Key Questions to Ask When Ordering Inspection Surface Plate

Sorry I forgot to say how much I paid for it. It cost me $75 bucs and a one hour drive to go get it. I was either going to buy this Enco 24x18x4 with ledges (http://www.use-enco.com/CGI/INSRIT?PMAKA=640-&PMPXNO=&PARTPG=INLMK3) but then I saw this Starrett one on Craigslist.

John/Hemlock that is good to know. So basically it is normal wear I'm assuming. I don't think I would need to resurface it or re-calibrate it for the hobby stuff I do, but I would like to see how it is done since I have just started taking up scraping and I find it kind of fascinating all the things that are derived from having a flat reference.

On another note, when I brought the stone in with my cousin, it was a pain to move with a weight at 248lbs. It occurred to me how the hell did them Egyptians move those lime stones from miles away and how did some of the other ancient people flatten the stone so true at puma punku.

edit: I would like to note that there does have a sticker saying calibration not required. I'm curious to know why that is there because I have heard 3-5 years is the average before you need to re-calibrate a pink stone.
http://i14.photobucket.com/albums/a318/Micscience/100_.jpg

Sorry I forgot to say how much I paid for it. It cost me $75 bucs and a one hour drive to go get it. I was either going to buy this Enco 24x18x4 with ledges (http://www.use-enco.com/CGI/INSRIT?PMAKA=640-&PMPXNO=&PARTPG=INLMK3) but then I saw this Starrett one on Craigslist.

John/Hemlock that is good to know. So basically it is normal wear I'm assuming. I don't think I would need to resurface it or re-calibrate it for the hobby stuff I do, but I would like to see how it is done since I have just started taking up scraping and I find it kind of fascinating all the things that are derived from having a flat reference.

Pits, scratches, or even small chips do not effect the performance or accuracy of a surface plate. They may indicate abuse however. This plate was a fantastic deal. It's worth 10X what you paid (for new that is).

Couple words for you:
1) These plates are designed to be supported in specific places. You may find three marks in a triangle shape on the bottom of the stone where it was supported during final lapping.

2) Crystal Pink is very slow wearing. It has a high quartz content. I clean mine with gojo grit-less hand cleaner. Keep it super clean and covered when not in use. I also level mine. My surface gage will float across it when the plate is clean.

3) If you use it for spotting, you won't be able to get 100% of the dye out. If you use the blue paste, (prussian blue oil paint? or Dykem spotting dye) you will end up with a purple surface plate. I use mine only as an inspection plate, and not for spotting. I prefer the black granite plates for that. But mine is not as large or as accurate as yours (12X18 B grade).

Surface plates are used as reference surfaces to determine how straight or flat an object is, but also as a common (flat) surface used to compare objects, determining parallel or square.

For hobby or part time machinists like me, a good reference plate is really helpful. I recommend looking for exactly a plate like yours because they are expensive new, slow wearing and often super cheap (as yours was).

I buy a lot of used tools and it is impossible to determine what the tool is without a way to measure/inspect it. For example: every machinist needs a master square (Starrett No 20). These are super expensive new and worth little used since few people have the means to calibrate them to determine if they still retain their original accuracy (."/6"). Other examples include used parallels, used 123 blocks, and other tooling. Such things can be inspected accurately with a surface gage holding a good tenths indicator, or a planer gage holding something similar.

I just purchased a used Starrett 24x18x6 pink crystal grade AA surface plate which brand new it is certified to be within a 0. tolerance. It has five times better the wear resisting qualities of the dark granite versions.

Now, I do think I got a good deal but I do feel a few tiny little holes in the surface which are barely visible to the eye. I know there are many machinists here who have plates I was hoping you could throw your 2 cents in if ya don't mind. The plate came from a shop not sure if it was a machinist shop or some type of engineering shop and was given to the seller for free then sold to me.

My question is if it was a decent buy? This is my first surface plate so I don't know how they come brand new.

Also are those tiny holes normal wear?

You got a truly excellent deal on one of the best grade of surface plates (tiny twinge of envy, mine is an old black granite Collins)

The obvious question would be, how close do you wish to work?

Any used plate is an 'unknown', which may have been abused.

A plate of that quality, gotten at the 'give-away' price you paid, justifies the cost of having a specialist metrology firm inspect it, and re-lap if necessary.

Only then will you know what you have.

As has been mentioned, it should have three thick rubber pads, at specific points....if those are missing, get new ones.

cheers

Carla Larry- Thanks I just wonder why HP would use a grade AA for a stand when they could get a cheaper one but I get what you are saying and the plate could have had many different jobs during it's life at the shop.


Adam- It does have the three feet at the bottom and it stands level since my bench is level however I need to build a stand and some kind of casing.

I also am curious how to clean this plate. You mentioned gogo cleaner but I have read some soaps can be harmful to the plate. I am going to have to research what counter top cleaner to use which will clean and not harm the plate.

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("3) If you use it for spotting, you won't be able to get 100% of the dye out. If you use the blue paste, (prussian blue oil paint? or Dykem spotting dye) you will end up with a purple surface plate.")
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That is precisely why I bought the plate for. I was hoping to scrape some items and I was going to use the plate as the reference by prussian bluing it up. That kind of sucks there has to be a way around this.

Carla-Thank you, I was hoping to hand scrape my lathe bed to within a tolerance of .001, if I can get better then that, so be it. I wanted to have an accuracy of at least .001 on parts made from this lathe I am building. I have no machinist experience so my goals are set a bit low to accommodate my learning curve. Thanks for the feedback guys.

Adam- It does have the three feet at the bottom and it stands level since my bench is level however I need to build a stand and some kind of casing.

I also am curious how to clean this plate. You mentioned gogo cleaner but I have read some soaps can be harmful to the plate. I am going to have to research what counter top cleaner to use which will clean and not harm the plate.

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("3) If you use it for spotting, you won't be able to get 100% of the dye out. If you use the blue paste, (prussian blue oil paint? or Dykem spotting dye) you will end up with a purple surface plate.")
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That is precisely why I bought the plate for. I was hoping to scrape some items and I was going to use the plate as the reference by prussian bluing it up. That kind of sucks there has to be a way around this.

Carla-Thank you, I was hoping to hand scrape my lathe bed to within a tolerance of .001, if I can get better then that, so be it. I wanted to have an accuracy of at least .001 on parts made from this lathe I am building. I have no machinist experience so my goals are set a bit low to accommodate my learning curve. Thanks for the feedback guys.

With respect to my friends here, this is the wrong forum for this sort of discussion. You should ask in the metrology section of practicalmachinist.com Ditto for the scraping. And BTW, scraping a lathe bed to .001 produces better then .001 on a cylinder turned on that lathe. Remember what that bed does. You can easily produce parts to .001 on a lathe with a worn bed. Turning perfect cylinders is the problem. And scraping a lathe bed (depending on the bed, is about the hardest thing you can do. You will need other masters besides the plate.

For surface plate cleaners, many old timer machinists recommend the gojo. Some use windex, others plain old rubbing alcohol, some even recommend WD-40. I forget what the Starrett plate cleaner is, but it is nothing special.

There are significant issues with some other cleaners. I would not use granite counter top cleaners until you know they are safe. Some leave stuff that attract dust. Alcohol actually cools the surface of the plate which can be a problem for stuff on top of it.

Cleaning Dykem off a plate is different from cleaning a plate normally. You have to remove dust and oil from hands and tools off a normal plate. Cleaning spotting usually requires mineral spirits followed by gojo or something similar.

Couple thoughts:
1) Assuming your plate is AA grade, it might be a good idea to not touch it AT ALL. Don't use it for spotting, don't put your coffee cup on it. Use it for inspections only and only when you get the experience you need (assuming you aren't experienced) to use it properly. You bought a Ferrari cheap. Don't use it as your daily commuter car. I would assume you bought a $750 surface plate and treat it accordingly.

2) If you must use it for spotting, maybe use the red dye! You will never get it all out of the pores of the stone, but maybe it won't look so bad.

Be sure to check out practicalmachinist for more information on surface plates and scraping.

What lathe are you working on? Adam thank you for that response. I have tried asking or figuring out what you just mentioned about the accuracy of a cylinder from a bed that is flat to .001 and I never got a clear answer before.

I read yesterday that some guy who calibrates the plates uses gojo so I might give it a shot til I can get the real stuff.

Ok I get what you are saying that I shouldn't use this plate for basic stuff but if I would have known that from the beginning I wouldn't have even bought it. My main goal was to scrape many parts for my lathe and make my own straight edge. I tested an area and it came off better then I expected however if blued often the plate will eventually show a trace of blue on it.

I am in the middle of building the Gingery lathe and I am well aware of the lathes capabilities. I am almost half way done. If you notice in the background of the picture you can see the bed and with the ways installed on the plate. I was lucky someone sent me a box with all the patterns of the lathe so I didn't have to make them except for the bed, I did that myself. I am addicted to this build believe it or not, that is how much fun I been having. That being said I am still going to buy a lathe. I have been looking around craigslist and many lathes are selling really cheap. So it won't be long before I'm turning some parts.

With respect to my friends here, this is the wrong forum for this sort of discussion. You should ask in the metrology section of practicalmachinist.com Ditto for the scraping.

Couple thoughts:
Use it for inspections only and only when you get the experience you need (assuming you aren't experienced) to use it properly. You bought a Ferrari cheap. Don't use it as your daily commuter car. I would assume you bought a $750 surface plate and treat it accordingly.

Be sure to check out practicalmachinist for more information on surface plates and scraping.

Micscience,

I'll first say congratulation on getting a nice coveted pink Starrett surface plate. While we will all see used tool grade surface plates on cragislist going cheap and not so cheap, it's very rare to find a AA pink starrett especially a 6" thick one in that size and at $75 it's a practically free when compared to a new price and to what these typically sell used for.
For the most part random small tiny holes will most likely not effect the accuracy that you need. I do see what looks like a scratch but that maybe part of the granite?

Not trying to stir the pot but in regards to not asking on this forum and instead you should go to Practical machinist. I have never been to a more arrogant, rude egoistical forum site in my life, it's very sad. If your new and ask a silly newbie question they will rudely rip you apart, call you names and then close the thread. It's like they are licking there chops waiting for newbie's, they almost seem to live for it. While not everyone on that site is like that (they're some very helpful members) but the bad ones are very loud and sadly wide spread including the moderators. The site does give you enormous and valuable information but wanted you to warn you and be aware on how that site operates. I think your getting some good information here on GJ in regard to your questions


I recommend using it for whatever you want to use it for, its used and it's yours.
Don't be afraid to use it. Your eyes & fingers are an excellent source in seeing problems with a surface plate. Do you see any wear spots, maybe from repeated use in one area? Your probably not in the position to have two different surface plates so try using it. Does it really matter if it gets stained? I know its a bit sacrilegious but what are your options? If you feel that way then sell it and get a cheap new fresh black one from Enco or Grizzly.

**For the most part it's probably an excellent plate in the current condition for your level of metrology needed, I would leave it alone unless you see some glaring damage or uneven areas. This pink granite is very, very hard and it was made so well and accurately that you have a better chance of this plate being in good shape compared to any others you'll see on craigslist.

I've been down this path and I know how frustrating it is not knowing if your plate can be trusted or not. Getting a plate like that tested can be very expensive but not always. I suggest to call around and you might get lucky finding someone near you that will take pity and give you a smoking deal to test it.

I was able to get mine tested for $30 and a six pack at local calibration shop after asking if they could help me. I brought my 18 x 24 plate to there shop when they were slow, they were very kind to explain and show me how they test surface plates. They were happy to see a surface plate being saved and being used by someone.

The comment about don't drive a Ferrari as a commuter? I say go for it. I see plenty of $300K Rolls Royces being sold now for $15K being used every day, some with surf racks and a big smile on the driver.

EDIT: These also make for very good platforms for vinyl record players, audiophiles love them. I also highly recommend making a cover for it On our large plate at work, we used it off and on for bluing up parts. We always use Prussian Blue as it does not dry. Starrett make a cleaner which is basically a liquid cleaner wax. I found the best thing to clean it was to use WD-40 and a clean rag. Spray it on, wipe it off, then give it a slight buff. And no, it won't hurt the plate as the Calibration guy even uses it.

As far as getting one calibrated, they use a meehanite cast plate and a fine diamond grit, then check it with a laser I believe. All of our large plates were within . in the Toolroom, the smaller desk plates were calibrated to . or better. None of our desk plates set on three points, they just set on the tops of our benches. Out large plates just set in a metal frame. One was a 3x4 plate, and the largest was a 4x8 plate. The large plate was hauled out and moved to another room by riggers, and set on the floor with nothing leveled, then it was cal'd and all was still within specs. So they can take a little punishment. You don't want to pound on them, but if you set it down on a flat surface, it should remain flat for as long as you own it. Most home shops aren't working that close that even a half a thousandth out will affect the work. Just make sure that wherever you set it that the surface is relatively flat. A good way to insure that the the plate will take a flat set is go to WallyWorld, and back around the paint section somewhere, they sell round disk of felt pads with a sticky back. Get the ones that are about 1" in diameter. Put one at each corner and one in the middle. The weight of the plate will settle down to where it needs to be without distorting the plate. Remember that the granite plate is nothing more than a rock that has been ground smooth. Plus the felt pads will not scratch your desktop if you have to turn it 90 degrees for any reason.

On our large plate at work, we used it off and on for bluing up parts. We always use Prussian Blue as it does not dry. Starrett make a cleaner which is basically a liquid cleaner wax. I found the best thing to clean it was to use WD-40 and a clean rag. Spray it on, wipe it off, then give it a slight buff. And no, it won't hurt the plate as the Calibration guy even uses it.

As far as getting one calibrated, they use a meehanite cast plate and a fine diamond grit, then check it with a laser I believe. All of our large plates were within . in the Toolroom, the smaller desk plates were calibrated to . or better. None of our desk plates set on three points, they just set on the tops of our benches. Out large plates just set in a metal frame. One was a 3x4 plate, and the largest was a 4x8 plate. The large plate was hauled out and moved to another room by riggers, and set on the floor with nothing leveled, then it was cal'd and all was still within specs. So they can take a little punishment. You don't want to pound on them, but if you set it down on a flat surface, it should remain flat for as long as you own it. Most home shops aren't working that close that even a half a thousandth out will affect the work. Just make sure that wherever you set it that the surface is relatively flat. A good way to insure that the the plate will take a flat set is go to WallyWorld, and back around the paint section somewhere, they sell round disk of felt pads with a sticky back. Get the ones that are about 1" in diameter. Put one at each corner and one in the middle. The weight of the plate will settle down to where it needs to be without distorting the plate. Remember that the granite plate is nothing more than a rock that has been ground smooth. Plus the felt pads will not scratch your desktop if you have to turn it 90 degrees for any reason.

I worked with big plates almost every day. We had a 5'X10' Mitutoyo granite plate that wasn't used often, and two vehicle size iron plates about 20'X30'.

They were mainly used to set up and scribe vehicle bodies. Every vehicle had locating holes on the underside and using a print, we'd locate stands using the scribed lines on the plate, then drill and tap the plate to tie them down. Then, we'd adjust the height to design height, ending with a pilot tip the size of the holes.

The bare body would have a thin coat of white paint, and once it was on the pins, we'd scribe it using height gauges and large gauge blocks every 100 mm in X,Y, and Z planes. Using the scribed lines, it was then possible to locate any point on the vehicle, given the XYZ coordinates.

From time to time, the millwrights would come and check the plates using a transit and a target. Using the screw jacks spaced in the plate, they could usually get it within .005" plus or minus across the plate, which was all we needed.

Grade A Black Granite Surface Plate, 18" x 12".  About $160 on Amazon...

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Surface Plates are solid flat plates used to mimic a perfect 2D plane as accurately as possible to facilitate other measurements being made on top that plane. Typical uses are precision inspection (for quality control, for example), marking out (putting precise markings to guide machine work), and tooling setup.

Many of the most common measurements are done with a Height Gauge.  A Surface Plate with height gauge is an extremely handy thing for any CNC'er or machinist to have on hand, so I recommend getting the set relatively early on.

Surface Plate Material: Granite, Cast Iron, Steel, or Glass

Surface plates are usually made of granite, and they come in a variety of sizes and grades.  There are cast iron, steel, and glass surface plates available, but most prefer to stick with granite.

Granite came into wide use during World War II.  Before that time, most surface plates were metal.  Granite surface plates were conceived as a means of saving the metal for more critical applications.  The idea to use granite came about by the happy coincidence of Wallace Herman owning both a monument and a metal shop in Dayton, Ohio.

Granite has a number of advantages over cast iron.  It's more durable and won't rust.  These advantages are why granite surface plates stayed popular even post-World War II.

You can find both black and pink granite used to make surface plates, with black granite being more popular.  Pink granite has more quartz, and therefore more wear resistance than black granite.  Quartz is more than twice as resistant to wear as the other minerals in granite.  However, Pink Granite isn't as strong as Black Granite and so it is normally desirable for a pink granite surface plate to be thicker.  Black Granite has superior stiffness, vibration damping, and machinability.

Before granite surface plates became prevalent, Cast Iron was the most common material for surface plates.  It is still popular for certain applications because it is easier to work it to a particular degree of precision than granite.  In fact, cast iron surface plates may be used as a tool to help lap (resurface) a granite surface plate.

Surface Plate Grades and Precision

Surface Plate Grades determine their flatness.  Standard grades include:

• AA (Most accurate):  These are referred to as laboratory grade.  Their flatness is (40 + diagonal (inches) of surface plate squared / 25) x 0.".
• A: These are Inspection Grade.  They're the AA spec x 2.
• B: These are Toolroom Grade.  They're AA x 4.
• Workshop Grade (Least accurate):  These are not really suitable for CNC applications, so I won't quote a spec.

Try to purchase an "A" grade granite surface plate in the largest size that you can afford and that fits comfortably in your shop.  This grade is intended for inspection work (quality control), and so covers the broadest range of use.

The ASME (American Society of Mechanical Engineers) has published ASME B89.3.7 - which is the current standard for granite surface plates.

All surface plates must be calibrated regularly to ensure their accuracy has not been compromised by chipping, warping, or excessive wear.

The flatness of a surface plate is defined by two parallel planes-the base plane and the roof plane.  The distance between the two planes is the overall flatness.

Standard Surface Plate Sizes

Standard surface plate sizes are 18 x 24 in., 24 x 36 in., 36 x 48 in. and 48 x 72 in, but you can get most any size custom made if need be.  I have an "A" Grade 24 x 36 in A Class granite surface plate for my metrology needs.  It's a very nice surface plate for a small shop, but larger shops will want even larger surface plates and will likely have more than one.

Reducing Wear, Cleaning, and Extending Life of Granite Surface Plates

Reducing wear is all about keeping granite surface plates clean.  Airborne abrasive dust is the greatest source of wear, and it will also tend to embed in work pieces and the contact surfaces of gages.

Cover your plate to protect it from dust and damage.  Rotate it periodically so that wear isn't always in the same place.  Wherever possible use carbide pads on gauging as it is much less likely to have embedded abrasive dust.

Surface Plates should also be kept as clean as possible.  Swarf and other debris can easily interfere with accurate measurements.  Special Surface Plate cleaner is available relatively cheaply (about $30 on Amazon for a gallon) and should be used to prevent any chemical erosion of granite surface plates.

Ideally, clean your plate at the beginning and end of the day with appropriate cleaner.  Note that the evaporation of the cleaner can cause chilling, which may distort the surface.  Allow an hour for small plates to normalize and 2 hours for large plates.

Depending on how porous the granite in your plate is, retaining water from cleaning can be an issue that makes precision tooling more likely to rust.

Surface Plate Calibration

Surface Plates can experience chipping, warping, and just plain wear, so they must be calibrated regularly.  Often, the wear will be localized due to a particular tool, such as a height gauge, being used in the same place constantly.

Here's a fascinating video that walks through the process of testing and calibrating a surface plate in the field:

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A process called lapping can be used to increase the flatness of a plate as needed.

Don't Use Your Surface Plate as a Counter!

Surface Plates are not counters.  Don't set anything on the surface plate other than gages and the pieces being measured.

Say you let someone put a Coke can on a Surface Plate.  That can may have dribbled so there's a little Coke on the bottom of the can.  But Coke is acidic and it will eat into your Surface Plate, damaging it.

Placing random things on Surface Plates can also transfer harmful debris that upsets delicate precision and abrades the Surface Plate.

Surface Plate Accessories

There is a huge variety of accessories used with surface plates.  We won't try to cover them all, just the most commonly used ones.

Ledges and Threaded Metal Inserts

Ledges are a part of the surface plate and must be specified when you order your plate.  Ledges are used to facilitate work clamping.

Threaded metal inserts may be embedded in the granite to facilitate clamping.  Keep the metal below the level of the granite to facilitate accuracy.

Surface Plate Stands

Proper support is essential for surface plates to minimize warping.  Granite Surface Plates should be supported at 3 points, ideally located 20% of the length in from the ends of the plates.  A precision surface can only be supported accurately by 3 points, not 4 or more.

A variety of stands are available to support all types of surface plates.  Smaller surface plates can be placed on any bench, but larger plates benefit from proper suspension.

Here's a typical fabricated stand for the sort of large surface plates found in the inspection and quality departments of many machine shops:

Temperature Controlled Room

It doesn't do you much good to specify a laboratory grade surface plate and then keep it in a room that isn't temperature controlled.  For the highest degree of accuracy and repeatability, the temperature must be consistent.  That's why so many shops keep their surface plates in a temperature controlled inspection room.

Surface Plate Covers

If your surface plate is not in continuous use (most are in commercial shops), it's nice to protect it with a fitted cover:

Surface Plate Cleaner

Special Surface Plate cleaner is available relatively cheaply (about $30 on Amazon for a gallon) and should be used to prevent any chemical erosion of the plate.

Height Gauges, Sine Gauges, and Gauge Blocks

Height Gauges, Gauge Blocks, and Sine Gauges (Sine Bars and Sine Plates) are probably the most common measuring accessories used with a surface plate, though there are many more.

This article is part of our Complete Guide to Metrology.

For more information, please visit Inspection Surface Plate.