I'm still stuck on whether P4 means GROUND ball screws or whether P4 could also describe ROLLED ball screws. In looking at some of the Personal CNC Design V1, in the ball screw section it says:
"P3 allows 12um error within any 300 mm. In imperial units, this is 0.0005" per foot. Cumulative error at 900 mm is limited to 21 um (0.0008 at 35")
"C3 allows 12um error within any 300 mm (0.0004" per foot). Cumulative error is not specified, thus allowing as much as 36 um at 900 mm.
"P4 allows 16 um error within any 300 mm. In imperial units, this is 0.0005" per foot. Cumulative error at 900 mm is limited to 22 um (0.0008 at 35").
Later on it specifies that the P4 grade was selected for the Tormach, but the only reference that I can find about ground vs rolled is that the manual states that C7 screws "allows the screws to be manuafactured by a rolling process rather than grinding process"; however I can't find a statement that the ball screws in the Tormach are GROUND. It is implied, but not stated in the Design document. Perhaps it can be found somewhere else. (I can't open the data sheet. It keeps crashing the evince reader on my Linux box; however, the other documents opened properly, as well as all documents from other sites.)
The VFD and motor are also a concern. On my Shopbot Prt-alpha, I have a 3hp spindle that can run at speeds up to 18,000 RPM; however, the torque curve shows that the spindle is only at full torque between 12,000 and 18,000 RPM. That works great when routing wood, since I normally run the spindle at about 13,000 RPM and use a feed speed of 400 to 500 inches per minute. What is the torque curve for the Tomach VFD and motor? Does it give full torque at all speeds? If not, what is lost at the low end? Would I have to be worried about cutting steel at slow RPM? (That is one of the reasons that I'm using the IH in manual mode for a while. I need to see what kind of finish I can get at 1,500 RPM. According to the chip load formula, I should get equal results if the feed speed is adjusted using the formula RPM X flutes X chipload give 1600 X 4 X 0.005 = 32 ipm with the IH and factory motor, or 16 ipm with a 2-flute cutter. If I used Tormach's VFD and motor running at 4,500 RPM, I should be able to cut 4500 X 4 X 0.005 = 90 ipm or 45 ipm with a 2-flute cutter. Unfortunately, correspondance with Tormach told me to expect cutting speeds around 14-ipm. What gives? The math says I should be able to go at least three times faster. Is the machine limited by stepper torque or motor torque or is the chip load formula defective?)
Before everyone thinks that I'm trying to start a war between the two camps, let me point out that I think that if we were talking cars instead of milling machines, we'd be comparing Fords and Chevys. Both would be excellent choices for basic transportation. However, I would be concerned if the sales literature from one car company stated that the engines used in their vehicles were all "blueprinted" when, in fact, they were using standard, off the assembly line motors, just like everyone else.  |