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Serious thoughts on DRO or new screws x and y Series 1 variable 2hp

Discussion in 'BRIDGEPORT MACHINES INC. & B'PORT CLONES' started by expressline99, Jul 29, 2017.

  1. expressline99

    expressline99 H-M Supporter - Premium Content H-M Supporter-Premium

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    OK so I've got the head mounted on the mill and will be posting that on the other thread hopefully at the end of the weekend. For those that know me I'm sneaking in some work here and there while I do everything around the house...wife down due to surgery.

    Anyway, I am moving on to the table repairs on my machine. I should be removing the table in the next week or so...unless I manage to get it off there tomorrow. So at the point there is way too much backlash. Started off at 70 thou. After adjustments it's still 45 thou in the center with 35 thou on the outsides.

    So it's either new screws or 3 axis DRO. I've seen lots of positive thoughts on DRO but if the idea is just good results but the machine doesn't end up in better shape when I'm done?? I just don't know which direction to go.

    Please sway me one way or the other so I can start ordering parts!

    Thanks, Paul :)
     
  2. kd4gij

    kd4gij United States Active User Active Member

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    Replace the screw and nut. Then get the dro.
     
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  3. RJSakowski

    RJSakowski H-M Supporter - Premium Content H-M Supporter-Premium

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    The new lead screws will reduce the backlash. The DRO will essentially eliminate the concern about backlash. My old mill/drill has .047" backlash on the x and .053" backlash on the y but I am able to do precision milling because of the the DRO. Good mechanical condition is always desirable and if I had the budget, I would do both but given an either or option I would pick the DRO first.

    The DRO does more than provide precise positioning information. Firstly, it removes the need to count turns when traversing large distances. Secondly, the use of subdatum coordinates allows you to move to coordinate x, y and set a subdatum to zero and conveniently mill a slot centered on that coordinate point. I also use that feature to make non-standard holes.

    If I need to make a hole, say .506" in diameter, I would move to the center of the hole and set a subdatum to zero. Then I would drill a a 31/64" hole and follow with a 1/2" end mill. Then I move .003" plus and minus, first in x and then in y. Finally, I will move to .002, .002, then .002, -.002, followed by -.002, -.002 and -.002, -.002 (this is approximately .707 x .003 and clears material on the 45º diagonals.) I could do this without a DRO as well but the calculations plus keeping track of any backlash would make it a nightmare.

    Finally, a DRO will permit you to cut arcs, use bolt circles, and drill hole patterns on a diagonal. Cutting arcs by DRO is a real pain and will move the CNC mill up on your want list but it does enable you to do something that would be difficult otherwise. The operations could be done without the DRO, using a spreadsheet or CAD program to perform the coordinate calculations but again IMO, it would be a nightmare.
     
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  4. Dabbler

    Dabbler H-M Supporter - Premium Content H-M Supporter-Premium

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    The excessive wear on your lead screws/nuts will affect all of your milling operations in terms of surface finish and amount of force you can use in your cutting operations. (especially for climb milling) Even with a DRO, it may be less satisfying to mill accurately because of this.

    Just an idea: Sometimes it is just the nut that is completely done, but the lead screw is pretty good. You might have a low$$$ save here by making a custom lead screw nut (usually 2 made together and then separated). You might then be able to afford the DRO and have good machining for 5 years or so before replacing the whole lead screw/nut combination.

    How you tell: the width of the Acme thread should be a little less than the space between them. You can look up the pitch/spacing in the Machinery's Handbook and check how much steel has worn away. If it is less than 30% I'd make a custom threading tool and make a new nut for now.

    Cake and eat it too!
     
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  5. expressline99

    expressline99 H-M Supporter - Premium Content H-M Supporter-Premium

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    Dabbler, great thinking. I've got to do a couple of things on the mill then I can take the table and saddle off as planned. There is a horrible amount of "twist" that I get by simply pulling or pushing on either end of the table. I can see and hear it move on the saddle ways. I tightened the saddle gib in as far as it would go without forcing it and still has the same amount of movement. So I'm hoping its just a worn gib. I will run it all the way in towards the column tonight and try the twist again to see if it's the same or tighter. If tight then my ways must be terribly worn?

    At least the head is nice now that I've completed the total rebuild!

    Once the table is off I can measure those threads. I need to clean it all up anyway and clear the oil ports out.

    If I can get away with new nuts that would be wonderful!
     
  6. expressline99

    expressline99 H-M Supporter - Premium Content H-M Supporter-Premium

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    DROs seem to do a lot more than I thought! Bolt circles would be great. I've got tons of projects that require that. A DRO is high on the list. Once I get the table off and can see and measure the screws I'll be quick to make the decision to move on this. 30% or less wear in the center of the screw like Dabbler said and it gets a new set of nuts and DRO. If worn more than that. I think it will get both a DRO and new screws etc. Unless the saddle ways need grinding....

    Paul
     
  7. Doubleeboy

    Doubleeboy Active User Active Member

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    IMO worn out screws with a lot of backlash leave you in a weak position if you like climb milling. Climb milling with worn screws leads to broken endmills, and mangled parts if you loose your focus for even a second. With new screws adjusted correctly climb milling is fun with limited downside. If it were me I would get new screws and then later get a DRO.
     
  8. RJSakowski

    RJSakowski H-M Supporter - Premium Content H-M Supporter-Premium

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    There is a whole legion of old time machinists that would say that climb milling should never be done for that very reason. Even with near zero backlash, lighter machines can have a problem with climb milling due to lack of rigidity.

    I also prefer climb milling in most instances but only on the Tormach where I have less than a thousandth backlash.
     
  9. RJSakowski

    RJSakowski H-M Supporter - Premium Content H-M Supporter-Premium

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    Another way to determine the source of backlash is to measure the backlash at the center of travel and to measure it at either extreme of travel. There should have been very little chance of wear a the extremes of the lead screw so the measured lash can be assumed to be due to the lead screw nut. The difference between the backlash at the center and the extremes will be the wear on the lead screw. My old mill has a difference of about .004" showing that most of the wear is in the nut.

    To measure backlash accurately, the gibs should be loosened to eliminate drag. I would mount a test indicator on the quill and clamp a block to the table and measure the position coming from either direction to reach a particular dial setting. The difference will be your backlash at that position. The reason for loosening the gibs is that tight gibs requires more force to move the which creates lost motion due to flexing of the frame, torquing of the lead screw, etc.
     
  10. Dabbler

    Dabbler H-M Supporter - Premium Content H-M Supporter-Premium

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    On the climb milling front, and to amplify that climb milling should never be done with a lead screw with a lot of backlash, Conventional wisdon is that climb milling is for very light finishing cuts in order to leave the best possible finish. I personally never climb mill with cuts greater than .005. It is just too hard on my lead screws, and I have a fairly beefy machine.
     
  11. expressline99

    expressline99 H-M Supporter - Premium Content H-M Supporter-Premium

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    The backlash is .010 difference from the center to the outside after the nut is tightened the entire way. .045 in the center .035 outside edges. At that are you thinking it's the nut or screw? I am still going to take it completely apart. The saddle gib won't tighten enough to keep the table from twisting by hand.

    I was using the machine tonight making a t-nut for my qctp...I didn't finish it because I'm working on horrible learning curve with a machine that moves in the wrong directions. However, even as bad as it is the single dimension I finished came out within .002 of what I was shooting for so not terrible. The machine needs leveled...head needs trammed.. table mechanisms fixed.

    The wear on the saddle ways bothers me. I hope that twist is only from a worn out gib. Otherwise, that knee and saddle...and table might go out for grinding.

    Paul
     

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