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Straight edge suggestions (for checking flatness)

Thinking about getting a decent straight edge for testing my hones. Been using a worn out dmt 325 for atleast a year or two. Feel like my stones are good and flat but struggling with my jnats and figured a straight edge would not be a bad investment.
 
Congrats. Straight edges at an estate sale are awesome. Just make sure it's straight. I was thinking about checking one of my older diamond plates for flatness, perhaps I'll take some pictures and post them here.
 
whenever I have a straight edge come in I will post some pics of my larger stones and how flat they are. Majority of my razor edges forever have just been my coticule edge which is a smaller stone (5x2). I think my stones are probably flat enough, but I feel like it would not be bad having a straight edge laying around
 
So in case it's of any interest, this is how we check things for straight/flat in a machine shop. First photo shows a precision granite flat. This is a reference surface. This particular one has been inspected and certified to be flat within +/- 50 millionths of an inch across the entire 18" × 24" surface, from highest to lowest point.

So basically, using a dial indicator resting on a stable stand in contact with the granite plate (an indicator measures movement of its point which is in contact with the item to be measured), we compare the flatness of the granite plate to the flatness of the item to be measured. Notice the tissue paper between the diamond plate and the dial indicator tip - this prevents the diamond plate from abrading the indicator point. If I was VERY worried about super precision I'd cut a small piece of paper and use that same little piece at each position. Usually this tissue paper is uniform enough not to worry about that though.

In this particular case, my Atoma 1200 is bowed concave about .001" low in the middle on one side and about .002" low in the middle on the other. This is about the quickest and easiest way to check anything for flatness, but the equipment is costly. A straight edge is good too as long as you're sure it's flat.

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I use an Empire steel rule I picked up at a local Home Depot and some .001" feeler thickness gauge tape from McMaster-Carr. So if the stone/hone is out of whack before I lap it, and passes the tape slide test after I lap it, my take is that the rule is straight--otherwise it would continually fail to pass.
 
I guess a lot depends on how straight a straight edge you really require.

Veritas makes some very good quality steel straight edges, in Canada, for a very decent price.


I've never tested them to see how hard they are, but they have obviously been hardened to some degree because they last.

Free shipping on them right now too, which only happens a few times a year. Maybe once per quarter.

My experience with some of the Made in China straight edges has been rather poor. They are not very straight and are much softer than the Veritas straight edges, so their heat treatment is obviously lacking.

Another somewhat inexpensive option is a large size Rafan machinist square. They are made in Poland, are hardened, and are very straight and square. However, the cheap stuff from China put them out of business a few years ago so you'd probably be looking for something used on eBay. But they can still be found rather inexpensively. Probably about the same price as the Veritas straight edges, for something nice.
 
Well, not so much a matter of how flat or straight a straight edge you need, more of knowing the actual status of the equipment you use. Most reliable vendors that sell new straight edges will probably be safe to trust. However, you never know what you're getting at an estate or yard sale.

BTW, anything made of aluminum is bad juju for a straight edge. It moves too much too quickly from thermal expansion - just holding it in your hand for 10 seconds can throw it out of whack due to temperature differential.
 
You'll want to make sure you don't touch the straight edge before you use it, maybe wear an insulative glove. And be very careful with it on abrasive surfaces, because the aluminum is generally quite soft.
 
Well wack, that's exactly what I got.

Thought it would be fine since it would be staying in room temperature spot all the time
You can still use it, just take care to work around that issue. You can wear some cotton gloves to insulate the aluminum from your body heat or you can just set up the straight edge with some insulating material glued to the sides for a "grip." You could also set the straight edge in a vise or something them hold the stone against the straight edge instead.

And yeah, be extremely careful not to slide it against an abrasive stone or you will wear it out of flat quick.
 

duke762

Contributor
So in case it's of any interest, this is how we check things for straight/flat in a machine shop.
Very interested!! It's nice to see a skilled tradesman showing folks how things are done.

Nice write up eKretz. One thing you didn't mention about your diamond plate and set up is this only works if both surfaces are parallel. Natural stones and most water stones will not be parallel and need a different approach. I've checked my Arks by bridging the surface with precision parallels and using a small surface gage and indicator, I checked lengthwise and across, up down the length.

I also check with my shop made straight edge. D-2, 12"x3"x3/16" ground it checked it, and checked it on the granite plate with a .0001" indicator. Needle didn't move. It gets crazy at this level of precision. Think about it this way. The average human hair is about .003" and .0001 equals 1/30th of a hair. In metric that would be..... .003 of an inch equals .0762mm, and .0001 of an inch equals .00254mm. I guess that's why they call it splitting hairs.
 
I use a Starrett stainless straightedge, beveled, no markings. I keep it away from everything, all it does is check stones. I would not buy one in a yard sale because they can be knocked out of spec easily.

I have an aluminum straightedge from Chine that I use for cutting long pieces of whatever - it's was not straight on arrival and now its almost useless for cutting straight lines. Need to get a new one of them but the next one will be stainless and made in the US if possible.

I keep my Starrett in the protective sleeve it came in, in a draw in my tool chest. it lives inside a long slender box to keep stuff from banging into it.
I swap them out every few years just because; they get used nearly daily, and they get put on abrasive stones. While I am not intentionally wearing the edge, I don't need to be concerned with it either.
 
Very interested!! It's nice to see a skilled tradesman showing folks how things are done.

Nice write up eKretz. One thing you didn't mention about your diamond plate and set up is this only works if both surfaces are parallel. Natural stones and most water stones will not be parallel and need a different approach. I've checked my Arks by bridging the surface with precision parallels and using a small surface gage and indicator, I checked lengthwise and across, up down the length.

I also check with my shop made straight edge. D-2, 12"x3"x3/16" ground it checked it, and checked it on the granite plate with a .0001" indicator. Needle didn't move. It gets crazy at this level of precision. Think about it this way. The average human hair is about .003" and .0001 equals 1/30th of a hair. In metric that would be..... .003 of an inch equals .0762mm, and .0001 of an inch equals .00254mm. I guess that's why they call it splitting hairs.
Yep, you got it. :thumbsup:

In the case of a natural stone or a part with irregular geometry on bottom, just like you mentioned I'd just set it up with the side to be tested facing down against the parallels and use the indicator underneath instead of on top. That's definitely the easiest way to deal with that particular issue.

If you paid close attention, you'd see that the diamond plate's top surface isn't perfectly parallel with the bottom surface either. Only by .001" though, easy to interpolate that - if I lower that one high side mentally by. 001" to even it up with the zeroed starting point, the middle reading needs to be lowered by half that, or .0005". So the -.0015" is then a -.002".

Additionally, the readings have to be taken at the peaks of the little diamond "heaps" on an Atoma. Easy to quickly find by probing with the indicator to find the maximums, or highest points.

How long ago did you make your straight edge? Did you heat treat it yourself or send it out? I just got my own little machine shop area set up finally, so I might make a few small tools. Got a Wells Index turret mill, a 14" × 40" lathe and an 8"×18" Brown and Sharpe surface grinder. Looking forward to getting them wired up and running.

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I normally use this ruler. It is inexpensive and very accurate.
It claims to be within .010 but the ones I have have been checked against a granite surface plate guaranteed to be within 1/10,000 " and they are very flat - within .001
The are made by Shinwa, a Japanese company known for quality.
At only 20 bucks they are plenty flat enough for checking hones.
Granite plates are surprisingly cheap and worth purchasing even if only to flatten hones with. Mine is a 9x12 but 12x18's are available and under 50 bucks here in Canada
 
Japanese tools are generally pretty darn good. And definitely agree that a granite plate is a very useful tool. I currently have two. The 18"× 24" depicted above that I use in my basement workshop and a 36"× 48" for machine shop use. The little import ones are pretty cheap and reasonably flat - certainly better than most home users will ever need.
 
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