Burning issues

[precaster]

I'm cutting 3/4" ac plywood on a prt alpha, and having some difficulty controlling the smoke and burning... as well as breaking bits. I'm running 1/4" Onsurd 2-flute upsiral at 12000 rpms/1.7-2.0 ips and steping down anywhere from .1-.35. Not sure if that information is helpful, or even if I'm using the correct terms, I'm pretty new to this machine, and running a shopbot in general. Has anyone else run into this, and if so, could you offer any solutions?

[richards]

Rob,
At 2-ips, 12,000 RPM and 2-flutes, your chipload is only 0.005. I like a chipload of at least 0.015, which requires a feed speed of 6-ips. At low feed speeds, I use a 1-flute cutter and still run at about 3-ips.

I've found that a cutter can dull in a very short time if the chipload is too low. A dull cutter will burn its way through the wood.

[harryball]

Agreed, you are cutting too slow. I cut 3/4" ply at 14k RPM with a 2 flute up cut at 5 ips in 2 passes. That's about 0.11 chipload, which I've found to be OK for 2 passes.

If your bits are smoking and heating up they are probably already dull, I'd start with a fresh bit at the new speeds and pass depths to see the best results.

Robert

[precaster]

Thanks for the feedback. I have tried a new bit, 1/4" 2 flute upcut, and increased the feedrate to 5 ips at 14000rpm's, and seem to still be getting a significant amount of burning, and the machine is actually "stuttering" which causes a fault. Is there anything else I could try to resolve this?

[fleinbach]

Rob,

Still getting smoking!

Guys this is the same Shopbot I asked the question about smoking while cutting HDO. Now it seems it is smoking on the AC plywood as well. This is very strange as I know the bits are new sharp bits. There is still another problem and that is with the computer. I had a difficult time getting it to recognize the usb port. It may be necessary to connect a USB hub. When I ran the speed test it always failed. Even though it failed the Bot still is able to get through a complete file.

[beacon14]

It could be the router - I had some strange cutting behaviors that went away when I swapped out the router for a recently re-built one.

[fleinbach]

David,

This Shopbot has a Columbo 3HP spindle and has had very little use. They picked it up from a leasing company.

[harryball]

Have you checked to make sure your RPMS are correct? Not some way it could be turnning much faster than you think. If it's burning at the bit on contact with wood... it's friction. Also, is the travel speed correct? i.e. are you really getting 4 or 5 ips or something slower and don't realize it?

Robert

[donchapman]

You are on the right track to solving the problem by eliminating all other possible causes until you're left with what must be the actual cause.

If you are sure you are using the correct sharp/clean bit at the manufacturer's recommended feed rate and rpm, the problem must be either with the material being routed or with the router itself somehow causing the bit to wobble, which would leave a kerf wider than the bit.

You can compare the bit and kerf with a caliper to eliminate bit wobble as a cause, which would leave you with the matierial itself being the culprit.

Over the years I've found all sorts of problems with plywoods including high moisture content, bad glues, weird trash contaminants, voids, etc.

Some glues can be very abrasive and some filler plys (especially in cheaper and lower quality plywoods but sometimes in better grades) can have many knots and lots of resin which can dull or gum up router bits and saw blades very quckly.

High moisture plywood and solid wood grabs and burns bits and blades too. I test suspect materials with an inexpensive moisture meter and generally don't rout any wood or plywood that registers any moisture at all.

Try routing a similarly hard/dense material (e.g. MDF or a hardwood plywood) and if the other material routs ok, the problem must be with the AC plywood itself.

[bill.young]

As Robert suggests make sure that the tool is actually moving at the indicated speed. With the problems indicated by the failure of the speed test and the "stuttering" I'd bet that the speed you're actually moving is slower than indicated.

You could try a different computer to see if that helps, but a call to ShopBot tech support is probably your best bet.

Bill

[precaster]

Thanks guys, I'll be putting a call in to tech support tomorrow to hopefully get this speed test issue resolved. The material problem might make sense, the trouble seems to occur in the same areas of the wood after it cuts the first pass with no problem, so maybe it's a knot or something in the inner plys...

We're using the parts we're cutting to fabricate molds for wet and "dry" cast concrete, which, before we acquired out bot, have been building by hand out of AC-plywood and HDO (high density overlay). MDF might be a way to go, except that it's porosity and moisture absorption after it's cut and the abuse our molds get have led us in the direction of AC.

Once we get through these few emergency jobs we'll try out some different materials, and I'm sure I'll be posting more


This was my first post and I appreciate all the input. Great forum, great resource.

[harryball]

Rob, some stuff you might try if you get bored which would help you answer some of the questions...

try running an air cut, no bit installed running the full length of the table. Make a square about 12" x 96" and cut it in 3 passes. Run at 12 ips. Time how long it takes for the bot to ramp up as it heads to the end of the table and them ramps down to turn the corner. On my bot it's about 9.5 to 10 seconds and it's smooth. I'm tuned farily aggressive for ramping, an acceptable typical result might be 11 seconds. The theoretcial time is, of course, 8 seconds without ramping.

Watch it closely, if it start bucking and stuttering stop and check your PC, that's a definate sign your communications are too slow.

If it runs smooth but you find it takes more than 10 or 11 seconds for it to start moving and reach the other end of the table you could have a speed problem or your ramping could be set too conservative.

In fact... have you checked your ramping values? Under Values - Ramping in the shopbot software... I think default is all .4 for all ramp rates on a standard PRT Alpha.

Hope that helps you stumble onto something.

Robert