View Full Version : Dealing with Mistakes

06-28-2007, 08:00 PM
The metal machining club I belong to is taking up the subject "Dealing With Mistakes". We hope to discuss practices that will help us avoid common mistakes. This is not a Safety in the Shop exchange, even thought performing an unsafe operation in the shop is a big mistake.

We will share some examples of salvaging parts that are out of print but my question to the group is, "Is there a technique you use to avoid a common blunder". For me , a dykim layout directly on the part has been time well spent. One fellow I worked along side of used masking tape adjacent to his dials to indicate, in plane view, the direction of table travel.

Shouldn't there be a list of common shop mistakes and how to avoid them, or would that list be too long to garner attention?

06-28-2007, 08:20 PM
My most common and hated mistake is to forget which way to turn the dial on a Bridgeport when making long slots or milling around something that turns thru 90 degrees at the end.

DRO would eliminate this ...as you get instant indication of direction change.

all the best.mark

06-28-2007, 08:30 PM
I'll play along. Most of these were learned the hard way.

-I always try to move the workpiece on a mag chuck before starting to grind.
I flung a workpiece and blew up a wheel in my face when I tried to grind a piece of mystery steel that happened to be non-magnetic stainless.

-Always check to make sure the vise is tight and parallels won't move before starting to mill.

-Mark the zero or reference corner with a centerpunch or red marker. Especially on multiple pieces with similar widths vs. lengths.

-Turn the rapids down and creep up to the workpiece in single-block when proving out a new program. Machinist's typos don't go away with whiteout. A quick lay-out helps here also.

-Make a few practice pases away from the workpiece when threading to get the rhythm back on when to pull out.

_Make a scratch cut on the first threading pass to make sure you are cutting the right pitch.

-Run your hand over the bottom of the workpice before setting it in the vise/chuck to check for burrs or dirt.

-Never trust the second shift guy when he tells you the program and set-up are all ready, all you have to do is hit Cycle Start.

06-28-2007, 08:36 PM
Always make sure your end mills are center cutting before plunging.

06-28-2007, 08:48 PM
All my mistakes have turned into opportunities to make smaller parts.

06-28-2007, 08:56 PM
After dialing in a vice and finding the edge set the mills cross feed dial to 0.

That way you can zero a dro as many times as you want and still find the 0 again.(too many times people dont do that and have to put an edgefinder back in to find zero again and again)

Again time your tools on a lathe, set the dial on zero, loosen the set screw on the tool holder and slide the tool up and tighten them down on a known diameter. do this with all your tools and you can have a gang of tools all set to +/- . 005 or less.

With all the tools timed you can put a black marker mark on the cross slide and a black marker mark on the carrage and that works like an "idiots guide" so you know you are getting close.

06-28-2007, 09:01 PM
My most common and hated mistake is to forget which way to turn the dial on a Bridgeport when making long slots or milling around something that turns thru 90 degrees at the end.

DRO would eliminate this ...as you get instant indication of direction change.

all the best.mark
No it won't. Don't ask how I know.

Well, at least I'm not the only one. I think I'll go with the masking tape idea, or draw on the mill with a Sharpie. As for CNC-related mistakes, my shop is so far too poor to worry about that stuff.


Mike Burdick
06-28-2007, 09:13 PM
All my mistakes have turned into opportunities to make smaller parts.
Soooo true! :D But you must be a pro.... my parts just end up in the trash. Seems I have to make three of everything.

...okay on the serious side.

It seems most mistakes come from machining order which may be as simple as how to hold a part for a simple c-clip...but if the part can't be held for that last operation, it's trash. Most of the things I make I add to the plans a list stating the machining order for each part. I'm a big believer of good plans and careful thought. Fixing a mistake with an eraser is pretty inexpensive.

06-28-2007, 09:21 PM
I find that I can avoid many mistakes if one sits and think about what is are going to be done. Which cut do I make first? If I make this cut, how will I hold the piece to make the angle cut?

If you can't get a mental picture of what is to be done, sketch out the part and look at it from all angles. Just yesterday, I was looking at how to make the curved portion flow evenly into the straight part. I sketched it out and by adding 1/4" to the one side of the part, there was no need to make a special cut to flow the two parts together. The additional material made the joint flow together.


06-28-2007, 09:22 PM
Just learn from it. Make a rule to Never make that mistake again. and think about why you made it.

06-28-2007, 10:28 PM
If it weren't for mistakes machine parts would have never been damaged in
the first place, and I'd be out of a job....
As for mistakes in the machining of parts, we can stick to the tried and
proofed ways. A part that took 6 hours to make in 1962 will take 6 hours to
make today. Now if we try using a wiper carbide insert that allows us to take
a .100 depth finish cut, or a 3/8-16 spiral tap that we can spin straight in at
450RPM that part will only take 2 hours. Will we f......, make mistakes?
Sure we will. Will we cut the times and become more productive. No Doubt!
I bring those learned lessons home to my basement shop and it makes my
hobby easier and more enjoyable.

If it doesn't involve blood it's not a mistake. It's a learning process.

06-28-2007, 10:54 PM
Here is one that starts pretty much out of our control but can have serious consequences.

Interruptions, by phone, friend, co-worker, boss, even the shop pet wanting out, makes no difference, getting back to exactly where you stopped takes a deliberate and concentrated "Where was I" and not just some casual lets get back to it.

06-28-2007, 10:58 PM
This may sound stupid ,and Common Knowledge,But...
Stay alert, the more tired you get the more mistakes you do.

If you get too tired walk away, get some sleep,or coffee, It will pay off believe me.

06-28-2007, 11:23 PM
Don't get in a hurry . More mistakes happen because we get in a rush. Do things slow and deliberate.

06-28-2007, 11:51 PM

06-28-2007, 11:54 PM
I think out and maybe draw up a "plan", then I think about how I will actually make "the thing", what order to cut, drill, etc. first, second, third, and "write" that down. Then...

While machining the part, I think, well it would sure save "set up" time if a just cut, drill, etc while I have "the thing" at "such-n-such" point, and go ahead an machine it "in the heat of the moment"...

Only to realize, latter in the project, that I have machined off a "reference point/edge", or a portion of "raw material" that I "orginially" intended to remain as "work holding area", "leveling", etc.

Clear as my muddle mind can be...
When I "glanced" at the "print", that "extra material" had "nothing" to do with the finished part, but that "extra material" was going to be needed later in the machining process. So much for "time saving short cuts".

06-29-2007, 12:21 AM
My Best Friend in the shop is my pocket scale. Measure your tool position or a "whisper cut", then READ the number on the print. Too many times I have had the wrong number in my feeble brain...........

06-29-2007, 12:30 AM
Evan: I've got a shirt with almost exactly that flowchart on it! Philosophy to live by...

06-29-2007, 03:52 AM
Evan: I've got a shirt with almost exactly that flowchart on it! Philosophy to live by...

I'd not like to consider the number of trees than have been converted to paper so that flow chart could be preserved and distributed, and that's just at Boeing.

06-29-2007, 06:27 AM
Remember to move across half the width of the edgefinder (DOH!)

Your Old Dog
06-29-2007, 08:48 AM
Stay alert, the more tired you get the more mistakes you do.

Usually when things are going along fine and you make a mistake I find the above to be true.

Also, when making parts that don't have to fit up to something else, I find that more often than not, mistakes can be viewed as opportunities in disguise. You just have to give it some time to figure out how to make it work for you. Maybe it's because of the old adage, "Need is the Mother of invention"

Still being a rookie at all this machining stuff, I usually make three of all the simple stuff I'm making. That way I can toss out the errors, use the best of the remaining two and have one for a spare. Tedious yes but gives me more experience and saves me from all the setup time.

06-29-2007, 09:51 AM
One thing that I have noticed is that after years of making stupid mistakes the mistakes I make now are becoming more sophisticated and harder to see when incorporated in the final product. ;)

Many times over the years when making up a quick part to fix something non critical I will eyeball the size. Frequently I find that the last rough cut turns out to be the final finishing cut, a few thou undersize. As I gain experience I tend to make those rough cuts closer to the desired finish cut size. I still haven't figured out how to make the rough cut look like a finish cut though...

06-29-2007, 10:20 AM
I can't speak for others, but for me, mistakes are directly proportional to the amount outside distractions. I'm not sure if the amount of demands made on a person are any greater today vs 20-30 years ago, but with fax machines, cell phones, beepers, IM and others modes, a person today is a lot easier to get ahold of (or bother). For those who have other jobs, and do this as a hobby or part time job, the demands from the "real job", SWMBO, kids, grandkids, dogs and neighbors, can be daunting. At times I have to shut the phone off, lock the doors and sent SWMBO and family away, to get anything done or done right.


06-29-2007, 10:40 AM
if you are concerned about making mistakes in your metal working , then i humbly suggest you, take a tempory break and do abit of boat building , every thing you do will be mistake well maybe on a good day a controlled mistake, then when you finally marshall all the mistakes into some semblance
of order and get her launched , you realise that it was a mistake to have built that boat, and you need a bigger one, then its back to metal working!

seriously there is a long steep learning curve here with this stuff and you have to run metal past the machine over and over again for years .even then you will still screw up.

07-19-2007, 03:32 PM
This one just bit me: When trying out a new procedure, test it on scrap first rather than on the actual part.

07-19-2007, 03:53 PM
The root cause of many of my mistakes can be traced to not stopping when I'm fatigued and aggravated. I tend to be bull headed and just want to plow through the last 10% of a job to "get it done".

When I'm fatigued and aggravated I'm not thinking clearly and end up being more focused on just completing the darn job then I am on doing a good job.

07-19-2007, 04:18 PM
So much truth here, I'd almost think you were talking about me. :) Seriously, I have had most of the same mistakes you speak of bite me in the butt over the years. I used to agonize over the fact that most every part ended up being different than the original idea. It later dawned on me that some of the designing of a prototype is done at the last minute. We all started calling screw ups "engineering changes." At a young age we all had to try and save what was left of our egos.

Something I started doing early on was marking machine dials with my grease pencil. Easy to rub off and change marks as you proceeded. Especially on the surface grinder downfeed dial, this is a lifesaver. As the wheel wears or gets dressed the diameter changes and you can keep up with where the touch off point is and where to stop. I have also found it handy to make notes on the machine table, of critical numbers and dial readings, especially when you are leaving a setup over night where someone else may lean on the dial or move it. I always would recheck the dial readings against my "notes" before resuming.

Good thread guys!

07-19-2007, 04:24 PM
The worst distraction in my shop is my woman , she just talks non stop. :D