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View Full Version : Knee Mill CNC Conversion- Your thoughts???



Syil_Australia
09-12-2007, 06:49 AM
Hello...

I have had a Knee Mill for a few years (pic below) and have often considered converting it to CNC.

The problem to me has always been how to convert the X Axis. Moving the knee is too heavy, moving the quill reduces Z travel and has some rigidity issues.

I like the machine as it is robust, has good solid tables and enough X & Y travel for my needs but it is manual.

This afternoon I sat down and had a good look at it and an idea came to mind on how best to convert the Z axis.

Here is: (Hope this is clear)

The column is very rigid and has Z Ways built into the column for the knee to move. Now if I move the knee to a low position and lock off that will ensure the table/knee is rigid.

Remove the head and Ram and fit an extension to the top of the column thereby extending the Z axis ways by say 300mm or so. Do away with the ram. machine a steel plate to fit onto the Z ways (with gibs etc) and fit a 1inch ballscrew to the Z slide. The head can then be re-fitted still with the tilting mechanism. This will require some mods to the pulley drive system however I figure I can use a direct drive belt and fit a VFD to the existing motor (3hp).

I know I will lose some Z travel but from my rough callcs will still have around 450mm (18inch)Z travel. Will probably also lose around 1 inch or so of Y travel. Overall the mill will have more travel that any bench mill etc.

Would appreciate your thoughts on this proposed Z axis conversion. The CNC drives/electrics etc are all covered so please do not comment on this.

Look forward to your replies.

cheers

Frans

Splint
09-12-2007, 09:16 AM
Hi Frans,
have you considered a pnumatic ram or a counter weight on a cable system to help take the load of the knee, and keep the head/spindle in one location and get the z travel from the knee moving?

Cheers
Splint

macca1001
09-12-2007, 09:48 AM
Hi Frans
Try typing in Hafco Metal Master HM-52 CNC Conversion , I dont know how to post the thread but this is a conversion done by Chich2 in Queensland. He used the pneumatic ram on the knee, it is a great conversion
Macca

epineh
09-12-2007, 09:53 AM
Here is the link Macca mentioned :

http://www.cnczone.com/forums/showthread.php?t=25895

Chich has done a great job, his build log should answer a few questions for you.

Cheers.

Russell.

Syil_Australia
09-14-2007, 07:50 PM
Thanks for your reply guys... much appreciated.

Had a look at Chich's conversion. looks great... from reading his posts (some) it seems he started to convert the Lead screws and then changed to Ball Screws?.... which is what i would do...

Still not convinced on the Knee conversion though..

Want to futher explore using the Quill. I know this will require sleeving the spindle and having it re-ground for fit. Do you know anyone who has done this?... what is involved in sleeving etc.... What do you sleeve it with, Brass or.... would need to take into account wear and lube factors....

Look forward to your replies.

cheers

Frans:)

chich2
09-18-2007, 09:45 AM
Frans,
Which Hafco do you have? HM 50, 52 or 52G? I priced getting my quill hard chromed up to size but it was going to be too expensive. Since my HM-52 has a horizontal spindle as well as the vertical one, the Z knee choice allows me to use both spindles CNC. Thats why I chose to automate the knee. Works great.

Chich

Moondog
09-19-2007, 06:40 AM
Chich.

Mine is the HM50 mill.

After all my research etc, I have decided to CNC the quill. Just deciding on what method I will use. Will have to start a new post when I make a start.

Cheers

Frans

chich2
09-24-2007, 08:58 PM
Frans,
Yes the HM 50 does not have a horizontal spindle so in YOUR case, the quill will work fine. Provided you can eliminate the slop in the quill.
You will be happy with a CNC machine of this type.

If you like, feel free to read the bit of a speel in my HM-52 thread of how I was going to do mine. (Before I decided to do the knee) The main reason for desigining it this way was so that the ballscrew was protected from swarf and chips more than other designs I've seen. I also wanted a quick release system on the quill so I could quickly switch from CNC to manual without any fuss.

I look forward to seeing your CNC conversion,

Chich

Moondog
09-25-2007, 06:39 AM
Chich..

I read all of your thread.... great conversion and I agree with your decision to convert the knee.

For my conversion I am considering the quill. Thinking about removing the quill controls and attaching the Z drive to the side of the head. I will make a large clamp arrangement to attach to the spindle and use a Ballscrew to control the spindle movement. this will add some rigidity to the spindle however, I think it will also be necessary to sleeve the spindle bore as well.

If I sleeve the spindle bore I will also add oiling grooves as part of a one-shot oil system.

Have to find a machine shop to do the boring though....

cheers

Frans

Moondog
12-25-2007, 05:23 PM
Hey guys.

Still thinking of how to design the Z axis.

Had a thought about removing the Head from the Ram and fitting a Z Axis Linear Slide similar to a router Z setup and then re-attaching the Spindle head to the slide.

If I used Alum plate say..30mm together with THK linear slides and kept the Z travel to around 200mm, Would this type of set provide enough rigidity??

Interested to hear your thoughts and advice.

cheers

Moondog

chich2
12-26-2007, 10:02 AM
Great idea. I don't know how rigid it would be because I have never used liniar slides before.
I wonder if you could lower the knee all the way down and lock it off, Then make a new spindle that slides up and down the column dovetail with a counterweight just like the industrial machines do. You could then use the knee movements as a manual coarse adjustment for different size jobs. The screw would have to fit in close to the column. It wiould be nice and rigid. Just a thought.

Chich

Moondog
12-26-2007, 03:53 PM
Hello Chich2.

My idea wasn't to use the existing column dovetail as it doesn't extend high enough unfortunately.

1.) I was looking at adding an Z linear slide assembly directly onto the end of the ram (ie, 2 plates with Linear bearings and ballscrew) and then re-attaching the spindle head to it. With this setup you only need around 150mm of travel as the existing knee can still be used in its current config.

Linear Bearing systems are used on a lot of high end VMC's.

2.) There is another idea as well....(full of ideas) using the existing quill but fitting a collar to the spindle. On one side of the head fix a Ballscrew that attaches to the collar and on the other side of the head fix a Linear guide system to the collar. The would provide additional rigidity and control of the spindle and should eliminate any side play whilst providing the Z axis movement. Hope you can understand what I mean...

cheers

Frans

davo727
12-26-2007, 05:23 PM
I like your idea of making a new upper unit with linear rails and attaching the head to a Z slide. I am looking forward to seeing your design. I will make a few drawings too and will post if I come up with anything that looks good. Cool idea, using the Nice heavy bridgy base and lock the old knee down and make new upper with linear rails and slide Yeah! :)

Moondog
12-26-2007, 05:37 PM
davo.

Thanks for your reply. glad u like the idea.. think it has some merit.

Assume you are referring to idea no 1 ???

Was thinking to use a 20mm ballscrew, 25mm linear rails and 30mm Alum plate. The base plate would be bolted very securely to the ram.

Would need to change the motor mount as it currently hangs over the back and would probably interfere with the Z movement downward??? maybe...

Do u think it would be rigid enough? any machine engineers out there?

cheers

Frans.

davo727
12-26-2007, 07:00 PM
Hey moon, Will your head have an extention between the mill head and your new Z axis slide? Also where do you figure on putting the ballscrew? If you make a square column to bolt on top of your base, the ballscrew can be inside the column. Can you post a rough drawing of a side view of your mill with measurements of the base, head and everything so i can get an idea on where things might fit? Also Im thinking 3/4 ballscrews are too small. 1 inch much better. I added a pic of a SW Ind mill to show how they use a head similar to a bridgeport type on a bedmill. Also please post a pic taken of a direct side view of your mill if you can.

Moondog
12-26-2007, 08:10 PM
Davo...

next time I go into work I'll take a pic.

The photo of the mill in your post would be ideal, but on my mill the column (dovetail) is not long enough to fix the head to it.

What I'm proposing is a Z Axis setup that is used on Routers. 2 * 30mm Alum plates with the Linear rails/bearings and ballscrew in between. One plate is fixed to the Ram and the Spindle Head attaches to the other plate. do you know what I mean?.

cheers

Frans

davo727
12-26-2007, 08:20 PM
Umm , No I have to see a drawing to see what your plan is.

Moondog
12-26-2007, 09:11 PM
Davo.

here is a real quick drawing, not to scale of the proposed Z axis.

cheers

Frans

davo727
12-26-2007, 09:30 PM
OK, thats cool. simpler than what I was thinking. I was figuring on a longer vertical rail setup and having to space the spindle assy out but with your machine design thats not neccesary. What length rails and Z travel do you think you might do? So next you can figure out height of linear rail and block assys and figure out if thats enough gap for the ballscrew and nut or you might have to space the rails out a little. Anyway I bet it will work good. Dave

Moondog
12-26-2007, 09:40 PM
OK, thats cool. simpler than what I was thinking. I was figuring on a longer vertical rail setup and having to space the spindle assy out but with your machine design thats not neccesary. What length rails and Z travel do you think you might do? So next you can figure out height of linear rail and block assys and figure out if thats enough gap for the ballscrew and nut or you might have to space the rails out a little. Anyway I bet it will work good. Dave

Davo...Trying to keep it simple. Lenght of rails etc would only be around 400mm or less. Only want travel of around 150mm to lessen flex. the height of the rails, blocks and Ballnut will determine if the rails fit flush etc... Next will be to re-draw everything to scale to determine sizes of components etc...

Hopefully it will be stiff enough.

cheers

Frans

davo727
12-26-2007, 09:47 PM
Sounds like a good plan, I look forward to seeing it go forward.

chich2
12-27-2007, 06:28 AM
Yea I understand. Like you say, under that configuration it would pretty much eliminate all quill play the machine may have.

Great Idea. Now to build it.

Chich