Mill Table too tight
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  1. #1
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    Default Mill Table too tight

    Hello. Our Mill is actually a Birmingham but they are modeled after Bridgeports. I hope it's okay that I post this here.

    I recently replaced the both x and y screw nuts. The issue I'm now having (which did not exist previously) is that when taking the table (x axis) all the way to one end the handle movement is extremely tight, but this is not the case when traveling the table to the other end. I have played with the gib in different configurations to have a lot of slack to being snug but this does not seem to have any noticeable affect of loosening the tension on the handle.

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    Mill ways do get worn in the middle. I guess someone could do a lot of work on one end. The problem with adjusting the ways to the felt resistance is, the resistance you are feeling is it from the ways or the screw and nut. I bought a CNC kit for a bed mill once. the installer set the ways to a readout on the computer. Manufacturer of the kit said that was the wrong way to do it. They said side to side play should be adjusted to .001 inch. On a mill with worn table I guess you do that on the tightest spot and live with whatever in the middle. Once you have the table free of excess tightness, address the screw and nut. Might be a better way but this is how I would start till someone suggested a better plan of attack.

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    Sounds like a lot of wear on one end of the table.

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    Fred C Had hinted at the same. However, I did not have this issue before replacing the screw nuts. The table had a lot of slop in it and the nuts were worn. Previously the travel was great and was very responsive the any gib adjustments but now its very very tight and just at one end. Very strange.

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    Screw can have taper from wear also. I think I would eliminate the table ways binding first then tackle to possibility the screw is binding on one end next.

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    Quote Originally Posted by sandar View Post
    Hello. Our Mill is actually a Birmingham but they are modeled after Bridgeports. I hope it's okay that I post this here.

    I recently replaced the both x and y screw nuts. The issue I'm now having (which did not exist previously) is that when taking the table (x axis) all the way to one end the handle movement is extremely tight, but this is not the case when traveling the table to the other end. I have played with the gib in different configurations to have a lot of slack to being snug but this does not seem to have any noticeable affect of loosening the tension on the handle.
    It sounds to me like your nut bracket is misaligned. Sorry to say it, but you will need to remove everything again and take a look at the alignment of the screw from end to end. It doesn't have to be perfect but it does need to be very close. If it is the same end to end you should be ok.

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    Couldn't this be answered very quickly by loosening the hold down bolts for the screw bearing on the tight end? If the problem is corrected (not that you can run this way) you know it is a screw/bearing either misplaced laterally or too short from the table. If the bearing mount is too tall, it will probably sag down as you run the nut on the screw away from the bearing. Slip a shim under the bearing mount and see if it gets tight sooner.

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    So I have the table slid off. I adjusted the lead screw nuts again and then hand cranked the screw all the way to the positive end of the x axis (where I had been having the tightness). it then started to get very tight without any bearings or anything on the leadscrew, just a floating screw in the nut bracket. I then adjusted the leadscrew nuts while the screw was at the positive end and hand cranked it down to the negative end and the leadscrew began to had a lot of play in the bracket. What does that sound like to you guys?

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    A few pictures would be handy. I like to be a Detective and ask questions before helping. Otherwise it's a guess.

    Did you replace the nut bracket? Does the new screw slide into the "original" bracket?
    Did you measure the replacement screw and compare it to the old one. Can you take some photo's of the screw. Or at least the old one so I can picture what you have. One Bridgeport's many times they had a shim pack that spaced out the bearings to bracket and screw. I would use a dial caliber and check the lengths of the bearing flanges. If you have to use a tape measure and check the length of the screws to compare. Ball screws use thrust bearings, do you have a sheet of directions. Take a picture of it or scan it in so we can read it. If you replaced the nut bracket, did you measure to see if it was the same height of the original bottom to center ? You may have to set them on a surface plate and use a height gage.

    I would assemble it and remove any dowel pins in the end brackets and leave the bracket bolts loose. With the gib loose say you can slide a .002" feeler gage in behind it or do the shake test on the table with .001 to .002" shake on both ends when the table is centered. Then crank the table to the original tight end making sure the brackets are loose but not sloppy. after you get to the end, tighten the bracket. Then crank it to the other end and tighten the bracket. If that cures the problem then drill the dowel pin holes to the next spring pin size and tap in new spring pins.

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    The problem you are describing sounds like a mislocated table bearing bracket.

    I agree with Richard, I would loosen both brackets, be sure to remove their roll pins, and move the table back and forth. Next, on a bridgeport, I think those bearing brackets are similar. I’d swap them. Any chance you installed yours wrong?

    I’d yank your gib and inspect that. Any chance you have some swarf in there?

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    You're right, pictures do help:
    img_20200422_071105.jpg img_20200422_071114.jpg


    The bearing brackets are not identical unfortunately so I cannot switch them. The only parts that I replaced were the set of brass screw nuts for the x and y screws. What's strange though I that I currently have the screw just floating in the nuts with no brackets connected (thus no load on either end) and I'm still getting a very tight movement from one end of the screw and not the other.

    You'll notice shims between the adjustment nut. The Taiwan manufactured part did not tighten all the way against the feed nut bracket so I had to shim it out so that I could make proper tightening adjustments

  14. #12
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    I re read the # 1 post. You ONLY replaced the nuts. Not the feed acme screw. The screw is worn in then middle and not worn on the ends. The nut has a slot in it to compensate for wear on the screw and eliminate the back lash. You will have to adjust the back lash on the ends not the middle. It will always be loose in the middle. You will never eliminate the problem of a few thousands back lash all over unless you replace the screw or chase the screw so the pitch is the same all the way down length. The photo's were needed in the first place, so no guessing was needed.


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