

Scraping is a combination of quantitve and qualitive machining.

In regular machining there is a surface spec with surface finish samples for comparison, in scraping there is qualitive analysis concerning spotting and quantitive analysis regarding spots per inch.

Correctly there is no difference except in method used to achieve a result.

Dresden · Jun 22, 2015

The biggest bearings I ever worked on were on a Westinghouse
Steam Turbine, the alignment pads were scraped the bearings were 16 " bore, the Turbine rotors were 100 tons, the generator rotor was 100 tons, the high pressure rotor was 40 tons

The hook on the crane with block weighed 7 tons.

Alex McGilton · Jun 22, 2015

Great post as always Rich, I guess the take home message is that for straight or flat references heat isn't so much of a concern as consistency of heat is.

Putting foreign manufacturer optioning aside, how much would you trust their inspection lab environment.
https://www.youtube.com/watch?v=x9-8jfOimx8

dsymes · Jun 23, 2015

I always like to read Richard's stories and enjoyed the comments and knowledge that has been shared on this topic. It seems to me that the wisdom shared so far adds up to:

(1) Don't let one side of your surface plate/machine/reference tool/scraping object get hotter than the other side or it will warp.
(2) As long as your surface plate/machine/reference tool/scraping object is in equilibrium with the surrounding temperature and is stress-relieved, you are probably OK to assume that your accuracy in spotting your work is not significantly impaired.
(3) Humidity alone can change the shape of a cast iron straight edge.

The first of these is obvious, but falls into the category of things that are easily missed. Richard's examples could apply to large shops and small and fall in with his frequent admonition to "be a detective".

Regarding the second insight, it would be nice to have an estimate of how (and if) Richard's confidence in the flatness of a granite surface plate decreased while it was heating up at the rate of 4F per hour (80F at 7:30am and 100F at 1:00pm) - is that slow enough to assume equilibrium? And at equilibrium, do you assume that an "A" grade granite plate certified at 68F is still "A" grade at 100F, or do you assume that it is probably "B" grade or worse at that temp?

Finally, I would like to suggest an alternate explanation to Richard's conclusion that his SE changed shape due to humidity in his story about scraping a machine in Malden Missouri. I don't question that the SE changed shape relative to the granite surface plate, but I can't think of any chemical or physical reason why a cast iron SE would change shape due to changes in humidity without changes in temperature. As Sherlock Homes famously said, when you eliminate the impossible, what remains must be true. I would therefore suggest that the granite surface plate changed shape due to the change in humidity, not the SE. Granite is porous while Cast Iron is not. If the underside of the surface plate was slightly more humid than the top in the high humidity, either because of a difference in ventilation or simply because it was shaded, is it possible that surface plate could have expanded more on the bottom than the top, becomming more convex?

Screwmachine · Jun 23, 2015

I was working in a place where one of the jig borers would catch direct sun from midday to late afternoon. Asked the boss if we could cover the window, he said it didn't matter. Zeroed out on a bore before lunch and showed him, checked it a few hours later and it was way off. Window got covered. :cheers:

Richard King · Jun 23, 2015

Douglas I was scraping a Cincinnati Grinder with aged castings and we would see the problem on the ways while using the SE. The same issues I would see with temp changes. I know it sound nuts that the cast iron SE changed on the ways and when we check the SE it showed the same distortion. I was not alone here we had degreed Federal Mogel Engineers involved with this issue too. We were rebuilding production machines and only had a small window to rebuild the machines We tried everything imaginable trying to figure it out.

Checked the plate with levels. I was also double checking the ways with a double bubble King-Way and the ways we were scraping. We figured out the 2 SE method worked and we did it.

I just nodded off reading the following. The scientists, professors and Engineers, can debate this. I doubt it was the plate as you can read on page 558 7.2.2 paragraph 2 mentions how 20 - 60 deg of humidity showed insignificant change in the granite plate. If it was in water it changes... Also some great info on cast iron stress relieving in 7.2.1.
http://doc.utwente.nl/69005/1/Bruin82dimensional.pdf

As I was reading it, I started to think I wonder if the water would evaporate on the iron causing the SE to cool a bit and that changed the SE straightness?

We tried wearing cotton gloves, but it still changed. i read someplace yesterday that 40% humidity and 68 F was ideal for inspection. Just not sure where i read it. enjoy. Rich

PDW · Jun 23, 2015

Richard King said:
As I was reading it, I started to think I wonder if the water would evaporate on the iron causing the SE to cool a bit and that changed the SE straightness?

Now, *that* is a very good thought and more likely, IMO, than the humidity itself causing a change to the CI.

I hate late autumn in my shop. The temperature swings always give condensation on my heavy tooling and I have to keep oiling everything to keep rust at bay. If I had the time/money I'd build a small enclosed space with AC just for doing the reconditioning work. It's on the job list....

PDW

gcude · Jun 23, 2015

Richard King said:
As I was reading it, I started to think I wonder if the water would evaporate on the iron causing the SE to cool a bit and that changed the SE straightness?

That's what came to my mind as I read the thread Rich. I think you have the answer now.

mloser1729 · Jan 14, 2020

I had an experience like this with my Herman Stone 48" x 96" x 16" grade A surface plate. I lapped one of the long sides to AA for scraping/checking straightedges.
One winter day when furnace was running a lot, I checked it and it was noticeably concave. After scratching my head for a while, I realized that the ceiling fan
was causing a warm draft underneath the surface plate and the top was insulated by the padded cover, causing a temperature gradient.

Forrest Addy · Jan 14, 2020

Richard said it all on the topic of precision scraping in heated environments, mitigating heat sources, thermal equalibrium, etc.

The only thing I could add is don't let your sweat fall on the work. Instant rust spot.

pressbrake1 · Jan 15, 2020

Screwmachine said:
I was working in a place where one of the jig borers would catch direct sun from midday to late afternoon. Asked the boss if we could cover the window, he said it didn't matter. Zeroed out on a bore before lunch and showed him, checked it a few hours later and it was way off. Window got covered.

In the installation guidelines that are in my sip jig bore user manual it states to avoid installing in a area/room with direct sunlight.
Its manifests itself audibly on a car getting hot on one side pinging in the heat

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