Fixing a 24-tooth Module 1.5 Mill Head Gear For My Emco Maximat V10 Metal Lathe

Difficulty Level (Easy, Medium, Hard, Insane):
Hard

Process:
I was working on my SSR Heatsinks for my kiln project when all of a sudden I heard this horrible noise. I turned off the mill, took off the mill head covers and immediately saw that one of the tufnol gears had broken off 3 teeth.

I spent the next few minutes poking around and thinking of all my options and decided that I'd have to take it apart first to have a good look at it.

After taking it apart and looking at the design of the mill head gears I realized that I MIGHT be able to press off the tufnol gear, make a new one out of nylon and press the new one on.

I sent a message to my Emco Larger Lathe Yahoo Group to see if anyone has run into this issue before and there actually were several individuals who either made a new gear, had a machine shop make one for them or bought one and pressed it on themselves.

One of the gentlemen even wrote his own Documentation when he had to go through this same exercise which, after readng, I was quite a bit more heartened than before and decided I was going to tackle this project. It was certainly not what I was expecting to do but at least I saw some hope that I'd be able to fix it myself.

After cleaning everything really well I re-assessed the situation and noticed that there was a LOT of play in the middle gear which ran on needle roller bearings.

I looked at the parts list document for my lathe and determined that I need two HK1210 bearings. Luckily, Motion Canada had them in their Toronto warehouse and they were under $4 bucks each and I got them to Moncton in only 3 days.

Once I received the bearings, I removed the old ones, which I ended up having to completely obliterate and even use a dremel tool with a grinding stone to grind through the outer race because they were pressed in so hard and the outer races were hardened steel.

Once I finally got them out, I used the Koyo Bearing Install Guide to make an installation tool and used my DIY 20-Ton Shop Press With Upside Down (Inverted) Bottle Jack to press the bearings in place.

After referring to a very helpful document entitled Cutting Involute Gears With Form Tools (taken from: http://metalwebnews.com/howto/gear/gear1.html), I took a piece of nylon, turn the outside to size, bored the inside to size and then used my Dividing Head for Indexing and Cutting Gears to cut the actual teeth.

The last thing I had to do was to press the old tufnol gear off the shaft and take the finished nylon gear and press it onto the shaft (I used some 5-min epoxy).

Originally, I was shooting for about 10-15 thousands of interference so I could press the nylon gear onto the serrated iron shaft, but after I pressed the old tufnol gear off the shaft I realized that I ended up only having about 1-2 thousands of intereference. Because of that, I decided to drill a 3/16" hole through the nylon gear and part of the wall of the iron shaft and install a roll pin in case the 2 thou interference wasn't quite enough to lock the nylon gear in place.

In retrospect, since the tufnol gear was shot anyways I should have pressed the broken gear off the shaft and taken proper measurements; that could have avoided having to use a roll pin.

Once the new gear was pressed onto the shaft, I put the gears back in the gear box and filled it up with Motomaster Moly Extreme Pressure NLCI #2 Grease and was finally able to continue making my two DIY SSR Heat Sinks for my Electric Kiln.

Videos:

 
Pictures:

The top portion of the mill head with motor

The first gear (16/24 teeth)

The cleaned up gear with the key

The bottom portion of the mill head with the moly grease

Pulling off the broken second gear (24/16 teeth)

The cleaned up gear with the 3 broken tufnol teeth

Another view

Gear specs with the broken tooth area circled

The third gear (35/43 teeth)

The third gear assembly

The cleaned empty gear box

The 940 RPM setting (24 > 16 > 43)

The 420 RPM setting (16 > 24 v 16 > 43)

The 770 RPM setting (16 > 24 > 35)

The 1740 RPM setting (24 > 16 ^ 24 > 35)

Calculating the different gear ratios

The raw stock of 2.500" nylon

Getting the outside turned to size

The replacement gear specs

Test fitting the nylon gear blank on the shaft

The outside turned to finished size

Using a boring bar to cut a recess into the gear

Another view

The dividing head set up and ready to cut gears

Cutting the gear

The finished gear

Installing the fixed gear

Filling the mill head with grease

Stock picture of the moly grease I used for the gear box

The finished mill head

Tools:
Metal lathe & accessories
Metal band saw
Screwdrivers
Wrench
Hammer
Cordless drill
Crow bar
Adjustable wrench
Diamond sharpening saw
Angle grinder
20-ton shop press
Parts washer
Form tool
Fly cutter
Dividing head
Dremel
Chisels
Awl
Snap ring pliers

Materials:
Motomaster Moly Extreme Pressure Grease
Rags
3" of 2.500" nylon
5-min 2-component epoxy
3/16" roll pin
Two HK1210 drawn cup needle roller bearings

Cost:
$15

Time:
10-15 hours

Savings:
$100 at least

Conclusion:

It works, but the gear is definitely a bit louder than it was before. I'm assuming that the gear will wear in over the next few hours of using it and even if it didn't, I'd rather have a little louder mill than no mill!