Thread: DMTs & does anyone have a micrometer?

Hope that piqued everyone's interest.

There was a Norton 4000/8000 lapping question recently where Randydance mentioned that he checked a newb's DMT 325 grit with a straightedge and the plate was NOT flat. That got me thinking in a concerned way since I have a DMT 325 & a new 1200. I took both to a pal's machine shop and we laid them both on a grade A pink granite surface plate.

Long story short, both the 325 & 1200 were about .001" off. The 1200 was .001 higher in the center (convex) and the 325 seemed to have a .001 slope upward.

It's been said that .001" or less will not affect honing a razor's edge. It's also been said, and fervently believed by many on SRP that taping a spine, for bevel setting, etc then removing the tape for polishing is a bad bad idea since the removal of the tape lifts the edge off the hone by the width of the tape which is enough to prevent polishing of the edge thereby accomplishing little (think of the tape as a shim. Removing the shim lowers the spine but raises the edge OFF the hone).

So........How thick is a piece of black electrical tape? How thick is scotch tape? If the thickness of tape can interfere with the overall honing process (unless the tape is kept on the spine from start to finish), is a tape's thickness dramatically far off from .001"??? I wonder. I have no micrometer to check, but maybe one of us does?

Why do I bring this up? What do I want to find out? I want to know if in fact .001" variance from flat will have absolutely no effect on honing a razor (this assumes the razor is flat). Maybe I'm making a mountain out of a mole hill, but I'd like to know.

No need for a micrometer, 3m specs theirs. The Scotch® Super 33+ Vinyl Electrical Tape is 7mil or 0.007" thick.

As for the DMT plates, I'm pretty sure the spec on those is within 0.001" of total flatness. If you are really really concerned about flatness, you might as well spring for a Shapton Reference Plate, either the glass or cast iron based diamond one... either way you're looking at a significantly higher price.


Regards

Christian

Originally Posted by Chrisl

..
So........How thick is a piece of black electrical tape? How thick is scotch tape? If the thickness of tape can interfere with the overall honing process (unless the tape is kept on the spine from start to finish), is a tape's thickness dramatically far off from .001"??? I wonder. I have no micrometer to check, but maybe one of us does?

My black tape mikes out to right around 0.007".

Why do I bring this up? What do I want to find out? I want to know if in fact .001" variance from flat will have absolutely no effect on honing a razor (this assumes the razor is flat). Maybe I'm making a mountain out of a mole hill, but I'd like to know.

Removing 0.007" of tape from the spine will change edge angle (per side) by about 0.5348 degrees on a 6/8 blade and about 0.4584 degrees with a 7/8 razor.
EDIT: Half a degree on a 7 to 8 degree bevel is quite a change when sharpening.

So a 0.001" change from edge to center of a 8" DMT is an angular change of about 0.014 degree. Fourteen thousandths of a degree variance is plenty good enough for razors (IMO). I know, TMI...

My DMT plates are all perfectly flat as indicated by my machinist straight edge. You can easily see the effect of honing a razor with tape. Just set the bevel with tape on the razor then color the bevel with a permanent marker. If you take the tape off and hone the razor you will find that it will take a significant amount of work to remove the marker all the way to the edge unless you torque the edge into the stone/plate.

Well, I build pipes in my spare time, so I just happened to have a digital micrometer laying about...a piece of electrical tape that I use for...well...electrical stuff comes in at 0.0065 of an inch (measured in three places, all came out the same). Just thought I'd confirm the 3M data.

Mark

That does not sound like much of a variance on the DMT. I would not be concerned.
The one I used made really bad grooves in the hone, not little ones but really big, wide ones.
I am sure it was a fluke. Many people here are using them and none have reported a problem.

I just checked the DMT website and indeed, I was correct on the flatness claims:

Here's a quote regarding the D8 series (8 x 3)

"The precision-engineered surface flatness to + / -0.001” will remain flat forever, offering the sharpest edges with no maintenance."

and in reference to the coarsest stone:

"With a surface flatness to +/- 0.001" Double-X is perfect for flattening water stones, Arkansas stones, and synthetic stones."


Now, considering that +/- 0.001" over 8 inches is pretty much the standard for basic machinists squares, it will have no discernible impact on razors or flattening hones. There is of course always the chance that a defective hone might get out and if so I would expect DMT to make it right. Personally, I think the D8 series is one heck of a bargain.

Regards

Christian

Originally Posted by heavydutysg135

My DMT plates are all perfectly flat as indicated by my machinist straight edge.

Nothing is perfectly flat. I am reasonably sure that your DMTs have more than 0.00000000001 " error to them.

When a surface is flat to .001, that is the specification for its flatness. It's not "off". If you want to get it to .0001, you can do that but it will cost a lot of money. "Flatness" is just a question of money. Look at your friend's surface plates or better yet, look them up online in the Starrett catalog or MSC or Enco. You'll see that as they go from Grade A to B, they're certified at lower levels of flatness.

Flatness is spec'd to the performance requirements of the tool. I don't believe that going beyond .001 will result in a better shave. Your face varies a whole lot more than that!

I have a bunch of new digital micrometers in boxes if anybody wants to start measuring their stuff.

I spent a few years in optics (for an aerospace company) then I became a quality engineer for a medical device company that makes optical devices. The subject of flatness is a very serious one in the world of precision manufacturing. All proper metrology labs have at least one lab grade granite surface plate that is calibrated to be "flat" and more importantly, "repeatable" as specified by Federal Specification GGG-P-463c. What does that mean? Here's an article that explains it in great detail:
http://www.qualitydigest.com/aug03/a..._article.shtml

What does that mean to us razor owner/honers? Realistically not much. While we may strive for perfection, the world in which we live is so full of variables that our stones and razors are probably moving more than .001" over any given twenty-four hour period while sitting on a shelf. I have to agree with Kaptain Zero when he says that we can be happy flattening our stones with wet/dry sandpaper on a piece of glass.