Replacing Gears - CH-10M mill/drill

I’ve succeeded in repairing the transmission of my old CH-10M mill/drill and thought it might be instructive to others if I shared some of my experiences gained during the process.

As already reported in a previous post - **LINK** - I had the misfortune to overtax this unit and tear out the teeth of the low end of the two-speed drive. In the picture just below you will see the two original plastic gears in somewhat sorry condition. Behind them will be seen a tub of grease which some previous owner had stuffed into the transmission and which I had to dig out of there to even see what I was doing. What a mess that job was. This is perhaps 80% of the grease, the rest being smeared over an assortment of rags and paper towels in the trash bin. The transmission casing had never been designed (nor sealed) for that amount of lubricant and the mill had been constantly weeping and flinging gobbets of grease during operation ever since I purchased it. More on that later...

The plastic gears in these little mills is something of an Achilles heel, being prone to breakage and also very inconvenient to replace. Fortunately, there are metal gear replacements.

My original thought was to just use a metal replacement gear on the main spindle, that to be running against one of the plastic ones on the intermediate gear in the hopes of quieter operation. Too, should it break, the plastic gear on the intermediate shaft is much simpler to replace as compared to the drive gear on the main which entails a lengthy operation to pull the main spindle and bearing(s). But ultimately I decided that the effort of disassembling the mill’s head unit was something I would just as soon totally avoid in the future if at all possible and so opted for metal gears on both the intermediate shaft as well as main spindle. These metal replacement gears are the same ones as fit the old Sieg X2 Mini Mill and were secured from Arc Eurotrade.

The gears, upon arrival and inspection, looked quite reasonable. The intermediate gear had a bit of hobbing flash, but a few minutes of work with some needle files brought that round right. The main gear, however, proved to be a problem.

The original, plastic gear for the main spindle on the CH-10M is a one-piece injection molded unit. The metal replacement offered by AET is actually a three-piece stack-up arrangement of two gears with a spacer between. Two difficulties immediately presented themselves.

The first problem was that the total height of the three-piece, gear-spacer-gear stack-up was not equal to that of the original one-piece, plastic main spindle gear - measuring 2.25mm too short.

The second problem was rather more vexing. It turns out that main spindle shaft key on this unit is not quite as long as required for the metal replacements. The keyway length is not an issue at all if using a one-piece plastic main spindle gear as originally designed. Unfortunately, the three-piece stack-up arrangement of the metal replacements means that when assembled on the main spindle the topmost gear in the stack is doing little more than just kissing the rounded end of the drive key and would never be able to transmit driving power to the spindle.

Now, of course, the simple approach would be to mill the main spindle keyway a bit longer, slip in some longer key stock and ‘job done’… or at least, that’s what I would have chosen to do. Obviously, with my only milling machine in pieces this approach was out of the question. Instead, I opted to solve the two problems using my TIG welder and metal lathe.

My first task was to make up the the missing height in the gear stack-up. To this end I added filler metal to build up one end of the metal spacer.

The spacer was made from some unknown steel mix that seemed perilously close to pig iron and did not play nice under the heat of the torch - spitting and smoking most unpleasantly. Nevertheless, after six or seven passes around the rim I had built up sufficient metal after which I chucked the spacer in the lathe and turned it back down to the correct size - it now having the necessary 2.25mm extra length.

Edited By Ignatz on 09/07/2018 07:59:54

The next step was to join the spacer to the larger of the two main gears. I temporarily slid the spacer and larger gear onto the main spindle to line them up, and tacked them together. I then removed them from the spindle to finish the rest of the weld-up on the bench. You will see in the photo that I elected for a series of deep, spotting welds around the circumference of the spacer. I chose for this approach instead of a continuous weld seam in order to avoid potential distortion. I’m certain it is strong enough for purpose. The welds did pull the gear ever so slightly out of line, but a very light skimming cut with the boring bar on the lathe allowed the welded-up pair to slide back over the spindle.

The three-piece stack having been converted to a two-piece stack with the topmost gear driving the shaft through the welded-on spacer, the short spindle key was no longer an issue. So I proceeded to reassemble the transmission.

But what to do about lubricating the new metal gears? Easy enough to apply grease while putting it together, but what about later on when that original grease gets flung off the gears to leave them run dry? I didn’t want to jam all of that original grease back into the transmission again. So at this point I made one alteration to the mill transmission housing. I drilled and tapped a hole (6mm threading) into the side of the transmission housing just in line with the intermediate drive shaft. A short bolt with a rubber washer does duty as a plug to seal the opening.

This hole serves as a handy lubrication port through which I can squirt discrete amounts of grease directly onto the intermediate shaft and gears as required. I really wonder why the factory never thought to include lubrication access in the original design.

Now it was finally time to reassemble the mill head and bring everything back to running condition. But again another fly in the ointment…

The transmission and main spindle case needs to be bolted to the mill head section that rides in the dovetails on the column. Naturally, the spindle must be aligned axially with the column, but sadly the two units are not keyed to one another and lining them up proved most annoying.

In his article, “Improvements to an X2 Mill”, Neil Wyatt refers to this procedure with the following:

“Before completing the setup, you need to refit the two parts of the mill head to each other. Careful inspection seems to indicate the upper surfaces of the two parts are the reference faces for their alignment.”

Well, that is how it is done for the old Sieg X2.

On the other hand, owners like me of the even older CH-10M will be most disappointed when they realize that there are absolutely no flat, clean reference faces on the two parts of the mill head to facilitate this operation. Obviously there must have been a jig for assembling these parts in the correct relationship to one another in the factory. But for the end user, out in the field, the re-assembly of these parts is a matter of some frustration.

My solution for lining up the two mill head sections was to mount my ER25 collet holder in the spindle. Into the collet holder went a length of hardened 8mm shafting from an old computer printer. The two parts of the milling head were then lightly bolted together, this assembly then being slid onto the column and locked in place

There is absolutely no provision for delicate adjustment of the relationship of the two parts one to the other, so it was then a matter of tap-tap-tapping the head this first this way and than that while holding a machinist’s square from the X-Y table against the length of hardened shafting.

Complicating this procedure is the fact that the bolts which secure the two parts of the mill head to one another are not accessible for tightening while the mill head is mounted on the column. So of course it took repeated iterations of tapping to what seemed to be correct alignment, pulling the mill head up and off to tighten the bolts a bit more and then sliding it back onto the column once more to again repeat the checking and tapping. I achieved what appears to be a reasonable alignment, but, please, don’t anyone ask me to hold a dial test indicator to the end result.

The mill is now completely reassembled and running (and rather nicely, at that). It will serve me for a while longer while I make a decision as to which newer, larger model will replace it

Good to know you're up and running again

Lionel

Lionel, see PM