A few questions about cutting birch ply and UHMW

[guitarwes]

I can't seem to get my bits to stop screaming at me when I cut 1/4" birch ply. Full depth pass at 3ips and fluctuating between 11500 and 13000rpm adjusting to try to find the less sqealing Hz. I'm new to cutting so I error on the side of conservative with feedrate. I usually profile cut this 1/4" birch for those squirlly letter doorhangers that are popular. Am I just going waaaay too slow with my feedrate? Also some of the big radius corners are a little chattery where it looks like the bit skips down the wood. I've tried using compression mortise, spiral up, and downcut bits. I use screws for my hold down method with tabs.

On the UHMW, I cut some for the first time last night. I cut 65 parts out of a 1" thick piece using a new upspiral single O-flute bit from Centurion. All of the recommendations I've found said go 1.5-1.7ips at 18,0000rpm. I cut it in 2 passes and my first pass was .51 deep. THIS STUFF MAKES A MESS! GOOD GOSH! The bit was squealing at me at first and I lowered my rpms to about 16000 or so but it still had a good amount of squeal and a good amount of chatter on the smaller pieces I cut. I was using screws as my holddown and tabs. The chips on the first .51 deep pass packed into the groove pretty tight and looked like they were ejecting about halfway out of the groove. This is a no-no, but I tried to rake out the chips from the first pass as the machine was going around for the second pass thinking that there may be some rewelding happening, which really didn't happen. Should I feed faster or decrease my pass depth and feed faster?

So, any suggestions? Thanks guys.

[jTr]

A search for feeds/speeds will produce dozens of links and advice on this topic. For a quicker answer:

Your conservative approach is wise, but probably too slow on feed speed. Seems the common wisdom is if the bit is screaming, feed it more.

Typical for me is a 2 flute bit .25" diameter running 12,000 rpm and 5ips. Still a little noisy, but falls within parameters of most chip load calculators. This applies for any plys I use, and many solids.
I usually do not drop to the 3 ips range unless cutting 1.5 - 1.75" thick hardwood slabs. Though there are multiple shallow passes, kerf can get very packed with undischarged shavings, so I like to keep it slower to avoid breakage as that brittle 1/4" bit is digging deep on final passes.

Sorry- no plastic cutting experience to share.

I would stick with a down cut for cleanest cuts in .25" material. While I'm sure it's already on your list, I highly recommend a vac table ASAP. This will not just be convenient, but will likely reduce noise and chattering, as it pulls the work piece tight to the table. This will be especially helpful stabilizing and controlling the thin, flexible materials you're currently working with. First time you use it you'll be completely amazed.

Hope that helps.

Jeff

[gundog]

I can't answer your wood question but I cut a lot of UHMW I usually feed 1.7 to 2.5 IPS at 18000 for 1/4" bit 1 diameter depth usually 16000 for my 1/2" bit don't expect a great finish other plastics will have a nice edge finish but UHMW always seems to have a little chatter on the edge finish. I have tried feeding faster slower higher and lower RPM it just does not produce a nice edge finish on the router. I use vacuum hold down. UHMW also has a huge amount of thermal expansion - contraction it is great stuff for certain products that need to be tough in a wear type application think conveyors cable shives Etc. I use UHMW on my anchor brackets for it superior wear properties. I use HDPE plastic for most of my products it cuts better looks better and resits warping and holds closer tolerances while being machined. Unless you need UHMW for your application I would suggest a different material if edge finish is important. UHMW does better on edge finish in my milling machine or when being turned on the lathe. UHMW is very grabby when cut on saw so make a wood fence with a near zero clearance so the piece does not cant when cutting or it will kick back worse than any piece of wood.

Mike

[Davo]

Just posting here cause I think it's good information... Maybe this file is already on this site and I don't even know it..

https://db.tt/nRg0P1FB

[jerry_stanek]

Normally a screaming bit is a hunger bit try doubling your feed rate You can adjust the speed with the shift + > keys while it is cutting.

[guitarwes]

Thanks guys. I had this gut feeling all along that I was feeding to slowly, but being a brand new shopbotter and not wanting to screw something up royally bad I was just babying it until I got more comfortable with it.

GREAT link Davo! thanks!!!

Gundog, I've done a lot of reading on your posts about cutting plastic and have learned a lot (just haven't applied much of it yet). Your products are very nice and hope to get into some of that some day as I'm near the Ga coast. The parts I'm cutting have to be UHMW at least 1" thick. The business I'm cutting them out for has another guy with a smaller Shopbot that cuts smaller parts and his edge finishes are great. I don't know what bit/feeds/speeds/holddowns he's using as he doesn't know that I'm cutting parts out also for the same business, so it's kind of a hairy situation.

[dlcw]

Be careful with increasing your feed rate to much. If 1/4" plywood is not held down with a strong vacuum system, you will get tiny vibrations of the plywood as it's being cut and that makes a LOT of noise. I cut 1/4" ply for drawer bottoms and cabinet backs at 5ips at 12K RPM. I have a strong vacuum holddown so the noise is not to bad. It does, however, make more noise then cutting 3/4" plywood at the same speeds.