A Microscope Repair.

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vtsteam:
Pete, I know exactly what you mean. Finish up something, and that night, think about how it could have been done better, but still, it's usable........and then a day later, you're making another one. I made five valves and 3 different valve cases for the pipe engine, and 4 pistons. Probably the first pieces would have worked after a fashion.

Pete W.:
Hi there, all,

I've done some more work and have some photos to post but I can't get in to Photobucket at the moment! :bang: :bang: :bang: :bang: :bang: :bang:

Pete W.:
Well, I don't know how I did it but I did it and now seem to have access to my Photobucket pictures, so here goes:

As I wrote in a previous post, I made another spindle. Here's the sum of my spindle efforts so far:

The top one is what I received with the microscope, on the next one down I took too much off the tapered section while the next one broke because I didn't support it properly. (While looking for something else in my chaos department recently, I found a 'miniature centre' that might have avoided that failure.)
The next spindle suffered a dimensional error in length.
The bottom one is, I think, good enough to use, though I was disappointed with having to use a junior hacksaw to make the screwdriver slot in the end! I deliberately made the lower spigot of that one longer than the original for a reason I'll expalin after the next photo.

This shows the spindle in position in the microscope's sub-stage bracket:

With the original spindle dimensions, the thread below the tapered section and the end of the lower spigot would need to be engaged simultaneously. This is fiddly and awkward when fitting just the spindle but I felt it would be really difficult when feeding the auxiliary condenser lens, spring washer and filter holder etc on to the spindle during the complete assembly. With my modified spigot length, that can be well engaged in its bracket hole before trying to start the threaded portion.

You can also see in the photo a washer under the larger end of the tapered section. Although I made the length of the tapered section the same as that of the original spindle parts, the replacement polariser unit fouled the bracket. I chucked a length of rod, faced it, drilled an axial hole that just cleared the thread OD and then parted off a 40 thou slice. The mini-thin grooving tool did this really well - I feared when I bought it that I was being extravagant but it's really performed well for far more operations than I originally thought would use it! 40 thou was thicker than it needed but I was being cautious - I think the mini-thin groover would have easily coped with parting-off a thinner washer. That first washer was a touch too big on the diameter so I skimmed the bar and took another slice! Here they are:

It's a bit out of focus but you should be able to see a dimple in the end of the spindle spigot. My lovely but shy assistant and I added that dimple to avoid the grub screw burring the spigot and making it difficult to dismantle in the future. We removed the grub screw, fitted the spindle (and washer) fully tight and then LBSA held the bracket while I presented a 1.5 mm drill down the grub screw hole using a hand-brace. I then enlarged the dimple once the spindle was removed from the bracket. We seem to have avoided damage to the thread in the grub screw hole. Refitting the 8 BA grub screw in the bracket was a challenge - despite my interest in microscopy, I regard 8 BA as small!!

The remaining operation on the spindle is to make a flat on the other spigot to take another 8 BA grub screw securing a collar that keeps the upper filter holder on the spigot (see the photo on the initial post).

But first, I'll make the collar.

Oh, I did find what was stopping my access to Photobucket - it was a FireFox add-on called Bluhell Firewall 2.5.0. It's now disabled!!

Pete W.:
OK, now, about this collar.

It's just a 3/16" slice off the ⅜" mild steel bar (previously reduced to 0.350" OD, drilled axially 0.150" then cross-drilled and tapped 8 BA. Doesn't sound difficult, does it?!?! Still, I bet some of you are thinking 'I know which bit of this goes wrong for him!'.

Well, the first operation mostly went well, like this:

What you see in that photo is raw ⅜" mild steel bar, I omitted to previously reduce it to 0.350" OD! Mistake #1!! Not too serious, that one, I can skim the OD just before parting-off the slice.

In weighing up how I would approach this job, the principal difficulty I had anticipated was cross drilling the tapping size hole for the 8 BA grub screw. If I parted-off the 3/16" slice first, it would be difficult to hold. I wrote off any idea of cross drilling the hole using my drilling machine (it's only a drilling machine, not a vertical mill, so no tables with graduated lead-screws). So, how was I going to ensure that the cross hole was accurately on a diameter? The answer to both of those problems was to drill the cross hole using the lathe and before parting-off the collar from the stock - this would give plenty of material to hold while positioning and drilling.

First thoughts were to use the vertical slide and the little Myford machine vice. But rigging that seemed to be a cumbersome process.

Then I realised that maybe I had the answer right there in my Dickson pattern quick-change tool-post! I fitted the length of rod in a boring bar tool-holder, the type with a vee-groove, with a fair length extending out of the holder. I could then adjust the height of the tool-holder to bring the centre of the rod to the lathe centre height. The conventional way to set this up would be to use a clock gauge, again a rather fiddly procedure.

Instead of that, I used a Myford accessory comprising a 2 Morse taper shank with a Vee pad. I fitted this in the lathe mandrel taper and turned the mandrel to level the vee. Then, having checked that the rod was square to the lathe axis, I offered the rod into the vee groove, adjusting the tool-holder height as necessary. When I thought that was OK, I turned the lathe mandrel through 180° and checked the alignment again. It turns out that, on my drill-pad, the vee-groove is slightly off so I 'split the difference'. I then repositioned the rod in the tool-holder with just enough overhang for the drilling operation. I then replaced the vee-pad with a drill chuck.

I forgot to take photos of these operations - in the one I did take the camera auto-focus locked-on to the cross-slide rather than on the cross-drilling operation. :bang: :bang: :bang: :bang: :bang: :bang:

Next time I get in the workshop, I'll restage these operations and take more photos.

To summarise, the cross-drilling went well, as did the parting-off of the 3/16" slice. However, catastrophe struck when I tried to tap the cross hole. Maybe I drilled too small a hole, maybe I was just clumsy. The bottom line is I have to do it again! Watch this space.

Pete W.:
Another day, more swarf!!

I didn't actually say in my previous post what catastrophe had befallen - not difficult to guess, it was a broken 8 BA tap. I couldn't remove the broken-off stump of the tap so I started again. I tried to remember to take more photos this time.

Well, I parted off the end of the ⅜" rod, faced the remaining end and deepened the axial drilled hole, here's a photo:

Then I transferred the ⅜" rod to the boring bar tool-holder, with quite a large overhang, and put the 2 MT vee drill-pad in the lathe mandrel:

Then I offered the rod to the vee in the drill pad, adjusting the tool-holder height to get the rod snugly in the vee, this puts the axis of the rod level with the axis of the lathe, ensuring that the subsequent cross-drilling is through the rod diameter. Actually, the vee in my drill pad is not quite truly central so I rotated the drill pad through 180° and 'split the difference'.

Next, I reduced the overhang of the rod in the tool-holder and replaced the 2 MT vee drill-pad with the tail-stock drill chuck. Starting with a small centre drill, I touched the end of the rod against the side of the centre drill, zero'd the cross-slide dial, moved the saddle until the end of the rod cleared the centre drill and then wound the cross-slide forward by ½ the centre drill diameter plus 3/32". I then centre drilled and cross-drilled the rod (right through) with a #55 drill followed by a #50 drill (GHT's recommended tapping size for 8 BA).

I re-chucked the centre drill and moved the rod back to touch it, loosened the tool-holder screws and carefully turned the rod through (by eye) 90°, maintaining contact between the rod end and the flank of the centre drill. I then repeated the cross-drilling sequence. I refitted the 3-jaw chuck to the lathe mandrel and chucked the ⅜" rod and fitted the mini-thin grooving tool to the tool-post. Having touched off the side of the tool against the end of the rod, I wound the cross-slide out to clear, advanced the saddle past the rod end by the width of the tool plus the desired thickness of the collar and parted-off.

Here's the collar, after tapping the holes 8 BA (by hand):

And here it is, partially assembled on the spindle with the filter holder and bracket:

and

I'm not happy with it, the hand tapping is crooked, the collar is too thick and I forgot (again) to reduce the ⅜" rod OD to 0.350" before parting-off. I'm hyping myself up to contrive some sort of tapping device - it won't be GHT's Universal Pillar Tool, magnificent though that is!! :drool: :drool: :drool:

I shall treat this collar as temporary and proceed with assembly of the microscope sub-stage components (see opening post photo). Because the collar is the last part to be fitted, I can substitute a better version later without having to completely dismantle the microscope.

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