Accuracy

[richards]

Dirk,

Great spreadsheet!

[Brady Watson]

PRTs are 1/4 stepping. Alphas are micro-stepping (1/10). A PRT will have approximately 800 steps per rev (4 X 200 steps) and the Alphas have 2000 steps per rev.

Micro-stepping is a lot smoother when compared to it's 1/4 stepping variant...BUT real positional accuracy is the same. Any stepper is physically limited to 1/4 step accuracy due to the way a stepper's rotor is designed. Areas in between a 1/4 step position don't have a good magnetic lock, and as a result can drift anywhere in that (let's say) 12 to 3 o'clock position. Even with an encoder on the stepper, there is no gaurantee that you will land on a spot in between 1/4 steps.

As Mike R. points out, by increasing the gear reduction, you can minimize the effect that this would have on accuracy...at the expense of speed. If you had 3.6 or 7.2:1 gearboxes on an alpha...that would add some serious accuracy to the setup, ruling out other mechanical gremlins.

-Brady

[dirk]

Brady
According to your expanation above it sounds like your saying the owners of PRTs with gearboxes have better "real accuracy" then the Alpha because the Alpha's not geared. Is this what you mean?
Dirk

[richards]

Brady,

Gear reduction gives greater accuracy ONLY if there is no backlash. In my former life as a process control computer designer/programer, I ALWAYS used belt drives instead of gear boxes.

Gearing a motor increases torque and decreases the length of each step - depending on the gear reduction. Both factors are big pluses IF there is sufficient speed to run each axis as desired and IF there is no backlash.

I've been playing with 1/10 step drivers and motors for several months and still have not reached any final conclusions. Every test that I've run agrees with the White Paper released by Maris at Gecko, but in real life, I'm still playing with the toys.

As far as a general purpose CNC router goes, I haven't found anything that beats the Shopbot Alpha for speed and for precision - at an affordable price. When I add up just the hardware costs to build my own machine with ball screws, tight tolerance rails, servo or steppers with feed back, the price is several times the cost of an Alpha direct from the factory. Which means to me that sometimes it's better to tweak the Alpha as tight as possible and then get out the sandpaper to smooth the cuts when necessary.

[Brady Watson]

Dirk,
To be honest, I really don't know. The Alpha motors seem to be very unique in the sense that they have a VERY wide power band for a stepper. This could be partly due to the fact that they run @ 170v. I believe that the alphas also run different sized pinions than a PRT (smaller).

Mike,
That's why I said, "...that would add some serious accuracy to the setup, ruling out other mechanical gremlins" I should have clarified the 'gremlin' part!

Contrary to what belt manufacturers will have you believe about their 'zero-backlash' belts...they DO stretch and DO exhibit backlash in the real world. The ShopBot gearboxes on the PRT are the more expensive variety of gearboxes from Oriental with taper-hobbed gears and very low backlash. I am still impressed by my PRT motors & am sure glad that ShopBot didn't cheese out on the motors. I am not saying that one configuration is better than another. If belts were truly zero backlash, I would put money on ShopBot using them on their tools, over the expensive gearbox motors. I do agree that your machine is only as tight/accurate as your greatest degree of slop/backlash.

I suppose the big question all of us are asking ourselves is, just how much resolution (accuracy) do we really need on a CNC router? I could see us balking at .005" or even .0015" on a metal mill...but what are people cutting that need that degree of resolution? I would venture to say that my PRT's cutting accuracy (when run at the 'right' speed) is WAY better than the advertised .015" number. Repeatability of my PRT is excellent...granted I have make a few refinements to tighten up the machine, but all of the running gear is untouched, except for new pinions on all 3 axes replaced this year.

-Brady

[mikejohn]

Back to the theory.
If the published tolerances are plus or minus .015" then an "error" of .03 is possible, and still be within specs.
I still don't know if you also add the positional error, .02" (twice times published .01") to the .03 to get .05".
Take Mr. Doveys dovetail joint glue gap, where he feels .01" excessive. Is it not possible that you can cut tails and pins, and they will not even fit but still be within ShopBot tolerances?
This thread was started to investigate the theoretical accuracy, following ShopBots published data. I recognise its relevance (or lack of it) in day to day cutting.
I just want to know how they arrive at these tolerances, and what practical effect they have.


..........Mike

[mrdovey]

Brady...

How much accuracy do we really need? Good question - and the answer (IMO) depends on the work we're trying to do.

For American kitchen base cabinet boxes we don't need much. For euro-cabinets a bit more might be needed. For most signs we don't need much. For furniture-grade joinery like dovetails the requirement goes up several notches, because cut edges need to mate cleanly and not show glue lines.

I've found cutting situations where conventional/climb cutting choices at different parts of a joint seemed to produce better or worse fits - and I've had situations where, after trying to rout something on the 'Bot, I switched to a hand-held router and tinkered to get a (measured) depth of cut somewhere between 0.2500" and 0.2505" - because that was what was needed to make a joint meet requirements.

Like you, I'm seeing accuracy on my PRT that's much better than +/-0.015" - some of the stuff I've cut has measured closer to +/- 0.0015", but I only measure when I feel tolerances in the work are critical and I'd be hesitant to advertize that I could routinely produce work that met that tolerance.

If I had a factory-built CNC milling machine that couldn't do better than +/-0.0005" I think I'd be pretty unhappy with my purchase decision.

Mike...

On the theoretical side I'd guess that you're absolutely correct; but keep in mind that:

"In theory, there is no difference between 'theoretical' and 'real world' - but in the real world there often is".

There's a bit of wry humor that describes the process of "calculating to seventeen significant figures, measuring with a yard [meter] stick, cutting with an axe, and adjusting (as necessary) to fit."

In the dovetail example, the practical solution to adjusting the fit appears to be making a choice between: (a) climb-cutting both tails and sockets for tightest fit, (b) climb-cutting one side of the joint and conventional-cutting the other side for medium fit, or (c) conventional cutting both sides for loosest fit.

Sometimes, it's just a matter of figuring out how to do the "adjust to fit" part.


...Morris

[dvanr]

Brady,

Your comment,

"Micro-stepping is a lot smoother when compared to it's 1/4 stepping variant...BUT real positional accuracy is the same. Any stepper is physically limited to 1/4 step accuracy due to the way a stepper's rotor is designed. Areas in between a 1/4 step position don't have a good magnetic lock, and as a result can drift anywhere in that (let's say) 12 to 3 o'clock position. Even with an encoder on the stepper, there is no gaurantee that you will land on a spot in between 1/4 steps."

is interesting. Would this be a possible cause of rough cuts on curves or circles when the Bot microsteps around a curve or circle and is unable to hold (or drifts) at 1/10th microsteps?

( Sorry Mike for the thread drift )

[mikejohn]

Dick
No apology needed ( I am less ambivalent of Ozzies after last years success
)
The thread drift often gives better information than the questioner intended.
Maybe ShopBot will pop up with a definitive answer for me.
............Mike

[Brady Watson]

Dick,
Actually, I should not have used the analogy of '12 to 3' because it paints the picture that a 1/4 step is a 90° rotation of the shaft...not true. On a 200 step per rev stepper, there are 200 'grooves' around the rotor and magnet-backed grooves in the case of the motor. Depending on the position of the rotor, this determines if there is full torque (each 1/4 step) or a position of less torque due to the magnetic/electrical flux holding the motor's position. Think of it as a sort of 'electrical backlash'.

As far as troubleshooting your rough cuts on curves, there are a few things that I have encountered that contribute to this...although I do believe that (on an alpha) there would be a benefit to gear reduction to decrease low-speed cogging on the 1:1 motors. (Not that I have witnessed low speed cogging...but other 1:1 steppers would be cogging pretty badly with this type of load)

The machine really needs a good going over to check that ALL of the v-roller bearings are seated properly in all parts of their travel. Wiggle every part of the gantry & Z assembly (with steppers on) and check that EVERYTHING is tight. Even a little looseness will transfer vibration to the cut. The new alpha/PRT gantry is leaps and bounds better than the older style one that came on my PRT. A tight gantry can be a double-edged sword as it brought a lot of things to light on my machine. The old gantry soaked up a LOT of vibration. After I welded it, it immediately started to see ridging on the edges of my parts, even on stright cuts. This turned out to be my pinions. It only cost me $60 to replace all 4 to fresh ones and the difference was remarkable. (This is after 3+ yrs on my PRT doing a LOT of 3D work...back and forth rastering. If you have an alpha and use the machine frequently, inspect your gears for signs of wear. The increase in speed, and in turn force on these gears is pretty substantial. Depending on what you are cutting and how well you greased the tracks, it might be a safe bet to say that you'll want to swap these out with new ones in the 1st year...or at least inspect them for wear. The other big thing that adds vibration to the cut is a mushy table. The alpha-style reinforcement plates are a MUST even on a PRT. Try air-cutting a 24" circle with your hand near the end of the c-channel, feeling for Y-direction movement. This is *very* minor on an alpha...and of gargantuan proportions on a PRT with no reinforcement plates. I will be adding these to my PRT soon! I have a 5X16' PRT...and it does shake...and those shakes DO transfer to your cut. So check to make sure (by air cutting or while working) that the table isn't moving etc. An older PRT would also benefit from a new set of XY motor mounting plates from ShopBot. They are MUCH stuffer than the single piece of angle holding them right now and will increase your pinion life. ALSO...you will get better circle cutting performance out of PW/Pro if you use the Arcs post processor and limit the tolerance to .001" or more.

-Brady