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Taig Mill not that precise Please Help
« on: July 26, 2008, 04:24:30 PM »
Hi Guys

Having trouble with my cnc Taig mill, have mach3 latest revision and i am trying to machine a precise part for a model jet engine talking sizes at 0.01, in mdi i type in a value say 6.37 for example and my machine will travel that distance on the odd occasion other times it will be short by 0.02 or more, backlash is enabled. With the backlash that sometimes ends up out by .1 or so when it comes back to its original point. Can someone please help me? Also when trying to cut a circle with backlash enabled the circle ends up with a short for a better word "nobble" on both sides due to the backlash overcompensating? im not sure.

Many thanks

Ryan
Re: Taig Mill not that precise Please Help
« Reply #1 on: July 26, 2008, 05:25:05 PM »
Not sure this will help, but check your gibs for looseness, tightness, etc.  I use a Taig and can repeat to the same start and end points when have to re-cut parts to deeper depths than originally called for.  In the past, I had trouble similar to yours and fought to get the corrected (bad circles, lost steps, etc.).  Adjusting the gibs corrected the problem.   If you are not familiar with adjusting the gibs do a search for Nick Carter and Taig.  He has a website with lot of Taig information

Another thing to check is Axis Calibration  in the Settings Menu - the Axis Calibration is at the bottom left of the screen, click on steps per unit to use the function.  That is something else I had to work with in order to get accurate movement. 

Hope some of the above helps

Bob@BobsShop

Offline RICH

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Re: Taig Mill not that precise Please Help
« Reply #2 on: July 26, 2008, 06:12:46 PM »
Ryan,
Just a general comment on keeping accuracy in perspective. You can adjust and refine just so much until you hit the tolerance capability of the machine. Every part of the machine contributes to the overall accuracy. So refinements are carefully done to make the mechanical side and software side as good as it can be. You can waste a lot of time and not achieve anything for your efforts. You can adjust the outcome of the machined piece by changing anything that is done to machine the piece.
PS: Make sure you post on that jet engine as a friend of mine is making one for his RC helicopter.
RICH

 

Offline jimpinder

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Re: Taig Mill not that precise Please Help
« Reply #3 on: July 27, 2008, 04:15:01 AM »
Are you saying that some of your moves are 20 thou of an inch out, which is how I am reading it. We have jet engines over on this side of the pond, a local firm in Barnsley (of all places) markets them - and they are looking for accuracy down to 1 thou.

I think - instead of messing about with bit and pieces, if you are to get down to the accuracy required, you will have to set out a plan and stick to it, otherwise you will do one bit, and it will upset another.

The first thing, as has been said, is to check the gibs on your table. Depending on where the table is pushed or pulled, if the gibs are slack then the table will "lurch" as it changes direction. On my lathe, it lurched on the Zaxis, and affected the X axis diameter it was cutting. If you have not done gibs before, by all means read up on them, but they are relatively simple - a series of screws pushes a metal plate in, against the running surface of the table, taking up the slack. Unfasten the locknuts, and (mine have an allen key) gently tighten the inner screw up - not tight, just take up the slack and a fraction more. Tighten the locking nut again (without allowing the inner screw to move).

It is a bit difficult to measure the effect of this, but if all is nicely "gibbed up" movement of one axis should not alter the setting of the other, particularly on a change of direction, but you should still retain smooth, even movement. Be careful on the table extremeties, where there is less wear, becasue it is easy to overtighten these and cause the table to stick or at least loose steps.

The next thing is to set your steps per unit correctly. This does not mean measuring anything - it means calculating (sorry BobShop - I do not use the Mach 3 function - it is open to inaccuracy). The calculation is simple - (motor steps per rev) multiplied by (microsteps - see drive) multiplied by (gearing - direct drive = 1) multiplied by (pitch of leadscrew). This should round off at a simple number (unless your leadscrew is something you picked up on a second hand stall) - mine is 60,000 - 200(motor) x 10(Gecko microsteps) x 3(gearing) x 10 (leadscrew 10 turns per inch), You can see from this I have 60 steps to 1 thou of movement - which is good for accuracy. You may not have the gearing so you might be down to 20,000 - still 20 per thou, (although you cannot guarantee the accuracy of microsteps)

Once you have done this you can test your movement. With a digital caliper, you can test the movement of you table over say 6 ins, using the MDI. move to the right, zero your DRO's, then with the MDI go in 1 inch steps to the right and check each distance. They should be accurate to within say 2 thou. The purists will say this is not good enough - but, quite frankly, I defy them to measure any more accurately with a caliper. The "error" if any should be consistant (+ - 2 thou)  throughout the length. If it is consistantly accruing  or loosing thous as it goes along - then there is a fault - and this is probably in your leadscew - all other things being simple numbers.
If this is the case, then (and only then) can you try making small adjustments to your steps per inch to correct the matter. This is extremly rare, and really I do not like even mentioning it, becasue it confuses the issue.

The above measurements can be taken with the "backlash compensation" on or off, but to adjust for backlash you need the compensation OFF.
It is very simple. Starting at 0.0 - on the MDI go to X1 (to get rid of backlash). Set up your caliper and zero the DRO's. Do a G1 on a low Feed Rate say 4 inch per min and go X2 then X1. In theory the table will move right then move left and be at the starting point. It will not be and you can read the distance on your calipers. The missing distance is the number of steps that the gearing and leadscrew took to get their shoulders ready to push the table the other way. You can, if you wish repeat this and take an average - since you are measuring - but if you are careful one will do. This is the backlash compensation and should be entered in the backlash table and compensation switched on. If you do averages - do not forget the initial move to get rid of backlash each time.
The only things about backlash is that it will not be consistant over the length of the table because of mechanical wear. If you job entails you using one particular part of the table, then measure the backlash over that part for better accuracy for that job.

You should find now that you can move your table up, down, in and out to a good accuracy - and it is worth taking the time to do this, becasue you can sit down and machine something with a bit of confidence.

This has nearly taken longer to write than to do it - except maybe for the gibs. My table is accurate to a thou (and you want to see my backlash) and I regularly machine steel to within a thousanth of size - i.e. fitting a wheel on an axle requires a two thou gap for the glue.

The other "trick" when machining parts to a fine accuracy is to, as we used to do manually before a computer did it for us, always do the cutting move in the same direction - i,e, run back a little over, then come in again from the same direction every time, not just run back to the spot and start machining. This makes sure that any innacuracies still in the system always happen (or dont happen) on every cut. It dosen't matter which, as long as you are consistant.

Somebody also put in another post that mills are not as accurate as lathes - and I do not know the Taig machine so you may have to be especially considerate to good machining practise to get the accuracy you seek, but good luck.If you already know most of this I apologise - once I get typing I don't know when to stop.

If the household authorities were more forthcoming then I would love to have a go at a little 7 H.P. turbo prop engine - coupled to a 175 amp 24 volt generator - which would make a fine basis for a gas turbine locomotive - and be totally authentic. I have the generator - now.............................. ???




















« Last Edit: July 27, 2008, 04:25:48 AM by jimpinder »
Not me driving the engine - I'm better looking.
Re: Taig Mill not that precise Please Help
« Reply #4 on: July 27, 2008, 08:29:26 AM »
Good response (lot better than don't try to adjust anything!).  My Taig was relatively new when my problems reared their head.  In my case the gib's screw adjusters were loose.  Tightening really helped - and, as you said, it wasn't that difficult - and not rocket science!

With regards to using the Axis calibration - It worked for me, but one experience does not validate its precision for everyone's requirements.

I have made pieces as large as 11X5 inches out of .75 aircraft aluminum  with my Taig.   Pieces this large really push the limits of the machine, but I have been most pleased with the results.  As Jose Rodriguez says - precision and accuracy are possible with a small lathe or mill.

Bob@BobsShop - back to cutting
Re: Taig Mill not that precise Please Help
« Reply #5 on: July 29, 2008, 02:18:18 PM »
Hi guys

Thank you so much for your advice and help it has been much appreciated, my mill is dc servo driven so dont know about its step loss, the motors drivers just return an error and stop the motors if it misses a line on its encoder, as for the gibs i have checked out Nicks website (thank you anyway BobsShop) and followed his method and feel that they are fairly well adjusted. jimpinder im from the Uk too down in hampshire ;) not really up to scratch on my imperial system but i think it is 40 thousandths, not sure if its right i divided 0.01 by 25.4 and got 0.0003937 which i assume is 40 thousandths excuse my ignorance! as for the steps per unit what do you actually put in the box? 60000? my servo driver aparrantly multiplies the encoder output by 4.

For the backlash i tried your method but for some reason once set the compensation over exaggerates and continues to move into the stock the other side of the cutter and mess up the nesting design. I must say though however i do find it difficult trimming my Taig, getting the headstock tangent to the table, its a bit of a neusance the swivvel head.

RICH, i shall post some pictures soon of my jet, as i bought most of the parts and am just assembling it, i am however going to try and machine a KJ66 diffuser. ill do this and post it up, but for now i have found this great little website this guy has on making a Wren 54 turbine it shows him machining all the parts, check it out http://www.5bears.com/gt.htm

Many thanks guys for your help

Any more tips would be really appreciated!

Ryan

Offline Sam

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Re: Taig Mill not that precise Please Help
« Reply #6 on: July 29, 2008, 04:01:31 PM »
Your a couple decimal places off... 0.01 is indeed 0.0003937, which is almost "half of one tenth of one thousandth", or "4 tenths" for a shorter term. For ease of understanding, 40 thousandths is 1.016 mm, or 0.04 inches. Waaay out of tolerance for making any functional turbine engine I would think. Or even a radial for that matter. I never thought such a small and lightweight machine such as a Taig would come close to achieving tenths repeatably. Rich, don't you have one? What kind of accuracies are you used to seeing with yours?

And kind of off the subject here, but has anybody heard of the 5bears guy recently? TONS of good info on his site, and it seems it was updated regularly, untill about 2005. Then it's like he just fell off the earth. Every time I see a reference to his site, I think he's probably taking a dirt nap. Maybe hes just waiting to unload a heap of great material for us to look at.....doubtful, but hopeful.
« Last Edit: July 29, 2008, 04:06:39 PM by Sam »
"CONFIDENCE: it's the feeling you experience before you fully understand the situation."

Offline jimpinder

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Re: Taig Mill not that precise Please Help
« Reply #7 on: July 30, 2008, 01:05:44 PM »
Ryan - glad to see you are in the UK and actually use metric. We did go metric a long time ago. I was taught Imperial, but then I was an Instructor at a Police Academy at the time we changed, so I got the benefit of both. My lathe is set up in Imperial - bacause it has Imperial dials and an Imperial leadscrew, so it seemed logical to set it up that way.

However - I do most of my work in Metric - and just type G21 at the beginning of most of my  programs. I thought you were across the pond, so I was talking Imperial, if the backlash etc. you are talking about is metric then it is not that bad.

The rest of my diatribe still holds good, however, just use metric if that is what you are set up in.

The Wren engines are the ones I was referring to - they are made a few miles from where I live.
Not me driving the engine - I'm better looking.