increasing resolution

[jon]

I have the 7.2 gear head steppers on my bot, and now I am looking to further increase my resolution. Does anybody have any ideas of the next step? I am seeing the resolution 'chatter' on aluminum parts cut on the angle (using both x and y axis to cut a vector). I would like to see those lines go away and for my parts to resemble things made on say a Haas or other metal specific CNC.
Has anyone installed helical rack on their machine? I'm not sure if its the straight toothed racks and gears that are causing this chatter, or the stepper motor resolution.

[Gary Campbell]

jon...

If you have the 7.2 gearmotors on your bot you are not suffering from a lack of resolution. Possible pinion lash or some gearbox lash inherent with all gearmotors that increases with use.

"I would like to see those lines go away and for my parts to resemble things made on say a Haas or other metal specific CNC."

You can reduce the edge chatter, but if you want "HAAS cut" looking parts, you must buy the HAAS. You cant expect a 20K+/- router table to cut like a 80K+/- machining center.

You may be getting some chatter form the pinion/rack combo, but usually not noticeable enough to justify the cost of helical rack. I am not sure if the motors you have will work with helical rack, due to axial loads.

You ask for the next step, but you do not say which model or year machine you own. PRT or PRS, standard or alpha?

See picture below. This is about the best you can expect to get from a router.

[jon]

Gary, I have a 2005 PRT alpha upgraded with; 7.2 gear motors on all 3 axis, new control card, hsd spindle, and lots of bracing and extra bearings riding on BWC hardened rails. New pinions and the racks are not worn.

I am in the process of building a single y axis gantry beam to further stiffen the chassis. I am hoping for a motor upgrade, (at the expense of speed is ok for me). Best case is using current drivers as well. Thinking of helical rack because I feel the vibration from it when I touch the machine as it runs. Backlash on the motors when I tug on them seems consistent and ok.

[Gary Campbell]

jon...
I would suggest that you use a dial indicator and find the play in your machine. In most cases you will not be able to feel an amount that will return scalloped cuts in aluminum. I am attaching a doc that shows setups for a PRS, use it as a guide to check your PRT. Especially the lower Z rollers when the z is down near the table where you usually cut.

[jon]

Thanks for that Gary, I will look into that first. Here is a recent part cut from 0.5 inch material

[Gary Campbell]

jon...
Make sure you define if or how much deflection in your machine and remove all on the mechanical side. I would be more than willing to offer you belt drive, motor and drive upgrades, but in most cases the OEM 7.2 alpha motors will not limit cut quality as much as the adjustments and the condition of your rails and rollers, especially if you have the 4 roller (vs 8) Z assembly.

It does neither of us any good to throw hundreds (or thousands) of dollars at something and not improve the cuts

Carefully inspect the pinion to rack interaction. In some cases the brackets that hold and pivot the motors are not orthogonal to the racks, allowing one end of the pinion to "dig in" causing a toothy sound.