Best engineering practice to fit parts together?

OneTenner

LE
Book Reviewer
Is there any chance that there's a ridge (manufacturing defect or caused by you interference fitting) inside the cast part? It wouldn't need to be much to stop a tight interference fit.
From how you described having to get the old part off I'd guess it's more interference than fit as it were. May well have been hydraulic pressed on in the first place.
I have a small hand operated hydraulic press for removing drive shafts from hubs, it's rated at 10 tonnes, there is a 'HD' version rated at 25.... The amount of (controlled) force required to put the two components together will likely exceed several blows with any size hammer, concentrated, irresistible force with a press is likely to be the original assembly method (possibly with the application of liquid nitrogen to the steel component), can the manufacturer give any guidance on replacement? after all, they - I assume - sold you the replacement part...
 

OneTenner

LE
Book Reviewer
My little problem was about 40mm diameter!
My set goes from 3mm - 50mm nominal, mostly used on white metal replacement gearbox bushings
 

anglo

LE
Ladies and gentlemen of Her Majesty’s Royal Electrical and Mechanical Engineers (or anyone else reading the thread with ideas!), I seek some advice please.

I have a part for a kitchen mixer that I need to replace. This is the planetary gear assembly. There is a shaft coming down from the gearbox, that goes through the main mixer housing, and then into a receptacle in the planetary assy.

View attachment 589480
The problem is that it appears to be an interference fit. As shown in the pic above, the shaft will go no further into the receptacle without doing something to either force it, or ease the fit. It has to go about 3/4” further in, so that I can insert the locking pin through the holes.

The receptacle is just cast into the mounting plate:

View attachment 589479

A few things have occurred to me:

1. Heat the receptacle hole with a heat gun. Tried that briefly last night, didn’t give me much of an improvement, maybe 1/8” further down. Also need to pack the gears with grease, and heat would be an issue there.
2. Press them together. I have a hydraulic press, but unless I get the holes aligned before pressing, I’m in trouble. No idea how to ensure the holes are exactly aligned.
3. Ream out the receptacle. My guess is it would require a very specific drill bit that I don’t necessarily have.
4. Reduce shaft diameter. I’m reluctant to do that, because the shaft is obviously machined, so is probably accurate. The hole is more likely the problem, at least to my eyes.

I could just get to it with a file or Dremel, but I don’t want to end up with a slack fit and it wallowing out.

I’m guessing you chaps spent many hours in SEME and then in the field doing exactly stuff like this, probably in the pissing rain under an MK in 3 inches of mud. So I’ve got it easy, that is recognised.

Virtual Yeller Handbag on offer to the DS solution here.

Thanks in advance!
Measure the shaft diameter and the bush diameter,
if the shaft is actually too big, it will smash the casting if fitted,
ream out the bush, so you can slide the shaft in,
it doesn't have to be a perfect fit
Do not use force or it will bugger it up
if you've measured it you will know how much of the bush internal
you have to remove
 

Joshua Slocum

LE
Book Reviewer
I had to do the same when I fitted a new motor to my cement mixer, the pulley would not quite fit
so I found a bit of dowel, some fine production paper, and tickled it in the drill for a few seconds until it fitted
a round file wrapped with emery will do the same
stuff my dear old dad taught me
stuff he was taught by the lads who had been right in the line of fire and needed to find a solution
Some of those old motorcycle and car books have useful tips
one I can recommend is the Vintage Motorcyclists workshop,by RADCO it shows all sorts of little tricks and dodges, yes their are often proper ways to do the job or special tools, but if only doing it once, make a simple cheap tool up
 

OneTenner

LE
Book Reviewer
Probably dont need NASA's engineering works to repair a food mixer.
Yer, NASA were great at machining things to tolerance and fitting seals....
 

964ST

War Hero
Heat the housing and freeze the shaft as previously said and extreme force, then hope you can mate the securing pin.
 
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RTU'd

LE
This holds the Army together.
1626553347620.png

Standard issue by the QM in REME Reserve Units
 
Sorry for delay in reply here, been out all day. Did not manage to obtain plumber’s freezing spray. If I need to resort to that, then it’ll be Monday and a proper plumber’s merchant rather than Home Depot.

My digital calipers have taken a shit, the display is smashed, so all I have is a dial caliper, which goes to 0.01” accuracy. The shaft is 0.54”, the hole goes to 0.55”. So it should fit, but doesn’t.

Good earlier point about pressing it in, the gearbox would take the force, so that’s a no. Same with freezing, I’d probably have to freeze the whole mixer, and it has the control board under the motor. I’ve no intention of freezing the electronics.

Given the small difference between shaft and housing, I am tempted to try @Joshua Slocum’s idea on the old one, and see if that makes it sloppy.

Cheers for all the help so far!
 
Couple photos.

3E976A9B-09CB-43F2-B3BE-91350E9C1748.jpeg


Old one, showing the crack in the casting that prompted the replacement.

New one, now I have the camera on it, there is scoring in the housing, so there must be a small burr on the shaft that I’ve caused this scoring with.

9BCB0446-EE61-456F-AC5C-E467D201FB8E.jpeg


Both old and new measure 0.55” ID.
 

OneTenner

LE
Book Reviewer
0.25mm is quite a bit to force in (ask the Mrs.) My concern with opening it out with anything but a reamer is that it won't necassarily be equal or straight. Think how the planetary gear will move around if the shaft is not perfectly aligned in relation to it. Possibly liquid nitrogen will shrink the shaft enough, with fitting lubrication. If it were me, I'd be reaming it out to size (but I already have the kit to do it).
There's no reason not to use abrasives, if you want it to last, I'd recommend machining.
 
0.25mm is quite a bit to force in (ask the Mrs.) My concern with opening it out with anything but a reamer is that it won't necassarily be equal or straight. Think how the planetary gear will move around if the shaft is not perfectly aligned in relation to it. Possibly liquid nitrogen will shrink the shaft enough, with fitting lubrication. If it were me, I'd be reaming it out to size (but I already have the kit to do it).
There's no reason not to use abrasives, if you want it to last, I'd recommend machining.


Well I think I may have found my culprit. Seeing the scoring in the housing made me think there’s some small burring on the shaft, because the measurements say it should fit. Looks like there is, right on the end.

1A965C34-46A5-4C84-A7DB-6CB51987DD69.jpeg


So I think I’ll clean up the shaft with some 400 grit paper, and perhaps run my my Dremel round the housing a couple times with the sanding sleeve, but not actually turn the motor on, just a “round sanding”.

But that‘ll be tomorrow. Quite fancy a beer or three now, not farting about with a mixer :)
 
You begrudge spending $40?
Which part of Scotland are you originally from?

If your repair isn't perfect, you do realise when you turn it on you'll break some other part, thus adding to the cost of replacement parts you'll need to buy.

Also, spending $729 in the first place on a food mixer is only something a woman or someone good with colours would do.

Of course I don’t begrudge spending $40, I just want it to work.

They’re $729 to replace it like-for-like today. And I paid less than half of that 8 years ago. Fcuk that, if I can fix it for $40, some swearing and (shared) ingenuity!
 
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OneTenner

LE
Book Reviewer
Well I think I may have found my culprit. Seeing the scoring in the housing made me think there’s some small burring on the shaft, because the measurements say it should fit. Looks like there is, right on the end.

View attachment 589608

So I think I’ll clean up the shaft with some 400 grit paper, and perhaps run my my Dremel round the housing a couple times with the sanding sleeve, but not actually turn the motor on, just a “round sanding”.

But that‘ll be tomorrow. Quite fancy a beer or three now, not farting about with a mixer :)
Enjoy your beer and tomorrow run a file around it, no more than 3mm down from the chamfer, I did see some scoring on your original photo but it wasn't clear if that was pre or post 'insertion'
 
Just to close the loop on this; it’s now fixed. It turns out that it was indeed that little burr on the shaft, and the scoring it caused in the bore. I filed off the burr, and went round the bore with a bit of scotchbrite. It was then a tight fit rather than an interference fit. Bit of fiddling to ease the shaft into the bore (oo er!) and all is well in the world.

Thanks to all for their help!

AB90D189-6E5A-401D-980B-9F019B1D56DA.jpeg
 

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