IH CNC vs. Tormach for my situation

[Richards]

Phil and Runner,
You've both made valid points. Since the mill is steel and the parts that I cut are aluminum, there is a significant difference in expansion due to temperature. Heat generated by cutting and by the movement of the axes would have to play a part in that expansion. Fortunately, the parts that I cut have few areas that need to be held to sub 1/1000 tolerances. For those areas, I have turned Go/NoGo gauges in aluminum on my mini-lathe to be sure that the size of bearing holes and other critical holes are proper size. The Go/NoGo guage may not be the most scientific, but, to date, it has worked perfectly.

Although I, as a Chip Sweeper, would like to find a way to cut everything to exact size, I'm enough of a realist to know that some dimensions just don't matter very much and that the dimensions that do matter can usually be handled with a little creativity. This is not meant to reflect on the inherent accuracy of any mill, IH, Tormach, or any other make, it is just to point out that even though the mill may have been built to high standards, there may be other factors that could significantly reduce the quality of parts cut on that mill. The old saying, "A poor carpenter always blames his tools" might be modified to say, "A good carpenter knows the limitations of his tools and his environment, and knows how to work around those limitations."

[Tormach]

On the issue of ballscrews, just looking to ground versus rolled is really missing the point. As a side note, the screws in the Tormach PCNC 1100 are, in fact, ground. The real point for the specification of a ballscrew is precision. This is generally specified in a two part designation, a number and a letter. While there is actually quite a bit more to it, in brief:

grade 4 has an allowance of 0.0006" error per foot of length
grade 5 has an allowance of 0.0009" error per foot of length
grade 7 has an allowance of 0.0020" error per foot of length

A prefix of T or C indicates cumulative error is allowed. This means that a T7 can have 0.002" in a foot, 0.004" in two feet, 0.006" in three feet, et cetera

A prefix of P is a precision grade, where there are strict limitations on the accumulation of error over the length of the screw.

Milltronics and Haas use C5 grade screws. Tormach PCNC 1100 uses P4. You will have to get detailed specifications from your supplier, but in most cases retrofit kits use rolled screws which generally in a T10 to T7 class. Rockford screws state 0.0070" to 0.0030" per foot and cumulative, which makes them no better than T8 (ref http://www.rockfordballscrew.com/pdf...s-catalog.pdf).

For a more detailed understanding, look to published standards ANSI B5.48, DIN 69051 or JIS B1192. Remember, rolled or ground just tells you a process, it does not specify the actual precision or quality. The machines for rolling screws keep getting better, some can occasionally roll a T5 grade. In addition, lots of rolled screws have a finish cut on a grinder. I don't know if those would be called rolled or ground. For some photos of a Chinese ballscrew factory, check out the page at http://www.tormach.com/quality_overview.htm

Greg Jackson
Tormach

[MichaelHenry]

Greg,

Thanks for the info on ballscrews. For those that want to check out the Rockford site, note that there is a trailing ")" that must be deleted from the link to get to the PDF file.

Mike