CB CNC Part 23 – Tramming Plate Design and Fine Adjustment Mechanism

 

In the previous video I tested the CNC machine with the new acetal plates by milling some aluminium, and also tramming the spindle so it would move square to the wasteboard. The process of tramming got me thinking about how I could designing some kind of mechanism to fine adjust the orientation of the spindle along the x and y axis, to make the process a lot easier. So in this video I present some of iterations of the design.

So this section will go on the gantry, and these four holes will be tapped and this is a centre point. Then the z axis c beam will be bolted onto this section here. And the holes here are counter sunk so the machine screws sit flat so these two section fit together like this. And these elongated holes line up with the holes that are tapped on the rear. So I just tapped the holes in the MDF. So that can pivot and what I’ll do is check this with the spindle, when it’s all put together and when I’m happy that it’s lined up properly I’ll tighten that up. Just on MDF I’m really struggling to twist that back. So I think that these four machine screws if this plate was made out of aluminium, with a centre pivot point will be strong enough to lock into place after tramming the spindle. Tramming is the hardest part of setting up a CNC machine because it means you have to dismantle certain parts and constantly adjust as you’re testing how accurate your cut is.

I’m now going to show you the second iteration in which I’ve designed a fine adjustment mechanism into the plates.

I put the machine screws round the back so you can see the movement in the elongated holes here. What I’ve also done if I pull these apart is machine these ears – these unusual shapes which fit into the openings. The little windows there. They are actually back to front. It’s meant to sit this way around but for some reason when I exported the files I flipped one by accident. The way this sits with the machine screws up here, the eccentric spacers go at the top and the fixed wheels stay at the bottom. So when the spindle pushes down the fixed wheels stay in position and there’s less likelihood the eccentric spacers would shift or twist. There’s two degrees of pivot either way. So four degrees in total. And that’s just designed in fusion 360. I’ve written a note there to remind me to flip or rotate, check when the hell I did with the vector files. I definitely made a mistake. I’m not sure if this shape here can be improved or whether I should place the grub screw or machine screw that’ll eventually act as the fine adjustment in a different position. So ignoring the top and bottom holes which have been reversed, what I might do is come off at a 90 degree angle as best I can off the surface of this curve here and change the shape so I have a flat edge, and this has better contact. At the moment because I’ve drilled straight in 90 degrees from the edge of this the machine screw sinks in the corner instead of the flat edge at the front. That also increases the amount of material that I can put a tap on the aluminium and also give me a flat edge where I can put a wing knob or a nut which can lock this in place. And essentially when I go to tram the machine I just loosen one and tighten the other and it will go one way and if I go the opposite way, visa versa. I think that’s going to work but I’m going to think about it a little bit longer before I cut anything out. It’s on of these things where you go to do it and you realise that you’ve overlooked something when you go to use the mechanism. This should be a one time mechanism that initial time of tramming the machine. I shouldn’t have to redo this. Anyway so that’s the prototype version two.

I’m cutting a segment of the pivot plate to test the concept in some scrap 8mm aluminium. I’ve actually ordered some 6082-T6 because I’m feeling confident this will work… but the scrap I’m using is of an unknow grade. I was struggling to drill holes with a drilling toolpath, with pecking set at 0.2mm using a normal single flute carbide bit. I stubbornly kept going and later changed over a Europa 5mm single flute carbide bit to area-clear the pivot window and elongated holes when…

Ok something happen there. While I was cutting the machine just completely stopped and the bit was held right in that point there and the spindle stopped as well. And it seemed to coincided with when I turned off that bloody heater. This isn’t as well shielded as I thought it was which is a bit worrying but on a positive note. Everything is cutting really accurately. That’s meant to be 20mm. The only thing I still have a problem with is doing the smaller holes. It’s almost as if the bit works better when it is moving. if it just plunges it just doesn’t seem to cut so well. What I’m thinking is using a centring drilling bit. The sort that you get with metal lathes and just pointing out all these areas and drilling them out on a drill press instead. So I only use the CNC machine to do the unusual shapes and outside. I kinda roughly know where my starting point is so I’m going to try and cut the two section out. This is a waste. It should still line up and I’ll work out what I’ve got left on the off cut and see if I can cut another pivot block out. That’s really annoying because it was doing really well.

Because of the interruption in the cutting the outside of the two segments are not aligned with the holes within. What I forgot to do when turning the machine on was homing it first, and then setting the new origin before cutting. Had I done this I could have realigned my cuts.

But this shouldn’t matter as I will still be able to reassemble everything together – minus the pivot centre and the wing of aluminium which I was planning to tap for the fine adjustment screw. The protrusion of aluminium which will be tapped for the fine adjustment is also off by about 1 to 1.5mm from where it should be in relation to the pivoting window. It’s a little higher and a little further away.

I’m having a bit of trouble drilling into the aluminium now. I don’t know what… well I know what I did here. I accidentally clicked the toolpath for plastics instead of metal but when I changed it over it still wasn’t cutting ok. I think what I’m going to do is change over to this centring bit and the swap over to the 5mm Europa just to cut the outside. It is a much nicer bit to work with.

I decided I needed to get the right tools for drilling the holes on the CNC plates – ones which I could either use on the CNC machine itself or on a pillar drill after marking out.  These would have to be shorter and more rigid stub style drill bit. The point angle on these types of bits can vary. And I’ve read that you should mark up with a centre or spotting drill with a larger angle than the drill bits you’re planning to use, if you want to get accurate holes.

I had a choice between slightly cheaper HSS cobalt bits and carbide ones. Either type had different point angles. If I buy the cobalt ones their point angle would be of 135 degrees which would have meant I needed an equivalent or larger spotting bit to mark up. These were hard to find on ebay and the ones on engineering supply websites were very expensive. Alternately if I bought the carbide bits, their point angle would be 118 degree and I could use centring or spotting bit with a 120 degree point angle which are much easier to find but the carbide bits are a hell of a lot more expensive. I think I can get away with using a large cobalt drill bit as a spot drill but for now I will use a normal centring drill bits. These bits have two angles. The smaller is set at 120 while the larger one is set at 60 or 90 so providing I don’t go too deep this should be fine.

It looks like I have to drill through these. It hasn’t gone through. When I drilled this section here out, this shallow recess, it’s meant to go around this way, eeeer, for the cough… It almost feels like my vectors for this plates are coming out backwards. I also made a recess for the machine screw which would eventually hold the c-beam for the z axis and now that’s on the wrong side. So what I thought was the left section of the pivot plate is now going to be the right side. When designing the plates I used the mirroring plates in Fusion 360 which should mean that the plates can flip around and still marry up together.

I filled down the bridges from the aluminium segments and tapped and assembled the everything. When I go to do the actual plates, I will use the drill press to tap and try minimise cocking it up.

I also marked and drilled a hole, which I again tapped for the find adjustment screw. This is a little off from the CAD model because of the cock up with cutting but I can still use it to test the mechanism. I can also adjust the model based on how this works by taking measurement with my vernier caliper.

Ok this is the final section, the test. I’ve put the machine screws through the rear just so you can see the mechanism a little bit better. There obviously was a few mistakes coz of the recess that I cut out here being on the top of the plate. So essentially it meant I was actually cutting out the opposite side of this pivot plate but, as you can see the pivot block fits in the window really well. That’s really really satisfying. When this section pushed up and essentially the spindle has moved counterclockwise, if I need to adjust it I can simply unscrew this and you can see this dropping. And obviously when I’m happy with the position I can lock it down with that locking nut there.

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