What's new
What's new

Questions on easiest designs for CNC machining

jinx18

Plastic
Joined
Oct 27, 2014
Location
Long Island, NY
Hello, I am needing to get a part made on a CNC machine. I am obviously looking to keep costs as low as possible so I am assuming if I can keep the design as simple as possible it will lead to less time on the machine which is less money to make it. Hopefully that assumption is right. Anyway I am totally unfamiliar with CNC Machines so please excuse my ignorance about them. I am wondering how I can keep this part the simplest and cheapest.

I am assuming flat sides are easiest? 90 degree angles? Are curves much more time consuming/costly? Anything I should know specifically?

The part is fairly simple and small.
About 1.5"x1"x1"
Stainless steel. Could be 304, 316 or 303. I am assuming 303 would be my best bet for machineability.

Thanks
 
Well there are two ways off the top of my head.

1) Make it so simple it doesn't need a CNC.

2) Order so many you get a bulk price.
 
You will cut out days of random guesses on our part and cut straight to concise answers if you post a picture or rough drawing of what you're trying to design.
 
Curves with straight sides are no problem. Make sure inside corners have a slightly larger radius than common cutting tools. I put inside corner radii of 0.3, 0.15 and 0.8 or bigger on everything possible. Stay away from anything that requires 3D milling if you can.
Jordy
 
Its not at all obvious cost should be as low as possible.
Depending on use, resistance to rusting, appearance, reliability, thermal factors etc coudl all have an effect 10x more than the base cost.

In short, the more material removed, and the tighter the desired tolerance, the more expensive the part is.
BUT.. this is ALL irrelevant until you are paying in the 2000$ plus quantity.

IE no matter what, other factors are going to be much more important, until and unless you are doing work for which it is worthwhile to do it well.
Thus, the question is too generic to be answered correctly.

A single piece setup time might cost 2000$. Or 10.000$. Or 5$. Depends.
How many do you need ?
What is the accuracy required ? (Is there any ?)
Timeframe ? Cosmetics ?

CNC means nothing in and of itself, apart from the fact that people who can actually make stuff efficiently, will charge you about 100-200$ to look at your part.
Ie expect to pay, happily, a minimum of 200$ for 1 piece, plus whatever costs are related.
This cost will likely be hidden, because people assume that manufacturing professionals like to look at clients who talk about volumes, but only want one.

Until you can show a sketch, rendering, picture or sample, no-one can advise you on how its efficient to manufacture the piece.
CNC is efficient, but not in all cases.
Curves are cheap - essentially zero cost.

Accuracy, finish, tolerances and other factors affect the job. So does runtime. So does equipment needed to do it.
 
First thanks for all the replies. It is very helpful.

2) Order so many you get a bulk price.

I thought with CNC machining whether you make 1 or 100 or 1,000 there is no real bulk discount as the method does not necessarily lend well to mass production (as the process is what it is and if you needed a lot of an item another method would most likely be more cost effective) and there isn't really a penalty by making 1 since there is no large setup cost for a special tool that would need to generate a lot of parts to recoup the cost of making the specialized tool.
 
The way this works is you have to program that part so the machine knows what to do, decide on and setup tools to make the part, figure out how ur gunna fixture that part (work holding or fixturing), etc. All this leg work takes time. Most people just hate seeing X amount for the part then XXXX for the fees to get to that point so most people just roll it into a per part cost so yes, 5 parts WILL cost more than 500.

As well, if I am making 5 for you, I will plan that job a LOT different than 500. Type of cutters, programmed paths, fixture work, etc. There are TOOOOO many variables.

If you don't need many, I too agree that for people on the SUPER cheap, you have to think about manual machining. Lots of guys in garages with Bridgeports and no care in the world if they lose money playing, and us CNC guys cannot compete "go out of business" practices.
 
Not really true... there is programing time, which will be the same for one or a million. Also fixed set-up time (figure out how to hold it in the vise, load tools not commonly used, etc.), and the fixed overhead of dealing with quoting, shipping, and billing. So, One can never be the same price as many.

Dennis

Damned, huleo types faster! :D
 
I generally wouldn't bother with automatic tool changes for one or two off parts. When the quantity gets larger than that (5 was a probable cutoff point for me) then I would set up tool offsets, ATC etc.
When several parts are to be made, now we could start thinking about spending an afternoon making up some sort of fixture.
You get the idea, so, yes, more parts will almost always be cheaper than fewer.

And for future parts please avoid inside pockets with 1/32" radii 3" deep.

(In Hasteloy!)
 
Last edited:
Everything they said above. Keep your interior radiases larger than the "commom" tool which will cut it. Avoid deep pockets, unless you have really deep pockets. Cutter deflection AND long cutter availability AND finish required . . . especially for machinists with only 3 axis machines. Keep in mind the shops with $250,000 + multi axis machines HAVE to make money to offset the costs of those wonderful monsters.
Fixturing: Plan your part to be able to be held with common fixtures I E vise, collet, angle plate. Indicate if and where fixturing holes would be permissible.

Arcs & rounded corners are a non issue to CNC, but keep in mind, inside radiases need to be larger than the cutter to allow chip clearance.

Don't specify 400 stainless when 304 or aluminum will do. 2-56 holes will cost more than 3/8-16, oh and do not spec threaded holes deeper than available taps nominal length.
 
I thought with CNC machining whether you make 1 or 100 or 1,000 there is no real bulk discount as the method does not necessarily lend well to mass production (as the process is what it is and if you needed a lot of an item another method would most likely be more cost effective) and there isn't really a penalty by making 1 since there is no large setup cost for a special tool that would need to generate a lot of parts to recoup the cost of making the specialized tool.

Be sure you are sitting down when you get quotes back for machining just 1 of your parts. :D
 
First thanks for all the replies. It is very helpful.



I thought with CNC machining whether you make 1 or 100 or 1,000 there is no real bulk discount as the method does not necessarily lend well to mass production (as the process is what it is and if you needed a lot of an item another method would most likely be more cost effective) and there isn't really a penalty by making 1 since there is no large setup cost for a special tool that would need to generate a lot of parts to recoup the cost of making the specialized tool.

CNC machining is a serial process but like all things, the shop guys will cut you a discount for higher quantity items with economies of scale.

But if you're speaking in a academic sense, thats not true. More time is allocated into programming a part, and so there are fixed costs to perform certain applications. Such as fixturing, CAM programming etc. If you need only a few parts, these fixed costs are likely going to be the same, hence price per part will be higher with low quantity, whereas as the quantity goes up, the fixed cost per part goes down and you're down to variable costs related to quantity of material used and electricity and all that good stuff.
 
First thanks for all the replies. It is very helpful.

I thought with CNC machining whether you make 1 or 100 or 1,000 there is no real bulk discount as the method does not necessarily lend well to mass production (as the process is what it is and if you needed a lot of an item another method would most likely be more cost effective) and there isn't really a penalty by making 1 since there is no large setup cost for a special tool that would need to generate a lot of parts to recoup the cost of making the specialized tool.

Lots of great advice so far. Just adding my $.02 - I work a lot with small startups and inventors so I basically regurgitate the following over and over

"time, effort, and materials are amortized over the quantity of parts you have made. So a qty of 1 is almost always more costly per part than quantities of 5, 10, 100, 1000."
 
Lots of great advice so far. Just adding my $.02 - I work a lot with small startups and inventors so I basically regurgitate the following over and over

"time, effort, and materials are amortized over the quantity of parts you have made. So a qty of 1 is almost always more costly per part than quantities of 5, 10, 100, 1000."

Yea. Thats economics 101. There are fixed costs and there are variable costs. Fixed costs arn't going anywhere. Obviously then the cost per part is higher with lower quantity and cost per part goes down with higher quantity.

In the case specific to CNC machining, fixed costs are machine operator programming and fixturing and material delivery costs.
 








 
Back
Top