CNC Source for project, general hints/tips/suggestions

snoop77 · Nov 17, 2020

Hi,

An electronic engineer here, weak on mechanical engineering know-how.

I've a case about 70mm cubed and a relatively low quantity of around 200 (initially). Long term view is unknown until initial sale figures can predict it. Certainly I don't want to order 10,000 up-front.

It looks like CNC is the way to go as I believe casting could have additional expense (sliders?) due to the strap groove going around the side. The case is to be annodised or similar which I'm told is another benefit of CNC (don't need additional polishing).

I ran it through 3dhubs and it's come back at about £3k for 200 pieces. Can I ask;

1. any cheaper and respected sources people use for CNC?
2. any hints and tips you can suggest - I'm learning and soaking up everything I can pick up on.

ChipSplitter · Nov 17, 2020

Manufacturing on Demand | Instant Quotes, Custom Parts | Xometry

:popcorn:

Finegrain · Nov 17, 2020

That part will be a right royal PITA to machine in a CNC mill, on account of the sharp internal corners in the internal slot and slanted floor, and thin walls. Maybe 5-axis machine can mitigate some of the sharp inside corners, but the thin walls will be a problem regardless.

Regards.

Mike

DDoug · Nov 17, 2020

Why not CNC mill wax or foam, then lost wax/foam cast it ?

snoop77 · Nov 17, 2020

digger doug said:
Why not CNC mill wax or foam, then lost wax/foam cast it ?

The walls are 1 or 2mm, I wasn't expecting any cnc issues...

I'm not fully up to terms, doesn't wax cast have low tolerance?

snoop77 · Nov 17, 2020

Unable to edit previous post. To clarify the floor isn't slanted that's probably the picture. The top cut out is a problem, I've since rounded the edges which isn't ideal but has to be so for CNC.

Finegrain · Nov 17, 2020

Part of the floor surely looks slanted :confused:

If you send me a solid model (STEP or IGES) I can illuminate trouble spots. PM me for e-mail address.

Regards.

Mike

mhajicek · Nov 17, 2020

snoop77 said:
Unable to edit previous post. To clarify the floor isn't slanted that's probably the picture. The top cut out is a problem, I've since rounded the edges which isn't ideal but has to be so for CNC.

First of all you need a toleranced print before it can be meaningfully quoted. Any model-only quote is for a part that's only kind-sorta close.

I think you'd be getting a steal at £15 each, but what you receive may or may not be suitable to your needs.

When machining the inside of this, the side that's to the right in the picture would be up, with rotating cutting tools coming down. Try to imagine any shape of rotating cutting tool that will cut that interior slot, or the sharp 90° between the sidewall and the portion of what is now the floor, which appears to be at about a 45° angle. With milling only, your supplier will only get as close as the radius of the ball endmill they use. To make that part as modeled would require a sink EDM, and you're looking at hundreds of dollars per part or more.

Hardplates · Nov 17, 2020

3K for 200? either some quoting algorithm is whacked or someone is starving for work

DDoug · Nov 17, 2020

snoop77 said:
The walls are 1 or 2mm, I wasn't expecting any cnc issues...

I'm not fully up to terms, doesn't wax cast have low tolerance?

I don't see any tolerances on your model sir.

Instead of this pulling of teeth to drag out the exact sizes & tolerancing, please
take an hour and make that print.
You already have the model.

snoop77 · Nov 18, 2020

Just realised I can attach a zip, so now the step is attached which will make it easier.

DDoug · Nov 18, 2020

snoop77 said:
Just realised I can attach a zip, so now the step is attached which will make it easier.

Don't doo this. People are rightly concerned with opening up .zip files from the 'net.

Simply make a pdf or jpg screen shot of your properly detailed & dimensioned drawing.

Finegrain · Nov 18, 2020

I looked at your solid model. Yes, you added a .15mm corner radius to the rectangular slot, but this is not helpful, as an endmill making this slot would have to have >25mm reach, so a 167 L/D ratio :nono:

. An L/D of 12 is pushing it, 20 is extreme. Most endmills are 2 or 3 L/D.

You might could nibble away at it with a steep-angle, pointed endmill in a 5-axis machine, but this would take significant machining time.

On the floor corners, the sharp inside corner where the angled part of the floor meets the vertical walls (and the angled floor itself for that matter) cannot be completely done with rotating cutters, even with 5-axis. The opposite wall blocks access. You would need to add a corner radius (like 2mm) to make this feasible as a milled part. A long, tapered, small-diameter ball endmill could get close, but again, requiring significant machining time.

Regards.

Mike

snoop77 · Nov 19, 2020

Finegrain said:
I looked at your solid model. Yes, you added a .15mm corner radius to the rectangular slot, but this is not helpful, as an endmill making this slot would have to have >25mm reach, so a 167 L/D ratio . An L/D of 12 is pushing it, 20 is extreme. Most endmills are 2 or 3 L/D.

You might could nibble away at it with a steep-angle, pointed endmill in a 5-axis machine, but this would take significant machining time.

On the floor corners, the sharp inside corner where the angled part of the floor meets the vertical walls (and the angled floor itself for that matter) cannot be completely done with rotating cutters, even with 5-axis. The opposite wall blocks access. You would need to add a corner radius (like 2mm) to make this feasible as a milled part. A long, tapered, small-diameter ball endmill could get close, but again, requiring significant machining time.

Regards.

Mike

Excellent post, very informative.

Regarding the upper slot, that ideally needs no corners at all as it''s to hold a position critical PCB in place. It sounds like the ideal in principle is out the window?

The rear inner corners have no purpose (they're actually a negative) other than to allow a consistent 1mm outer wall on the outside. If I removed them I'd probably have to make the rear wall 3mm. This is actually feasible.

Some comments from Chinese suppliers are that the 1mm walls are too thin to CNC well. They've not said anything about the two aspects above and this is my worry, they are always too quick to say yes.

snoop77 · Nov 19, 2020

Damn it, wrote a reply and it's disappeared.

---

Excellent post Finegrain, that's the sort of detail I need. The Chinese of course haven't mentioned it...

The inner rectangular slot at the top is location critical as it acts as a placeholder for a PCB which must be precisely positioned. I can't see another way around it other than a bolt through the top but I'd really like to avoid that.

On the second point could you scribble a circle over the offering corner?

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