MechMate CNC Router Forum

Go Back   MechMate CNC Router Forum > Personal Build Histories > Introduce yourself and start planning
Register Options Profile Last 1 | 3 | 7 Days Search Today's Posts Mark Forums Read

Reply
 
Thread Tools
  #1  
Old Fri 30 October 2009, 21:08
salewis
Just call me: Stan #67
 
Littleton, NC
United States of America
Another NC-USA, New Guy...

Hi everyone! I've been lurking, reading, and following the mechmate for a while. Still trying to decide whether to build a mechmate as I have roughly a 2'X4' cnc router. I've also built other cnc machines (foam cutters for friends in R/C modeling).

But, I have a couple of friends that need (rather want) a larger machine and the mechmate fits the bill. If I build one (pretty sure, since they'll help pay), I will build a 5'X10'.

I am located in North Carolina (watch out JR, I might want to see your's up close and personal) near the Virginia Border.

I have read much, downloaded what I could, working on a BOM (thanks to everyone who worked to get a BOM). Wise people count the cost....

I am also working with a laser cutter fab shop in to get the laser cut parts made (at least an estimate of cost).

Now for the first question....

belt drive vs gear motors....

gear motor more expensive and is working, but so is belt drive. belt drive is what I'm using on my current machine (with AC servos). So the question is simple, what speeds can one expect with belt-drive? I've seen JR's video, but I am not sure what drive (gear or belt reduction). Is the belt-drive slower (in terms of IPM) than the gear? Keling's motors are not 6 (8) wire so the half-coil option doesn't apply to those motors. I'm looking at a machine that is a good all-around (2D, 2.5D, little 3D) for a variety of materials (mainly wood and composites, some alu).

More questions as they come...

Stan
Reply With Quote
  #2  
Old Sat 31 October 2009, 08:31
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
The question requires a lot of "it depends". I'm going to assume that you have motors, stepper drivers and power supplies large enough so that the steppers have adequate torque to do their job.

1. What is the pitch diameter of your pinion gears?

If the pitch diameter is 1.25 inches (25-tooth), each time the gear turns, you move an axis 3.925 inches.
If the pitch diameter is 1.5 inches (30-tooth), each time the gear turns, you move an axis 4.71 inches.

2. What is your belt-drive reduction? 3:1, 3.6:1?

At 3:1, the 25-tooth pinion gear will move 3.925 inches / 3 = 1.3083 per motor shaft revolution.
At 3.6:1, the 25-tooth pinion gear will move 3.925 inches / 3.6 = 1.09 inches per motor shaft revolution.
At 3:1, the 30-tooth pinion gear will move 4.71 inches / 3 = 1.57 inches per motor shaft revolution.
At 3.6:1, the 30-tooth pinion gear will move 4.71 inches / 3.6 = 1.308 inches per motor shaft revolution.

3. What is the pulse rate?

Using Gecko G203v stepper drivers that require 2,000 pulses to turn the motor's shaft one time, Mach 3, when pushing pulses out at 45,000 times per second (2,700,000 times per minute) means that the motor's shaft will revolve 2,700,000 / 2,000 = 1,350 RPM.

Multiply the numbers:

1,350 X 1.3083 = 1,867 inches per minute
1,350 X 1.0900 = 1,471 inches per minute

1,350 X 1.5700 = 2,119 inches per minute
1,350 X 1.3080 = 1,766 inches per minute

At 1,350 RPM, most stepper motors don't have much torque, i.e., they're being pushed beyond the shoulder portion of the torque curve. I would multiply the numbers by 50% so that you're only asking the stepper motor to do what it can do best.

The last part of the problem is the resolution. Worse case, with a 3:1 belt-drive and a 30-tooth pinion gear, you would get 4.71 / 3 / 2,000 = 0.000785 inches per step. Anything under 0.001 inch is excellent for a CNC router that cuts wood.
Reply With Quote
  #3  
Old Sat 31 October 2009, 08:56
salewis
Just call me: Stan #67
 
Littleton, NC
United States of America
Talking

Ah, the "it depends" answer.... Ran myself right into that one AGAIN

I haven't both anything yet, still planning as I don't wont to come up short on the funds later nor buy motors (apparently single biggest expense) and then decide/go a different direction.

As for power supply, it would be whatever is needed for the motors (half-coil or full coil) and the drives would be G203V.

Pinion gears haven't been purchase (part of the larger question, I suppose) as I want good clean cuts (back to the it depends).

The question requires a lot of "it depends". I'm going to assume that you have motors, stepper drivers and power supplies large enough so that the steppers have adequate torque to do their job.

Looking over the numbers...

For the 'fastest' moves looks like the belt ration of 3:1 with a 30 tooth pinion gear would yield about 1059 ipm (at 50%rating of steppers) with a resolution of 0.000785" which is probably not going to the "true" resolution considering TIR of router and alignment of moving parts.

Now the questions continue....

1) how does this rate with gearhead steppers?

2) is there a "good" base torque number to look at/for, in addition to inductance (for power supply rating)?

3) is this "accomplishable" with 4-wire motors? (I guess I'd have to look at the torque/speed curves).

I can look up the torque of the gearhead motors, but just wanted to double check that torque of gearhead as compared to belt-drive (figuring that 3:1 would triple the torque) was done. My initial thoughts are steppers in the 400-600 oz.in. (seems like you, Mike, posted this somewhere) in a suitable inductance for G203Vs would work (the question is would they work well).

Back to the "it depends," which is kind of like evaulating n-order polynomials with a couple of missing variables...
Reply With Quote
  #4  
Old Sat 31 October 2009, 09:32
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
A belt-drive has no backlash, but depending on the stretchability of the belt, it could possibly have so "rubbery" effect. Belts are available that have Kevlar strands that keep the belt from stretching.

A gearbox has to have a little slack or the gears will bind up. I use 7.2:1 tapered hob gearboxes on the Shopbot PRT-Alpha and have no problem with backlash. Most of the users here on the MechMate forum are very satisfied with the quality that they get with the SH style gearbox that comes with the Oriental Motor PK296A2A-SGxx motor.

With the proper components, a belt-drive multiplies the torque of the motor by the gear ratio. 3:1 gives 3X as much torque. 3.6:1 gives 3.6X, etc. The gearboxes on the Oriental Motor stepper motors are underrated for torque, even those TH gearboxes that I have on my PRT-ALPHA. Oriental Motor rates that stepper/gearbox at 80 lb*in. They rate the PK296A2A-SG7.2 at about 40 lb*in and the PK296A2a-SG3.6 at about 20 lb*in. A 3.6:1 belt-drive attached to a 450 oz*in motor gives 101 lb*in of torque.

My prefered motor is the Oriental Motor PK299-F4.5 wired half-coil, which gives about 600 oz*in of torque. The smaller and cheaper PK296-F4.5A motor, wired bipolar parallel, gives about 440 oz*in of torque. Parallel wiring produces more heat.

I've found that motors with Inductance ratings between 1.5mH and 2.5mH work very well when used with a properly sized power supply.

Assuming that you're cutting wood, you should realize that most people using traditional tools would be very proud of 1/32nd inch tolerance. That is about 30X worse than the nominal tolerance that you would get with a belt-drive. Also keep in mind that wood changes dimensions depending on humidity. I've learned to loosen things up a little so that I don't turn the part into split kindling when I use a dead-blow hammer to connect two parts.
Reply With Quote
  #5  
Old Sat 31 October 2009, 11:40
salewis
Just call me: Stan #67
 
Littleton, NC
United States of America
OK, Now I must ask about the performance of the PK299 vs PK296 other than heat produced, which is not a real issue as long as motor stays below threshold.

How does the voltage reduction feature of the G203Vs play into this?

I've seen the formula for the proper sizing of powersupplies which is important because too much is just wasted in the heating of the motor (as you et al have pointed out).

Stan
Reply With Quote
  #6  
Old Sun 01 November 2009, 09:47
Richards
Just call me: Mike
 
South Jordan, UT
United States of America
As motors get larger, generally they are slower to respond and their torque drops off faster. If you look at the torque charts for the PK296 motors and compare them to the torque charts of the PK299 motors, you'll see what I'm writing about. At the speeds that I run stepper motors (~500 RPM), either the PK296 or the PK299 works perfectly.

With a gearbox or a belt-drive transmission, current reduction doesn't seem to matter. I believe that current reduction WAS a factor on my non-geared/non-belt-drive Shopbot PRT-Alpha, but that was just a supposition on my part. Stepper motor drives use current reduction to reduce heat build-up. It is a standard feature/function on every stepper driver that I've ever used. Although the rate of current reduction differs between stepper drivers, the concept is a constant.

Heat is related to power. The more heat, the more the power that was produced. A stepper motor is built to handle a lot of heat. The forumla that Mariss has provided us keeps that heat within the specifications for the motor. Personally, I normally reduce the voltage by 10% to 15%, but that is only because I neither need 100% torque nor do I like to risk searing a finger tip each time I touch an exposed motor. The heat will not hurt the motor, it just makes me a little uncomfortable.
Reply With Quote
  #7  
Old Sun 08 November 2009, 15:46
salewis
Just call me: Stan #67
 
Littleton, NC
United States of America
when to move the discussion to the construction forum?

Ok guys and gals (I can't imagine that there are no women enjoying the devilishly fun nature of building/using a MechMate) now that my buddy and I have the "kitchen table parts" on order along with zslide (from castone, thanks leo) and laser cut parts from joe I have a simple question....

When does one move from here to there... the construction forum as I have several questions already, most are related to the chicken/egg syndrome and is eating me up..... (can't wait to the day we go get the steel..)

For example, does one drill the 7" beams (after using them to cut/grind rails) before welding (same question for the y-cross)? I think I know the answer (yes) but want to make sure that there is nothing left out of my reasoning.

Stan
Reply With Quote
  #8  
Old Sun 08 November 2009, 16:09
Jeffn11
Just call me: Jeff
 
Greensboro, NC
United States of America
Welcome to the party Stan - I'm in Greensboro - It's a great project - Don't worry after shes done you will wonder what to do first too.

Jeff
Reply With Quote
  #9  
Old Sun 08 November 2009, 16:13
chopper
Just call me: chopper
 
Big Lake Minnesota
United States of America
Quote:
Originally Posted by salewis View Post
Ok guys and gals (I can't imagine that there are no women enjoying the devilishly fun nature of building/using a MechMate) now that my buddy and I have the "kitchen table parts" on order along with zslide (from castone, thanks leo) and laser cut parts from joe I have a simple question....

When does one move from here to there... the construction forum as I have several questions already, most are related to the chicken/egg syndrome and is eating me up..... (can't wait to the day we go get the steel..)

For example, does one drill the 7" beams (after using them to cut/grind rails) before welding (same question for the y-cross)? I think I know the answer (yes) but want to make sure that there is nothing left out of my reasoning.

Stan
Stan,
you know you answered your own question.. cut and ground the rails, then I welded the channels to the legs and cross-members, straightened them out on the frame,( mine had a little twist in them) then drilled and tapped the rails, after the frame was assembled..
//chopper

Last edited by chopper; Sun 08 November 2009 at 16:18..
Reply With Quote
  #10  
Old Sun 08 November 2009, 19:19
salewis
Just call me: Stan #67
 
Littleton, NC
United States of America
Yep, my buddy and I are already dreaming... I've been dreaming since I built my first CNC in grad. school (I was bored, OK). I've built several CNC foam cutters, a flowing gas CO2 laser cutter, and a router.

Now it is time to get serious ;-)....

All this to feed my other addiction -> r/c aircraft.
Reply With Quote
  #11  
Old Sat 14 November 2009, 06:51
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Quote:
Originally Posted by salewis View Post
For example, does one drill the 7" beams (after using them to cut/grind rails) before welding (same question for the y-cross)? I think I know the answer (yes) but want to make sure that there is nothing left out of my reasoning.
You are asking when to do drawing 10 10 300 D. That drawing is nearing to the top of the pile of drawings than 10 10 300 W. You start at the bottom of the heap of drawings and work your way to the top
Reply With Quote
  #12  
Old Sat 14 November 2009, 12:11
salewis
Just call me: Stan #67
 
Littleton, NC
United States of America
This one statement is worth a whole lot of questions.....

This will indeed help me to plan the workflow as parts will come in at different intervals.

Stan
Reply With Quote
Reply

Register Options Profile Last 1 | 3 | 7 Days Search Today's Posts Mark Forums Read

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
From China,new guy delongcnc Introduce yourself and start planning 3 Wed 28 October 2009 18:05
"Nuther New Guy" in NW Illinois eclayton2 Introduce yourself and start planning 1 Wed 28 October 2009 09:52
Integrated PC & Control Cabinet - Wilmington NC cbboatworks Construction started, but not cutting yet 51 Sun 05 April 2009 16:19
Help for a null electric guy Gato Richy 702. Power Supplies 12 Thu 26 March 2009 00:38


All times are GMT -6. The time now is 03:40.


Powered by vBulletin® Version 3.8.3
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.