Thread: How do I test it for flatness?

How flat do you consider flat?

If you have access to a satisfactory flat piece of plate glass, you can blueprint it yourself. And the coating doesn't really need to be blue, nor bearing blue. Slow-drying acrylic paint, felt-tip marker, or china marking crayon on the plate glass may suffice. Wash off with water or rubbing alcohol, as appropriate

Cheers,
Joe

Being far, far, far from an expert - but I have heard with three pieces you can make all three flat. This could be wood or metal.

The theory being that two pieces might have canceling dips and ridges - but the third cannot meet flush with both - therefore can be used to identify the flat.

This will not work with glass unless you can flatten the glass yourself.

For example, take three bits of wood, find the high points, flatten them down, continue until all three meet evenly. Then use these to check the cast iron plate. The wood may warp over time, but if you check that all three meet well, then they are flat at that time. Once used to flatten the cast iron, then it no longer matters if they warp as the cast iron is now your true surface.

Of course, that all being said, I reckon that glass is just fine for anything that I am likely to need!!

Cheers

Cam

Thanks Joe and Cam for your responses.
I do have a piece of plate glass somewhere in the shed, so I will see if I can find that.
At the same time, I think I will use my jointer and my handplanes to make three wooden straightedges.
I can then:
1. use them to test the glass and the surface plate and see if they give the same result
2. use the glass to mark the surface plate
3. scrape the surface plate (and repeat 2 and 3 again and again)
4. use the straightedges to test how well the scraping and the glass testing went.

This could be a fun project.

Yes indeed - but I would use a cabinet scraper to true up the wood rather than a handplane. I'll be interested to hear how it goes.

What are you using for the hand scraper for the cast iron?

One of these:
Attachment 137292
Which I got from Greenwood tools in the UK.

There has been quite some discussion of use of scrapers to flatten plane soles in this thread. It's also worth reading mic-d's blog posts on scraping plane soles and other bits

Thanks Jeremy,

I have been following Mic-d's stuff a bit - which got me onto looking for flat surfaces!! Since posting this question I noticed that you had said that you had got this from the UK... Oops.

However, what I don't understand is the need for a fancy (expensive) scraper. Maybe I am missing something?

1. I hear people saying that they used to use ground files. Maybe there are better tools now - I am happy with that.

2. The better tools seem to be better on the basis of having solid tungsten carbide inserts. HOWEVER, there are a heap of scrapers out there now with TC inserts that are a whole lot cheaper than $100+.

So what am I missing - why wouldn't something like one of these two options work?
Bahco Carbide Scrapers : Buy Bahco Carbide Scrapers at Axminster Power Tool Centre
[ame=http://www.amazon.co.uk/Stanley-Tungsten-Carbide-Scraper-640/dp/B0001IW65S]Stanley Tungsten Carbide Scraper 0 28 640: Amazon.co.uk: DIY & Tools[/ame]

Yes, Cam, I looked at those scrapers too.

My process of reasoning went like this:

• When you go to the Bahco and Satanley (spelling error that I decided to leave because it's just too good not to share!) websites and look the descriptions of the tools, they are paint scrapers for removing paint from timber, glass or metal surfaces.
• I also had my doubts whether they could be used in the same way as an engineer's scraper, especially when you look at the techniques that are shown on the youtube videos in mic-d's thread.
• You can get ordinary engineers' scrapers from Blackwoods, but I didn't want to go through hassles trying to use them and more importantly learning how to sharpen them. Similarly, with a new technique to learn, I wanted to know that it wasn't the tool that was the problem, so the idea of making my own from a file (although it appeals to my latent Scottish side) was rejected.

So I chickened out and bought the Sandvik.

Fair enough. It would be nice to compare these things side by side though. Unfortunately that usually means buying both! Sigh.

I have the other two tools - which actually do wonderful work on shaving down wood finely. I was hoping to be able to use the same tool for multiple uses.

Maybe I will leave the idea of super flat for now and actually make some useful things...

Originally Posted by CameronPotter

Fair enough. It would be nice to compare these things side by side though. Unfortunately that usually means buying both! Sigh.

I have the other two tools - which actually do wonderful work on shaving down wood finely. I was hoping to be able to use the same tool for multiple uses.

Maybe I will leave the idea of super flat for now and actually make some useful things...

I reckon that having a surface plate could well be a worthwhile thing to add to the collection of devices needed to make useful things. So it is on the To Do list.

On the other hand, SWMBO wants some pantry doors and I need to make some more bookcases for my study, so the surface plate may have to wait for those two projects to be done and dusted first.

Yeah - I spend a lot more time making tools than I do using them... <Guilty>



Mind you, my workshop is getting close to properly set up... (as always).

Like the frog jumping across the pond with each jump 50% of the previous one?

Guilty of that too!

Yep. If I ever make anything it will be absolutely square and measured to the nearest .XX of a millimetre... And probably possibly made quickly.

The catch is that I only seem to make stuff when it is "needed" - and tools often seem "needed" more than things like coffee tables.

On the weekend I made a 0.8% taper reamer for tuning clarinet barrels. Good fun - no idea I could be that precise on my wood lathe - but I have not yet used it.

Hi Jeremy,
Sorry not to reply earlier, but I have been thinking about this. To be frank I think you are wasting your time with wood straight edges for the following reasons

1. wood is unstable
2. the thinnest removal (shaving) you could hope for in planing is just sub .001", too great a tolerance
3. The generation of standards requires spotting amongst three plates. How are you going to spot timber?

I would avoid using the glass as a reference just yet. It's possible that it's less flat than the surface plate. So you'd need to check it first somehow, so you might as well go straight to the plate and check it huh?
How about this. Have you got a good straight edge that's the length of a diagonal? Well you should get one. I would grid out the plate in say 10mm increments (x,y coordinates) then use your straight edge along each grid line and as many diagonals as you wish to see if you can rock it. That is, test for convex. If it is, buggered if I know a simple way to quantify that! Let's say it's not, then you could repeat the check this time putting your 0.001" feeler gauge under the straight edge at each point on the grid and see if you can rock the straight edge. That'd be done with the straight edge running on each of the grid lines and as many of the diagonals as you wish. That way you'd build up a map of the plate. If you can rock the straight edge at every orientation of the straight edge and with the feeler gauge at every position then it must be flat to less than 0.001" and that'll be good enough.
If it's outside that limit then you are going to have to get hold of a surface plate and scrape it flat.

Does that make sense?

Cheers
Michael

Originally Posted by jmk89

Thanks Joe and Cam for your responses.
I do have a piece of plate glass somewhere in the shed, so I will see if I can find that.
At the same time, I think I will use my jointer and my handplanes to make three wooden straightedges.
I can then:
1. use them to test the glass and the surface plate and see if they give the same result
2. use the glass to mark the surface plate
3. scrape the surface plate (and repeat 2 and 3 again and again)
4. use the straightedges to test how well the scraping and the glass testing went.

This could be a fun project.

Cameron,
Scraping tools have been around quite a while in the basic form of the scraper Jeremy pictured. I mean a scraping arris at the end of some tool handle like a dedicated one or old file etc. So I'd expect there's some very good reasons those cheap scrapers aren't used. Just a couple I could think of are chatter and the wrong shaped blade.
There's pros and cons to different scrapers. An old file needs constant honing, a TC one can be quite expensive. If you can silver solder then you can make your own TC scraper. The only thing better than that is knowing someone who can make one for you

Cheers
Michael

Originally Posted by CameronPotter

Thanks Jeremy,

I have been following Mic-d's stuff a bit - which got me onto looking for flat surfaces!! Since posting this question I noticed that you had said that you had got this from the UK... Oops.

However, what I don't understand is the need for a fancy (expensive) scraper. Maybe I am missing something?

1. I hear people saying that they used to use ground files. Maybe there are better tools now - I am happy with that.

2. The better tools seem to be better on the basis of having solid tungsten carbide inserts. HOWEVER, there are a heap of scrapers out there now with TC inserts that are a whole lot cheaper than $100+.

So what am I missing - why wouldn't something like one of these two options work?
Bahco Carbide Scrapers : Buy Bahco Carbide Scrapers at Axminster Power Tool Centre
Stanley Tungsten Carbide Scraper 0 28 640: Amazon.co.uk: DIY & Tools