machining a bearing housing to size

[picclock]

Don't know if anyone has any good tips on this. I'm machining some 22mm recesses for a bearing and they always seem to come out too small or too big. I've just done one which seemed just right, I pushed the bearing in, without excessive force (so i thought) and its too small having compressed the bearing to the point that it won't turn. A thou or so larger and I'm using epoxy to hold them in.

Is it just me or are there other tricks/tips that I should be using to get that perfect fit ?

Any advice much appreciated

Best Regards

picclock

(shame about HMEM)

[David Jupp]

Loctite (or similar) bearing retainer - saves the day if you can't get the sizing quite right.

[BillTodd]

One way I used is to make a gauge; turn a piece of scrap with steps at 1/2 (0.0005") and 1 thou" under size (much easier to measure a O/D than a bore) Bore carefully to the 1 thou" under-size, then cut dust (i.e. repeated passes without moving cross-slide)  'till the 1/2 thou" under-size just fits (this'll give you a press fit )

[jiihoo]

Hi,

Any possibility that the housing was hot after machining when you measured it and it then shrunk some as it cooled? Dunno how hot it needs to be for the expansion / shrinkage to be significant, but if you are getting into the hair splitting domain in the accuracy department then you need to take this into account too.


Jari

[loply]

How are you boring them, on a lathe? What setup/tools/etc?

Couple of tips I have learned:

1) When you get real close to final size, measure your tool post movement using a dial indicator, not using the markings on your dials

2) Beware of a burr on the bore when you do your measurements and trial fits, remove it with a file or deburring tool or suchlike as it will make the bearing appear to be a tight fit until the bur breaks off, then you realise it's oversize

3) As hinted above if the part is real hot it can grow but in my tests at this kind of size you'd only get 0.01-0.02mm growth with a LOT of heat

4) How are you measuring the bore? Telescoping bore gauges are probably your best bet, inside calipers won't work.

[mzt]

Hi Picclock,

what Jari said about workpiece temperatures. Either work using coolants, or allow some time (it can take a lot!) for the workpiece to cool down before the final cuts. I've had some luck with bores made slightly oversize that shrunk to dimension, but I would have not tried that on workpieces that required more than 5 minutes to make again.

What works for me, especially when working with inserts, was making the final cut almost the the same depth than the roughing cuts. I/e: if I had a consistent diameter reduction when removing 1mm/dia during the roughing cuts, that will be the diameter reduction (now I think of it, that would be increase, in bores) I will use on the finishing cut. I may go 0.9 or 1.1, but I've quit shaving the wpcs. 0.1mm each passage. Be confident, take courage (and practice on scraps, first).

Another point I take special care into, is never making cuts less deep than the insert point radius. You advance the tool, say 0.1mm and the tool does not remove material. Another 0.1 and nothing again. On the third 0.1 it removes 0.4 and You're undersize.

Marcello

[loply]

Hi Picclock,

what Jari said about workpiece temperatures. Either work using coolants, or allow some time (it can take a lot!) for the workpiece to cool down before the final cuts. I've had some luck with bores made slightly oversize that shrunk to dimension, but I would have not tried that on workpieces that required more than 5 minutes to make again.

What works for me, especially when working with inserts, was making the final cut almost the the same depth than the roughing cuts. I/e: if I had a consistent diameter reduction when removing 1mm/dia during the roughing cuts, that will be the diameter reduction (now I think of it, that would be increase, in bores) I will use on the finishing cut. I may go 0.9 or 1.1, but I've quit shaving the wpcs. 0.1mm each passage. Be confident, take courage (and practice on scraps, first).

Another point I take special care into, is never making cuts less deep than the insert point radius. You advance the tool, say 0.1mm and the tool does not remove material. Another 0.1 and nothing again. On the third 0.1 it removes 0.4 and You're undersize.

Marcello

If, like many on here, you're using a cheap or somewhat imprecise lathe, the above is certainly true but care must be taken.

On my small lathe for instance, moving the tool post by (say) 0.15mm will almost always result in 0.17-0.18mm coming off the piece. I have no idea how. You can repeat it over and over. I presume it's something to do with flex in all the parts sucking the tool in, because the gibs/backlash/etc are all as best as can be.

What I tend to do is two measured cuts - I move the tool and measure it's movement using a DI, take a cut, check the size, and repeat. This way I can accomodate for the relationship between toolpost movement and cutting depth and usually end up bang-on.

[picclock]

Thanks for the replies. I'm boring the recesses on the mill with a rotary table and DRO. The finish I get is excellent, but that may be part of the problem, because if it was ridged as it would be in a lathe the ridges would deform to the outer size of the bearing. I'm using a telescoping gauge for measurement but these things are not good when you are trying to measure to such accuracy levels. The way you hold it, insert it and lock it causes small differences in the final measurement which are significant in the accuracy I'm trying to obtain.

For the record its 6800 bearings into mild steel, and the work temperature is always cool and I'm finishing it with a 10mm endmill (cos that way the maths is easier  ).

Many thanks for the input.

Best Regards

picclock

[loply]

Thanks for the replies. I'm boring the recesses on the mill with a rotary table and DRO. The finish I get is excellent, but that may be part of the problem, because if it was ridged as it would be in a lathe the ridges would deform to the outer size of the bearing. I'm using a telescoping gauge for measurement but these things are not good when you are trying to measure to such accuracy levels. The way you hold it, insert it and lock it causes small differences in the final measurement which are significant in the accuracy I'm trying to obtain.

For the record its 6800 bearings into mild steel, and the work temperature is always cool and I'm finishing it with a 10mm endmill (cos that way the maths is easier  ).

Many thanks for the input.

Best Regards

picclock

With some practice and possibly some refining of your bore gauges you should be able to get pretty bang bang on (down to 0.01mm).

My bore gauges had slightly faceted faces when I received them, so I polished them using a fine compound on a soft polishing wheel on the bench grinder.

In terms of technique it's best to practice on the ID of a bearing of a known size, if you have a spare bearing of sufficient size anyway.

Are you aware of the right technique? Pop the gauge in the bore at a slight angle (say 15 degrees) and pop it open, lock the screw down tight and now bring it up to vertical and then beyond, whilst simultaneously wiggling it gently back and forth in the perpendicular direction.

Measure it really light with your mic.

Practice on the ID of a bearing and soon you should be able to get the same reading more or less every time.

Cheers,
Rich

[picclock]

Hi Loply
Yes I know the technique. I think its always trickier because you are measuring the part on the mill in order not to disturb the setup. I have X and Y DRO's and a DRO on the Z axis for drilling so it should be easy, however like so many of these skills it isn't. I suspect I may end up being an expert after the first few hundred but I was hoping to cut down on the learning time. For one which was too tight I remounted the part on the lathe and despite having no DRO it was easier to get the correct size. I suspect that the lathe allows the counterbore sides to be more parallel as there is less tool deflection - wouldn't have thought that would be an issue with a 10mm endmill and fine cuts but I suspect something else may be going on, possibly the endmill rubbing and not cutting properly due to the shallow tool contact angle and low depth of cut.

Many thanks for the comments though.

Best Regards

picclock