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Old Wed 21 March 2012, 06:28
Just call me: Mike
South Jordan, UT
United States of America

A single-stage belt-drive limits you to a ratio of 4:1 maximum, and that's if you use an 18-tooth XL pulley on the motor's shaft and a 72-tooth XL pulley on the drive shaft.

Fifty pounds X 16 ounces = 800 ounces. A PK296-F4.5A motor wired bipolar parallel produces 450 oz*in holding torque. When it is geared 4:1, it produces 1,800 oz*in. Even if it is wired half-coil, it would produce 1,200 oz*in.

In comparison, the PK296A2A-SG7.2 motor can only handle slightly more than 1/3rd of the holding torque (about 700 usable oz*in). Many MechMaters use that motor to drive gantries that weigh much more than 50 lbs.

Contact Mike (Metalhead) about belt-drives. He offers everything that you'll need, including Keiling motors that cost less than the Oriental Motor steppers that I use. (The Keiling motor that I tested was a very nice motor.)

The biggest problem that you're going to face is building a one-of-a-kind machine. I don't know anyone who got the design perfect the first time around on any electrical/mechanical device. Be prepared to spend much more than it costs to build a single machine. Look at all of the posts showing the various MechMate builds. Read carefully the posts that showed modifications to the basic design. Read even more carefully how many times costly mistakes were made when the basic design was changed.

My advice is to build a standard machine first. Your only major modification would be how you mount the cutter. Making that single modification would be much simpler than redesigning the gantry.
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Old Wed 21 March 2012, 06:48
Just call me: Sean - #5, 28, 58 and others
Orlando, Florida
United States of America
...ditto on Mike's advice.
I heavily modified #5 machine after I had a perfectly working standard machine.
- added a 48" z-slide with 36" of usable stroke
- added a 96" x 48" usable 4th axis in the table bed (Yes, that is a HUGE block of foam)
- any guinea pig for many other things you see in the forum today.

I can tell you, I used the working machine to make the parts to modify it - like many others here on the forum.

Clearly easier and more cost effective. Not to mention, fewer headaches.

Good luck with your choices.

Last edited by smreish; Wed 21 March 2012 at 06:51..
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Old Wed 21 March 2012, 08:00
Just call me: Ken
My rule of thumb in designing a machine from scratch which works 1st time.
If you need to ask for advice, don't bother.
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Old Wed 21 March 2012, 12:06
Just call me: Alan (#11)
Cape Town (Western Cape)
South Africa
Send a message via Skype™ to Alan_c
Oh Ken, you crack me up. If we followed that line of thinking we would still be walking behind donkeys!
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Old Wed 21 March 2012, 19:51
Just call me: Guillermo
Puerto Ordaz
Hi Mike.

Very grateful with your advice.

Hope to see you in few days in BOB thread.
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Old Wed 21 March 2012, 21:32
Just call me: Ken
Alan, We sure didn't build our stepper motor from scratch...
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Old Thu 22 March 2012, 16:43
Just call me: Mike
South Jordan, UT
United States of America
There are many ways to build any device. A good cook knows how to alter a recipe to make things just a little bit better. Someone who has built a lot of mechanical things has a "hunch" about the best way to do things before he ever puts pencil to paper. I have designed stepper motor projects since 1982, so I have seen many of the things that work and some of the things that don't work.

When I designed my first printed circuit board, I made a lot of mistakes. Even after having made more than 100 original designs, I still make careless errors. Whenever possible, I start a design by looking at something that I know will work and I change only those things that must be changed. It keeps the possibility for error lower.

There are probably dozens of stepper motors and stepper motor drivers that could do an adequate job. None of them are perfect. There are always trade-offs between size, price, and availability.

When I'm building a "one-off" project, I first check my "junk box" to see whether I have a motor that might work. If it looks like it might work, I start with that motor. Then, after trying that motor out, I can easily tell whether another motor might be better suited to the application. There's nothing magic about it. It's mostly trial and error, based on a little math and a little experience.

This forum has all of the information required so that anybody, who is willing to follow instructions, can build a good workable CNC machine. It assumes that the newcomer has never built a CNC machine. It assumes that the newcomer has never worked with metal or with electronics. It assumes that the newcomer has never done anything more than "wish".

By reading the information and then asking questions, anyone should be able to be successful.

Many of you have to translate the posts from English into your native language. I spent two years in France and Belgium, so I know how hard it can be to follow technical articles when little of the vocabulary is understood. That's why we're here. What is clear to one person may not be clear at all to someone else. I hope that nobody gets the idea that they should not ask questions. Each of us had to ask a lot of questions when we first started out.
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Old Tue 26 February 2013, 05:22
Just call me: Brian
United States of America
Here's a short paper that does a reasonably good job of describing the interactions of voltage, amperage, inductance and speed on stepper performance:

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Old Tue 26 February 2013, 07:27
Gerald D
Just call me: Gerald (retired)
Cape Town
South Africa
Thanks Brian.

"Adding more supply voltage to the drive is the only way to increase the critical step rate and supply more current to the stepper motor, so more torque develops at higher speeds."

If you have selected too high a voltage, giving you very hot motors, then it is simple and cheap to reduce the drive currents.

My advice today would be to select voltages about 20% above the Mariss formula and then fiddle with drive current settings to keep the motor temperatures under control.
(This applies only for MechMate router tables - based also on lots of practical experience from various users)

See also this thread:
Confusion on power supplies for Pk296A2A-SG7.2 motors . . . . .

Last edited by Gerald D; Tue 26 March 2013 at 09:56..
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