Starrett 440 Depth Micrometer, HB400, or CMM? A Metrology Field Guide

I'm not going to pretend there is one right measuring system, because there isn't. For the past 11 years, I've handled inspection-related work for machine shops, prototype builders, and small manufacturers. I've also documented seven of my own measurement-tool mistakes that added up to roughly $22,000 in wasted time, calibration fees, and rework. I'm not proud of that number. I'm just glad I still have the checklist it paid for.

The same three questions seem to come up on every shop visit:

• Starrett 440 depth micrometer or the depth rod on a Digimatic caliper?
• Starrett optical comparator HB400 or a full metrology CMM?
• How does a megger insulation tester work, and does a mechanical shop actually need one?

Those all look like single-choice questions. In reality, the right answer depends on the geometry, the tolerance, the part volume, and the operator. Let me walk you through the branch I use now, with the expensive parts included.

Scenario A: Starrett 440 Depth Micrometer or Digimatic Caliper Depth Rod?

If you have a 6-inch electronic caliper, the depth rod on it can do more than people give it credit for. The Digimatic name is tied to Mitutoyo's digital product line, but I'm using it the way most buyers do: any standard digital caliper with SPC output. It is a fast and comfortable tool for deburring checks, shallow steps, and one-off measurements.

For a single prototype part, I'd probably use the caliper and pocket the money. But before you make it the production method, listen to the story I tell in every new-hire intro.

I first-pieced a stepped pocket in an aluminum fixture with my caliper depth rod. It read 0.505 in. The machinist across from me measured the same feature with a Starrett 440 depth micrometer and got 0.501 in. I told him his tool was off. He zeroed the micrometer with the base on a surface plate, let the contact touch the plate, then measured the pocket again. Same reading: 0.501 in.

The problem wasn't his micrometer. It was my caliper. I was holding the depth rod at a slight angle, which made the edge of the rod ride up on the shoulder. The error was about 0.004 in, and with the print tolerance sitting near 0.002 in, that was not a rounding issue. The 440's flat base sat on the reference surface the way the part would sit in the assembly. That stability was the whole difference.

So my scenario advice is not romantic. Use the digital caliper if the tolerance is loose, the batch is small, or you are only looking for a go/no-go warning. If the depth callout repeats and the tolerance is tighter than around 0.003 in, I'd put a depth micrometer in the crib. The Starrett 440 is a good example because the base is rigid and rods are available for different depth ranges, but the brand is less important than the calibration certificate behind it.

One more caveat: a depth micrometer cannot measure a deep recess in a corner if its base won't fit. In those cases, I use a small-base depth gage or a digital indicator on a fixture plate. Don't force the micrometer into a space it was not designed for.

Scenario B: Starrett Optical Comparator HB400 or a Metrology CMM?

The comparator-versus-CMM question usually comes from a shop that just won a job with complicated profiles or hole patterns. Let me separate the two uses clearly.

The Starrett optical comparator HB400 is best when the geometry you care about is visible as an edge, silhouette, or surface profile. Stamped parts, formed brackets, turned profiles, inserts, and cutters all show up well on a comparator. You can check radii, angles, and contour shapes quickly, with a clear image the operator can see.

A metrology CMM is best when the important numbers live in three dimensions: bore positions on different faces, true position relative to a datum reference frame, perpendicularity of a hole to a face, or scanning a free-form surface that doesn't sit flat relative to an optical axis.

I learned this the expensive way in 2021. I chose a comparator for a housing with critical bores on two perpendicular faces. The part looked clear on the screen, and the XY readings were beautiful for the face I could see. The second face, the one that needed a Z coordinate and a different axis alignment, could not be measured on that instrument. The parts were not bad. My equipment selection was bad. I ended up sending the housing out to a lab with a CMM and paying a rush inspection fee that ate whatever I thought I saved.

Don't read this as an attack on the HB400. If anything, the comparator is the more honest tool: it shows you what it measures. A CMM can create false confidence if the operator does not understand part alignment. I don't have hard data on how many CMM errors come from software setup rather than machine accuracy, but my sense is that operator setup and software alignment outnumber hardware problems.

Here is the branch I use:

• Use an optical comparator when the callout is a 2D profile or an edge condition and the part can be staged in the field of view.
• Use a CMM when the print uses true position, 3D datum relationships, or measurements at multiple heights and angles.
• If you are a small shop that only needs CMM data occasionally, rent calibrated CMM time before you buy one. It protects the budget and shows what the volume actually looks like.

Scenario C: How Does a Megger Insulation Tester Work?

This one feels out of place, but I get it from maintenance people and shop owners who work on their own motors. A megger insulation tester is not a dimensional tool. It answers a different is-this-still-good question about electrical insulation.

Plain-English explanation: A megger applies a high DC voltage, usually 500 V, 1000 V, or 2500 V, between the conductor under test and the motor frame or ground. An internal current meter measures the small leakage that flows across the insulation. Using Ohm's law, resistance equals voltage divided by current, so the display gives you an insulation resistance value in megohms.

Why high voltage? Because common insulation faults do not always show up at low voltage. A megger stresses the insulation, which is why you should never use it on an energized circuit or on sensitive electronics unless the equipment is rated for it and you know what you're doing.

In 2022, I skipped the insulation-resistance test on a rebuilt spindle motor because I was focused on runout and thermal expansion. Eight hours after installation, the machine tripped on a ground fault. We pulled the motor, meggered it with a 500 V tester, and watched the reading sit at 0.2 megohms. Ten minutes of testing before installation would have caught the problem. That one cost me a weekend plus about $700 in contractor time. I don't wish that experience on anyone.

So the scenario advice: if you repair, install, or store electric motors, buy or borrow a megger insulation tester before you need it. If your shop only does mechanical measurement, keep the megger out of your own toolbox and let an electrical contractor handle it.

The Practical Branch Finder

If you're still in front of a catalog and the cursor is hovering, run through this:

1. One depth feature on a one-off part with a loose tolerance? Use the caliper. Repeated depth feature with a tighter tolerance? Depth micrometer, and a Starrett 440 is a safe starting point if it fits the feature. If not, small-base depth gage.
2. 2D edge or profile callout? Optical comparator. Starrett's HB400 will carry that load well. True positions or dimensions on multiple faces? CMM. If this is the first job, don't buy a CMM to win it; rent CMM time and save the capital for calibration data and fixturing.
3. Electrical insulation or winding-to-ground condition? Stop asking about calipers and get a megger insulation tester.

Small orders don't deserve second-class treatment, and I mean that in both directions. The first vendors who treated my $300 inspection orders seriously were the ones who got my $5,000 orders later. The same logic applies to tools. You don't need the grandest instrument on the wall; you need the instrument that fits the measurement, the operator, and the honest uncertainty budget. If the reading cannot be traced to a reference standard, all that resolution is just a story.