Z axis Ballscrew back drive concerns - CNC mill conversion

[Bally123]

Hi

First post (of many I suspect).

I am converting an adcock & shipley 1es mill to cnc. I have raised the vertical head, added a dedicated vertical drive motor. Having serviced and set the gib strips I am happy with the machine and its time to look in more detail at the CNC conversion.

Right now I am favouring Linux CNC with MESA motion control. I feel stepper motors will suit my budget and expectations, combined with ballscrew conversion. Nema 34 for z axis and probably Nema 24 for x/y

The z axis carries a fair bit of weight, I did some calculations based on 5mm pitch ballscrews and this showed holding torque would be ok using a nema 34 motors. However I am concerned that the axis will back drive when the machine is powered off.

Anyone had any experience with similar problems

What kind of holding torque might a stepper give when not powered.

Regards Bally

[magicniner]

Counter-balance with a gas ram, go for a few kilos less than your weight to avoid backlash and free play issues,

- Nick

[Bally123]

Hi Nick

Interesting suggestion. Was thinking about counter balance options but gas struts hadn't crossed my mind.

Better go figure out how much this thing weighs......

Thanks andy
..

[johnsattuk]

Could use a stepper with built in brake

[Bally123]

Hi John.

Yes fair point. I was also thinking similar lines at one point. An electromagnetic brake certainly seems to resolve the back drive issue but pushes up cost and control complexity. (my budget suits basic range components ;-))

My other concern is the stepper sees a high load trying to lift the weight of the knee and a much lower load lowering it.
I am therefore tending toward the counter balance solution as it means the stepper sees more equal loads in both directions. This possibly means a smaller motor, or at least less maximum load on the motor.

A brake will be plan B if plan A doesn't come off.......may come down to actual weight and stroke details...that's my mission to find out today.

Cheers for your post....

andy

[johnsattuk]

Hi John.

An electromagnetic brake certainly seems to resolve the back drive issue but pushes up cost and control complexity.

andy

I have Yaskawa sevo's on my mill with built in brake controlled by the driver. I also am building a router at the moment and using a servo with a brake. I also have Vexta stepper motors with built in brake controlled by the driver. They are industrial stuff but can be aquired relatively cheaply with plenty of Ebay searching.

Gas struts can be quite a good solution, (need to get one with no built in damping ), c/balance weight can be a bit problematic finding room for it to travel in and also control any bouncing with fast movements. Both solutions are IMO second choice solutions.235882358623587

[Bally123]

Hi John

Thanks for reply and photos, very helpful.

On closer inspection a gas strut is going to be awkward to mount because of stroke length. Not impossible but a bit of a hassle. I haven't discounted this yet as I like the "assistance" it provides to the motor when lifting the table. Maybe I will end up with both strut and brake working to complement each other.

In regards to the braked stepper set up I have a couple questions:

I am interested to know, in terms of £, what is relatively "cheap" in your mind? - based for example on the Vexta example you show. - what would you pay?

Secondly if I sourced stepper and driver without a brake I think I could add a brake set up, I guess I could control the brake either from my motion control board or simply release brake when drives are powered up.

Got some work to do on slideway, I found some play in the vertical axis that needs sorting first.

Cheers

Andy

[magicniner]

On closer inspection a gas strut is going to be awkward to mount because of stroke length.

Post a photo and explain the nature of the problem, there are simple engineering solutions which can place you a mounting point in "thin air" ;-)

[johnsattuk]

Hi John
I haven't discounted this yet as I like the "assistance" it provides to the motor when lifting the table. Maybe I will end up with both strut and brake working to complement each other.
I am interested to know, in terms of £, what is relatively "cheap" in your mind? - based for example on the Vexta example you show. - what would you pay?
Andy

I do actually have strut and a brake servo on my mill for the same reason.

Cost of vexta steppers is very variable depending on how long you search for, very round figures £100 each, 5 phase, 500 steps, useful torque to 3000 rpm, direct 230v connection, I believe the steppers run at 162 v

.23629

[Bally123]

Just to be clear thats including the drive and motor in that estimate?

Cheers andy

[johnsattuk]

~£100 per motor, ~£100 per driver.

If you do a search for VEXTA 5 phaseon eBay. Just had a quick look and as I said they vary a lot, there is a braked motor on there at the moment for £130 with offers, and drivers similarly priced.

[dazp1976]

Take off the head.
Weigh it.
Get a gas strut that is rated just a little more than the weight of the head.
Fit.
Done.
Or get 2 struts that add up to the weight when combined.

The head on my little X2 weight about 15kg so I went with a 20kg strut. Works.

[picclock]

Not sure if this helps but ..

I am converting a smaller mill and one of the options I have is to use a pneumatic cylinder fixed to the rear of the column and pulley on top with a steel cable connecting to the head. The cylinder will connect to a small pressure vessel, and a pressure gauge. When filled with compressed air at the correct pressure the weight can be offset.

The advantage this offers is that assistance is pretty much constant over the full range of head positions (ratio of cylinder displacement to air tank volume), the amount of assistance can be varied by adjusting the air pressure, and there is no damping of the movement (subject to size of pneumatic connection between cylinder and tank). If damping is needed just restrict the flow of compressed air between the cylinder and pressure vessel.
This doesn't solve the inertia of head movement (dynamic load) issue but goes a long way to improving the static loads. Additionally it significantly reduces the bending load on the column.

It is also possible to overcompensate the load such that the stepper drives the tool into the work and when the power fails the head rises to the top, possibly with a short damper for the last few centimeters or so.

Best Regards

picclock