Unless there is a very good reason to get the 940M I would ask PM if they can change your order to the 940V. I know it costs a little more but I suspect this is because of the VFD and maybe it is better made? Eitherway, you should be able to get a lot of use out of either machine.
I have the PM940M-VS-CNC. This was a short lived version a few years back and is no longer available. Since that time they have made a few changes to the PM940M, but it appears to basically be the same as mine. (The most obvious difference is that the x, y, z crank handles are different.) The M version is gear driven while the V version is belt driven. My VS (variable speed) is VFD powered, but still has a gear box similar to the PM940M you selected. My gear box only has a high and low setting and with the VFD it goes from ~ 90-1000 rpm and from ~ 1000-3200 rpm. Hence, my machine probably has a little less weight in gears. My machine does NOT have the power down feed as it has all three directions of controlled motion via the CNC . Anyway, power down feed on the M or V will probably add weight. Of course filling the head gear box with oil also adds weight. Long story short, I think the geared head is too heavy for the machine. The PM940V is has a lighter weight head, has variable speed via a VFD, and may have different spindle bearings as the top speed (5000 rpm vs 2000) is faster. Having a higher top speed is a very real positive feature. Over all, with motor and no tooling I have estimated that 940M head weights between 250 and 300 lbs.
The best I can tell there are two kinds of knod. One is just alignment, is the head perpendicular to the table. This you can shim away assuming that the machine does not bend in use. The second is more dynamic and is what
@Sticks seemed to be referring to. If you loosen the clamps the head tilts, knods. As he notes this is related to the gib being tight enough. To tight though and you cannot lift or lower the head. Likewise it tilts differently if you are using the crank to lift the head as oppose to lowering the head. To make this more repeatable you need to always set the final height by approaching it from the same direction, i.e. crank the head up above and lower it back to the final position. So this knod has a lot to do with this head weight. You can tighten the gib up so that a lot of the knod goes away, but on my machine it seems that this gib tightness is a function of the position of the head along the ways. That is, the knod is sensitive to where it is positioned along the z-axis. I think this is because the separation in the ways actually changes, the ways flex (bends together such that the way width varies differently as a function of z-position). The z-axis ball screw bearing is attached to the head via an arm which extends between the ways to clamp to the lead screw. Hence, there is a gab between the ways extending from near the bottom to near the top. You can see this gab in the second photo of the induction heating picture on the PM web site:
https://www.precisionmatthews.com/wp-content/uploads/2017/06/Z-Axis-Way-Hardening.jpg But the two halves of the way rails are solid at the top and the bottom of the Z positions as the lead screw arm does not travel there. So no way flexing at the top and the bottom, but way flexing in between and the most flexing seems to occur at the center position in Z. This position is right where you are more probable to be operating. Since the head essentially hangs on these ways it bends the two halves together. So the gib is effectively loosened. If the head weights less there is less flexing! As
@Sticks noted this does not have to be very much. A couple of thousands makes a big difference in the effective gib tightness. Folks with the 940V do not seem to have as much a problem with this as those who get the 940M. On the other hand, perhaps the manufacturer has beefed this structure up since I purchased my machine, but the pic looks similar to mine. The flexing would be a strong function of the weight as well as the thickness of the walls of the way box.
By the way, I never use the Z-axis locks. In order for them to hold the head in position, the two lock bolts have to really bear down on the gib. When I removed the gib to study it I found that these bolts had left large dig marks into the back of the gib and appear to have bent it even thought it is cast iron. (maybe it was bend into a C shape at the factory before I got it.). Anyway, my CNC motor holds the z-axis position while I am milling and when I power it down I just use some blocks to support the head and keep its weight from turning the stepper motor/lead screw and falling.
A couple of other things, I have no experience with the V, but I think that the geared version is probably noisier. The fan on the motor is very loud so the gear noise maybe unimportant.
@Sticks is right, check the tightness of everything, but remember the cast iron that is uses is a very porous sandy like material and it is easy to strip the bolt hole threads. This is especially true for the smaller bolt holes. The nut at the bottom end of the z-axis lead screw on mine and a couple of other folks machines actually fell off. The machine will run without it, but this nut holds the end bearings for the lead screw in place. I actually ran my CNC machine before I took it off the pallet. When I lift it from the pallet I found the nut, washers, and the nut locking cover laying on the pallet. Initially, I had no idea where it had come from!
Dave L.
