Tuesday, June 10, 2014

Mill: Power Feed

I've wanted a power feed for the X axis for a while now, not only because it gets pretty tiring cranking away, but also because a power feed is much smoother than I could ever be and that will produce a better finish. I decided to wait until I installed the large table upgrade since that would effect how the motor mounted to the table.

I had been looking at using a car's power seat servo as motor for a while, and I finally committed when I saw apointofview's post at Home Model Engine Machinist use the exact same motor to create a very elegent and simple power feed: http://www.homemodelenginemachinist.com/f28/x2-mini-mill-power-feed-21466/

The new large table's lead screw extends through a bushing at the left end of the table and has a slot cut in it. I pinned a steel key in place in the slot. I then took a 14" flanged nut and milled a slot in the middle of it, just large enough to fit onto the key.

Key fitted to the slot in the X leadscrew.
14mm nut with slot milled into it.

For the motor itself I used a widely available servo for a power seat in a car. You can get them from several places including Amazon: http://www.amazon.com/dp/B005IR1NBA/ The motor is reversible from left hand to right hand orientation. I used this for the speed controller: http://www.amazon.com/dp/B00CW82ZPG/ since it was cheap and had good reviews. If I were to do it again, I probably would have bought this to save a couple of dollars: http://www.amazon.com/dp/B00CBBIG34/

For the direction switch I bought a DPDT DC switch that's center off. That way I can set the switch to center to stop the feed. I found a DPDT switch on eBay specifically design to be used as a direction control for DC motors, so that saved me a couple of wires and a bit of soldering. For the table jog I decided to just use the speed control and wired in a switch set the control's speed adjustment pot to maximum.

Wiring diagram.

For the power I used an old 15v laptop power supply since they tend to be pretty good quality. I wasn't worried about using 15v since most cars run between 13-15v anyway.

The whole mess of wiring was then packaged into a project box from Radio Shack and attached to the side of the lathe. The extra switch on the top right of the control box is the on/off for the spindle light.

Upper left is power, lower left is jog, center is direction.

A piece of aluminum plate was machined to bolt to the end of the table, and to accept the bolt pattern of the motor's housing. The back of the motor mounting holes in the plate were countersunk to provide clearance for the end of the table. The motor's leadscrew was removed and four sides milled flat so that a 3/8" drive socket can easily slide onto it. The leadscrew was then cut down so it would clear the end of the table's leadscrew's key when mounted in place.

Leadscrew machined down and motor bolted to mounting plate.

A 12 point 14mm socket was slid onto the leadscrew and then everything was mounted on the table. To engage the power feed I slide the socket forward until it engaged the 14mm nut. To disengage I simply slide the socket back. I purposefully made everything with very loose tolerances to allow easy engagement and to prevent any binding from misalignment. I cut some grooves into the socket on the lathe for a little more traction.

Power feed assembly bolted to the table. In the picture the power feed is engaged.

The latest beta version of Android DRO (now called Touch DRO) will display the feed rate for a selected axis. When using that with the power feed I can set the feed rate very exactly. As the power feed is configured now it will hold a speed to within 1/2" per minute, which I'm quite happy with. I think if I remove the X hand wheel's handle before using the power feed that variation will go away, as I suspect the weight of the handle is speeding up the feed for half a revolution and slowing it down for the other half.

FOLLOW UP: After a couple of days I started getting some vibration in the motor when running in a certain direction. I was able to fix the issue by taking out the plastic screw which supports one end of the motor's shaft and pressed a small ball bearing into its bore. This gave the motor's shaft a solid and smooth surface to push on and rotate against.

UPDATE: I've since relocated the control box to the side of the top of the column. It's a lot easier to maintain the mill and access the controls.

UPDATE: I added a small cover to the leadscrew engagement to keep chips off it. It's mounted via magnets on the underside of the cover.

Cover in place over leadscrew.

Cover upside down to show the mounting magnets.

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