Hmmm - sure looks familiar!
I've been really happy ever since I filled mine.

 

Mark, I didn't expect any "credits" for the idea - I was just poking you in the "I told you so" ribs.

 

btw, lead shot works better. I got a 25# bag from BassPro and as I remember it added about 4 lbs. to the weight of the pressure rod. I'd weigh it, but it's 350+ miles away right now.

 

Mark, are you doing this with the two front rollers, too?

 

I just did mine, it was a SNAP to do it! Thanks Dorsal for the great idea, thanks mark and trish for the video!!! :mrgreen:

 

Will do.!!!

I did some cuts today and man the mod makes a HUGE difference in holding the media in place. I can get much closer to the edge of the media against the roller now. So I can use more of the media now then before, doing some light ply cuts tomorrow and I think for the wood and balsa this mod is going to be off the hook. :mrgreen:

-Peter

 

Does this pressure roller mod help prevent shifting of the foam at higher feed rates? I am having a ton of problems with the foam shifting now that I have increased my feed rates and have started to do a lot of pocketing and stepped bevel edges for control surfaces. I especially have problems with cuts far from the center of the foam. I have to decide between this mod and the spring mod and was hoping someone could tell me in more detail what these mods help improve.

 

I added sand to my roller and it really helps with the wavy foam but I still have shifting problems at higher feedrates. If I am cutting around 100 to 150 there are no problems. When the cutting speed gets up around 200 to 250 gantry motion tends to skew the foam slightly and if the pressure roller is moving at the same time it can push the foam side to side with no problem.

I think a lot of the skewing has to do with the slower spindle speeds I use to keep the noise down. I will do some test cuts with the Rotozip at full speed and see if the sand helped the skewing. I would like to increase the cutting speed more since the MKII can do rapids at 1500 with upgraded steppers just fine.

 

My observations have led be to think the skewing I've experienced was due to slippage of the drive roller against the FFF, despite my lead-filled pressure roller. I'm thinking about replacing the sandpaper with a foam tape, to see if that helps. Maybe I should just make another roller with foam, so I could swap them in or out - speed rate calibration should be within a couple percent.
OTOH, the wavy blue stuff varies in thickness front/back and side/side. Imagine how a fully tapered piece of material would track, if the left end of the roller was sitting on 10mm foam and the right end of the roller was sitting on 5mm stuff. If the piece was long enough, it would track in a circle!
What I'm saying is: As long as we're using crazy-wavy material, if it tracks straight at all, you're just lucky. :geek:

 

I do understand how it works but that is an excellent post. I tend to underestimate the cutting force on the foam and it does explain exactly what I am seeing. I was cutting some very long rectangles and all the skewing was from the right of the PP to the left. That is also the direction the gantry is moving when cutting the short edges of the rectangles toward the front of the PP.

 

I took a look at my rollers tonight and noticed they were all loose in the Y direction. This includes the drive roller which had about 0.042" of play left to right. In addition both upper pressure rollers were very loose in the Y direction and could basically slide on their shafts left to right. Maybe this is part of the reason I am having shifting problems during cuts :?: This looseness was probably not a big deal when I was doing once around a part perimeter cuts, but now with traveling back and forth doing lots pocket cuts and control surface bevels I think the looseness of everything was starting to stack up.

So ...I went to the hardware store and got a bunch of washers and 1/4" set screw collars (saw these on Marks MKII ) to hold everything in place better in the Y direction. I also made a shim from 0.0325 thick x 1" brass stock to take up the slop in the drive roller. I have not tested this set up with cuts yet, but will report back soon.

I will see if this collar/shim mod reduces skew and then add sand to the rear pressure roller and test again.
Attached files

 

Tested my MKI tonight with the new collars, washers and brass shim on the roller shafts. Man what a difference!

All my cuts are now spot on dimensionally! One set of tab slots is spot on to another cut edge even after running back the foam back and forth through the rollers over 14 times between each. All the Y direction slop in my rollers must have been the root cause to my shifting problems.

For the test I cut 1/4" blue foam at a feed rate of 50ipm with a 1/8" dia fishtail cutter.

Next I am going to add steel BB's to the rear pressure roller. This should put my roller weight somewhere between lead shot and sand, maybe that will be the perfect balance of weight? I am hoping I will be able to speed up my cutting feed rates a little with the additional roller weight.

 

I totally agree with you Randy. It seems like there is a balance between how much holding grip we have with the rollers and what kind of chip load we have based on what cutters we are using (diameter, flute shape, depth etc.) and the feedrate used. I would imagine a certain flute size and shape would be optimum for the fastest possible feedrate without overcoming the grip we have on our rollers. I am guessing the worst condition as far a skew having the potential of happening is when we make a 45 degree cut at a high feed rate. This is because we are rolling the foam through the rollers in X and applying cutting force in the Y direction at the same time. The time where skew would be least likely to happen would be when we make a cut straight in the Y direction since the foam is not rolling through the rollers.

Maybe a good skew limit test would be to cut diamond shapes with all 45 degree cuts at progressively faster and faster feed rates. It would be cool to create a whole sheet of these diamonds and then step up the feed rate each time one is cut and then measure the final cuts to see at what point the skew gets bad enough to be a problem. Then we would know the limits of each specific machine and could gauge the effect of mods or improvements to prevent skew! Hmmm I see some diamond drawing in sketchup in my future!