Advice on a Compound Lead Screw

tjb

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Okay, Everybody.

After reviewing everyone's input, I decided to take the following approach. Strategy 1: Try to fix the lead screw. If that failed, I've essentially lost nothing (as several have suggested). Strategy 2: If I broke it, cut off the Acme threaded rod and weld and pin on a new section. If I blew that, I'm still out essentially nothing except I would then own a remnant of Acme threaded rod that might come in handy someday. Strategy 3 (and actually an ongoing process since the beginning): Keep looking for a replacement.

I gave Strategy 1 a try in the shop today, and I'm happy to report it was quite successful. As an expression of gratitude and hopefully as a benefit to anyone else who runs into this kind of problem, I'd like to present the following pictorial documentation of the steps I took to fix the lead screw. Any of you real experts out there with suggestions on how I could have done it better, PLEASE respond for the benefit of all of us. Here we go.

Step 1: Try to identify exactly where the bend is in the lead screw. A visual examination didn't help much. The 5/8" shoulder in the middle made it challenging to lay flat, so I drew a 'plus' sign on the end of the shaft (difficult to see in the photo) and marked the points beginning at top and going clockwise 1-2-3-4. Using a surface plate, v-block and height gauge, I registered the readings in these four positions. I was careful to take all the readings on the crown of the same thread rotation. I normalized the readings so that the lowest value was exactly zero. I also took some readings further up the shaft to help me identify where the bend started. The first six attached pictures shows how I did it.

Step 2: Straighten the bend on the shop press. Since the 1-3 (vertical) dimension was far worse than the 2-4, I started there. Before moving to the shop press, I used a ruler to determine approximately how high the center of the shaft should be above the bench when still mounted on the v-block. I straightened the shaft until I achieved that point in the 1-3 dimension. Photos 7, 8 and 9 show the process.

Step 3: Assemble the compound to check preliminary results. After adjusting on the 1-3 dimension, I did a dry assembly to see how close it would be. Amazingly, the compound worked fine. If I forced myself, I could sense a very slight binding, but it's not enough to make me want to fine tune it more. (Like Yogi says, "If it ain't broke, don't break it.") I finished the assembly and called it a day (see photos 10, 11 and 12). Start to finish: about two hours.

Again, I'd like to thank all who responded with very good advice and counsel. This was a good learning experience for me and what appears to be the last step in getting this fine machine in good and complete working condition. Any suggestions on how I could have done this more efficiently? There may be a 'next time' for me or some of us other neophytes.

Regards,
Terry

P.S.: I'm not sure if I'm attaching photos correctly. If any of you have problems seeing them, let me know, and I'll give it another shot.


IMG_1245.JPG IMG_1246.JPG

IMG_1245.JPG IMG_1246.JPG IMG_1247.JPG IMG_1248.JPG IMG_1249.JPG IMG_1250.JPG IMG_1252.JPG IMG_1256.JPG IMG_1257.JPG IMG_1258.JPG
 

mikey

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I took a slightly different approach and used an aluminum V-block on either end and measured the high point of the bend. Then I press, check, press and check until the bend is eliminated. I shoot for 0.001" TIR or less if I can get it. I've only done this twice so not a lot of experience with it but this worked for me.
 

tjb

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I took a slightly different approach and used an aluminum V-block on either end and measured the high point of the bend. Then I press, check, press and check until the bend is eliminated. I shoot for 0.001" TIR or less if I can get it. I've only done this twice so not a lot of experience with it but this worked for me.
I thought about doing it that way, Mikey, but the 5/8" shoulder prevented that. The shoulder starts about 2" in on one side (OAL = @6"), and the bend started about 3/4" beyond it. Really wasn't enough room to place the press. Two v-blocks would certainly have been easier in most circumstances, though.

Thanks for the reply.

Regards,
Terry
 

Glenn Brooks

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Yeah, Randy. I'm not too uncomfortable in trying to straighten it. Just thought I'd see if it was easy and relatively inexpensive to buy one. Then I'd stick the old one in a scrap bin for a future who-knows-what project. I suspect I'll end up trying to fix this one. My shop press is pneumatic, and I've never tried to do anything quite this precise with it. I'm concerned that I won't have enough control on the foot pedal to not 'overcorrect'. I have a good heavy duty vise, so that might be a better way to go. What do you think?

Regards
I recently had a new cross feed screw and nut made up for around $150. Best investment I ever made. Made my lathe like new.

If your screw is worn, it won't hold dimensions consistently. So sure, try to straighten, but if your not satisfied, you can have a new part made up to the exact dimensions,of the old one. And no guess work with Grizzly.

Glenn
 
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f350ca

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Well done Terry!
The way you did it worked as the bend was at the start of the tread.
I had a part come out of the chuck (think it was still too hot from welding and as it cooled it shark and loosened), it pushed the carriage back, bent the lead screw and broke the casting holding the nut. The bend here was in the middle of the thread so had to be straightened using v blocks and pressing between them.
Got it straight enough to use while making a new one. The acme thread was a metric dia and form with inch pitch and the adjusting thread to preload the bearings was metric. My other lathe doesn't cut metric.

z7jtU2fZ8eXvSC-erFvC6Jq5EMrHZWjYAXf1KBmQrod6VZsosQJXLOJMGyAGmoHPXvfWMfRw51xqRS7le0XaIE70PWA6hhsuJeQh-ao-Jrr4AG8vVSsmJj4tx_q46BDW92pFn0YfIH0jV6h1Q8AJ8RZ0ggZ5wCTAY-F6mW5_70_mpP6incIQ7cXcibjyUQohF0y3b6TPyF7ofA39aMUjSjftaMy1fTX6kXZHH3s5-E-z5GKa2c7QYGibYyk9iu60l9DEplKyH3pxG1in1RIfzVcRCKZiTBR5sH16-P3F-bBsP0LEM3TdX5Ddyl7zsgJuV2cUfGg-22z1mTlaY9ZCqNY8wMqlqzFhWCsfaaI5EyRlGNPe2EDx4edpC4B28pfqNicL5GdJQhF6ae9QlCbXt7t9uf9pIVH_GZ0NM5KmO726CEMPc_Y6Gu72WoZ2TdxWbPjq_2J8BeKcIPx-a4_8ip_kPGDaylMVtnE6gGr56vsZ99swOorUSNneH1inIZLxPXhKSjqgwcEZ0c3qd3DQuxOpzn17wCPz64vytdDonBdcJN3A0N8lXjyN_CcjdcRUt15KmQaPHd9ZdFgzCjeILSQmrncLbSOOpyOrxvYECZiClwIMkD9MFI9J=w1702-h1276-no


Greg
 
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tjb

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I recently had a new cross feed screw and nut made up for around $150. Best investment I ever made. Made my lathe like new.

If your screw is worn, it won't hold dimensions consistently. So sure, try to straighten, but if your not satisfied, you can have a new part made up to the exact dimensions,of the old one. And no guess work with Grizzly.

Glenn
Thanks for info, Glenn. As you may know if you've followed the thread, I was able to straighten the lead screw and the compound is working fine now (never was an issue with the lead screw nut). It's good to know, however, that it's not impossible to have a set made.

Thanks again for the info.

Regards,
Terry
 

tjb

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Well done Terry!
The way you did it worked as the bend was at the start of the tread.
I had a part come out of the chuck (think it was still too hot from welding and as it cooled it shark and loosened), it pushed the carriage back, bent the lead screw and broke the casting holding the nut. The bend here was in the middle of the thread so had to be straightened using v blocks and pressing between them.
Got it straight enough to use while making a new one. The acme thread was a metric dia and form with inch pitch and the adjusting thread to preload the bearings was metric. My other lathe doesn't cut metric.

z7jtU2fZ8eXvSC-erFvC6Jq5EMrHZWjYAXf1KBmQrod6VZsosQJXLOJMGyAGmoHPXvfWMfRw51xqRS7le0XaIE70PWA6hhsuJeQh-ao-Jrr4AG8vVSsmJj4tx_q46BDW92pFn0YfIH0jV6h1Q8AJ8RZ0ggZ5wCTAY-F6mW5_70_mpP6incIQ7cXcibjyUQohF0y3b6TPyF7ofA39aMUjSjftaMy1fTX6kXZHH3s5-E-z5GKa2c7QYGibYyk9iu60l9DEplKyH3pxG1in1RIfzVcRCKZiTBR5sH16-P3F-bBsP0LEM3TdX5Ddyl7zsgJuV2cUfGg-22z1mTlaY9ZCqNY8wMqlqzFhWCsfaaI5EyRlGNPe2EDx4edpC4B28pfqNicL5GdJQhF6ae9QlCbXt7t9uf9pIVH_GZ0NM5KmO726CEMPc_Y6Gu72WoZ2TdxWbPjq_2J8BeKcIPx-a4_8ip_kPGDaylMVtnE6gGr56vsZ99swOorUSNneH1inIZLxPXhKSjqgwcEZ0c3qd3DQuxOpzn17wCPz64vytdDonBdcJN3A0N8lXjyN_CcjdcRUt15KmQaPHd9ZdFgzCjeILSQmrncLbSOOpyOrxvYECZiClwIMkD9MFI9J=w1702-h1276-no


Greg
Whoa!!! That's ugly! As you can see from my photos, mine was not bent nearly as badly as yours. From your picture, your compound looks to be just like mine. It makes sense that you have an Imperial pitch on a metric rod. That's exactly what it seemed like to me. That's the main reason I was anxious to either fix it or buy a replacement. Does anybody sell that kind of threaded rod (McMaster-Carr, etc.)? Were you able to fix the casting?

Regards,
Terry
 
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f350ca

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I'd be surprised if you could buy it as rod. Mine as mentioned was a metric OD forget size but 30 deg angle rather than 29 then through inch pitch on top of that.
Made a new part out of cut iron to replace the mount.
i0NXwlVso9ibz6K18tdP5WvNye9fRAB2FFwGdx2z65xYJn-zfeRfS2_1kzjU8E7hXg1K1ZdQV57QipVH2v_FJuFNmW4C-nzbZxT0UUBlvusjVMOB1Nfw3qT3cL9FR0CPVT6qrlyqKSORdMZsui22Xl0FSXEkept-SuJ9tcNGUPV-U3Re9fODxrrGX6dxkSeyY-nz48Ia182J_LpOVaZofcuvVpQCipHTeSa4SCe4y6dlPv0C0ImGOP3vznL_S_LSswBFJtgaKiC_bVfHKeEfj4-ErcPycyxtVYVMWh2Yjw2QS-LVgSsEF2Qkxzy9Cp72SMa43b10dgD_QkagFs0f-Hj7yPZ0Di6jc4IBWGV2uOTPhjLVXliEXRBfXH7nXvYkaGB6Bd2PXUbtWS5TiGvFlfaDXxChUIYsEoCtuCANbRbLTj8iSGV0rRI2GI17Z5pw2nwDSyEdEbyelfs9mvgKU5xqOilVRglzDpEP-6QOFR5NesNNLEuf-Rcx56dAdRrd-mwe8KV7l9xGfTuAaqMnrL-fRdj7d9f_MMTbdyD97_wRlVVlMeWa86qXi6Rb3aPxXZX_W5nE5voUCpkU4-w0OLGW0t4nmV-HgQEU_zsBz0Jm7CeyL4na7uxu=w1702-h1276-no


Greg
 

tjb

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I'd be surprised if you could buy it as rod. Mine as mentioned was a metric OD forget size but 30 deg angle rather than 29 then through inch pitch on top of that.
Made a new part out of cut iron to replace the mount.
i0NXwlVso9ibz6K18tdP5WvNye9fRAB2FFwGdx2z65xYJn-zfeRfS2_1kzjU8E7hXg1K1ZdQV57QipVH2v_FJuFNmW4C-nzbZxT0UUBlvusjVMOB1Nfw3qT3cL9FR0CPVT6qrlyqKSORdMZsui22Xl0FSXEkept-SuJ9tcNGUPV-U3Re9fODxrrGX6dxkSeyY-nz48Ia182J_LpOVaZofcuvVpQCipHTeSa4SCe4y6dlPv0C0ImGOP3vznL_S_LSswBFJtgaKiC_bVfHKeEfj4-ErcPycyxtVYVMWh2Yjw2QS-LVgSsEF2Qkxzy9Cp72SMa43b10dgD_QkagFs0f-Hj7yPZ0Di6jc4IBWGV2uOTPhjLVXliEXRBfXH7nXvYkaGB6Bd2PXUbtWS5TiGvFlfaDXxChUIYsEoCtuCANbRbLTj8iSGV0rRI2GI17Z5pw2nwDSyEdEbyelfs9mvgKU5xqOilVRglzDpEP-6QOFR5NesNNLEuf-Rcx56dAdRrd-mwe8KV7l9xGfTuAaqMnrL-fRdj7d9f_MMTbdyD97_wRlVVlMeWa86qXi6Rb3aPxXZX_W5nE5voUCpkU4-w0OLGW0t4nmV-HgQEU_zsBz0Jm7CeyL4na7uxu=w1702-h1276-no


Greg
Beautiful work.
 

jouesdeveaux

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Hi Terry
I had a similar problem on the compound rest for my Grizzley 8689. The lead screw did not rotate smoothly through 360 degs and became tight once every 360 degs. Early on, I eliminated a bent lead screw from the crowd of "suspects". To make a (fairly) long story short, the problem came down to the two discs that formed the dials attached to the end of the lead screw and to the top part of the compound rest. Their inner faces rotate against each other. The two faces that met simply were not parallel, and so there was one spot in each 360 deg rotation that significantly tightened up. The inner face of the outer dial disc was skightly raised at about "10" mils and "30" mils (on the dial). I lapped the face until it appeared to be flat. When I put the compound rest together again. It rotated smoothly through 360 degs.
Cheers,
Gerry

.[/QUOTE]
 
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