View Full Version : To calibrate or not to calibrate ?

12-12-1999, 09:18 PM
I just set up my sb (finally) and I noticed when I sent the y axis to 48 it only made it to about 44 inches. I thought that the rack n pinnions did not require calibration. Well I calibrated the y axis and then it was right on. The values for the y axis were -114.5916 and the recalibrated value was -127.3609. Logically it there should be no calibration needed, is
-114.5916 the correct default value?

12-13-1999, 08:22 AM

My guess is that you have different sized pinion gears than the default, which makes the calibration figures different. This is probably more information than you're looking for, but for an example if you have the aluminum gearboxes with belt drive, a 72 tooth internal drive gear, and 20 pinion gears, the calibration settings are 127.32395, which are mighty close to yours. I have 18 tooth pinions, so my calibration numbers are 141.47106. The pinion gears can be changed to give you more speed or more power, but the calibration figures have to change to fit the new gear ratios.

My suggestion would be to email ShopBot with the particulars of your gear boxes (type of gearbox, number of teeth on your pinion gear, etc.) and they can give you the precise calibration figures.


Ted Hall, ShopBot
12-14-1999, 06:53 AM
Hi David,

Bill has steered you correctly. No calibration is required, but you do need to know the gear ratio for your specific tool and axis. Here are the current list of ratios and Unit Values. They can be found at the back of the User Guide (downloadable from the web-site FTP area). They are also printed on the ShopBot QuickRef Sheet (also there in the FTP area). [We'll be sending out a copy of this new User Guide to all ShopBotters shortly.]

Unit Values for GearHead Motors:
Gear Type...18 tooth pinion...20 tooth pinion...22 tooth pinion
Std 3.6 Ratio....127.3240.....114.5916.....104.1742
HiRes 7.2 Ratio....254.6479.....229.1831.....208.3483

Unit Values for Thrust Box Motors (with Belt drives):
Large Pulley...18 tooth pinion..20 tooth pinion..22 tooth pinion
Yel 60t 3.33:1.........117.8926.........106.1033.........9 6.4575
Blk 72t 4:1...........141.4711..........127.3240.........1 15.7491

Here's how to derive the Unit Values FYI. The Unit Value for X & Y ([VU]) = #steps-per-unit(e.g.inch) * 0.25

To make your computation, the basic thing you need to know is that there are exactly 400 steps per revolution of the motor.

This all means, for example, that for a Unit Value of 120 there are 480 steps per inch ... if you are running in inches.

The Unit Values for Z and Acc are just = #steps-per-unit ... so a Unit value of 2000 means 2000 steps per inch.

12-15-1999, 06:29 PM
I entered the correct values according to the specs, but over the span of about 20 inches the real world messurment were off about .06 inches. Could this discrepancy have anything to do with the distance the pinnion gear sits off the rack? ie the further away from the rack the greater the diameter of the pinnion. I ran the calibration routine and its correction made things work just right. I'm having other trouble though. I pulled out my handy fish scale and my Shopbot is only pulling about 25 pounds before it lurches backward on the preasure of the fish scale. My old cable drive had about 40 lbs of force. I did the tech notes modification of the resistor on the driver boards could this be a reason for my decrease in power? The stepper motors are the original ones. My voltmeter is checking in at about 28 volts on each of the leads to the stepper motors. Oh one more thing I also only have one driver on the x axis as per the original.

12-16-1999, 09:51 PM

Not sure what's going on woth your calibration, but I experienced the same power decrease when I changed resistors. I played around with them because I made some other drive changes (from cable to a timing belt drive system) and wwasn't sure what was causing the problem. I have gone back to the mid range resistors for now. I also have a 5" fan blowing over my shopbot control board so there's lot's of airflow over the driver chips and their aluminum bar heatsink. In the future I plan to add heatsink fins from radio shack to the aluminum bar and possibly going back to the original resistors (i used them for three+ years with no problems) to get back to more power and maybe higher jog speeds before stalling. I do have two drive boards for the x axis so the problem is not as noticeable as it might be.

Good Luck


12-18-1999, 09:41 PM
Has anyone tried using a bigger stepper motor then ShopBot currently uses? I was doing some research and found out the amplifier on the controler boards can handle 3 amps and the current steppers are 2 amp. Can you use a 3 amp 2 phase stepper motor. I found one that has 611 oz-in holding torque. The one on the currently shipping ShopBot has 340 oz-in. And the Upgrade stepper motors for the thrust boxes is 187 oz-in.
Any Ideas about this?

Ted Hall, ShopBot
12-20-1999, 07:58 AM
Hi David,

Jim's suggestion about moving the resistor values back down will help some. This is especially the case for your X axis that has one board for two motors -- it should be a lower value. But we also worry that the larger power supply is delivering more effective current to the drivers and thus running them warmer than they were for you previously ... so check the temperature.

The rack and pinion system has approximately the same gear ratio as the cable drive, but it has considerably less friction. So you should have no problem achieving the same power that you had with the cable system. You need to be certain that you are comparing the power at the same speed. We usually reported the power of the cable tool at .5in/sec. Make sure this is the speed you are running the R&P version when testing. Then decrease the resistor values a bit.

Note that the reason for switching to the higher voltage power supply is so you can achieve higher speeds. The power of your tool at slow speeds will not be affected by the voltage of the power supply. You will, no doubt, be able to run your Y and Z axis much faster with the 28v power supply. But the X axis will not benefit with both motors being run on the same driver board. These top out at about 2in/sec with one driver.

On the question of bigger motors ... well, with steppers you can always get more speed and power with bigger motors and higher voltages ... however, the prices for power supplies and drivers also go up, and at a much faster rate. Keep in mind too, that you are always struggling with power and speed, a motor that delivers more power, may have coils that are less efficient to load and thus require a higher voltage in order to get full speed.

So speaking more practically, you will need to aggressively cool the drivers if you decide to run them higher than the approximately 1.8A we are currently operating at. On the issue of motors (see my discussion of this elsewhere here), we like the 300oz motors because they give a good bit of power (about 80 lbs cutting force) and pretty good top speed while still operating at reasonable currents and voltages. The 187oz motor that we also use is a high performance motor that while not as powerful as the 300oz at low speeds, holds its power to much higher step rates (e.g. the power curve drops off more slowly) and so it works well for us too. Thus we are working at about 80 lbs for the 300oz and 60 lbs for the 187oz with jog speeds in the range of 3-4in/sec, which seems pretty reasonable.

I don't know why your calibration is still not correct. This is not a problem we've had with R&P tools or conversions, to date. Perhaps we have mistablulated one of the values for the setup that you have. But it should be a fairly straightforward issue of Steps and Gear Ratio.

12-20-1999, 09:36 PM
Hi Ted, Thanks for all the great information. I think I'll probably stick with the 187 oz motors. The power/price ratio seems the best and the speed is good too. :-) I have a couple more questions about the motors. The oriental catalog list the voltage for all of the above mentioned motors as very low around 5 volts. Why the higher voltages? I also noticed a motion dampening device that can be mounted to dual shaft motors. Would this be of any value with very short segmented lines and quick moves?

12-20-1999, 10:10 PM
Mr. Fisk:

Something I can answer here. Stepper motors have
something called INDUCTANCE. This basically is
resistance to changes in current. Current flows
from center "tap" to one end, then as the motor
takes another step, the current has to reverse

Now, the higher voltage allows the motor to change
the current's direction "quicker" because there
is more force to change (voltage).

Manufacturers of stepper motors rate their motors
in "volts" but what they are really rating them is
in current. Now, at 5 volts, a motor with say 2.5
ohms of resistance will flow 2 amps of current.

If you were to hook up 5volts to said motor, it
would give you the rated holding torque, but as
you started to step the motor faster and faster,
you see that the actual torque output of the motor
would fall DRASTICALLY, because of our enemy
inductance which want to resist the change in

Enter the ShopBot's driver. It works on principle
of "chopping". Instead of supplying 5volts to the
motor, why not supply 25V (or more--but 25v for
simple math) and apply that voltage 1/5 as often.

What the driver does is "sense" a set current
level and shuts off at, in our example, 2 amps.
Since the voltage is higher, the drive is better
able to overcome the inductance and give more
torque at a higher speed of the stepper motor.

Lastly, motion dampeners are designed to act as
flywheels (or mechanical inductors) which help
smooth out rough stepper motor motion. They are
generally used in applications where there is
very LIGHT load and/or LOW friction.

In my personal experience, I have not needed to
use motion dampeners, but here is what one stepper
motor manufacturer recommends. Put a Jacob's
style chuck on one end of a dual shaft stepper
motor. If the motor runs smoother with less
vibration and/or you notice an improvement in
output, then your motors are candidates for motion

Hope this helps,

Bruce Clark
bwclark@centurytel.net (mailto:bwclark@centurytel.net)

01-08-2000, 04:30 PM
Ok, so now I have installed the extra driver and the 187oz motors. I still have the resister modification in place. The machine is doing better it has 40 lbs of force before springing back. But not close to the 60 lbs it should. Also my mysterious calibration problem went away with the change in motors. I did not touch the thrust box mount I only removed the old motors and installed the new ones. Should I just live with 40 lbs or should I go back to the old resistors or ....?

07-06-2003, 06:36 PM
How do you figure out what your unit values for the PRT should be if you have 7.2 high res gear head with .25 degree steps and a 20 tooth pinnion gear? One of the posts above says 229.1831 but what about the differant microstepping settings?

What should they be for 1/2 stepping and 1/4 stepping?

What will the actual resolution be in each mode?

07-08-2003, 10:18 AM
PING!! Does micro stepping affect the speed and Unit values?

07-09-2003, 12:00 AM

From what I know - very little but have built several different CNC machines, the micro, macro, 1/4 step, 1/2 step and other 'stepping stuff' have NO relationship to the UNIT values or SPEED.

I haven't resolved resolution. I just know the machine is far more accurate than I am.

07-09-2003, 12:39 AM
Thanks Ron, I just was curious.
Ted Hall had wrote earlier in this thread... "Here's how to derive the Unit Values FYI. The Unit Value for X & Y ([VU]) = #steps-per-unit(e.g.inch) * 0.25

To make your computation, the basic thing you need to know is that there are exactly 400 steps per revolution of the motor.

This all means, for example, that for a Unit Value of 120 there are 480 steps per inch ..."

Soo . . . I was wondering, since this formula has in it the number of steps, if micro stepping effects it?
Should the 400 steps now be 800 with 1/2 stepping or 1600 with 1/4 stepping? I'm also curious what distance the machine is moving for each step or micro step. And since my steppers are the .25 degree per step type what the ultimate resolution of my machine would be in each micro step mode.

The PRT upgrade has been a tremendous improvement in the smoothness of the cutting. I can see the differance in the way the machine moves. Much less jerky and a lot less vibration.

07-09-2003, 08:03 AM
There have been many changes in gear rack and pinion that will also effect "unit value". I have "Home-Built" belt reduction boxes and Martin gears and racks on my machine with one of the "new" PRT Boards. I have a 24 Volt power supply which I will sometime upgrade to a 48 volt supply. ShopBot cannot compute my unit values. I just run the calibrate program and refine with a 12" dial caliper.

The folks at ShopBot are working on the "WinDoze" software right now. They are claiming this will be an upgrade. I like and use the "preview" mode of the Beta software and the new Typesetter program.