jeffp51

I am finishing up cad plans for a .60-sized biplane. My son's shop teacher has a laser cutter that we want to use to cut out parts. He says the kerf appears to be .0085 inches offset to the inside. As a junior high shop teacher, he has mostly used the tool to burn football logos into pinewood boards and has not done a lot of precision work. I know I could send my plans off to a cutting service, but I like the idea of doing it myself (I can't plant my own balsa trees, but I can do as much as possible). I have two questions:

1. is a tolerance of .0085" anything to worry about? I am not sure I have ever been that precise with my own scroll saw and sander. But doubled on a part like a rib spar notch, it would equal about 1/64". There is also a wing tube to worry about.

2. How can you have an "inside" kerf if you only have a straight line? My son says the machine cuts with a scanner stylelike an inkjet printer instead of drawing each part.

suggestions?

stay tuned for a build thread

kenh3497

Concerning the kerf, here is my take on it. If you draw a circle, the cutter will cut on the "inside" of the line. You just have to plan where the cut will be to maintain your tolerances. It's no different from you using a table saw of band saw to cut wood. You have to plan on "which side" of the line you are cutting on. In this case the software driving the laser cutter makes the choice for you.

LesUyeda

Seems like Monzano Lazer had a tutorial on how to design for lazer cutting. I'm in the throes of a new computer/ISP/system and don't know right now how to go look for the link, but they would be worth researching.

Les

Chad Veich

My Feedback: (60)

I have found that an offset of .005" works very well for most of my wood parts, be it balsa or ply. There are many variables that can make a difference including the type of material, how thick it is, the condition of the mirrors in the laser, the wattage, etc. However, .005" seems to be a good compromise for what I do. The offset needs to be to the outside of the part except on interior cut outs where it will need to be to the inside of the part. In the example below the blue line indicates the part as designed while the red line is the offset cutting line. If all goes as planned the finished part will be reduced back to the blue line due to the material being removed by the laser, ie, the kerf.



Regarding question #2, I can think of very, very few instances where you would be cutting nothing but a straight line which would result in a very thin slit cut into you material. If that is the case then no offset is needed as you want to cut right down the middle of the line.

LesUyeda

The only thing you left out Chad, were the ocassional breaks in the cut, so the part does not fall out of the sheet. Been there, done that:-((((((((((

Les

Chad Veich

My Feedback: (60)

Absolutely Les! Makes for much faster and simpler turn around when cutting if you don't have to pick up the parts and remove the throw aways from the laser bed every time.

LesUyeda

Sorry. Still doing battle with my new computer; new RCU format; etc, etc, etc,.:-((((((((((((((((((((

Les

BMatthews

Don't know much of anything about the laser cutting options but if this setup does printer like scan lines that sounds like it's more of an engraving sort of machine than a cutter.

On the kerf allowance? I guess it depends on your personal standards. But for me if a part fit with a .0085 inch gap I'd throw it out and start again. Let alone if the kerf allowances added up between two parts to form a .017 gap. That sort of thing is getting into "fill it with hot glue" territory. Faced with product that had gaps which added up like that I would just toss the whole lot in the trash at that point. So all in all to obtain the nice slip fit where it won't fall apart is shaken lightly sort of standard for sizing you will most certainly need to alter the files to provide offset lines such as indicated above to deal with this size of kerf. Such a slip fit sort of thing is the standard that I feel is expected from a laser cut process.

Chad Veich

My Feedback: (60)

You hold to a pretty high standard BMatthews when 1/64 of an inch of gap is "round file" material! Some laser systems have software which will automatically adjust for the laser kerf, or so I've been told. None of the ones I have used have it so I've gotten used to offsetting my laser files and don't find it to be too much of a hassle.

jeffp51

Thanks for the feedback. Guess it is time to start redrawing my templates. Here I had hoped I was almost ready to start cutting. The other option is to send it off to a service that does offset for kerf.

jeffp51

Thanks for the feedback. Guess it is time to start redrawing my templates. Here I had hoped I was almost ready to start cutting. The other option is to send it off to a service that does offset for kerf.

Chad Veich

My Feedback: (60)

Jeff,

Maybe let your laser guy cut a few test parts for you and see how it goes. Many times I have used parts that were cut without accounting for the kerf and while they may not provide for the absolute perfect fit they are still useable. (Unless the laser is just wildly out of focus or something) Most likely they will still be more accurate than anything you could do by hand.

BMatthews

Chad, I hold myself to those standards as well.

When I cut out wing ribs I'll start with a template which is slightly oversize by about a pencil line worth. And no spar slots. If the plans don't call for it I'll also increase the length at the trailing edge of the ribs by the thickness of the ribs. Once cut out the ribs are stacked and pined or bolted. The profile and leading and trailing edges are sanded to the final size to even everything up. Spar slots are cut and filed out using "safe edge" coarse files until the spars need a firm finger push to fit. Once done I unpin or unbolt the ribs and number the ribs as they come off the stack. By numbering the ribs I can use them in order from tip to tip. This way even if it's a constant chord wing any minor runouts in thickness, sanding shape or spar slot position is evenly graduated over the span of the wing. The trailing edges are then notched to a nice firm fitting size so that the extra length added to the rib trailing edges fits into the notch for a stronger glue joint.

This all really pays off when I assemble the wing. The ribs slide into the trailing edge notches and are located chorwise by a pinned down spar or the leading edge and the spars can be placed and will hold themselves in place with the light friction fit until I touch the joint with some thin CA. I've even learned that if I want to use aliphatic resin glue instead of CA that I'd best go with a very light press fit since the glue causes the spar wood to swell slightly and things can lock in place a little TOO well while inserting and positioning the spar over the span. In such cases I go for an easy fit when making the spar slots.

When I build my stick and tissue models for free flight or old timer RC flying the uprights must fit with a light finger pressure between the main longerons. If they are too tight they get sanded a hair. If I go too far and it's not able to stay in place by wood to wood friction then it gets used for the next smaller upright.

Overkill? Some may think that. But my wings and fuselages seldom warp or suffer from parts coming adrift. Glue joints are stronger when the fibers are in contact rather then bonded to big wads of glue that bridge the gaps in less well fitted cases. And glue is heavy. By using well fitting joints and minimal glue the model can come out both lighter AND stronger. And the longerons and stringers on my built up models all lay fair to the lines instead of having odd ball dips and lumps in their paths.

Now i'm not above using a bit of lightweight filler here and there. Few of us are consistently that good. And there's always the odd booboo where we least expect it. But I refuse to compromise or fill gaps where it really counts. For example, if I were to find a stab platform with a wiggle in it so that the stab has a long arched gap I'd fill it in with balsa and sand it back down to a straight line or the airfoil shape as needed. I would not fill so much as a 1/64 gap with glue for something that critical to the model's alignment.

The bottom line is that I really enjoy building. But I only enjoy it when stuff fits like it should. If I have to adapt to poor joints and mis-alignments and pull or push to make somthing fit so that I know things are twisted then it takes away all the fun.

jeffp51

Sorry for the double post earlier. I really love the building and designing aspects of this hobby. There is nothing more rewarding for me than seeing an idea progress to a plan and then to a functioning airplane. I tend to be a bit of a perfectionistit is part of my training. The guy that taught me years ago made me redo my first plane five or six times before he would let me fly it. That attention to detail has paid dividends many times over. My conflict comes when my impatient character comes into conflict with my perfectionist character. I never know which will win out.