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Repairing Cross Slide Screw, Need Suggestions
- Thread starter David2011
- Start date
Yes, I already have a 36" piece of 5/8" LH Acme screw in 4140 from McMaster-Carr and the tap is on the way.
After a lot of digging I found some very useful data. The Henkel/Loctite website was next to useless. The shear strength of Loctite 603 is 3,770 PSI. The area of a .400" diameter pin, 1" long, is 1.256 sq inches. That works out to a shear strength of 4,735 pounds. Without running numbers, that's probably stronger than the steel key between the handwheel and the shaft. Guess I can do without a mechanical pin in there.
If it ever has to be replaced again it can be cut off and bored out. One YouTube of this procedure shows the guy machining out the bore and discovering that it had already been repaired that way before his repair.
I think you are correct, back in post 14 I calculated 75 in-lb strength with a pinned joint. The locltite works out to 950 in-ib with the 0.4" dia. more if you go to 1/2" pin.
I do think most of the locktites can be removed with heat at a point that will not damage the steel but I have not had to do that very often.
I do think most of the locktites can be removed with heat at a point that will not damage the steel but I have not had to do that very often.
After a lot of digging I found some very useful data. The Henkel/Loctite website was next to useless. The shear strength of Loctite 603 is 3,770 PSI. The area of a .400" diameter pin, 1" long, is 1.256 sq inches. That works out to a shear strength of 4,735 pounds. Without running numbers, that's probably stronger than the steel key between the handwheel and the shaft. Guess I can do without a mechanical pin in there.
Partially, I agree with your thinking there. In practice, what I described is a "non standard" way of making a good joint, outside of a laboratory environment, in the real world, "getting it done". The loctite won't stay in the pressed areas, so it won't be a 100 percent coverage. Or if you do a close fit, but still free, that's what the loctite excells at. Doing it "my way" leaves loctite only in the machine marks. Then you're back to calculating the press fit's effectiveness. And your ultimate strength (assuming you don't go "too" thin on the outside part, is gonna be the small diameter of the "pin" on the end of the screw.
Use those numbers, they're all good stuff when comparing like to like, but for quantitative absolutes, it's very atypical that field repairs yield theoretical results no matter how careful you are. Adequate, Good, Spectacular, and Miraculous are all very plausible results, but as you found int he "spec's", the numbers are surprisingly high. There's a reason for that, and that reason is the idealized "theoretical" joint that it's tested on.
No matter how you slice it, whatever way you approach it, as long as you get it kinda good, and don't screw it up horribly the pin is not necessary. If it were me I'd pin it it anyhow. But I'm just like that sometimes.
603 will relax with heat. As will a light press. Hopefully you'd never need to pursue that for maintenance, as it takes a LONG time to wear out a leadscrew, but if you did, it'd come apart with reasonable heat. If you broke it, that little stump is gonna break right at that hypothetical infinatecimal gap between the acme threads and the original shaft. That gap in the outside dameters will be the weak spot. Not the joint it's self. Thread a quarter inch bolt into the broken "stump" so you can get a hold of it, it'll still come back out.
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- Dec 23, 2012
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Most cross slide screws I have made was single point threading.
After thread then buff the thread.
This done buff or a soft wooden block and lapping compound.
Just remember the screw and nut wear out from no lubrication not the type of steel used.
Dave
The transplant was almost successful. I thought that the fit was so good that I should heat shrink it so I tried. Heated the socket until it started developing color. The threaded part with the pin was still in the lathe chuck. The two went together 3/8-1/2 inch and stopped. Tapping with a hammer did nothing. It was stuck and not going anywhere.
After considering options the best one seemed to be to cut the pin off flush with the socket and drill/ream the pin out. After facing the saw cut I spotted it and started drilling. Switched from the spotting drill to a carbide stubby to finish drilling through the stub of the pin. Expecting to break through any second, instead I got sparks for a second or two and then the carbide drill exploded. Having no way to machine the carbide out of the hole it was clear that I would be making a replacement for the end of the screw that I didn’t intend to replace.
Making the new handwheel end of the screw was less complicated than making a complete screw with the Acme threaded portion as a single piece would have been. The major diameter was not critical at all for function but needed to be very close to .625” because it had to be held in a 5C collet. The thinner portion was really only critical where the power cross feed gear sat and that dimension was 0.590”. The thrust bearing bores are much larger than the shaft so as long as the gear will go over it, it’s good. Lengthwise there is only one dimension that has to be accurate; from the end to the shoulder for the gear. The location of the keyway for the handwheel isn’t critical either since the handwheel floats axially and is secured by an 8mm button head capscrew that has a 5mm setscrew running down the centerline to lock it in place.
Making a too long story even longer the socket end worked out well. Drilled and reamed it as described earlier because my snap gauges only go down to 1/2”. I scored the reamed bore with a carbide tool. The shoulder on the Acme screw was cut back a little to regain some of the length I cut off. Fortunately I left enough extra on the Acme screw for “just in case.” I used a Sharpie and a file to fit the pin to the socket. It made a satisfying “pop” when withdrawn when done. I made sure to leave some texture on the pin so the Loctite 603 would have a good grip and degreased both pieces with brake cleaner before joining them. After applying 603 to both surfaces and started to slide them together I remembered that I thought about drilling an air escape hole because suddenly it would be nice to have one. The air escaped, bubbling through the Loctite. I hope there isn’t too much air trapped inside the Loctite.
The assembly is curing now. Once it’s done I’ll probably make another one to correct the mistakes of the first one. Pictures coming.
After considering options the best one seemed to be to cut the pin off flush with the socket and drill/ream the pin out. After facing the saw cut I spotted it and started drilling. Switched from the spotting drill to a carbide stubby to finish drilling through the stub of the pin. Expecting to break through any second, instead I got sparks for a second or two and then the carbide drill exploded. Having no way to machine the carbide out of the hole it was clear that I would be making a replacement for the end of the screw that I didn’t intend to replace.
Making the new handwheel end of the screw was less complicated than making a complete screw with the Acme threaded portion as a single piece would have been. The major diameter was not critical at all for function but needed to be very close to .625” because it had to be held in a 5C collet. The thinner portion was really only critical where the power cross feed gear sat and that dimension was 0.590”. The thrust bearing bores are much larger than the shaft so as long as the gear will go over it, it’s good. Lengthwise there is only one dimension that has to be accurate; from the end to the shoulder for the gear. The location of the keyway for the handwheel isn’t critical either since the handwheel floats axially and is secured by an 8mm button head capscrew that has a 5mm setscrew running down the centerline to lock it in place.
Making a too long story even longer the socket end worked out well. Drilled and reamed it as described earlier because my snap gauges only go down to 1/2”. I scored the reamed bore with a carbide tool. The shoulder on the Acme screw was cut back a little to regain some of the length I cut off. Fortunately I left enough extra on the Acme screw for “just in case.” I used a Sharpie and a file to fit the pin to the socket. It made a satisfying “pop” when withdrawn when done. I made sure to leave some texture on the pin so the Loctite 603 would have a good grip and degreased both pieces with brake cleaner before joining them. After applying 603 to both surfaces and started to slide them together I remembered that I thought about drilling an air escape hole because suddenly it would be nice to have one. The air escaped, bubbling through the Loctite. I hope there isn’t too much air trapped inside the Loctite.
The assembly is curing now. Once it’s done I’ll probably make another one to correct the mistakes of the first one. Pictures coming.
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I learned the hard way about weep holes for blind hole pins. They wouldn't set at the same distance. In my case they were 2mm pins. I had to drill radial holes (in the new part) for the air to escape. Then all the pins were pressed at uniform depths. The earlier part had pins that for some reason were trapping more air and resisted being set to depth. I used a 1.95mm drill for 2mm pins for the interference fit. The simple pin spanner took a lot longer to make than I expected, due to the necessary iterations. The thing is, I could have sworn I had set the pins to uniform depths, since I used a spacer block. Not sure if some moved afterwards or not, but I was surprised looking at the tool later seeing how nonuniform the pins were.
Looking back I'm glad that I replaced the entire screw even though it was still a graft of the purchased Acme threaded screw and the remainder that I made. There was considerable wear along the contact points of the handwheel end of the piece. While I was reassembling it I realized that my major concern about the size of a pin was already answered in the original screw. The gear is pinned with a 3mm (~.118") tension pin. If that's enough for the power drive, it would be enough to secure the pin and socket. Only mentioning it to complete that line of thought since Loctite 603 is the only thing holding the parts together now.
Today's job is to make a new nut. There is a sleeve around the screw that secures the cross slide nut. It projects beyond the surface on which the nut should rest, enabling the nut to rock as the cross slide is moved. It only projects .024". It's not long enough to serve to locate the nut and it has an OD of almost 3/4". I can't see any purpose for it so will mill it off so the nut can seat on a flat surface. No idea why there is a dark rectangle around the sleeve unless the present cross slide nut is not the original. There is no relief on the nut to receive the sleeve.
Today's job is to make a new nut. There is a sleeve around the screw that secures the cross slide nut. It projects beyond the surface on which the nut should rest, enabling the nut to rock as the cross slide is moved. It only projects .024". It's not long enough to serve to locate the nut and it has an OD of almost 3/4". I can't see any purpose for it so will mill it off so the nut can seat on a flat surface. No idea why there is a dark rectangle around the sleeve unless the present cross slide nut is not the original. There is no relief on the nut to receive the sleeve.
The cross slide screw and nut project is finished and I could not be happier with the results.
While I had things apart I also shimmed the cross slide gib. I had seen the cutting tools dip several times on heavier cuts and had broken 2 or 3 carbide cutoff bits. The gib was adjusted as tight as it would go so it clearly needed some help. Since I bought the machine I've felt like it was probably used for very little other than facing or other cross slide type operations for a very long time. That is reinforced by all of the wear in that area while the rest of the machine seems to be much tighter. These repairs may reveal weaknesses elsewhere, though. I measured the space between the gib and the dovetail in the cross slide with feeler gauges and thought a 12" x .008" feeler gauge would be about the right thickness. When I test fitted it, it turned out to be a little too thick and I ended up using a 12" x .006" feeler gauge. I shortened the long feeler gauge as required and smoothed the ends. After degreasing the gib and feeler gauge with brake cleaner I glued them together with Loctite 603, installed them and let the Loctite set for 10 minutes. Everything around the back side of the gib had a generous amount of oil on it so I wasn't concerned about the Loctite joining things that shouldn't be joined. Just to be sure, I pulled the gib out after 10 minutes, reassembled it and the cross slide and adjusted it. A dial indicator showed no side to side movement after shimming yet the cross slide could still be move in and out by hand pressure. Great start!
The protruding piece in the above photo was removed and a barely touching skim cut made on the surrounding area so the new nut would have a solid place to rest. Apparently I didn't take a picture of the flattened area. I'll back up and show some photos pf the process. The old nut was well worn as as mentioned earlier, seated on the round piece above so it wanted to rock back and forth as the screw was turned. The gap between the nut and cross slide is visible here.
The power cross feed gear was pinned with a single tension pin and was easy to remove.
The wear in the old threads is pretty obvious. The peaks of the threads are much narrower in the middle than at either end.
I cut the threads off of the screw with a hacksaw. It cut pretty easily; definitely not very hard steel.
After facing the rough end I drilled and reamed the end to form a socket. It was at this point that I realized I had no way to measure the ID of the bore since my snap gauges only go down to .500" and my target was .402". I had previously written .401" but that was just a mistake on my part since the reamer was not as I remembered.
It was about this point where I incorrectly thought that I could do a good shrink fit and only got about 1/2" of the pin into the socket before it was stuck. I recovered by machining back a bit of the previously prepared Acme screw and got a perfect slip fit on the replacement. I glued it with Loctite 603 and it had three full days to cure before I got the nut made. The nut was made of aluminum bronze instead of the usual 936 bearing bronze because I couldn't find a small piece of 936 bronze. The aluminum bronze came from Speedy Metals and was about $34 plus shipping for a piece 1" x 1.25" x 3". It was about 1/8" oversized in all directions so there was plenty of excess to clean it up to my 1" x 1.25" x 1.5" starting point. After truing up the entire 3" piece I cut off 1-1/2" and cleaned up the sawn end. The blade is 1/16" x 4" and I went slow, spinning it at 70 rpm. Somewhere around 150 rpm would have still been plenty slow.
After cleaning up the end I chamfered the corners. The chamfer bit is a Bassett carbide tool, made in the USA. That was the first time I had used it after buying it late last year. It worked very well. All of the milling and drilling was done on my Jet-16 mill-drill since the quill and spindle are out of my Index mill at the moment for new bearings and a regrind of the R-8 taper. The other half of the bronze was in the other end of the vise jaws to keep the load balanced.
Tapping the block went well enough for the screw that secures the nut. I drilled to where the centerline of the cross slide screw would later be drilled and tapped it with an M8-1.25 spiral flute tap. That was my first indication of the toughness of the aluminum bronze. I used lots of Tap Magic EP-Xtra. The thread was beautiful even if a little difficult to cut.
The big hole for the leadscrew was located by putting the nut on the cross slide, assuring that it was squared up and using a shop made transfer punch to mark the location of the Acme screw hole on the end of the nut. The hole was spot drilled and then completed using progressively larger bits until the desired diameter was reached. Some of the larger bits had unknown histories and got pretty warm in the process. The hole for a 5/8" acme thread is just over 1/2" and a size for which there is no bit shown on my Starrett chart so I went to the next larger Imperial size. The new nut is 1/4" longer than the original for a little more surface area in the thread so I wasn't concerned that the hole was a few thousandths oversize.
Tapping was a chore. I got the tap started straight while the nut was still in the mill's vise but once it had made several turns I laid the nut on its side for more clamping surface. As it got deeper it tool a lot of force. Unfortunately I didn't have a tap handle big enough so I ended up using a socket and ratchet. That saved a lot of work and time, though since I was only turning the tap 1/8 turn at a time and that took my full body weight leaning into it. Finally the pressure began to ease and the tap went all the way though.
After deburring it was picture time.
With the original nut and screw the backlash measured .122"; almost a full turn of the hand wheel. With the new parts installed it measured just .001". The lead screw gets tighter just before is screws all the way through the nut so I expect that as it wears in the backlash will increase to a more normal for new machines level.
Time to make more chips and see what else the machine needs.
While I had things apart I also shimmed the cross slide gib. I had seen the cutting tools dip several times on heavier cuts and had broken 2 or 3 carbide cutoff bits. The gib was adjusted as tight as it would go so it clearly needed some help. Since I bought the machine I've felt like it was probably used for very little other than facing or other cross slide type operations for a very long time. That is reinforced by all of the wear in that area while the rest of the machine seems to be much tighter. These repairs may reveal weaknesses elsewhere, though. I measured the space between the gib and the dovetail in the cross slide with feeler gauges and thought a 12" x .008" feeler gauge would be about the right thickness. When I test fitted it, it turned out to be a little too thick and I ended up using a 12" x .006" feeler gauge. I shortened the long feeler gauge as required and smoothed the ends. After degreasing the gib and feeler gauge with brake cleaner I glued them together with Loctite 603, installed them and let the Loctite set for 10 minutes. Everything around the back side of the gib had a generous amount of oil on it so I wasn't concerned about the Loctite joining things that shouldn't be joined. Just to be sure, I pulled the gib out after 10 minutes, reassembled it and the cross slide and adjusted it. A dial indicator showed no side to side movement after shimming yet the cross slide could still be move in and out by hand pressure. Great start!
The protruding piece in the above photo was removed and a barely touching skim cut made on the surrounding area so the new nut would have a solid place to rest. Apparently I didn't take a picture of the flattened area. I'll back up and show some photos pf the process. The old nut was well worn as as mentioned earlier, seated on the round piece above so it wanted to rock back and forth as the screw was turned. The gap between the nut and cross slide is visible here.
The power cross feed gear was pinned with a single tension pin and was easy to remove.
The wear in the old threads is pretty obvious. The peaks of the threads are much narrower in the middle than at either end.
I cut the threads off of the screw with a hacksaw. It cut pretty easily; definitely not very hard steel.
After facing the rough end I drilled and reamed the end to form a socket. It was at this point that I realized I had no way to measure the ID of the bore since my snap gauges only go down to .500" and my target was .402". I had previously written .401" but that was just a mistake on my part since the reamer was not as I remembered.
It was about this point where I incorrectly thought that I could do a good shrink fit and only got about 1/2" of the pin into the socket before it was stuck. I recovered by machining back a bit of the previously prepared Acme screw and got a perfect slip fit on the replacement. I glued it with Loctite 603 and it had three full days to cure before I got the nut made. The nut was made of aluminum bronze instead of the usual 936 bearing bronze because I couldn't find a small piece of 936 bronze. The aluminum bronze came from Speedy Metals and was about $34 plus shipping for a piece 1" x 1.25" x 3". It was about 1/8" oversized in all directions so there was plenty of excess to clean it up to my 1" x 1.25" x 1.5" starting point. After truing up the entire 3" piece I cut off 1-1/2" and cleaned up the sawn end. The blade is 1/16" x 4" and I went slow, spinning it at 70 rpm. Somewhere around 150 rpm would have still been plenty slow.
After cleaning up the end I chamfered the corners. The chamfer bit is a Bassett carbide tool, made in the USA. That was the first time I had used it after buying it late last year. It worked very well. All of the milling and drilling was done on my Jet-16 mill-drill since the quill and spindle are out of my Index mill at the moment for new bearings and a regrind of the R-8 taper. The other half of the bronze was in the other end of the vise jaws to keep the load balanced.
Tapping the block went well enough for the screw that secures the nut. I drilled to where the centerline of the cross slide screw would later be drilled and tapped it with an M8-1.25 spiral flute tap. That was my first indication of the toughness of the aluminum bronze. I used lots of Tap Magic EP-Xtra. The thread was beautiful even if a little difficult to cut.
The big hole for the leadscrew was located by putting the nut on the cross slide, assuring that it was squared up and using a shop made transfer punch to mark the location of the Acme screw hole on the end of the nut. The hole was spot drilled and then completed using progressively larger bits until the desired diameter was reached. Some of the larger bits had unknown histories and got pretty warm in the process. The hole for a 5/8" acme thread is just over 1/2" and a size for which there is no bit shown on my Starrett chart so I went to the next larger Imperial size. The new nut is 1/4" longer than the original for a little more surface area in the thread so I wasn't concerned that the hole was a few thousandths oversize.
Tapping was a chore. I got the tap started straight while the nut was still in the mill's vise but once it had made several turns I laid the nut on its side for more clamping surface. As it got deeper it tool a lot of force. Unfortunately I didn't have a tap handle big enough so I ended up using a socket and ratchet. That saved a lot of work and time, though since I was only turning the tap 1/8 turn at a time and that took my full body weight leaning into it. Finally the pressure began to ease and the tap went all the way though.
After deburring it was picture time.
With the original nut and screw the backlash measured .122"; almost a full turn of the hand wheel. With the new parts installed it measured just .001". The lead screw gets tighter just before is screws all the way through the nut so I expect that as it wears in the backlash will increase to a more normal for new machines level.
Time to make more chips and see what else the machine needs.