Figured out screw pitch

Last night, after messing for 30 minutes with a caliper, issuing commands, etc I have figured out that the screw pitch of the
ballscrews, is 2.5mm. I thought originally that it was 10 TPI. So the precise value of INPUT_SCALE is 40640. Amusingly, my own estimate of it (before I guessed that it was a metric step) was 40629, a close guess.
i
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Aw, C'mon, Ig. What's 0.04mm between friends?
LLoyd
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On 2010-07-20, Lloyd E. Sponenburgh <lloydspinsidemindspring.com> wrote:

that AK receiver may shoot sideways instead of straight?
i
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Ignoramus23878 wrote:

Wait until you get to the tedious fun of measuring and entering the pitch error map...
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I would need that on a very worn nachine? How can I find screw pitch error without super complicated tools?
i
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Ignoramus23878 wrote:

Nope, it's used on pretty much all machines. No ballscrew is perfect, and by measuring and mapping the error the control can correct for it and make the machine that much more accurate. The ballscrews typically come with a pitch error chart and table from the factory that you enter into the control. I presume your machine likely doesn't have this anymore, so you'll need to measure the error.

A good dial indicator will get the job done, just in tedious 1" steps. Your axis travels aren't too long, so it shouldn't be that bad. You have to get out the dial indicator anyway to measure the backlash.
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wrote:

FWIW, I've never done this on any of my machines. Depends on what you make, nearly all my work +/- .002 is fine. Bores would be an exception, but the accuracy here comes from the boring bar, not the CNC. Now, if you intend to make production molds or tooling dies, all bets are off.
Karl
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Karl Townsend wrote:

Certainly you can live without doing pitch error mapping, but for the modest amount of time it takes to do one time, don't you think the increased accuracy is worth it? Back when I did CNC service I replaced a few crashed ballscrews and each time spent the time to enter the pitch error table provided with the new ballscrew into the Fanuc control.
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Ignoramus23878 wrote:

Very interesting. This is on your CNC machine?

All you need is a good dial test indicator and a stack of gage blocks. The bigger a stack you can collect, the longer a section of the screw you can map. You align a straightedge with the axis to be measured, and place a stop at the end. A big machinist's square is good. Then, you indicate and take a reference off the crossbar of the square, and then move the table and insert gage blocks and read the difference on the dial. Once you get the hang of it, it actually goes pretty fast. Coming up with 18" of gage blocks may not be trivial, however.
Jon
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Yes, on my machine. It is Bridgeport brand, but was made in England.
I tried to very carefully calculate desired input scale by using the depth part of a caliper to measure the moves of the table.
The number seemed odd, until I decided to try a metric conversion.

I hope that I will be OK without it. I will se how it goes.
i
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Ignoramus23878 wrote:

Backlash is what could give you the most problem. It is usually caused by bad thrust bearings but a worn ball nut will also cause it too. IF you want to get into axis compensation, cross compensation between the x and z would be the thing to look at. If your table ways are worn the table moves in an arc rather than in a straight line. The way to measure this is with a straignt edge that an indicator mounted on the table rides on. Ideally it should not move but in most cases it will increase the measurement when the indicator moves off of the center line of the machine. All machines that the table moves beyond the bottom ways will eventually wear this way. YOu cannot measure this by putting an indicator on the quill and moving the table x back and forth.
When you get your cnc mill running fine I have a job for you. retrofit a nice OM VBM for me. Its been sitting in the corner waiting for some loving attention. :) Every time I start on it something else comes up. I'm finishing up rebuilding the transmission on the Niles vertical boring mill and last Thursday the 15 HP spindle motor blew out on the Monarch CNC vmc. The rewind shop had the thing done Monday night and I reinstalled it this morning.
JOhn
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I could figure out backlash, by powering the mill, making sure that servo motors are holding, and then by pushing the table back and forth against a dial indicator, right?

I am not losing sleep over wear. Someone told me that the machine does not have a lot of wear, it has a nice lube system and the ways look good, so I believe that. I do not want to increase accuracy beyond which the machine was designed for. I think that I can get under 0.001" and that is all I want.

Sounds fun, but I wonder, were there any overloads on the motor?
i
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wrote:

This works, but not my preferred method. Put a small part in the vice and machine all four sides with a smiple Gcode program. measure part and adjust backlash values to make it on target dimension.
Karl
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Ignoramus9140 wrote:

The easiest way to figure out the backlash is to put a dial on the table. Then increment the table in .0001 steps and watch the indicator move. Then reverse the increment movements and see how many it takes before the indicator moves.
To check the backlash in the nut attach an indicator on the table and put the tip on the ballscrew where the balls run. Move the table from end to end and watch the indicator. Any wear or backlash in the nut or wear in the screw will show up as movement on the indicator. Ideally the indicator should not move at all.

The problem on the motor was that the field winding shorted out to ground and was blowing the field fuse. I seem to remember that one time way back it blew the field fuse but its been running for a long time since then. The one job we did before it blew was deep pocketing some steel plates with a insert cutter at high speed. The vibration probably moved the field coil a little and took it out to lunch. The rest of the internals of the motor looked fine. The commutator had almost no wear but I put new brushes in it. The bearings were in good shape so we left them in. I did notice that the bolts for the housing end bells were too long and just bottoming out when tightened so I went with a little bit shorter bolt. The previous rebuilder didn't pull the grease seals off the motor so all the grease pumped into the grease fittings did no good and just came out the other plug opening. The machine was up and running since Tuesday and catching up on the parts backlog.
John
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Good idea. I added it to my To-Do list seen here:
http://igor.chudov.com/projects/Bridgeport-Series-II-Interact-2-CNC-Mill/08-Spreadsheet/

Sounds great. I have a CNC rotary table that seems to have a nonworking servo motor. I may tap into your knowledge a bit. I have not done anything to it because I want to do one thing at a time.
i
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    [ ... ]

    Hmm ... reminds me of the way a particular type of machine like an ultra-precise milling machine -- called a "jig borer" IIRC is set up.
    Start with a 'V' the length of the axis with the narrow end down. It needs to be truly parallel to the axis. (I guess that good angle iron will be a good start if you don't have a 'V' machined into the cast iron.)
    Then add a dial indicator to read position at one end of the V.
    Then there is a set used with it -- a micrometer thimble at 2-1/2" long fully closed (IIRC) and 1" travel, and a set of bars with domed ends and lengths of 1", 2" 4" 6" 8" and perhaps 12" (two of each and two of the micrometers (color coded one for each axis) to provide precise positioning for both X and Y axes).
    All of these have collars near the ends to support them in the 'V' at the same height so the centers touch.
    So -- you can set up any position within the accuracy of the dial indicator and the micrometer thimbles.
    I've got a boxed set from a Pratt & Whitney jig borer. However, I can't find a set on eBay currently. This set was picked up at a hamfest about thirty years ago or so.
    The original purpose of the jig borer was to manufacture drill jigs to allow accurate drilling. But for a lot of work, it became more efficient to use the jig borer to directly make the holes in the workpiece, skipping over the jig.
    And the way of measuring everything from a common datum was adapted into NC (and later CNC) machining.
    Enjoy,         DoN.
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Absolute overkill for iggy's application.
Setting backlash would be as far as worth going.
Wes -- "Additionally as a security officer, I carry a gun to protect government officials but my life isn't worth protecting at home in their eyes." Dick Anthony Heller
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