Bit Chatter on xy moves only

[barrowj]

Hi,

I have just gotten my Shopbot setup and running finally (bought a used 2004 PRT Alpha w/3hp Spindle last month). I was cutting a piece with some really nice curves (move speed 1.0, spindle 13000) using a 1/4" bit and the wood is Bradford Pear. The straight cuts were fine but any curves had chatter and you could feel the chatter on the machine.

Any ideas what would cause this? I've called the tech support team quite a bit lately and figured I would come here first this time.

Thanks for any help.

[scottp55]

Joe, is the machine tight with no play? Have you tried shallower/faster passes and played with feed/speed? Some woods even 500RPM and .1IPS makes a big difference for me.
Have you thought about offsetting VERY slightly and doing a full depth pass to clean up the .015-.02" "skin".
Just thinking and not caffeinated:)
Got pics of problem area. We can do a lot more from a pic.

[barrowj]

Scott,

Thanks for replying, I haven't played with that yet but the chip carving has a second cleanup pass. This is a Vintage Wooden Spoon Wall Rack from the project site on Vectric. I just went with their settings and while I have cut a number of straight cuts, I haven't tried anything like this ever.http://www.joebarrow.com/download/chip%20carved%20piece.jpg

[barrowj]

http://www.joebarrow.com/download/Chip%20Carving%20Problem.jpg

[Brady Watson]

If your Alpha has the AS911 motors, you are never going to completely get away from some 'edge chatter'. However, dollars to donuts your pinions are probably DRY and WASTED. Those early Alphas had 20 tooth pinions and they don't last very long. Even less if they are not properly greased. Call SB & order (4) 20T pinions & make sure they know the motor model number.

Replace those pinions, grease the racks and tighten each motor turnbuckle only 3.5 turns from being just loose. Then your cut quality should be way better than what you are getting now.

-B

[scottp55]

Joe, What depth of pass were you running? Should be able to get a lot cleaner than that.
It's VERY uniform and one of the full size guys should be able to help more.
The more info we have the better. Bit/climb cut? etc.
should be able to get a little more like these. I'd rather the machine cut longer and me sand less, but I'm lazy:)

[scottp55]

Oops, Brady with better advise:)

[barrowj]

Scott/Brady,

I have just ordered new pinions, glad you mentioned it. I remember John (the seller) mentioning that it might need new ones but they looked good. The rack was very dry when I got it and a layer of rust on the rails so I cleaned it up and lubed them, then ran the exercise file (per Frank) to loosen it up. That helped a good bit and got me through the surface planing. The file I was cutting uses 4 passes so it wasn't a single deep cut. I also set the ramp too.

[barrowj]

All of you guys produce really nice work, hope to get to that level. The last shopbot I owned was only used to cut shelves ( I sold 100's of mdf crown molding shelves back then) so we were too busy to get into the really cool stuff. I sold it when my company moved me to TN from Atl , and stayed at the plants 90% of the day. My new position with my company (Hewlett Packard) let's me work from home so i'm not so tired at the end of the day. My shop is 75' from the house and I will be retiring in about 3-4 years so I want to start using this shopbot for more cool things, still it's a play toy right now.

[bobmoore]

Joe. Try programming your cut out to onion skin depth with an offset allowance of .020" and a climb cut. Use as many passes as you are comfortable with. Finish cut out at full depth with no offset allowance and a conventional cut. you should have a lot less sanding to do.
Bob

[Brady Watson]

Unless you have the pinions off of the machine and held against a new fresh one, you really cannot gauge how worn they are unless you have changed a lot of pinions over the years. Pinions on non-Desktop tools directly affect the cut quality and grease cushions the gear mesh absorbing micro-vibrations that get transmitted to the cut. Grease & wipe...and wipe and wipe until they are schlucking down the rack without throwing globs off of the gear. You wouldn't run the steering box or rack & pinion in your car dry would you? Protect & maintain your moneymaker...they aren't cheap.

To properly lubricate these tools, you need to apply a glob of grease every 3-6" and slowly run the tool back and forth the entire travel of the axis at slow speed (<2 IPS) and wipe the rack. Increase speed and do it again, incrementally up to a Jog speed of 15 IPS, wiping each time. It's steel on steel and nothing in a spray can is going to work as well as what the factory gave you for grease. There are plenty of alternatives to grease, but they all require routine maintenance that most will never do - so greasing them is cheap insurance and the path of least resistance.

For those re-greasing, rake the old shmutz out of the rack first with a brass or steel brush (cheap @ Horrible Freight etc) and then apply fresh grease. Popsicle stick works well for applying...WD40 to dissolve the stubborn parts. Caked in wood/foam/plastic chips in the rack DOES cause a positional deviation.

-B