(Old) Lead Screw Upgrade Kit (OLD THREAD)

X-Carve Upgrades
191

It would be helpful to also state the power supply, motors, drivers and ustepping used to achieve the performance mentioned above in order to provide the complete picture.

Different voltage, current, inductance and ustepping result in different step time (and therefore RPM and therefore speed and acceleration).

What works for you might not necessarily work for someone else, unless we are taking about stock motors and controllers. And even in this case there is Gshield and X-Controller.

In any case, having some numbers for a given set of components used is good for reference and comparison.

192

All stock motors and using x controller. But on screws not belts.
I wouldnā€™t push this speed on belts as they would snap.
Iā€™m kinda hesitant to run at this speed all the time until further testing.
Iā€™m only getting this speed cuz I tuned my machine to within a hair of itā€™s life.

193

microstepping? $100 & $101?

194

i believe 1/4 step 100stp/mm
i felt the resolution to power ratio at that setting was good enough.

195

Critical speed was mentioned earlier in this thread, so letā€™s dive in a bit deeper.

Hereā€™s a nice definition:

Critical speed is the first resonant frequency (speed) of the rotating shaft. Resonance in a rotating shaft can be catastrophic and even break the shaft.

Hereā€™s a nice example:

Hereā€™s the formula for calculating leadscrew critical speed:

N=(4.7610^6) dr*C/L^2

where

N: speed (RPM)
dr: root diameter (in)
L: shaft length (in)
C: constant (0.36 for fixed-free, 1.00 for simple-simple, 1.47 fixed-simple, 2.23 fixed-fixed)

Notice the inverse square (L^2)? Doubling the length drops the critical speed by 4.

EDIT: And another interpretation of the above. Assume you use a certain length and certain thickness at a certain speed. If you wish to maintain the same speed and try 2x the length, you need to compensate with 4x the thickness.

Out of the three:

  • thin
  • long
  • fast

you can only pick two.

In our case, itā€™s a Tr8*8 leadscrew with a 6.20mm root diameter, 1000mm long in a simple-simple bearing configuration. Therefore

N=(4.7610^6) (6.2/25.4)*1/(1000/25.4)^2=750RPM

Manufacturers recommend speeds between 60% (conservative) and 80% (aggressive) of critical speed. The aggressive value would be

0.8*750=600RPM

Multiplying with 8mm/rev lead:

600*8=4800mm/min

If you run at 9000mm/min you are 87.5% higher than the max recommendation, 50% higher than the critical speed.

The bottom line is everyone is free to experiment, and most of us have learned great lessons by breaking stuff. But, please do not encourage others to do so without some sort of warning or ā€œat your own riskā€ note :slight_smile:

5 Likes
196

How would the formula change for a tensioned acme rod with center support and both ends supported?

197

These factors have been already taken into account by selecting the ā€œsimple-simpleā€ C value.

Hereā€™s a visualization of all 4 values.

EndFixity.png444Ɨ521 42.3 KB

198

U seem knowledgeable. Lemme ask this. If the center support is a constant variable since it keeps moving how would that affect the resonant frequency?

199

Knowledge is endless, so Iā€™m not :slight_smile: . I just happened to research the topic when I needed to select ballscrew thickness and speed a few months ago.

The formula above (for more details see here, here, here) only considers the screw length between the bearings and not the position of the nut. The calculated speed will even excite the natural frequency of the screw in the reverse configuration (driving nut, driven screw).

My understanding is that the position of the nut (as well as the time spent at the critical speed) will affect the amount of the damage. If someone can shed more light here or is willing to perform any FEA, they are welcome.

In any case, lead and ballscrew manufacturers advise to stay at 80% of the critical speed or below to avoid premature wear or failure.

EDIT:
One more thing to consider. When you hit a guitar chord your left hand is an end stop. When you rotate a lead screw the nut is not. The whole length will rotate.

200

Another idea for more stationary lead screw.
Screw doesnā€™t turn, belt driving nut turns instead.
Might interest.

201

Iā€™m trying to avoid belts completely.

1 Like
202

He means ā€œhawking.ā€ Two words that are often confused.

2 Likes
203

Doing neither :stuck_out_tongue_winking_eye: Itā€™s already released for sale.

204

Agreed, back in the day , the kid who sold newspapers on the street corner was known as a street hawker. ( I did that myself for a summer job one year in my youth)

205

yeah but what about stephen hawking?

206

Iā€™m not sure about his history as a salesman. Was a kid with big ideas, butā€¦

207

The frequency component is the vibration. This is a acoustical vibration that will cause the metal to crack or fracture. The sound comes from the friction and as well as the stepper motor. (Steppers make a squealing noise when they are driven.) This frequency is then transferred to the metal. All metals are crystals and have resonant frequencies at which they can shake apart.
Use of a belt to drive the screw will help minimize the acoustical from the drive motor. You still will have friction based vibration. So pay attention to the studies Elias did on this.

208

This seems like the right placeā€¦ would a tr12 lead screw be able to extend past the 1000mm mark?

209

A Tr12 has 8.5mm root diameter. So your critical speed at 1000mm length and simple-simple bearing configuration would be

(4.76*10^6) *(8.5/25.4) *1/(1000/25.4)^2=1028RPM

and your safe max speed would be

80%*1028=822RPM

Multiply this with your lead (mm/rev or in/rev) to find your safe max rapids (in mm/min or IPM).

One thing worth noting here is that depending on your power supply, drivers, motors, you might be able to reach this speed or your steppers might stall at a lower speed.

EDIT:

Do it yourself :slight_smile:

Hereā€™s an online calculator if someone wants to experiment with different lengths and thicknesses.

And here are a couple of tables for finding the root diameter needed for the calculation. Look for rod minor diameter. I usually take the max and then dial down from the critical speed. Use the min if you want to be more conservative.

http://www.amesweb.info/Screws/AcmeScrewNutThreadDimensions.aspx
http://www.amesweb.info/Screws/MetricTrapezoidalScrewThreads.aspx

210

Thank you. Is there a calculator that could help in determining max length of a screw?