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| Dicing with Delrin - A Repair Job. |
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| Pete W.:
Hi there, all, Despite some trepidation about offering my latest efforts alongside the posts of the stalwarts of Mad Modders, I've decided to have a go. If only because some Modders may be able to tell me where I went wrong and how they would tackle the job. So, here goes: A friend managed to damage a rather high quality binocular loup and asked me if I might be able to fix it for him. I said I'd attempt a repair, strictly on a 'best efforts' basis with no guarantee, but that wouldn't be a quick job - I'd have to finish some other jobs first and also to make some tooling and fixtures along the way. In this first post, I'll describe the diagnosis, identify the damaged part and summarise my repair plan. (Photos at the end of the post.) The loup comprises a pair of magnifying optics which mount on to a bar with an adjustment for inter-ocular distance. That bar, in turn, mounts on to a head-band. The optics resemble rifle sights but have a shorter focal distance (about 30 cms) and the mounting bar is shaped to angle their optical axes to converge at that distance. My friend had managed to drop the loup which had caused it to become 'boss-eyed'. I gave the loup a thorough inspection and was relieved to find no damage to the optics units themselves but the mount that attaches one optic to the bar had been distorted by the impact. This component (aka 'the mount') is shown in the photos. It is plastic, I assume it is a moulding, and it has a stepped hole through which the ocular end of the optic fits, retained by a threaded ring and an M2 grub-screw. On its upper flat face, there is a tongue that is a sliding fit in a slot in the mounting bar where it is secured with a washer and a plastic-headed M2 thumb-screw. The tongue on the damaged component had suffered a permanent twist which had caused the loup to become 'boss-eyed'. I don't have facilities for plastic moulding so the only way I could see to make a replacement mount was to hew it from solid. My measurements suggested that 8 mm sheet would be the suitable raw material and I opted for Delrin (I'll come back to choice of material later in this thread). I bought a piece of Delrin sheet on eBay - it was billed as 8 mm but turned out to be nearer 9 mm which had me scratching my head at first! I sawed off a 1" wide strip using my table saw and divided that into 1½" lengths. This gave me five blanks (aka 'dominos'). I decided that it would be easier for me to make a matched pair of mounts than to exactly duplicate the undamaged original. So I resolved to work on all five dominos in parallel in the hope of achieving two usable mounts rather than having to start over if I suffered any problems along the way. The tongue seemed to be the trickiest feature so I opted to start with that, then progress to the stepped hole and finally to profile the mount to something like the originals. As Bob says, 'OK, photos': |
| Pete W.:
To continue: I don't have a mill but I do have a vertical slide for the Myford ML7. (A friendly neighbour did say I could use his mill but my modus operandi is to do a bit and then think a bit and I don't think it's workable to time share a mill!) After some trial and error, I reached the stage where I had five dominos, each with a usable tongue on one end. (One domino also has an unusable tongue on the other end - if at first you don't succeed ... ) The procedure was as follows: Fit a length of 12 mm PGR in the lathe collet, insert the domino into the machine vice with its best edge against the rod and tighten the vice. Remove the PGR & collet and mount the milling chuck with a four-flute end mill. With the lead-screw half-nuts closed, take a clean-up facing cut across the end of the domino, noting the lead-screw dial reading. Move the domino clear of the end mill and advance the carriage by the length of the tongue. Touch off the rear edge and underside of the domino, zeroing the feed-screw dials in each case. Wind the vertical slide down by the width of the lower shoulder and feed the cross-slide in by a pre-calculated distance. Then feed the vertical slide down by the thickness of the tongue plus the cutter diameter, this cuts the 'near' edge of the tongue and positions the end mill ready to cut the upper surface of the tongue. Next feed the cross-slide out by a pre-calculated distance and then feed the vertical slide up until the underside of the domino is level with the top of the end mill. This SHOULD give a rectangular tongue of the right width and thickness! It sounds easy but real lathes have back-lash. In one or two cases I must have miscounted the turns of the cross-slide dial which put the tongue 100 thou out of position. Furthermore, my milling arrangement gave lots of swarf and paper edge that obscured visibility of the job (see photos). I guess a four flute end mill at the maximum open speed of the ML7 is sub-optimum for machining Delrin but it's the best I've got. Before removing the domino from the machine vice, I replaced the milling chuck and end mill with a drill chuck and drilled and tapped the end of the tongue for the M2 thumb-screw. The original tongue has rounded ends. I wasn't able to reproduce these so resorted to bevelling the ends of the tongue with a craft knife. This gave a shape that works but isn't pretty! Similarly, removing the paper edges, again with a craft knife, wasn't a precise operation. OK, photos: |
| Pete W.:
That's all I've got on photos at present, though I have done a bit more work. I plan to drill three holes in each domino so I've made a gizmo to position the holes in the hope of getting all the drilled dominos the same - it doesn't warrant the title of 'jig' but maybe it's fair to call it a template. It's made from mild steel sheet, about 2 mm thickness. After carefully marking the hole positions I used my recently purchased optical centre punch to prepare for drilling. I also plan to cut a square of 3/8" aluminium alloy plate to mount in the four jaw chuck. I'll use the template to position holes in that plate. One hole will mark the position of the stepped hole in the mount - the other two will be tapped and used to attach the domino to the plate for boring. I'll probably contrive some sort of clamp for the tongue end of the domino. If that sounds confusing, wait for subsequent posts. |
| krv3000:
hi Pete I see nothing wrong in the way you is attempting the fix brill |
| Pete W.:
--- Quote from: krv3000 on October 15, 2016, 03:57:52 PM ---hi Pete I see nothing wrong in the way you is attempting the fix brill --- End quote --- Hi there, Bob, Thank you for your encouragement. I've progressed a bit further - I'll post a description and a few more photos. |
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