[Newbie] Cnc Questions

[HEAVYMETAL87]

Hello everyone,

I am purchasing a PM727 and PM1022 mill and lathe. I was considering in the future making them where they could be used for both CNC and manual machining if that was possible. What would be the cost of making a basic, three axis CNC milling machine- roughly? I have a computer for it already, and had planned on using servo motors if I go this route.

Also, is there a way for a CNC machine to check to see if the cutting bit is still viable- so that if it breaks it simply stops trying to make the piece? Would a fourth axis in the form of a rotary table mounted so that the work piece was horizontal work in lieu of having a CNC lathe?

Sorry if any of this seems kind of dumb, I really don't know all that much about CNC- but it does look interesting.

Thanks all!

 

[wrat]

Also, is there a way for a CNC machine to check to see if the cutting bit is still viable- so that if it breaks it simply stops trying to make the piece? !

I've worked with this feature years ago, but i've no idea if it's common today. It also takes some extra lines of code.
The idea is to monitor the spindle motor amperage. If something is cutting, there's amps being expended. If a cutter is snapped off, the idea is that it's basically freewheeling. Trouble is, WHEN do you monitor? You have to load up the cutter flutes, then tell the machine to start the monitor. Then, before the cutter is unloaded, turn the monitoring off. Otherwise, it'll think every cutter is broken before it touches the material. It can't be made automatic (in any scenario we could envision). Worse yet, if the cutter is just torn up badly, you still get amps and the unmanned machine will plow through the part like all is fine. During roughing, this might be recoverable, but during some finishing moments, maybe not.
In so-called 'high speed machining' (as if anything was ever 'low speed'), or meltdown machining, this is easier what with only one cutter (or a few) doing everything in small and easily predictable passes.
While it's true that in most cases, there's adequate coolant splash that an operator can hardly see if things are going great, anyway. And often there's enough noise to not hear if things are doing well. Still, he can usually catch things earlier than any monitoring system.

Would a fourth axis in the form of a rotary table mounted so that the work piece was horizontal work in lieu of having a CNC lathe?

Not generally. Certain moves, yes. But not for most of what you might want dependably round.


Wrat

 

[JimDawson]

Sorry if any of this seems kind of dumb, I really don't know all that much about CNC- but it does look interesting.

When on The Hobby Machinist there are no dumb questions. We were all there at one time, we are here to help.

I was considering in the future making them where they could be used for both CNC and manual machining if that was possible.

Yes, that is very possible and highly recommended. My mill can be operated as a manual machine, or as a 2 axis with manual quill, or as a full 3 or 4 axis. When I CNC my lathe, I will keep full manual capability.

What would be the cost of making a basic, three axis CNC milling machine- roughly? I have a computer for it already, and had planned on using servo motors if I go this route.

I just replaced 4 mill sized servo motors on a CNC router, the parts cost was about $4000, including all of the cables and a new power supply. Add to this a CNC motion controller and software. Then there is the ball screws to replace the lead screws. I guess I would budget around $4000 for a 3 axis conversion on a mill. A lathe would be less because it is only 2 axis.

Also, is there a way for a CNC machine to check to see if the cutting bit is still viable- so that if it breaks it simply stops trying to make the piece?

Yes, it's possible, but would require a pretty sophisticated control system. Normally when the tool bit breaks the machine is just ''cutting'' air so no real problem. Really high end machines, $1,000,000 +, have systems that detect broken and dull cutters. This is not normally found on lesser machines. I thought about adding that to my mill, but I don't think it's worth the effort. It would be a lot of programming work and require a lot of experimentation to get it right. A better solution when running the machine ''dark'' (unattended) is to simply to mount a camera near the machine so you can watch the progress on your I-phone, maybe even with a remote E-stop.

Would a fourth axis in the form of a rotary table mounted so that the work piece was horizontal work in lieu of having a CNC lathe?

In some instances, maybe. I have found that trying to do lathe type work on the 4th axis is not really practical. It is mostly to do with cutter geometry. Consider that a lathe tool bit is more or less a sharp point, where a milling cutter presents a relatively wide flat surface to the work. Each machine is pretty much designed to do a specific type of work. There are high end horizontal machining centers that do both very well, but again you are looking at $1,000,000 + machines.
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[HEAVYMETAL87]

Holy cow! A conversion can cost up to $4000.00?
Is it possible to do one for less- or is there some surprise expense I am not aware of- aside from the servos, power supply, mounts- what other parts are required to do the conversion?

 

[JimDawson]

.............aside from the servos, power supply, mounts- what other parts are required to do the conversion?

When you are talking servos, you are talking some real money. The word servo implies a full closed loop system, where you have feedback from a positioning device via some type of encoder, and normally today when used in this context means BrushLess DC Motors. BLDC motors and drives are expensive. Just to put it into perspective, I did some major upgrades to my machine and the total was about $7500. And I already had the ball screws, and the X and Y axis motors & drives. I also went with full on industrial hardware, the motion controller card and associated electrical hardware was about $3000, then add about $750 for 1 micron linear encoders.

Cheap/ accurate/ fast: Pick two

Having said that, going with an open loop stepper system is much much cheaper. You still really need the ball screws for the X, Y , and Z axis, but they are not terribly expensive from an Ebay vendor.

Below is an example of a 3 axis stepper motor kit, if I were going to cnc your mill this is the specifications of kit I would use. This is just the first one I came to in a search, there may be a better price. I have used a lot of Wantai hardware and have been happy with the performance.
http://www.ebay.com/itm/Free-ship-w...701345?hash=item33bd7210e1:g:4KYAAOSw3YNXY6~h

The above stepper kit also comes with a breakout board and cable that is compatible with Mach3

Below is an example of a ball screw kit, this is only an example. I have no idea if any of these would work on your machine.
http://www.ebay.com/itm/Anti-backla...803347?hash=item3d07cbd2d3:g:KlUAAOSwAYtWNvOL

Mach3 CNC controller software is about $200, CAM software is about $150, then there is the CAD software. The good news is that Fusion360 by Autodesk is free to hobby users and is a full professional 3D CAD/CAM package.

The rest of the parts, mounts, and other stuff can be built in your shop.

 

[HEAVYMETAL87]

Honestly sir cheap and accurate sound like the two that matter to me the most.

I really don't mind slow all that much, mostly because it's going in my garage, which will be air conditioned, likely have a computer in there anyhow, so I think that I would be fine if it's not going to get anything done in much of a hurry. Just as long as it is a good piece when the job is done.

Is it true that a manual mill converted to be CNC/manual will never perform as well as a dedicated machine? I am not trying to build parts for NASA here, just live steam engines and every day little projects.

Which is easier to convert by the way- the lathe or the mill?

 

[Metal]

Heavy: dont let the cost dissuade, you, you have a mill, you can make many of the parts for the conversion WITH the mill that you already have!

you have two options
Everyone will suggest servo's, these are a closed loop system that can self sense their location, they are also expensive
Steppers only turn (usually 1.6 degrees) when sent a signal, if they can't turn due to load, the software will think that they did and you will get inaccurate parts.

I've done both now, I would highly suggest steppers, oversized for the job and geared down with a timing belt, if things go bad you can get bad parts, but they are /dramatically/ easier to setup, and cheaper

 

[JimDawson]

Is it true that a manual mill converted to be CNC/manual will never perform as well as a dedicated machine?

Not true at all. It all depends on what quality of hardware is used to do the conversion.

I'm not sure which machine would be the easiest to convert. There are less parts to contend with in the lathe conversion. But I have only worked on mills and routers thus far. The lathe is on my list, as is my surface grinder.

 

[HEAVYMETAL87]

The machine that I was looking at converting is the PM727M milling machine.
It's basically another Rong-Fu- 7x27, about 700 pounds etc.

I take it that with CNC instead of learning to manually machine and go a little bonkers from that, it's CAD that makes you a little bonkers with CNC?

 

[T Bredehoft]

For what it's worth I ran two different mills which had been converted to CNC, one vertical light weight, one horizontal, heavy weight, I programmed them by punching in machine code, wrote my own sub routines, etc, used tool offsets, etc. for one-off pieces, never did production (more than one piece) on them. I ran those machines for over 10 years, taught others to use them, did lots of tool-room work on them. The management bought Mazak lathes and mills, I ran them, too, but preferred my old manual CNC machines. Yeah, tool changers were great, but the machines were no more versatile than my to manuals.