Question 4th axis tilting upwards

That's what the label says. when I apply some pressure from the backstop to help keep things in place, the 4th-axis assembly tilts upwards visibly. Once again here is another nice example lesson on How-Not-To-Do things. Please look at the image.



I can't provide a photo of my own so I had to ransack Stepcraft US. If you look carefully at the bottom you'll see a hard edge where wall side meets bottom, but about midway the line becomes a shadow. This is because the bottom is in two levels: one level for bolting, and then a slightly recessed bolting. You can see how the bolts passage are about 1/4th way from the face wall.

So when the tailstock screw is tightened against the workpiece it exerts a force against the headstock which is transferred to the fastening bolts and increased by lever effect. In other words the headstock is forced backwards and this bends the workpiece centerline upwards.

This could be mitigated by making the base a flat plate just like the top surface, instead of cutting it into a slightly raised level for no good purpose. The only remedy I have for this is to insert a shim under the recessed part.

Another possibility would be to invert the damned thing head down, but then the I'd have to get me a new tailstock as centrelines would no longer match!

And why the headstock has a rotational slack of 1 degree or 2 when I handle it is something else I'll have to see, but I'll leave that for another day after I dismantle it and inspect how the gears are meshing.

MMorao,

I agree this is not what you expect,or perhaps just exactly from Stepcraft.
I think the easiest solution is to glue the shim to the tailstock. Locktite 496 will do the job.

But this is not what you want for such a pricey part.

good luck

MMorao wrote: So when the tailstock screw is tightened against the workpiece it exerts a force against the headstock which is transferred to the fastening bolts and increased by lever effect. In other words the headstock is forced backwards and this bends the workpiece centerline upwards.

Haven`t been to this forum for a while. Do you really need that much pressure from the tailstock? The tailstock centre cone should match up with the centre hole and not need a lot of force for support. Haven`t used the 4th axis assembly myself so don`t really know how much force we are talking about here. Maybe use more rigid hardened steel bolts for clamping the headstock if not already.

Thank you CNC Miller, I'll have to follow your suggestion...

Tikka, you're absolutely right, it shouldn't do that considering that the tailstock is there to guarantee alignment only. I do mark end centres on the stockpiece, and punch a tiny hole on the tail side to lodge the point. However with the lightest pressure I can see it buckling already. As Galileo Galilei said, "e puor si muove"!

I swear to you that apart from welding all to an I-beam I can't see how I can make the assembly more rigid than I made it already!!

Another thing: to make sure once and for all, I closed the chuck jaws and brought the tailstock close. Surprise, surprise, the tailstock point is 1mm below the jaws closing point!!!

But don't pay much attention to this because this chuck movement is really not the smoothest thing on earth, very much cranky in fact. It's all done by Proxxon, but when I opened it as it when it wasn't moving when it arrived, the lube was so dessicated that it looked like it was from the time when Proxxon made fish hooks for the Kaiserliche Kriegsmarine!

Ah well, Happy Easter all!

MMorao wrote: Tikka, you're absolutely right, it shouldn't do that considering that the tailstock is there to guarantee alignment only. I do mark end centres on the stockpiece, and punch a tiny hole on the tail side to lodge the point. However with the lightest pressure I can see it buckling already. As Galileo Galilei said, "e puor si muove"!

I do not know all tricks of trade but I think you are supposed to drill a centre hole with a proper geometry that maches the tailstock cone so that the tip does not touch the material and sides take the load (probably 60 degree HSS centre drill). Centre hole should be drilled with a lathe.

MMorao wrote: Another thing: to make sure once and for all, I closed the chuck jaws and brought the tailstock close. Surprise, surprise, the tailstock point is 1mm below the jaws closing point!!!

Wow 1mm is clearly too much. I assume you dismantled the chuck for cleaning, maybe the jaws are shifted after reassembly. Do not know the exact construction of this particular chuck but if it has a spiral disk then maybe there is a proper order of inserting the jaws in addition to having them in proper numbered positions. Is the distance between chuck outer diameter and chuck yaws more or less the same when measured without anything clamped in?

MMorao wrote: But don't pay much attention to this because this chuck movement is really not the smoothest thing on earth, very much cranky in fact. It's all done by Proxxon, but when I opened it as it when it wasn't moving when it arrived, the lube was so dessicated that it looked like it was from the time when Proxxon made fish hooks for the Kaiserliche Kriegsmarine!

I have done some reading about hobby lathes in the past and it seems common that you have to clean all metal parts in kerosene before first use because all parts are covered in anti rust substance that is no good for lubrication.

MMorao wrote: And why the headstock has a rotational slack of 1 degree or 2 when I handle it is something else I'll have to see, but I'll leave that for another day after I dismantle it and inspect how the gears are meshing.

Do not know if the following is the same product: link ,
but that one is said to have an anti-backlash worm gear (adjustable), so you should be able to adjust it somehow.

Hi Tikka, thank you for the well thought out reasoning in your reply. So let's see...

1) Of course you're right and I was being simply lazy. No further argument needed!

2) & 3) It is indeed a spiral disk and the "ports" are numbered and so are the individual chuck jaws. When I received this part I contacted the seller (Rory) and made some posts here. I had to dismantle the chuck, clean and lubricate, AND set the jaws in proper sequence. The jaws metal, by the way are softer steel than the chuck disks probably to avoid damage to these, and rather roughly finished. I'll have to dismantle this again to check the workings, and see if the chuck disks are aligned with the tailstock or not, and/or there is a problem with the jaws - which wouldn't be a surprise.

4) I can't say for sure that it is exactly the same product but it looks like it very much, at least the tailstock is undoubtedly the same. Use of a substantial collet has advantages over a dubious chuck. I'll bear that in mind for someday. Meanwhile I am meaning to dismantle the unit anyway to see if there is any problem with the wormgear meshing or something loose, right now I have no idea.

Also, to avoid problems with the chuck jaws gripping the workpiece I devised this simple shape to be machined on the head end and it works. I milled this vertically but that's impossible after some length, but then it will be easy to make some sort of a triangular or hexagonal prismatic bed to machine this tenon horizontally.