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Part breakout /fixturing

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Dave S03/07/2020 21:54:29
45 forum posts

Evening all,

Now my CNC has potential to work nicely I'm thinking about making things

I will be making some small watch parts, mostly 2d in general shape. I think I can effectively cut these out of sheet stock, but what's the best way to fixture it?

Additionally what do you do to prevent the part breaking out and jamming/snapping the end mill.

For general size the would be parts that require 1mm or smaller endmilll to get into internal radiuses, and typically 1mm or so thick.

I occurs to me that I could use (say) 1.5 mm gage plate,surface the top by 0.25, cut around the part leaving 0.25mm bottom stock. Then fill the "gutter" with shellac and flip over, probably on my surface grinder to remove the 0.25 left.

Any better techniques?


Brian Oldford03/07/2020 22:05:21
677 forum posts
18 photos

Use the third axis to leave a few small tabs to hold the piece in place.

blowlamp03/07/2020 22:15:42
1377 forum posts
85 photos

You could cut right through, but leave a few small & thin holding tabs to keep the part in place.

The next stage would be to generate a toolpath that cuts away those tabs, but before running the code, apply some hot-melt glue in the already cut areas to provide support whilst the tabs are cut.

So similar to you own idea except no need to grind the the other side to release the part.


Clive Hartland03/07/2020 22:18:58
2576 forum posts
40 photos

I have seen the watch movements being machined, they do them in strips of 4 or 5 at a time. They are fed into a slot and the strip lightly clamped and the metal then machined. The component is left with a small thin fillet that they can then breakout.The metal looked like it had a high copper content as it had a red look to it.

JasonB04/07/2020 07:07:19
18332 forum posts
2024 photos
1 articles

As you are using Fusion that has the facility to add "tabs" to the contour cut, when setting up the contour geometry there is a box to click if you want tabs and that then allows you to enter length, thickness and positions.


Tool will lift and drop as it goes around the work. Simply file off the thin tab when cleaning up the edges.


Some people apply good quality painters tape (blue 3M) to a sacrificial surface and then use superglue to hold the stock to that which can be peeled off after cutting but you need to be careful with thin items that you don't bend them.

Andy Carlson04/07/2020 08:55:40
260 forum posts
105 photos

I've used a couple of ways with thin brass and PB sheet (10 and 20 thou)...

1. Mill a sacrificial backing piece of aluminim flat and then fix the job to that with double sided tape from B&Q. Press the stuff down hard. If you really want a strong bond, leave it overnight (I didn't do that). Clamp the aluminium to the bed and run the job, To get the tape undone, hit it with a hairdryer until it is a bit hotter than you can hold and the tape will soften nicely. Clean the remaining goo up with IPA. This works very well for parts where the surface area is large compared to the length of the boundary.

2. Use brass as the sacrificial backing and solder the job down. I've used this where the dimensions are mechanically critical or for fiddly parts that lack the surface area to hold well with the tape method.

I haven't tried tabs myself - the sheet is very thin and for me it's better to have it stuck to a backing piece.

John Haine04/07/2020 09:51:21
3185 forum posts
172 photos

Elsewhere on here I've described applying blue masking tape on surface of a backing plate and on the underside of the material; then gluing the latter down with superglue. Relatively easy to peel the blue tape off.

Another approach which should work is just to superglue it direct - this is more secure but harder to remove. However I once by mistake forgot to apply the blue tape to the material so had to remove it with heat - I found that a relatively short boil in water did an excellent job. As people use superglue as a replacement for "wax chucks" (see Clickspring videos) it's only the same idea applied to milling.

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