rcflyer11

My Feedback: (4)

Hello Everyone,
I have a new kit laid out on 5 balsa sheets in AutoCAD R14. I learned that when fed into a laser cutter, the machine cuts in the sequence that object/items were drawn.

How can I identify the drawing sequence and then modify it for the most efficient laser cut sequence? Also, how can I tell which direction the cutter will take between break-tabs?

I guess the order would be to first arrange the cut sequence within a given part, such as a rib, and then arrange the cut sequence among parts within a sheet?

Any and all help is greatly appreciated. I plan to run these kits in lots of 50, so the cut efficiency is important.

Thanks,

CoosBayLumber

My Feedback: (1)

Randy

Well, about six years ago, I discovered what it took to make the movement of the laser cutting arm more efficient. Or lessening the "Light Off" time, and thus lessening the overall time a sheet of wood was set onto the vacuum table, and thus lessening the per hour rate. I can go to a favorite place with an armload of balsa and plywood and get it all cut in about 30-40 minutes, including the file conversion within the operations software. If anyone else hauls in the same ammount it would take a bit more than an hour, due to wasted set up time, or light out traveling.

First off, you have to be familiar with Autocad R11/12. This version was about the last to optimise for pen plotters. The subsequent versions were optimized for ink jet plotters. I have a few versions of Acad set up here, but in general always revert to the R11/12 standards. They simply cut faster than later versions. Using pen plotting software will also speed up the whole operation by about 30%. I have done several jobs with 300-400 parts in the semi-kit. They take maybe thirty sheets of wood per kit, and by reverting back to R11/12 pen plotting standards it takes about 1-2 minutes per sheet. The mention by others that 1/2 the cost of cutting is the wood is incorrect. By lessening the cutting and processing expenditures, the wood can become a greater percentage of those costs. I think in the end, it got down to those 300-400 part kits could be cut in about 30 minutes. Too many kits are done on a one off basis, and not looked at on a high volume basis. Saving a few seconds per sheet adds up in the final total. Including materials costs, shipping costs, computer conversion time, I had been getting sheets cut for as low as 90 cents, but most were in the $1.50 range. I also found that DesignCAD files seem to cut equally as quick as Autocad R11/12 files. You are not going to find out the magic bullet on your first ten tries, but will eventially come across the method as I did.

As for your other question, in R-14 you can do the old reverse direction command for a line on the parent part. This was maintained in Acad for those who work in CNC or G-code languages. As you will find there is a lot to learn and expend in order to determine waht lowers the costs. Some of your other comments is mentioned at my web site instructions for laser cutting.


Wm.

rcflyer11

My Feedback: (4)

Wm,

Thanks, but what I got out of this is that you know how to reduce laser time and that I need to figure it out. I guess if I could have done that I wouldn't have posted the question

Admittedly, there is much to learn. I still think there must be a way to see the draw sequence in an R14 file and change it.

CoosBayLumber

My Feedback: (1)

You must not have read the whole document at the web-site referred to in the earlier note, as it outlays information therein. There are quite a few "P's and Q's" which have to be watched when developing the cutting files before getting them ready to send off. There are some things which only work within an Autocad R11/12 environment, and these are what have to really be watched. To trying to use late model commands on earlier based equipment, will only blow the effort. This is why I stress using a reversion to the earlier standards. In the earlier mode you cut and paste the elements in the plotting file into the desired plotting (cutting) order. This is done at the EDIT mode. Sometimes the files get huge, like if you have 30-40 parts on one sheet, and are thus difficult to sort out. However, you will readily notice via coordinates and the -99 file commands where one set of lines begins and another ends. The -99 is the command for pen up or light out, -90 is for a curve center, and -9999 ends the whole project.

Autocad used to supply a plot optimizer, but with the ink jet printers then becoming popular in the middle 1990's, it was dropped as useless. I think also upon release 14, they did not automatically insert the ADI 4.1 drivers upon loading up from the CD-ROM. You had to manually select the option, or get one of the WinLine drivers. From these you could then edit the plot files. You can also get a plot editor at www.cadalog.com for free, and do the work yourself. It will not work on Autocad x.PRN files, but only x.PLT files, thus again why I stress you work at the earlier basis. I ship not only the DWG files, but the sorted and arranged PLT files to the laser cutter, as these cut so much faster than a standard unarranged file as you may be creating. Once you get used to it, it will take maybe 3-5 minutes to process the whole job, but will save you maybe 10 minutes per kit.

If you want to watch the development process, you need to have access to a pen plotter that you can alter the set-up and then see what is out of order. Just do an XREF over of all of your finished cutting files into a new drawing file. Then, set the plotter into motion and the lines of the pen will show you exactly to what order the pieces are getting drawn (or cut) in. Before you begin plotting, most of the plotters made during the early 1990's had a liquid ink pen option. Just set the speed to 2 or 3 IPS, and you can then watch the sheet be plotted out in the order in which it will be cut, and about the speed it will take also. The high dollar laser cutters will work the surface at about 5 IPS, whereas the lower cost one may chugg along at about 1 IPS. Use the onboard plotter timer, and you can tell how long the plot takes, and how many dollars you are going to get charged at an hourly rate. If there is a whole bunch of pen up movement, the timer still runs, and thus you still know you are paying for time without parts being cut out.


Wm.

rcflyer11

My Feedback: (4)

Wm,
Thanks for the info. I have only R14 and do not have access to R11 or R12. Also, I don't have access to any type of plotter. Does this rule out any hope of reducing cut times?

One cutter told me to convert everything to polylines which would help a lot. I spent a whole day doing that, and it did reduce his quote. However, other cutters are still quoting the higher cost... even with the polylines.

KenLitko

rcflyer11,

I guess it depends on how you are able to work with your laser-cutter. When I quote out a job, I base it on total line length and a few other factors... not actual time on the laser cutter. I would waste more time quoting than actually cutting if I did it any other way.

If you are going to be getting runs of 50 kits cut, I would ask your vendor to cut a sample kit and quote it based on your current drawing and the actual time to cut. This way you have a REAL baseline. Maybe your vendor will charge for this and maybe not. Then, try optimizing your drawing and see how much time you can save.... unless you have something to compare, you're optimizing before you really should.

Just something to think about.

rcflyer11

My Feedback: (4)

Hi Ken,
Thanks for info and advice. A few people have quoted for me, based on "phantom cut" with the laser beam turned off. Since the costs are running more than double the cost of the wood itself, I'm thinking that my files aren't very efficient for the laser.

At one point, I converted everything to polylines which appeared to help with one cutter. However, the other cutters came in at the same cost with the polyline version that the first cutter came in without polylines. All were quoting via the "phantom cut" method.

I anticipate at least a couple of iterations of test cuts just to get the dimensional fits worked out. My first cut had zero offsets for laser kerf, and the parts fit was way too loose. I have since offset everything by .003" for the kerf.

KenLitko

That is probably not the best measure of efficiency since it will depend on the geometry of the particular parts you are cutting. Also, every laser cutter will have a different cost structure on how they quote you, from differing hourly rates to different material markups (or none at all), to inspection charges, etc...

Straight line cuts are the easiest. Splines are also very speedy. Circles can be problematic, depending on how the machine interprets the geometry. I generally go by the total line length when quoting... this has an averaging effect and doesn't punish any one person for a bad drawing. When it comes to as many kits as you are talking, phantom cuts or an actual cut is best.

I've seen some machines (my own included) occasionally jam the head and lose it's zero position. This results in offset cuts after the jam occurs.... and worthless parts. You can't see this when you do a phantom cut. Usually it means that you need to re-position a part on the drawing. Don't trust phantom cuts before ordering 50 kits... get a sample.

That's exactly what you should be doing. Kerf will depend on the material/thickness-> 1/4 inch balsa will have a wider kerf than 3/32 inch. Don't forget too that micro-cut wood is not perfect. 1/8 inch wood is "about" 1/8 inch... sometimes a little less, sometimes a little more... so don't worry about getting a super-tight fit... leave that to the modeler.

rcflyer11

My Feedback: (4)

My concern isn't about the thickness-related fits; rather, the length and width of laser-cut keels fitting into their respective laser-cut notches in the mating parts. Those fits should be relatively controllable.

CoosBayLumber

My Feedback: (1)

I seem to have different interests as to what model A/C is to be reproduced and cut up than others. They usually are small ones which require a great deal of builder's effort or are of the now "in-between" size A/C which is those having a WS of the 36 to 72 inch size. I don't like large A/C as they don't fit into the trunk of my car.

My A/C get developed in stages, in that I have a prototype cut, or maybe a second one too, and then a final. For example, last month I completed one of the Between size, with 72 inch span, using thirteen different full sized sheet files, made up of 1/8", 1/4" balsa, and 1/8" and 1/4" plywood materials for a total of nineteen. Average cost for materials and cutting was $2.36 each sheet and it takes nearly an hour to cut one kit.

I used Bob Holman for this as he has a large sized cutting machine which will handle Acad files through R-14 directly. He obtained a special driver from Kern which works in AcadLT, and will compact the linework and create the driver for cutting files in about 2-3 minutes. That first prototype kit took a full 90 minutes to cut. No effort was made to streamline the operation at my end, as it was moreover for check on quantity of parts, fit and precision. Afterwards, the modifications for better buildability were taken and I took aims to optimize the cutting proceedure, compact the parts, and where possible make one line cut along two parts. Often times I forget to add in one part, and include too many of another. Although I have done several thousand parts now using Acad, I still have trouble with the inventory for a kit. Omit one part, and all hell breaks loose.

My smaller electric powered A/C, take a bit of thinking too. I may need 1 1/3 sheets of 1/16th per kit. So I cut three prototypes, such that I use the entire stock of one sheet. Also using the laser to cut into irregular shaped sheets if necessary to save on raw wood costs. Most of these light weight A/C will cost me about $5 per semi-kit for materials and cutting.

I purchase wood from one of the popular advertisers in the magazines, and they are not too consistant as to thickness. As mentioned, don't get too concerned as to fit, as the wood shrinks and swells before and after cutting. However, most plywood is metric based, not fractional and thus runs a few thousanths thinner and you do need to adjust tollerances there. At this point, don't get concerned with kerf as much as count, for if the wing chord was 0.003" smaller than designed, you can make up for such by specifying a thick Mono-coat covering.




Wm.

DICKEYBIRD

Hey Wm, you shouda put a after that one! I 'bout choked on my coffee reading it!

ps: Thanks for all the tips....I'm not likely to go into kit production myself but understand exactly what you're talking about. I only produce parts for my own designs in a production run of ONE on a home-made CNC router but it's neat for you to share your experience with us.