Making a CI surface plate

[rmcphearson]

Greetings all,

Some of you may remember my post about a commercially successful tool I designed for my fellow golf course mechanics called the Roller Tamer. Anyway, I have designed another tool, again, specifically for my fellow golf techs. This one is called the Height of Cut Gauge Calibration Plate which I’ll refer to as the plate. It is a surface plate that allows me to 1) check flatness of my HOC gauge 2) calibrate my HOC gauge and 3) other typical uses a surface plate is used for. A HOC (height of cut) gauge is the gauge that we use to measure the height of a reel mower above the ground. Most greens mowers are set anywhere from .070” to .125” above the ground. HOC gauges for a long time have been cheaply manufactured and accuracy and proper calibration methods have been a growing concern since the height of greens mowers has been reduced and the demand for precision has increased. I know bla bla bla, I’m getting to the point! So I designed this plate and some of my fellow techs that have seen the prototype are drooling and want one.

The current prototype is 24” long, 4” wide, 3/4” thick. There’s a counterbored hole in the middle with a minor diameter of 1/2” and major diameter of 1”. There are only two critical deminsions
1) The surface on the smaller diameter of the counterbore side should be flat. I haven’t decided yet to what degree. This is not going in the space shuttle, we’re mowing grass here, so let’s say “less than .001” over the entire surface”. Cost of production will be the primary determining factor when deciding the degree of flatness.
2) The thickness of the counterbore “lip” must be known. Similar to a “standard” rod included with mics. The lip simulates the blade height. It should be within the typical HOC of a mower, about .100”. Will the lip thickness of all my plates be the same? I don’t know. I have a feeling that would drive costs up, so right now I’m assuming they will all be different and the thickness will just be measured and stamped or engraved on each plate.

I’m going to get some quotes from grinding shops for a quantity. I’m thinking 100 for the first batch. I made the prototype from CI, had the counterbore drilled and both surfaces milled. I had a hard time finding a shop with a surface grinder that would accommodate 24” so I just “ground” it by rubbing it (for hours) on long strips of belt sander paper stretched across my giant granite surface plate. I’m researching scraping and lapping techniques, don’t know much about them. If you were going to make a quantity of these in your home shop…

1) What material would you use? (Type of CI)

2) How would you make them? Is scraping after milling an option or will grinding be required at some point?

3) Is there a class available you’d recommend for me?

4) If committed to fulfill 100 orders by hand scraping, how many would it take to go from “wow, this is neat, I can drink beer in my garage and get paid to hand scrape!” to “ holy ****, what have I done, I want to cry”?

Thanks
-Roland

[nc5a]

If you have already committed to 100 hand scraped units you will in short order be saying (holy crap what have I done). Then you'll find a quiet corner somewhere and slit your wrist.I have a bit of experience with hand and power scraping machine tools and masters such as Vee blocks, squares, straight edges etc. You really don't want to go the hand scraping route, trust me on this.

Like you mentioned, this is a grass cutting fixture/gauge and I have to admit I don't have a clear picture of what you want to build or how it works. But from your description it's very unlikely that your product needs anywhere close to the flatness that scraping can provide. Without knowing exactly what the product is and how it's used I would say that milling a 4" X 24" X 3/4" thick bar stock might be the right choice at least for the prototypes. Then grinding if you need a better finish or more accuracy and lastly scraping. Keep in mind that a 3/4" thick bar scraped or not will not remain flat to the degree you mentioned (0.001"). Also, if you go the scraping route the material will need to be cast iron ($$$$$) because it's difficult to scrape steel.

Richard King provides scraping classes in his shop in Cottage Grove MN and various other locations. Google him to find out about his scraping classes and cost if you still want to go that way.

For some reason I can't cleanup the paragraphs in this post, sorry.

Good luck with your new product.

[mikey553]

If I understand you correctly, you need 2 flat and probably parallel to each other surfaces 4" x 24" in size. And the thickness of your gauge is 3/4". First of all you need to decide the flatness tolerance on your flat sides. If it is .001" or less, you will have an impossible task in front of you. The cast iron bar of this size is still very flexible and to make it rigid enough you will need to increase the thickness to 3-4". That alone can make your project out of reach.

But if we are talking about 100 plus quantity, you can probably justify making a proper light weight design, making a pattern and outsourcing the gray iron castings to a foundry. I would recommend to forget about scraping and finish the castings by milling or possibly grinding. You need to be realistic about the required tolerances - accuracy costs money.

[mickeyf]

we’re mowing grass here, so let’s say “less than .001” over the entire surface

I'm not a golfer, but that sounds like pretty high precision grass to me. I suspect you could get by with a much lower tolerance and it would still be vastly better than whatever is in use now.

[alanganes]

I don't have any special insight here, so this may be either a stupid suggestion or one you have already considered and discarded, but would having something like this made in granite in the manner of a surface plate be feasible? They can be made in custom shapes and sizes for all sorts of applications. I have no idea how the economics would work out, but I'd guess that 100 is a non-trivial quantity and the cost may be workable if you don't require "metrology lab" grade flatness or dimensional accuracy.

Just a random thought.

[reggie_obe]

If you could draw up an image of the "plate" in use, we might be able to make some helpful suggestions.
The 24x4" length as already mentioned isn't going to be very stiff, you might consider turning that into an I-beam to gain rigidity over it's length.

[eKretz]

Flat within .005" should be plenty good enough. Mill it and if you want it to look a little fancier have it surface ground. Do it in a big batch and you can get them done on a Blanchard or similar for a pretty low cost.

[rmcphearson]

Yes, if I can have them made of granite that will work. I don’t know where to go for that and I suspect CI will be adequate and cost less. My customers want their gauges to measure to an accuracy of 001”. So my plate that they are using to calibrate their gauges needs to be of similar accuracy. The prototype was milled, there was a .003” bow in the middle. That was not acceptable. Y’all have me wondering if 3/4 is not thick enough.

Thanks for the suggestions. Even if I farm this out to a grinding shop, I need and want to learn to scrape to make more prototypes and maybe even sell some prototypes. I’m ordering the scraping tools and supplies now.

I’ll post a video of the tool soon.

[Bented]

Are you actually talking about cutting grass to an accuracy of +-.001"?

[J Tiers]

................... Y’all have me wondering if 3/4 is not thick enough.

...................

It's not. For 24" you'd want at least 2" or 2 1/2" thickness. With CI, that would not be solid, but would be the depth overall including the stiffening ribs on the back.

[sid pileski]

RMcphearson- what ever happened to your situation at work where they were harassing you, or whatever it was?
Did you get that straightened out or did you move on?

Sid

[Mcgyver]

4) If committed to fulfill 100 orders by hand scraping, how many would it take to go from “wow, this is neat, I can drink beer in my garage and get paid to hand scrape!” to “ holy ****, what have I done, I want to cry”?

If you are really a steadfast and determined guy, probably 1.5

[Paul Alciatore]

He did not say that they cut grass to +/-0.001". Only that the gauge should be that accurate.

But he did say:

"Most greens mowers are set anywhere from .070” to .125” above the ground."

And I see two things from that number. First, the +/-0.001" suddenly appears more reasonable. But even there, it would seem that +/-0.003" or even +/-0.005" would be a lot more reasonable.

And then there is the matter of the materials the grounds keepers are dealing with. The grass and the ground. Grass does not have a "surface finish" that could even be called that. A layer of grass that is only 0.125" thick could not even be relied upon to actually cover the ground at all places. There would be holes in it where it's thickness would be essentially zero. And just what does a layer of grass that is only 0.125" thick look like? Then, just what does one that is about half that at 0.070" thick look like. I would love to see some photos - up close and personal please.

But these grounds keepers are judging this grass thickness from THE GROUND. Frankly I strongly doubt that you could not find an area of ground, only equal to the size of one of their mowers, on any golf course in the US, heck, in the world, where the ground is that flat (0.070" to 0.125"). No ground is that flat. And even if it were at one time, the first rain or use of sprinklers would allow it to continue to be so. That just isn't going to happen.

On top of all that, just how does the mower sense that ground level? I mean, REALLY, HOW? Are they equipped with some kind of sensor to measure where the surface of the ground actually is? I won't even ask how that works as it probably is possible, But does an individual mower just measure at only one point over it's total footprint? That would be incredible inaccurate. Two? Four? How many?

I fear the answer is the exact same number as the number of wheels on that mower. In short, it is the wheels that determine where the ground is. And those wheels ride on both the mowed and the un-mowed ground. They ride ON THE GRASS, some before it is cut and some just after it is cut.

I looked at some high end mowers for gulf courses. They cost as much as a new car or truck. They brag about how flexible they are. They brag about how wide a swath they can cut. They brag about power and fuel economy. Where there is any reference to the depth of the cut, it is called out as "Bench HOC range". I must conclude that this is only a bench adjustment and has nothing to do with the actual depth of the cut above ground.

I further suspect that the actual height above ground is something that is very difficult to impossible to determine.

I further noticed that the actual height of the cut is possibly called the "deck height" and is variable, IN THE FIELD in a range like 1" to 5". I find that to be a much more reasonable range in actual use.

I have to wonder if this 0.070" to 0.125" range is something that is perhaps just at one end of the overall and larger range. Perhaps at the low end as in: set the deck to the lowest possible (1") and then set something (perhaps the cutting blades above the plane of the wheels) to that 0.070" to 0.125" range. And the person using the mower can count on that 1" field setting to be 1.070" to 1.125" above some ASSUMED average value for the ground. If that is the case, one must wonder just why a specification of +/-0.001" is actually necessary or even meaningful at all.

I say get some steel that is about 1.5" thick. Anneal it. Mill it flat on both sides and flip it over on each successive 0.005" pass until it is about 1.455" thick. Then do two 0.002" passes, again on opposite sides, to 1.451" thick and a final pass of 0.001 to 1.450" thick. Then check it on a surface plate. It should be around +/-0.005" and that should be good enough for those mowers. Use toe clamps on your mill to avoid excessive warping when it is clamped.

If the above does not yield acceptable results/yield, then do a second annealing pass when you are at about 1.460" thick and see how that works.

That's how I would do it and I would bet that there isn't a single grounds keeper in the world who will complain. Just do not publish any ridiculous +/-0.001" spec on it. If you must, then just say it is "the most accurate gauge available", which it will be.

Of course, if you can find a surface plate company that will make it in stone for less, then, by all means, get them from there. Stone breaks easier so there will be more repeat sales.

Are you actually talking about cutting grass to an accuracy of +-.001"?

[alanganes]

Yes, if I can have them made of granite that will work. I don’t know where to go for that and I suspect CI will be adequate and cost less..

I don't know relative cost and I am not pushing the granite approach, just food for thought. If you do decide to look into that, there are a bunch of places that make stuff like this for custom gauges and machine bases and the like. A google search for something like "custom granite surface plate" brings up a number of fabricators and avoids most of the granite countertop results. Probably not the cheapest source, but Starrett has an entire division focused on stuff like this:

. https://www.tru-stone.com/pages/oem_custom.html

It will be fun to see where this all leads, looks like an interesting project!

[mikey553]

Yes, if I can have them made of granite that will work. I don’t know where to go for that and I suspect CI will be adequate and cost less. My customers want their gauges to measure to an accuracy of 001”. So my plate that they are using to calibrate their gauges needs to be of similar accuracy. The prototype was milled, there was a .003” bow in the middle. That was not acceptable. Y’all have me wondering if 3/4 is not thick enough.

Thanks for the suggestions. Even if I farm this out to a grinding shop, I need and want to learn to scrape to make more prototypes and maybe even sell some prototypes. I’m ordering the scraping tools and supplies now.

I’ll post a video of the tool soon.

The reason I suggested to think about near final shape castings is you have to buy the material no matter what. Gray iron castings are usually priced by the pound. Properly designed casting may weigh 10 kg, but a chunk of 4 x 4 x 24" will weigh 48 kg if you can find this size at all. To make the heavy chunk lighter you will need to remove a lot of material, which equals a lot of money. In my opinion the near final shape casting is the way to go for the quantity you indicated. Gray iron is inexpensive, readily available and stable material (after stress relief). It would be my material of choice for such a gauge.

If you do a rough machining, stress relief and final milling with "smart" clamping, you may be able to get .001" flatness without grinding. By the way grinding operation does not automatically guarantee good flatness.