A sufficiently strong machine

**Boyan Silyavski** · 27-10-2013

Thanks for the detailed explanation.

However i would like to design mine machine quite bigger, up to the point that i am tempted to reach and surpass the length limit of the ball screws on the long axis. I don't intend to do steel though. I wonder if i am right that with this gantry design + 2 nuts at each of the long sides/x/ i could go with longer 1610 screws, say like 1800mm or even 2000, cause if i separate the bearing blocks that move the gantry say 400mm that gives screw lengths like - (1800-400) /2 = 600mm from each side if gantry is in the middle and 1400 if gantry is at one end, which more or less i assume is the limit before whipping occurs.

So, from your point of view, what are the things i should concentrate if i upscale the machine?

Apart from the obvious, like sturdy frame and gantry. I intend to use 100x100x4 profile and 100x100x? for the gantry.
and possibly size 30 Hiwin rolller bearing slides, instead of ball. I am planning to mount possibly an impact air hammer, so it seems i will further strengthen vertically one side of the gantry adding one more profile
For now it seems to me the limiting factor to all will be the gantry weight/strength ratio, so if i am right i could upscale until one reaches limit/weight for the given motors or strength for the chosen length/

**JAZZCNC** · 27-10-2013

Originally Posted by Jonathan

As I'm sure you're aware, you can improve the response of a resonant system in just two ways - either raise the stiffness to reduce the magnitude of the deflection, or increase the damping so the error dies away faster. Therefore comparing the stiffness of each option is very much related to comparing the overall response for resonant conditions.

The linear guides do have some damping effect, due to the oil layer between the bearings and rail. Adding rail with two bearings, as you suggest, would therefore increase the damping factor, as it increases the surface area in contact. Instead of adding an additional rail and two bearings, you could increase the stiffness and damping by adding one bearing to each of the existing rails which is likely more cost effective. This also has the added slight bonus of evening out errors in rail straightness and generally aligning two rails is a easier than three. Either way, this damping effect isn't that large as the surface area in contact is small, so you're probably better off improving damping in other areas such as joints in the frame. You can also add non-load bearing sliding contact bearings to further improve the damping effect, which will make a bigger difference as sliding bearings will have a much greater surface area in contact.

If you take the load ratings from the Hiwin datasheet and plot them versus rail size, it seems that the load rating is proportional to the rail size raised to the power 1.7. So if we assume the stiffness of the rail is proportional to the load rating and that the magnitude of the force on each linear bearing is similar, to increase the stiffness by the same factor as adding one additional rail (i.e. 50% as you're spreading the load between 6 bearings instead of 4), you only have to increase the rail size by 27%. So for example going from a 15mm rail to 20mm, or 20mm to 25mm would gain slightly more stiffness than adding the additional rail. Similarly if you want to double the stiffness, then instead of going from two rails to four, you could increase the rail size by 50% (e.g use 30mm rails instead of 20mm).

Looking at it a different way, if you plot the price of the linear rails and linear bearings versus their size, it's a pretty convincing linear relationship. So by increasing the rail size you have an exponential gain (x^1.7) in stiffness for a linear increase in price. If you add more rails you have a linear gain in stiffness for a linear gain in price.

Wow that's lot of waffle just to say you don't agree Jon (well think that's what your saying.?) so I'll keep it short and simply say don't agree with your Waffle.!!

I'm sure if you where to model accurately this full machine in Cad and add the rails like I suggest so load is spread across 3 areas then do a Stress analysis on it in all directions and compare deflection at the cutting tip you'll see a difference worth the effort compared to just adding another bearing on each rail or increasing rail/bearing size.?

All said and done thou Whether it's worth the effort is dependant on it's use and it's current design it isn't exactly flimsy is it so probably fine for 99.9% of any HD-DIY work.!!

**george uk** · 28-10-2013

Thanks Jonathan.

exactly the level of detail i needed to factor both the advantages and disadvantages and the associated costings. If ever am up your way, i owe you a coffie

**superpot** · 21-11-2013

Absolutely brilliant machine and many thanks for posting the project Jonathon. Will you be posting the files and costings on here?

**gavztheouch** · 21-01-2014

Fantasic machine Jonathan.

**ramsbury** · 22-01-2014

Ive read The thread a couple of time and watched the video A very inspiring thread.

**andrewuk** · 26-01-2014

That is impressive well done.

**cropwell** · 03-02-2014

This is a great machine and it has sparked off some ideas for the design of my first build. Thanks for the detailed posting. VERY useful.

Ta

Rob

**PAULRO** · 29-03-2014

hi Jonathan , super build, well done. where can i access the dwgs and BOM for this build. i am putting together a costing for a home build similar to this baby and would love to incorporate some of the ideas. any help to get me down the track that bit faster would be greatly appreciated. Dermot.