When lasers are focused, the beam coming out of the lens is, simply put, tapered or slightly curved as you move towards the focal point.  The sides of the beam after the lens slopes in this curve.  The energy distribution across the beam as well is usually a form of Gaussian curve.  

So if I program a cheap laser cutter to drill a hole though a thick sheet, unless it is a very expensive multi-axis cutting head that can compensate for the beam physics, you will find your drilled hole has a slight taper to its walls, and may have a second reverse taper in the bottom third or so of the length. Laser operators refer to this as undesired  trepanning of the hole. 

Likewise as you cut, without some very specialized optics, if you look on the micro scale, you will find lasers never make exactly straight sides on the cut. More expensive machines and collimation optics can compensate for this, but most low cost laser cutters in the wild  leave something to be desired in terms of edge quality. 

Look at it this way, the focus lens produces a cone of laser light. That cone leaves a transform on the hole if fired blind without compensation. 

Bad explanation, but it's 1 AM here and I've had a long day. Poor choice of words on my part.

Steve

On Sunday, July 26, 2020 at 10:57:10 PM UTC-4, srober...@gmail.com wrote:
> The laser will cut both kinds of plexi if you have enough laser power for a given thickness and enough assist gas pressure to clear the cut,  Up to say 3/8ths inch in Plexi if I remember right for an under 100 Watt Co2 Laser. . There is a physical depth limit for a one pass cut in many materials, at which no matter how much energy you throw at it, you'll hit a limit.  
> 
> It will cut the plexi at an angle, but it will trepan the edges due to inherent issues with the laser optics.  In other words the edge will have some slightly different angle then what you set the optics head for, and the edge may not be perfectly flat. Wiki on trepanning is quite good. 
> 
> You can go to the Synrad web site and download their materials calculation software. 
> 
> I'm an avid Synrad fan because the low cost Asian hobby grade tubes are only good for 400 hours or less. They have never got the catalyst right for long gas lifetime, and I doubt they will ever bother to invest in that research. 

Not sure what you mean about trepanning the edges.  When I look it up the definition is to make a larger hole by cutting a ring around the edge of the hole rather than boring the entire area.  

When you talk about cutting the plexi at an angle you mean like tilting the blade on a circular saw, right?  

-- 

  Rick C.

  -+- Get 1,000 miles of free Supercharging
  -+- Tesla referral code - https://ts.la/richard11209

The laser will cut both kinds of plexi if you have enough laser power for a given thickness and enough assist gas pressure to clear the cut,  Up to say 3/8ths inch in Plexi if I remember right for an under 100 Watt Co2 Laser. . There is a physical depth limit for a one pass cut in many materials, at which no matter how much energy you throw at it, you'll hit a limit.  

It will cut the plexi at an angle, but it will trepan the edges due to inherent issues with the laser optics.  In other words the edge will have some slightly different angle then what you set the optics head for, and the edge may not be perfectly flat. Wiki on trepanning is quite good. 

You can go to the Synrad web site and download their materials calculation software. 

I'm an avid Synrad fan because the low cost Asian hobby grade tubes are only good for 400 hours or less. They have never got the catalyst right for long gas lifetime, and I doubt they will ever bother to invest in that research. 

Steve 



Steve

On Monday, July 20, 2020 at 10:14:18 PM UTC-4, Ricketty C wrote:
> I had someone today tell me it would be hard to bevel laser cut plexiglass.  Anyone know if that is true? 
> 
> I know little about the material really, but they talked about it splintering, etc.  The issue is that it has a corner on the edge that is causing some wear to a plastic object it is pushing on.  
> 
> The ideas they talked about to solve this issue included some that struck me as odd like adding rollers instead of a flat surface.  
> 
> I don't know much about laser cutting of quarter inch thick material, but if the laser can be tilted, a 45 degree bevel would be easy.  Even a 30 degree tilt would do a lot to relieve the corner.  
> 
> BTW, multiple pieces of this material are assembled to create a wide piece attached to the arm which then presses on the plastic bag.  So it's only to the two outer pieces that need to be beveled. 
> 
> -- 
> 
>   Rick C.
> 
>   - Get 1,000 miles of free Supercharging
>   - Tesla referral code - https://ts.la/richard11209

PONOKO  ask them

On 2020-07-22, Ricketty C <gnuarm.deletethisbit@gmail.com> wrote:
> On Wednesday, July 22, 2020 at 5:32:41 PM UTC-4, Jasen Betts wrote:
>> On 2020-07-22, Ricketty C <gnuarm.deletethisbit@gmail.com> wrote:
>> 
>> > https://slack-files.com/TUTSYURT3-F017H6M216H-6c3fe964c0
>> >
>> > Seems the outer pieces are smaller than the rest and the only spacer is the arm in the center.  The arrangement ends up being 70 mm wide, not counting the screws.  
>> >
>> > Presently I'm looking for a knob that will do this same job without making anything.  The ones I've found only go up to about 40 mm.  Gear shift knobs or drawer pulls.  
>> 
>> Above a certain size they become pommels. 
>> 
>> Have you considered the float from a float valve? possibly it'd not be durable
>> enough. Or maybe some sort of wheel?
>
> Heck, I've tried to explain the problems of an internal battery
> since they can't be left for long periods without charging (SLA). 

If you want a store a lead acid battery for a long time store it dry.

> To be honest, I think the issue is moot at this point because enough
> time has gone by that whoever needs ventilators can now get them.  
> The problem was everyone wanting them at the same time.  Even if
> there is another spike in the fall, will there be another crunch
> like there was in the spring?

Only if there's a perceived profit in it.

> Even with all this time to prepare?                

>
> I'm not even sure this will be ready by the fall.  
>
>
>> It seems a stange goal to design an inexpensive machine to be
>> manufactured using the second most expensive manufacturing method.
>
> Are you suggesting we use knives and whittle ventilators? 

I'm suggesting that NC methods are one step up from that.  injection
moulding, and pressing are cheaper.

-- 
  Jasen.

On 7/21/2020 6:34 PM, rangerssuck wrote:
> On Monday, July 20, 2020 at 10:14:18 PM UTC-4, Ricketty C wrote:
>> I had someone today tell me it would be hard to bevel laser cut plexiglass.  Anyone know if that is true?
>>
>> I know little about the material really, but they talked about it splintering, etc.  The issue is that it has a corner on the edge that is causing some wear to a plastic object it is pushing on.
>>
>> The ideas they talked about to solve this issue included some that struck me as odd like adding rollers instead of a flat surface.
>>
>> I don't know much about laser cutting of quarter inch thick material, but if the laser can be tilted, a 45 degree bevel would be easy.  Even a 30 degree tilt would do a lot to relieve the corner.
>>
>> BTW, multiple pieces of this material are assembled to create a wide piece attached to the arm which then presses on the plastic bag.  So it's only to the two outer pieces that need to be beveled.
>>
>> -- 
>>
>>    Rick C.
>>
>>    - Get 1,000 miles of free Supercharging
>>    - Tesla referral code - https://ts.la/richard11209
> 
> I have cut a lot of plexiglass with a CO2 laser It cuts very well and does not leave particularly sharp edges. I have cut both clear and "smoke" colors. The laser is infrared, so visible colors aren't likely to make a difference.
> 
> You can soften cut edges with a flame, but you can also round them with a scraper - it goes way faster than you might expect.
> 
> How thick is the plexi you're looking to cut? what kind of sizes and tolerances?
> 

Quick aside if you don't mind, know where I can get something like this
made? (not that large)

<https://youtu.be/OmodASEvdKU>

On Wednesday, July 22, 2020 at 5:32:41 PM UTC-4, Jasen Betts wrote:
> On 2020-07-22, Ricketty C <gnuarm.deletethisbit@gmail.com> wrote:
> 
> > https://slack-files.com/TUTSYURT3-F017H6M216H-6c3fe964c0
> >
> > Seems the outer pieces are smaller than the rest and the only spacer is the arm in the center.  The arrangement ends up being 70 mm wide, not counting the screws.  
> >
> > Presently I'm looking for a knob that will do this same job without making anything.  The ones I've found only go up to about 40 mm.  Gear shift knobs or drawer pulls.  
> 
> Above a certain size they become pommels. 
> 
> Have you considered the float from a float valve? possibly it'd not be durable
> enough. Or maybe some sort of wheel?

Rollers are what the lead guy is planning based on the most superficial examination of the wear on the bag.  During a conference call another guy looking at the same limited images I saw assured us that this was from friction rubbing and not from the edge of the plexi scraping the bag.  As the bag is compressed the it changes shape a pretty good amount so that the ends curl upwards.  I think this produces a scraping effect on the bag.  Also, there is not much to hold the bag in place.  

I would want to examine the problem further before trying to effect a fix.  In any regard, the bag is going to suffer fatigue in rather short order.  It may last weeks, but it won't be months of use.  I don't think they have accounted for this yet.  This is something you would want to consult someone in either the manufacturing field or the user field to know what is acceptable for wear and maintenance.  

Heck, I've tried to explain the problems of an internal battery since they can't be left for long periods without charging (SLA).  I don't really think this has an impact on any decisions.  I expect these units to be used one of two ways.  Either they are bought in advance and stored for who knows how long before being used, or they will be built in a panic when another pandemic hits.  To be honest, I think the issue is moot at this point because enough time has gone by that whoever needs ventilators can now get them.  The problem was everyone wanting them at the same time.  Even if there is another spike in the fall, will there be another crunch like there was in the spring?  Even with all this time to prepare?  

I'm not even sure this will be ready by the fall.  

> It seems a stange goal to design an inexpensive machine to be
> manufactured using the second most expensive manufacturing method.

Are you suggesting we use knives and whittle ventilators?  Should we use all through hole electronics so they can be assembled by hand?  Or maybe like one guy here who was trying to design electronics using parts they could get from scrap devices in the third world?  The 8051 is back!  

There are a lot of inconsistencies.  At one point in the previous group I was suggesting something other than an Arduino like one of the many tiny MCU boards some of which have second sources.  It was pointed out to me that initially in the pandemic shipping was a problem.  They claimed the Arduino was available from lots of local vendors so that was the smart choice.  Another issue was about enough of a selected part being available from stock to make 10,000 units.  I don't think the local shops would have 10,000 Arduinos.  

This is why engineering has formal planning and management techniques.  The actual design work is usually the easy part unless maybe you are designing an SR-71.  We aren't designing an SR-71.  

Yeah, in many ways this is a maker group.  

-- 

  Rick C.

  --+ Get 1,000 miles of free Supercharging
  --+ Tesla referral code - https://ts.la/richard11209

On 2020-07-22, Ricketty C <gnuarm.deletethisbit@gmail.com> wrote:

> https://slack-files.com/TUTSYURT3-F017H6M216H-6c3fe964c0
>
> Seems the outer pieces are smaller than the rest and the only spacer is the arm in the center.  The arrangement ends up being 70 mm wide, not counting the screws.  
>
> Presently I'm looking for a knob that will do this same job without making anything.  The ones I've found only go up to about 40 mm.  Gear shift knobs or drawer pulls.  

Above a certain size they become pommels. 

Have you considered the float from a float valve? possibly it'd not be durable
enough. Or maybe some sort of wheel?

It seems a stange goal to design an inexpensive machine to be
manufactured using the second most expensive manufacturing method.

-- 
  Jasen.