Thread: Where to get flat reference surface

I hear some people use some thick mdf but others can definitely provide a better solution.

Regards

Adam

Float glass is glass that is allowed to harden on molten metal. Most modern glass is produced this way.
Suggest a glazier to start with and enquire what they might have in the scrap bin that would suit you. Some sort of toughened glass offcut might work ok.

To confirm flat look at straight lines in the glass reflection, if still straight then probably pretty good. If wavy then the glass isn't flat.

Originally Posted by BoneInspector

To confirm flat look at straight lines in the glass reflection, if still straight then probably pretty good. If wavy then the glass isn't flat.

can you please elaborate?

Marble or granite benchtop offcuts are flatter than you’ll ever need. Most of the smaller companies selling these have loads of offcuts and broken pieces that they have to pay to dispose of so go and ask, you’ll probably get stuff for free. You can use an angle grinder with a diamond disk to resize them if needed.

I use the marble or granite off cuts but am lucky enough to get them for no cost so if possible shop around.

[QUOTE] Originally Posted by BoneInspector
To confirm flat look at straight lines in the glass reflection, if still straight then probably pretty good. If wavy then the glass isn't flat.[QUOTE]

can you please elaborate?

By "straight" he means "undistorted"
Only a truly flat surface will produce an undistorted image. If an images is skewed, irregular or has ripples, then the reflecting surface is not flat.
Using your eye to check for flatness this way is not that accurate but it will be more than accurate enough for WW operations/tools.

BTW $87 for the Carbatec surface plate is good value. I've seen similar size plates will cost double and triple that and larger plates can cost thousands.
I have one for MW operations - they come with a certificate of flatness which is in the micron (0.001mm) range.

Exactly what is said above.

Think of the carnival mirrors as an extreme example of distortion for an idea of what I mean.

I expect that float glass is good enough for most use cases as I know of several machinists that use it.

I use the tablesaw top to flatten plane soles. Buy a couple of big sanding belts, cut them at the seam and clamp them down. Having a good long run speeds things up. I know the tablesaw top is not perfectly flat but more than good enough for the task.
Regards
John.

[QUOTE=BobL;2146040]

Originally Posted by BoneInspector

BTW $87 for the Carbatec surface plate is good value. I've seen similar size plates will cost double and triple that and larger plates can cost thousands.
I have one for MW operations - they come with a certificate of flatness which is in the micron (0.001mm) range.

30cm is too small for hand planes

No commonly available material such as stone countertops, glass, tiles or metal plates are flat to the degree of flatness you need when you're talking < 0.0001" per foot flatness.

You can make your own flats by using three pieces of countertop or thick mild steel plates and some diamond abrasive but given the amount of work involved a small granite machinist plate is a bargain. Start with something like 60 micron grit. Label your plates A, B and C. First lap A and B together until they make full contact. Then lap B and C together. Finally lap A and C together. Then proceed to the next finer grit. When you get down to 1 micron grit (several days later) you'll have three flat surfaces. This procedure is used for truing lapping plates and it works very well.

Originally Posted by yoboseyo

I've heard about float glass, but I'm not sure what float glass is, and it doesn't help that whatever I get that's supposed to be flat I have to trust that it's flat because there's no way I can know.

Float glass has been the standard window glass since the '50s. It's made by pouring molten glass onto a bed of mercury and it's as flat as a sh*t carter's hat. All my planes are flattened on wet & dry over glass, especially new ones. The glass plate will flex, so I support it on a large piece of marble

The abrasive transfers witness marks onto the metal so you can easily tell if the flattening process is finished.

mick

Originally Posted by Glider

Float glass has been the standard window glass since the '50s. It's made by pouring molten glass onto a bed of mercury and it's as flat as a sh*t carter's hat. All my planes are flattened on wet & dry over glass, especially new ones. The glass plate will flex, so I support it on a large piece of marble

Mercury cannot be used to make float glass as it boils at about 370ºC and molten glass is well ~1200ºC when poured onto molten metal, which is tin.

Using float glass for flatness has been discussed and measured extensively in the "Practical Machinists" forums.
The responses range from one member who claims to have checked out a number of pieces of float glass for flatness and found a few deviations from level of up to 0.25mm and many up to 0.1mm.
Other members have measured it to be at < 0.003mm/" (please excuse the mixed units).
It might just depend on the individual pieces.
A "B"grade surface plate will be about 0.0004 mm/" better than this.

IBTW 'm in intrigued as to why in your post does "carters" show up as a link to a baby clothing website?

Molten glass is "floated" on molten tin

I have a couple of surface plates and a 1/2" thick piece of float glass I use to flatten stones. I've had the float glass for more than 10 years but it has seen little use.

I cleaned everything up and placed the glass on the granite plate like so.

Setup.JPG

Then I prodded the contact edge between the surface plate and the glass with thin feeler gauges to see what size if any would easily slip into any gaps.

Like this - which shows that this corner is 0.1 mm lower than the middle of the glass plate in contact with the granite.
In fact I could relatively easily spin the glass on the surface plate about the centre of the glass indicating that the centre of the underside of the glass plate was raised.

I got the same feeler gauge into the gap on the opposite corner of the plate as well.
ISide1004.JPG
On the other pair of opposite corners I got 0.178mm and <0.04 mm
Side1007.JPG

All other edges were <0.04mm

The other side of the plate was a bit better.
Firsly the glass would not spin.
One corner would let a 0.1mm feeler slip in and everywhere else was <0.04mm
There could also be a dip in the middle of the glass plate and I would not detect that.

So based on these side measurements, one side is about 0.1 mm out of flat across the diagonal and the other appears to be somewhat less.
Is this good enough for WW? Only you can judge this but clearly this piece of float glass is not flat enough for metal work.

I also tried it out on the bigger cast iron plate and got similar results or maybe a bit less, but I have no certificate for the iron plate and it has seen some hard times.