Added steel channel to X-axis c-beam

Hey folks,
I thought you might want to see the results of adding a stainless steel channel inside of a c-beam. I took extensive measurements from various places on the machine from 5 to 65lbs of force. The results are not what I was hoping for but I'd be curious to know if any one has taken a similar gamut of measurements from their CNC and how it compares.

Here is a video of the process:
https://youtu.be/SwHb75_GWwM

And here is a link to the data spreadsheet:
https://docs.google.com/spreadsheets...it?usp=sharing

Cheers,
Tyler

Hi Tyler,


I took your google data sheet and used the deflection which I believe is closest to the collet to calculate the stiffness. It is around 1125 N/mm where you took that measurement (router plate Y direction). This would be lower at the collet itself which is really the only value that matters.
Attachment 24291

A few of us have done something similar. Here is my thread showing my 'old' machine:
http://www.mycncuk.com/threads/7155-...ss+measurement

Stiffness was:
X 1250 N/mm (the long axis)
Y 1000 N/mm (across the gantry)
Z 2450 N/mm (vertical)

These are not that high but the machine worked OK. Measurements were taken with spindle centred on the gantry (i.e. mid-span), and the spindle Z 50mm down from the home position (i.e. a notional cutting position). Force was applied on the collet nut (in X, Y, and Z directions), so does not include the stiffness of the cutting tool. Others have made stiffer machines than this, see post.


If you want to compare basic section properties (C-beam is poor in torsion), here is my calculator:
http://www.mycncuk.com/threads/2214-...ess-calculator

Ooooo, this is very helpful! The take away is that I don't believe all my efforts and expense to add steel to an aluminum extrusion had much of an effect. Would you agree? Next I'll try replacing the X assembly mini wheel setup with linear bearings on some form of steel X beam. I think this will be the most significant upgrade I can make presently. I'll fashion it in such a way that I can use it moving forward.
Can I post all or parts of your reply on my YouTube channel and various other forums? I think it would be helpful. I'm going to recreate your calculations so I understand how the math/physics work and do a follow up video getting into a bit more depth. And also, can I link to your calculator?
Thank you for doing this!
Oh, I should add that I think some of the other measurements are also important. For example, the measurement from the rear of the gantry gives a bit of a window into how much error is being introduced in that area. In terms of thinking about design and/or upgrades, it's good to have a big picture. I do agree that the most significant number is closest to the bit. How did you measure side forces at the collet in the X and Y directions? I didn't see any photos of that. A spring luggage scale?
Cheers,
Tyler

Hi Tyler,

OK, each one in turn.
1) Benefits of your upgrade: I didn't see any data in the 'before' condition, and I think in your video you have only measured after? So difficult to say, but you are currently in a modest DIY machine territory. For reference technical papers mention commercial machines of 50,000 N/mm upwards if I remember correctly . . .

2) Wheel upgrade to linear rails, plus steel beam: The linear rails upgrade may not show much stiffness, as they are probably not the limiting factor. But they will reduce the maintenance, and may reduce the vibration a bit. But when combined with the steel beam it will start to help.

3) You can post my reply on YouTube if you like. Just reference the 'mycncuk' forum for the source.

4) You can link to the calculator, again quoting the 'mycncuk' source for the reference. Just one thing - I wrote it many years ago to help myself out with some decisions. I have a Mechanical Engineering background so these calcs are OK for me but I really do not have time to explain the calculations or theoretical background for the worksheet. There are instructions on how to use it contained on the sheet but that is all I can offer. Hope you can understand.

5) Measuring force/deflection around the structure: The only one that matters is the force/deflection at the collet (or tool if you want), and ideally with the spindle in the middle of the gantry and at some depth away from home to represent a cutting position. Force/deflection at other locations doesn't help any. But what does help if you want to explore is keeping the force FIXED at the collet, and measuring deflection at other parts of the structure. This will build up a sort of deflection map of the structure and show you where in the chain the worst deflection is happening. Keep the force FIXED at the collet. Then measure the deflection at the collet, the spindle, the Y axis, the gantry, the gantry sides, and the bed in turn. You will probably see a sudden delta change at some point in this chain, that is where you need to improve things.
For applying force to the collet in X & Y I used kitchen scales (the flat electronic type) with a short length of wood in between to get onto the collet. I was then pushing until the scales read 5kg (~5N). You can also pull with luggage scales for X and Y. For Z I put the scale under the collet and pushed up. I'm not sure how you would do that will luggage scales other than perhaps with a pivot and beam, then scaling the results. As your readings showed the machine is fairly linear so any force (within reason) will do.

Great! I'm going to put together a part 3 summation video and I want to think about the next step/upgrade. I do want the machine stiff enough to mill aluminum, but maybe I keep this machine for wood an other materials and start from scratch, save my pennies and build a stout little milling machine.
In turn:
1. There was no before data, unfortunately. If I had to guess, I'd say that my modifications both to the Z axis: steel V-track, bearings and stainless channel, and to the X axis: steel channel, added very little rigidity where it was needed, in the middle of the X beam.

2. OK, so v-track is probably fine to stick with if I try an X beam upgrade to steel.

3. Should I use: http://www.mycncuk.com/threads/2214-...ess-calculator URL for the calculator and the general mycncuk.com URL in general elsewhere? I'm happy to drive traffic here if I can.

4. I entirely understand that your time is limited and I appreciate that you've already helped me so much! I can't believe you ran my numbers, that's so kind.

5. What you're describing is actually what I did, I think. I applied say 20lbs to the scale by pushing the router collet down, which amount to upward force. Then I did the same 20lbs for 7 points all over the machine. That is what the data represents. Maybe I'm misunderstanding.

I do have more questions about the next step, steel. But, I'll start a new thread so all can benefit.
Thank you again, routercnc!

Hi Tyler,

1) OK, but read 5) below before concluding (although you might still be right!)

2) No, what I meant was just upgrading to profile rail would not help unless the underlying structure was improved. So I agree if going with the steel beam upgrade (which is your plan) then I would recommend doing both the steel beam and the profile rail.

3) I'm OK with you including the URL in the text box below the video on YouTube for the calculator and the forum site. Lee Roberts is the site owner and I'm sure he will chip in if there are any issues with that.

4) All I did was divide the force you applied (converted to N) by the displacement you measured. The only with any real meaning is the force and displacement applied at the collet/cutting tool so make sure you include that.

5) To get the bigger picture when the force is applied to the collet - what I understood you had done is apply force at one location (say location A) , and measure the displacement at that same location (location A). Then applied the force to ANOTHER location (say location B), and measure the displacement at location B, and then the force at location C and the displacement at location C.

I'm saying don't do it like that. It is much more helpful to apply the force to location A, then measure at A (A being the collet/tool). Then apply at A again, and measure at B. Then apply at A again and measure at C, etc. Do not move the force location, only the measurement location for the deflection. Then by looking at the numbers, or plotting them on a graph, you can see where the stiffness falls away and where the improvement is required in the structure. You can also apply at A, and measure the displacement at A,B,C,D etc if you have enough DTIs. If this is what you did then carry on.

The short answer to all the above is:
Used closed sections - e.g. rectangular hollow sections [], not [, U, or I

Maximise their outer dimensions (this gives stiffness improvements to the ^3 for a linear size increase)

Then up the wall thickness until the weight is getting too high (this gives stiffness improvements directly proportional to increase so is not as effective as outer dimensions)

Minimise offsets, these create moments which you will not fully recover through beefing up elsewhere

Be practical - consider the principles, but you have to be able to build the machine !

Ah yes, I did take measurements the preferred way. For each measurement, I simply plunged the router down onto the scale, the same way for all measurements taken. Sorry I wasn't clear about that. I will also take some deflection at the collet while applying lateral force in both the X and Y directions.
Yes, from what I'm learning, closed structure for beam is best. Presently I'm thinking about using a 2" x 5", 3/16" thick steel beam (8.15lbs per foot) with 1/2" steel end caps welded on, for mounting to the gantries. The way I see it, since the worst of the deflection is clearly from twisting in the middle of the X beam, I'll start there. I can upgrade the rest in stages, as money allows. The idea is that I can reuse the steel beam in whatever configuration I end up with. If I go thicker on the steel tube walls I think it gets too heavy for my design.
I may go with linear bearings, but I'm also considering v-track and v bearings. The the prices are $300 and $200 respectively, in the long run, not much of a difference.
Again, thank you for all of the feedback! I'll post a part 3 video soon in which I discuss some of things we're talking about here and consider options. On another note, I'm seriously considering leaving this CNC machine as is and saving my pennies for a mini mill for metal work and leaving the larger machine for wood and softer materials. The CNC, as is, is quite adequate for such purposes. Taig, I found them today, makes some incredible tiny mills capable of milling mild steel for short money, and you can order them CNC ready, just hook up your steppers and off you go. Have a look: http://www.taigtools.com/mmill.html
Pip, pip!

So, after much deliberation, I've decided to go with a 2" x 6" x 3/16" x ~750mm long piece of mild steel for the X beam. I found a local machine shop willing to flatten the face where the linear rails will go and make the bottom perpendicular along with plates that I'll weld onto the ends for mounting. I found a good source on eBay for the steel. The reason I opted for 2" x 6" x 3/16" was both budgetary and design based. I couldn't source the size tube I wanted locally so shipping was prohibitively expensive after a certain wight/size point, massive jump in price. Literally from around $36 shipping to well over $100! And, since I'm doing this upgrade in increments, I'll eventually switch over to a fixed gantry with Y table movement, I want to keep the weight somewhat reasonable. This beam is 9.42lbs per foot making the total weight around 23.5lbs. That's a goodly chunk of steel to be throwing around along with the whole X/Z assemblies and router! Ooftah. Let's see what these poly carb wheels can take (on the gantry plates). The total upgrade is going to be close to $500, not including all the swearing I will do. I'm teaching myself to weld with a pitiful little DC arc welder I picked up, and off to the races! Wish me luck! Video to come.

Attachment 24453

So, here is my current plan for the X beam, 2" x 6" x 3/16" steel beam with linear rails. I'll weld at least .25" thick plate onto the ends for mounting to the gantries. Maybe thicker? Not sure if it's necessary, will just add more weight.
Over on the Openbuilds forum, M90Ranger suggested I split up the rails and use a design like this: Attachment 24452 Certainly two .25" wall 2" or 3" square tubing would be cheaper than the single 2" x 6" I purchased. Somehow I like the overall feel of the single rectangular tube. Which do you think would be more rigid?

Two tubes could be almost but not quite as good if the end plates were of infinite stiffness, the guy who suggested that obviously isn't aware of the concept of Triangulation in structural design or the properties of different cross sections in torsional load.
I don't find that surprising any more though ;-)