Common Machining Mistakes

A note on mistakes made early: Back in the early 1950's my pop worked with a toolmaker who had been working on a very complex part for the Nike Ajax air defense rocket for over two weeks. The toolmaker went into the bull's office with a downcast face and said he had scrapped the work. The old man put his hand on the toolmakers shoulder and told him it was alright and not to worry. "Go ahead and start over," he said, "but this time if you're going to scrap it, don't take so dammed long!" Cheers from NC/USA

I'm a full-time lathe operator, and I do a surprising amount of measurements with my calipers. One way I improve the accuracy of them, is by using a gauge block set to "zero" them, or more accurately calibrate them, in the vicinity of the size I am going to measure. Say I am turning a final diameter of 1.875", then I grab the 2" gauge block and get that dead-nuts; that gets me quick measurements with a reliable precision of maybe .0003" or so! These are good quality Mitutoyo calipers, which also helps. If I need better precision (I sometimes get tolerances that demand .0001"), then I do similar checks with the micrometer. Say the 2" block reads 2.0004"; then the actual dimension is four-tenths smaller than the reading. It's possible to calibrate the barrel also, but it takes time and some of those older ones have really sticky barrels, so in practice I just keep the correction factor in mind most of the time.

Excellent This Old Tony impression.

Biggest and most common mistake I see students/beginners make is alignment of measuring tool to the surfaces to be measured. Which you covered. Good job.

One of the best bits of advice I got was from Abom - don't look at the gauge on the calipers or micrometer as you're measuring to prevent that bias, forcing the tool into the measurement you're looking for

Quinn, Regarding micrometer feel; Grab a feeler gauge set, set the mic to 0.004” for example and pull a 0.004” feeler through the mic. Pretty good so far, eh? Now set the mic a half a thou under, feel what too tight feels like, set half a thou over and feel too loose, immediately return to just right to reinforce the lesson. Next, select the thinnest feeler gauge of your set, often it's 0.0015” or 0.002”, this one should push through the mic's anvils without buckling. Caution, if the mic is set too tight on these thinnest feelers they are subject to tears. (tears pun unintentional). Repeat daily or weekly until the mind/muscle interface is adequately programmed. Side benefit, you've also just programmed your feeler gauge 'feel'. Nifty innit? From a old mechanic hoping to pick up on some machining.

Being an old, emphasis on old racer I love the metaphor. Having some experience with machining (retired gunsmith) I really like your approach. It's not simplified exactly but very easy to understand. It never hurts for old dogs to refresh the memory banks from time to time. I like what you are doing very much.

I found another mistake. You didn't say "tappy tap tap" 9:26

Two Blondihacks videos on one day?! You're not drunk, this is happening. This video needed to get published right now, for reasons you'll see at the end. Enjoy the two-fer! 😀 Consider it a gift of appreciation to all my awesome viewers. Or perhaps an American Thanksgiving Miracle, if you're into that sort of thing. 🦃

Excellent Quinn, if I may suggest an aide to ensuring holes are on dimension, scribe a circle using a pair of dividers slightly bigger than the required hole, this shows its position, before drilling, and at the start. Never heard of "Pleasant dimensions/numbers" before 🤗. Thanks for sharing.

"Hey Tony, fix these parts for me, would ya?" Love it

At 9:45 I thought you were going to mention the additional (deliberate?) marking-out mistake that each scribed line measurement is is being referenced to a different datum edge. As I'm sure you appreciate, if you want four perfectly spaced holes then it is best to measure and mark-out all their positions relative to just two of the block's edges. [Or more pedantically, all measured in same coordinate system, as defined by three perpendicular reference faces meeting at one corner of the stock. ] Otherwise, by taking each measurement relative to a different side of the block, the machining tolerance errors on the stock size will get added to the hole spacing distances, and will make them less accurate. (Yes, this is a mistake I have made myself) Good CAD drawings will have "Datum feature symbols" on them to tell the machinist which are the part-faces that all of the dimensions should be measured from.

Just adding an additional proviso, to the use of the ‘ratchet’ knob on a micrometer. Don’t be tempted to use it to ‘rapid’ the mic down to the final measurement, as the mass/momentum of the approaching anvil face, can also apply a clamping force greater than the ‘ratchet’ would have applied.

At the end hand shot, I was thinking to myself, "This reminds me a lot of This Old Tony"

If you're going to the trouble to scribe your layout for your holes, you should use a prick punch to mark the intersection of the lines. As you note, you can definitely feel when the point is at the intersection of the lines; then you give the prick punch a light tap to mark the position. Then you come back with your center punch, either automatic or manual, and mark the center with a more confident tap of the hammer or triggering of the auto-punch. The prick punch is lighter than a center punch and is ground with 30º included angle to the point. It's purpose is to make a light indentation to preserve a layout for future use or to receive the points of a divider. A center punch is ground with a 90º included angle and is intended to serve as the starting point for drills. A center punch will pick up a prick punch mark much more positively than it will the intersection of scribed lines.

I’m new to your channel. I’m loving your video on measuring and your honest answer to common problems with measuring. I have a passion for machinery and working on machine tools. I’m really glad you are invested in your craft. It’s not easy teaching measuring and how to troubleshoot when the measuring process is seemingly working properly. When I used to work in the machine shop we developed feel for the amount of drag on a caliper or micrometer. I’m saying that you can sense the amount of torque on a given part measurement. Good job keep up the lessons.

Quinn, As a newbie thank you for showing me the simple mistakes that I will endeavor not to replicate. I appreciate the teaching method you use. State the issue, show the issue, review the issue... throw in the self deprecation and humor.

Among my multitude of roles at my shop, I am also the specialty welder. So, I see in-shop and out-shop mistakes and reworks often. In our case, it's typically cost effective to have a guy like me try to save a part that has a lot of machine hours into it, not to mention the cost of the exotic stock materials (specialty steels, Beryllium alloys, etc). Typically, that means hours of LASER or GTAW welding to bring things back. All that said, preventing the "hurt" early, as you rightly said, is the best way to avoid thousands of dollars in welding bills and rework.

Love your videos, I'm an amateur machinist raising two girls and it is refreshing to see a lady machinist to inspire my girls.