Flappy machine - Advise needed please!

[cncJim]

Haha! I'll do my best to stay off your list of fools Jazz! No guarantees mind!

Would you mind taking a look at the parts of my machine and give me your opinion? I don't anticipate that I can economically improve the machine but I would like to learn more on what to look for in the future.

My machine's frame is made from approx. 100mm x 40mm welded steel box section
X axis is 1200mm of 25mm unsupported rail
Y axis is 2 x 700mm of 20mm unsupported rail
4 x Zapp steppers
Ballscrews on all
1.5kW Chinese Spindle + Matching VFD
2 x Leadshine SPS705 power supplies
4 x Leadshine Microstep Driver M752
Spindle V3 Board
PCPPS bob

From what I have read on this forum, most of the parts are pretty good (I am only cutting thin wood) but the rails are the big area of issue, mainly from an accuracy point of view. Would that be right?

Also, back to my original issue, does it look like it comes from the flexing on the y axis?

Here is a pic...


If I feed too fast the red points end up being curved rather than a sharp right angle?

I don’t think putting it back on ebay is not an option at the moment though - Not sure how much it would fetch but for the price I paid I wouldn't be able to buy a decent complete machine to replace it. I also really enjoy using it at the moment and feel like I am learning lots every time I fire it up.

Cheers,

Jim

[John McNamara]

Hi Jim

I have used this little program for several years.


You may find the following link useful.....
Straight beams with constant cross sections (Straight beam)


MITCalc is a small Easy to use tool for calculating beam strength and more importantly deflection. It should help you to understand how "floppy" a steel bar really is. Don't confuse strength and deflection. For most machine design deflection is the enemy, as you have found out your 20mm bars easily support the spindle they will never fail in bending. They do however sag and deflect under load.


Many years ago time ago I helped my uncle and a mate restore a 6 cylinder car engine with a cast iron block. we re bored and honed the cylinders to size. While he was testing the bore size he placed a micrometre across the bore longitudinally that is along the block and adjusted it until it just held by its own weight in the bore. He then placed his two index fingers on each side of the particular cylinder and pressed hard. The micrometer fell out. That was my first lesson about steel deflection. a heavy casting is not immune to it.


Get a free trial version of the "beam" module and try it out Only a few Euros if you like it. (I have no connection with the company.)


The program will give the deflection at various points along the length of the beam for a point load, as with a router. This is a static load calculation tool, meaning it does not allow for vibrating or moving loads. Software to do that is not for the feint hearted. My solution is to add a reasonable margin of safety.


Please refer to Beam (structure) - Wikipedia, the free encyclopedia
Note the different kinds of supports..... Use the fixed setting in MITCalc for firmly attached ends. Although steel fixings also flex a bit!


Experiment with different beam depths and widths. MITCalc will allow you enter your own RHS sections You can also enter your 20 and 25mm bar and compare that to other sections.


Regards
John

[njhussey]

How about some pics of the actual machine to see if it is possible to modify it?

[cncJim]

Thank you John - Very interesting, I will check out MITCalc.

I will upload a pic or two tonight!

[Jonathan]

Can you post a picture of a part which has rounded off corners, which you propose are caused by deflection? The reason I ask is if you have the acceleration set too low for the X and/or Y axis, that will consistently cause the machine to cut noticeable curves in corners if it's in constant velocity mode. Search 'constant velocity' or 'G64' on the forum and you'll find plenty about it. If your acceleration for the Y axis is set too low, then the corners you indicated will be rounded, so I'd start by checking that.

I agree that the list of parts you have are not bad except for the rails, so if you replaced all the rails and make a rigid gantry you'd have a good machine. The 2.2kW spindle would have been nice, due to the larger collets, but that's minor compared to the rails.

With £100 you're struggling, as that's not enough to get rails for both the X and Y axes. Without enough funds to replace every 'weak' part, we need to work out which is the least stiff and replace that as it will have the biggest effect overall. The deflection of unsupported rails for a force applied at the center is proportional to the length cubed, and inversely proportional to the diameter raised to the power 4. That means your X rails will deflect (1200/700)^3*(20/25)^4=2.1 times more the Y rails, so I'd be inclined to replace those first. Having said that, the difference is not that great so you would still notice some gain if you replaced either.

Bear in mind that even if you replaced the rails with just 12mm supported rails, this would still be stronger than your current setup as it is the strength of the supporting structure that matters most when using supported rails, not the rail itself. I would advise using at least 20mm though, especially if wanting to cut harder materials like aluminium, since the extra stiffness of the bearings does help.

Another option would be to invest the £100 in getting the open bearings that the supported rails use (SBR25 & SBR20) and adding your own supports to the existing rails by drilling and tapping holes in them. That could be a courageous decision though...

[JAZZCNC]

Would you mind taking a look at the parts of my machine and give me your opinion? I don't anticipate that I can economically improve the machine but I would like to learn more on what to look for in the future.

Agree with Jonathan and Pretty sure this will be tuning and CV problem. Post the feed rate in G-code or better still the G-code file it's self so we can check.

Regards the rest of machine then agree you have a half decent kit of parts so I'd just look to upgrading the Frame. . BUT. . Don't try to do it on the Cheap because there's very little gain for the Money and time invested.!! . . . Best to Save up and buy decent rails for all axis and Beef up the frame where needed.

Post a picture and lets see what's what.!!

[firetrappe]

I had similar problems with corners cutting as curves. I changed from CV mode to 'Exact Stop' mode in Mach3 and it solved the problem. I know there are probably better ways by tuning CV mode etc, but it might be a quick way of proving whether it's mainly a software or hardware issue.

Si.

[routercnc]

Hi Jim,

Glad to hear you are enjoying the machine despite the problems. The rounded corners sound like the software setting already mentioned, but on the subject of unsupported rails - my first CNC machine used 900mm unsupported 20mm rails for the Y axis (it was a 'landscape' machine rather than the more usual 'portrait' machine) and 600mm unsupported 20mm rails on the X axis. The Z axis was around 300mm unsupported round rail.

I paid about £50 for all the rails and bearings (from a colleague) just to have a play with CNC and see if it was just a passing phase. Well the machine did work, but it flexed badly. The cutter was almost dragged through the work (plywood) at a slight angle so feed rates had to be low. However, I was hooked so I sold the mechanical bits on ebay and designed the machine I have now which is much better (although about to upgrade again!).

I remember playing around with temporary supports clamped to the rail at intervals (if the part being cut was small) but this didn't help alot. As Jazz as mentioned I think you might go the same way - trying bits and pieces but not making too much difference. Jonathan's suggestion might work (change bearings for half-bearings and try to support the rear side of the rail), and I had the same idea with my original machine but didn't do it in the end for fear of grinding the toughened surface to be able to drill into the softer core (to tap) and releasing in-built stresses and distorting the rail. I might have been worrying over nothing there - but it would also have made them more difficult to sell on e-bay after use so I didn't pursue.

About 2 years ago I bought a pair of 16mm supported rails around 1300mm long for ~£100 from Chai on LinearMotionBearings2008 (ebay), so this is probably the best sort of upgrade for your £100 and you could use it on your next (!) machine, but the unsupported Y axis will hold you back.

By the way what rails are on your Z axis because this is also part of the chain?

[EddyCurrent]

The term "Mission creep" springs to mind.

Edit: Jim your original 'rough diagram' did not do your machine justice, it's quite a bit better than that.

[cncJim]

The term "Mission creep" springs to mind.

Edit: Jim your original 'rough diagram' did not do your machine justice, it's quite a bit better than that.

There is always mission creep...... :) though in my experience with IT projects the term "mission creep" is usually applied when the requirement gathering/scoping stage has not been fully completed!

Looking back I guess my diagram is a bit lame!