scmi drill bank

[wcmboss]

My record 220 has a drill bank that is driven up and down by a stepper motor gear driven into a cogged metal lined belt.
The problem I am having is as the drill head does a line boring operation or any multiple drilling ops the depth that the drill go gets to be less and less. They may start out drilling to a depth of 3/4" but by the time it is done it may barely be touching the workpiece. If I reset the drill head to the home position it will start out ok but by the time it is done same outcome. I have replaced the stepper motor and the drive belt that it uses. So these 2 items should be back to new condition. If I do not reset the head to home position after each run the drills go deeper with each succeededing run. After a few the drills wont evan clear the workpiece. Have called scmi with the problem with no good help. They have a rack system to replace the belt. At one time mine worked fine so if I have the belt replaced and the motor replaced this should eliminate the mechanical end of it. I hace no exp with the stepper motor drives. I sure could use some help on this or a good tech to talk to about it. Thanks to all who respond. Sorry so long winded.
Jay White
White Custom Millworks
Belle Plaine Ks

[NC Cams]

May be an issue of "step loss".

Steppers don't feedback their position. The computer sends out a step and merey assumes it (the stepper) executes it. If resistance is greater than force of step motor, step is sent but force overrides it.

First thing to check is for binding. If the system binds or has enough residual friction, the stepper may not have the "oomph" to overcome it.

By sending it "home" you reset the system and it all starts over or it loosens stuff up.

How so??? If you're dithering/oscillating back and forth and have a sliding bearing (as opposed to rolling element bearing) supporting the device, you could be 'punching a hole' in the lube locally and the system is binding as a result.

I'd first do a thorough cleaning and maintenance/lubing of any sliding elements. Check, relube or replace any rolling element bearings that are not moving freely. Check for linkage binding or motor bearing binding.

In this case, friction is probably your unseen enemy.

[ger21]

Are there any pneumatics involved?

[wcmboss]

Gerry
The drill bank has pneumatics on it for the drop down of the drills themselves. You can call or drop any or all of the drills. So when I line bore I call 3 5mm bits to drop. As far as I know the pneumatics just extend the drills down. I checked to see if they were being pushed back up into there bores but they did not move. Thanks for the reply.
Jay

[ger21]

Our masterwood works the same way, and we have had issues with moisture in the airline causing some drilling problems. Increasing lubricant and a bit more pressure worked for us, but ours weren't drilling deep enough.

[arch1]

but the Horizontals would fall to deploy...

This unit from my opinion is pretty dumb it has no feedback.

waiting for the mechanic to rebuild the unit so it has sufficient air pressure.