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full travel on compound feed 7x mini lathe reveals wobble

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pds65

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#1
I haven't noticed it before until now; for a year I've been machining parts only 2-3" max for a few thou' and generally have not used the compound feed handle that much. today, I chucked a 4" piece in and turned out the full length of the compound as I wanted a fine feed for the whole length of my piece. I wanted to turn it down .080 in 4 or 5 passes and have a great finish. it's only 3" dia round, 6061 so not expensive if I ruin it.
But as I got about half-way thru and stop to check if I was turning it square to the center, I saw my cut was more than the .020 I started with, it cut about .028. Then I thought, well, I'm ok I might've bumped the cross slide and can get back on spec next pass. So I finish my turning cut and check the end close to the chuck and it's back to the .020 I planned on. scratching head. my ways are really straight (within .0015) and the main coarse feed doesn't so this, just the compound.

I don't understand what I can do to compensate or maybe I need to get a new compound? the cross feed is right on 90deg for it's full travel to the chuck.
thanks for any tips!
 

4ssss

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#2
Try putting your compound parallel to the piece and crank it back in tight. You may have enough range in your cross slide then to cut the diameter. A 4" piece is stretching your lathes abilities regardless if it's just aluminum.
 

pds65

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#3
thanks I'll try that.
yes my 4 jaw chuck won't hold much bigger with the outside jaws. I usually do really small parts. this is a one-shot deal to turn it down, drill and tap the center.
 

homebrewed

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#4
Is the problem repeatable? Compound/cross feed gib screws adjusted for minimum play? Did you notice any binding while turning the compound crank that might correspond to the region with the unexpected depth of cut?

You might also want to take a look at the compound gib and dovetails to make sure there are no dings or divots (or embedded swarf) on the surfaces that might deflect the compound as it is being advanced.

A workaround to consider while you troubleshoot the compound is to use the power feed to advance the carriage, since it appears to be OK. Set the change gears for the highest ratio (20:80:20:80 I believe). This ratio is so-so for a finish, but the result can be improved if you use a tool with a large radius on the tip. The high ratio will make it easier to disengage the half-nuts before the carriage crashes into the chuck.
 

catsparadise

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#5
Any particular reason you're feeding with the compound rather than the carriage? The compound angle is adjustable and those little graduations are not a recipe for precise adjustment. If you must feed with the compound and don't want any taper you'll need to chuck up a piece of material (eg drill rod or something accurately sized) and put an indicator in the toolpost and adjust the compound angle until there's no movement in the indicator when you feed with the compound.

Making that angle adjustment will be more difficult if the bolts that tighten the compound onto the slide are covered by the moving part of the compound (like on the minilathe I have). Best to use the carriage and either use power feed as has been suggested or manually feed trying to keep the tool progress constant.
 

pds65

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#6
well, it's a minor problem. I started taking the tool post down and yes there is chips all in it but I thought I was keeping it clean! they get in there... It's a Grizzly G8688, and I'm thinking about a QC toolpost. there seems to be more play in the gibs than when I first set it up. I'm finding about many things can combine to introduce error!
Being it's made in China, I wouldn't put it past them to have some defect on say the compound screw too.
Oh I didn't like to use power feed since I only turn maybe 2" cuts and actually like to feel the machining. brass is my fav, the sound of it and nice shower of sprinkly chips lol

Thank you guys for you insights and suggestions! I'll let you know how I'm getting along and have other ?'s too I'm sure
now that I look back, the lathe was the cheapest part of me machining stuff lol. I have more in tooling and inspection than the machine itself
 

homebrewed

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#7
I use a small machinist's square to set the compound. The result depends on how square the back of the cross slide is (and how parallel the exterior of the compound is to the dovetails), but it works OK for most of the stuff I do. And machinist's squares are relatively inexpensive, unless you spring for a big-name brand.

Swarf and chips are adept at finding their way into just about any nook & cranny of your machine, so don't feel too bad about discovering the stuff in weird places!
 

pds65

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#8
ok with carriage torn down, I find there is play at the compound feed screw where the dial/handle mounts though the handle is tight to the screw. I know it's kind of tricky with the dial retainer spring, so is there a common issue like a bushing or spacer worn? not sure if a pic would help. I'd hate to disassemble and have it apart for more than a day
 

homebrewed

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#9
When I remove my compound the feed screw can be wiggled quite a bit in the perpendicular direction. Due to the need to have some clearance between the compound body and dial (otherwise you couldn't turn the handle), there always will be some "wiggle" that is amplified by the length of the feed screw. The more-important performance issue is how much backlash you've got when the compound is mounted on the carriage. If it's more than you like, you could install a shim washer between the dial and mount. The thickness of the washer will depend on your particular machine.

Some folks have taken it a step further by making a new mount that incorporates a bearing -- see compound bearing mod. If I were to do this, I'd try using a thrust bearing -- VXB has a 10mm x 24mm x 4mm thrust bearing that should work. The 10mm ID matches the feed screw, which would greatly simplify the mod. BTW a thrust bearing makes for an easier mod because you don't need to provide clearance for the inner race to turn, unlike the mod detailed in the link I provided.
 

tomfiii

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#10
I had to move nut as it would bind close to the end of travel towards the crank handle. Also some mini's machine work can be off from one to the other. Also see "The complete mini-lathe workshop" by ted Hansen, there are also some youtubes on the mini.
 

pds65

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#11
I guess I should not rely so much on the top crank and make more use of the carriage feed. the compound itself is rigid and the cross feed is good. The bearing upgrade may be a future to do project... many thanks homebrewed for the link!
 

homebrewed

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#12
If you like the smoother motion provided by the compound, another approach would be to put a hand crank on the end of the lead screw. To use it, just engage the half nuts and turn the crank. You can use your lathe to drill & tap a hole on the end of the lead screw for mounting a hand wheel/crank. I did this and use it fairly often. I used a shoulder bolt to install the handle so it turns freely as I turn the crank. This really isn't a necessity but does make it a bit more user friendly.

Warning: you want to remove the crank if you are using the power feed or threading. The crank handle thrashing around could be hazardous to your health! More sophisticated hand crank designs provide some way to disconnect the crank.
 

hman

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#13
Warning: you want to remove the crank if you are using the power feed or threading. The crank handle thrashing around could be hazardous to your health! More sophisticated hand crank designs provide some way to disconnect the crank.
McMaster-Carr (and I'm sure other sources) sell handwheels with fold-away handles. That might be a safe alternative. The way McMaster's website is set up, I can't offer a working link. But if you go their site, search for "handwheel folding handle" and scroll down till you see:
Spoked Hand Wheels with
Unthreaded Through Hole
and Fold-Away Revolving Handle
 

homebrewed

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#14
Good suggestion about the fold-away handle, but McMaster sure is proud of those -- $65 and up. I did find one on Ebay for a little over $10. Made from aluminum, it probably would work OK.
 

Firstram

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#15
This is a machining forum, you're not allowed to buy something from McMaster that you can make on your lathe! Do the bearing mod on the cross and compound, it's worth the effort.
 
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