I was going to put this project in the Burke mill forum, but I put it here hoping more folks would see that you can replace your worn lead screws and nuts in you machines, even if they are not available and make them BETTER than new.
The knee on my Burke mill was well worn and had about .040" back lash. Some of this was in the miter gears and some was in the nut and screw. This back lash has caused more than one scrapped part or broken mill from the table lifting ans jamming. I had to always lock the knee tightly but then I couldn't machine using it as I sometimes needed to do. Since there are no parts available , I looked at my options. I can buy an acme nut from McMaster Carr that costs about $180 for the nut and flange and I would still have to machine it to fit my machine. The other option is to make one. Guess which option I chose.
I ordered 3 feet of precision acme 5/8-10 left hand lead screw from McMaster Carr for $13. ( this was the total cost for this project as I had a piece of bearing bronze lying around).
This is the 1 1/4" X 1 1/2" long piece of bearing bronze. I machined a hole through it at .545". This is the minor diameter for the 5/8-10 acme thread. NOW.... just to let you know, even I screw up now and then....I worked for 3 hours making this part and the last operation was cutting the acme thread. I cut a perfect thread in this part and when I was done, I realized it .....I MADE RIGHT HAND THREADS INSTEAD OF LEFT HAND THREADS... I had one more piece of bronze big enough to make another one .
I machined the acme threads first this time and made them right. A snug but smooth fit on the screw...ZERO back lash. Next I machined one end to 1.125" diameter for a distance of .500" . then threaded it 1 1/8-20. This is to screw in the mounting flange.
You can see a .400" long relief machined in the bottom end of the nut. The only reason for doing this was my threading tool was not long enough to thread the full length. I still have 1.100 inches of thread length.
I next machined a steel flange 2.5" diameter and .500 inches thick. I bored the center and threaded it 1 1/8-20 so the nut would screw in it. I try to make all my single point threads a class 3 fit. It is not any harder to make a prefect fit and it just feels so darn good when you do.
The nut was screwed into the flange TIGHTLY and faced off on the bottom so the surfaces were perfectly flush with one another. You can see here the three mounting holes are center punched for drilling. To locate these perfectly, I took a short piece of the acme screw and threaded the old nut and the new nut back to back and tight up against one another. Then used a transfer punch to mark the hole locations. I then drilled the 3 holes.
After drilling the mounting holes, I drilled a #21 hole on the seam between the nut and flange then tapped it 10-32. I put a set screw in the hole tightly. this keeps the nut and flange from ever coming loose.
I forgot to take a picture of the part , but I made a new end for the lead screw where the miter gear mounts. It was machined from a piece of 1 1/4" diameter CRS 3 1/2" long. I machined a .625" shaft on one end 2.875" long. This is what goes through the knee casting and the miter gear mounts on the end. The other end was machined to 1.100" diameter and faced off to .750" long. A 1/2" reamed hole , .625 inches deep was cut in this end. this is to connect the new acme screw. I cut a piece of acme screw 11" long and turned one end down to .501" diameter for a press fit in the hole. The screw was pressed int the hole and a .187" roll pin was pressed into a hole drilled through. You can just see the edge of the roll pin at the top in the photo below.
I put the original thrust bearings back on and now had to come up with a way to locate the roll pin hole for the miter gear. This is a critical dimension. If the hole is too high, the screw will move up and down in the casting. If the hole is too low, the pin won't go through or the shaft will bind because it is too tight. It has to be just right! To accomplish this, I made a spacer as seen in the next photo.
This is just a piece of 5/8" ID tubing, faced on BOTH ends to be the length of the thickness of the knee casting plus .002". It is put on the shaft after the thrust bearing. Next I put the gear in my drill press vise so that a 1/4" drill bit goes through the pin holes in both sides of the gear. It is now aligned for drilling. I put the lead screw shaft into the gear and drilled through for the roll pin. I was fortunate , as my miter gears are in excellent shape. On a worn machine, you may have to make a shim for between the gear and casting to reduce back lash, but I didn't need to.
This is the complete lead screw assembly with the nut ready to go in my mill. I put the gear in place , slipped the shaft up through the casting and gear and the put the roll pin in. Next put the screws in the nut to secure the nut to the machine base. Before tightening these screws , I lowered the knee almost all the way down so the nut will be aligned and then tighten the three screws gradually working around.
Here is my completed repair. A brand new nut and screw. I made it a little tight. I can turn the hand wheel, but it is a little stiff. not bad though. it will loosen with use. There is .0005" back lash.... that's right....0005" in the knee looking at the hand wheel numbers and that tiny bit is in the gears. I couldn't ask for a better repair.
The knee on my Burke mill was well worn and had about .040" back lash. Some of this was in the miter gears and some was in the nut and screw. This back lash has caused more than one scrapped part or broken mill from the table lifting ans jamming. I had to always lock the knee tightly but then I couldn't machine using it as I sometimes needed to do. Since there are no parts available , I looked at my options. I can buy an acme nut from McMaster Carr that costs about $180 for the nut and flange and I would still have to machine it to fit my machine. The other option is to make one. Guess which option I chose.
I ordered 3 feet of precision acme 5/8-10 left hand lead screw from McMaster Carr for $13. ( this was the total cost for this project as I had a piece of bearing bronze lying around).
This is the 1 1/4" X 1 1/2" long piece of bearing bronze. I machined a hole through it at .545". This is the minor diameter for the 5/8-10 acme thread. NOW.... just to let you know, even I screw up now and then....I worked for 3 hours making this part and the last operation was cutting the acme thread. I cut a perfect thread in this part and when I was done, I realized it .....I MADE RIGHT HAND THREADS INSTEAD OF LEFT HAND THREADS... I had one more piece of bronze big enough to make another one .
I machined the acme threads first this time and made them right. A snug but smooth fit on the screw...ZERO back lash. Next I machined one end to 1.125" diameter for a distance of .500" . then threaded it 1 1/8-20. This is to screw in the mounting flange.
You can see a .400" long relief machined in the bottom end of the nut. The only reason for doing this was my threading tool was not long enough to thread the full length. I still have 1.100 inches of thread length.
I next machined a steel flange 2.5" diameter and .500 inches thick. I bored the center and threaded it 1 1/8-20 so the nut would screw in it. I try to make all my single point threads a class 3 fit. It is not any harder to make a prefect fit and it just feels so darn good when you do.
The nut was screwed into the flange TIGHTLY and faced off on the bottom so the surfaces were perfectly flush with one another. You can see here the three mounting holes are center punched for drilling. To locate these perfectly, I took a short piece of the acme screw and threaded the old nut and the new nut back to back and tight up against one another. Then used a transfer punch to mark the hole locations. I then drilled the 3 holes.
After drilling the mounting holes, I drilled a #21 hole on the seam between the nut and flange then tapped it 10-32. I put a set screw in the hole tightly. this keeps the nut and flange from ever coming loose.
I forgot to take a picture of the part , but I made a new end for the lead screw where the miter gear mounts. It was machined from a piece of 1 1/4" diameter CRS 3 1/2" long. I machined a .625" shaft on one end 2.875" long. This is what goes through the knee casting and the miter gear mounts on the end. The other end was machined to 1.100" diameter and faced off to .750" long. A 1/2" reamed hole , .625 inches deep was cut in this end. this is to connect the new acme screw. I cut a piece of acme screw 11" long and turned one end down to .501" diameter for a press fit in the hole. The screw was pressed int the hole and a .187" roll pin was pressed into a hole drilled through. You can just see the edge of the roll pin at the top in the photo below.
I put the original thrust bearings back on and now had to come up with a way to locate the roll pin hole for the miter gear. This is a critical dimension. If the hole is too high, the screw will move up and down in the casting. If the hole is too low, the pin won't go through or the shaft will bind because it is too tight. It has to be just right! To accomplish this, I made a spacer as seen in the next photo.
This is just a piece of 5/8" ID tubing, faced on BOTH ends to be the length of the thickness of the knee casting plus .002". It is put on the shaft after the thrust bearing. Next I put the gear in my drill press vise so that a 1/4" drill bit goes through the pin holes in both sides of the gear. It is now aligned for drilling. I put the lead screw shaft into the gear and drilled through for the roll pin. I was fortunate , as my miter gears are in excellent shape. On a worn machine, you may have to make a shim for between the gear and casting to reduce back lash, but I didn't need to.
This is the complete lead screw assembly with the nut ready to go in my mill. I put the gear in place , slipped the shaft up through the casting and gear and the put the roll pin in. Next put the screws in the nut to secure the nut to the machine base. Before tightening these screws , I lowered the knee almost all the way down so the nut will be aligned and then tighten the three screws gradually working around.
Here is my completed repair. A brand new nut and screw. I made it a little tight. I can turn the hand wheel, but it is a little stiff. not bad though. it will loosen with use. There is .0005" back lash.... that's right....0005" in the knee looking at the hand wheel numbers and that tiny bit is in the gears. I couldn't ask for a better repair.
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