Mill, CNC conversion questions (quite likely daft!)

[John Hill]

I have been thinking about power feeds on my turret mill, one like this:
http://www.scottmachinery.co.nz/dbimage.php?meth=norm&img=MTUR-0-0001__1

.... so laying awake at night, thinking, thinking, (always dangerous) "If I am going to make power feeds I might was well jump straight to CNC".

Why do hobbiest seem to always do their CNC projects on smaller machines?  Cost of the steppers and controllers? Cost of conversion to ball screws etc?

Steppers should not be a problem as I have a few Fisher and Paykel stepper type washing machine motors, about 1HP at peak by my estimations,  controllers I would have to build myself but surely they are just small controllers with current amplifiers required for the big motors.  That leaves the ball screws.......  am I right in thinking the ball screws are desirable to avoid backlash?  Any other reasons? If I can operate it by hand without falling foul of backlash cant code be written to do the same?

Big question then "Am I overlooking something?"

[Rob.Wilson]

Hi John

If i had your size of mill it i would convert it to CNC ,,,,,,, most lads buy  a hobby mill learn to use it then decide to convert it to CNC ,, space is a big factor with with larger machines .

Bigger machines ,,,,,, larger cost to convert to CNC   

Ballscrews are desirable  to eliminate backlash and are allot smoother  in operation ,,,,,,, Mach3 dose have an anti backlash compensation feature ,were you program in the backlash of your screw ,,, suppose this feature is only going to work if the screw is new and is not worn more in the middle section of the screw were most work is done on the table . Also having backlash in the system causes problems as the tool path changes direction , causing tool dig in or breakage.

Hope fully someone with more CNC experience will chip in .


Rob       

[John Hill]

Hmmm, thanks Rob.  Actually, I had to spend all my lifes savings to have a new house built so that I would have room to have things like that little mill..

I assume ball screws would be quite expensive but fortunately the feed screws should be hardly worn yet.  Now I need to sit and think how I can connect these motors, that are about as big as large dinner plates, to the X, Y and Z.  Incidently, is there any reason why I should not plan to lift the table rather than move the quill for the Z axis?  (The quill movement feels particulary crappy on this machine).

[Rob.Wilson]

  thought your mill was a full sized one John ,, looks a big bugger in the photo 

Ballscrews are not so expensive ,,,, but the nuts are   
http://www.zappautomation.co.uk/product_info.php?cPath=1_61_62&products_id=606


I would go for moving the Knee ,,,,,,,,, not allot of point in having limited Z travel    ,, i will be locking off the quill on my 626 mill and using the knee , when done i will still be able to use the mill manually if needed via the hand-controller and as a drill .

Rob 

[kvom]

Ball screws make manual milling more difficult as you have to lock all the other axes to use one.

Adding a power feed  to just the X axis is good for almost all manual mill purposes.  On my BP mill I use an electric drill to raise and lower the knee when more than 1" movement is needed.

[DMIOM]

Happy-almost-Christmas-Day to John

Ball screws make manual milling more difficult as you have to lock all the other axes to use one....

yes, ballscrews are way more efficient - but they can back drive as well (from cutting forces or gravity) so are less frequently seen on purely mechanical machines.

Once you've got a good CNC setup with a pendant or some form of jog shuttle, you'll be unlikely to use manual handwheels again.

......Steppers should not be a problem as I have a few Fisher and Paykel stepper type washing machine motors, about 1HP at peak by my estimations.....

Also just on those steppers - if you're planning on using the machine with power feeds from those steppers which have a more motive-power than machine-movement heritage, you may find (a) they don't have that many steps/revolution, and (b) if you try to use the machine manually by just powering-off the steppers, you may discern some cogging as you try to nudge the handles...

In terms of selecting the type/size of motor (stepper/servo) there's a good white paper written by Mariss on Gecko's website.

cheers /  Dave

[John Hill]

thought your mill was a full sized one John ,, looks a big bugger in the photo 

I thought it was big too Rob, until I saw one in someone else's workshop where it looked like something for them to hang their coats on and somewhere for the lunch box and electric kettle....

[John Hill]

Ball screws make manual milling more difficult as you have to lock all the other axes to use one.

It is becoming very unlikely that there will be ball screws in my mill!

Adding a power feed  to just the X axis is good for almost all manual mill purposes.  On my BP mill I use an electric drill to raise and lower the knee when more than 1" movement is needed.

Thats about what I was thinking, I started out thinking of powering the X axis then began to think "Why not go the whole hog?"

[John Hill]

Also just on those steppers - if you're planning on using the machine with power feeds from those steppers which have a more motive-power than machine-movement heritage, you may find (a) they don't have that many steps/revolution, and (b) if you try to use the machine manually by just powering-off the steppers, you may discern some cogging as you try to nudge the handles...

In terms of selecting the type/size of motor (stepper/servo) there's a good white paper written by Mariss on Gecko's website.

cheers /  Dave

Selecting the steppers will be pretty much a process of looking in my junk cupboard at whatever is there,  I have the washing machine motors and a variety of very small steppers from computers etc.  I do have two or three big(ish) DC motors from old computer tape drives (the big ones we used to see in movies), there were used in some sort of servo mode in the tape drives so thats an area I can study but steppers would be breaking less new ground.

Here is a picture of the washing machine motors on someones' mill

There are 47 steps per revolution which might be too coarse for a mill?  Yes, they do cog but it is very easy to turn them by hand and it is the ribbed plastic bit that is the rotor.

[Later, after standing in the shower!]  Since I have DROs on the X and Y and presumably I could fit a scale to the knee for Z, do I really need steppers or just a way of moving the table by small increments?  I am thinking of the tape drive motors..

[Swarfing]

John

I have to e honest these motors will just not do. 47 steps per revolution will give you very course cuts. A standard stepper will give you 200 per rev. This will divide down to 1600 if you micro step. The time and effort you will waste getting these to work (god knows how many amps they will draw?) i would just get some new ones?

You may also get confused on whether you were drive the car or mill with them 

[Davo J]

Hi John,
There are power feeds on ebay from Taiwan for $300 delivered. It's either that or your not going to get much change out of $3000 for us guys down in this part of the world.
I am going to CNC my mill but so far have only bought the ball screws and double nuts, which cost me $650 off ebay from linearmotionbearings.
Kelling have the DC servos for around $150 each, then you need the encoders, drivers, power supply etc, etc.

So it either buy a $300 power feed and spend a day fitting it, or spend the next 6 months or more converting it to CNC, plus 10 times that money. There is also the steep learning curve for CNC.

I am not trying to put you off, just putting it in black and white.

My machine is a similar size to yours, so have a look at Chich's conversion in the link below to give you an idea. 
You will see he went with the standard screws and custom nuts at first, but found they where not really suitable so he went back on it all and installed ball screws. If your going to do it just go for ball screws strait up and be done with it.
http://www.cnczone.com/forums/vertical_mill_lathe_project_log/25895-hafco_metal_master_hm-52_cnc.html

Dave

[kvom]

"So it either buy a $300 power feed and spend a day fitting it, or spend the next 6 months or more converting it to CNC, plus 10 times that money. There is also the steep learning curve for CNC."

You get no "feel" using a CNC mill, so you need to be proficient in feeds and speeds or you'll break a lot of tools. 

[John Hill]

OK, washing machine steppers are no good................

Now then, back to the tape drive motors.  It would be an achievable project for me to gear those down for powered X,Y and possibly Z.  The question "If I have DRO scales I can read does that negate the need for encoders etc?"

Regarding buying power feeds and CNC kits, yes I could do that and although it might be a hit close to the waterline I doubt it would really sink the budget.  For me, the activity is "shop play".

[Davo J]

Hi John,
Like you all my gear is just for fun as well.
A small CNC router project might be better to start off with. You would be able to engrave and do aluminium etc on it. This would be a cheap way of getting into CNC, and once you learn the basics you could move on to bigger things.
I am going to go this way, and that way I will have a CNC machine to do engraving any time I need it, and if I have a crash (which I am sure I will) it wont be expensive like it would be on my big mill.
My daughter goes to the markets and said there is demand for name plaques etc, so I could even make a couple of dollars out of it. I am only looking at it to learn the basic CNC operations and have a bit of fun building it.

That price above for CNC was just to do it on the cheap like I am, a kit would be a lot more as they do all the working out for you and then their mark up on top.

Dave

[John Hill]

Thats right Davo, all my stuff is for fun  which in fine but it can lead to confused priorities!