Trying to turn a sheetmetal shop to machine shop? 1

[engmechs]

Hi all,

Recently, I started as a CAD engineer in a sheetmetal fabrication company. In my first week, a QC manager came and asked me to go over future drawings carefully for one particular company. He said if necessary, better to say we can't do it to the company. As they used GD&T specifications for all their sheet metal parts. And some of the tolerances/ spec. is quite high, which resulteed some parts were rejected. To tell a long story short, they tried to make fabricated sheet metal parts & bolt it together to construct a frame assembly (which another precision machinery sit on the top of it). However, due to imperfection of bending of flanges and hole locations positioning, the assembly was off.

I understand this is a challange to our shop as some of the parts will not work, if straightly go by the spec., but on the other hand, I am questioning if there is a straight yes/ no answer to this question. As I saw there are similar parts made some time ago (on the drawing database). Being a new guy, I don't want to reject the potential purchase order right away, as the management will get mad on me.

My department has a production manager (he basically in charge of everything technical under the roof, he is pretty busy), and a project manager (he is more focused on administrative tasks and schedules).

Any thoughts on this? Any advices help.

engmechs

 

[tygerdawg]

It sounds like the QC Manager is trying to get you as CAD Engineer to be the person to pre-emptively reject a job so he isn't burdened with it after the fact. The QC Manager should have this discussion with the Production Manager who should discuss this with his Manufacturing Process Engineer who can explicitly state the shop's manufacturing capabilities and the tolerances & quality levels that can be achieved there. Not the CAD Engineer. This is a difficult situation for you. Better call a meeting to make sure you aren't forced to make a decision upon which you have no knowledge or experience (and get burned later).

On the other hand, perhaps those customer tolerances were created by another CAD Engineer who probably also did not discuss them with a Process Engineer. This may be an opportunity for you to go back to the Customer and have a talk with them to relax their drawing tolerances to something your shop can produce.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[ornerynorsk]

The design, the process capability, and the shop capability must all 3 be in harmony. If you feel that the design fits the general capability of the processes that you have chosen, then it's a matter of finding a capable vendor. With you only alluding to "high tolerances", it can only be guessed what might be meant by it. Sheet metal can be very precise in the hands of a capable shop with competent people and good equipment, but like with any other process, the price tends to rise as the accuracy tightens up.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[hendersdc]

The tight tolerances would ideally be flagged by whoever quoted the job and discussed internally to see if it was feasible.

Years ago when I had a similar job to yours we would send back exceptions to the tolerances on the drawings that we didn't think we could meet with the quote. For example a hole dimensioned off of a bend that was originally specified at +/- 0.003" might be changed to +/- 0.010" or 0.015". If the customer was ok with the exceptions we did the work to the new tolerance, if not we didn't get the job. This worked better for us than rejecting the job as it still gave us the opportunity to win the work.

GD&T can be difficult in sheet metal depending on what they chose for their datums, hopefully they used holes or edges and not flanges.

As a side note, see if they will let you work at different jobs around the shop to better understand what you are programing for. If you can spend a week or two working on a turret punch and a laser you will make life much easier for the operators!

 

[berkshire]

Hendersdc's idea is a great one if the company will let you do it. Don't forget a week on the press brakes, Most of the problems holding tolerances in a sheet metal shop come from flat pattern allowances and the bending operations. Time in the bending area will show you if your Cad operations are using the correct K factor and what the press-brake operator has to fiddle to get your part to come out right at he very least watch what they are doing and ask questions point out to the operator that you are trying to make his life easier.
B.E.

You are judged not by what you know, but by what you can do.

 

[KENAT]

GD&T doesn't actually mean tighter tolerances. Done correctly it should mean same or looser tolerances than for robust +- tol schemes.

While never plan A, sometimes drilling holes/machining features after forming can be the only option.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[engmechs]

Thanks hendersdc, for your suggestion to work at the floor. After quick interactions with the press braker and the punch team, get info about material thickness variation can be +/- 0.005" or more. That could add up extra dims off on the part too.

engmech

 

[berkshire]

engmechs,
You had better believe it, If you dimension from the outside of the part. the press brake folds from the inside.
B.E.

You are judged not by what you know, but by what you can do.