Guide
What is a good buy-to-fly ratio on a machined part, and how do you lower it?
The buy-to-fly ratio is the weight of raw material bought to make a part divided by the weight of the finished part, so a ratio of 5:1 means four-fifths of the metal you paid for leaves as chips, saw kerf or bar ends. There is no single “good” number: a ratio is good when the cost of the metal you throw away, plus the machine time to remove it, is lower than the cost of buying a blank closer to the final shape.
By Saola Manufacturing engineering team · Published · Updated
The term is used mostly in aerospace, where “fly” is the weight that ends up installed on the vehicle, but the arithmetic applies to any machined part. It matters most when the alloy is expensive, such as titanium, nickel alloys, stainless, bronze or aluminum-lithium, and when the part is a thin-walled housing, a ribbed bracket or a flanged shaft carved out of a solid block. This guide is for design engineers and buyers who see a quote dominated by material, or a scrap bin full of costly chips, and want to know how to calculate the ratio, where the metal actually goes, which levers lower it, and what to put on the drawing and RFQ so the supplier can use them.
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What is a good buy-to-fly ratio, and when is a high one a problem?
A good buy-to-fly ratio is the lowest one at which the total part cost still falls, so for cheap free-machining alloys a high ratio is often fine, while for titanium or nickel alloys every point of ratio is real money. A high ratio becomes a problem when material is the largest line in the quote or when long roughing cycles are spent turning paid-for metal into chips.
NASA programs give a sense of how far the ratio can drift on hardware machined from solid. A Lockheed Martin report on aluminum-lithium launch-vehicle barrel panels, filed on NASA’s technical reports server, says that the conventional route of machining thick plate into a tee-stiffened panel produced up to 90% scrap, which works out to a buy-to-fly ratio of about 10:1, and that extruding near-net-shape panels was studied because it lowers the buy-to-fly cost (NASA NTRS, Hartley). A NASA Marshall-sponsored near-net forging program for aluminum-lithium Y-ring adapters gave the same reason: the process was meant to cut the material scrap rate, which the report equates with the buy-to-fly ratio, because aluminum-lithium alloys typically cost about five times as much as 2219 aluminum (NASA NTRS, Hall).
Outside aerospace, the same pattern shows up in three ways a buyer can see without a calculator. The material line on the quote is larger than the machining line. The supplier proposes a stock size that is visibly bigger than the part envelope in one or more directions. Or the part has a thin web, a large flange or deep pockets, so most of the block is removed. Any one of these is a reason to work out the ratio and ask where the metal goes.
The ratio is not a quality metric and a low ratio is not automatically better. Moving from bar stock to a forged or cast blank lowers the ratio but adds tooling, a second supplier step, a different material specification and new inspection risks. The useful question is always the cost of the metal removed, at the price you pay for it minus whatever the chips are worth as scrap, compared with the cost of the alternative blank.
What causes a high buy-to-fly ratio on a machined part?
Geometry causes most of it: a part with a small cross section in one place and a large one in another must start from stock as big as its largest feature. The rest comes from stock-size steps, cleanup and workholding allowances, saw cuts and bar remnants, and parts scrapped during setup.
Ranked by how much metal they usually account for:
- Shape mismatch. A flanged shaft, a bracket with one tall boss, or a housing with thin walls has to be cut from stock that encloses its biggest feature. Everything outside the part envelope is chips, and on pocketed parts most of the inside is chips too. This is the dominant cause, and it is fixed only by changing the design or the blank.
- Stock-size steps. Bar, plate and extrusions come in standard sizes. If the part envelope sits just over a standard size, the supplier must buy the next size up, and the whole difference is removed.
- Cleanup allowance. Hot-rolled, sawn or scaled surfaces usually have to be machined off to reach sound, dimensionally controlled metal. A drawing that requires every surface to be machined forces cleanup even where the stock surface would have been acceptable.
- Workholding and cut-off. Turned parts need grip length in the chuck, a parting cut and a bar end that cannot be used; milled parts often need extra material for a vise, a fixture tab or a reference face that is removed in a second operation.
- Setup and yield loss. First-off parts, try-out parts and scrapped parts all count as bought material that never flies. On short runs of an expensive alloy this can move the ratio noticeably.
The first two causes are design and specification decisions, which is why the drawing and the RFQ are the cheapest places to lower the ratio.
How do you calculate buy-to-fly and find where the metal goes?
Divide the weight of stock bought per part, including its share of saw cuts, grip length and bar remnant, by the weight of the finished part. Then split the difference into the causes above, because the breakdown tells you which lever will work.
A practical sequence a buyer or engineer can run on one part number:
- Get the stock definition. Ask the supplier for the stock form, size and length per part they quoted, and the material specification they will buy to.
- Get the finished weight. Take it from the CAD model with the correct alloy density, or weigh a finished part.
- Get the bought weight. Weigh one cut blank, or calculate it from the stock size and length, then add the part’s share of saw kerf, chucking length and the unusable bar end.
- Divide. Bought weight over finished weight is the buy-to-fly ratio; its inverse is material utilization.
- Split the gap. Estimate how much of the difference is envelope (stock bigger than the part), interior removal (pockets and bores), cleanup and workholding. Whichever is largest points to the lever in the next section.
- Put a price on it. Multiply the removed weight by the stock price per kilogram minus any chip credit per kilogram, then add the roughing time it takes to remove that metal at the quoted machine rate. That figure, times annual volume, is the most a near-net blank or a design change can save, and it is the number to compare against die or tooling cost.
A quick way to split envelope from interior removal: compute the weight of the smallest standard stock that encloses the part, then the weight of a solid shape that just wraps the part’s outline with no pockets or bores. The first difference is envelope and stock-size loss, which stock choice or a small design change can fix; the second is interior removal, which only a hollow or near-net starting form, or a split part, can reduce.
The table below runs that arithmetic for one hypothetical aluminum bracket made three ways. The masses are illustrative, chosen to show how the ratio moves, and are not measured data from any part.
| Starting stock | Bought per part (kg) | Finished part (kg) | Removed as chips (kg) | Buy-to-fly ratio |
|---|---|---|---|---|
| Rectangular bar, next standard size above the envelope | 2.40 | 0.40 | 2.00 | 6.0:1 |
| Extruded profile close to the bracket cross section | 0.80 | 0.40 | 0.40 | 2.0:1 |
| Die-cast blank with machining allowance on functional faces | 0.48 | 0.40 | 0.08 | 1.2:1 |
The ratio alone does not pick the winner. The extrusion needs a die and the casting needs a die-casting tool and a different alloy, so compare the material and machining saved per part against that tooling across your real annual volume, as the break-even guide shows for casting versus machining.
Which levers lower buy-to-fly, and when is each worth it?
Start with the free levers on the drawing and stock choice, such as fitting a standard size and allowing stock surfaces to stay unmachined, and move to near-net blanks such as extrusions, forgings or castings only when volume and alloy price pay for their tooling. Splitting a badly proportioned part into two simpler parts is often the cheapest fix of all.
Use the decision table to match the lever to the cause you found. Each blank form comes with its own material specification, which matters because a forged, extruded or cast blank is not the same material as bar of the same alloy name, and the drawing may need to change to accept it.
| Lever | Works best when | Cost or risk to weigh | Stock specification to name |
|---|---|---|---|
| Resize the part envelope to fit inside a standard bar or plate size | The envelope sits just over a stock size | A small design change; check fit and strength | ASTM B211 or A108 bar; B209 plate |
| Cold-finished instead of hot-rolled bar | Cleanup of scale and size variation drives the gap | Higher price per kilogram; different stress state | ASTM A108 cold-finished steel bar; B211 aluminum bar |
| Allow unmachined stock surfaces where function permits | Bar diameter or plate faces could stay as supplied | Stock tolerance and finish then govern those faces | Size tolerances of the bar or plate specification |
| Tube instead of bar for a part hollow through its length | The bore is large relative to the outside diameter | A second raw-material stream to buy and qualify | The relevant tube specification for the alloy |
| Split the part into two simpler parts | A large flange or boss sits on a slender body | A joint to design, assemble and inspect | One stock specification per piece |
| Standard or custom extruded profile | The part is prismatic with a constant cross section | Die cost for a custom profile; temper and properties per the extrusion specification | ASTM B221 extrusions |
| Closed-die forged blank | Strength-critical parts in costly alloys at repeat volume | Forging dies; grain flow and properties requalified | ASTM B381 titanium forgings; matching specification for other alloys |
| Cast blank with machining allowance | Complex housings that carry modest loads | Tooling; casting alloy and porosity replace wrought properties | ASTM B85 aluminum die castings |
Two cautions apply to every near-net option. First, the blank form changes the material: annealed titanium bar and billet fall under ASTM B348/B348M while titanium forgings fall under ASTM B381, and a die-cast aluminum alloy is a different alloy from wrought bar, so a drawing that names one specification does not allow the other without a change. Second, near-net blanks bring their own defects, such as forging laps or casting porosity, that can open up when the blank is machined. NASA work on electron-beam deposition makes the same trade explicit from the other direction: building protruding details onto a simplified casting or forging avoids machining a large volume of material away from an oversized forging and can significantly reduce the buy-to-fly ratio (NASA NTRS, Glaessgen and Schoeppner).
What should the drawing and RFQ say to keep buy-to-fly down?
Specify the material and its required properties, not the stock size, and say which product forms are acceptable. Then ask the supplier to show the stock form, size and weight per part in the quote, and to offer a near-net alternative when it would pay at your volume.
- Material by specification and form. For example, “6061-T6511 extrusion per ASTM B221 or 6061-T651 plate per ASTM B209 acceptable” gives the supplier room to nest parts efficiently, where “6061-T6 plate only” may force a thicker block.
- Unmachined surfaces. Mark faces that may remain as supplied stock, with the stock tolerance as the governing limit, instead of a general note that all surfaces be machined.
- Grain direction only where it matters. A grain-direction requirement on plate restricts how blanks can be nested; state it only for fatigue- or bend-critical features.
- Datums and tolerances that do not force extra stock. A datum on a face that must be cleaned up, or a tight flatness on a thin web, can force the supplier to start thicker and remove more; the CNC machining cost guide explains how tolerance choices drive cycle time.
- Volume and program length. State annual volume and how long the part is expected to stay in production, so a supplier can tell whether extrusion, forging or casting tooling would pay back.
- Quote breakdown. Ask for stock form, stock size, bought weight per part and finished weight, so you can calculate the ratio and compare suppliers on the same basis.
- Chip value. On expensive alloys, ask whether a scrap credit for clean, alloy-segregated chips is reflected in the price.
For castings that will be machined, the allowance left on each face is the casting equivalent of stock cleanup; the machining allowance guide covers how to set it without leaving extra metal everywhere.
Who does what on a Saola order when the stock or blank changes?
Machining from bar, tube or a blank is done by CNC turning and milling at our lead CNC plant, T.K Viet Nam Precision Company Limited, in operation since 2017. Cast blanks, extruded profiles and surface finishing come from partner plants we manage, and every quote states the production site and the country of origin.
Turned parts are cut on conventional CNC lathes from bar and tube; the CNC turning page lists the bar and tube materials that run today, and titanium and nickel alloys are not on it, so for any other alloy say so on the RFQ and ask before assuming. Milling at the lead plant is mostly a second operation on castings, and the CNC milling page lists the substrates machined today, so a part to be milled from solid plate or bar is one to raise on the RFQ rather than assume. Forging is not among the processes we list; if a forged blank looks like the right route, say so and we will tell you whether we can support it. When a review of your drawing shows that most of a block would become chips, the quote can include a short manufacturability note on stock size, splitting the part, or a near-net blank.
If a cast blank is the better route, the casting is made at a partner plant through our die casting program, and that plant is not named. That plant is not necessarily in Vietnam, and moving from bar to a cast blank can change where the substantial transformation happens, so check the origin stated on the quote against the country-of-origin guide for cast and machined parts. ISO 9001 certification is in progress, not held. Send the drawing, the material specification and your annual volume through the quote form, and ask for the stock form and weight per part to be shown so you can see the ratio for yourself.
Frequently asked
Is buy-to-fly ratio the same as material utilization?
They are the same measurement turned upside down. Buy-to-fly is bought weight over finished weight; material utilization is finished weight over bought weight, usually written as a percentage. A ratio of 4:1 is a utilization of one quarter.
Does buy-to-fly matter on cheap alloys such as free-machining steel or 6061 aluminum?
Less than on titanium or nickel alloys, but it still costs machine time. On cheap stock the cycle time spent roughing away metal is often worth more than the metal itself, so a lower ratio can still lower the price even when material is a small part of the quote.
Will a lower buy-to-fly ratio always lower the part price?
No. Extrusion dies, forging dies and casting tools add one-time cost, and a near-net blank adds a supplier step and new inspection risks. A lower ratio pays only when the material and machining saved per part, times the volume, exceed that added cost.
Can the supplier switch from bar to a forging or casting on its own?
Not if the drawing names a bar specification. The blank form changes the material specification and can change properties, so a switch should go through a drawing or specification change that you approve, with first-article inspection of the new route.
Do machining chips have any value?
Usually yes, if they are kept clean, dry and separated by alloy, because mixed or contaminated chips are worth far less. On costly alloys it is reasonable to ask whether a scrap credit is reflected in the price.
Sources
- Production of aluminum-lithium near net shape extruded cylinders (Hartley), NASA Technical Reports Server, 1995
- Near-Net Forging Technology Demonstration Program (Hall), NASA Technical Reports Server, 1996
- Materials (Glaessgen and Schoeppner), electron beam freeform fabrication and buy-to-fly, NASA Technical Reports Server, 2006
- ASTM B348/B348M-25, Standard Specification for Titanium and Titanium Alloy Bars and Billets, ASTM International, 2025
- ASTM B381-26, Standard Specification for Titanium and Titanium Alloy Forgings, ASTM International, 2026
- ASTM B211/B211M-23, Standard Specification for Aluminum and Aluminum-Alloy Rolled or Cold Finished Bar, Rod, and Wire, ASTM International, 2023
- ASTM B221-21, Aluminum and Aluminum-Alloy Extruded Bars, Rods, Wire, Profiles, and Tubes (scope), ASTM International, 2021
- ASTM B209/B209M-21a, Standard Specification for Aluminum and Aluminum-Alloy Sheet and Plate, ASTM International, 2021
- ASTM A108-24, Steel Bar, Carbon and Alloy, Cold-Finished, ASTM International, 2024
- ASTM B85/B85M-25, Standard Specification for Aluminum-Alloy Die Castings, ASTM International, 2025