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Archive for January 2nd, 2018

Reading a Vernier Height Gage

The first time around, I simply set both pairs of MPCNC rails to equal heights using my height gage (*) as a reference, rather than as a measurement tool:

MPCNC - Rail height measurement

MPCNC – Rail height measurement

By now, I assume all the plastic bits have shaken themselves down and the rails have settled into their more-or-less permanent locations, so it’d be useful to measure the actual rail heights and adjust as needed. The scale along the vertical bar of the height gage gives the height of the top surface of the projecting arm above the bench:

Brown and Sharpe 585 Height Gage

Brown and Sharpe 585 Height Gage

Normally, the gage base would sit on a surface plate. Building an MPCNC on a big granite slab would certainly cut down on the shakes from overly enthusiastic acceleration settings!

The nicely reshaped and polished lathe bit transfers the top surface of the gage arm to the top of the MPCNC rail, so whatever height shows up on the vernier gives the rail height. The exact value, of course, doesn’t really matter in this situation, but when you need an actual measurement, it’s got you covered.

The two brackets slide along the height gage, with the thumbscrews on the right locking them in position. To measure a height, you loosen both thumbscrews, slide the whole affair to put the arm bracket at about the right height, tighten the top thumbscrew to anchor the adjusting bracket, twirl the knurled wheel to precisely position the arm bracket, then read the height from the scale.

This requires reading a vernier height gage scale:

Vernier Height Gage - 132.20 mm

Vernier Height Gage – 132.20 mm

The other scale on the other side has inches, but nobody uses those any more. Right?

Things I didn’t get quite right the first time around:

  • The numbers along the right side are in centimeters
  • The smallest lines on that scale mark 0.5 mm increments
  • The numbers on the vernier have units of 1/50 mm = 0.02 mm

So, to read the scale:

  • Multiply centimeters by 10 to get millimeters: 130
  • Add the number of whole millimeters below the 0 vernier index: 2
  • Add a half millimeter if needed: 0
  • Find the matching vernier increment: 10
  • Multiply the increment by 2: 20
  • Slap the decimal point two places left and add: 132.20

OK, try this one:

Vernier Height Gage - 159.84 mm

Vernier Height Gage – 159.84 mm

As I see it:

  • Read 15 cm
  • Count 9 ticks
  • Add the 0.5 mm tick
  • Match vernier tick 17, multiply and slap decimal = 0.34 mm
  • Add: 150 + 9 + 0.5 + 0.34 = 159.84 mm

There, now, that wasn’t so hard, was it?

There’s obviously a parallax issue between the edge of the vernier scale and the main scale; it’s easier to get it right in person than in the photograph.

I pronounced the reading as “160 minus point 5 is 159 and a half plus point 34 is point 84”, but I also take eight photographs as I work my way around the MPCNC frame to review any suspicious results.

Obviously, reading a digital height gage would be much easier & faster, but we don’t want to deskill the workforce, do we?

The maker’s mark on my height gage says it’s a Brown & Sharpe 585 with a 19 inch scale; B&S has long since been Borged. Back in the day, this painstakingly applied etching distinguished it from all the other height gages in the shop:

Brown and Sharpe 585 Height Gage - D.E 1-I-3 etching

Brown and Sharpe 585 Height Gage – D.E 1-I-3 etching

We’ll never know the rest of the story.

(*) When Starrett spells it “gage”, it’s good enough for me.

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