Pin Gage Question

[Rex Walters]

I'll chime in since my gage pins are one of the most used tools in my shop. I use them CONSTANTLY for all sorts of stuff (far more than simply measuring holes). They are incredibly useful for setup work. I prefer to use them over gage blocks whenever reasonable (no chance of screwing up the stack accidentally -- just grab the size you need).

As far as what sizes you'll want: as silly and obvious as it sounds, it's worth pointing out that you can put TWO pins in a hole to gage a diameter. (I went an embarrassingly long time before I realized this!) A set of pins up to 0.5000" let's you trivially gage holes up to 0.9990". Add a single 1.0000" pin (or a carefully mic'ed dowel) and you can measure up to 1.5000" (anything bigger and telescoping gages are accurate enough).

The most common use for my pins are positioning parts and fixtures relative to a stop. I also seem to chuck them up in lathe chuck or Jacobs chuck in the mill fairly often to position things off the spindle axis (oh: it's also incredibly useful to know the exact diameter of your mill spindle for the same reason!). They are particularly handy when boring holes on the lathe to a critical depth with a carriage stop. Just face a hair off the part with the boring bar (or just gently touch the face), then position the stop with an appropriately sized pin between the stop and the carriage.

Note that pins give you a good tactile check for any taper in a bore, too.

As far as sizes, I've got two "minus" sets (which is what you want to measure bores): one set from 0.0610" to 0.2500", another set from 0.2510" to 0.5000", and a single 1.0000" pin. That's been perfect for my needs. Anything smaller that 1/16" and I'll use feeler gages and shim stock. Anything larger and I'll use telescoping gages, gage blocks, adjustable parallels, or whatever.

I don't remember where I bought mine (probably Enco or MSC) but they were pretty cheap and my experience was similar (all were about one or two tenths under the marked sizes (as well as I can measure). They were a LOT cheaper than gage blocks. I'd rather have an expensive micrometer I trust and a cheap set of pins than vice versa (for my hobbyist shop).

 

[woodchucker]

As far as sizes, I've got two "minus" sets (which is what you want to measure bores): one set from 0.0610" to 0.2500", another set from 0.2510" to 0.5000", and a single 1.0000" pin. That's been perfect for my needs. Anything smaller that 1/16" and I'll use feeler gages and shim stock. Anything larger and I'll use telescoping gages, gage blocks, adjustable parallels, or whatever.

well hold on there... it depends on what bore.
say you are measuring a bore on a pulley to fit on a shaft. Then you would want a +.
A minus would be useful for holding a dowel pin...

so it really depends...

just saying... just my opinion.

 

[Rex Walters]

so it really depends...

Well, yes. But I think it's safe to say you more often want a tight hole than an oversized one, and on the rare occasion that you do it's better to sneak up on it (lapping is easier than cutting if you care about tenths -- I rarely need that level of accuracy).

As the old saw goes: if you cut a rope too short you can always splice some more on, but if it's too long there's nothing you can do about it.

 

[woodchucker]

Well, yes. But I think it's safe to say you more often want a tight hole

yes, I always want a tight hole... um, wait are we talking machining?
I mean no, I want the right size hole.

so it's measure once, cut twice? or is it measure once, cry twice?

 

[Rex Walters]

Heh.

I'm serious, though.

Pin gauges come in minus sets (typically tolerance of +0/-0.0002") and plus sets (tolerance of +0.0002"/-0). For the work I do in my home shop, the former are far, far more useful.

Boring operations increase the size of a hole, so pins that are slightly over the marked size simply don't make sense to me. If I was doing some sort of additive manufacturing process where the bore diameter gradually decreased, then plus pins would make sense. Plus pins do have QA/metrology applications, of course, but for machine boring operations, especially hobby machining, I stand by my statement that minus sets are what you want.

For normal use cases, plus pins would be misleading (you'd tend to bore all your holes very slightly oversized, which is often worse than slightly undersized). It's extremely common that I want to bore a hole exactly to a given diameter (to the closest 0.001") without going over. I can't think of of a single occasion where I wanted to continue boring a hole without going under my target diameter! And that includes pulleys on a shaft (where you would normally want ~0.0005" radial clearance for a slip fit, so I use a pin marked 0.001" larger than the shaft and then use a grub screw and a flat on the shaft to keep the pulley from rotating).

For my projects, I sometimes want an exact (light press) fit where the bore and the shaft diameter are identical, sometimes a tight slip fit (say 0.0005" radial clearance), sometimes a loose fit (a thou or more of radial clearance), sometimes a slight interference fit, and sometimes a heavy interference press fit. In every case I measure my progress with pins marked to the closest 0.001" pin to my target. The amount of slop or interference I desire depends on the overall diameter of the hole (more for larger holes), but I'm invariably boring to the closest thou and using pins to ensure I don't go over my target. For loose clearance fits, minus pins aren't a problem as I'm targeting a thou or more of diametric clearance. For press fits and tight slip fits, though, plus pins could cause real problems.

I'm pretty confident that vendors sell a LOT more minus sets than plus sets, for exactly this reason.

---

Oh: One more thing re: actually using gage pins. Get in the habit of ALWAYS reading the marking on the pin itself. It's stupid easy to grab a pin from the wrong row, below the markings on the box! And even though nobody else has access to my shop, I have accidentally swapped a couple pins when putting them away.

 

[RJSakowski]

My preference is both plus and minus pins. Having both allows one to advance in roughly half thou increments (.0004/.0006). I'm not there yet but the second sets are on my want list.

For measuring out of range holes, I use three pins, as indicated in post #14 above.. Two pins requires them to both be on the same diagonal, If one or both of the pins are slightly cocked, it can result in an error in reading. Three pins ensure that the pins are parallel to the bore.

 

[rabler]

Two pins requires them to both be on the same diagonal, If one or both of the pins are slightly cocked, it can result in an error in reading

I can see that for a very shallow hole (depth vs diameter), but it seems that two pins that fit well are going to be forced to be parallel to the bore for any significant hole depth. You should be able to rule that out by seeing if the next size up of two pins will even start into the hole. Obviously what level of accuracy you need plays in, trade that off with doing the trig for three pins vs. simple sum for two. And throw in bore taper, etc as muddle factors.

 

[Rex Walters]

Having both allows one to advance in roughly half thou increments (.0004/.0006

Everyone has different needs. Personally, I often find myself wanting to measure a bore of, say, 0.4870" +0/-0.0002. But I can't think of any occasion where I'd want to measure a bore with a +0.0002/-0 tolerance, or measure, say, a 0.4876" bore (tenths accuracy). I might want to lap a bore/shaft pair to get the desired fit, but I wouldn't often (ever?) need to actually measure a bore diameter to tenths.

I use three pins, as indicated in post #14 above.. Two pins requires them to both be on the same diagonal, If one or both of the pins are slightly cocked, it can result in an error in reading.

Really?! Three pins requires some math, but even I can add two values in my head. And gaging a bore is a feel thing even with a single pin. I've never had any difficulty measuring a bore with two pins, even fairly shallow holes. For me, measurements with two pins are FAR more accurate and repeatable than with telescoping gauges, for example.

 

[RJSakowski]

I can see that for a very shallow hole (depth vs diameter), but it seems that two pins that fit well are going to be forced to be parallel to the bore for and significant hole depth. You should be able to rule that out by seeing if the next size up will even start into the hole. Obviously what level of accuracy you need plays in, trade that off with doing the trig for three pins vs. simple sum for two.

The math is an impediment, certainly. I get around that by drawing the three pins and hole in CAD. By setting the required tangency of the four circles, I have only to dimension the hole. I usually will try to select three adjacent pin sizes. The error in using three pins separated by .001" rather than three identical pins is less than a tenth of a tenth. The hole diameter will be 2.1547 x the middle pin diameter.

 

[Ischgl99]

Everyone has different needs. Personally, I often find myself wanting to measure a bore of, say, 0.4870" +0/-0.0002. But I can't think of any occasion where I'd want to measure a bore with a +0.0002/-0 tolerance, or measure, say, a 0.4876" bore (tenths accuracy). I might want to lap a bore/shaft pair to get the desired fit, but I wouldn't often (ever?) need to actually measure a bore diameter to tenths.

A lot of things have plus tolerances, holes for bearings and couplings for example. Metric dowel pin holes are normally H7 fits, that's a plus tolerance.

How are you measuring a hole to 0.0002" tolerances? Do you have a set of pin gages with 0.0001" intervals or smaller? Or are you assuming if you lap until the pin just fits you are within your tolerance?