Comparing PRT/PRS Resolution

[gerryv]

I currently have a 2006 PRT Alpha with 1:1 steppers. I've been "assuming" that a PRS Standard with geared 3:1 steppers would give me a noticeable improvement in resolution, especially when doing 2.5/3D carving, cutting arcs and circles, etc. Speed is not a priority. I've read here on the forum that this may be most noticeable on the Z axis.

I'm also "assuming" that my 1:1 alpha steppers likely depend on micro-stepping rather than gearing (which may do nearly/just as well? It also seems I've seen units older than my 2006 alpha that did have Shopbot installed geared motors - it's all a bit confusing

Recognizing the gantries are a separate topic, can anyone with experience on both machines offer some advise on what actual difference I'm likely to see?

[Brady Watson]

There are only 2 types of Alpha motors used on PRTs:

1:1 or 7.2:1 (same as PRSAlpha)

PRT Standards came with:

1:1 non-Alpha motors
3.6 or 7.2:1 SG series motors
3.6 or 7.2:1 TH series motors (practically the same as PRS Standard)

Newer PRS Standards run 2 amp 3.6:1 TH motors, which offer more torque than the older 'early' PRS Standards and late model PRT Standards that used 1.5A motors. That extra 0.5A makes a noticeable difference in torque.

If you just look at the unit values of a 1:1 Alpha vs a 4G or newer PRT S with 3.6:1 motors - the Alpha will have a UV of 1273.xxx and the Standard will have a UV of 1833.xxx - So, yes. You would gain nearly 1.5x in step resolution. Additionally, you will gain some smoothness because of the 25 tooth pinions with more teeth engaged in the rack on the Standard, compared to the Alpha with 20T pinions, which also tend to wear much sooner than the 25T.

In the long run, the gearbox motors can become sloppy, where as the Alpha 1:1 will not. I think the gearboxes have gotten much better over the years (Oriental) because my 7.2:1 Alpha isn't any where near as sloppy as my 3G PRT was before I retired it. The absolute ideal situation would be to run 3:1 belt reduction boxes on the Alpha 1:1 for an effective UV of around 3800 (3x current resolution) AND 3x the current torque, which is about 560 oz/in in 1:1 configuration. Of course, you really don't need that much torque and resolution, so I would bump up from a 20T to a 30T pinion which increases the life and smoothness of the rack/pinion quite a bit.

After about 2500 UV you start to reach the law of diminishing return. When I was beta testing the PRTAlpha 7.2:1 drive system before it hit prime time, I had a UV of 3819.xxx - this was just too fine and there was no noticeable difference in cut quality over the standard 2482.xxx resolution. The tool would max out around 6 IPS cutting speed. This is of course way before SB increased com speed in software etc.

So...on the surface, a Standard has more resolution...but I personally would never step backwards from an Alpha to a Standard. I'd rather step it up and go with belt reduction or just get 7.2:1 Alpha motors...but that's me...

-B

[richards]

There are so many apples to oranges comparisons between the standard and the alpha models that sometimes we get lost in the possibilities.

The PRT-Alpha 1:1 requires 1,000 pulses per revolution. The 4G had Gecko stepper drivers that requires 2,000 pulses per revolution. So, if PK299-F4.5 motors (wired bipolar parallel) were used with the Gecko stepper drivers (and if the drivers were remounted on a heat sink and if a 50VDC power supply were used and if the current limiting resistors were resized), a machine with the Geckos would have 1.5X the torque (880 oz*in compared to 560 oz*in) and 2X better resolution.

Having more torque would negate the advantage of the Alpha's "stall sensors".

Four PK299-F4.5 motors and four G201x stepper drivers cost about the same as one Alpha motor and driver.

Using the less expensive PK296-F4.5 motors and a 3:1 belt-drive transmission would give about 2.25X more torque than the Alpha 1:1 motors and 6X better resolution (0.00052 inches per pulse compared to 0.00314 inches per pulse on the Alpha 1:1).

Then changing the pinion gear from the 20-tooth to a 25-tooth would still give you 1.8X more torque than the Alpha 1:1 and 4.8X better resolution than the Alpha 1:1.

At 25,000 pulses per second, the PK296-F4.5 would have a top speed of about 16-inches per second which compares very favorably with the top RECOMMENDED speed for the newer Alpha 7.2:1 gearbox.

The fly in the ointment is that going the PK296-F4.5 w/Gecko route and belt-drives means that you'll be on your own to do the proper electrical interfacing and belt-drive construction. It's not a big deal if you're patient and willing to fix the mess-ups as they become evident. I designed several 3:1 belt-drives for the Alpha 1:1. Each one worked without a lot of rework. Later on, I designed a separate stepper interface that totally eliminated the Alpha motors and drivers. That took about one full month of working hours to design and test (because my health was far from ideal, it actually took several months on the calendar, but most of that time I was floating around in the pain-killer world). So, if an old white-haired guy who is high on pain killers can design the electronics and the mechanics to "hot-rod" a Shopbot, anybody can.

[gc3]

Here's the 3:1 ratio belt-driven-transmission cut out of aluminum on my Alpha.

Cutting speeds in the 1/2-inch thick 6061 aluminum were 0.60-ips XY-axes, 0.05-ips Z-axis, 12,000 RPM, 0.075-inch step down per pass, using a HSS 3/8-inch cutter designed especially for cutting aluminum. Those speeds required flooding the cutter with tapping fluid made especially for aluminum. If you look carefully, you'll see some edge marking in various places. That was caused by letting the cutter get too dry (thus getting too hot).

After using the 3:1 ratio for several weeks, it seems to work perfectly for the kind of work that I do. There hasn't been any problem with 'chatter'. The open frame has not been a problem - no dust on the belt or gears. In short, with the 3:1 transmission, my Alpha is cutting like a real CNC router.

above posted by Mr. Richards a few years back


...amazing what you can find online!

[fozzyber]

i built 3.2:1 belt drives, andI am verry happy, even more happy when i went to the pk299-4.5 motors.

[gerryv]

Thanks much guys, this gives me a better understanding of where my machine fits in. Also, thanks for the reminder Brady that as good as the evolving Standard machines are, I should not undervalue the ongoing benefits of my 5-year old Alpha.