POLL: Relative Machine Ridgidity

[basskitcase]

Obvously, the more ridgidity the better in any CNC machine, but i think many folks here may have different opinions of what exactly is a "ridgid" machine.

So here is my proposition.

I would like to hear the approximate ridgidity of your machines via the following method.

Apply approximately 30-40 lbs force (no need to be exact, just guestimations) perpedicular to the Y axis (front to back). about how much movement do you see (1/16th inch, 1/8th inch, no movement, etc). Be sure to apply the force relative to the table, such that any movement of the frame is not aparant, as movement of the frame should be a net zero movement as the table and router should both be physiclly moving the same amount.

Do the same, but this time, go up and down, parallel to the z axis. How much flex do you see?

I will go first: In my current machine, which is not yet done, i would say i have around 1/16th inch of movement, but thus far, this has not seemed to affect part quality (when doing .2" cuts at about 30-40 IPM in wood)

[hemsworthlad]

Erm...call me dim!!. .but i,m not sure i get what or where you want the force applied.? . . Are you after checking for racking etc or flex of actual frame or gantry beam.?

Also which axis are you calling the Y.? I call my longest axis the X and the across the gantry Y.

Look at the pic from at top down view and where do you want the force applied.? A B or C

[wildwestpat]

This is easy to do with the spindle running if you place a cutter in the spindle - withdraw cutter and check hole dimensions using twist drils as a go no go gauge (The marked sizes of cutters can be a bit out)- then - plunge a fresh hole - use a push stick to push the lowest stationary part of the spindle in all directions in X and Y assuming Z is vertical!. The increase in size of the hole compared to the first one is a good comparison between machines. If you are careful this can also be done pressing on the shank of the cutter with the push stick but I do not recomend this unless you have experience.

Like Helmsworthlad I am confused by your notation. However it is the unwanted movement as seen by the work piece at the cuting edge that is important for the accuracy and finish of the work pieces. The unwanted movement will have several components - resonances in the chassis - static deflections in the chassis and axis structures - elasticity in the drives - backlash.

The dynamic unwanted movements will require a comparison of possitional accuracy and size by machining repeated patterns (squares for example) using different directions of travel. Yes you could also provide external forces but why? Leaning against a machine whilst it is in use sounds like a very bad habit so stay safe.

Regards Pat

[basskitcase]

Originally Posted by hemsworthlad Erm...call me dim!!. .but i,m not sure i get what or where you want the force applied.? . . Are you after checking for racking etc or flex of actual frame or gantry beam.? Also which axis are you calling the Y.? I call my longest axis the X and the across the gantry Y. Look at the pic from at top down view and where do you want the force applied.? A B or C

I was thinking in A and upward B. Not realy considereing racking so much as static deflection, but as you both pointed out, there are multiple things to consider for ridgidity. I just think the term "ridgidity" is pretty loosly used, and one person may consider a certian deflection ok, while others expect milling machine stiffness. I do like the idea of plunging a hole the checking to see how much the hole changes when applying forces.

[hemsworthlad]

Ok, this is my findings.
Pushing for all i'm worth with all my 15st(210'lb) weight at the very outer edge's of my gantry(Y axis) perpendicular to it(A direction) and direct in line with cutting direction which would be like extreme racking it deflects at most between 0.20mm(0.00784") and 0.30(0.0118") measured using a dial gauge.

Pushing direct end on at the top of the gantry in the B direction again with all my weight it moves very little between 0.05(0.00196") and 0.11mm(0.00433")

Pushing in the B direction direct onto the Z axis like pushing from the side 0.08mm(0.00314") and with the Z fully extended 190mm(7.5") and pushing directly parallel in the cutting direction (A) again with all my weight between 0.15mm(0.00590") and 0.25(0.00984").
With Z at the top so very little extended 12mm(1/2") 0.11(0.00433")

Like i say this is pushing with all my strength and weight and Dont think i will put anything like this amount of force in either of these directions while cutting hard woods or even aluminium with small DOC's and sensible feeds.

Also got to bear in mind my gantry is very low only 150mm(6") from top of rails and also sits on profiled rails so it's very squat and stiff which helps.

Cant do any tests by pushing while cutting like pat suggested because i'm in the middle of rebuilding my controlbox so no movement at min!!

Edit: pics as requested by Stangtjk.

[stangtjk]

Maybe when replying with results everyone could post a picture of their machine as well for comparison.

[stangtjk]

That looks like a pretty nice machine, what would you estimate the force was that you were applying? I was curious so I took a bathroom scale, put it up against a wall and pushed on it pretty hard with a steady amount of force and it was only 80lbs. I was expecting more, I can squeeze the scale with my hands and reach 160 without any trouble.

[hemsworthlad]

I cant say exactly, but i was pushing at an angle and braced and pushing off the bench behind me with all my weight and force bearing on it.

Erm you got me wondering now, So I will check my scales to see what they go upto and try pushing again with them in between.?

regards
Dean

[hemsworthlad]

Ok just been and try'd with scales, pushed in exactly same place and way as before and they flickerd between 11.4st (154lb) and 13.6st(190Lb).

Like you i expected more, dont know how them forces compare to actual cutting forces at the tool.? . . . but i,m still happy with the movement i've got for the kind of work i intend to do with it, mainly hardwoods and thin profile Ali work.

Now how about your's.? . . . . or others . . . .Come on join the party dont be shy.

Cheers
Dean

[stangtjk]

I would post mine but all I've got as of right now is a box of parts and a cad model. I plan to start building it next weekend and I'll post my results when I'm done. I've I have a max deflection of around .01" in any direction I'll be satisfied.

[hemsworthlad]

Originally Posted by stangtjk I would post mine but all I've got as of right now is a box of parts and a cad model. I plan to start building it next weekend and I'll post my results when I'm done. I've I have a max deflection of around .01" in any direction I'll be satisfied.

Yep i just come across your cool looking build so found that out.

.01" Sounds easly do able to me from what i,ve seen.

Good luck.

[basskitcase]

I finally tried it out by plunging it down into some wood and pushing in all directions. In a single direction, my deviation was about 0.025", but i am going to redo my z axis bearing mount, which is currently the weak link in my machine. After doing that, i hope it will be a bit more accurate. The problem is, without making do with the less stiff z, i couldn't have made the parts to stiffen the z so...we'll see how it goes.