[tulsah]

I am in the midst of a CNC conversion to my IH mill. As part of the conversion, I am switching to belt drive.

This means that the mill head will be drained of all oil, the gears will be gutted out an the cast head cover removed - thus removing some weight. The belt drive assembly will then add approximately 22-25lbs back into the head assembly.

My real concern is the weight of the new motor. I have found two motors - one that is a real gem and the other that meets most of the requirements. The trouble is that the better motor weighs in at 102lbs, while the other weighs 62lbs.

I will be using 850oz-in servos through a 3:1 pulley reduction, so I'm confident there will be plenty of torque at the ballscrew. I guess I'm just wondering if the weight of these motors is excessive and something that I should be concerned about with the setup I have described. If it's not deemed excessive, my preference would be for the heavier of the two motors, but would go to the other motor if the general consensus was that the weight was too high.

Thanks for any help.

-tulsah

[Runner4404spd]

counterweight the head and then use the better motor.

[Cruiser]

What I see is that with the 3to1 ratio you are wanting speed of axis movement. If you went to 5to1 you would be in a much better suited power band and not need extra hoops to dance through. It would still provide a little more than 100 inches per speed and that is plenty. Also, you will find that especially when drilling with large bits, you will want the power in the feed rather than speed. All larger tooling will benefit from the power increase. I have found that even this is marginal at times !

[tulsah]

Thanks for these responses. I just pulled the trigger and purchased the larger of the two motors as it was just too good to pass up on.

I was told today that the conversion to belt drive reduces the head weight by approx. 60lbs. That said, the net increase in weight should be somewhere in the order of 40lbs with the new motor installed.

I already have my servo motors, pulleys and belts, so changing those ratios now is not entirely preferable. I suppose I'll just have to give it a try and see what it does. If the Z-axis servo winds up being underpowered, it would likely be easier to switch it out for a more powerful unit.

Thanks again.

-tulsah

[RustedOut]

I'm in the process of building parts for my own belt drive conversion. And I have two very nice motors to choose from, a lighter weight 2 hp and 100 lb 3 hp motor. I'm leaning hard toward the larger, heavier motor because of the extra torque it brings to the conversion.

I've never weighed the pieces that will be removed, but the list is exactly like yours including the cast top plate. But I'm also wanting to build a quick change draw bar too and am going to run two belts if I can pull that off. Those pieces all add more weight. I've decided, now that I have it fixed, my Z servo motor can handle the weight with the 4 to 1 reduction it has and the larger 1125 oz. in. motor until it's turned of. So I'm thinking I need some kind of brake there to keep the head in the air when things are powered down. Still working that, but have been thinking it can be on the smaller diameter end of the servo motor shaft. It's not going to take much to keep it from sliding down, just something.

But I agree with the comment about the drive ratio. You may find it wise to change to a greater ratio there. I would have no problem changing to a 5 to 1 ration if that would allow it all to work, especially the issue I see with it after it's powered down or dealing with the extra weight I'm throwing on it.

Bob

[tulsah]

I've decided to heed the advice from a couple of the above replies and convert the z-axis drive to a 5:1 reduction rather than the 3:1 for the other axes.

It turns out that getting this ratio is a big pain as most of the timing pulleys max out at either 72 or 80 teeth. This makes the driven pulley very small. I was able to make it all work out with 2mm pitch pulleys for the Z as opposed to the 5mm pitch for everything else.

Use of a brake was also mentioned to keep the head from dropping when the power is off. I don't see much mention of this for any of the build thread I've checked. Is it a common issue or just peace of mind?

Thanks again.

-tulsah

[RustedOut]

Use of a brake was also mentioned to keep the head from dropping when the power is off. I don't see much mention of this for any of the build thread I've checked. Is it a common issue or just peace of mind?

Thanks again.

-tulsah

For me, it comes after over a year of problems with my Z and may be a peace of mind thing as you say. But fighting with the Z components for that long and watching the occasional crash into the vise or work piece does something to you mentally. Or it did me at least. Keep in mind my mill currently still is a gear head with the stock IH motor, so there's no extra weight up there. And the crashes were of a violent nature at times. A real dramatic and stressful event.

I have to park it at it's home position or there's a risk the head will slide down on it's own after powering it down. Some may say I need to tighten the gibs, but I have them where there isn't any slop in the ways and the .003" backlash in the ballnut is taken care of by gravity. So, yep, I'm thinking if I change nothing in the drive system there's going to be a problem when I go adding weight to the head.

There is another direction I could go in changing the single ballnut to an anti-backlash version and tightening the gibs, but for now I'm looking at some way to just keep the servo motor from spinning after power is removed from it. I'm thinking it would be like a parking brake on your car. Might be completely mechanical. Certainly doesn't mean I'll go that way and doesn't mean I'll actually have a problem when the conversion is finished. Just means I'm wary of it.

I certainly got no idea what others have done or if they've had similar problems. But adding counter balances to the heads may indicate some concern there.

Bob