PM Research Dynamo

[EricB]

I decided my little engines needed something to do, so I bought a PMR Dynamo kit thinking it could be an easy build. I started making chips about a month ago. My first step of course was to search youtube to see what others had done. Among others I found mrpete222 has 3 videos for his dynamo build. I learned a couple of things from him, the most important being he used a Bridgeport (because he has one) and an Atlas lathe. The other builders used larger machines too. I have Sherline machines so I could not use many of their holding techniques.

I studied the diagrams and measured the castings to decide where to start and figure out how to hold them. For me the logical choice for alignment of the frame casting's features are the two mounting lugs that would secure the dynamo to a platform. I wanted the tops of the lugs about the same height and parallel to the bottom surface. Once machined the bottom would become the master surface. To accomplish this I set the casting on a pair of 123 blocks as shown and gently filed the high spots from the top sides until it did not rock, then clamped it to the mill table again using the blocks. It's at an angle because the casting was 1 5/8" end to end and the t-slots on the table are only 1 1/2" between centers. It worked ok. The only problem I had with the setup was the back block came in contact with the mill column. A better setup would have been on a tooling plate, which I had at the time in a box.

For the ends I faced the boss for the lifting ring, found the center of that and marked it with a punch. Then I measured and marked half the finshed length to each end from that mark. After that it was secured in the vise. I used the straightest edge of the casting against a square to find the first end. Then it was just flipped over for the other end.

This seemed like a good place to check and measure, and it became apparent that I needed to correct for the built in table error in my 5400 mill. The immediate solution was replace it with a 2000 mill I picked up last summer (because it was local and inexpensive). It came with it's own set of quirks but it's a higher quality machine than what Sherline made in the 90s.

More later.

Eric

[propforward]

Excellent project - I will enjoy seeing this progress. Seems like you are off to a good start.

[crueby]

Following along here too, very interested to see how it goes.


 

[EricB]

Part 2.

The next operation was boring the frame casting to fit the magnets. In addition to the mill I needed a longer boring tool than I had on hand. While waiting for that to arrive I investigated the problems with the other machine. That's still on going but I've already replaced the table and repaired some damage to the base which I thought was only cosmetic. It's better now but not quite where I want it yet.

One of the reasons I had not switched to using the 2000 mill is my work bench (a small child's former desk) is shorter front to back than the machine. I had to cut out spaces in my chip guard for the base board and the column ram to pass through, and the machine hangs over the back of the bench. It's still stable enough for most things. Boring might not be one of them.

Back to work. The casting was held in the vise, lifted to give clearance at the bottom of the bore, and indicated square. I centered it roughly using the tool and took out the crust and about half material needing removal before crashing the tool into the vise. OOPS!

Time to stop, remove the part, clear the chips, dress up the damage on the vise, examine the tool, and start again. The bore was looking good at this point and the tool was ok so I put the part back in the vise with more space at the bottom, indicated it square, located the center and continued. All was going well until those last few light passes, then vibration vent up and the finish went away. The bore is to size and a cylinder but it looks like hell. I later found that the motor bracket had worked loose and was bouncing around with the off balance boring head. I also found the tool had chipped.

Next up, the end bell castings.

Eric

[propforward]

Well looking at the pictures that seems to have gone very well.

[crueby]

On the 2000 mill I picked up used last year, also had issues with how long it was front to back. Wound up cutting the ram arms a few inches shorter to fit on the bench. Also got the rigid riser block, the normal riser column block kept slipping and rotating when milling.


 

[Roger B]

Looks to be an interesting build with your Sherline kit  I like to see engines driving something   

[EricB]

Part 3.

Figuring out how to hold the end castings was interesting. My youtube "mentors" all had larger lathes and they all used their larger chucks along with some type of custom spacer for setting up, griping the castings with the very tips of the jaws. The jaws on standard Sherline chucks are not long enough to grip the castings in any useful way so I needed a fixture.

I took the general idea of their spacers as a starting point and made a glue chuck. Since the pulley end had fewer features to deal with I started there.

The chuck was carved from a block of 6061 held in the 4 jaw. It would stay there until I was done with both parts. In the center of each casting is a bearing support. Both were about the same diameter so I drilled and bored the block to clear the bearing on the deepest casting. Then I removed material to just clear the outer edge of the bolt bosses on the mounting flange. From there it was off to the rotary table on the mill to relieve the areas for the bolt bosses. I wanted the outside of the mounting flange in firm contact with the machined surface in the fixture. I had to remove a bit of material in the center to clear the spokes for the bearing support to get the fit I wanted. The final touch was to cut some grooves on the contact surface to give the glue a place to flow.

Once the CA glue was cured I finished the flange features and drilled and reamed the bearing support. The part was released using a kitchen torch.

[EricB]

Well looking at the pictures that seems to have gone very well.

The good news is the surfaces that I want to see look good. I was hoping to get better results in the bore. I figured if I ever wanted to build a bigger engine this would be good practice for that.

[crueby]

Great solution for holding the parts. Jigs and arbors are well worth making!


 

[EricB]

On the 2000 mill I picked up used last year, also had issues with how long it was front to back. Wound up cutting the ram arms a few inches shorter to fit on the bench. Also got the rigid riser block, the normal riser column block kept slipping and rotating when milling.


 

My 2000 mill was an opportunistic purchase. I didn't need it but it was local and at a low price. The most common complaints I've heard is the ram moves under a load or that it's less rigid than the 5000 series machines. I've tested with and indicator and it is indeed less rigid. I cleaned all the friction surfaces on the ram and my movement problems went away, so far. I think the 2000 is just an odd collection of compromises. For example, it has more y axis table travel but less room between the column and spindle, so you have a larger work envelope close to the table but shallower working in front of the column. I could go one but that's another story. The jury is still out on which machine I like better.

[EricB]

Part 3. Continued.


The brush end casting has to be bored out to make room for the brushes. To make that easier I bored the fixture to
the finished diameter and depth needed for the casting. That way I would only have to remove material from the
casting itself.

Next I needed to make room to clear the arms that hold the bearing support so back to the rotary table on the
mill. I cut slots across the fixture with a .250" end mill, 45 degrees from the bolt boss reliefs. The arms on the
casting are wider where they connect to the mounting flange so the slots had to be wider there too. I took
additional cuts rotated 2.5 degrees each way from the initial cuts, resulting in a bow tie shape. The gods must
have been smiling because the part dropped right in with no further adjustment.

A little CA glue and back to the lathe to finish it up.

Next up, drilling and tapping.

[EricB]

Part 4.

Not counting the bearings, the three castings have 23 more holes that need drilled and 8 for tapping. The prevailing method I've noted seems to be to drill one part and then transfer the locations to the mating part. I decided to combine as many operations as I could.

The locations of the holes are shown in the diagrams with bolt circles and spacing, but I figured the easier way was to just find the centers of the bosses on the end bells and drill there. Starting at the brush end, because for my build it has the best fit with the frame casting, I found the orientation I wanted and marked its position. Then clamped it down and went to work. First I located the rough position of the boss under the spindle with a short length of 1/4" rod held in the drill chuck. Then touched off on the boss with a center drill. Once happy with the location it was center drilled, followed by the tapping drill through both parts to depth, and then the clearance drill just through the end bell. Repeat for the other three positions. I hand held the frame when tapping the four holes and used the clearance holes in the bell to guide the tap.

For no specific reason I drilled the hole for the eyebolt at this point, then setup, drilled, and tapped the pulley end the same as the brush end.

Back to the brush end. The mounting holes for the brushes are centered on the bearing and a measured distance apart. They are also a different drill size then the clearance holes so I did them after all the others to avoid confusion.

The last operation was the mounting lugs for the frame. My vise is just wider than the part so I did two on one side and slid it over for the other two. Spot faced with an end mill (held in the drill chuck), the spot drilled the center, and finally drill to size, times four.

This is all the work needed on the castings. The remaining parts are fairly straight forward turnings. I plan to make some hex bolts instead of the fillister head screws. I'll post some pictures of the completed dynamo.

Eric

[Admiral_dk]

Looks nice so far - what kind of amatures / magnet are going inside ?

Per

[EricB]

Looks nice so far - what kind of amatures / magnet are going inside ?

Per

I should probably show that too shouldn't I. I still have a few more bits to make then I'll show the assembly.

Eric