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

Another difference is that the tape affects the angle of the razor in a consistent way, while a slight variance in the hone will probably be gradual enough that the razor can adjust to it as you hone. A good X pattern stroke should compensate for some slight unevenness. My Belgian blue hone is way off flat when checked against a straight edge, but it works fine with razors of all sizes and shapes.

Josh

Well, if the consensus then is that .001" variance has no effect on adversely affecting a razors edge during honing, I'll live with that and chalk up my tribulations to my inexperience rather than being able to blame my lackluster results on the plates and hones

SRP member Sticky had mentioned in an recent post that the average thickness of a pencil mark is four billionths of an inch. With that in mind, aside from seeing where a hone is "out of the box", I see no need whatsoever to go through the trouble of ever doing a pencil grid for re-lapping a hone after the initial lapping. Why even bother taking the time to remove 4 billionths of an inch if a .001 variance has no ill effect? I ask this rhetorical question because I've used the pencil grid when re-lapping hones in the past. I'll do nothing more than some brief refresher passes on the diamond plates to clean the surfaces of my Norton 4000/8000 more than anything else and call it plenty good enough.

Originally Posted by Chrisl

SRP member Sticky had mentioned in an recent post that the average thickness of a pencil mark is four billionths of an inch. With that in mind, aside from seeing where a hone is "out of the box", I see no need whatsoever to go through the trouble of ever doing a pencil grid for re-lapping a hone after the initial lapping. Why even bother taking the time to remove 4 billionths of an inch if a .001 variance has no ill effect? I ask this rhetorical question because I've used the pencil grid when re-lapping hones in the past. I'll do nothing more than some brief refresher passes on the diamond plates to clean the surfaces of my Norton 4000/8000 more than anything else and call it plenty good enough.

The pencil mark was somewhere between 4 to 17 billionths as measured electronically with a Wheatstone Bridge. Geometric analysis agreed with the Bridge.

The rational behind drawing a second grid and re-lapping is because the first lapping will introduce errors (much larger than 4 billionths), from the operator, the initial unevenness of the hone, and the DMT plate. The worse your hone is at first, the bigger the introduced errors. If your hone starts out pretty flat, then the errors will likely be small. Many times small enough to be fine for razor use (depending on the operator's skill at lapping).

I only re-mark and lap twice under 3 conditions:

• This is the first time I am lapping this particular hone. OR-
• Using a 6" machinists rule, before beginning to lap (new or previously lapped-by-me): I see more than about 0.006" or 0.007" of daylight, at any point, between the rule and hone.
• I have the slightest suspicion/feeling that I might need to...

The only way a lapping job could be w/in 4-17 billionths of an inch is if all of your grid marks disappeared equally and at the same time. This will be impossible on a DMT plate because the DMT is not that accurate. All of us operators are even worse...