Measurment tool from Freight Harbor

Are you talking about a single tool which serves both purposes, or two separate ones?

I guess that it you mean the latter, just putting them in the opposite order "Digital caliper and combination square" would eliminate the confusion as to how far the "combination" applies.

I don't know the Harbor Freight tools from experience, though here are the things which I would consider important:

1) Combination square:

a) Are the numbers and lines engraved, or simply stamped into the scale? If the latter, the accuracy is a lot more questionable.

b) How accurate are the angular relationships between the scale and the heads? Check the 90 degree parts against a rigid machinists square. Check both the head alone against the machinists square on a surface plate, and the head and rule against the machinists square separately. Also -- check the 90 degree and 45 degree points on the protractor head against the appropriate surfaces of the square head.

c) How smoothly does the scale slide in the heads, and how firm a grip does the lock achieve?

d) Is the scale straight, or does it have a curve?

e) What is the material of the heads? Steel or cast iron are good ones. Aluminum is not likely to retain its accuracy. The rule should be hardened, ground, and engraved.

f) For a used one -- check that you don't have a "shrink rule" calibrated for something like making patterns for casting metal, where there is expected to be a certain amount of shrinkage, so the rule causes you to make the patterns oversized by the proper amount.

2) Digital caliper:

a) How smooth is the travel of the head?

b) Is there a thumb roller to allow fine adjustment?

c) How common a style of battery does it use?

d) Are there provisions for turning it *fully* off? Some turn the display off automatically after a few minutes of inactivity. On these, when you move the head, does the display turn back on with the proper reading?

Ones which can be turned fully off give better battery life, if you are not using them every day. My older Starrett does not turn fully off, and to get good battery life, I have to slide the battery holder on the back just far enough to disconnect the batteries, and to re-zero it when I turn it back on. The Mitutoyo, in contrast, goes into "sleep" mode with no easy way to turn it fully off, but gives pretty good battery life (much better than the Starrett without the sliding battery holder trick, but not as good as with). However, the Mitutoyo wakes up in proper calibration whenever and wherever it is turned on -- unless the batteries have died or been replaced since the last time it was turned on. Then, you have to re-zero it.

Cheap digital calipers (e.g. the $8.95 one form a hamfest which a friend got) tend to really eat batteries.

e) What is the material? Mitutoyo makes both stainless steel bodied ones which are quite accurate, and inexpensive ones with a fiberglass filled plastic body (light green, IIRC), which is less likely to be accurate over time.

f) What about absolute accuracy? The specs on some of the inexpensive ones may be 0.0005" resolution (the smallest change which can be measured), but only 0.005" (ten times worse) accuracy when you check all points along the length with gauge blocks. (Others turn out better than expected -- but you can't tell with the cheap ones.)

But -- the absolute accuracy may not matter that much, unless you are very careful how you use it, because closing the tips of the jaws on a workpiece can cause enough tilting of the head so the reading can be several thousandths off (more likely in the longer ones like 12" length with longer jaws).

So -- for serious measurements, you will want a set of micrometers (0.0001" vernier scale and carbide anvils) to cover the range of workpiece sizes likely in your lathe. I've got them from 0-12" for my 12" swing Clausing, but in reality 0-8" is probably more reasonable to cover just what will swing over the carriage -- unless you are making precise things which are too large to fit over the carriage -- as perhaps a flywheel whose surface is also expected to precisely drive a flat belt.

Digital calipers normally have a resolution of 0.0005", but can hardly be depended upon to provide that accuracy in general use. (Good ones, perhaps, used very carefully may approach that, but not in inexperienced hands.

Regular micrometers have the barrels calibrated in 0.001" steps (except metric ones, of course). Better ones add a vernier scale to take the resolution down to 0.0001". Digital ones will take it a bit past that, with a resolution of 0.00005". However, careless use will still not get accuracy even as good as the first style. You need to ideally learn the proper "feel" (how tight to tighten the micrometer onto the workpiece). Better micrometers will have either a ratchet thimble (the small "tit" on the end of the spindle will click and not turn the thimble at a certain torque), or a "friction thimble", in which the whole knurled grip on the thimble will slip to limit torque. Note that there is always a way to set it or to grip it to exceed those settings. The friction thimble (in good condition) can probably be the best choice for an inexperienced user. The click limit is not quite as precise. But the best it to develop your own "feel" (grip it just tight enough so the thimble slips under your fingers before you get to the point at which the friction thimble or the ratchet thimble releases. And set the zero of the micrometer using the same feel.

Note that the reading which you will get on some surfaces (e.g. a grooved one from the wrong feed rate or a poorly sharpened lathe tool) will vary widely between the methods, because you are getting enough force from the screws to crush the ridges, thus changing the readings.

And the anvils of the micrometer should be cleaned whenever you zero it (or check the zero), and whenever you read using it. Tiny particles of grit can shift your readings significantly. Standard practice is to lightly close the anvils of the micrometer (or the jaws of the calipers) onto a piece of clean paper, and slowly withdraw it from between the jaws without opening them. This tends to carry out any grit which may be present.

All of this being said -- you can start out with the cheap ones, and plan to get better as you learn. This way, if you damage something you won't feel as bad if you damage an $8.00 digital caliper as you would with a $200.00 digital caliper by one of the big makers. I still keep a 0-4" Helios dial caliper which has a serious ding in the ID measuring surfaces on the jaws because it got too close to a rotating chuck on a Unimat lathe. I would much rather that had happened to an $8.00 one (if such were available back then.)

If the cheap tools survive, you can use them as the ones to lend to someone who wants to borrow your tools. Keep the good ones out of other's hands.

For anything digital, keep a full spare set of batteries in the case with it. The batteries *will* fail at the most inconvenient time.

And I keep at least one vernier caliper for use if all of my digital tools decide to die on the same day. It stays beside my computer for the infrequent use that it would get up here, where the batteries are more likely to die between uses.

The most frequent failures of the batteries are in my older 12" Mitutoyo digital calipers -- simply because I don't *need* that size very often.

I hope that this is some help -- in spite of it not dealing with the explicit products which you asked about.

Good Luck, DoN.

I picked up one of the 6" digital units on sale and it was definitely worth it at half price! Actually I WAS suprised by it's quasi-quality and it serves its purpose well of something more accurate than my tape measure.. that's all. Just open it up before you buy it and make sure it isn't busted. Buy your squares from Lowes or HomeDepot.

- Loren

Alex wrote:

Freight Harbor?

Good site. I tend to buy the silver oxide 357s for some calipers (the Starrett takes a larger "coin" battery), micrometers, and the HP 15C and 16C calculators, so I buy a dozen at a time, along with some other orders from MSC.

And if you want to extend the storage life of batteries not yet in equipment, store them in the 'fridge (for most, not in the freezer, just normal cooling a bit above freezing).

Enjoy, DoN.

Wow, a hearty "thank you" for that; I've been wondered why my caliper batteries bonk out at me so fast, and now I know why. I'm gonna go spend the $2.79 at Batteries Plus to get the one I need today (need one anyway).

Thanks!

Jon

The 6" digital calipers are a great deal at the usual $20 sale price and an even better deal at the recent $15 sale price.

The combination square is of poor quality IMO. You may not be happy with this one.

Maxell or Sony silver oxide SR44W - $.97 + $1.89 sh. Free shipping for

5 or more. I bought 5 and was dismayed when they came and they appeared to be no-name. But it was just the package, the battery itself was marked "Maxell".

Bob

I have been pleased with my cheapy HF digital caliper. That said, I wouldn't buy a combination square from them.

As you know, Starret has always been the big name at the higher end, although Mitutoyo and Brown&Sharpe also make very high quality squares. Another name to look for, and what I bought for our lab, is PEC Tools. Very nice, American made squares and rules, for 20-30% less than the big three. And no, I have no other affiliation other than being a satisfied customer. Bought my set through MSC.

All that said, your best deal might come from a monthly special at your favorite vendor. I don't think you will be dissapointed with a square from any of the above manufacturers.

Also, you might find an older square at a neighborhood garage sale. I have an old MIller Falls, which is fine for wood dorking activities and general home use. It is a step above the junk Stanley and General squares sold at the BORG.

DLGlos