Puff The Magic Dragon

My Feedback: (2)

I'm working on a set of Cad files to send off to get laser cut. I know that the laser has a .08 Kerf. So when I'm designing, say the wing rib spar slot or former stringer slots, what should be the dimensions of say a rib with a .250 spar slot. Would the spar slot offset on the cad laser file need to be larger or smaller. ? Do I take the size of the Kerf and half it for my offset? Would the .250 slot need to be .254 or .246 ???

Jim_Purcha

My Feedback: (8)

That is something you should be asking the person who owns and is compiling the machine code for the laser cutter and doing the cutting.The offset for kerf also depends on the direction of the cut. Usually G41 (left kerf or cutter compensation), outside profiles are cut CCW and inside profiles (holes) are CW. Most programming software will compensate for the kerf at the machine controls.

Jim

limeybob

My Feedback: (6)

I think we have a terminology problem.
The actual laser will cut an approximate .008 to .015 cut in the wood, depending on the wood thickness and type..
The "Kerf" is a space placed on the part to stop the part falling out.
For the spar size, draw to the actual dimension and the laser will cut it slightly oversize.
Leave at least a 1/8 space around the edge of the standard wood size your cutter will be using.
Also check what color the cut and engrave lines need to be.
Hope this helps.
Bob

CoosBayLumber

My Feedback: (1)

This is sort of a loaded question, for is no memtion as to exact thickness of material being used. Last week had a saleman show me his Fine Cutting, but the laser displayed a .015" wide kerf in thin Plywood. On balsa the very same part gave a .008" wide kerf. Sort of depends you know. But I usually draw the line in Autocad to center of line, and disregard any kerf. That, if required, can be handled via software inside the laser machine.

I usually set lines from parts about .050" apart from one another. As the thin resulting section in between two can catch fire, and then you got junk. Only use the normal 1/8 inch at the exterior borders. Thus with the reduced tolarances can pack the parts in tighter to one another and posibly eliminate one whole sheet. It wokrs now for past ten years.


Wm.

Chad Veich

My Feedback: (60)

I suppose you will get as many different answers as you get replies but I have found on the laser cutter I use that .005 offset to allow for the kerf works pretty good for most balsa and plywood parts in the thicknesses generally used for models. I simply draw the part to size and then offset the line in CAD by .005 to the proper side. On outside cuts you need to offset to the outside, i.e., make the part larger. On inside cuts you need to offset to the inside, i.e., make the part smaller. Below is an example. The blue line represents the actual size of the part as drawn in cad while the red line is the laser cutting line. Please note I have exagerated the offset in the sample below to make it obvious.




No offense Bob but I think the terminology error lies with your use of the word. Here is the definition of "kerf":

Main Entry: kerf
Function: noun
Etymology: Middle English, action of cutting, from Old English cyrf; akin to Old English ceorfan to carve — more at carve
Date: 1523
1 : a slit or notch made by a saw or cutting torch
2 : the width of cut made by a saw or cutting torch

limeybob

My Feedback: (6)

you are right of course.
I meant to say tabs.
Thanks
bob

Lnewqban

Could you explain this a little better for me (I have never ordered from a laser service)?
Does it mean that the AutoCAD file (drawing) that you send to the laser cutter has to have two lines in different colors?
Thanks

dreadnaut

My Feedback: (5)

Concur.

Also, I use a .004" offset for all of my parts; 1/16-1/18 balsa, 1/16 A/C ply and 1/8 lite ply and everything fits real nice. The laser guy I am using has never done R/C (balsa) before and commented that he found the variances in the wood density to be a little troubling. If he slows the rate, the cut will go all the way through the dense parts, but widen the kerf through the soft spots. If he speeds the rate, the hard spots will not always cut all the way through. I advised him to set his speed based on the softer balsa. I can x-acto the parts that don't cut all the way through, and a little sand paper takes care of any tight fits.

dreadnaut

My Feedback: (5)

This depends an the laser cutter you use. Often they want you to color code based on cut lines, stock outline, text marking etc. The guy I use lets me determine the color code as long as I provide a legend. Generally, all the ones I have encountered want everything on layer 0. I have a macro that changes the color of the lines I offset. This is useful for when I go back to delete the original so that I do not delete the kerf'ed profile by mistake.

CoosBayLumber

My Feedback: (1)

Per #8 noted above, to which side does the offset go when using one line to cut two parts? I get called up quite often to have one part line cut for two parts. Such as to the wing rib as illustrated. A forward half and a rear portion. Usually at where dihedral braces go in at or at wing tips. And thus you have now created a gap. I think something is missing here then, for one example does not fit all situations.

And as per #6 noted above the term locking TABS is correct, for we have been using such since the late 1980's era. Years ago, had a LISP program wrote up to create these, and gap varies between hard woods like used in plywood, and very soft woods like on balsa.


Wm.

tweedy

My Feedback: (1)

I use a product called SheetCam to generate my offsets and holding tabs, it's not an expensive program and well worth it if you do much of this type work.

Richard

Chad Veich

My Feedback: (60)

I have this same issue and it really is irritating. Almost all balsa sheets have dense spots that resist cutting more than the rest of the sheet. Some times 90% of the parts will cut in one pass but there will be a few spots that take multiple passes with the laser to cut completely through. This is one of the reasons why I use Poplar light plywood for the vast majority of the framework in my designs. With judicuous use of lightening holes (made easy with the laser) the light ply is not much heavier than balsa and cuts much, much better.

Chad Veich

My Feedback: (60)

This is no problem, unless there is something I'm missing. Below is an example of two mirror images parts which share a single cut line between them. Once again, the blue is the original while the red is the offset cut line. Notice that the shared red line is offset the same amount from both original parts. Just don't forget to eliminate one of the overlapping red lines or the laser will cut it twice.

Chad Veich

My Feedback: (60)

That is a handy macro dreadnaut. Did you create it yourself? I generally change the color of the original line to one that we do not use for cutting. That way I do not have to delete the original line and I can tell at a quick glance whether I have remembered to offset the part or not.