Plywood fiber springback

[dansley]

With the advent of hot humid summer weather I am experienceing a fiber springback when cutting 1/2 and 3/4 inch plywood - poplar and luan respectivly. Inside corners seem to hold tolerance but as the cutter (1/4-dia single and double flute down sprial) leaves the corner the fiber spring back resumes. Parts end up being 0.025" +/- too big in most cases.
Has anyone else grappled with this and if so have you developed any structure means to account for it - i.e. table based on moisture contect for reducing part drawing dimensions?

[bleeth]

I just experienced a similar problem and noticed that the springback was happening whenever the bit changed from cutting completely through the material to having less than full width to cut. I assume that this is due to less resistance being met when the bit is only, in essence, cleaning up rather than completely cutting. (it is all being done full depth. On one set of parts I had to go to a double path leaving 1/8" for a cleanup pass and on the other my tolerances allowed me to go ahead and just hog it all. There is a definate bounce in the car on the y rails when the spindle goes from full cut to partial cut. It's irritating as it effectively adds twice as much cutting time to multiple parts machined out of sheets and I had to do 2o sheets with 12-20 parts each. I expect that this is less of a problem with the car redesign for the alpha, but I'm running a PRT. Any solutions beyond double pathing guys?

Dave

[dansley]

I think your diagnosis is closer than mine - some form of y-axis misery. I just encountered another version of this same problem I think.

I need to cut 3/4-inch plywood into strips 1.58" by 87". To reduce cutting I created a dxf drawing that sent the cutter on one continuous machine along vector zigzagging path 1.83-inches apart. Then two runs across the y to cut the ends of that we're not cut. A consequence is that each stick has a climb cut and a conventional cut which may be exacerbating the y-rack problem if I understand things correctly. What I got was half the strips at 1.56" and half at 1.63".

I hate the idea that I must do half depth cuts to avoid this. I am running a PRT as well and if this issues proves true I will have to do something to stiffen the carriage.

[bleeth]

It seems to be more in the hold down system of the carriage(or lack of it) than anything else. I've been tweaking the adjustments to no avail. Since I've been using a 1/2" compression bit for panel cutting partial depth is not an option.

This could be the consequence of value designing and not readily repairable. The location tolerances of the machine in general become secondary to stiffness under load in this type of project. I have noticed for quite a long time when cutting panels with large arcs that the pieces were not perfectly symmetrical either. The problem seems to be getting worse with machine age (or perhaps my fussiness).