Interfering with fits?

Very tired today, but progress has been made and I am very happy once again.

I started off looking at what tools I had already for the lathe. There was a quarter-inch square boring bar/ keyway cutter that I purchased last summer in a job lot, which I had almost forgotten about. There was just enough HSS left to grind down the opposite end of the tool into the 3mm cutter I needed.

I didn't bother grinding it to exactly 3mm as it was getting hot and I was getting impatient as it had taken a while. Then from the bench grinders fine wheel I took it to my tormek whetstone grinder and finished it off on there. I ended up with a 3.3mm cutter. Not the easiest to take pictures of, The camera makes it look rougher than it was. Most importantly, it was quite sharp and relieved on the front and sides.

Then I put the tool into a tool holder and set it to close enough to center. I did this by eye and looking at where the scratches were in the bore. Once the scratches were even I stopped.

I started off by taking smaller passes, especially since the full cutter was being used in the center pass. Starting with 1 thou depth of cut, although once I was a bit more confident I tried some larger cuts up to around 5 thou.

I did find that I occasionally got a bit of deflection, so I did more than one pass on 1 depth to ensure it was cutting parallel to the bore, this seemed to work fine for me.

Once the first slot was to depth, I used an indicator to move the tool up 2.4mm from the center. Then I cut another slot. This slot was smaller so I attempted a few heavier cuts of 7 thou or so, but generally, I tried to just do 2.5 thou cuts.

I then lowered the tool and cut the other path. I found this to be easier than cutting two slots on either side and then taking out the middle. This way I started in the middle, just my preference.

I can't imagine how much I would have struggled if I hadn't upgraded to a QCTP this summer. Thanks again to Phil for his help.

I found this to be quite enjoyable. Definitely not as quick as broaching the hole, but much lower cost (zero in fact) and the satisfaction of doing it myself.

I thought the chips pinged off quite nicely.

I deburred everything with a file and deburring tool, then I attempted to fit the wheel. It fit perfectly!

I stuck an indicator on the flat face(about where my thumb is in the picture above) and the runout is now 2 thou, a significant improvement compared to the 10-20 depending on shims before.

The circular face is now 1.5 thou out of round, compared to the 10 thou before. Also a significant improvement.

I turned on the motor and put it up to full speed, the vibration has gone. It's honestly like a new machine, it works a lot better. And the belt seems to wobble a lot less.

Opinion time-

I am tempted to turn a mandrel and remount the wheel and skim the outside face to true it to the bore. Is this likely to make it better, or am I more likely to do worse than the 1.5 thou already achieved?

I am also considering crowning the wheel to the same as my tracking wheel so the belt sander *should* track better.

I will also be adding the "braces" of the grub screws at some point once the tuning is done.

Thank you to everyone so far for your help. If only I had made the wheel myself originally and enquired earlier!

An alternative to cutting a key way would be to mount the "Wheel" onto the shaft with as close a fit as possible, and then to drill and tap two diametrically opposed holes on the interface between the parts, and screw in grubscrews to act as "dutch" keys. Grubscrew length, ideally should match the thickness of the "Wheel"

For good measure, use Loctite or a similar anaerobic locking agent on the dutch key grubscrews so that they are unlikely to come looses.

Howard

That turned out well and a good example to a certain Centec owner on how it should be done

I’m not sure that he will be seeing this!

No bodging here, and no youtube either... This may have been quite a challenge for me, but that's why I've found it so enjoyable.

Today's progress should finish the job completely (as far as the wheel is concerned. The belt sander still needs fixing up somehow, but that's a problem for another day and thread)

The day commenced with turning a 24mm mandrel to remount the wheel onto. I make sure it was a very tight fit with no slop. Then I double-checked the runout on the outside to make sure it was the same runout that I'd seen on the motor shaft. This meant that it was accurate enough and taking a cut was likely to result in a better concentricity, so I decided to take the plunge and skim the outside.

After skimming the outside I decided to put a tapered crown on the wheel to *hopefully* help the belt sander track a bit better. This went quite well, just like when I did the same thing to the tracking wheel.

Next step was to create the "dutch keys" for the 'braces' half. I put some layout fluid on the surface where I thought the holes needed to be, then I marked the holes and drilled to 4.2mm, ready to tap to M5.

Once tapped and deburred, I dropped a blob of locktite 271 onto each of the M5 grub screws then tightened them down.

Howard, I understand you were saying to use long grubscrews or as long as the thickness of the wheel. I just used what I had lying around already, It's better than nothing. I just hope they're not needed at any point. The shrink-fit will hopefully do enough and the belt sander won't have a hard life anyway.

With that completed, the wheel was remounted onto the motor. All finished.

I checked the outside for the concentricity and it looked to be about 1.5 thou out. This may not be perfection, but it is significantly better than before. Plus, the vibration is practically gone, which is exactly what I wanted. No shims are required either...

Thanks to everyone for their help, I'm very grateful for your knowledge.

Perhaps the centec owner should read this and see how easy the proper fix is!

Result!!

Cheers Phil, I hope the tractor restoration is coming along well. I still need to catch up on last week's videos!

I'm just placing an Arc order since there's 5% off. Just for fun, I looked at the price of the tools to broach an 8mm key in a 24mm bore (not the complete broach kit). It came to a total of £70 or so! I think I'll stick to my current method...

Meanwhile, I think it's about time I cleaned my lathe. The belt sander build has involved a lot of turning, so it's time to tidy. I suspect I may have another leak in the lathe to solve too...

Hi Richard, sorry for the non subject related question, I had an account with arc for over 2 years, did you receive a 5% off code through email? I can't see anything on their site?

No worries Pete. Yes, I received it through email. The code is ARC821-5

It's 5% off all accessories until the 30th of August 2021

Thanks

Thanks you very much Richard.

Looking back through some old M E s came across a formula to aid arriving at the correct interference.

A = (1.00625 D) + 0.5 where A is the interference in Thous, and D is in inches.

Presumably A would be microns, and D would be mm if working in Metric units.

Seems a bit large to me, being overcautious.

0.001" on 1.000" steel into cast iron seems to work OK for me, so maybe 1/1000 on diameter might be an adequate rule of thumb..

Anyone like to confirm?

Howard