. . . a screw looks an awful lot like a nail.My 8 year old son is in Cub Scouts as a Wolf this year, and we just experienced our first Pinewood Derby. I wanted this to be Mike's car and not daddy's car, which meant that I had to take a step back and let him succeed (and fail, if necessary).
|The official kit. |
Image by Grika.
The first thing that I noticed was that the package didn't have details as to what was actually included nor how to use what was in the package. There was a lot of French language on the outside, but nothing to help the purchaser make an informed decision on whether the contents were useful, how to use the contents, or even really what the contents were at all.
When I got home, I opened things up and found that the speed kit consisted of sandpaper (which I already had), a mandril, graphite lubricant, some polishing paste, and four extra axles. So I paid $12.00 for a mandril and some polishing paste. What I was expecting to find was four polished and prepared axles. I wasn't real happy with the kit, but it is what it is.
I've had a scroll saw on my "low priority buy" list for a year or so, but this project required either a band saw (which I want but is expensive), or a low cost scroll saw. The scroll saw, available at the local hardware store for $99.00, was a win over a $700.00+ band saw which wouldn't be delivered until sometime next week (past the weigh in date).
In the meantime, Mike started learning how to draw patterns. I had him freehand the design of his car, which he did a good job of. He was very clear that there was going to be a spoiler in the rear, and that there were going to be doors as well. He even drew an extra view just to label the doors as "not opening".
Next, his Mom and I taught him how to draw at a 1-to-1 scale on graph paper. That took longer than expected, but his mother has the patience of an angel. She figured out that Michael needed to learn about symmetry. Once he got the idea of symmetry he did a great job laying out his pattern. We also watched a video by Oxtools on YouTube where Tom shows how he printed out a pattern, glued it to a piece of stock aluminum, and then used a bandsaw to cut it out.
|Practicing with the scroll saw.|
After Mike had his finished pattern created on the graph paper, Mom cut it out. Unfortunately, the first time she cut it out, she followed his pattern instead of just cutting out the rectangle to paste onto the wood. This led to Mike having to re-do the pattern. I think the two of them redrew that pattern three or four times before we finally got it glued correctly onto the wood block. Of course, they wanted to glue it on with regular Elmer's glue, and I had to send them out for glue sticks, but it got done.
Unfortunately the block of wood had a bump, almost a reverse groove, going right down the center. I didn't want to run the block through the jointer for fear of it breaking out the axle groove. This was a mistake on my part as we would later find out.
|The 'line' going down the top of the center line was left behind when the block|
was cut out. It should not have been there, and it caused us problems.
When Mike (and I) had gained as much experience as we were able in the time available, we did the cuts on the car itself. I was happy with the finished product, which was pretty amazing considering the design was made by an 8 year old!
|The completed car.|
The next day after school Mike and mom are back at it, as dad is still at work. Mike is given paint brushes and the paint he selected and he goes to work painting the car. After the car had been cut out he decided the car look a little bit like a rattlesnake so that's what he went for. That included eyes and some snake decals.
Once the car was painted and drying, we went off to the shop to work on the wheels. The limited instructions said, ". . . wrap a drill in a rag and then hold the drill in a vise, put the nail/axle in the chuck, then use . . ." Well, when all you have is a hammer, a screw looks an awful lot like a nail. LATHE TIME!
Except that my three jaw chuck has a minimum size of about 0.125 - 0.250in. Okay, you might start to see the problem there. We were working on a wood item and I was thinking in terms of thousands of an inch when being concerned about 1/32nd was overkill. I spent a little bit of time thinking, and figured out how to mount one of my drill chucks in the three jaw and we were off to the races (I could have removed the three jaw and used a MT3 to MT4 adapter to turn the drill chuck directly, but did not).
Image by K. Murray.
Mike did a great job making sure the parts were moved from the holding area to the work area and back as needed. Next year I'll grind a left hand tool to be able to reach in and turn the axles, which will do a much, much better job. It might also help to have a better set of files, maybe some second cut and smooth files. I'll also cone the axles a little. If legal, I might even groove the axles.
With the axles complete, it was time for the wheels. The wheels went on the mandrel and we started sanding them. This just wasn't right, so I set up a good turning tool and turned them. Amazingly enough, the wheels were not true to begin with. In the end, we wound up with some very nice looking wheels and some better-than-stock axles. Next year, I plan to fill the axle holes with something, drill and ream them to size, then put them on the mandrel and turn them.
Then we went back inside for assembly. Out came my triple beam scale to get the weight just right: 141.74g (you can see that OCD coming out again). I set the weight to 142.25g just to make sure that if the official scale was a little off it would still work out. And with the car, wheels, axles, and all the extra weight on the platform, I couldn't reach 141g. Oh bugger. So back to the shop, this time to the reloading station. Ah, there it is, a 170 grain .308. That plus the other stuff put the car up over 141.7g. This meant that we could do it with what we had on hand.
The next day, Mike and I got back in the shop, this time at the drill press. After a quick setup we had a hole right where he wanted the bullet to be. I'm thinking, "I'm such a cool dad. A bullet for a weight. And it's sticking out looking cool! No other kid will have a bullet for a weight." Well, when we got to check in on Thursday night what did I find but a half dozen cars with bullet weights. I guess we live in part of the country where hunting and reloading is a normal thing.
|Weighing the finished car.|
The car weighed in at 5.0oz so I knew their scale was not as accurate as mine. I spent a few minutes and adjusted the wheels, then tried again. This time the wheels fit, but the weights on the bottom of the car were not clearing. The weights were not giving the car enough clearance! (It turns out that the reason the weights would not clear the track without risers was because of that aforementioned bump, which pushed the carefully sized weights out of kilter just enough to drag.)
|The weights wouldn't clear the track until we added risers.|
|Close-up of axle and groove.|
Image by K. Murray.
In regards to the axle groove rule, there were some interesting points to note. One pinewood derby site talked about how to cut the body into different chunks and then glue the pieces back together to move the axle grooves but still retain the "original grooves" and to be able to place the axles in the original grooves. Following the letter of the rule while totally breaking the spirit.
So it was time to design. If we were going to build the risers then we had to figure out exactly what was needed. Research on the net showed that the lane guide is 1.750. The hub offset is 0.050. That meant the risers should be 1.650 wide. (OOPS, my second error. I didn't leave any clearance.) We only needed the risers to be as wide as required, that is 0.500. The height would be 0.250, just stock size. The first piece of stock I pulled up was 1.750, but I didn't notice.
Mike and I put the bar stock in the horizontal band saw and cut off a chunk 0.625 wide. I am new to machining so I left a machining allowance of 0.125. I had watched Tom of Tom's Techniques and he talked about leaving 0.075 for a machining allowance. That Lazy Machinist had talked about 0.4mm for a machining allowance from stock material. (He was making a 25m, 50mm, 75mm block from 1in stock.)
I started the process. Following directions from Tom's Techniques I put the piece on a parallel and took the least amount off the first secondary surface. Since the primary surface was against the vise face this meant that the secondary surface was now square to the primary surface. If I had been really worried I could have surfaced the primary first but that level of precision was not required.
The next step was to flip the part and surface the second secondary surface. Because the down surface was machined, it was banged down firmly. Once we had the second surface machined the part was removed and measured. The total amount of material to be removed was calculated and the dial set so that it would be zero when at size. THANK YOU, Tom and Marc (That Lazy Machinist), for showing me how to square material and set my dials.
A few passes later, the part was squared and parallel for the primary and secondary surfaces. In the past, I'd had trouble with the tertiary surfaces and I could see I was having problems again. In order to square the first tertiary surface the piece has to be set square in the vise. To do this the primary surface is placed against the vise face making the piece square in one dimension. A reference square is then placed against the top of the vise jaw (or bottom) and against the secondary surface. Doing this squares the piece in two dimensions so that when the first tertiary surface is machined it will be square to the primary and secondary surfaces.
|Diagram of risers.|
With the piece squared and parallel on four surfaces, and with the part square in the vise, I surfaced the fifth surface (first tertiary) and so had five square surfaces. The work was flipped over and the final surface was machined. I measured it and machined it to size. YEAH! My first piece squared correctly!
One of the things my teachers have taught me is that burrs and dirt are the enemy of good work. I de-burred and de-burred again every time I pulled the work piece from the vise. This led to very good results in the squaring process. I was happy with the results.
The next step was to drill the axle holes. This was a more difficult operation. The main reason is edge finding. Until now there had been no need to know the X,Y location of anything. The closest we came was in moving the Z axis to machine the work to size. Since I was using a 1/2in end mill in a 1/2in collet, I changed in a 1/2in edge finder and quickly found the right most X edge. Using a bit of math I moved to the center and then off the edge in the Y direction. Then I locked the X axis and found the edge in the Y direction.
|Illustration of off-center axle.|
Image by K. Murray.
Removing the work, I flipped it over and used the drill bit to guide it into place. I reset and flipped the work again. Still off. A bit more work and more and more and I could NOT get the work to line up as expected. Finally I opened the vise up and what did I find? A chip sitting on the parallel.
With the chip cleaned off and things in place again, I was able to get things lined up using the drill bit as a guide. I kept flipping the work piece and checking, and it looked good. I started drilling using a pecking method. I was almost to depth when the bit snapped off in the hole. I had just totally ruined the work. I couldn't even figure out how to get the bit out.
Mike was with me this whole time. He'd been raising and lowering the tables, supplying "kid power feed" to the cross feed, moving parts as needed, and holding the flashlight when I needed extra light to read the dials. We were done for the night. Plus it was cold as sin in my shop, and we were out of propane for the heater.
Thus ended Friday. The race was at 1300hrs on Saturday and I still didn't have the risers and the car still wouldn't pass inspection.
|One of the risers, ridge up for sliding into the original axle groove.|
Next up was drilling the axle holes. Following Marc's rules I was meticulous in keeping the work area clean. This time when I flipped the work to put in the axle hole it lined up perfectly. I know this because when I drilled down, the two holes met somewhere in the work piece and when I probed the hole, I really couldn't feel the joint. Once the axle holes were in place I repositioned the work so the primary surface was up. I found the edge and centered the work, and drilled a hole for a #8 wood screw.
The work then went over to the drill press. I thought it would be fast and easy to put the counter sink in the drill press and change to the slowest speed. That was true. Unfortunately that was still too fast. I got chatter. It wasn't until much later that I remembered that the mill had back gears and could have gone slower than the drill press.
Finally the work went back to the mill. I found the edge on the Y axis. Once there I adjusted the work to leave a 0.065 wide ridge to use for registration with the axle groove in the car. I adjusted the height of the table and cut the width of the piece. Flipping the work end for end I cut the other side of the work. When I looked I saw that the registration ridge was too wide. I was about to go back to math when I remembered Tom's instruction on cutting a square headed bolt.
|A riser, ridge down.|
Oh, the fact that I left the cutter in place and removed and put the work back is also from Tom's example. The swap was MUCH faster than re-positioning the tool on both sides. At that point the only things left to do were more deburring and cleaning the hole for the holding screw. From there it was clean up and time to put everything together.
As expected the axle holes were a little large. The cure was blue Loctite. The risers went on and the wheels went on. While the Loctite was setting, the axles were sliding. Unfortunately with too many people moving the car around, what I had expected to happen did happen. One wheel was pushed in far enough to rub against the car body and also against the lane guide. When the Loctite set up the axle could not be moved. As I stated earlier, if I had machined the risers 1.850 instead of 1.650 there would have been enough space and the wheels would have had the smooth surface of the machined riser to ride against.
What I learnedFirst, it always takes longer than expected to do anything. Machining the risers took 5 hours of shop time. I learned that I need to allow myself a lot more time than I generally estimate. The wife suggests multiplying by a factor of 2.5 or more . . .
The second thing is always always draw your pictures and write down your dimensions. Because I was measuring and working on the fly, I made a couple of mistakes. Most of those mistakes could have been avoided if I had drawn up a dimensional object.
My goal of putting the work back on the lathe to turn a hub around the axle holes was a correct one and I should have done it. This would have given the wheel something to ride against as well looking nicer than having a big chunk of riser sticking out.
Last, don't be afraid to start over if you make a mistake. What I just realized is that I should have made three pieces instead of just two. The amount of time it takes to do three pieces is hardly more than making two. Most of the time was spent on set up. Taking a piece out of the vise and getting it back into the vise in the same place is not difficult. All it takes is a vise stop and taking the time to do the set up.
In closing, I want to go back to the shop and remake both risers "right". That would start with a detailed drawing in a CAD package.
What Mike learned
I'd like to think that the most important thing that my son learned is that he can count on me. We put a lot of work on this, and he held my hand and I held his hand, and we "got her done." I'm very proud of the effort and patience that he showed.
He learned some practical things, like how to use a scroll saw and how to use a triple beam scale. He also learned another practical lesson: that doing something like this is Hard Work. It wasn't easy, and there were times when it wasn't all that fun (especially in the cold workshop), but he walked into that race knowing that it was HIS car. So he learned that there is a pride that comes along with a job well done.
|Me and my son.|
His final commentary on the whole experience was this: "It was 25% boring, 50% okay, and 25% with my Daddy!" And when he told us this, he was grinning madly. Basically, it was hard work, kind of boring, maybe a bit interesting, but all of it was something he got to do with daddy, and that made the whole thing worth it. Yeah, I'm grinning, too.