Knee Mill Power Downfeed

[cathead]

I have a standard 9x42 Step pulley Enco vertical mill that was purchased new. It has been in my
posession for about 20 years but have rarely used the power down feed feature. I decided to use it
to drill a 1/4 inch hole at the slowest down feed speed at maybe 250 rpm drill speed. It started drilling
fine but the engagement bar kicked out after getting started. I took the assembly all apart and
made an adjustment there. Now the overload clutch slips when trying to drill a 1/4 inch hole.
I see a compression spring that apparently exerts pressure to the clutch overload cogs and
advanced that a little bit but it already seems fairly compressed. That didn't help much. I don't
want to break something inside so am wondering if drilling a 1/4 inch hole in mild steel would
be asking too much for this machine. The material thickness was .375 of an inch.
Just from estimation, it would probably down feed a 1/8 inch bit, however I prefer to manually
feed small bits as it is good practice to clear the chips using the down feed handle. Is this
something to look into or leave the machine as is and use the power down feed for only boring
operations?



Edit: After a little research on the web, I found that the maximum drill size recommended for a standard Bridgeport type mill is 3/8 inch for power down feeding.....

 

[talvare]

250 RPM for a 1/4" drill seems pretty slow. I typically run a 1/4" bit somewhere around 750 to 1000 RPM, depending on the material being drilled. You may try increasing the speed and see if it improves.

Ted

 

[cathead]

250 RPM for a 1/4" drill seems pretty slow. I typically run a 1/4" bit somewhere around 750 to 1000 RPM, depending on the material being drilled. You may try increasing the speed and see if it improves.

Ted

Yes, I agree that 250rpm is somewhat slow, however, at 1000 rpm, the rate of feed would be quadrupled. I will give it a try
and and see what happens. Thanks for the reply.

 

[Wreck™Wreck]

Yes, I agree that 250rpm is somewhat slow, however, at 1000 rpm, the feed rate would be quadrupled. I will give it a try
and and see what happens. Thanks for the reply.

Actually the feed rate remains the same as it is gear driven from the spindle, .xxx inches per revolution regardless of spindle speed. Be aware that locking the spindle lock adds to the force required and will trip the overload mechanism on some machines.

An X axis power feed with a stand alone motor and gear box is a different story as the spindle speed and the feed rate are independent.

 

[cathead]

Actually the feed rate remains the same as it is gear driven from the spindle, .xxx inches per revolution regardless of spindle speed. Be aware the locking the spindle lock adds to the force required and will trip the overload mechanism on some machines.

An X axis power feed with a stand alone motor and gear box is a different story as the spindle speed and the feed rate are independent.

If I speed up the spindle speed, the rate of down feed increases with it. I think you are right that the advancement per RPM remains the same.
Just for fun I tried drilling at 1000 RPM and the same feed rate as before. The drill advances about 3/16 of an inch and then the
engagement bar kicks out. Maybe one would be best off not to use the automatic down feed for drilling and just use the handle...

 

[Bob Korves]

Make sure you are using the smallest of the three possible feed rates, at least to start.

 

[Wreck™Wreck]

If I speed up the spindle speed, the rate of down feed increases with it. I think you are right that the advancement per RPM remains the same.
Just for fun I tried drilling at 1000 RPM and the same feed rate as before. The drill advances about 3/16 of an inch and then the
engagement bar kicks out. Maybe one would be best off not to use the automatic down feed for drilling and just use the handle...

Yes, the feed is geared to the spindle, the feed per minute changes but the feed per revolution remains the same so nothing has changed but the spindle speed, this feed rate can not be changed outside of the ratios built into the gear box.

A NC mill with a Z Axis drive is a different game, one may push the Z axis at maximum speed when programming.

I am not familiar with Bridgeport copies but the feed stop is supposed to do this when it hits the knurled wheeled stop (like the stop on the side of many drill presses) on the front of the spindle, this will not hurt anything, controlling depth is why it is there at all.

 

[cathead]

Yes, the feed is geared to the spindle, the feed per minute changes but the feed per revolution remains the same so nothing has changed but the spindle speed, this feed rate can not be changed outside of the ratios built into the gear box.

A NC mill with a Z Axis drive is a different game, one may push the Z axis at maximum speed when programming.

I am not familiar with Bridgeport copies but the feed stop is supposed to do this when it hits the knurled wheeled stop (like the stop on the side of many drill presses) on the front of the spindle, this will not hurt anything, controlling depth is why it is there at all.

Yes, all that makes sense so I may take one more look at the feed stop and see if I can adjust it so it kicks out at a little more pressure.
The knurled wheel is not used much. I generally keep it rolled to the bottom so I have full travel of the quill. Thanks for your comments.

 

[12bolts]

What happens if you run the downfeed but not actually drilling anything. Does it still kick out?

Cheers Phil

 

[cathead]

What happens if you run the downfeed but not actually drilling anything. Does it still kick out?

Cheers Phil

Sure, it will kick out when it reaches the end of the quill travel.