[HawkJET]

Lee's post is a good contrast of how we all have different perspectives as well as different abilities and experience. It is likely there are different aspects of something I design and build that wouldn't work for Lee and vice versa. We all make our own call.

In an attempt to give you as many ideas as possible, here's mine:


Use a 10 x 33 piece of the 1.5" plate for the table and two 5 x 26 pieces of the 1.5" plate for the double beam of the base. I showed 1/2 inch plate for the ends and webs between the double beam. The base should also be closed on the top and bottom to complete the shear path for torsional rigidity. In my own design for the new base of my X3, I plan to cast a similar double beam (sans the afore mentioned top and bottom) into epoxy/granite. The E/G casting will provide the torsional rigidity and effectively dampen the structure.

The column is bolted from the bottom, through the double beam.

Joe, if you have access to a surface grinder, you could make your own dovetail ways and adjustable gibs. This could save you some $$$ but would be more work than buying linear bearings.

Cris

[Joezx10r]

Ah its all clear now thanks for the drawing. I had first planned on linear rails and bearings to save some time but after seeing the prices for them I may go with a dove tail.

[Joezx10r]

Thinking about widening the tower to 8 inch by 1/2 thick instead of the 6x3/4.

[LeeWay]

This is just a guess here, but I think the smaller box in this case would be stronger than the larger. The larger would have a better bolting at the base, but overall a weaker structure. That would be something to test out in the software that figures stresses on structures. Interesting to see I think.

[HawkJET]

The larger would have a better bolting at the base, but overall a weaker structure.

It's good to ask these kind of questions. I get your point, however the weakest link in what you mention, IS at the bottom bolts. The stresses in the rest of the structure are generally relatively low (given the material, thickness and geometry). Plus the increase in size of the column will almost offset the decrease in wall thickness (which was quite generous to begin with).

The biggest advantage I see is in a different area. The wider column will allow for wider Z axis ways. This is most likely a weaker link (in regard to stiffness) than the column bolts.

Cris

[Joezx10r]

Ill have to check prices tomorrow maybe I should go with a thicker wall.

[HawkJET]

Ill have to check prices tomorrow maybe I should go with a thicker wall.

I doubt you could tell any difference in the completed mill. A chain is only as strong as its weakest link. The weak links in the area we've discussed are the column bolts and the Z axis ways. These two places are going to be where you lose rigidity. It is inherent in the design.

As a previous poster pointed out, bolts are not as stiff. But with appropriate allowances in the rest of the design, they can be made to work just fine. The change to an increased column size is going to help this.

The ways are always going to be a weak link. Any bearing system will be. Again, proper design will make them work just fine. The change to an increased column size will help this also.

Something you WILL notice is incorporating Epoxy Granite into your design. Spend your money on it. Steel is horrible in damping. Just rap on the side of your column after it is built and it'll ring like a bell. This can wreak havoc on surface finishes and tool life. There is an excellent thread in CNCzone that provides a wealth of information on how to make your own E/G mix.

The positive effect of damping is generally not intuitive as it is a dynamic condition. Dynamic conditions can behave much differently than static conditions. After you finish your machine, you can put an indicator in the spindle and watch the deflection as you lean on the column. This is a static condition. In a dynamic situation, you can get resonances and vibrations that may seem small but they can really mess with the tool. Every time you cut a pocket with an end mill and you dwell for a second in the corners as you change directions, invariably, the tool chatters. This effects more than the part you are machining.

I know I have mentioned Epoxy Granite a number of times in this thread. I keep bringing it up because it will make a difference and it is something that doesn't seem to be of much interest. Yes, your mill will work without it. But it is relatively easy to do and the benefits are well worth it in my eyes.

Cris

[The Blight]

You don't have to use EG. Machine builders have been using regular concrete, sand and lead to dampen vibrations.

I don't believe the bolts holding the column in place will be the weakest link here. Lots of big VMC have bolted on columns. As mentioned earlier in this thread, they should be combined with pins.

[HawkJET]

You don't have to use EG. Machine builders have been using regular concrete, sand and lead to dampen vibrations.

Great point! The key is there is a need to dampen vibrations. I personally like the advantages of EG over the other ones you mention. But yes, there are other ways to dampen.

I don't believe the bolts holding the column in place will be the weakest link here.

By weakest link I really mean least strong. It can be quite strong but the other parts are likely stronger. Just compare the cross sectional area...

Lots of big VMC have bolted on columns.

Yes exactly, they probably all do.

[Joezx10r]

This may be the craziest dumbest thing you guys have ever heard but what if I made a webbing vertically inside the column witch boxed in the ball screw creating a hollow area to fill with some sand or something. Would that absorb vibrations?

[HawkJET]

...what if I made a webbing vertically inside the column witch boxed in the ball screw creating a hollow area to fill with some sand or something. Would that absorb vibrations?

Based on my understanding of your description, depending on what you filled it with, it sounds like it would.

Cris

[Joezx10r]

Well Ive reevaluated my needs and am thinking a smaller machine my be better for me so heres the new design using 6x6x25 1/2 inch wall tower and 5x1x20 base rails. Enco compound table and X2 head.

[LeeWay]

Nothing wrong with that. Should work great.

[Joezx10r]

I haven't worked out everything yet but I'm thinking of making box ways for the column from A2 flat steel instead of linear ways.

[Joezx10r]

Or would it be better to bolt the stock dovetail column to the column im building for support?

[HawkJET]

Or would it be better to bolt the stock dovetail column to the column im building for support?

It depends on what you mean by "better". Square ways are generally more rigid than a dovetail. But you would have to deal with mounting the X2 head. Using the stock dovetail would likely make less work though. Which is "better"? Only you can really say.

[LeeWay]

Box ways would be nice. They would take a bit more work of course, but if you have machines available to grind them or face flat, then they would be a good choice. The X2 dovetails don't need near that much extra support to be rigid enough to use. An angle plate would likely be sufficient behind it.

Structural tube can be used as well. That might save a little cost and work. All would need milled or faced flat.

I would consider linear ways though personally. They are cheap enough sometimes on ebay or you can affordably buy new Hiwins now. Far less work or adjusting after the fact on those. I would go for 20 or 25 mm sizes for Z, but X and Y could use 15's. They have very little friction and are pretty rigid and carry a long life with low maintenance.

If cost is an issue (when isn't it?) then the dovetails will get you by fairly well. Less work overall to build, but a little maint. needed from time to time. All machinery needs that anyway to some extent.

[Joezx10r]

So assuming im using linear ways for the z axis what diameter ballscrew would I need to get? Sorry for my ignorance I know nothing about ballscrews.

[LeeWay]

Ball screws would be nice, but again not necessary. You can get some nice enough ballscrews from China on ebay fairly cheap. You can buy rolled ball screws from Mcmaster Carr or there are other places that have them made to fit X2's and X3's already.
You would need some machinery to put your own together from Mcmaster Carr. A decent lathe and perhaps use of a mill for the bearing supports.
I recently bought a very nice 27" long ground nsk ball screw off Ebay for $57. It had the ball nut mounts as well as bearing mounts on both ends. You just have to watch what they have. That one is 14mm diameter. The other ones I use are 5/8" or 16 mm.

[Joezx10r]

I was looking at a hiwin 650mm , Diameter: 16mm . 12mm.. Good for this?????