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Surface plates

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Guy Lamb18/09/2019 12:35:24
78 forum posts

Can I take it that the corners of a surface plate are ground at 90 deg to one another?

I've just bought an oldish second hand cast job (12" x 10" and according to the one square I trust implicitly it's as spot on, or is this just a fluke?


Pete Rimmer18/09/2019 12:41:54
564 forum posts
25 photos

You can expect it, or you can inspect it. Only one of those can you trust.

Hopper18/09/2019 13:00:42
3988 forum posts
85 photos

Probably "spot on" as measured by eye with a try-square but probably not "spot-on" to the tenth of a thou like the top surface of the plate if measured with precision measuring kit.

Mike Poole18/09/2019 14:55:53
2331 forum posts
53 photos

The edges of my surface plate appear to be machined to a high standard but not ground, although it can be useful to use the edge as a reference when marking out (and the pins on a surface gauge can be useful to allow this) I don’t think they are included in the specification for the surface of the table. For average home workshop uses they are probably straight and square enough but if you are working at a high level then it may matter. Calibration only seems concerned with the surface and not the edges. Straight and square will need to be obtained from other references.


old mart18/09/2019 20:33:06
1101 forum posts
113 photos

Surface plates usually have straight sides to allow easy use of a surface gauge. As for squareness, I cannot be dead sure, but its likely they are within a milling machines tolerances.

Vic18/09/2019 22:39:39
2402 forum posts
12 photos

I’ve got a granite surface plate, only a small one. Once I’d constructed and painted a nice wooden box and lid to protect it it’s never seen the light of day, well maybe once or twice! It’s now a raised platform for storing stuff ...

Chris Evans 619/09/2019 08:57:09
1565 forum posts

I have a 2 foot square cast iron surface plate that never gets used. If I mark out I use the mill table, saves moving all the junk and wooden cover from the surface plate which has morphed into a shelf.

Bazyle20/09/2019 08:46:51
4905 forum posts
195 photos

The way to handle it is to make the cover as a tray so that it can be easily lifted off with all the clutter and to specifically and deliberately use it to store lightweight items such as micrometers and height gauges.

SillyOldDuffer20/09/2019 09:31:06
5138 forum posts
1074 photos

Can some kind person explain why a hobby workshop needs a surface plate?

My understanding is they are mainly used for precision metrology - inspection of parts to ensure they are within manufacturing tolerances, gauge making and other tool-room work requiring 10x more accuracy than needed for production - like jigs and fixtures. In modern use, surface plates are often part of a Coordinate Measuring Machine, which are completely over the top! For high precision work surface plates have to be calibrated regularly.

Surface plates are also used for marking out but this is a technique little used in my amateur workshop. Exceptionally I might use a glass sheet and a height gauge, but the latter is much less useful than expected. Instead my milling table and bench-top are 'good enough' as flat surfaces and - most of the time - a DRO fitted milling machine supported by micrometer, digital caliper and DTI replaces marking out. So I don't use lots of marking blue, scribe lines, and centre pops: rather, I establish a reference point on the job and work from that, creating new reference points as necessary to check and maintain accuracy.

It's not just surface plates I've rejected! I don't use buttons or other high-precision tool-room methods either.

Perhaps it's me - I rarely need measured accuracy better than 0.02mm (about 1 thou). Close fits are achieved by using one part as a gauge and shaving the other down to match as tightly or loosely as necessary. Others may need better, but for what? What am I missing that makes it worth owning a surface plate?


Clive Hartland20/09/2019 10:35:38
2503 forum posts
40 photos

You would be surprised at how useful a surface olate is from scribing rising dimensions to checking flatness of parts being made.

I had a bronze casting and as I worked it it distorted and I only found it by using the plate and a V block.

If you have a Scribing/measuring device it is somewhere to store it ready for use.

not done it yet20/09/2019 11:17:42
3946 forum posts
15 photos

Re checking for square. Your square will be pretty well nigh on a right angle if you scribe two lines, from a straight edge, one from each side. If the lines drawn neither converge or diverge it is likely good enough for most of us!

If your surface plate will be very close to square if the sides are exactly the same length and the same for the diagonals. Need sharp corners for measuring the diagonals, of course.

Use comparative measurements rather than a steel tape measure! If rectangular, the same applies but comparing short and long sides as pairs.

Mike Poole20/09/2019 11:27:17
2331 forum posts
53 photos

I suppose a surface plate is useful if the work you do and the way you work makes one useful. A vernier or digital height gauge is much more useful if you have a flat surface to use it on. Machine tables and plate glass are handy if you don’t have a surface plate but there are no tee slots in a surface plate. DROs on machines can save a lot of marking out. The surface gauge was a classic apprentice piece and I suppose that was because it was likely to be useful and the expectation was that a surface plate would be readily available and they certainly were in the tool room I worked in although being an electrician the were only useful for homers like checking the cylinder head of my motorcycle was flat (it wasn’t) .


Pete Rimmer20/09/2019 11:45:21
564 forum posts
25 photos

You can't use length and diagonal measurements for checking the squareness of surface plate corners to the tolerances that surface plates are supposed to be good for. Taping corners would get you at best ten thou, plates are flat are square to ten thousandths (100,000x less). If you're going to use such a crude method you might as well check a bench corner.

If I wanted to check the squareness of the corners I would stand the surface plate on a long edge on top of a larger surface plate and use a DTI on a surface gauge to make comparative measurements against the opposing vertical ends. If those measurements match then both corners are square to the 'bottom' edge. If I didn't have a larger plate to stand the test piece on I would put a box square against one edge and run a surface gauge along the square face and check the measured face for parallelism to the box square. That would let you prove each or any corner individually.

Edited By Pete Rimmer on 20/09/2019 11:46:03

Andrew Johnston20/09/2019 12:17:23
5115 forum posts
594 photos
Posted by SillyOldDuffer on 20/09/2019 09:31:06:

Can some kind person explain why a hobby workshop needs a surface plate?

You don't, but he who dies with most toys wins!

I've got a 3' by 2' cast iron surface plate; £40 on Ebay. While I don't mark out that often the surface plate is useful when I do. The milling machine tables are too small, they've got T-slots, have stuff bolted to them and I'd need to move everything on and off each time. A surface plate is also useful when taking measurements of anything from heights to tapers.

I've never measured, or worried about, the edges of the surface plate being square, or even straight.


IanT20/09/2019 13:13:45
1415 forum posts
140 photos

I think the answer is that you can certainly get away without one Dave - and in some ways it might be better to do so - after all ignorance is bliss - and what you don't know, probably won't worry you!

I'm afraid I don't have DROs etc on my machines yet, so still do my marking out on either a small CES cast-iron plate I "made" (e.g. surface ground at college) several decades ago - or for longer work I also have a lump of 1/2" plate glass sat in a wooden holder.

However, as Pete points out there are some things that are so much easier to check/measure on a surface plate. Moreover, once you start checking your work on one - I think you will find yourself doing so much more often (especially if you are bit obsessive about such things). Work/Tools I had assumed were flat on key surfaces were not and using machine tables doesn't necessarily work if (like myself) you are not certain how truly 'flat' they are. All of my machines are quite old and vary from a little to very well worn - and I've always known this. So apart from being able to measure 'flatness' accurately - I also need a reference surface when trying to correct wear.

So, I would agree that most folk don't/won't need a surface plate but I do think that anyone working with older or worn machinery will find a use for one eventually. My largest plate is an old commercial 18" square one and is probably long past 'laboratory' standard these days but it still the best reference surface I have available and it does get used fairly regularly to check both my tools and work.



old mart20/09/2019 13:21:12
1101 forum posts
113 photos

We have a float glass plate of about 1 foot square on a chipboard base with some 1/4 square beading around the edge and its own lid at the museum, which is really useful. I recently got hold of a 2 foot square cast iron plate which is really good for the extra size, but we have room for it, and the glass one would be a good size for most model makers, and flat enough for 98% of work. The glass one has the advantage of being light enough to move out of the way if you have little room available.

Vic20/09/2019 13:34:54
2402 forum posts
12 photos

I bought mine for marking out. Some time later though I bought a vertical bandsaw that has a very nicely ground cast iron table and that’s good enough for most jobs.

Mike Poole20/09/2019 15:50:41
2331 forum posts
53 photos

Of course once you have a surface plate if you want to do anything apart from just blue it up and check for flatness then you will need some accessories, many of them are already likely to be around like angle plates and vee blocks but there are plenty of other toys that go nicely with a surface plate and can make it into a very useful addition to the workshop. I know a guy who insisted on having one on his bench when a new workshop was set up but that was how he worked and after 35 years why change?


Pete Rimmer20/09/2019 16:35:47
564 forum posts
25 photos

All of the people who have been guests at my workshop to learn a bit about scraping have had a revelation when they saw just how little bearing their cross slide actually had on a flat way, and how easy it was to fix. An easily made scraper, some blue and a small surface plate can improve things a huge amount, for just a few hours work.

Howard Lewis20/09/2019 16:39:10
2747 forum posts
2 photos

A Surface Plate can be used for marking out, but mine tends to be used more often to find Centre Heights relative to a datum face, or centre distances, and the like.

Not a "Used every day" item, but many of our measuring tools fall into that category. When did I last use a 0 - 6 inch mic? Like a jack when the car has a flat tyre and the wheel needs to be changed, INVALUABLE!

Mine is covered by a wooden lid, lined with green felt, (helps to hold the oil with which the plate is coated after use  ).

Unfortunately, it supports the chest containing nearly all my measuring equipment, so needs a few minutes preparation before use!


Ah! Inserted a space to get rid of the smiley

Edited By Howard Lewis on 20/09/2019 16:40:33

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