Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: Davek0974 on September 28, 2016, 01:32:51 PM
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I'm just at the final stages of testing on my high speed spindle attachment for my Bridgeport, running cut tests now.
Would backlash make holes smaller or bigger?
The holes are 3.1mm being cut with a 2mm single flute endmill and are coming up a little tight.
They are ok on the other machine so my guesses backlash?
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Backlash would tend to make the hole out of round so it could well be like it is smaller.
Hood
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Hmm i guess so, does backlash comp actually work?
Any tried and trusted ways of assessing actual backlash - I know its there, just need to assess and maybe adjust or compensate.
I know it needs new bearings on the ball-screws, one axis is new but wrong type, other axis is old OEM type so still not ballscrew style but better than the new ones!
List price is over £100 per pair so i need to find the right ones here.
Have read that any angular-contact bearings that fit would be ok with the right shims - the originals are pre-ground so there are no shims needed.
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Backlash is supposed to work in the CSMIO but I have never used/needed it so can't say from personal experience.
First thing to find out is if it is consistent along the length of travel, if it is then just plug the numbers into the backlash page and enable.
Easiest way to test is place a clock on the table and against the head, somewhere flat preferably and then with the smallest MPG or Jog increment move onto the clock and keep going until it is zero then Zero the X DRO in Mach and then MPG/Jog the opposite way and watch the clock to see when it starts to move, the value in the DRO will be the backlash. Repeat for Y but this time you can have the clock on the knee and have the tip against the saddle.
Hood
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Thanks, i'm sure there will be some variation as the ways are not worn evenly as normal on Bridgy so there will be more load on the screws at each end.
I'll run the tests and get some numbers but I really do need to find some bearings.
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Try Bearing King and see what kind of price he can come up with for a matched pair of angular contacts in the required size. I got a good price for the Spindle Bearings in my Beaver Mill, half the price I could get anywhere else and they were actually a better tolerance class as well.
I bought original bearings for the manual Bridgeport years ago and never used them, weren't cheap either LOL but for the life of me I can not remember where I put them.
Hood
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Bearing king seems ok, still a lot of ££££ at £122 per pair - ouch
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Would backlash make holes smaller or bigger?
Don't know as it's more complex than just a yes or no. ;)
I used backlash compensation (BC) for a long time. It's a work around that if applied appropriately
can compensate for the mechanical inaccuracies of machine components. The best solution is to
get rid of the backlash (minimize backlash is a better term ). BC coupled with good machine code
can accomplish machining a hole such that a bearing fits just right.
BUT ..... BC settings can vary and other component inaccuracies come into play when accuracy is desired.
Can elaborate more on this if you so desire.
RICH
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Feel free to elaborate at your leisure :)
I knew there would be some backlash, in fact i was pleased how little it is really for an old girl.
The holes are 3.1mm and a 3mm screw 'almost' fits so its not like its a country mile off size or coming out square :)
Yes I could drill them but that means another tool change and TBH the end mill makes a neater job out of them.
I will measure my slop and save up for the new bearings.
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Ok, seems there is a reasonably stable backlash of 0.1mm in Y and 0.13mm in X
Not so bad i think but can we improve with the compensation?
Are those the figures that are entered into it?
Backlash speed set at default?
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Yes, the compensation can help.
Here is a link to some info worth looking at and play with the settings to see how things work for
your machine:
http://www.machsupport.com/forum/index.php/topic,29149.msg204717.html#msg204717
I will assume that you checked the backlash at different feedrates and different places along the axis travel.
So the "stable" numbers you mention reflect the above.
You may be able to optimise the adjustment of the table gibs and thrust bearings.... just note that you can't
have no backlash since the table won't move and as you take out play you also will use up motor torque to move
the axis.
For now,
RICH
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Thanks,
no , i only used micro-jog to check the readings, i did check at 3 points on each axis though.
I'm not sure how much can be got rid of but i do know the thrust bearings need replacing, however the cost id holding that off for now.
On larger work, 0.1mm won't matter but on a 3mm milled hole its a big difference.
I'll read the linked thread.
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For the time being can you clean / lube / and re pre-load the thrust bearings?
See how much they contributor to backlash.........
Load up the bearings so movement requires some torque / hard to turn the screw ( one can call that zero play). Measure with torque wrench. then reduce pre-load by 1/2 the torque value and see what the difference the bearing contributes to the total backlash.
Just a thought,
RICH
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They were cleaned and repacked when i assembled it all, the original bearings are face to face and extra torque on the nut makes no difference, the new crappy ones i got with the new end plate were just bog standard bearings so i made some shims to enable me to tighten the nut solid. they were quite firm but not tight.
I would not want to pull them apart unless i had the new ones to go in really.
I will put the dial indicator on the screw-ends and measure end float - that can only be attributed to bearings. i have a feeling it will be probably 90% of my backlash figure.
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I quess you have what you have and until you upgrade components BC can help improve accuracy.
The best approach is finding just what each component adds to the backlash.
One needs to isolate the component, and test each, adding the numbers should approx the total backlash.
Usual mechanical items that cause some degree of backlash:
- Motor coupling - anything other than hard coupled will add to the BC, it's just a matter of how much
torque is applied to it.
- Thrust bearings - Can approach zero play, but will never be "zero"
- Screw - lost motion can be anywhere, vary, and linearity different along the screw and may not be
the same in the oposite direction
- Screw Nut - it is what it is
- Older Machine - One could fix / replace all of the above but conditon of the different machine surfaces
can affect accuracy.
Just some comments:
I hard couple the motors to the screw shaft. Requires good alignment, but, have found that over time the
bellows and rubber type can cause problems in repeatability of movement.
Thrust bearings are something that a lot of folks just don't pay attention to. They need to be preloaded
properly such that they minimize play with the least force / torque required for movement. I measure
the installation to confirm.
I have never used the screw mapping in Mach so don't know how well it works for a worn screw.
To profile a screw can be a PITA, rather easy on my end because I have the equipment to do it
opticaly and very accurately in rather short order. What would you do if you found that linear tolerance
spec is not over the length and is all in a small section of the length? OR Accuracy is different in the
travel directions?
For "hobby" use adjustable ball screw nuts are an option such that it is easy to minimise any play.
You pay for good quality screws and nuts. The accuracy of your machine is only as good as the components
used. For a lathe I recommend the best quality screw one can rationalize buying for the X axis ( the smaller
the more expensive they seem to be ).
Practicaly speaking on a retrofit, one can replace the axis components and find that the a machine surface
causes inaccuracies or even installation problems.
Just some thoughts,
RICH
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Yep, all noted, thanks.
I'm not chasing sub-microns here, at the end of the day she's an old Bridgeport with the stability of a wet lettuce leaf;)
I did not spend the most on mega-quality ball-screws - it just did not seem worth it as the ones i got were pretty costy and better C rating ones just went astronomic!
The thrust bearings are most definitely crap, I fully admit that and they will be pulled as soon as I have the cash for the ground and matched angular contact ones - the ones in it are just regular radial groove ones, I was pretty surprised how good it actually works and was expecting a lot worse TBH.
The belts could maybe be tighter as that would be another source of lost motion, not sure how tight is too tight on a 20mm HTD belt ?
Overall I'm very impressed with it, but need to get the 3mm holes to a better size as these are a part i make regularly on the mini-mill which i hope to sell-on soon.
I will add 0.1mm BC and try another test this weekend just to see what happens :)
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belts could maybe be tighter as that would be another source of lost motion
Timing belts are surprisingly accurate....even when home made timing gears are used and driven axis movement may be off a little but it's repeatable / rather consistent per revolution.
Been a long time since we did testing of timing belts.
Basis was measured computer and driver output pulses as compared to varing belt tensions to see the affect on an encoder output connected to the axis side to see the differences between the three pulses. Difference were small values of steps per unit as they relate to axis movement.
RICH
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Replaced both sets of bearings on X & Y - got a great price from BearingBoys of £60 per pair of Nachi graded bearings.
Amazing difference, really quiet when doing rapids now, backlash is sub 0.1mm on each axis.
A little bit of compensation should get rid of that i think :)
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Whats teething with backlash compensation and the shuttle acceleration?
Is there a recommended setting for servos and a CSMIO controller?
Currently have it on 50% acceleration and 0.001 shuttle accel, but the shuttle thing does not seem to make much difference - what should it do?
All values are sub 0.1mm so its just a tiny but sharp knock when changing direction on jog, n to tried any code yet.
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Never used servos, only steppers with BC a long, long time ago.
BC at that time never worked with the SS and would think an external controller would need to support BC.
My experience is expressed in my link posted in the other reply I made.
Reread the info and try starting very slowly and observe the motion.
The faster BC is implemented you will get a knock / thump sound as the slack in the axis is taken up very rapidly.
Sorry can't be of much more help,
RICH
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Thanks Rich,
I have re-read it and the reason for asking was that it goes against a couple of videos on the 'tube - nothing like a bit of confusion :)
Their idea was to go 100% speed and something 0.001seconds
I will have a play with it and see what makes a difference if anything :)
You can see taking the backlash out so i guess the controller is ok with it.
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Dave,
Can you post a link to one or two of the videos so I can see what they are doing?
RICH
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Here's one link...
https://youtu.be/A27nOkTFYDY
A bit long but skip to around 5:30
Dave
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Thanks for the link. Watched the video.
My 50-50-50 rule is a "suggested starting point" for settings and one needs to adjust based on the machine and program settings.
Note that BC is implemented at change in axis direction thus the Gcoding comes into play. Motor tuning values define the rate of possible axis motion and are set by the user....and hopefully the settings allow for reliable / repeatable operation. So BC values
could be at 100% and some small time value.
Note also that in the video he adjusts BC to get a more perfect circle. What would happen if another circle with a larger diameter ( ie; 10x) was done in at different axis position? There will be a change in cut diameter tolerance. Just a matter of degree....
A few comments on backlash.
The first step is to know how much you have if your going to use BC to remove it. Backlash is directly related to axis / controlled point repeatabilty.
Each component of the axis can have backlash and each adds to the total. The total backlash may not apply over the complete travel of the axis. So the backlash value can vary depending on where and how it was measured.It's all a matter of degree of non-movement and what is causing it and additionaly where it is happening.
Any component of the axis is not perfect in it's motion.
An example is the thrust bearing you just replaced. You can only preload so much and must leave some play or have a motor capable of turning the screw.
RICH
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Thanks
I'll try it out on some code, do a test piece.
I've measured BL as good as i can, at three points on each axis, all axes show <0.1mm backlash at all points.
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Hmm....see what happens with repetitive short moves back and forth.
Would think it's the equivalent of beating the poop out of the machine! ;D
Similar having a car stuck and your constantly going from forward to reverse with the pedal to the floor to rock it.
RICH
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Yes, this was caused my recent ePid fault query, i was trying some V-Carve engraving for the first time and i had the following error set a bit too tight.
So, whats best to do - I can't really reduce backlash anymore physically without spending stupid money on surgical grade ball-screws which is just not worth it on this machine. Is it best to leave BC off and live with it or leave it on with the risk that it will hammer the backlash worse than it is?
Or set a slow comp time maybe?
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I have an interesting experiment for you to try. I'll bet your Z axis is no problem with backlash, because it is gravity preloaded againt the screw and bearings in one direction. So try the same thing on the X&Y. Get a gas spring with a force greater than the cutter thrust and preload the axis in one direction. An initial experiment could be done with enough bungee cords to provide the force required and tie the ends to the wall, another piece of immovable equipment.
I bring this up because I once owned a Spindle Wizard knee mill conversion that had double nuts with bellville spring washers loading a second nut against the first so it stated against one side of the screw. That machine was not fast, 20 ipm cutting speeds, but would hold tenths with no problem, and round holes were like they were were bored!
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Yes that would be the only way to guarantee zero backlash, but at speed its not so easy to do. The thing with gas springs or springs in general is that the force is not linear, only a portion in the middle of travel is reasonably linear.
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Tried a few different settings, I think it will be ok on the standard mill setup with 50% speed and 0.1s but on the high-speed setup its far too violent when doing V-Carve stuff as its hundreds of very short lines and it just sounds and looks rough on the old girl.
Will do an engraving test tomorrow and see how it is with the new thrust bearings in and the rebuilt Z drive.
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Actually a gas spring is quite linear, which is why you use one instead of an ordinary spring. A 90 lb gas spring with say a 10" stroke requires 90lbs of force to move it off the stop. At full compression the force is only about 10% higher or about 100 lbs! On my Z axis I have two gas springs back to back for a longer stroke because otherwise it drifts right down when the drives shut off. The Z stroke is 18"
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FWIW Dave - I can't for the life of me see how BC can co-exist with CV - particularly with lots of V small moves.
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Thats probably why it sounds so rough, I think i'll leave it off and do some accuracy tests, I may be chasing ghosts here.
V-Carve G-Code is evil, probably the nastiest code possible as it is thousands of minute moves in all axes all at once.
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Dave Vcarve is the best thing to test with if it's bad after a 1000 lines of code it will be ok on 2D only, if it's good you're all set. I test with a V carve file that has around 1000 moves on each axis I can get it to 0.002
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Did some V-Carve tests tonight, 28000 lines, 38mins run time, it will run with BC on but sounds happier with it off, physical results look the same.
Will do some proper mill cuts ASAP.
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I reckon you're trying to make a silk purse out of a sow's ear here Dave. Looking at your test text toolpath you posted elsewhere, some of those moves are an order of magnitude less than your backlash. Is it really sensible to use a 0.2mm engraving bit with thousands of moves of a few 100ths of a mm on a Bridgeport I wonder?
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:) I like pushing the envelope :)
It seems to work ok so I can't see any harm in it, if it was too bad I would expect broken tools but this one seems fine after running these tests and it wasn't new when i started.
I know it's not an engraving machine as such, but it does seem to function pretty well as one, I don't do much engraving luckily but needed to know if it was possible and it seems so.
I still have the mini-mill but really need to sell that on now to recover space in the shop as well as cash.
I think she will do just fine but being me I like to know exactly what limitations i have :)
Just for completeness, the "test" is pictured below, the T is about 18mm tall, this picture was taken before i fitted the new thrust bearings on X & Y, its even better now.
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FWIW,
Backlash becomes a killer when the size of the engraving / machining is real small.
A good "size test" would be to do a simple letter like "E".
Tool tip radius comes into play, so for the "E" a depth of say .002" with width of machined line .003" the smallest overall size would be
.015" ( using high speed spindle and slow feed).
Tool breakage with small end mills becomes a problem and BC doesn't fix it.
I built a machine just for engraving.
http://www.machsupport.com/forum/index.php/topic,6904.msg45266.html#msg45266
Most will not go to that extreme in size.
http://www.machsupport.com/forum/index.php/topic,13398.msg87957.html#msg87957
RICH
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Must have got the wrong end of the stick - sorry. I thought the point of the thread was that you had issues. If you're happy - I'll get back in my box.
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Stirling,
You can't leave, one need only hit the top of the box and you will come popping up. :D ??? ;)
RICH
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Nah, no problems as such - I just like to know 1-am i getting the best out of my machines and 2-am i doing any damage by doing whatever on them :)
I knew there would be backlash - I have not used maximum quality ball-screws and its a jelly-like Bridgeport mill with many decades of abuse under her belt :)
Below is a picture of my mini-mill, it was built for cutting aluminium parts which have now been moved to the Bridgport, the engraving was a bonus really, the picture below was done on the mini-mill, settings were likely far from correct as it was a while back now. I was surprised how similar the result is, the Bridgeport was slightly better i think most likely due to the mass of the machine.
The mini mill has 20mm HiWin linear guides all round, dual ball screws on Y and single on X and Z, runs on a UC100 controller.
Your engraving machine looks a nice little build, excellent detail work.
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Stirling,
You can't leave
That's actually a bit scary...
as in... You can check-out any time you like, But you can never leave!
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LOL ;)