New Lead Screw For Burke #4 Mill

Mark_f

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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. :grin:

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).
1 Bearing bronze.jpg 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..:bawling:. I had one more piece of bronze big enough to make another one .

1 machined and threaded.jpg 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.
2 machined and threaded.jpg
3 machined and threaded.jpg 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.
4 nut and flange.jpg 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.
5 nut and flange.jpg 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.

6 nut and flange.jpg 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.
9 assembly for drilling.jpg 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.
7 spacer.jpg 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.
10 complete assembly.jpg 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.
11 a job well done.jpg 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.

Burke mill knee lead screw.jpg
 
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I was wondering about the tolerance of the lead screw nut.
Mark has a talent for answering the question before it is asked.
 

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I was wondering about the tolerance of the lead screw nut.
Mark has a talent for answering the question before it is asked.

When I make parts to thread together, I cut one piece and cut the other to fit as perfectly as possible. I always try for a good class 3 fit. NO PLAY but works smoothly. It is not difficult when making your own threads. If I buy a threaded part, I will cut the other part to fit.

I bought new nuts for my lathe to the tune of $45. After installing, I went from .040" back lash to .008" back lash. BUT, if I make my own nuts, and I WILL, there will be .001" backlash....or less.
 
I wanted to ask if the aftermarket lead screw nuts had such tight tolerances but once again,
answered before asked. Thanks again.
 
Since the new lead screw for the knee on my Burke mill came out so great, I decided it was time to do a new screw for moving the saddle in and out. The acme nut on this axis is large and an odd shape. I checked on a block of bronze the size needed for making this nut and the price almost gave me a stroke! Soooooo....since that ain't gonna happen, I started digging in my junk box....er....I mean inventory room :rolleyes: and found a piece of 1 inch diameter brass about 1 inch long ....just right to start. I chucked this up and put a 1/2-10 acme thread hole in it. Now how to make the rest of it.
1 saddle nut.jpg I took a block of steel and cut it the shape of the original bronze nut base. I milled a flat and a small protrusion on the brass and ta notch to match in the steel.( you can see it in the photos)
2 saddle nut.jpg With the two pieces held together, I drilled a hole through the steel and tapped the nut 1/4-20. The bolt pulled the two together and held them tightly. The notch and matching peg provide stability to take all the load.
3 saddle nut.jpg
4 saddle nut.jpg After bolted together all the excess material was milled away. It came out great and I didn't have to buy anything. To get the right height for the acme screw hole, I put a piece of acme screw trough the new nut and the original nut and set them on my surface plate. Both nuts set flat on the plate indicating I did a good job and both are the same height. To get the distance out from the base for the hole , I measured close as the final setting will be marked at assembly on the mill. I meant to take photos of the process machining this nut, but got wrapped up in it and forgot. I took about 5 hours to make this nut with lots of measuring and milling. The only difference between mine and the original is the original is solid bronze.

I started making the new lead screw and have it almost finished.
-Burke #4 mill W axis lead screw (1).jpg I had made a print for the lead screw about a year ago. I am going to make one change. I am going to add a lock to this axis so I can lock the setting while milling. Next I intend to make a new nut and screw for the table. then my mill will be tighter than a new one. I was so pleased today using the new screw and nut in the knee. there is .001" or less backlash and it was so much nicer to use. less vibration and no movement not intended when milling. accuracy is much easier to achieve.

I was going to make a print for the nut on this axis but decided against it because these machines were more or less hand built and the saddle nuts vary a little. The nut from one machine will not directly interchange with another without some re-fitting ( I tried).
 
Nice work! After bolting them together, I wonder if silver soldering the two pieces of the acme thread nut together would be an advantage? Perhaps the heat would distort the thread??
 
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Nice work! After bolting them together, I wonder if silver soldering the two pieces of the acme thread nut together would be an advantage? Perhaps the heat would distort the thread??
I had thought about that but it is totally not needed, besides, I would worry about heating the brass nut that much. The 1/4-20 bolt drawing the nut into the step is more than strong enough. That step takes all the load. It won't go anywhere.
 
I finished up the w axis lead screw this morning. it works smooth but a little snug. it will loosen up quickly. the knee loosend up nicely with just a little use. this will do the same. I make the nuts to fit the screw with absolutely zero back lash. this means if anything is one or two thousandths off ( which we know it will be ) the assembly will be tight to operate. but as soon as the high spots wear off, the screw will work easy and smooth with NO backlash! . this is why there is slop in a nut you buy. they have to have a little clearance so Joe Everybody can install it and it will work easily, but you end up with 3 to 10 thousandths back lash which many are happy with. even most new machines have a few thousandths back lash. But making your own, you can make them extra accurate.
When I milled this nut, I milled a slot and had to move the knee up .010" and then down .010" to get the proper width after the first cut with the end mill. It was so nice to be able to dial either direction without having to compensate for back lash.
1 w axis.jpg 2 w axis.jpg This is the new screw and nut installed.They work great. Next is to do the table and my little mill will be better than new.:grin:
 
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