This wing is 36" long along the X axis and presents a severe challenge for the machine since it has matching indents along both sides of the parts, tabs and slots that all must match up perfectly in order to assemble the wing.
With the addition of the new guides and some careful preparation, these parts came out flawlessly.
By careful preparation, I mean that the 6" strips of balsa need to be planed down to exactly 6" or 5.95 so that both sides are exactly parallel. This was easily done by dropping a burr bit down past the depth of the balsa and bringing the Y axis in to just shave a small amount of balsa as it was fed into the rollers and kept butted up against the opposite edge. Then once that was done, I could place the opposite guide rail in place and secure it so that there was zero side to side movement the entire length of balsa.
After sweet success, I decided to get out the video camera and do a video tour of all the mods I have made.
I added one more mod after shooting the video, and that was a delrin plate to hold the Z axis bearing in position. I had that on my list of things to do, but it had slipped to the wayside.
So if you have 23 minutes to spare, check out the video on youtube...
Great blog, and very helpful for diagnosis of skew problems! I've recently finished building a Phlatprinter 3, and I'm pretty sure that I'll have to deal with a skew problem. A quick question: would the addition of a fence make your previous modifications of the pressure rollers unnecessary?
ReplyDeleteThe best solution is to get the rollers true. A fence does not always guarantee a fix. If there is a lot of skew, the side force on the fence eventually means something has to give. It puts tremendous pressure on the material and at some point slippage must occur between the material and the roller.
ReplyDeleteDifferent materials generate different skew problems. 1/2" foam skews worse than 1/4" for instance. There will probably always be some skew present no matter how hard you try otherwise.
Uneven pressure from the upper rollers can also contribute to skew.
I don't use my fence much, but have used it on balsa from time to time to try and keep skew to a minimum but I find using a good flat sacrificial base such as 3mm Depron gives me the most accurate cuts when cutting balsa or other materials. The rollers grip it well and the tight tolerance of the Depron depth help keep the pressures equal from side to side.
It is a hobby quality machine. That is the best it will ever be. It is fine for cutting out foam planes and even some wood projects, but overall, the accuracy due to slippage and skew is unacceptable for precision cutting.
Thanks for your honesty and your objectivity! My main objective is to make reasonably accurate bass relief sculpture on 1.5" thick balsa, but I'll be adding a "skin" of wax or clay for the last 1/8" or so (after the CNC work is finished, of course).
DeleteIn light of your observations, I'll true up the rollers as much as possible, and use the 3mm Depron as a sacrificial base. Probably replace the current springs on the rollers with some with a smaller k value, so there isn't so much normal force on the balsa.
I'll let you and your readers know how it turns out. Thanks again for all your work!