[letmefixit]

I have finaly got my z axis up and running but im a little concerned for safety. If I were to lose a belt, the head would come crashing down. OUCH!!!! So we have talked a little bit about counter balanceing the head and I think im gonig to go that way. But i need to know if anyone might know the weight of the head. My 600"oz. motor seems to move it quiet easily but still the safety factor bothers me a little bit. ANY HELP????

DJ.

[jetflatline]

Would it really come crashing down?

I'm guessing mine, sans motor, weighs around 75-100#, after lifting it on my own a couple of times.

[Cruiser]

That stock belt drive is really robust on the stock setup. If the gib is set snug enough the head won't crash down.
physics dictates that any mass added to the system will effect the drive. It seems to me the weight of the head is close to 300lbs as it is.

[letmefixit]

I have seen ball screws free wheel with a load on them on some applications that i work with at work. So that's why it concerns me. The other thing that bothers me is that Im over compressing my preload springs on my ball nut. Would it be better to not run preloaded on the Z axis????? It really throws me some funky numbers when i try to dial it in.

DJ

[03vert]

I am also in need of the weight of the head/slide motor combo. I decided to do a counterweighted head from the start. May buy a scale today and weigh all the parts and come up with a figure. If I haven't seen a weight by then I will post my results. For what it's worth, while I was mocking my Z up I was able to backdrive my ballscrew by standing on the headslide. I use EMC with a mesa 5I20 and 2 7I29's and don't really want to use any bias in EMC. (want to keep things predictable) Thanks for all of the ideas and help so far!! Steve

[Cruiser]

You do want to get rid of as much lost motion or backlash as is possible on the Z axis. Otherwise it will show up with direction change. I had close to .007" in mine, I run a single ball nut on my Z. My lost motion was with the pin mount that locks the head to the saddle, It had close to .01" slop so since it was a steel pin I welded a ring around it and turned it in lathe, I also made a bushing to take up the space of the bolt in center hole. Now it is more like .0007" and I like it much better. If there is excessive lost motion in the Z axis it is possible for cutting pressure to push the head up by that amount and it made for some sloppy surface finishes as well as made it difficult to get a good set on tool length using my 3" standard to touch off.
I know I said above it was 300lbs all up on the head weight but actually I think it is closer to 380lbs. Others have done the counter balance system and nitrogen Cly's too. The fact is that when it is all set up proper, it will hold unless it is messed with, such as hand cranking it with power off. I always use a block of wood when I work on mine. When just setting static, it stays put and mine runs looser gibs on Z as I installed rails to draw the saddle up snug to slide way and the gib is only taking out any slop to control shaking and rotational error. I do not believe the counter balance is needed !

[03vert]

Head with no motor or slide is 176lbs, head slide is 38.5, 2 horse 3ph motor is 41 for a total of 255.5. The head won't come crashing down if you lose a belt or your drive faults, if you have good smooth motion it comes down slow enough that you can say a few profanities as you watch it slowly snap the only bit that's not stocked locally.
Me and Murphy are all too familiar with one another.
Steve

[jetflatline]

Hm, I forgot about the slide, but is the head really that heavy by itself or am I just stronger than I realized?

[RustedOut]

I've only got a single ballnut on the Z as well and run the gibbs only tight enough to take out any slop. Since I replaced the stock IH Z nut bracket with a steel one I fabbed, my Z heights from tool to tool have been excellent. Just got done pocketing out an area for a bearing, then ran the boring head right down to the bottom of the pocket with no discernible step off between the two tools.

For some reason, the Z drive faulted earlier in the day on a rapid up move and the head slid down. It came down slowly so I was able to get a block of wood between the vise and head. I use that same block when turning off the mill for the evening, though if I run the head all the way up it only comes down a little bit and stops. Mostly it's not an issue, so much not an issue that I have no intention of making changes. But I'll keep the wood block handy.

If I were inclined to change things, I would investigate a brake on the back of the motor that engaged when the drive was in fault or off. Not worried at all about the belt breaking.

Bob

[letmefixit]

Thanks for your input everyone. The project is coming along nicely. just a few rookie set backs here and there. Need to figure out the NC code thing so i can make something now. or at least test it.

dj

[jetflatline]

I've only got a single ballnut on the Z as well and run the gibbs only tight enough to take out any slop. Since I replaced the stock IH Z nut bracket with a steel one I fabbed, my Z heights from tool to tool have been excellent.

Why would a steel one be better (or worse) than the stock cast iron one? Or are you talking about the original ACME thread and not a ball nut?

[RustedOut]

Why would a steel one be better (or worse) than the stock cast iron one? Or are you talking about the original ACME thread and not a ball nut?

I modified the original to accept the ball nut and wiper brush last year when I converted to CNC. The original cap screw pulled right out of the original casting. It had a shear and tension load on just the cap screw that holds it to the slide. On top of that, the ball nuts use an extra fine thread. That thread may be too fine for use in cast iron, though I've not heard of anyone with a problem there.

The one I made transfers the shear load to the slide and nut mount. I pocketed out the back of the slide to accept a boss machined on the end of the nut mount. I also drilled and tapped nearly twice as deep as the original and replaced the bolt with a longer quality USA made 5/8" cap screw. It can be argued that there is a limited number of threads making full engagement though, but it makes me feel a lot better with it the way it is. It wasn't the material so much that made it better on it's own, but the improvements I made to it as well. And steel is by far stronger than the cast it replaced.

After going through that rework, my Z axis is finally very repeatable. It wasn't before that and I found myself always looking for the errors I was seeing. We're talking a few thou here, but a problem none the less. Now it repeats with no error.

Bob

[jetflatline]

The original cap screw pulled right out of the original casting. It had a shear and tension load on just the cap screw that holds it to the slide. On top of that, the ball nuts use an extra fine thread. That thread may be too fine for use in cast iron, though I've not heard of anyone with a problem there.

Ugh, I have the original IH iron casting drilled/tapped for a ballscrew. I guess "make a new Z mount" should be on my todo list.

[RustedOut]

I wouldn't make it a real high priority unless it's already open for the change, but keep it in mind when you start to see something that looks like backlash on the Z axis. Like I said, never heard of someone having trouble with the 1.125-18unef threads in the cast iron nut mount.

But it does make sense to me to have preventive maintenance as a priority. Several other guys have had problems with the nut mount attachment to the slide and that's been documented in this forum. I'm only advocating there are better approaches to the nut mount than the original IH design. Funny they would have a boss on the original Y nut mount and none on the Z.

Bob

[Cruiser]

Threads in iron v/s steel require a longer thread in the iron to match the thread shear for steel. There is a lot of potential shear force put to the threads by over torquing the bolt. Generally torque in iron is much less than for steel.
I used the original iron block but I drilled the hole deeper and run the thread to the bottom of hole and used a longer bolt from the get go. I easily doubled the thread shear resistance on that bolt. But, I did not go bananas when I tightened the bolt into it as it is just not needed nor did I use locktite.
If the threads are pulled out of iron then it is best to fix with a heli coil and If a deeper hole is possible, even a double heli coil application, and It will never be a problem again. If your not familiar with a heli coil, it is basically a thread in itself and the kit comes with the coils, installation tool, drill, & special tap. It is an easy matter to drill out the old threads with the supplied drill and increase the depth some, I'd use a smaller pilot drill first to deepen hole. Then tap the hole to full depth. Then put a coil onto the tool and thread the coil into the hole. If going for a double, then run it to the bottom and then put another coil on the top. It is necessary to break out the tabs of the coil and if any of the second coil is outside of hole grind it off or notch it and snap it off. then put in your bolt with some never seize and forget it. The coil fix is so much stronger than the base iron that it is now near indestructible.
Because of the shear strength of iron threads, fine threads are never used or should not be used.
When torquing bolts into iron, forget the bolt torque specs from charts and just use common sense, and FEEL the bolt. The shear in steel is 1.5 times (minimum) stronger than iron. Bolts in iron assemblies are generally 'more than' the 1.5 times deeper for a reason.
SORRY IF this comes off like a rant or something, but I saw the implication above that this might be beneficial to some of you. I hope that it is !
If your interested, I started life as a mechanic on various vehicles & jets (F-100) then 35 yrs ago, I got a lucky break (destroyed ankle) and got into machining for vocation. I've seen and worked on lots of iron and don't miss it at all !
I'll end this with a chuckle >>> I possess the education, training, skill, and expertise to invent an answer to any question !
DON

[jetflatline]

It's all useful information, thanks.

While I get that steel is stronger than cast iron, I hadn't considered that the cast iron wouldn't be strong enough for this application. (The vast majority of my metalwork has been auto repair and furniture fabrication where it's trivial to overbuild something.)

Given that I have a 7' ceiling and the only way to get the screw/ballnut out of the Z is to remove the column from the table (or tip the mill on it's side), I might just wait until it breaks to make a new one.

[Cruiser]

I drilled and tapped the iron mount while on the mill. It was tricky but then all I had to do was drill it deeper then run the tap.
Don't get the idea that cast iron is not strong enough to do the job. The deal is that when torqueing a bolt that the threads themselves will pull out and strip sooner than steel will.

[letmefixit]

I used a 2"x2"x4" chunk of 6061 t6 aluminum for my Z nut mount. I machined out the slide assembly to fit a 3/4" x 1.5" cap screw with a collar. Drilled and tapped the mount 1.5" deep. I hope that will be beefy enough. So far seems to be ok. Have not really put a load on it yet.

dj