How I made my own granite surface plate

Hey guys so I’ve been wanting my own reference surface/ flat surface and for the most part I’ve been using a piece of cast iron from my table saw, but when I seen these piece of granite am idea form in my head so use these and this is that process. How I took these piece of granite that where anything but flat and took them to within a couple of hundred microns of absolute flat.

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[colinellicott]

45 years ago in metrology class we measured a large granite flat plate with optical instruments. It was a wonderful introduction to the measurement sciences which I used throughout my career. What you have done there is an excellent effort to fabricate and qualify your own flat plates - Kudos!

[htarold]

When grinding 2 pieces, the upper piece tends to go concave, as when it overhangs the lower piece, the middle gets ground more but the sides don't get touched. Similarly the lower piece ends up convex. The folks who grind their own mirrors for telescopes make use of this phenomenon to get the correct curvature.

[scotthenry]

Makes sense why surface plates are so expensive, the work and the quality control to produce one is immense. Being a engineering technician i would love to own my own good sized surface plate.

[JimmyDAal]

Just a suggestion as to what may have been contributing to the uneven lapping results you were getting: while researching to lap a couple of plates myself, I found a number of people saying that rectangular plates are not ideal to lap. This is because when the top plate is 90 degrees to the bottom, you have a lot of overlap which tends to take a lot more off the edges which seems to be what you were experiencing. Square plates are best and when lapping, just make sure the top plate just goes past the bottom to help keep an even coverage. But so long as you are happy with what you got, that's the most important thing. Thanks for the video.

[jumpi]

I use silicon carbide for lapping granite on granite. It cuts pretty fast, but also breaks down fast. It is not embedding in the surface (compared to Diamond) and you can get it dirt cheap online.

[merc]

Loved this video. People interested in something, thinking "I'm gunna have a go", then having a go. Good stuff.
Like you said, it's good for what you're doing.
The methodical way you went about it was good to watch.
Cheers.

[JohnKlopp]

These DIY surface plates look really good, great job! You have a lot of grit and determination. 👍

[lukeng]

Great job, it's so good to see someone sharing what they can with the resources they have rather than trying to make something perfect. Amazing job though and feel like you can pretty confidently say it's flat!

[ronkellis]

Great job! I'm going deaf and use closed captions all the time. Half the fun is watching the AI trying to work with the accent. No worries, I know we in the States butcher everything 😊 Glad I stumbled across your channel.

[ohasis]

That was really very interesting and made me glad I just bought mine.
Thinking on your one, two, three plates, I was reminded of grinding telescope mirrors as a kid. The top plate (No2) tends to wear to concavity.

[k.hussain]

Good work Darren, hats off to you for having the patience to do it.

[hdl]

Very interesting and detailed video, nobody can pay you for the work you put into it. The deviations that still exist can simply be ignored for private use. I have a green granite, 500 x 500 x 70 mm, polished to a high gloss, with a deviation of 0.2 mm over the entire diagonal. I'm not going to sand it any further. The result you have achieved is excellent. A very exciting video. Even if it was sometimes difficult for me to follow your accent :).

[abitoffcenter]

Having just stumbled upon your channel, because what this vid is about. I've gotta say, Great work!! I like to think I'm pretty clever when it comes to making the tools I might need. Mostly because I can't afford to just go out and buy them. I'm not sure if I'd have come up with or attempted to do this. Outstanding Sir!!

[barry.w.christie]

Great to hear a fellow Glaswegian ... I'm from Dennistoun myself 😁... as to whether or not they are perfectly flat, yes it'd be great if they were perfect, but at the end of the day you tried to do it yourself 👍
I made kitchen worktops out of 2ft square by 1" thick granite slabs (doubled at edge to 2"), £240 instead of thousands for a 3/4" rip-off version!

[KarlMiller]

Wow! I appreciate that you showed us how you did that. It is always easy to spot someone who is smart at his trade, even if they don't have all the best tools, shop or video equipment.
Nice work.

[The_Fluid_Druid]

This is the first of your videos I've seen. Loved it. Cheers.

[bobvines]

Darren, as mentioned below, both Tom at oxtoolco and Robin at ROBRENZ made videos showing how surface plates are lapped. Also, at least Robin (maybe Tom too?) emphasized that surface plates needed to be supported on three points to keep them flat. For your uses and size of surface plates, it may not matter how you support them. Congratulations on the plates you made!

[fredj.scipione]

Darren Barclay, I am certain it is enough that you are happy with your results, but I may have a bit of bad news and some good. The bad news☹ - your metric conversions are off by 100x! For a Grade B plate the the limits of 120 & 200 u-in convert to 3.0 & 5.0 microns (u-m). The good news🙂 is that, if you are willing to make a few simple gauge pieces, I can tell you how to measure your straight-edge and surface plates with enough certainty to support Grade A (self) certification (or possibly even AA). Perhaps you could get a "re-do" video out of it. Another good news🙂 is that once you are able to measure and hand-lap your plates to the 20 u-m range, the loose-abrasive plate-on-plate grinding will then work and will assure smoother surface maps.

Local flatness variations over various spans can be detected/measured with simple 3 or 2-footed gauges. Here are some links about making and using such gauges for home-made optical flats and surface plates.
Note especially the demonstration of flatness detection with a resolution around 22 micro-inches near time stamp 8:30 in the first link -

Let me know if you want my e-mail address (or look it up via comp.os.cpm newsgroup).

[larkop]

At least you have attempted this practically and observed the deviations. The technique is similar to that used to produce parabolic mirrors down to angstroms. From my understanding it's exceptionally difficult to create flat surfaces by grinding 2 against one another as they will always tend to go convex and concave. Usually 3 plates are used, top and bottom are stationary and the center reciprocates. Alternatively you can use a smaller lapping surface to remove high spots

[RB-yqqv]

great effort and a good result, one has to remember you are not build space parts and what you have done is great for the DIY work shop