[3Dsigns]

I wired my house, 3 shops, do all of my own mechanic work and most of my computer upgrade and repair. I set up my own shopbot but This will be my first time actually building a CNC machine. I was contemplating buying one of the cnc kits on Ebay. Has anyone used one of these? Are they any good?

This is one that I was looking at....thanks!

5Axis Nema 23 stepper motor 425 oz & Driver DM542A CNC Router LONGS MOTOR | eBay

[ger21]

The worst thing you can do is buy motors and drives without even having a machine to put them on. Almost everyone that I see doing that ends up either replacing them, or living with a poor performing machine. Motors and drives need to be matched to the drive system to get the most out of them. Poorly matched components will result in very poor performance.

As for that kit.
The 425oz Nema 23 motors are almost never a good choice for a router. They are high inductance motors, and require close to 100Volts to get the most out of them. At 36V, you'll only be using 1/3 of their power.

[3Dsigns]

Thanks ger21

I just finished building the main part of my machine, yesterday. and will post a photo when I get back to the shop Monday. It's a 10'7" x 8'4" x 6' high hotwire foam cutter. The only resistance any single motor will have is to move about 30 to 35 lbs along the horixontal x axis. In the vertical y axis, each motor will just be mainly raising and lowering it's own weight plus a little extra to lift the wire, a linear bearing, and a small aluminum wire mount assembly. If any other resistance is encountered, such as an event where the wire loses heat and touches or stalls in the foam, I intend to install limit switches to solve that.

I will make the final decision on the motors after the y axis is installed and sliding on the rails. The control system will need to have five separate steppers because one will be an optional turntable lathe but, I mainly just wanted to know if the Chinese kits are any good. Are the drivers, breakout board and steppers any good? If so, do you know the best place to get a bigger power supply?. Thanks again for the help.

[ger21]

What's really important when choosing motors, is how fast you want to go. That and the drive components should dictate what motors and drives you want.

I mainly just wanted to know if the Chinese kits are any good

That depends. They can range from very good, to complete garbage.

Check Automation Technologies for good quality Chinese components. Automation Technology Inc. | Stepper Motors, Servo Motors, CNC Stepper Motors, Stepper Drivers, CNC Kits, Stepper Kits Automation Technology Inc

[3Dsigns]

What's really important when choosing motors, is how fast you want to go. That and the drive components should dictate what motors and drives you want.


That depends. They can range from very good, to complete garbage.

Check Automation Technologies for good quality Chinese components. Automation Technology Inc. | Stepper Motors, Servo Motors, CNC Stepper Motors, Stepper Drivers, CNC Kits, Stepper Kits Automation Technology Inc

Thanks
This machine will be moving quite slow; in the inches per minute category when cutting. Someone even suggested using reduction gears on the steppers, but I would like a lttle faster when rapids are needed for setting up and things like that. The guide system will be v-groove bearings and rails with a parallel adjustment feature. For drive, I will be using timing belts, which is what many of the 'storebought' machines have. Racks would be overkill on this.

I had thought about using a Gecko g540 to control it that will only handle 4 motors.
If I knew which of the Chinese products are not junk that would be good to know.

[H500]

I don't like the motors. A 9 mh inductance is 3 times higher than it should be.

[3Dsigns]

This is the basic frame. I have yet to add the lathe, turntable framing and the upright axis/gantry.

[109jb]

judging by the size of that frame, those motor aren't going to cut it.

[3Dsigns]

This one uses little 160 oz/in servos. They must be geared way down?

8700 CNC Specifications - Foam Cutter Hot Wire 3D EPS CNC - Architectural Concrete/GFRC Molds & Theming | Hotwire Direct

[H500]

Servos do not lose torque at high speeds. A 160oz servo can put out much more power than even a large stepper when properly geared.

[aarggh]

I would have thought for a light frame, hot wire foam cutter, even 280oz/in steppers would be overkill? Is this not the case?

cheers,
Ian

[3Dsigns]

Thanks for the input everyone. I looked at alot of servos and the cost is going to be a big factor with that.

If I use small steppers, would it be best to use reduction gears to get smooth accurate cuts at very slow speeds? and still get decent rapids when needed?

Would a 1:10 reduction be about right?

or would these work? The only drawback on this system is it doesn't have the 5 motors I need. 4 Axis Stepper Motor CNC Router / Mill Electronics Kit, Gecko G540 | eBay

If I go with small steppers, in the 200 to 400 range, which would be the the right Gecko drives? The g540 only handles 4 motors. Thanks

[109jb]

You would be better off with direct drive. Especially with high inductance motors. Torque on a stepper drops off as RPM increases. Too much gearing and themotor has to spin faster than it really wants to. You also run into problems with how fast the computer can talk to the driver.

[3Dsigns]

That's what I was hoping for: No gears. Are high inductance motors a bad thing?

[H500]

High inductance is bad for most systems. The only exception is when you need high torque but don't need speed.

I'm using 287 oz motors for all 3 axes on my sx3 mill. The head weighs 115 lbs. It gets 100 ipm with a 2:1 reduction. Do you really need high torque?

[3Dsigns]

Thanks
I tried a v-groove bearing on one of my homemade v-rails this afternoon and it rolled very smoothly. I don't see the machine needing high torque motors either since the gantry/upright axis is much lighter than the head on your machine. I was mostly concerned whether the steppers could move slowly enough to cut foam with a hot wire. And, I was concerned about whether a 5/16" shaft on a NEMA 23 motor could handle the tension of 10' long timing belt pulled tight.

Are you using gears or belts to get your 2:0 reduction?

[H500]

I prefer to minimize the load on the motor shaft. It should not be used as part of the machine structure.

There's no limit to how slow you can step a motor. Gearing is unnecessary unless you need more torque or resolution. The top speed of a low friction machine like yours will most likely be limited by resonance (motor vibration).

My x and y axes are direct drive. The z is gear driven, but I don't recommend it, due to the noise. I had a set of gears on hand so it was a convenient way to experiment with different ratios. I will be using a HTD belt drive. The pulleys are very easy to make.

[3Dsigns]

Thanks
how do you make pulleys?

[H500]

The tooth profile is cut using a 1/8" ball nose endmill. The Gates catalog list the dimensions.

[3Dsigns]

Cool!
Thanks