Backlash or motor tuning problem? How do I test for backlash?

When I cut a square frame of ¾” thick MDF using a ¼” cutter the finished piece is .05” larger than drawn on outside cuts but only .005 to.01” on inside cuts. Does this indicate backlash is present? I don’t feel any backlash when I try and push the gantry back and forth.
To give the background story I calibrated the machine yesterday (if I did this correctly) by drilling a hole in my base table, lifting the cutter and moving a set distance in MDI and drilling a second hole. I did this several times and finished by entering slightly different numbers (in the x and y axis’s) in the “steps per” field in motor tuning in Mach3. But now I question whether this method is correct as there’s no load on the machine when I’m just drilling holes. So my question is what do I change first, motor settings or backlash and how do I test for backlash if the motor settings might be incorrect? The old chicken or the egg question.

Brian

Steps per inch is a strict function of gearing and screw ratios. Once it is set up correctly, it is never altered for backlash.

Backlash is a certain amount added (or removed) from an axis movement during a direction reversal.

You can test as you were by drilling holes, drill one pair of holes in the normal method, then drill another pair on the same spacing, but insert an extra movement in front of the second hole, to cause the machine to first overtravel the second hole position, after which it will then approach the hole from the opposite direction.

Compare the seperations of the two sets of holes. The difference will be the backlash in that axis.

Here's a method I came up with. Take a dial indicator, they are cheap from harbor freight, set it up on your table, make sure it's secure. Jog an axis over until it moves the needle on the dial indicator. It doesn't matter what the value is we are just using the sensitive nature of the dial indicator to see when the axis moves. Now zero out your DROs in Mach and hit 'tab' to reveal the Jog dialog. Click Jog Mode button until Step lights up. This will enable step jog mode. Then jog off, reverse from what you just did originally, one step at a time until the dial indicator moves. Now reverse one step and read the DRO and see how far your axis is off. Then go into backlash settings of Mach and enter those settings. Redo the experiment until it compensates.

One thing to be sure of is to run a program, cutting just air, and look closely at all your table parts. MAKE SURE nothing is flexing. No matter how strong you think your table is or what it is made out of it can flex. I thought I backlash problems turns out I had flex from a 1" x 2" extruded aluminum on my Y axis. I will reinforce it when my parts arrive.

Are you cutting the same direction for your inside and outside cuts? You could be getting bit deflection.

Backlash comp is an acceptable computer ploy for dealing with backlash BUT, the best fix remains to eliminate it. You'd be amazed at the improved quality you'll get (and easier programming) when you rid your machine of backlash issues.

A neat trick for identifying tune and/or back lash is to simply cut a circle.

If it is oblonged, your tune is probably off in one of the 2 axis.

If you make a squarish hole, chances are that both axis are in need of tune as a circle is resulting in BOTH axis being put thru the/a worst case for velocity and change of same in order to cut a round hole.

IF you have flat spots at 12, 3, 6 and 9 o'clock, backlash - the worse the flat the worse the backlash.

When you rid yourself of backlashe via preloaded ball support bearings and backlash free acme or ball screws, the flats will go away and you'll get MUCH better cuts irrespective of anything else you do.

Spend the time and money - you won't regret it in the end.

Quote:

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Originally Posted by NC Cams

IF you have flat spots at 12, 3, 6 and 9 o'clock, backlash - the worse the flat the worse the backlash.

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i have x-axis at 12,6 oclock direction and y-axis at 3 and 9 o'clock direction.

now let suppose i have flats at 3 and 9 o'clock...then what axis am i getting the backlash???

If I understand you correctly then Y.

thank u Ger21... when i machine the larger circle on MDF, it seems almost okay..but when i drill hole say 10mm with 1/4" cutter the circle seems irregular and sometime i find flat areas at 3 - 9 oclock...
Now please tell me , if it is due to MDF or the MDF dust inside the hole that resist the cutter motion.???

Quote:

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Originally Posted by Khalid

thank u Ger21... when i machine the larger circle on MDF, it seems almost okay..but when i drill hole say 10mm with 1/4" cutter the circle seems irregular and sometime i find flat areas at 3 - 9 oclock...
Now please tell me ,

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I would have assumed it was in the X?
Anyhow, using a Handwheel, preferably, move the axis you are testing in say the X+ direction, place a mag based dial gauge against the X axis table or the router head, depending on what type of machine, set the dial gauge to zero, and then with low increments setting on the HW, move the X axis in the X- direction, stop as soon as you see the dial gauge move, The screen display should show the expected move distance, subtract the dial guage from the display, and you have your backlash value.
I think post three mentions this method also.
Al.
PS If a router, it could be the Y as I always think gantry tables have the XY backwards, as the axis designations assume you are are on the long side of the table, when usually the operator is on the narrow side Y, (backwards from convention).
Al.