When I inspected the problem, the culprit was a plastic gear. No parts were available for replacement ( obvious reason being $ minded Dremel wanted to make more money from newer models). It was bubbled wrapped and kept in the storeroom for nearly 2 decades and was slowly forgotten until recently.
I decided to repair it, not that I needed it badly, and to challenge myself to do something that I have never done it. It was no easy task. There was little information about gear making and I am not in this industry or a metal fabricator. As far as I know, production gears are made from cold forging techniques. They have dies with the opposite shapes and a whopping 200 tons of pressure is applied to a billet of softer material into the die . A bit like making jelly in a mould but instead if pourIng molten metal into a mould, excessive pressure is used. Examples are Shimano crown gears in fishing reels using this type of cold forging process. But who am I kidding, DIY project using 200 tons of force!
So the only way to make a replacement gear is the use of a gear cutter. I need just one gear and it's a one off thing.
From the broken gear, measurements were made, number of teeth were interpolated into the imaginary calculations. These were not difficult being a science student in my teenage days, I can still remember geometry and properties of circles very well. It worked out that the gear had 16 tooth and it's pitch was determined from the straight rack by way of vernier measuring. Believe me it took me many weeks to accurately determine the pitch of the gear. If the wrong diametrial pitch was taken, the resulting gear will never mesh properly. There are also many types of gears and the one I was working on is an involute gear. So call involute because the curves on the gears are part of an involute circle. Very confusing when I first started understanding this, but the more I read, the more interesting it got.
The easiest way to make this gear replacement is using an involute gear cutter. Gear hobbIng is also possible but ........
So making a gear is actually removing material from a complete round rod to form the tooth of the gears, so technically it's not gear cutting but that's what they call it anyway. Like why they call a skyscraper building when it's already completed ? Anyway I know to cut 16 tooth means I need to rotate the blank rod 22.5 degrees precisely for each pass, otherwise the finished product looks odd and of course it won't mesh with the straight rack.
7075 aluminium was chosen . If I were to do this again, I would select 60xx aluminium. 7075 being a harder Alu alloy it's was so damn hard to cut it. Next time stick with something that's not an overkill.
16 passes were made and the resulting gear was amazing.
There was a problem, I made the cut too deep so the mesh was sloppy. So second one was made ( I too have my fair share of failures ) and I cut it shallower. And there's a formula to follow when cutting the depth of gears . Lesson learnt.
The second gear made was much better and I move on to solve the rotating stem that must be attached to the gear. Dremel used a positive type of engaging mechanism. It's too complicated to replicate on the aluminium it has a shape much like a Mercedes Benz logo.
I went for an easier approach . Just epoxy the gear and the rotating stem assembly. I made a tight fit recess to receive the stem so that it would not be sloppy and I was wrong. Later I learnt that a bit of slop would assist in the epoxy stage.
Because the stem and the gear are two seperate pieces, to find a common center is difficult. So the recess part of the gear was made slightly bigger for adjustment just before the epoxy sets.
Before epoxy, I cut a 0.5,mm slot so that the return coil spring can be attached. This slot must be made exactly across the diameter otherwise the rotating gear will be off center and it will bind
With that done, the gear is mounted on the lathe and dial in to get the best possible runout of 0.10mm, zero is ideal. The stem was well made by Dremel and had a 0mm runout.
The stem was mounted onto the tail stock and epoxy (JB weld) applied and the 2 pieces are bonded together. Best possible bond is to have no air gap.
The next 6 hrs was checking for runout just before the epoxy sets after which no adjustments can be made.
After 24 hrs, the epoxy sets and the part is ready for assembly. Aluminium bushing was turned out to better support the stem during use so that the stress distributed on the bush. Later I made Delrin bush to replace the aluminium one.
And with the parts made, just assemble it together and use the press.
There was still some binding between the gear and the rack so I used engjneer's dye to locate the binding and removed some more material from the gear. And that little bit of material removed made a huge difference.
The final and finished product.
It was a good learning experience making this part. A lot of thinking was involved.
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