Here we go again . . . MK4

[routercnc]

I've explored the limits of the current MK3 machine and it's time for an upgrade ! I want to cut aluminium more successfully with a better surface finish.

Here is the design so far, generally happy with it but tweaking minor things here and there:




Z axis
Box-Z design clamping the spindle all round. 15mm 4off profile rails and 8 carriages. Went for the single Z ballscrew in the end as couldn't get a twin ballscrew design to fit and give reasonable travel etc.


This is how the spindle clamps, but still keeps the rails aligned:



Y axis
Twin ballscrew driven with box surrounding Z axis. All joints overlapped such that pre-load can be set on rails before tightening. Has 20mm profile rails on 5mm epoxy.


X axis
Twin beam gantry design using 60x100x5mm RHS steel sections. This sits on 16mm open bearings which are on 16mm simply supported rails. Cannot get finance approval for 1000mm profile rails so am re-using existing parts. I'm going to use 6 open bearings per side to get the most stiffness out of it. When the time comes there is only minimal work to adapt to profile rail.



The gantry is driven by twin ballscrews, and I tried lots of layouts to get them syncronised before getting to this one. Still not 100% decided - this one has 5M HTD pulleys 31.8mm dia with 15mm wide belts. To get the link across means bolting 2 pulleys back to back. Also thinking about a single 25mm wide pulley with 9mm belts shared between the stepper and the link across (2 belts per pulley), but need to read up about 9mm vs 15mm belt strength etc.

The energy chains are split down each side. LH side is for spindle power cable, RH side is for stepper power, home/limits, auxillaries, and water cooling pipes. Now that the control box is 24V logic for limits, (homes still WIP), and all cables are shielded I'm hoping that running them together in the RH side will be OK.

Not decided on water cooling arrangement, got several options which I'll post when I finally decide. The current radiator & fan is quite bulky and 'sticks out' where ever I put it.

[routercnc]

I've started making some of the parts for the MK4 machine. Thought I'd start with an easy and relatively quick one.
Here is the Z ballscrew lower bearing bracket. The standard bearing bracket is too large to fit in so I've made my own again. I'll re-use the ballscrew and bearing from the MK3 machine which is already turned down to 6mm shaft at the end allow a pretty small bearing to be used. Just needs a new housing to suit the new design:

Laying it out and machining the bearing pocket:


Machining the profile leaving tabs to hold it:


Cleaned it up and set up to cnc machine the M8 pilot holes (note that this is a drill press vice - a proper precision vice is on my wishlist !)


Started the taps off in the drill press (just to get them straight):


Then finished them off in the vice with the hand tap wrench:
Since I discovered these lovely spiral fluted taps there is no going back . . .


Chamfered the thread entry with my favourite snail countersinks, then cleaned out the threads with a bolt:


Done:


More to follow . . .

[routercnc]

Next part is the Z axis stepper motor bracket.

Laying it out:


Machining the pocket - no adaptive toolpaths I'm afraid so about 1 hour to machine this:


Then the slot for the motor shaft:


Small pocket to clear the boss on the front of the stepper:


Light skim to get a level surface:


Outer profile cut:

I had problems here as I had the tool stick when it was about 10mm into the part. It was on one of the curves (which I think increases the chip load a bit) and I had about 2 seconds to go from hearing the problem developing to the spindle stopping rotating. I cleared the tool and thought I'd got away with it but it had obviously missed some steps as there was a ridge in the part. I let it finish, going easier on the feedrate, then homed the machine and ran some new full depth toolpaths working up to about a 0.5mm offset inward. This made the outer surface 0.5mm smaller all round, but at least it was smooth again. For this part it did not matter so I was lucky.

Machining the bolt slots was pretty quick:


Set it on end and machined the clearance holes for mounting (thought this might vibrate but it was fine):


Skimmed the other side down to get the final 20mm thickness:



That will do !

OK, so it was not 20.00 everywhere . . . !


Counterbored to 9mm deep (DRO coming in handy here to get them to the same depth)


Snail countersink to finish off (love these things, so much better than the star type which are a waste of time):


Done. Worked out OK in the end:

[routercnc]

Next part is the Z ballnut bracket. All started well . . .

Laying it out


Pilot holes for the threaded holes:


Skim the surface to level it:


Machine pocket to house ballnut:
(Toolpaths are fairly basic from Cut2D - gentle ramp into material, then a series of simple circles. No spiral option. But it gets the job done.)



Profile machined most of the way through leaving a bit holding it in place:




All holes drilled to size, tapped, then on the very last hole backing the tap out - it snapped ! Aargh!

Tap was 12mm into the part and bashing and picking was not working. Flatten the end off the centre punch so that was no good.

Got some Alum powder (about £3.50 off ebay for 2x100g) and mixed up a solution with hot water:

Then dropped in the part, and you can see it bubble straight away (bottom right hole).

After a day there were lots of black specs and a general browny dust settled onto the part (hole now top left).


But even after a couple of days it had only eaten about 0.5mm groove in the top of the tap. At this rate it was going to take weeks.

I tried a masonary bit (3mm tungsten carbide tipped) on a very low speed in the pillar drill and this gnawed away at it for about 15 minutes. The drill kept trying to wander off into the aluminium at the sides, so I flipped the part over and put the drill bit into the hole hoping this would guide it. It made a sound like it wasn't enjoying it much but I pressed on. After another 15 minutes of drilling I got to within about 4mm of the surface, and then the drill broke and got stuck in the hole. Luckily I managed to get it out. I've since put the part back in the Alum solution. I think another go with a 3mm masonary bit will have drilled the last bit out, but I think the thread might be beyond use. I could probably live without that bolt and use the other 5 but it is annoying me now. I'll clean it out and see what to do. Could enlarge the hole then make a plug with a flange on it and press it in from the reverse side, then drill and tap into that. Probably tap it before pressing it in just in case !

[Clive S]

After a day there were lots of black specs and a general browny dust settled onto the part (hole now top left).

I read something about this in an earlier post and I thought it said you had to keep it on a low light to keep it warm for it to work

[komatias]

.... it on a low heat to keep it warm for it to work

Fixed.

Have you tried putting more alum powder in to see if your concentration is too low?

[routercnc]

Hi Clive, Komatias,

I'd also read about constant heat but I don't have a hot plate and using the oven/hob was not allowed so I periodically drained some off and topped it up with water from the kettle. It was wrapped in insulating cloths to keep it warm for as long as possible.

I added about 100g of powder initially, and changed the whole lot for a new batch of 150g after a day or so. There was always some undissolved powder in the bottom even with lots of stirring which meant it was saturated at maximum concentration.

It was working, just slowly. I think a hot plate or keeping it fairly hot would give it more chance, I'll see what I can get hold of.

I don't know if drilling with the masonary bit would have taken 15 minutes without the Alum powder treatment or if several days in the Alum had softened it. I'm nearly through with the drill so when I'm next in the workshop I should break right through.

[komatias]

Masonary bits are too blunt. You would also need to spin backwards to aid removal of the tap. I have used special carbide spade drills to remove taps. The only place in the Uk that has them is
http://www.drill-service.co.uk/Produ...60000&Tool=377
Seriously considering stocking some in 2016

[routercnc]

Good to know thank you. I'd ordered a solid carbide 3mm drill a few day ago, but these look better still.

[JAZZCNC]

Please Don't get me wrong on what I'm about to say has I'm not pulling down what you have designed, it looks and sure will work great. But This design seems an awful lot of work and complexity and extra cost for no major benifit over a fixed gantry design.!

By this I mean the wide Gantry and wasted space at each end mean it will have about the same foot print and give about the same cutting area of Fixed Gantry.
If used Fixed gantry design with lifting Y axis allowing none lifting Z axis meaning minimal Z axis extension, essentialy just tool length would have been much stronger and far simpler design IMO.

Like I say just an observation which for the sake of others thought I'd mention and not pulling down what you are building or doing. It's looking great sure will work great and i'm looking forward to seeing it come along.