[Zach_G]

I'm trying to stay away from as much precision machining as possible in my mill build since I don't have access to a machine shop (anyone in the Dayton Ohio area help with that?). I was wondering if 1.5" square bar cold rolled steel would be accurate enough to bolt 15mm linear rails to. This bar would also be bolted to a length of .75" thick pre-ground steel plate, the geometry is shown below in this concept CAD model (still working out the overall structure).

[jalessi]

Zach,


The pre ground plate sounds awesome however bolting rails to the cold rolled without surfacing is going to sacrifice a lot of the accuracy the rails provide.

You might even cause the bearing's to drag or lockup depending on how much you distort the rails mounting them to a uneven surface.

I believe the maximum mounting surface flatness tolerance for 15mm rail is
.0005"

See the attached .PDF for more information.

Good Luck with your project.

Jeff...

[Zach_G]

The question then is how accurate is cold rolled steel? I've always heard it's more dimensionally precise than hot roll but I'm guessing it's not 0.0005" accurate. Ohhh welllll.

[mactec54]

Hi Zach G

Use hot rolled EN36 & have it blancher ground this is better to work with & more stable than cold rolled

[LeeWay]

You can get the CR to work okay I think. It won't be the best option at all, but still useable. You would still need some precise measuring tools and a variety of thin shim material.
If you build in some slight adjustment features where it all bolts together, then eventually you could come up with a fairly straight setup.
I will say that it won't be much fun trying to do all the alignments. It could take 30 minutes or a week.
The way your design is, you could still have trouble even with preground plate.
If you are going to try and skip the final surface grinding, then you need to take more time on the design and building and especially alignment. I would say that it can be done though.

My steel router is fairly accurate now. I do notice movement in it just from the temperature changes it sees sometimes. The shop can heat up pretty good in the summer and the machine itself sits right in front of my shop AC. It certainly moves a bit.

[BillTodd]

I like the idea of using a milling table as a bed But won't the rails tend to get covered in chips and coolant? Could you not mount them underneath on the slide-way surface? (this might well be flatter than the sides of the table)

[keebler303]

I am in Dayton and have machine shop access. Shoot me a PM if you want.

Matt

[svenakela]

Keeblers offer sounds like a deal.
I buy pregrinded steel from a tool steel provider (www.uddeholm.com), it's more expensive than raw material but still worth it.

[mc-motorsports]

If your going to use a milling table, why not use the saddle also? Just an idea.

[Zach_G]

Hi everyone, thanks for the feedback and interest!

PM sent Keebler!

Concerning the cold formed sides, I think I'll try it. If the straightness is indeed a major concern, I figure I can try using the self-leveling epoxy trick discussed here http://www.cnczone.com/forums/showthread.php?t=55625 or failing that spend the effort to do it right.

BillTodd, I agree, the linear rails are exposed at the moment. I'm thinking about using some angle iron bolted to the sides to give some protection. I checked the sides of the mill table and they are flat and parallel as best I can tell to within 1 thou. To put them underneath I'd have to mill off the dovetail ways. Plus the current location helps reduce the moment load of the cutter just that bit more and provides a better orientation for attaching the vertical support columns.

mc-motorsports, the saddle is a good idea, although I only bought the table as a replacement part from Harbor Freight. It's from their 9x32 mill and only cost $150 shipped to my door! Course it took 6 months to get here...

Overall I like my design (not just narcissistically), keeps the machine footprint small and doesn't require the extra mass in a standard vertical column to maintain ridgidity. Two important things for someone like me who moves often between apartments. I'm not too worried about the noise either, it's kinda payback for the gang of 400lb gorrillas that live above me

[jalessi]

Zach,

Do you have the part number to the table handy?

Jeff...

[Zach_G]

Sure do Jeff and anyone else who might be interested:

The product number is: 33686 1-1/2 Horsepower Heavy Duty Milling/Drilling Machine
The part number is: 140 Table

Also, I incorrectly said that this was a 9x32 size earlier, it's actually 8x28. I wanted the larger version but at more than double the cost I chose this one. I know that Grizzly also had these in stock for double as opposed to HF which shipped from China.

[rokag3]

hello zach,
have you seen the work made by Billtodd with roller screw ?
http://www.cnczone.com/forums/showth...=13593&page=42
since you work a lot on this thread it will be nice for you to see "not in vaine"|

[Zach_G]

Hey rokag3, I have been watching Billtodd's progress with great enthusiasm. I gave up this pursuit since it didn't fit my needs but I still have an interest in its development.

Getting back to the task at hand, I have been tinkering with the design more. Turned out I didn't have the amount of flat ground plate I needed so I figured I'd just go with two massive cold rolled bars. Bridge is gettin kinda heavy now at an estimated 225lbs. Next major concern is racking of the z axis. Length:height ratio of the linear bearings is approximately 2:1. Should be the other way around I know. Work cube is going to be 18"x 11"y 13"z.