Boxford VMC - limits of blue tape adhesion

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Alex Gibson

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Jul 13, 2021, 11:48:10 AMJul 13
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I’m hoping to make a thing on the Boxford soon, and just checking whether a method is likely to work out well or badly…

 

The part I want to make is 40x40x12mm.  I have a blank 102x102x12.4mm aluminium plate.  (no big deal if only one side can be faced off).

 

If I completely recreate the induction part process, I would place the part somewhere within the blank, do drilling, do a facing pass, then mill the edges leaving tabs.

 

This will make the part at least 40mm+(diameter of endmill x2)+arbitrary extra waste wide – I certainly couldn’t get 4 pieces out of the blank L

 

I was very impressed by how well the blue tape, rubbed down and glued back to back, actually worked.

 

BUT – is it strong enough that I could get away with cutting out a 42x42mm square as marked, deburring it, and just tape+gluing that to the sacrificial piece, and be able to mill the edges without it lifting off?

 

On the one hand I could understand if that’s too much to ask of it – as the aspect ratio is higher than for the induction piece – but on the other hand the amount of material being removed from the edges would be <1mm and it could be done in multiple shallow passes.  This would also be way easier for clearing chips. 

 

The only worry is whether the adhesion of the blue tape is enough?  I’d be sure to maximise the adhesion – clean with IPA, rub down the tape all the way to the edges and have good coverage of superglue.

 

I feel that’s likely to be OK but I want to check in before I go ahead.

 

If not… part of my design involves holes drilled into the square surface – so to augment the blue tape, after I drill these 3mm holes, I could additionally drill  a 2.65mm hole down into the sacrificial piece, tap that to M3, and screw bolts down into it before doing the edges… it’s an extra step but not much extra bother.

 

Any experienced views on this?

 

 

Alex Gibson

 

+44 7813 810 765    @alexgibson3d    37 Royal Avenue, Reading RG31 4UR

 

admg consulting

 

edumaker limited

 

·         Project management

·         Operations & Process improvement

·         3D Printing

 

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Steve Rodway

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Jul 13, 2021, 12:19:36 PMJul 13
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I'd say just using glue and tape with no additional measures, you've
probably got better than 50/50 chance of it holding. Things you could do
to improve the odds, in no particular order

A) Don't mill the perimeter all the way through, leave maybe 0.3-0.5mm
on the bottom. Helps 2 ways, first you've got more surface area left,
second if it does lift, it's most likely to happen when the perimeter
cut breaks through because the cutter can catch the edge, no
breakthrough = no edge to catch

B) Make 4 at once and leave them connected by multiple tabs per side,
will add surface area and resistance to twisting forces

C) Bolt down as you suggest, pretty much guaranteed to work at the cost
of some additional effort, make sure toolpaths aren't going to collide
with the bolts!

D) Low value for maximum ramp stepdown, will reduce cutting forces at
the cost of increased machining time. If you have many to make it will
result in uneven tool wear and decreased life, but for just 4 of them in
aluminium it shouldn't be an issue

E) Hold it in the vice instead. Will give a much firmer grip at the cost
of having to get the vice set up and having to machine the part in 2
setups to deal with the bits you can't machine in one cause the vice is
holding them.

Steve
> Alex Gibson

Alex Gibson

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Jul 13, 2021, 1:53:55 PMJul 13
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Thanks very much Steve

Great feedback. Yes if I make 4 parts at once, with tabs, the amount of waste can be lower if I space them by one tool-width in the centre - and being connected by tabs they lend each other stability.

I'm prototyping and might change my design after the first iteration, so I think I will use the vice and bolt-down.

I've realised that as well as two holes through the piece, I also need one M3 thread tapped hole into it- and it being off by 0.25mm or so in X and Y is not an issue. So I can use this to bolt the workpiece to the sacrificial piece from the underside. In combination with the blue tape that should be plenty!

Cheers,

Alex Gibson

+44 7813 810 765 @alexgibson3d 37 Royal Avenue, Reading RG31 4UR

admg consulting

edumaker limited

• Project management
• Operations & Process improvement
• 3D Printing

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Stephen Rodway

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Jul 14, 2021, 2:37:46 AMJul 14
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It's best to make the space between them slightly larger than the tool
width (only slightly) because it makes for an easier cut.

Right now you might want to wait a little though..... It seems my
efforts to clean up the spindle bearings may have actually made things
worse :( I guess some of the dirt was structural! At the moment the
spindle has more vibration and run-out than I'm really happy with so for
anything accurate it might be wise to wait till I've had time to do the
full spindle rebuilt that's planned.

Steve

Alex Gibson

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Jul 14, 2021, 8:13:45 PMJul 14
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Thanks for the warning! Any help needed doing it/finding parts etc?
Structural dirt can certainly be a thing in old cars... is it possible some dirt/metal shavings got in between bearings in the replacement? Or something went back in the opposite orientation (that's in theory reversible but has worn a groove/oval?)

I don't think I need the maximum level of accuracy for my first prototype (it might have been made on a pillar drill!) and I was very impressed with the induction part so could detune quality expectations a lot - how bad is it really?
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toby.g....@googlemail.com

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Jul 16, 2021, 4:52:49 PMJul 16
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In terms of holding difficult parts, remember that multiple work holding strategies can be employed. For example, its perfectly possible: 

1. to super glue the part down 
2. then use strap clamps for a heavy milling operation
3. then remove the strap clamps and use a tabbing strategy
4. mill only the tabs away, relying on the super glue for the final work holding.

breaking through the skin of the material (especially aluminium) can be tough and very grabby, progressively milling away the tabs is an excellent strategy.
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