Compression cutters and slight step at bottom of cut

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Hi all,

On most of our mass produced parts we take components from our flat table machines to a point to point machine which will do a skim pass on the edge and then proceed to do all the horizontal drilling, slotting, vertical drilling and other machined features. This process works great for us and is very fast. On low production items or samples we cut everything on the flat table to finished size with all the vertically machined features done at the same time. We then horizontally bore or edge slot by hand. The issue we have is that unless we cut far too deep in to our spoil board, our compression cutters leave a small step on the bottom edge of our parts. It is fairly inconsistent as it seems to be related to very small inconsistencies in the cutter. The easy fix is to cut .050" in to the spoil board, but we typically process a couple of lifts a day and at that depth we would have to change out the spoil boards every day. What are other folks who are heavily invested in nested base processing doing to solve this issue? It is especially problematic with pieces we are edgebanding. One thought we had was to do a .010" finish pass with a straight flute, but I'm concerned about tearout and of course it's just one more tool in the tool changer.

cabinetmaker

We started reverse onion skinning everything just recently because we have cranked up the feet per minute rate and seen a lot of slipping in the parts- we run a 40hp vac pump pulling about 28Hg though the spoil board. This stopped a lot of problems, inconsistencies in the edge and slipping.

We can go right to the the bander from the table no dressing. A lot of people will say its a waste to reverse onion skin, but since we cranked up the router, there really is no time to band and run the auto doweller. We have timed the cnc router.

We are running a 3/8 compression 3130xp from Vortex. We are running @ 800 inches per minute @ 16000 rpm and our life expectancy went from 30 sheets to 100 + per bit and less chipping in the bit tip from running so many pre-layed up sheets.

We flycut about once a day taking off .125 mm @ a time. Although it does not get the spoilboard completely cleaned, it does take a lot of the high points off and gets rid of the shine from the compression of the sheets squeezing the fibers of the mdf. We run a 3" flycutter, not sure on the feed or rpm, but we had to fool with it.

We replace the spoilboards about once a month when we are busy.

Some people are going to pre-mil on the banders to fight this.

Good luck

JM

We saw a similar ridge when onion skinning parts....do you onion skin everything?

Our solution was to make the first pass slightly larger than the desired final dimension, then the part gets cut to correct size as it cuts through the onion skin. We found the cause of the problem was bit deflection.

Our first pass is cut something like 0.015" too large. Then our final pass cuts to finished dimension while cutting 0.005" into the spoilboard.

We use a 3/8" 2 flute compression bit, 700ipm, 18000rpm.

Edward Forer

Member

We are running similar feeds and speeds to both of you using 3/8" two flute spiral compression cutters (Amana). We run on bigger parts 750 IPM at 18k. We cut through leaving .0075" an onion skin and oversize the pass by .010" and then run a finishing pass that goes in to the spoil board .010". This strategy helps keep our very small parts from moving. We get the most noticeable step on the face down side of the melamine. It is very slight but if we feed it through the edgebander it crushes the edge in and chips it unless we sand it off. Again, on production runs where we make a lot of the same part, going to the PTP machines makes this a non issue because we can run cutter deeper so that the up cut part of the cutter better engages the bottom edge of the part, the issue comes when we cut just past the bottom of the part.

JM

Ed,
Try running your oversize pass a little larger that the .010.

Your problem sounds exactly the same as mine until I played with my cutting parameters to get that little lip/ridge gone. We were seeing it on the bottom side of the part as well.

Try running a part with your oversize pass set at something like 0.040". If the problem goes away, you know its just cutting strategy. From there, you can start reducing the amount of "oversize" required until the ridge comes back.

I just confirmed my cutting parameters, and we run an oversize offset of 0.015". I believe any less than that and we get the ridge.

Also make sure your tooling is as far into the collet as you can get away with, and use the shortest tools you can get away with. This will help minimize bit deflection.

Edward Forer

Member

I'll give that a try. My suspicion is that the very tip of the cutter wears and that after a few boards it is sort of chipped back an almost imperceptible amount, but because we are only cutting a little bit in to the board the little area on the tip of the cutter that might have been sort of knocked back is leaving the ridge. That's why I was almost thinking about trying a straight flute on the cleanup pass but I am worried about chipping.

Snaglpuss

I know what an onion skin is but
what is a "reverse" onion skin?

cabinetmaker

sorry Return Onion Skin. CAM option for us to onion skin everything

I 'll get off the crack vial now

JM

Ed,
Give it a try. I cut well over 100 sheets with a single tool, and have never seen the tip chip as you describe causing the ridge.

I found it to be 100% caused by bit deflection.

We leave a 0.030 thick onion skin.

The cutter offset 0.015" on the onion skin pass. So the part width and length grow by 2 times that amount, or 0.03"

We then make our through cut right on the line, and cut 0.005" into our spoilboard.

We only resurface our spoilboard when we start to lose a little vacuum (small parts moving), or if we start noticing a little chipout on the bottom side of the panel beacuse it wont sit flat enough on the spoilboard. Quick guess would be every 30 to 40 sheets.

Edward Forer

Member

I'll try that strategy tomorrow. Thanks!

jerry

Member

I do a climb cut to the onion skin then a conventional cut to cut through

Bernie

Same as Jerry. Try it, your problem will go away.

Bill

I agree on the bit deflection but it is really the machine that is deflecting. Carbide I was told by Vortex doe not deflect, it breaks if there is any deflection.

Another strategy if you can do it is to reduce the chip load on the cutter to take stress off the machine.

jerry

Member

I agree on the bit deflection but it is really the machine that is deflecting. Carbide I was told by Vortex doe not deflect, it breaks if there is any deflection.

I have seen solid carbide .125 flex about .125 before they break caught some before they broke and had to slow down

Will Williamson

A few months back, here on the forum , someone from Vortex, said they were going to conduct a study on bit deflection, and attempt to measure it. I never heard anymore about it. I have a feeling the results were unexpected and the bits were deflecting more than expected. I run vortex tooling most of the time. Wear on the bearings and slop in the machine has to come into play on this. I recently replace some bearings on my Thermwood and noticed an improvement. And Thermwood added a second pass in the opposite direction feature to the control nesting software. That feature was a big improvement.

Evan

Member

I recently discovered the benefits of Jerry's method. Running a climb cut will deflect slightly away from the part, coming back with a conventional cut through the onion skin pulls the bit slightly into the part. No offset required. We started doing this method with small parts and it results in a very clean, even edge, ready for the bander. With a faster machine I would do it on all parts.

JM

Ed,
Did you ever get this figured out?

eforer

Tried this strategy today. Seemed to work well, but we were cutting pre-finished ply all day which reveals the issue less. We'll see how it goes tomorrow. I like the concept!

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