Acme screw repair.

C

candeservices

Registered
Registered
Joined
Apr 8, 2021
Messages
15
Last year I acquired a Dah Lih GH-2200 dual head milling machine. I have been slowly cleaning and repairing all the dirty and loose parts on this mill. One of the items was the loose lash on the Y axis travel, it was .025-.035". It has two feed nuts opposing each other for this adjustment, with the table in the center of the travel if you adjust them it takes most of the lash out but then when you travel to the ends it gets very tight. With this machine being 40 plus years old I expected to have wear in the nut so I started to disassemble and found the brass threads to be worn to a knife edge. I then ordered 2 nuts and waited for them to come in, after they arrived when I tried to screw them on the acme screw I found out that it was very tight to screw in, it would only screw in about a half a turn. At first I thought I ordered the wrong part but after checking and measuring again I found out that it was the right thread. I proceeded to check the feed screw and found that it was worn in the center, but I guess that's ware all the wear happens anyway. I was about to machine the nut to make it fit the screw but then realized that even if I did that I would still have the tight issue on the ends of travel with a worn screw. So I proceeded to machine the screw to fit the nut instead, when I set up the screw in my lathe and dialed in my cut, as expected it would only cut the thread on both ends. I machined the thread so my cutter would cut the thread on the ends of the shaft and only touch the center of the screw ware it was worn the most. In my head all the thread should be at the same dimension and should eliminate the tightness on the end of my travel. When I then installed the new nuts on the shaft they went in pretty snug with a nice fit.

My question is? Is this an acceptable repair or did I just ruin the screw?
Are the replacement nuts machined undersize to anticipate the ware?
I am new to machining and I am tryin to learn. What other way or what is the correct way to repair this?
I know I could have ordered some new acme screw and weld it to my shaft but wanted to keep cost down.
I have other machinery I still need to refurbish so this wont be the only time I will see this problem again.
 
Unknown is the accuracy that your lathe is capable of in maintaining accuracy of thread pitch, this may not matter in making small adjustments of Y travel, but may not be accurate enough for long moves. If you have a DRO, it simply does not matter much. If the result of your work is accurate enough for what you do, then it was an OK way to do the repairs.
 
benmychree said:
Unknown is the accuracy that your lathe is capable of in maintaining accuracy of thread pitch, this may not matter in making small adjustments of Y travel, but may not be accurate enough for long moves. If you have a DRO, it simply does not matter much. If the result of your work is accurate enough for what you do, then it was an OK way to do the repairs.
Click to expand...
I was just trying to reduce the lash amount. I do have a DRO on the machine and I love the outcome. I just wanted some guidance's to what I do, I am just self teaching and besides looking at videos and reading forums I wanted to bounce ideas off from some one that knows more than I do.
 
Sometimes we have to live with the problems suppliers send us. Sounds like you've joined the ranks of the experienced machinists.
 
Worn dovetails and lead screws leaving travel tight on the ends is a very common wear issue to deal with, and should not have much affect on precision. Backlash is always there to some degree, and is as easy to navigate with .050 at the handwheels as it is with .005. It comes with the territory when we buy cast off machines that have been consumed by industry. It's also natural that we should try to correct those conditions. If you have nothing to lose by modifying your equipment, go for it. If it makes you nervous to scrap a one of a kind part, this is a great place to ask about it, as you have.
 
Thinking about your situation, you are concerned about accuracy of diameter.
Rarely will you be depending on cutting (at the same time) large and small diameters (The wear on your lead screw would cause problems there.). Only then will you be concerned with relative diameter Otherwise, lets imagine you are trying to hit. 1.000 diameter and have .030 to go. Take .005 on the dial, leaving .021 on the diameter. You now know that .005 on the takes .009 on the diameter on the first cut, leaving .021 to finish. As you have established the "work load error", Advance the dial .0105 and you will very probably hit your goal of 1.000.

"Work Load Error" New term indicating resistance of the work to being cut.
 
Interesting I will have to keep an eye on my dial and the amount it actually take off the material. I don't remember how much I took off the acme threads but it was not very much maybe .oo4- .005" to make all the tread uniform.
 
You must log in or register to reply here.
Back
Top