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PostPosted: Thu Jan 22, 2009 6:48 pm 
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Location: DC Metro
So I bought some goodies on ebay, including a 4” 3 jaw lathe chuck with an R8 adaptor to use in my Bridgeport. Well, the retarded chimp that I bought the stuff from (I’m temporarily withholding names till I see if he makes any real effort to fix things otherwise, I kind of doubt it based on my experience with him/the company so far) took everything, chucked it in a flat rate USPS box with no padding between individual boxes/items to keep them from moving, opened the 4” lathe box, took the top foam packaging/padding out of it, bolted the R8 adaptor on it crooked, left the packaging out and folded the box flaps around the R8 stem which smashed the rest of the contents in the box in the process of getting to me. (the outside box got to me fine with no significant marks on it, but everything inside it is mangled, even the chuck box ended up with a big hole in the side of it and all it’s parts were scattered around the inside of the USPS box)

Anyway, first I put it in my bridgeport and checked it with a dial indicator- it’s .047” out… grrr… I can actually see daylight between one side of the adaptor and the back of the chuck, and not the other.

I took and loosened the mounting screws from the backing plate/adaptor to the chuck and carefully tapped it loose, cleaned up the burrs made whoever slapped this thing together, cleaned everything , made sure the mounting faces were as good as they were going to get and put it back together (and noted that either that either the treaded holes in the back of the chuck or the holes in the adaptor plate are miss drilled, and when you line one up the other 2 are a little off, also one of the recessed holes in the adaptor piece is not drilled as deep as the others so one of the allen cap screws sticks out a little). Anyway, I went back to the Bridgeport and dial indicator:

Measured at the R8 adaptor flange: less than .001” runout, the needle just barely wiggles as it goes around.
Measured at the machined side of the chuck, about 2/3 of the way down, between the adjusting head and the grooves that the jaws ride in: right around .004” runout
I chucked up a polished steel round that I know is pretty good and chucked it up and loosened and did it again 3x (and then had to go to work) and got:.007”, .004”, and .004”
(I don’ t know what the deal was with the one .007” reading, it shouldn’t have been dirt, I’m hoping that it’s not a sign that the jaws aren’t repeatable …)

So I know that part of the answer here is “well, how accurate do you need it to be?” but realistically, how accurate can I expect average new parts like this to be out of the box? I was hoping for better than .002”, but I don’t know if that’s really realistic.


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PostPosted: Thu Jan 22, 2009 7:20 pm 
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A dumb question: for what do you intend to use a 4 inch 3 jaw chuck on a Bridgeport Mill?

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PostPosted: Thu Jan 22, 2009 7:43 pm 
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Now that you mention it, that's a good question. Well, it's really the R8 shank that gets my attention - if it were MTx, it could easily be for a rotary table. Some people turn simple things on a mill by holding a toolbit in a vise, but I always envisioned that being done with a collet.

Bill


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PostPosted: Thu Jan 22, 2009 8:21 pm 
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I would say if it will repeat at .004 that is as good as can be hoped for if it is one of those cheaper Chinese chucks. Try it with several different diameter test bars.

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PostPosted: Thu Jan 22, 2009 9:59 pm 
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schwabw wrote:
Now that you mention it, that's a good question. Well, it's really the R8 shank that gets my attention - if it were MTx, it could easily be for a rotary table. Some people turn simple things on a mill by holding a toolbit in a vise, but I always envisioned that being done with a collet


that's exactly it... I'm hoping to sort of turn something on my bridgeport. I guess I should try a bit to see if I can get it repeatable where i'm at, maybe even try mounting it in a few different positions to see if one is better then the rest and go from there.


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PostPosted: Thu Jan 22, 2009 10:42 pm 
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Joined: Sun Jan 18, 2009 12:15 am
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Location: Central Indiana
Hey Silver, I've found that cheap machine tool accessories are cheap for a good reason.
In this case, I see two choices. First, don't alter anything and return the items giving the sender the opportunity to make right the shipment. Second, accept situation and repair and/or rebuild to tolerances you find acceptable.

I am rebuilding a Harbor Freight 8" rotary table that was poorly manufactured and also damaged in shipment via FedEx but.....

We can rebuild it to be Faster...Stronger...More Capable... because the Home Machinist can & it is my choice :wink: -- and it sounds like you have made yours.

Regards,

Amigo


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PostPosted: Thu Jan 22, 2009 10:47 pm 
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If the 3-jaw chuck has more than one socket to tighten the jaws down then tighten each one wtih about equal torque. Sometimes that makes a big difference.

I don't know about Ebay these days. About 40% of my purchases have been bad deals the last 6 months. This week I bought some 2 lip carbide inserted endmills. One endmill an exact duplicate of one I had for 30 years and wanted a spare. The price was right but inserts screws were missing and one endmill had an insert screw broken off then someone drilled a hole through the broken screw. I got the broken screw out with a 3/32 carbide endmill. The cost of my time and $20 for new screws I could have bought a new endmill.

Jim

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PostPosted: Thu Jan 22, 2009 11:12 pm 
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Silverback wrote:
that's exactly it... I'm hoping to sort of turn something on my bridgeport. I guess I should try a bit to see if I can get it repeatable where i'm at, maybe even try mounting it in a few different positions to see if one is better then the rest and go from there.


A little runout won't bother you if you do not remove and re-chuck the work. If you do want that freedom, then a 4-jaw chuck would be a better choice, or you might use your 3-jaw to turn a collet-friendly "shank" on the stock, after which you could use (I guess - others are invited to jump in with corrections/additions) just about any collet (R8, 5C, etc.) to hold the work by that shank.

Bill


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PostPosted: Thu Jan 22, 2009 11:21 pm 
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JimGlass wrote:
If the 3-jaw chuck has more than one socket to tighten the jaws down then tighten each one wtih about equal torque. Sometimes that makes a big difference.

True, although it may be that one socket alone will yield the best operating condition, while tightening all three will yield a getter grip. When a different socket is used to tighten a chuck, the scroll is loaded differently than it is loaded by the other two. In almost all cases, one of the sockets will yield a closer running fit than the other two, and it often is true for all sizes. It is for that reason that one marks a socket on a chuck and uses it exclusively when running soft jaws.

Harold


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PostPosted: Thu Jan 22, 2009 11:26 pm 
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Location: Onalaska, WA USA
schwabw wrote:
A little runout won't bother you if you do not remove and re-chuck the work.

That's assuming that the piece being machined excludes the portion that would be chucked. If it does not, the amount of error in the chuck would manifest itself as eccentricity, and will likely include at least a little error in perpendicularity. It the piece is machined 100% on location, it makes no difference as long as there is stock adequate enough to provide truing cuts to eliminate the amount of error in the chuck.

Harold


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PostPosted: Thu Jan 22, 2009 11:44 pm 
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Harold,

Harold_V wrote:
schwabw wrote:
A little runout won't bother you if you do not remove and re-chuck the work.

That's assuming that the piece being machined excludes the portion that would be chucked. If it does not, the amount of error in the chuck would manifest itself as eccentricity, and will likely include at least a little error in perpendicularity. It the piece is machined 100% on location, it makes no difference as long as there is stock adequate enough to provide truing cuts to eliminate the amount of error in the chuck.


I am thinking of using the 3-jaw just long enough to put a clean shank on the work, and then switching to collets. The 3-jaw comes off the mill and the "clean end" of the work goes in a collet, after which the rest would be turned. If the part is then flipped to turn more on the "shank," it would be subject to error in the collets, not the chuck - right???

Bill


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PostPosted: Thu Jan 22, 2009 11:58 pm 
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schwabw wrote:
If the part is then flipped to turn more on the "shank," it would be subject to error in the collets, not the chuck - right???

Correct.

Harold


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