Well here goes... Aluminium frame router design

[CharlieRam]

Maybe i am being thick but i cant see how flipping the rails will introduce vibration, looking at the conventional method then the spindle is mounted away/lower from the bearing blocks whereas my design is a constant length of zero from the base of the spindle clamp to the bottom of the bearing block. Is it the spindle clamp causing the issue or is the spindle too low in the clamp? How low can the spindle be in its holder?

Also think i may use a plate across the back of the machine like you suggest, I suppose one open end is enough if I need any overhang
Cheers

[Boyan Silyavski]

Maybe i am being thick but i cant see how flipping the rails will introduce vibration, looking at the conventional method then the spindle is mounted away/lower from the bearing blocks whereas my design is a constant length of zero from the base of the spindle clamp to the bottom of the bearing block. Is it the spindle clamp causing the issue or is the spindle too low in the clamp? How low can the spindle be in its holder?
Cheers

Whatever you do if you dont:

-use long size 20 bearing blocks
-space the bearings that the front plate slides on at least 260mm from outside ends/each long block is 90mm long/
-fix the rails on the front plate back

you will finish with a machine incapable of working Aluminum and maybe not so precise on hard woods.

There not so much to understand, the rails +the bars they rest form ribs that prevent long axis bend. That simple.

Front plate must be 12mm steel backed with steel bars below the rails, or at least or 20mm aluminum backed with aluminum bars. Using 2 spindle brackets would make things even better as the spindle will add stiffness.

Linear rails are cheap, so that will add very small amount to the build. See 200mm travel axis made same way as we speak that reaps/i can not help say it / fully extended , through aluminum without any chatter . here post #109 and how looks finished from 12mm steel plate a bit up at the same page

[CharlieRam]

Hi Silyavski, the rails and bearings are 20mm profiled from chai, they are 75mm long plus 15mm for grease nipple which I haven't added to my sketchup model yet. I originally spaced the bearings 250mm apart from outer dimensions but was told 200mm was fine, this then gave me 150mm depth of cut instead of the original 100mm I planned so I have no issue going back to 250mm spacing if it will help.

The front Z plate is 20mm thick steel with 15mm X 50mm steel spacers glued and bolted to the plate and then the carriages are bolted onto this, the way I see it is the spacers should give me the added rigidity, would it help if they were surface hardened to make them stiffer because that is the only difference I see between having the rails mounted to the Z plate.

I am just finding it hard to understand why flipping the rails will introduce vibration as dean stated and I can't understand how the plate can bend as it has the bearing blocks on each corner. I am planning on having two spindle clamps as you suggested, in fact, it was there on earlier iterations, I must of deleted it!

Jazz, yes it is the workpeice that resonates in my example but the rubber absorbs a lot of it and I am thinking if I tensioned the rubber over the front of the Z plate it would absorb some of the resonance created in the steel Z plate if that is indeed the issue (steel instead of ally?).

Thanks, Charlie

[Boyan Silyavski]

Given the short length of your desired travel it will work ok. Bus as you have drawn it, the bit should touch the finished machine bed. Cause lower it will not go obviously. Is it so?

The case is if say you want it to travel 200mm for example or longer , then the 4 blocks on the Z will go higher than the gantry, hence the tuning fork.

[CharlieRam]

The case is if say you want it to travel 200mm for example or longer , then the 4 blocks on the Z will go higher than the gantry, hence the tuning fork.

So, is that the issue that will cause the vibration, the rails going above the gantry top? If it is then I will have a look at what I can do.
Cheers, Charlie

[CharlieRam]

Well... I'm at a bit of a crossroad in my design, I have altered the Z axis to the conventional method which should please Jazz! but the machine is now 1004mm wide by 1400mm long with overall cutting area of 620mm x 1100mm and I'm now thinking do I need it that big?
The original idea was to have a machine capable of machining soft metals or even steel if it could cope but I also wanted it to be big enough to machine 600mm wide sheets of MDF so I could machine my arcade cabinets, the problem is I would need a machining area of 1800mm lengthways to produce them in one cut but I am now thinking should I make a smaller machine with say a cutting area of 400mm x 600mm for aluminium and possibly steel (Is steel doable or a definate no go?) and when that is built, use it to make parts for a dedicated mdf/wood router that would take the full length sheets at a later date, This machine could hang of the wall with a slight incline to get over the space issue.
I am also thinking that 1610 ballscrews with 1.8 deg steps will only give a resolution of 0.05mm, If I built it for metals and plastics then 1605 ballscrews could be used bringing the resolution down to 0.025mm.
What are your thoughts on this?

One other thought, If I only have a cutting width of 400mm would I get away with a single ballscrew, if not, what is the maximum width I could get away with or is it just better to go with 2 when machining metals?
Cheers, Charlie

[irving2008]

For machining metals, esp steel, you need rigidity, so a mill arrangement, table that moves in X & Y and a rigid fixed Z column is the best solution. a rigid router style machine can mill ali if strong enough but its a compromise at best.

To improve resolution for machining metal you need to gear the motors down. a 4:1 reduction improves resolution by 4 and increases torque, necessary to overcome the inertia of a heavy moving table and increased cutting forces.

[JAZZCNC]

I am just finding it hard to understand why flipping the rails will introduce vibration as dean stated and I can't understand how the plate can bend as it has the bearing blocks on each corner.

Charlie it's not rocket science mate. It's all about leverage and vibration. Quality of finish mostly comes from trying to get the lowest amount of vibration at the tool. So we use the shortest tool we can with the least amount of stick out from the spindle to shorten the lever.

Your design is a very long constant length lever that never changes no matter what depth we cut.? Your lever length is distance the spindle extends from the spindle mount plus the tool extension. Lets say total extension of 150mm.

Now if you swap the rails around your lever is variable length lever and only ever reachs 150mm when cutting really thin material and at full extension.
Every where in between and the leverage drops and the stiffness increases so there's less deflection at the tool. At full height our extension is pretty much just the spindle nose and the tool length so order of magnitude better than your design.

May seem like a small thing to you but it makes all the difference when things get hard.!! . . . . Your design is ONLY good for wood anything harder will show it's weakness quickly.
Also with your design then using thick steel for the back plate will add very little to the design other than making the Z axis motor work harder lifting more weight. Your gaining very little strength over aluminium because it's supported at the 4 corners and not extending, if it was extending like a lever then yes it would give more strength.

Regards the size and use of machine then I'll repeat what I've said many many times it's always best to build a machine designed to do the job intended and do it the best it can. JACK of ALL TRADES is always a compromise in some department and this compromise increases with size.
Small machines do it best because they are easier to OVER build.!

[routercnc]

Hi Charlie,

Good to read that you've switched to the conventional Z axis, although I hope you've done it due to understanding the problem rather than peer pressure.

To to lay it out again, using the calculations for the simplified example previously shown:
Conventional Z axis deflection of between 0.19um - 10um depending upon Z axis extension
Unconventional Z axis deflection of 10um - FIXED (i.e. the worse case conventional at all times)

Your unconventional sketch in post #39 looked like it might get around the 'tuning fork' problem but it would not be very practical as it was drawn at full extension and this meant cutting would be up near the gantry! Your only option for practical work would be to drop the spindle down in the mount to hang down out of the clamp, but I think this would not be especially stiff. Your only other option in the design would be to raise the lowest bearing block to allow the Z axis to drop further but then you are back around the loop of an unconventional Z axis as per the above.

[CharlieRam]

Hi Charlie,

Good to read that you've switched to the conventional Z axis, although I hope you've done it due to understanding the problem rather than peer pressure.

Hi, TBH I changed because that many people can't be wrong...Can they? :-P
I understand what they are saying, although I don't fully agree because although with the conventional method at miniumum travel it is stronger because the tool tip is about 40mm from the bottom bearing block, at max travel it would be far more flimsy than my design, there must be a 'magic' point on Z where aluminium can be machined ok and if I knew that distance then I could try and work it out in my design.

None of that matters now ( I think!) because I have gone back to the drawing board with a completely different design using a fixed gantry based off a Video Jazz posted in another thread where the Y axis raises and lowers with Z. It may come to nothing and I may change my mind again!

Jazz, I know what you say about jack of all trade machines, it would help if I knew what I intended to use the machine for but I don't! I like working with wood as much as metal, I think now I am leaning more towards metal machining because more practical things can be made but I would also like to have a play with my artistic side with woods and plastics, This was why I was going for a router type build initially but the size required for some of my wood type projects have put a bit of a dampener on that :-( unless I go for a vertical machine like yours.
Cheers, Charlie