How straight are your cuts ?

General discussion of laser machines

How straight are your cuts ?

Postby J45on » Sat Oct 29, 2011 12:00 pm

I presume I have an alignment issue ?
I cut a 50mm plywood square and looking at it from the top down
The left edge and bottom edge have a 2°angle to them sloping out like a pyramid
The top and right edges are fine to my eye :?
What would cause such a issue ?

I am trying to design a "da vinci code"style codex/cryptex but obviously I need straight edges :(
J45on
 
Posts: 258
Joined: Thu Mar 31, 2011 11:30 am
Location: Kent,united kingdom

Re: How straight are your cuts ?

Postby lasersafe1 » Sat Oct 29, 2011 3:02 pm

You may have your lens at a focus because you are cutting effectively, but if you are hitting the lens off center then it will steer the focal point in the opposite direction of center. Check that the incoming beam is exactly centered in the final lens.
lasersafe1
 
Posts: 599
Joined: Fri Nov 27, 2009 8:23 pm

Re: How straight are your cuts ?

Postby bdring » Sat Oct 29, 2011 3:33 pm

The lens should focus to the same point regardless of where the beam hits it. That is why you can do the trick with the pointing laser beam running next to the IR beam. If the beam is striking the lens at an angle you could have problems.

Is it more likely you have a squareness or alignment problem. The square cutting more like a pyramid is hard to wrap my mind around. A parallelogram is the more common issue.
Bart
"If you didn't build it, you will never own it."
bdring
Site Admin
 
Posts: 2966
Joined: Sun Nov 22, 2009 7:33 pm
Location: Chicago, IL, USA

Re: How straight are your cuts ?

Postby J45on » Sat Oct 29, 2011 4:24 pm

Ok I tried to take some pictures to explain better
So I cut another 50mm square from 6mm ply wood
And lined them up on a granite surface plate with a 123 block

Image

The top of the wood is always on the left
this is X- down

Image

X+ down

Image

Y- down

Image

and finally Y+ down

Image
J45on
 
Posts: 258
Joined: Thu Mar 31, 2011 11:30 am
Location: Kent,united kingdom

Re: How straight are your cuts ?

Postby lasersafe1 » Sat Oct 29, 2011 5:30 pm

bdring wrote:The lens should focus to the same point regardless of where the beam hits it. That is why you can do the trick with the pointing laser beam running next to the IR beam.


Not true at all. It works for 750nm and 1064nm only because the focal power of the lens is so different (over a power of 10) for the different wavelengths. If you hit the lens 1/8" off center with IR, lets say downward in our Y direction, then the focus of the IR will occur 1/8" upward in the Y direction. Remember the candle example in common physics books where it is upside down on the other side of the focus.

His problem is not hitting on center.
lasersafe1
 
Posts: 599
Joined: Fri Nov 27, 2009 8:23 pm

Re: How straight are your cuts ?

Postby J45on » Sat Oct 29, 2011 5:34 pm

I shall have a tweak tomorrow , but I have the maker slide upgraded coming so it all has to come out soon anyway :D
J45on
 
Posts: 258
Joined: Thu Mar 31, 2011 11:30 am
Location: Kent,united kingdom

Re: How straight are your cuts ?

Postby dirktheeng » Sat Oct 29, 2011 5:45 pm

There are 2 things going on here, I think.

1) the laser beam naturally cuts in a V shape with the widest part at the top.... this is unavoidable, but can be minimized to a certain extent by making sure you only put in the minimum amount of power to make the cut

2) bart is correct, no matter where the beam hits the lens (if the optics are perfect) it should make it go through the focal point. However, if it is off center it will come in at an angle.


Consider the flolowing 2 beam paths... the top one the light goes directly into the center of the lens, the second comes in off to the side

_____|_____
|
|
.

_________|_
/
/
.

Granted, this is a gross oversimplification, but you can see that this can affect the angles of your side cuts. If the system is aligned correctly, the cut angles should be the same on the +x and -x side as well as on the y's. They will never be perfectly square to the edge, but both should be at the same angle tilting into the piece. Also, x's and y's may not have the exact same angle as the end grain cuts differently than long grain.

So in the regard of making sure the ends are near to square, it is important to not use any more power than you need, to find the right focus, and to try to get that beam in the middle.

Oh also, I just thought of something else... you should take your lense out and make sure the mirror is actually directing your beam in as near of a perpendicular course as possible. I did this with my red laser when I did the primary alignment. Mine was WAY off. This will aslo affect (probably more than getting it into the center of the lense). I had to rotate the head and shim one side to get it right.
dirktheeng
 
Posts: 616
Joined: Thu Sep 09, 2010 4:49 pm

Re: How straight are your cuts ?

Postby dirktheeng » Sat Oct 29, 2011 5:48 pm

shoot... the thing took the spaces out of my diagrams and squished it against the wall. anyhow you should get the idea
dirktheeng
 
Posts: 616
Joined: Thu Sep 09, 2010 4:49 pm

Re: How straight are your cuts ?

Postby lasersafe1 » Sun Oct 30, 2011 1:47 pm

If what you say is correct, then the beam steering with a lens I have been doing for the past 15 years at a National Lab hasn't worked and it has all been a figment of my imagination. I have a 2 meter focal length lens mounted in a stepper controlled XY stage. The laser enters at normal incidence. When I move the stage 2 cm left and right or up and down, the focused spot 2 meters away moves by the exact same amount. Prove it to yourself by using a laser pointer and a short focal length lens and sweep it across the lens in a flat plane and watch the shift of the focal point. It will be a 1:1 ratio with distance off center.
lasersafe1
 
Posts: 599
Joined: Fri Nov 27, 2009 8:23 pm

Re: How straight are your cuts ?

Postby dirktheeng » Sun Oct 30, 2011 2:46 pm

you actually just proved the point in a round about way. In your case you are keeping the beam stationary and moving the lense around... that is you moved the optical axis of the lense and thus the focal point of the lense. Now reverse the scinario.... keep the lense stationary and move the beam behind the lense... the spot on the wall will not move... it may change shape a little because the angle it comes into the wall/object is changing and there are also undoubtedly abarations to deal with, but the spot wont move. The focal spot never moves relative to the axis of the lens.

This is the situation we are talking about here... if the beam comes into the back of the lens at any point but the center, it will come to the same focal spot and necessarily come into the focal spot at an angle off normal to the lens plane. If the beam has any width to it (which it does) it must also come in at some conical shape for the same reason.
dirktheeng
 
Posts: 616
Joined: Thu Sep 09, 2010 4:49 pm

Next

Return to General

Who is online

Users browsing this forum: No registered users and 1 guest

cron