I recently made a pair of gears for Thomas, who is one of the readers of this blog and my main website . Thomas contacted me ast week to say that whilst he was pleased with the gears the dimensions he supplied regarding the length ot the teeth was wrong and could I make another pair this time with 15mm instead of the 12.75mm initialy asked for.

Because the greatest part of the work required initaily had been done, it didn't take too long to produce the set shown below. The time taken to work out the cuttter required (and not found at a sensible i.e less than £135!) as well as calculating the holes, and number of turns for the Vertex dividing head and grinding a cutter with the correct profile, was the largets part of the project.

Set of extension gears for a childs electric jeep

So it didn't take too long to make the new pair. I made these in aluminium this time since I had some bar available. they will be posted off tomorrow (Monday) and hopefully Thomas will get them by Tuesday at the latest. Thomas has promised that he will send some pictures of the jeep when we have sorted out his problem.

Today I managed to get out into the workshop and complete the two gears that Thomas, one of my website readers, asked me to make for him. He intends to upgrade a electric Jeep for children increasing both top speed and torque by gearing and new electric motors. Hopefully Thomas will send some pictures and perhaps a short video of the completed jeep in action.

Back to the gears; I decided to grind a second tool with the tooth form and compare the results of both on a piece of scrap aluminum. Once this was done and the best profile found (as well as the number of teeth cut) I started on one of the brass blanks. The first gear was cut without too many dramas’s and checked against the gear it should drive. Everything seemed ok so the second blank was put into the chuck on the Vertex dividing head. Because the top of the teeth on the first blank could have had a better involute shape to the top of each tooth I decided to cut the second gear a little deeper, and this did improve the second gear slightly.

Rather than try and re-chuck the first gear and get to line it up (very difficult…) then take a slightly deeper cut I decided to take the top edge off each tooth with a small needle file. When testing both gears they seemed to work well enough so they should work fine.

Once the teeth were cut all that remained was to cross drill, then tap the hole for the grub screw. This was tapped 5mm because that was the smallest grub screws I had and the only deviation to the plans sent to me. Again hopefully this will not be a major problem and should offer better security than the 4mm asked for.

I will let you know what feedback I get for Thomas and whether I need to remake anything. As always….time will tell…………………

Having taken a few days to recover from the last workshop sessions; and since I get up early in the mornings, I decided to go into the workshop before my brother Steve came over to take me sea fishing in our boat.

The reason for my early start was the gear cutting request from Thomas, which at first sight was a simple task, but it turned into something both more complicated and a good learning experience. The main lessons for me were the lack of availability of reasonably priced cutters for the size required, and having to refresh my knowledge on gear cutting in general.

I did the sums in both imperial (DP) and for metric (MOD) since Thomas required metric finish sizes to enable me to find a cutter. I thought I had solved the problem when I found the right hobb for a very reasonable price. In the past I have used Tracy Tools based in Torquay (only a few miles from me) but they didn't have the sizes required. So Arc Eurotrade are certainly worth a visit for metric hobbs at very reasonable prices.

The Hobb purchased from Arc Eurotrade

However the hobb has a thread form so the blank to cut the gear on must be free to revolve, something the design would not allow for this project so it was back to square one. I decided therefore that it may well be easier to go back to the original plan and make a form tool of the tooth myself.  This was soon made and I decided to make a new tool holder for the milling machine since I found the one I was going to use had a 20 thou 'wobble' which obviously would not do.

New tool holder for gear cutting

The last workshop session saw me make the tool and holder but because I was unwell, I could not for the life of me do the basic calculations (my excuse)to use my Vertex dividing head. Unfortunately the tables that came with the Vertex have decided to hide themselves, so I have to work them out each time unless they are in the data tables books I have. So of course the 15 divisions Thomas want's doesn't appear anywher since the lowest given is for 20.

Now it is relatively simple and John Stevenson has written quite a bit on gear and gear cutting (search Google) and there is an excellent book available written by Ivan Law (Workshop Practice 17 Amazon link on my main website) and anyone new to cutting gears should consider buying a copy. So once I had sorted out the numbers for the Vertex I wanted to check that they were right, hence my early morning start. I have decided that I will put gear cutting for novice engineers as part of my main website, hopefully adding a novices perspective.

After finding a scrap piece of brass of the same approximate diameter I secured it in the chuck and started to cut using the specially ground tool. I didn't want to take too deep just enough to count the teeth. So after 15 cuts I checked the piece and was more than just pleased to see the correct results. Whilst the tooth shape is not correct because of the depth of cut, trying the unfinished scrap piece on the gears Thomas supplied and it all looks fine so as soon as I am well enough to go back to the workshop it shouldn't take to long to finish the job.

The scrap piece of brass used as a trial for checking it is 15 divisions