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Backlash vs DRO

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8mpg

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#1
Im brand new to machining. Havent actually taken my first cuts on the mill I bought (been building the rest of the shop). I know I have a couple issues with the mill to fix but started going over the mill. I found the backlash on the mill to be .022" in the x axis, .037" in the y axis. The table will pivot about .040 when pushing and pulling on the table. Im assuming I can fix part of that by the gib but its tighter towards the end.

So I was thinking of ordering a new set of split nuts and possibly the x and y screws. Then I thought, would it be wiser just to spend that money on a DRO? Im thinking since the DRO is independent of the screws and only measures when the table actually moves, is backlash as big of a deal?
 

JimDawson

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#2
The Y backlash seems a bit excessive, the X is not too bad, but could be tightened up. Your nuts may be adjustable, but the screws may be worn in the middle so if you tighten them up it may be tight on the ends.

The table is my biggest concern, 0.040 is a lot, tighten as best you can without having it over tight at the ends.

While the DRO ignores the backlash so hole pattern layouts will be pretty accurate, the play in the system will affect the accuracy and finish of your milling work.

Bottom line is I would tighten up the machine, then get a DRO.
 

RJSakowski

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#3
Im brand new to machining. Havent actually taken my first cuts on the mill I bought (been building the rest of the shop). I know I have a couple issues with the mill to fix but started going over the mill. I found the backlash on the mill to be .022" in the x axis, .037" in the y axis. The table will pivot about .040 when pushing and pulling on the table. Im assuming I can fix part of that by the gib but its tighter towards the end.

So I was thinking of ordering a new set of split nuts and possibly the x and y screws. Then I thought, would it be wiser just to spend that money on a DRO? Im thinking since the DRO is independent of the screws and only measures when the table actually moves, is backlash as big of a deal?
A DRO assumes that the table moves in a well defined direction determined by the ways. If there is .040" of play in the ways, the DRO will not give you a correct position. I could live with the backlash, having a DRO but not the loose ways. Unfortunately for you, having a loose in the middle, tight on the ends of travel situation usually indicates uneven wear and can only be corrected by remachining the ways. Practically, this would mean scraping them. This isn't a trivial task.

My old mill/drill has .035" backlash on the x and .052" on the y but, because of the DRO, I can still turn out some decent work on it. Locking down the axis not being moved helps. You can also improve finish by slightly tightening the locks on the moving axis. On my machine the locks work on the gib and act in the same manner as adjusting the gib. I tighten them to the point where the increased drag is just noticeable.

In my case, the major backlash is in the lead screw nut. You can determine whether it is the nut or the screw by measuring the lash at the ends of travel.. Since that region of the screw is seldom used, it will not have wear. In my case, the x axis shows only about .003" which is attributable to the lead screw. The y axis is worse, with about .014" attributable to the lead screw. On a full restoration, I would probably replace only the y axis screw. The mill is an off brand machine and replacement parts are not available so I would be faced with making my own parts. Since I have a Tormach, the urge to do a rebuild isn't there though.
 

Janderso

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#4
I have an old Bridgeport. The DRO I installed made it much easier to operate a worn table/Ways. I lock down what I can and so far I have been able to accomplish my project needs.
 

8mpg

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#5
Thanks for the responses. The Y axis gib is maxed out so I am going to shim it. Possibly shim the X as well. I will need to remeasure as I didnt realize I needed to lock the axis when measuring. The H and W videos on youtube have been a blessing. Just ordered parts for the quill feed from them.

I measured the spindle and have less than .001" of runout which Im very happy about. Hopefully I can tweak this machine up. I cant complain as it only cost me $300.
 
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