Machining Parts - What school didn't teach me?! 1

[ryandias]

I am new to the industry and working as the only Mechanical person in my location.

I have been asked to design a part (heat sink adapter) and have it prototyped (say 20 pieces).

I know how to draw/Model. But what school did not teach me is , Tooling time, ways to save money/ lower cost, other ways to improve the product while reducing time/cost.

Any suggestions? or books, or other info sources?

 

[tomwalz]

Holy Moly,

That is a huge subject. With any luck at all there are others there who can help you. A good engineer listens to machinists.

If not put it out for bids and see if anyone wants to help.

We do brazing and sell carbide. We just suggested to a customer that removing a bit of metal would add some flex to a structure and prevent carbide cracking. We got some nice work out of it so we did the redesign for free and charged for the machining.

Other times we will supply design expertise alone for a fee.

I think your best bet is to talk to the people who will actually build the parts. Be nice to them and don’t expect them to work for free.

Tom

P. S. Have good, clear, complete drawings and be able to answer questions about what the material is, what the device is supposed to do and not do.





Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

 

[swertel]

A good engineer listens to machinists.

If only more universities taught that lesson. Just because the machinist went to a tech school or other associate degree-level trade school does not mean (s)he is not good enough for the 4-year or more engineering degree to talk AND LISTEN to.

I have had the luxury of having a machine shop in each of the places I worked so I just had to take a short walk to tap one of the machinists on the shoulder and ask them what they think of this design. They usually come back with a way to fasten it differently, allowing for less machining time and fewer setups (big time & $ savings) and still accomplish the same function and ease-of-use.

If you don't have a machinist or tool-and-die maker handy, consider taking a few machining courses at the local community college.

--Scott

For some pleasure reading, try FAQ731-376

 

[NickE]

Start talking to the machinists. Draw something, make a B/P then take it out to the floor and get some opinion.

Some simple suggestions are:

Keep the tolerances big, but make sure you get your nominal. (And make it easy to hit.) Most shops like +/- 0.010" and will love +/-0.020".

try to avoid thin sections.

Hand deliver the B/P to the shop and see if you can sit down with the estimator and talk about your design. That will assure you get what you want.

I'm a materials guy who over the last 3+ years has learned enough design to draw our own check gauges and design components. I even got to design parts as suggestions to customers. You'll just have to practice and listen to people with more experience than you.

Good Luck!


Nick
I love materials science!

 

[ryandias]

all great suggestions.. But something i was sorta looking for is maybe a "rule of thumb" set.. that i could start working my designs around.

After chatting with a buddy in another firm and showing him a design i did previously, He mentioned that tool changes are automatically 20-30 minute charge, changes in axis (for basic mill setups) are also a basic charge. After hearing this, I simply changed my design from 12 tools to 2 (either big or small) and saved about $250-300 on a $450 quote.

Now I enjoy the oppertunity to have an open slate to design with but, honestly i like having a well defined problem as well. This makes it much easier to do exactly what is required.

 

[swertel]

You summed it up right there.

Limit the number of tool changes and setup. In other words, when making holes, try to make them all the same tap size or clearance size. Granted, function outways convenience, but if it is possible, do it.

Second, picture the orientation of the cutting tool. Any time the cutting tool has to enter from another face, that's a new setup and extra cost. If you can machine the entire profile from one direction, thus one setup, you have saved quite a bit of money.

When I was doing tool design, that was my rule of thumb for designing my component parts in the assembly (such as were to add fillets to allow for larger end mills). Once I had that designed, I walked the print down to the machine shop and had a design review one-on-one. My drawing checkers and management thought I was a genious because my released designs worked the first time and were cost effective. Little did they know!

--Scott

For some pleasure reading, try FAQ731-376

 

[wes616]

My college *required* a 1 credit course in machinine shop practice. You could repeat it up to 4 times for "elective credit." I did. It was the best, to this day, most useful course I took in college. I use something I learned in that class every day. It was taught by the head machinest at our campus. It was where I learned about tolerancing. Prototyping parts, translating a machined prototype into a casting, or a forging. It's where I got to see what work hardened steel looks like. It's where I learned how important is was to double and triple check the print before, releasing it to production.

I could go on and on.

It really is too bad more colleges don't teach this.

Here is my actual advice, since I don't know anything about the part you are designing. Keep your non critical tolerances as loos as possible. The tighter the tolerance, the more the cost.

Unless it is necessary (and for the case of your heat sink it might be), use an easily machinable material for your parts (the bigger the roughcut, the less time on the machine, the lower the cost).

And as everyone has said above... listen to your machinest/operations technician/whatever. You may not agree with what he has to say, but listen anyway.

Wes C.
------------------------------
Light travels faster than sound. That's why some people appear bright until you hear them speak.

 

[eromlignod]

You just have to imagine in your head that you are actually making the part. Setup cost is usually the killer.

Think about what it takes to saw out the raw block from stock, clean it up (I'm talking about milling or turning to remove saw marks or mill finish and to square it up and cut it to size...if necessary), and indicate it in a machine for the actual machining operation(s). If you can possibly design a part so that all or most of the machining can be done in one setup, that's a good thing. Try to see that he can get as many things done in each setup and use as few different machines as possible. The time it takes to actually remove the chips usually doesn't amount to that much (especially with NC work). It's the setup and indicating that burn time.

Try to design as many features into a single part as you can, rather than as an assembly of several parts. Why design a bracket to mount something to a part when you can machine the bracket directly into it. Fewer parts means fewer cleanups and setups.

Lathe work is usually faster and easier than mill work. The chuck is self-centering and you are cutting the cylinder and the two ends only. You would be surprised how many parts can be made cylindrical rather than prismatic. A pedestal for a tool or fixture piece, for example.

Don't machine any surface you don't have to. Unless you are designing for a consumer application, a machine part does not have to be pretty. If the surface is not designed to mate with another part, then it probably can be left as the as-rolled or rough-cut finish. Scale can be quickly and easily removed by glass bead. I see anal-retentive engineers (and machinists) all the time that want to make something look slick (or pad their time) by fly-cutting and grinding all six surfaces for no practical reason.

Usually the last concern is material. Often far too much thought goes into this end of the design. Unless you are using some sort of exotic, expensive material, or you are designing a part that you will be making 100,000 of, you aren't really going to save much money by meticulously planning your material usage. Steel is cheap in comparison to union wages. Size your part close to a standard bar/rod size, then machine away. The fact that a lot of material ends up on the floor isn't usually a big worry. It takes a certain size block to make a certain size part. Removing less metal from that block doesn't really save you any money in material...the block still costs the same.

But...though the material itself is of minor concern, the time to remove it is important. Look at the general shape of a part and determine whether it is more economical to use an already-produced shape, rather than cut it out of a solid block. If your part is shaped generally like an "L", maybe you can use angle iron, or bend it out of a sheet or bar. The same goes for other structural shapes, or extrusions. But remember: if you have to machine all the surfaces of the angle, you didn't really save much. Try to use the structural shape in as raw a state as possible.

And, as has been mentioned already, be sure to have a good rapport with your [experienced] machinists. They do this stuff all day and are a valuable resource.

Don
Kansas City

 

[drawoh]

ryandias,

Product Design for Manufacture and Assembly
Boothroyd, Dewhurst, Knight
Marcel Dekker

This is an excellent reference, although it is no substitute for listening to your fabricators. Ask questions before you design. Ask how good your drawings were after they fabricate. Learning your job by trial and error is a pain, but some times, it is unavoidable.

I took machining and manufacturing process courses in college, and this has made me a better mechanical designer. My hands-on machine shop stuff did not get very far beyond "if you leave the key in the chuck, you're fired", but I understand the machines that are out there, and I can visualize how material will be removed.

Handy 3D CAD Tip:

When designing a 3D model of a to-be-machined part, never add material. Start from a big block. Create your features by removing material. If you cannot see how a machine tool can do the removal process, then your part probably cannot be fabricated.

JHG

 

[ryandias]

My main problem is that I know how to use the machine shop and I very much enjoy it, thusly I do not value/estimate the time required to build a part easily.

I tend to say "yeah thats easy to build...." not really considering well, there are this many Mill bits required, that many taps, and 3 positions in the vise.

I guess it will be a visit to the machine shop/water jet guys to ask what they think.

 

[TheTick]

A good engineer listens to machinists.

I do not hold much hope for an engineer who does not figure this one out in the first month.

 

[MikeHalloran]

Without putting too sharp a point on it, you don't _really_ know how to use the machine shop until you _can_ estimate the machining time and setups required. The guys in the shop have been doing it for a long time, and they've worn out a lot of tools along the way, so they have a 'feel' for what can be done, but it's not in their genetic code, it's something they learned.

You can learn it too, by commuting between your desk and the shop, and asking questions, and listening, and ...

>>>> doing the math <<<<<.

By which I mean, take a part print or two, sit down with Excel and compute cut times from feedrate and surface speed, then get a tool catalog and sketch the fixturing and tools you'd need to make the part. As in making a detailed operation sheet. Yeah, it's a pita, and automated tools exist, but you'll learn more by doing it yourself with simple tools a few times.

Don't then go down in the shop and pretend you're a time- study guy; just ask the guys how they made the part, and listen to what they say. Chances are they made it in a 'smarter' way than you ever thought of.

One other thing. Don't _ever_ touch a machine or a workpiece in the shop without specific permission to do so.



Mike Halloran
Pembroke Pines, FL, USA

 

[ajack1]

Two good rules of thumb.

The most expensive thing you produce is swarf.

Half the tolerance, quadruple the price.

 

[IRstuff]

honestly i like having a well defined problem as well.

That is the crux of the difference between school and work. Welcome to the real world.

Work rarely has well-defined problems because problems in the real world are invariably bounded by stakeholders, cost, schedule, etc.

Your job, should you decide to accept it, is to recognize that fact and move on.

TTFN

 

[tomwalz]

Suggested simple rules

1. Try to design out of the Grainger catalog. In other words look really, really hard to find a mass produced part that will work instead of having one custom made. Two of my best designs are a filter system and a dog watering system. The filter system is all purchased parts except for one bent piece of aluminum with holes to serve as a frame. My very best design was a solution for the parks department. How do you use a drinking fountain to also provide water for dogs? We considered floats and valves and drains and all sorts of things. Eventually we went with a cup on a chain. It has been about 12 years and the cup is still there. No on has stolen it or vandalized it. It still works. It is only three parts from two sources. It requires no maintenance. It is user friendly and very intuitive. Everyone seems to figure out how to use it.

O.K I’m bragging. However it took a lot of us an embarrassingly long time to come up with this and we thought I was going to be temporary until we could do a “real” design.

Designing simple is really hard and not as much fun as custom design. However it is what really good engineers do.

2. Never ask for advice and then tell them you already know that. Everyone has a different perspective to offer. Trying to figure out how to learn from the other person when they are wrong is hard. It is even harder when they are arguing with you. Best of all is to go into a confrontational situational and figure out how everyone is right.

3. Heck yes, never touch anything without permission. Keep your hands in your pockets. When anyone loans you a tool return it properly closed, clean etc.

Finds out if your particular body chemistry causes rust or not. Do the same thing with your tone of voice and volume. Are they irritatng? Do they communicate clearly?

Learn all the shop safety rules and follow them exactly. Learnt what the lines on the floor mean.

Learn manufacturing etiquette. Stay out of the way of people doing the work.


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

 

[GregLocock]

well, don't exactly put your hands in your pockets. In one workshop I worked you got a belt round the ear if you did that.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[NickE]

"Finds out if your particular body chemistry causes rust or not."

yep, thats an important one that I've forgotten recently.

We have a new temp here to help with a move, had him measure a bunch of PPAP parts... The next day they were fingerprint rusted...

Now he wears gloves when he handles steel.

Has any one heard that that used to be one test to be a machinist in germany, handle a fresh ground block of A2, at ~60HRc, then the next day see if it was fingerprinted?

Nick
I love materials science!

 

[plasgears]

Get advice on finishes and tolerances.

One story told in GE manufacturing training:
A part was produced with a finish far superior to specification. The shop training mgr proceeded to roughen the part so that it conformed to spec.
Lesson learned:
Don't spend time and money producing more precision than specified.

 

[drawoh]

plasgears,

I have specified surface finishes of 500/250 microinches, meaning that I want a rough surface. The fabricator has to find a way to mess it up.

Otherwise, your story sounds weird. If I specify 125, and the fabrication process achieves 16, then the work has exceeded specification, and there is no need to do anything else. There may be an opportunity to use a cheaper process.

JHG

 

[scastillo]

My very best design was a solution for the parks department. How do you use a drinking fountain to also provide water for dogs? We considered floats and valves and drains and all sorts of things. Eventually we went with a cup on a chain.

[Marketing guy]OK, I understand the dog will be drinking from the cup...but...how does he hold it?

Will there be different sized cups for different breeds?

Will this design work for cats as well?[/Marketing guy]