A measurement of backlash

[dlaery]

I have a center stop on my mill.
It measures .476 with calipers.
when I use my edge finder, the "stop" measures .487, a difference of .011
Would this would be .0055 backlash?

Now I am going to cut a 2 part mold. In the next pic shows the molds side by side (to and bottom) using the center stop in the middle. With the backlash the 2 halves will be off .0055?
Would I be better off to cut them separate and use the same side rather than cut both at the same time?

[Jeff-Birt]

You can't measure backlash that way. What you need is a good test indicator mounted in the spindle. You 'preload' the indicator by moving the X-axis so that the indicator probe just touches your center stop and then keep moving toward the stop until the indicator has moved about 0.010" (ten thousandths). Now move the X-axis the opposite direction 0.001" (one thousandth) at a time. If you have no backlash of course the indicator needle would move back 0.001, in reality there will be some backlash.

Let's say you jog 0.001" three times before the indicator moves and it only moves about 0.0005" on that last jog. This would mean that you have about 0.0025" of backlash on that axis.

Hope that helps...

[dlaery]

thanks Jeff that makes sense. I have an indicator and I guess I am lazy.
I called it backlash and that may not be the correct term, but I am trying to understand what this means.
The problem I am having it getting the top and bottom molds lined up so I won't have a have a parting line or minimal parting line. So when I get this difference in what I get with a caliper and with the edge finder, what is that? I am sure the edge finder is not accurate, but is .011 a lot or normal for an edge finder?

When I use the edge finder I am using Mach3 and I set the jog at 0.3% so I find the edge on the left of the stop, then I move to the right of the stop and find that edge.

[desktoplathes]

The edge finder will vary in accuracy quite a bit, depending on how you use it. I have only ever used one in manual mode, with my eyes right up next to it. You can see where it begins to make contact, and then center itself, and then run off to one side after enough contact is made to force it off center. Ideally that happens right at the moment that you've crossed over the barrier at which it would be perfectly aligned with it's counterpart above. There can be several thousandths of travel in those transition areas and I doubt the push of a digital jog button will give you the resolution you need to make an edge finder perform all that well. I recommend powering down the stepper board, and manually moving in to locate the edge. Mark it appropriately in Mach3 and then turn the board back on and jog to the next edge... turn off board, repeat. -Keith

[dlaery]

thanks desktoplathes, I can't move my machine manual. That is why I post here to get a different perspective so maybe I can see it more clearly.
I use this edge finder a lot to find my 0,0 and to check for squareness.
It seems pretty accurate to me, for finding edges, that's why I am assuming that my difference in my original post about the difference in the mic measurement and the edge finder is in play in the nut and screw because I am checking the edge in a different direction on each side of the stop. When I am putting on my vise I line it up with the edge finder and check from side to side to see if it is running straight and can repeat my measurement from side to side without any difference.

[desktoplathes]

Aha... no manual mode. As an aside, do you have dual shaft stepper motors or some other setup? I tend to use my mill with the steppers off more than on. I made some quick and dirty handles out of 3/4 aluminum bar stock and socket head cap screws that clamp onto the shafts.

So back to the edge finder... I've never used one for aligning a vise. I recommend attaching a dial test indicator to your headstock so that it reads off the vise side and then run the x axis along. You'll be able to see a half thou difference from side to side while and in real time vs. spot measuring with an edge finder.

Some other thoughts... does the center stop material vary in thickness from top to bottom? If you take a mic measurement at various locations, does it vary? Also, with the edge finder in the spindle, it may be sensitive to how in-tram your spindle is; and then how vertical the milled center stop sides are would have a similar impact. All that said... backlash would add to the error as well. Can you measure the backlash as Jeff suggested and see how much it plays into the equation? Good luck! -Keith