First day on the job

First day on the job as a machine designer

Some of you reading this can remember your first day at the job, others will experience it in the near future. One thing that can not be left out are the mistakes that you will make when you start.

I was hired as a machine design engineer straight out of school. Some of my first assignments were mostly in the assembly and some design work. For a couple of weeks I was assembling a machine with a few workers and had an assignment to fix the model of the machine in SolidWorks. The model had some errors and the fastening elements were a mess, but it was a straightforward task. This was a great move by my superior. The SolidWorks task was a low risk, getting to know the software type, getting to know mostly SolidWorks buggy PDM, and the assembly task was a great opportunity to feel the metal, as my supervisor put it. And both of the tactics worked.

I really got a sense of what tolerances are in the “real word” not just on the screen and how some design decisions can hugely influence the assembly process. Not only that but I have learned how to manage a small team of workers.

My first “serious” design work was to make an adapter plate for a replacement servomotor on the machine that was in the prototyping phase. This was, I think, on the third or fourth day on the job. Our CTO called me to come by the machine and showed me a new servomotor and said to make a new adapter plate and launch documentation into the production without any further directions.

My plan was just to modify the existing adapter plate. I copied the model but didn’t copy the drawing (I didn’t know how to copy both together in SW PDM back then 🙁 ). I changed the hole radius for the flange and the position of mounting holes for the new bigger motor, and that was it!

Well not quite… As this posts name might suggest I made a mistake. I didn’t look at the part within the assembly, and I messed up the clearance pocket for the timing belt… Since the new motor required a bigger pulley the timing belt needed a larger clearance pocket. Since I didn’t look at the assembly I didn’t see this… This required the rework on the CNC… Not a good start.

The second mistake(s) that I made was in the drawing. Since I drew it from scratch. I didn’t look at the previous drawing, I thought that I new enough to make a simple drawing like that (Ego).I made some mistakes like putting a 0.01mm tolerance position on the threaded holes that our head of manufacturing caught right away and I redid it (the correct tolerance was 0.1mm, talk about not having a good feel for the tolerances). But I also didn’t put a tolerance on the bore for the servomotor flange. And that also required some rework, the motor flange could not fit. Let me explain:

Most servomotors have flange outside diameter in tolerance for example h8 or h7 like the one drawing I found online, picture bellow.

Since I didn’t put any tolerance on the bore of my part, it was made with the bore undersized by 0.1mm and it could not fit no matter what I tried to do. The servo motor flange is used to center the motor on its mounting plate. Sometimes the additional pins are used but the flange is almost always manufactured in tolerance. So to center it on my adapter plate I need to make a close fit bore. The correct tolerance is H8.

Let’s say that the nominal dimension of the bore was 100mm. So since the outside diameter of the flange is 100h7 that is 100+0 – 0,040 mm and the bore should be 100H8 that is +0,054 – 0.

So if the bore is correctly manufactured, the servo motor flange will mate perfectly, and the motor will be correctly positioned on the adapter plate.

Yeah, those were a couple of mistakes that I made on my first go as machine designer. One part that required two additional trips back on the CNC, not a great start… But a start nonetheless. After the rework the plate worked just fine.

That’s a thing, you will be making mistakes when you start. Important thing to remember is that most mistakes can be corrected, and in the end it is not a big deal.

Success does not consist in never making mistakes but in never making the same one a second time.

-George Bernard Shaw

Do you have some similar memories that you would like to share? Send me a message I would love to hear from you.  

Regards,

Aron

 

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