[linkn11]

Greetings,

I am an intern tasked with building a CNC mill that has the following characteristics:

- Small machining volume (approximately 3"x3"x3")
- Need to machine aluminum at medium feed rates (about 50 IPM)
- Small material removal rate (<1 in^3 /min)
- A precision/repeatability of 0.001 is essential
- Industrial, must be able to repeat a simple chamfer operation and milling operation quickly for up to 18 hours/day
- Must be able to perform a chamfer operation on two faces, and a milling operation on one face, preferably simultaneously
- I have access to industrial mills (and qualified machinists) to machine any required components
- Budget: Up to $5,000

Design Summary:

I am attempting to modify a standard gantry-type CNC mill, but instead of mounting a spindle on the gantry, mounting my workpiece. The gantry system will then be able to drive the workpiece in 3 axis into three stationary spindles. I envision that I will have three stationary spindles, that will be running continuously at about 15,000 RPM for 8 hours at a time. Mach3 will be only be used to control the stepper motors. Due to my very small required travel lengths, I expect to be able to make my system acceptably stiff and precise using quality components and custom-milled aluminum structural members. I started an earlier thread with the false assumption that I needed feed rates of up to 300 IPM. I now realize that this is totally unrealistic, and unnecessary. After a bit of research, I've put together a Bill of Materials that represents the best of my knowledge at this point. I am looking for feedback, and a check that this project is headed in the right direction.

I will loosely be basing my design on this Instructables post, with the exception that my build will have much smaller travels.


Bill of Materials:

Category	Description	Qty.	Price	Total	Website
Spindle	800 W Water Cooled CNC Spindle 220V	3	279.00	837.00	http://ugracnc.com/WATER-COOLED-SPINDLES/GMT-Water-Cooled-CNC-Spindle-0.75-kW-220V-R.html
Spindle Power Supply	2.2 kW VFD	1	249.00	249.00	https://buildyourcnc.com/item/spindl...-vfd-nospindle
Ball Screw	Ballscrew, 200mm travel, 5mm lead, C7 Quality	3	118.35	355.05	automation products Control Resources: 16mm rolled screw, 5mm lead, 350mm overall, 200mm travel, 16mm diameter, 5mm lead, RM1605-0350-0200-FS
Ball Screw	Fixed and Floating Endmounts	3	92.00	276.00	automation products Control Resources: Ballscrews & Accessories, Ballscrew end supports
Ball Screw	Motor Coupling	3	16.00	48.00	http://www.controlresourcesinc.com/store/products.asp?cat=1396
Ball Screw	16mm Bracket	3	38.00	114.00	http://www.controlresourcesinc.com/store/products.asp?cat=1381
Ball Screw	Motor Mounting Bracket	3	12.00	36.00	http://www.controlresourcesinc.com/s...s.asp?cat=1396
Linear Rail	HIWIN Extra Wide, High Moment	2	53.78	107.56	http://www.controlresourcesinc.com/store/products.asp?cat=1388
Linear Rail	HIWIN Miniture, 9mm Rail	4	34.59	138.36	http://www.controlresourcesinc.com/store/products.asp?cat=1069
Guideway Carriage	WEH27CAZ0, Extra Wide Bearing Block	2	115.37	230.74	http://www.controlresourcesinc.com/store/products.asp?cat=1388
Guideway Carriage	MGN09CZ0H, 9mm Bearing Block	8	30.00	240.00	http://www.controlresourcesinc.com/store/products.asp?cat=1069
Motors	320 oz-in, Low Inductance Steppers	1	539.00	539.00	http://www.cncrouterparts.com/3-axis...-kit-p-74.html
Sofware	Mach3	1	175.00	175.00	http://www.machsupport.com/shop/mach3/
Water Cooling	Water Pump	3	35.00	105.00	http://ugracnc.com/CNC-SPINDLES/Water-Pump-12V-79GPH-For-Water-Cooling-System.html
Water Cooling	Heat Exchanger	3	37.50	112.50	http://ugracnc.com/CNC-SPINDLES/Heat-Exchanger-For-Water-Cooling-System.html
Water Cooling	Vinyl Tube	15	0.75	11.25
Water Cooling	Misc. Water Cooling Hardware	1	5.00	5.00
Misc	E-stop button	1	14.95	14.95	https://buildyourcnc.com/item/electronicsAndMotors-electronic-component-e-stop-button-switch-safety
Misc	6 Limit Switches	1	30.00	30.00	https://buildyourcnc.com/Item/electronicsAndMotors-electronic-component-limit-switch-safety
Misc	Electritian to Connect VFD's	1	200.00	200.00
Misc	Cable Carriers	3	20.00	60.00
Misc	Alumimun Plate	1	200.00	200.00
Misc	Misc Hardware	1	50.00	50.00
Misc	Shipping	1	50.00	50.00
		Grand Total:		$4,184.41

My Assumptions:

• With my budget, a set of quality stepper motors would be more appropriate than a set low-end set of servos. These steppers will be able to accelerate a 5 pound gantry will ease.
• HIWIN rails, grade C7 rolled ball screws, and a beefy aluminum structure would be adequate to reduce backlash/slop to less than 0.001”
• An 800W Ugra CNC water cooled spindle will be able to run continuously for 8 hours, and be able to perform light milling and chamfering operations
• A single 2.2 kW VFD will be able to run 3 800W spindles simultaneously at about 60% power for 8 hours at a time

Questions:

• Am I crazy?
• What should I be most concerned about?
• Is my budget realistic? Am I missing anything?
  Will the final results produce smooth toolpaths, or sloppy, visible steps?

THANKS!

[louieatienza]

I couldn't find it on the website, but FlashCut CNC sold a Sherline mill with ballscrew upgrade that would be perfect start. I'd get one without the spindle and make a mount for your water cooled one, then add a trunnion so you can do the chamfers and facets (you don't chamfer a face, you chamfer an edge.). Last I saw the ad was maybe a year and a half ago.

[hanermo]

Some data:

The small rails are 100x too weak for the proposed use.
Your moment, with 3 sindles, will be approx 100-1000 kgf.
(With 800 W spindles).
MUST plan for 10.000 N kgf push force (for 3! spindles).
You need to build for 2x to 10x the maximum force encountered at spindle imptact.

You dont calculate on carry ability, you calculate on impact loads.
Milling, drilling and grinding are impact loads.

Spacing is important.
Rails must be widely spaced, to carry twisting loads.
This will increase rigidity, and reduced vibrations, for near-zero incremental cost, just mass in structure.

Forget the steppers. Small servos at 300€ / axis are much more appropriate.
This is debatable, but no industrial machines use steppers, since about 20 years ago.
Step sizes must be 10x smaller than proposed *mechanical* accuracy.
Aim for 1 micron steps size => giving you actual, real 0.01 mm repeatability.
Having some headroom is critical, *and every single commercial machine in the world does this, by 10: - 100:1*.
Fwiw.. I use 0.2-0.3 microns on high accuracy lathes and mills.

Your project mentions 18 hours a day.
None of the proposed components are appropriate, if you plan to do this on an ongoing basis, industrially.
Probable lifetime is 2 days - 20 days, depending.

Your proposed gantry mass is 10-100 times too low, for milling !
Alu is totally unsuitable.
Use cast iron.
Its 30x better at absorbing vibrations, 5x cheaper, 3x stronger, and 3x cheaper.

You need a motion control hw board, to deliver high step rates, at low-enough resolutions.

You need one vfd for each spindle.
Cheap VFDs will be fine.

Use bigger screws, for milling.
The rigidty on small screws, in kgf, is not good enough.
Aim for 25 mm or up screws, for 3 spindles, at above 0.5 kW power (in-use), for milling.

The screw mounts are not too good, as-is !
They will need proper angular contact bearings, and refit.
This is expensive in bearings.
You preferably neeed precision spindle mounts, for industrial use.
They are the most critical component, in achieving both rigidity, and repeatability.
You need machine tool spindle bearings for this.
Contact you supplier, and check on P4 quality angular contact bearings for this application.
Ac bearings from a good supplier might be good enough.
The mounts usually dont contain good AC bearings, as-is.

I know mine dont, and these must be specified at order.

*Every single machine tool* manufacturer in the world, uses P4 (or even P2) quality bearings on ballscrew mounts.
There is a reason.
Yet, for alu, and low accuracy, std AC bearings will likely work.

There is critical data missing - and some choices have 10x the effect on total cost and complexity of the build.
Milling = material removal at == more or less the max power of the spindle.

If you are only engraving, or the total mass removed is only 0.1 mm at most, then the situation changes.
Your total spindle power used is about 100 W, or 1/7 Hp.
Then, the components suggested might work or last, to some extent.

Mass, and linear rails, and ballscrews need to be changed, as I suggested above, for industrial use.

If I was signing off on the project, I would not accept rails less than 25 mm, screws less than 25 mm.
I would ask the supplier, will the spindles have warranties at 18 hours per day, day-in, day-out.
Running 3 spindles will be complex.
No std solutions exist, and you will need to do this through a motion control board.
Suggest using analog outs, as several boards have analog outs that could be adapted.

The sw interface and spindle control is critical.

Every component, every board, is different. None work together. All have limitations.
For industrial use, I would use CSMIO-IP.
Its the onyl one with 24 V IO (really, realy desirable) in industrial use.

The sw spindle control is not trivial.
Thus, choice of motion control board is critical.
This *will* drive everything else.

I also suggest that your budget is about half of whats needed, and the integration time will be 200-400 hours for a very experienced person, with that specific motion control board.
Again, for industrial use, for 18 hours per day, and for 3 spindles, and for milling.
These are drivers, that dictate what you must have and these then dictate what you *must* start with as a baseline.

For a similar project, my initial budget would be around 20k€.
Thus - component costs would be about half that.

If iterative development, ie failure on first tests, is acceptable.. ?
You might try this, and then proceed from there.

My first suggestions is to only try to get the sw part working, with 3 spindles.
You will be very surprised, and the learning experience will be interesting.

The motion control part is trivial, and if you only spend an extra 1000$ on first-iteration hw, you got off really cheaply.

I am *not* being negative on purpose.
My opinions are based on looking inside about 2000 machines, and about 10 similar projects, industrially.
Industrially is the key word.

I would first get a sign-off on some iterative development, with authorisation to lose the first 5k$ on tests.
And suggest a 20-30k$ as a probable total cost, with about 3-4 months timeframe.

If you change a parameter to make it work cheaper - this is fine.
But, as a supplier, I could not do that, so I would poropose on how to make it work within the parameters given to me.

There might be many ways to make it cheaper.
This might or might not be a good idea, again depending.
More data on the app would make this easier.

[GITERDUN]

5K budget, 3 spindles working 18 hours per day maintaining 0.001- there is a machine that fits your needs perfectly at www.fantasyland.com

[linkn11]

Thanks for the detailed reply. I think we are talking about two very different projects, with different loads. This is my failure to communicate this effectively.

First, the loads perpendicular to the spindle (in-line with the axis of the ball screw) required for this project are *very* small: much less than the 10,000 kgf you are proposing.

Here are my cutting parameters:

Milling Operation:
End Mill Diameter: 0.125"
Cutting Flute Count: 3
Depth of Cut: 0.050"
Radial Width of Cut: 0.050"
Feed Rate: 30-50 inches/min

I entered these values into an online calculator to obtain a tangential cuffing force of roughly 10 lbf, or 45 N.
Given these very lite loads, do you think my components could be appropriate if I used cast iron for structural members?

Additionally I am planning on designing my rails such that the 9mm rails are separated by about 100mm, with 4 carriages.
The large 27mm rails and carriages will be used as a single pair, instead of a set of four.

I contacted Ugra CNC to see if they have any statistics regarding the reliability of their spindles under continuous operation.

Finally, for the ball screws, do you think a C7 grade ball screw is appropriate?

Thanks.

[LeeWay]

Okay.
Just to start with, you will need better than rolled ball screws. The best specs on those that can be believed are .003". Start with ground ball screws of at least .5" diameter. You can be sure they will hold and move the load you need. Say 12 mm.
Now that this is out of the way. I will say that building your own mill can be a challenge.
If you have machinery available to help out, then all the better. Otherwise you outsource and one mistake there can cost money and seriously attack the budget.
It can be done on a shoe string budget. No doubt about it.
I built one with nearly the capabilities you are looking for.
It is my spare machine. It came in handy last year when I was without my main mill.
Good luck with it and let us know how it goes.

[109jb]

You may be able to use the rolled screws if you use a controller that can map the screws and compensate for the lead errors. Mach3 can't, but last I checked, LinuxCNC can. With screw mapping you can achieve precision ground ballscrew accuracy with common rolled ballscrews. A bonus is that LinuxCNC is free.