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Watch your "spring" passes!

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macardoso

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#1
I have been doing some precision boring on the lathe lately, routinely hitting +/- .0002" with ease. I was feeling pretty good about myself (problem #1). I have a part that I needed to bore a bearing pocket for a 26mm angular contact bearing which was 3.75" deep into the part. It needed to be concentric to the other bearing pocket on the backside of the part within .0005" and on size -.0004/+.0002". I thought to myself, no problem!

Here is where the problems began. The part only had a .625" hole through, which forced me to go to a 1/2" boring bar (Problem #2). The boring bar stickout was 7xD so I would chatter at anything above 100rpm (Problem #3). I can be a little stingy at times and didn't shell out for a carbide boring bar (Problem #4). I needed an accurate depth to the bottom of the bore 3.708 +/- .002" so I had to use the compound slide on the lathe rather than the carriage (I don't have a DRO, Problem #5). This meant I didn't have power traverse feed to control the surface finish.

As I brought the bore into size I made the 5 final passes at .005" radial steps to get a handle on bar deflection. After each pass, I stopped to measure with a telescoping gage and a .00005" mic. Each pass was coming out right where I needed, within a tenth or two. I noticed that as I was getting close to final size, my surface finish wasn't very good (Problem #6). I was feeding too rapidly into the part (I really was trying to go slow). I decided that I wouldn't worry about it until the final pass where I would feed as slowly as I could to get a nice finish. My next to last pass mic'd in at .0048 from final size, so I dialed it in and cut the final pass going very slowly. The finish was awesome. I took the bearing and tried the fit, and uh oh, it dropped right in. So I mic'd the hole again and wtf?! it was .0076" oversize!!!

I sat there and pondered life and the universe. Then it came to me. Approaching final size in even steps is great, but if you don't use the same feedrate, the force on the boring bar is different. The higher forces with the faster feedrates tended to deflect my long slender boring bar away from the cut, giving a smaller hole. This isn't a problem as you compensate for the deflection when you measure after each pass until... I took the final pass at a fine feedrate. The lower forces allowed the bar to spring back into the cut, removing much more material than intended. The effect was greatly exaggerated by the length and diameter of the bar.

So now I have to decide if I want to rebore it larger and press in a liner, or allow the bearing to float a little in the pocket.

Lesson learned: In precision (manual) boring, approach your final size in even passes and identical feedrates. Make each cut identical to the last.
 

benmychree

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#2
The most important lesson, not mentioned, is to measure the final cut by just boring in just far enough to get a measurement before proceeding to final depth.
 

tjb

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#3
I find it commendable that you didn't bash the part in with a sledge hammer.
 

Bob Korves

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#4
When finishing bores it is important to get the spring out of the bar completely before setting it for the final pass or passes. Still, you were working a 8:1 stickout with a HSS bar, which is a very springy setup. If the shaft goes all the way through the part, setting up for more support by line boring would have helped a lot.
 

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#5
that's how you learn. I also feed out on each cut. feed in and reverse the lathe and allow it to feed out. watch the chip size if it looks to big turn the cross slide in and you wont have ruined the part. try to get some carbide insert boring bars. bill
 

macardoso

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that's how you learn. I also feed out on each cut. feed in and reverse the lathe and allow it to feed out. watch the chip size if it looks to big turn the cross slide in and you wont have ruined the part. try to get some carbide insert boring bars. bill
Unfortunately this cut was done blind. Too far in to see the chips or the cut. It was a cut and pray situation... Didn't pan out this time.

Fortunately I learned a valuable lesson.
 

macardoso

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#7
I find it commendable that you didn't bash the part in with a sledge hammer.
Almost did... but I have 10 hours and probably 6 setups complete on this part. I'm not starting over...
 

benmychree

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that's how you learn. I also feed out on each cut. feed in and reverse the lathe and allow it to feed out. watch the chip size if it looks to big turn the cross slide in and you wont have ruined the part. try to get some carbide insert boring bars. bill
I do not think that "feeding out" is a good idea, or good practice; it does not suit the geometry of the cutting tool, and depending on the condition of the lathe, the straightness of the cut may suffer also; this would be especially true with lathes with flat ways. You likely never see a professional machinist do it.
 

tjb

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#9
Almost did... but I have 10 hours and probably 6 setups complete on this part. I'm not starting over...
You have the patience of Job. Congratulations.
 

benmychree

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#10
So, what is it that you are building?
 

T Bredehoft

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#11
Was the hole OD enough that you could have used a larger/heavier boring bar to finish?
 

macardoso

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#12
It is a mount for an 85mm AC servo motor (Allen Bradley MPL series) for my CNC. It is made from billet aluminum 4x4x5". The ballscrew support bearings are integrated into it (I messed up one of them). Similar to this one (X Axis), but more complicated. 4.jpg
 

macardoso

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#13
Was the hole OD enough that you could have used a larger/heavier boring bar to finish?
Yes, and now that you mention it, I should have finished with a 3/4" bar. Good Idea.

I'm thinking I will rebore/re-line the hole with a sleeve and hit the size correctly, in this case I will use the larger bar.
 

mikey

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#14
If I'm understanding you correctly, you're boring on the lathe and you have two bores back to back in the same part. Each bearing bore has to be concentric with the other within 0.0005" so you have to bore one and then flip the part and bore the other side to tight radial and axial tolerances, correct? To me, that is problem #1; not insurmountable but can be tough to indicate an odd shaped part. Assuming you can do it ...

Deflection with a steel bar is going to be a real issue, as you pointed out. Thing is, if you sleeve the bore and try again are you going to use the same bar material or are you going to step up to a carbide bar? I ask because you are trying to take very tiny cuts in a deep hole with a carbide insert. As you know, radial cutting forces rise until the depth of cut exceeds the nose radius and this is going to compound the deflection you already have with that bar material, leading to even greater uncertainty and inaccuracy. Best option is to go with a carbide bar and switch to the smallest nose radius you can get for your finishing passes. Minimum depth of cut you can expect some accuracy with will be about 1/2 the nose radius so smaller is better but ideally, you want to take a minimum depth of cut a few thou larger than the nose radius to provide enough support to overcome the radial forces. If you can find an insert that fits your bar with a nose radius of 0.002", that will probably work. What is your nose radius, anyway? What kind of insert?

You are using the compound to feed the bar because you need more precise control of bore depth? A good, solid carriage stop is a far better device for depth control. If you don't have one, you really should make one if you're going to do precision boring.

With regard to overboring on the final step, that was a big miss. If you were taking identical depths of cut and getting proportionally the same increase in ID each time then simply reducing the feed rate would not have caused that large a discrepancy. Something else is going on. Some things to ponder:
  • Did you allow the part to cool to ambient temp before taking the last few passes? As you know, metal heats up and aluminum in particular can expand enough to make an accurate measurement a problem until it cools down. A related issue: are you sure you measured the bore correctly? Not being a dick; this is a common reason for a miss and I'm using myself as the prime example because I KNOW this is a factor to consider.
  • Was the insert broken in? If you used the same point on the insert, was it worn or damaged? It should cut the same if it is intact. Look at the insert under magnification and make sure.
  • Are you certain your depth of cut was set correctly? I've made this mistake more times than I care to admit; when I miss a bore, this is usually why I missed the damned thing. The larger the error, the more likely it was me.
  • Did you drag the cutter out while it was still in contact with the bore on the final pass? I assume not but if you did then this is very possibly the culprit.
Bottom line: you did good all the way up to the end. I'm just not convinced that "spring" is responsible for that big a miss. If you had issues when roughing and mentioned spring then yes, I might agree there. However, with tiny cuts like you're taking at the end, spring is usually not an issue that causes this big a miss on a final pass. Radial cutting forces, deflection caused by the user and the bar material itself, the condition of the insert, the heat of the part, coolant or no coolant, adequate depths of cut for the nose radius in use all make a difference and, at least in my case, the very important pilot error. When you're boring for a bearing and the fits impact on internal bearing clearances, you're working into the low tenths and everything matters.

By the way, I'm not being critical in the least here. I know what its like to spend hours on a part and blow the whole thing with the final pass in a critical bore, believe me. My best advice is to sort out the issue before you attempt to bore that sleeve. Personally, I wouldn't attempt it without a carbide bar.
 

macardoso

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#15
If I'm understanding you correctly, you're boring on the lathe and you have two bores back to back in the same part
Mikey,

Thank you for the detailed response. I don't feel criticism at all, in fact I am still plenty new to all of this (<10 years) so I'm trying to learn everything I can (hopefully that never stops.

You are correct about the part, there are several features on this part, but the critical ones are the back to back bore which hold a pair of angular contact bearings. Concentricity is critical here as any runout between the features would cause uneven loading on the bearings and poor alignment of the ballscrew. I chose .0005" as an attainable value, however I'm trying to hold as tight as I can. The axial distance between the bores is slightly less important, but I wanted to hold better than +/- .005" so I opted to use the compound slide. I gave a .002" tolerance, but it was less critical than the bore.

I agree it was tough to hold these tolerances, but the smallest diameter hole through the part was cut in the same setup as the backside bore which allowed me to indicate it when flipped (The indicator was completely inside the part to reach!).

My plan for sleeving will be to use my 3/4" x 10" steel boring bar, as T Bredehoft commented above. I have confidence I will be able to hold the tolerances with a little more care than might have been given before. I will one day invest in carbide bars, but I can't stomach the costs right now, so a little more skill/luck/care will be required. My bar is a SCLCR with CCGX 31.51 (1/64" radius) or CCGX 31.50 (1/128" radius), I can't remember which I have mounted currently. I have been using these razor edge ground inserts for a while now with good luck. I really can't get them to rub even when taking <.001" cuts.

You are correct about the use of the compound slide for depth control. I have been thinking of making a carriage stop, so I may have to bump that up on the roster of projects.

In answer to your thoughts:
  1. Temperature was kept nominal by the very slow cutting speed, small depths of cut, and application of coolant. I agree it can mess with measurements, but in the case of a bore, wouldn't that tend to undersize the bore? I very well could have screwed up the measurement, we all know I have done that before. I personally don't love the "feel" type of measuring instruments like the telescoping bore gages I used, however I had no other way to measure the bore deep in the part. I would prefer a direct reading 3 point bore gauge, however the money is better spent elsewhere.
  2. Insert is relatively new but has removed some material. I don't know if the break in is as important as it is on inserts for tougher materials, but it is a thought. The insert is sharp and has no damage.
  3. I've goofed plenty before so I can't rule that one out! I didn't think I missed it, but 8 thou is a serious discrepancy.
  4. Cutter was backed off the material before backing out of the bore. I never reverse cut.
I agree that I was surprised to be that much oversize. I might be exaggerating the impact of the spring in the bar and there may be other factors at play here. I think with the equipment I have, I am happy when I hold within .0005" on diameter.

Thanks for all your thoughts!!!

Here is a model of the part:
Capture1.JPG
And a cross section showing the bores:
Capture.JPG
 

mikey

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#16
Thank you for not feeling that I was being critical; I am really trying to help. Boring accurately on a part like this requires that we control every single variable we can.

If this was my project, I would probably bore the through bore first so that I have a really accurate surface to indicate from. I would do the inner bearing bore second in the same set up that I used to bore the through bore. Then I would flip the part and dial in the inner bore to get it dead on or as close as I could get it. Done this way, you should be able to maintain concentricity between the two bores that is well under a thou.

I think part of the problem is the insert you're using. More specifically, the nose radius. The 32.50 insert has a 0.008" nose radius, which is great for general work but a finish cut would require a 0.003-0.004" deep pass and even that is a bit on the light side. A more accurate cut would be a 0.010" pass as this gives the nose maximum support. Of course, you would need to know how accurately the insert will take such a cut but if you stage your roughing cuts precisely enough to get you within 0.020" of final size, you can take two 0.010" passes. The first pass is to make sure the insert is cutting on size and the second pass will bring you in on size.

May I suggest you do some practice runs? Do a practice bore at the same depth and in the same material and do a few rough passes. Measure what the change in ID is with each roughing pass so you know how the insert likes to cut. Then do the same with your finishing passes but try using a 0.010" finish pass. Most commonly, a 0.010" finish pass will increase the ID by slightly less than 0.020" so either adjust your depth of cut to give you a number you prefer or just work with what you get. For example, if a 0.010" finish pass produces a change in ID of 0.0190" then you know that when you reach 0.0190" of final size, a 0.010" deep pass will bring you in on size. Every insert cuts the way it wants to cut so you have to sort out what yours wants to do. This is because the wear pattern on each insert differs so you have to accommodate that when boring to the tolerances you're trying to hit.

The other thing to do is to contact the insert maker and see if they make a smaller nose radius insert. While this is unlikely with an SCLCR bar, you might get lucky.

The cooling the part thing has to do with measurement. If the part is hot then the bore is going to be larger than when it is cool. How much larger will vary with each part and who knows what that measurement is? You would think that a hot part would produce an undersized hole but from experience I know that sometimes it does and sometimes it doesn't. I agree that being 0.008" oversize is not likely to be due to temp variation but you have to control what you can control.

The other thing is that you said that speeds were "very slow". Can you clarify this? Did you change speeds when roughing to finishing? In general, your tool will cut a particular way at a given speed but it will cut differently if you change speed. This is simply due to tangential cutting forces, which tend to rise as you slow down and that can really throw your accuracy off when you're working to the tolerances you're working at. My suggestion is to set a speed (probably max your lathe can run at) and leave it there. You can change your feed rate and depth of cut but leave the speed where it is.

The other thing I forgot to mention is that feed rates differ between roughing and finishing so when doing your practice cuts to sort out how the insert cuts for finishing passes, settle on a feed rate and don't alter it when doing the real bore. You will find that changes to any of your cutting conditions - speed, feed or depth of cut - will alter how the insert cuts so keep things as constant as you can.

So, make a carriage stop and learn to use it with power feed. Mark your bar about 1/4" or so before hitting the bottom of the bore and power feed until you hit that mark, then kick it out of power feed and feed manually until you gently bottom out on the stop. Try to keep your feed rate similar to the power feed rate and don't slam into the carriage stop.

Good luck on this. If this bore is important, and it sounds like it is, then you really need a carbide bar. Even with a 3/4" steel bar, your max depth is only 3" before you run into deflection issues. Something to think about.
 

mikey

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#17
I forgot to ask. Are you going for a line-to-line fit? This means the bearing bore is the same size as the outer diameter of the bearing. This is the recommended fit for most angular contact bearings because it locates and holds the bearing securely but minimizes the influence of the bore size on internal bearing clearances. If this is what you're going for, tolerances in the bore are typically in the range of +/- 3-5 microns - pretty tough to measure that, let alone hit it. You can do it but you have to hit the bore dead on so try to control every single variable you possibly can.

I did this kind of bore with this kind of fit for a live center I made for my Sherline lathe. I hit that bore exactly on size so I know you can do it.
 

GrayTech

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#18
Mikey,

Thank you for the detailed response. I don't feel criticism at all, in fact I am still plenty new to all of this (
You are correct about the part, there are several features on this part, but the critical ones are the back to back bore which hold a pair of angular contact bearings. Concentricity is critical here as any runout between the features would cause uneven loading on the bearings and poor alignment of the ballscrew. I chose .0005" as an attainable value, however I'm trying to hold as tight as I can. The axial distance between the bores is slightly less important, but I wanted to hold better than +/- .005" so I opted to use the compound slide. I gave a .002" tolerance, but it was less critical than the bore.

I agree it was tough to hold these tolerances, but the smallest diameter hole through the part was cut in the same setup as the backside bore which allowed me to indicate it when flipped (The indicator was completely inside the part to reach!).

My plan for sleeving will be to use my 3/4" x 10" steel boring bar, as T Bredehoft commented above. I have confidence I will be able to hold the tolerances with a little more care than might have been given before. I will one day invest in carbide bars, but I can't stomach the costs right now, so a little more skill/luck/care will be required. My bar is a SCLCR with CCGX 31.51 (1/64" radius) or CCGX 31.50 (1/128" radius), I can't remember which I have mounted currently. I have been using these razor edge ground inserts for a while now with good luck. I really can't get them to rub even when taking
You are correct about the use of the compound slide for depth control. I have been thinking of making a carriage stop, so I may have to bump that up on the roster of projects.

In answer to your thoughts:
  1. Temperature was kept nominal by the very slow cutting speed, small depths of cut, and application of coolant. I agree it can mess with measurements, but in the case of a bore, wouldn't that tend to undersize the bore? I very well could have screwed up the measurement, we all know I have done that before. I personally don't love the "feel" type of measuring instruments like the telescoping bore gages I used, however I had no other way to measure the bore deep in the part. I would prefer a direct reading 3 point bore gauge, however the money is better spent elsewhere.
  2. Insert is relatively new but has removed some material. I don't know if the break in is as important as it is on inserts for tougher materials, but it is a thought. The insert is sharp and has no damage.
  3. I've goofed plenty before so I can't rule that one out! I didn't think I missed it, but 8 thou is a serious discrepancy.
  4. Cutter was backed off the material before backing out of the bore. I never reverse cut.
I agree that I was surprised to be that much oversize. I might be exaggerating the impact of the spring in the bar and there may be other factors at play here. I think with the equipment I have, I am happy when I hold within .0005" on diameter.

Thanks for all your thoughts!!!

Here is a model of the part:
View attachment 274627
And a cross section showing the bores:
View attachment 274628
Looking at that part, wouldn't it have been easier to bore straight throughfor both bearings at one size and then press in a sleeve for the spacing between them?

Sent from my H3123 using Tapatalk
 

macardoso

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#19
Looking at that part, wouldn't it have been easier to bore straight throughfor both bearings at one size and then press in a sleeve for the spacing between them?

Sent from my H3123 using Tapatalk
Yeah, that would have been a good idea! Only thing would be that it would need to be a fairly heavy press fit as that shoulder will be taking all the thrust loads from the screw. Which, if everything goes well, will be very high. These are 1kW 5000 rpm servos for the axis motors.
 

macardoso

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#20
Mikey,

My experience, and please feel to contradict, is that you don't need a light cut to hit a critical dimension - you just need consistency. I have gotten away with holding tight tolerances in the past when I take my final cut to size at .005 to .010 radial depth of cut. In this way you full engage the nose of the insert and don't have issues with rubbing. If your semi-roughing passes are taken in an identical manner (same radial and axial DOC, spindle speed, and insert) then your final pass should land right on size.

I think that a smaller nose radius insert would be good here, but I think the bigger issue was the consistency of the cut. If you know anywhere to find aluminum specific sharp corner inserts (CCMT31.50), please let me know!

Temperature sure plays a big role in tolerances at this size. I used to work in a metrology lab where everything was temp controlled. We couldn't sign a measurement if the room was out of spec.

The "very slow" spindle speed (100 rpm) was due to bar resonance at higher speeds. I might have been able to go even faster, but I found the chatter to subside at 100 rpm. A carbide bar would really help here.

I will be practicing before I make the final cut! Re-boring it for the liner will be a good test too!

Since this is aluminum, those tight of fits really don't matter quite as much. It will stretch far more than a solid semi-hard steel would. That being said, I'd like to keep it somewhere around a light press to a very tight slip fit. I figure this is in the -.0004/+.0002 range. I'm not sure how much tighter I can hold with my skills and equipment at this time.
 

mikey

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#21
I totally agree with you - burying the nose radius is the best way to minimize radial forces for a consistent cut. I do not know of a source for CCGT inserts that fit a 1/2" bar with tiny nose radii. That is why I suggested contacting one of the insert makers; they may have some we don't know about.

If you choose to go with a carbide bar at some point, watch ebay for a Circle Machine QCMI bar. It takes flat-topped boring, grooving and threading inserts. These bars are really versatile and very accurate. You will also find that a flat insert will take a much finer cut than one with a chipbreaker. With the right speed and feed, they produce coiled chips instead of long strings, even in aluminum. I totally gave up on SCLCR bars a long time ago; they cut but not nearly as well as a Circle bar does. Moreover, Circle tends to make insert with smaller nose radii and that really helps at these tolerance levels.

Good luck with this project and let us know how you make out.
 

mattthemuppet2

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#22
you can get CCGT 32.50 inserts off eBay for not much money - those will help reduce cutting force and bar deflection. Also, if I'm trying to hit a critical bore dimension I'll do most of the cuts at the heaviest DOC I can up to say 0.5mm depth and width from final dimension. Then enough spring passes at that same ID until no material is removed, then measure. I'll then face the bottom of the bore to full depth and take a 0.1mm cut off the ID. Measure, cut again, until dimension is achieved. If you have the mating part, I often use that as a go-no go gauge when I'm getting close, just in case I do something stupid with the measuring.
 

ezduzit

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#23
...I needed an accurate depth to the bottom of the bore 3.708 +/- .002" so I had to use the compound slide on the lathe rather than the carriage (I don't have a DRO...
Do what I did...mount a long-travel dial indicator to get accurate carriage travel. I have one 2" and one 4" travel. I zero the boring bar insert on the workpiece's face and can control depth to within 0.001".
 

Meta Key

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#24
This is a really good discussion -- thanks to all contributors! I've learned a few things here this morning, much appreciated...

MetaKey
 

mmcmdl

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#25
Am I missing something here ? Why not bore your 3 diameters on the right side in one set-up and face perpendicular , remove your part , turn up a spud ( step mandral ) and locate off 2 bores and the face and bore your last diameter . Concentric and perpendicularity are guaranteed . ;) Do it all the time .
 

macardoso

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#26
Am I missing something here ? Why not bore your 3 diameters on the right side in one set-up and face perpendicular , remove your part , turn up a spud ( step mandral ) and locate off 2 bores and the face and bore your last diameter . Concentric and perpendicularity are guaranteed . ;) Do it all the time .
That probably would have been good. I have a lot of experience on the mill, but newer to the lathe. Learning what makes sense as far as order of operations takes a little while. In hind sight:
  1. Square Stock
  2. Drill 4 mounting holes, 2 through holes, 2 CBores
  3. Drill and bore all 5 features from one side (motor pilot, large bore, clearance bore, bearing bore, and through hole)
  4. Flip, drill and bore the opposite bearing pocket
 

macardoso

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#27
Project update!

Used the long weekend to get an extra hour or two in the shop. Turned a plug to 1.2503, bored the part to 1.2499, nice press fit with a few whacks from a dead blow mallet. Rebored to 1.0236 and hit it dead on size (as close as I can measure). Used the 3/4" boring bar with no trouble at all!

Then I turned a simple arbor and pressed in the angular contact bearing (it was a nice light press). I did some thinking and realized I shouldn't have put the bearing in since I have another op on the mill and then sanding, polishing, and painting. Went to tap the bearing out and BAM!, I separated the inner race and balls from the outer race :rolleyes:.

Now I have a bearing race stuck halfway in my hole with no obvious way to remove it (too small and deep to fit a bearing puller). I will have to think that one through. I still hear my dad telling me to slow down and think lol.

Thanks for all the advice!
 

mmcmdl

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#28
Then I turned a simple arbor and pressed in the angular contact bearing (it was a nice light press). I did some thinking and realized I shouldn't have put the bearing in since I have another op on the mill and then sanding, polishing, and painting. Went to tap the bearing out and BAM!, I separated the inner race and balls from the outer race :rolleyes:.

Now I have a bearing race stuck halfway in my hole with no obvious way to remove it (too small and deep to fit a bearing puller). I will have to think that one through. I still hear my dad telling me to slow down and think lol.

LOL ! That won't be the last time that happens ! ( And I'm laughing with you and NOT at you ! :hammer:

They have many part in here at wok similar to the part you are making . I take an end mill and cut 2 slots thru the bearing spacing bore to get a pin punch onto the outer race . Those little bearings sure go in easy but are a RPITA to get back out .:eek 2:
 
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macardoso

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#29
OK, so I salvaged the part in what I think was a creative way. I turned a collar on the lathe which was just small enough to slip through the outer race of the bearing, and I included a lip on it which went under the race (which I had already knocked out .050 before it fell apart). I split it into two pieces using a hack saw and filed them until they could pass through the bearing and be "assembled" behind the race. I turned a long mandrel which fit into the center hole of the collar and I was able to knock the race out without damage to the part. All told it took 30 minutes!

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