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Design for milling.

Design for milling.

Hello! I am a brand new Design/Stress Engineer at a large aerospace company. I have been here for 3 months or so and I'm with a great group of people and I have been able to carry several parts through the design and stress process and see my parts go to the floor. After just graduating college this is an amazing feeling. But, I have recently been given an incredibly difficult part to re-design, the Skeg. It involves very complex titanium machining and it is quite a large part. I have been having difficulty finding any good knowledge on design techniques for milling besides just the basics. I know about milling, I'm quite familiar with an old Bridgeport and a 3 axis Haas we had at school, however I am designing parts for an incredibly sophisticated machine shop. I have had some guidance from them on the part, but they suggested that some of the contours on the part right now are very inefficient to machine. I have no idea how to design a part that truly capitalizes on the benefits of the machines that we have, and I cannot find any company resources that are useful for this either. Does anyone know of any book/design manuals that could help me out? Thankyou very much. Also, if anyone has any emergency landing/ belly landing/ sliding reports those are also very interesting to look at.

RE: Design for milling.

If you can arrange it, spend a couple of days at that incredibly sophisticated machine shop.
Ask questions.
Listen to the answers.
Write down the questions and answers later; workers may be intimidated by clipboards.
Talk to the managers and CNC programmers, of course, but make it a particular point to talk to everyone on the shop floor. That's where the company's real knowledge store resides, and it's probably not written down anywhere for easy reading.

You may find extremely interesting workpieces in progress, on machines, or on workbenches. They may be in a fragile intermediate state, and may represent the expenditure of hundreds of hours of hard work, and they may have sharp edges that can injure you.
Ask questions about whatever strikes your fancy, but again, do not touch, and especially do not pick up anything.

Try to make your questions show just a little knowledge about what the particular person is working on, without telling them how to do their job. Once you get them talking about stuff they love doing, just shut up; you will learn more that way.

Mike Halloran
Pembroke Pines, FL, USA

RE: Design for milling.


Excuse my ignorance, but by skeg do you mean a tail skid as shown below? If so, is the large milled titanium part you are looking to reduce machining costs of fairly similar to the beam shown in the picture?

If you are working at a large aerospace company there should be some in-house manufacturing/materials specialists. So as a start you might consult with them.

Here are a couple general things you might look at to reduce the machining cost of a pocket milled titanium beam.
- First, while all titanium alloys are difficult to machine, see if there is some other titanium alloy that has better machinability than what you are currently using while still providing acceptable mechanical properties.
- Second, see if it possible to start with a raw material form that will minimize the amount of scrap and stock removal required during machining. For example, rather than starting with stock size wrought plate/bar, for a reasonable cost you might be able to get a simple hammer forged shape that is close to the finished profile. Titanium alloy is quite expensive, and the reduced amount of raw material scrap may save enough to cover the forging cost.
- Third, increase pocket corners/fillets to the largest size practical. Eliminate any large areas with thin walls, or thin ribs/flanges that are not well supported. Machining titanium typically requires slow cutting speeds and heavy feeds, which will create high cutter forces that can easily deform poorly supported thin section features. Thin section areas and small radius pocket corners/fillets require a finish pass using small size cutters taking a very light cut, and this is very time consuming.
- Lastly, the high cutter forces required for milling titanium requires rigid fixtures/tooling and high HP machines.

RE: Design for milling.

a skeg is more typically a large almost landing skid used to protect the airplane in case of emergency landing.

I agree, there should be experts (or at least more knowledgeable people) where you are.

I agree, understand the reason for the new design ... reduce part count, reduce assembly time ?

My 2c, you have a profile, some limiting surfaces ... make it so ! I think a big design concern should be the cutter size, and direction (ie cutters are cylindrical and can't easily do undercuts) and by aware of the impact of simply running the cutter fillet rad around all the corners of the pcokets ... ask someone there rather than asking us why, you'll learn better.

a thought from left field ... what about a casting ?

another day in paradise, or is paradise one day closer ?

RE: Design for milling.

Is this the same thing as what used to be called a tail skid,whose purpose was to protect the rear end from scrapping bottom,never heard this term before ?

RE: Design for milling.

Thankyou so much for all of your replies! I found an interesting article about a Boeing process, pockets with integral ribs/gussets are a big part of this design so this was somewhat helpful as well. The Skeg I'm referring to is the "auxiliary landing gear" type that is placed directly under the wing for belly/NLG down landings. The customer company we design and build the plane for does not like castings, that was of my first thoughts as well. Also we are nearing half way through production and they would take an engineering change that just involves altering the milling much better than one that involves totally changing the process. As far as going to people who are experts in machining, they are in the company but they aren't exactly easy to talk to, they have much more PDR and CDR projects to worry about than one that is already in production, even though this part is a large amount of money milling wise. My goal is to get to 50-50 billet to machining cost. That modern machine shop magazine seems like it would be worth a subscription, Info like that would be useful to have in a couple of years when we begin the derivative of this project.

RE: Design for milling.

i think you mean LG "up" landings ...

it seems like you're working for a smaller company than I thought. You're gathering good data, and the experts locally seem unreachable, give it your best shot ??

are you improving a current machining design ? in which case, can the people who want it "improved" point to what needs improving ? or can you talk to a machine shop about what looks expensive with the current design ?

or are you changing the design to a machining ? (from a s/m assembly maybe?)

From my limited experience, Ti isn't usually the material of choice. Most I've seen are Al.

Has someone done the study of how this ablates ? or it it sized by eye ?

How constraining are your surfaces ? I'd've thought the primary design was to put as big a lump of metal as possible ?

another day in paradise, or is paradise one day closer ?

RE: Design for milling.

Belly landings/NLG down landings is what I meant to say by that.
Yes the study of the wear surface itself was done years ago during the PDR/CDR of the program. I am just altering the actual structure that transfers the 100 ton load into the rear spar. However, the wear surface is not covered by the belly fairing as is common with most skegs (probably a good reason why most people don't know about them) and there is a small fin that is intended to move the initial contact load forward of the rear spar. This small rib is also made of very complex geometry and is difficult to machine. So that's why I am also looking for info on the wear surface itself to see how others have transferred the initial very very large load forward of the rear spar. Does anyone have any pictures of a skeg on an MD80 or 727? that would be interesting to see. There are probably multiple contact/wear surfaces so the design is probably much different. Would be interesting to take a look at though. Company is pretty large (15k employees) but very spread out and involved with many different programs, wouldn't really know where to start to look for advice in the company, all design knowledge of other programs is strictly off limits.

RE: Design for milling.

surely if the NLG is down the plane will land on the LG ?

or do you meant that the MLG is not down ?

this isn't an intentional landing skid ?

if the current machining is difficult to machine, ask those people "where is it difficult ?"

this design is unlike previous designs, right? if they went to machining as a process improvement, and the machining is difficult/expensive, then maybe it's time to consider castings. Modern processing (like HIP) have significantly improved the quality of castings; an exmaple is a type III exit on a large commuter jet.

another day in paradise, or is paradise one day closer ?

RE: Design for milling.


Based on your posts it sounds like you want to optimize the machining process to reduce cost, without any changes to the part design itself. The article you linked describing the cutter configuration developed at Boeing was interesting. But to make the technique really effective requires using large diameter cutters and high HP milling machines.

You stated that your goal was to reduce machining cost equal to raw material cost. You did not provide a sketch of the finish machined skeg, so there is is no way to know how complex the machining operations are or how much material stock is removed. Aircraft quality wrought titanium alloy plate/bar can be quite expensive (maybe $50/lb), and it is quite common with aircraft structural components for much of the raw material to end up as chips. On the other hand, the hourly rate for operating a quality, high HP milling machine in an AS9100 facility, including labor and overhead, can easily run $200/hr or more. And as noted, end milling fine part features, like blind pockets with small radius corners and thin sections, in titanium can be a very time consuming process. So I don't think you'll reduce machining costs to meet your goal.

Having said all that, there are a few things you can look at to reduce machining costs. The most basic thing you can do is figure out ways to use the machine time more efficiently. Things like reducing the number of set-up changes required for each part, or modifying tooling/fixtures to make changing out parts faster and more precise/consistent. You can also work with the NC programmer to optimize every part of the machining instructions. Making lots of small changes to tool paths, cutter speeds, feed rates, etc. can add up to significant reduction in machine time.

RE: Design for milling.

Its for an MLG up and nose gear down landing, as well as an all gear up landing.
tbuelna-Can't really show any pictures/sketches of the product. You are exactly correct about what my question is about, Are there any books/resources available to help with that type of a design process?

RE: Design for milling.

Also, I'm aware of the equipment needed to perform this type of milling and we have many facilities that are capable of it. I am just wanting to know how to design components that utilize the technology that modern machines can offer. We aren't making parts on 3 axis Bridgeports anymore, however most parts are designed as if we are.

RE: Design for milling.

I worked as a CNC machinist for many years. Even multi axis machines behave about the same as the old Bridgeports. You may be able to get away with less fixturing and fewer set ups with a multi axis machine, and have more rigidity and higher HP machines, but you still have the same metal removal limitations. Yeah cutters have improved, but not really that much. It still comes down to surface feet per minute and metal removal rates and cutter diameter. Chem milling might be an option, but I've never work a chem mill machine.

RE: Design for milling.

If you've got a 3 axis design and you want to make it 5 axis ... look for places where the limited cutter axis has left bunches of unnecessary material.

How expensive is the machining now ?

has the price increased (or decreased) with 5-axis machines ?

How much weight can you extract for what increase in price ?

another day in paradise, or is paradise one day closer ?

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