BF20 Mill conversion

[dave benson]

I'm finally done.
Since the last post, I've had time to do some more modelling, adding the saddle.
My main goal was to do as little material removal from the castings as possible
and have managed to use the existing lead screw hole and took 10 mm off the cross braces.

This now means that I can replace the lead screw for a better quality one at some time in the future
and it will be a bolt in swap so no further machining of the housing or any machining of the lead screw is required.

I spent an inordinate amount of time setting the AC bearing preload and alignment correct, a couple of days if I'm honest.
This did 'pay back in spades' as when assembled with the gib key set and a
dab of way oil, the saddle ran over its working length with my fingers, felt fine.

I used a 350 mm lead screw and a 25mm bearing housing, this has resulted in losing 15 mm
travel up close to the Z axis and gaining 40 mm on the motor end  as the mill throat hasn't changed
I have lost some symmetrical travel so will replace it with a 365 mm leadscrew.

This is the longest ballscrew that will fit the housing without enlarging the leadscrew hole at the front of the casting and gives you the original travel.

I also put a bumper at the end of the leadscrew so the nut cannot come off the screw.

When doing these small mills, people said there wasn't much room and they were not kidding.
In reality the X axis is the problem child, the Z and Y have plenty of room.

For the X axis I used the mills original bearing blocks, so the center line  of the ballscrew is the
same as the old leadscrew, which meant that I had no machining of the saddle to do.

For the Z Axis, I used the original 5301 bearing as it was a heavier duty bearing than the 7000
series, the only tricky part was measuring the drive dog height, and it's length is critical compared to the other axes, because of the way its mounted.

I have the Drives and power supplies and need to organise a control cabinet.
I set the drives up and have ran them to produce a new profile.
3000 mm\m vel and 350 mm\sec\sec acel, this is with 80 % motor power.

The last thing is to add a Y Axis homing\limit switch, I do not want to have the switch wiring moving with the table so am going to use a fixed switch
and a small custom G28 where the Axis
are homed in a particular order and the homing position is in the middle of the table for X.
I'm going to do this in CB's PP not mach3.



I had planned to make a new spindle drive interface the same as the last drive interface
but then saw that clough42 had a new video about cnc'ing a PM 728  where he mentioned that
PM had made a spindle interface card for the conversion, so I'll look into that card first.

Dave

[lloydsp]

Boy!  That is one HONKIN' motor for that small a table! 
L

[Tool-n-Around]

.......I have the Drives and power supplies and need to organise a control cabinet. Dave

Hey Dave. Progress! What did you end up going with for drives and PS?

Best,
Kelly

[dave benson]

Boy!  That is one HONKIN' motor for that small a table! 

Yes, it is Lloyd, I had originally planned to, and ordered the next size larger machine at the
vendor, and when I got home from the shop he called up and said that the machine would not be
delivered until 6 – 8 weeks later, and I was keen to make a start. They did have this machine
on the showroom floor so I took that one. I already had the Drives which were specified for the larger machine.

Kelly for the drives and power supplies I used Leadshine motors and drives and power supplies.
The motors are smooth and quiet, XY axis motors nema24 at 48 volt, the Z Axis nema34 at
60 volt.
A control box is on the way, so until then the computer and drives are housed underneath the mill.

This is my area, so I compared two options, closed loop and open loop, my head was telling me
that on this machine the open loop motors and drives were more than adequate and the extra
performance from closed loop drives wasn't a huge benefit for this mill, especially as they were
almost to the dollar twice as expensive but do not offer twice the performance.

For you with a router where you have a lot of real estate to cover they would be worth a look as
 the minimum spec, they are not servo's though which offer far better performance albeit at a
higher cost.

For the last machine I used single nut pre-loaded Tsubsaki ground ballscrews, and this is what
I will install as time and money permits, the last set I got from S Korea second hand around 400 dollars if
memory serves, a new one at the time was 1200 dollars.

The cheap and cheerful leadscrews commonly available are not preloaded so will have a little backlash, and subtle
positioning errors over their length which I'll tune in with mach3.

Of all the areas to cover during a cnc build the bearings and leadscrews will ultimately determine
how good the work off the machine is.

The first things I'm going to make are a tool setter and a digitising probe, with these I'll be able to commence
the Lathe build and have a look at the tool turret, I miss the convenience of auto
tool changes and the AI preventing me from making Muppet moves.

Dave

[Tool-n-Around]

..........Kelly for the drives and power supplies I used Leadshine motors and drives and power supplies. The motors are smooth and quiet, XY axis motors nema24 at 48 volt, the Z Axis nema34 at 60 volt.

Dave

Based upon our previous conversations I figured that to be the case. Is the reason for the N34 on Z mass or need for additional accel/speed?

Best,
Kelly

[dave benson]

No not really in this case, it's just the gravity thing, If I had an air spring or counterbalance
system then the Nema 24's would be fine for the Z axis
.
In the year off, while poking around the net, I came across a guy that compared two motors
one a Nema 34 @ 4nm and a Nema 24 @ 4 nm, he was testing the high speed performance
of the motors, he found that because of the bigger motors inductance that it stalled at a lower
rpm than the Nema24, so for example with a machine where the motors are on the gantry then
these motors would provide the same amount (or a little better) of torque at the pull out rpm
and are much lighter and cheaper. So, in this case it would be a win\win.

If the motors are on the machine frame, the mass of the motors is not so important.
I think that with your use case, fast light cuts with not a of cutting forces the Nema 24's
would be ok for the Z, on the millright they would be overkill, every kilogram you strip from the Z axis lets you up the acceleration rates.
When I upgraded my spindle a couple of years ago now.
I replaced a single phase 0.5 hp motor with an BLDC servo drive, this motor swap saved 11 kg
of the moving mass of the Z axis, allowing me to run larger accelerations which is great for things like peck drilling and 3d work.

Dave

[Tool-n-Around]

When doing the design trades on my next machine, the sheer mass of N34s was a bit eye watering.

No not really in this case, it's just the gravity thing, If I had an air spring or counterbalance system then the Nema 24's would be fine for the Z axis

I could easily incorporate an air spring, but but the router is fairly light compared to a larger spindle and......

If the motors are on the machine frame, the mass of the motors is not so important. I think that with your use case, fast light cuts with not a of cutting forces the Nema 24's would be ok for the Z, on the millright they would be overkill, every kilogram you strip from the Z axis lets you up the acceleration rates.

Yes, and the MR is R&P drive with two motors on the gantry. I modified/increased the Z-Axis stroke on the MR. It was a an easy mod. Doing so decreased the urgency for completing my next machine. The reason: all the things that make the hobby grade MR less attractive for more challenging duty actually make it pretty good for cutting foam.....it's very low mass makes for low inertial loads and it's quite fast, and since cutting forces are essentially nil for foam, I pay no penalty for the lower rigidity.

On the other hand, the  alternate machine with steel beams and superior motion control components, though much more rigid, is also much more massive, especially with N34s. I'm considering reducing the work envelope and having it dedicated to harder up to soft metals....like machining my castings....

It's the old chestnut....-Jack of all trades master of none.

The problem is the availability of shop space for both machines or it would be done.

Best,
Kelly

[lloydsp]

My opinion is that an air spring on the vertical axis is always a plus.  Sure, it adds complexities (like always requiring an air supply and having LARGE exhaust capabilities on both ends of the cylinder for rapids), but having a zero weight spindle is important to me, so that all the motor has to move is inertial mass.

Some (few) machine controllers know how to program drive currents to the motors to counteract weight, using higher current to lift.  Most don't, and some power supplies cannot cope, even if the controller knows how.

To that 'exhaust' issue, I provide the largest lines I can to and from a balance cylinder, and a large-capacity reservoir on the bottom end, so there's a place for the lift air to go rapidly on the down-stroke, and a place for it to come from rapidly on the way up.

That, because most low-pressure regulators don't have a high-volume relief path for excess pressure nor the ability to relieve tiny increases in pressure quickly -- they usually show some hysteresis.

Since the reservoirs work at relatively-low pressures, I just make them up from PVC pipe, caps, and an air fitting.  If the reservoir has a volume of +10X the cylinder, the cylinder never sees any appreciable pressure change during rapids.

Lloyd

[dave benson]

Yes I agree with your sentiments kelly, space issues aside it would be good to have a machine
dedicated to each part of your process, as the job requirements and the machining strategies
are so different.
The MR engineers designed a machine for fast light cuts and is well suited to cutting foam.

Lloyd TOT did a video a while ago about constant force springs\struts, did you consider those.

Dave

[lloydsp]

"Lloyd TOT did a video a while ago about constant force springs\struts, did you consider those."
---------------
Yeah, I'm familiar with them.  They simply combine cylinder and reservoir in one package, by having a tiny rod serve as the piston in a relatively larger outer container.  They're fine.  I have one machine that came from the factory with one to balance Z.

My issue with using them is availability in the specific range I need immediately!    I always have small air cylinders and regulators in stock, but don't stock 'one-off' items I might never have another use for.  So the build-it-myself option is more available.

Lloyd