Mini-VMC project

[JFettig]

I started this project intending not to have a log, but I think the input from you guys and what can be learned by all of us; it would be beneficial to have a log.

What I am doing is building a small VMC-like CNC mill. I first got this idea when I first started working in a machine shop 2 years ago while examining the Haas vf-3 I was working on then later the Milltronics VMC I used a few times. Both of these machines used linear rails and slides much like THK HSR and SHS so I figured I would definitely use those.
In a Haas, the table sits on the bearing blocks and slides across the rails back and forth, the y-axis is mostly conventional to what you would think.
The Milltronics was kind of a combination of a Haas and what you typically see. The motor and ball screw was attached to the saddle along with the bearing blocks and the rails attached to the table along with the ball nut.

I choose to use the Milltronics VMC style mainly because there will be more table space and more travel per shorter rail and more room for clamping at the ends. The Milltronics VMC design also covers up almost all of the ball screw and all of the linear rails at all times and is easy to incorporate way wipers/covers.

While designing I decided I wanted to eliminate the huge saddle or the added on extensions by doing some sort of spinning nut design. The advantages would be less larger expensive parts, more compact machine, and all of the ball screw would be covered at all times.

Disadvantages would be: a more complex nut assembly, centrifugal force acting on the nut tubes causing improper ball rotation and possible rigidity issue(I think that wont be a problem) and slightly longer ball screw(another not so much of a problem).
This spinning nut would only be for the x-axis.

The plan:
Make a very fast, rigid, precise machine,
Travel will be about 16x9x16 x,y,z.
I will be looking for tool changer spindle, otherwise I have designed some tooling around the R8 taper.
I would love to have a tool changer, but that will only shoot the cost out the roof.
Spindle will be fully programmable, I was thinking about using a 1000w ac servo coupled with a step/dir drive to have full control over it.
Oh yeah, did I say speed?

The components I have:
3 400w AC servos,
4 22" SHS rails, 8 bearing blocks
2 18" HSR rails, 8 bearing blocks (extra set)(4 through hole, and 4 tapped on the side)

So far, that’s all I have. What I intend on doing at this point:

I intend on using precision rolled ball screws with .5" lead, rolled because of the price and availability issue. I intend on using the .5" lead with a 1:2 gear ratio so I only have max of 1500rpm at the screw(nut) From what I am told and seen, ball screws aren’t supposed to really even take that or much more?
The gearing could change depending on how much speed and torque I feel I need, With that setup I have up to 750IPM which is flat out ridiculous.

I will be making the majority of this machine out of Cast Iron for simple reasons of rigidity and vibration dampening. I have access to this through a friends dads work who gets a lot of cast iron plate.

Enough talk, here are some screenshots from SolidWorks
A few things like the saddle and spinning nut assembly will change a little, that’s more of a mock up.

I would appreciate some good input if you have.
Enjoy,

Jon

[miljnor]

posted by jfettig and all of the ball screw would be covered at all times.

This is true with both designs. The advantage of a spining nut possible puts the motor and belts/couplers under the table too.

Bridgeports never uncover the ballscrew on the x. just on the Y.

The way you cover the ballscrew under all sittuations is to have the Table larger than the travel (as on the x of a bridgport). So if you wanted to cover the y-axis you would just need enough room for movement and a larger table.

Good concept though and I think the spining nut could potentialy be a higher load machine and maybe a little more compact.

good luck.

[DieGuy]

I am very interested in seeing how this project goes. I have been thinking of a bridge design that would be 18X24X10-12.

[JFettig]

Miljnor, I choose not to go with a regular manual mill style where the x motor is attached to the x-axis partially because I would be moving around with the machine more, I want this thing as stationary as possible(no wires will ever touch the table)

I considered a gantry(bridge) mill for quite a while, 2 main reasons I am not doing it:
1. Odly shaped big parts wont fit between the columns
2. I want a lot of z-axis travel, having the head hanging down not well supported could be an issue.


Jon

[gimbal]

Hi Jon,

Your project sounds cool. I am doing something similar (details here http://www.cnczone.com/forums/showthread.php?t=13881)

Hope you have a good back and/or a crane - My machine is Taig/Mini Mill sized, built from solid cast iron and will weigh 90kg (200lbs) when complete - once final assembly is complete, it aint moving.

One thing I found very handy is access to a shop with a large surface grinder - seems a shame to go to all the trouble of using cast iron and not ground mating surfaces and rail mounting surfaces.

Good luck!

Cheers,

Pat

[JFettig]

Gimbal, I have seen your setup, looks like a nice little mill. One thing I am doing with mine is making it semi-easy to take apart, I am putting my SHS bearings on the x-axis so I can unscrew the ballscrew and slide the table off, and remove the column or/and the head.

I do want to get access to a large grinder, I don't know if it will happen unless I pay someone to grind it.

Jon

[damae]

Jon, your machine is looking great. What sort of parts do you want to cut with such a large Z axis?

The simplest tool changer design would be to have the tool holders mounted in an array on the work table. You could simply move the spindle over the toolholder and then drop down and pick it up. The advantage of this design is that you don't need to mess with rotary carousels, or any extra moving motors and assemblies. You would, of course, still have the auto-drawbar.

The downside is lost table space. Since you're in the design phase, you could simply design around it and extend your table by the width of one row of toolholders.

Most colleges have large surface grinders in their machine shop, and if you're a student there, you should have free access to it. Or, you may try to call up the machine shop you used to work for and see if they would do you a favor, perhaps a reduced price.

[JFettig]

I want the large z-axis for such things as long tools, like a tapping head if rigid tapping isnt going yet, long drills, etc. I only make random stuff so something is sure to pop up where I need a lot of z-axis travel. Large fixtures are another thing, especially larger rotary tables with large work peices, those take up plenty of room.

The reason I wanted to go with a bridge mill style machine was mainly for the tool changer simplicity like you said, but it has its other downfalls that stops me.
The college here only has smaller surface grinders as far as I have seen, the shop I worked for didnt have one, This other shop I know of might have a large one, but gaining access or having them do it might be a problem.

Jon

[miljnor]

one way on a bridge mill to get the z-axis more ridgity (one I haven't seen yet) is to make the whole Y-axis on linear guides. you would of course have a different set of headaches to deal with. like clearance of the whole y-axis to the part but just throwing Ideas out.

[JFettig]

Now that think about what you said about the whole y-axis on guides, I have seen it, theres one in the gallery. quite a beefy router it is.

I made a small amount of progress.
All these designs are for if I choose to go this style of a machine, I am still considering a bridge mill for the sake of being able to add an ATC easily.

Heres a picture of the z-axis slide. Mounting of the head will be similar to how tormach, miljnor and a few others have done it.
I still need to put the screw holes in for the ball nut flange.

It is looking more like the column will be a 4x6" rectangular tube with a slot cut in the middle for the ballscrew/nut to go down inside(unless someone wants to cast me a oclumn). there will be rail mounts welded to it and machined flat, possibly ground if I get access to a surface grinder.
-edit-
-little update on hole placement-
the pic of what your possibly thinking of here:
http://www.cnczone.com/gallery/showp...00/ppuser/7518
Enjoy my small amount of progress

Jon

[JFettig]

I drew up a concept column, most dimentions are correct, a few obvious things arent, and the slot in there isnt definite.

Jon

[JFettig]

I am considering making or buying a spindle, something in the bt30 variety. Theres a grinder here at school that I know can do the OD, but not sure about the ID.
BT30 is a good size, I dont have much info on it more than this:
http://www.desktopcnc.com/useful/taper_spec.htm
I can find tool holders on ebay for decently low prices.
Does anyone have any bt30 spindle drawings(I'll consider cat30 too, maybe cat40 if it comes down to it).

Does anyone have any suggestions? I will also be needing a little information about tool retention and stuff like that. I have started on a tool holder for r8 that uses an air coupler type retention system that I am making myself, haven't done much with it in a while and wont for a while but I have a general idea on that but I dont know exactly what the dimentions should be for everything.

Thanks,
Jon