Interfering with fits?

Hello all, I could do with some help

I have a wheel from a belt sander I've been rebuilding, however, the wheels bore is too large and not concentric with the outside surface so I get about 10 thou runout at best after I've shimmed it. Not great when it's spinning very fast.

Video of runout

I'd like to bore the center out, then plug it and rebore it properly so everything is concentric.

I have a slug of around 40mm aluminium just the right size to do so, but before I jump in and start making chips I just want to ask about the fit. The motor shaft is 24mm so the hole will be reamed to that at the end.

I've looked in the zeus book about what to do, but having never done it before I could do with a second, third or more opinions.

If I want the plug to press in and never come out again, what sort of dimensions should I aim for?

Would it be better to drill out the majority of the center of the plug as well?

Will the center being removed after effect the fit?

Thanks

Link to album of building the belt sander

Just to get the ball rolling, in this situation Loctite is your friend.

Tony

I agree, make the joint with 0.001 clearance degrease both halves and use Loctite 601, 638, 620 or the equivalent. Leave the joint 24 hours and you are good to go. If you ever need to disassemble the joint, heat the assembly to 300C to destroy the Loctite. A turned finish, not polished will give the best strength of joint.

Edited By old mart on 15/08/2021 20:28:26

You may also want to bore the plug to fit rather than ream, the average reamer is usually H7 or H8 which is a plus size which may be OK for a running fit like a bearing but could be a little loose for your pully.

I have to agree with Loctite,

I'd go for a .001/2" thou clearance, as suggested by Old Mart,

Be sure to true the bore, after drilling, with a boring bar, before reaming.

Jim

Is it a solid wheel? If so, shrink fitting should be easy as the coeff. of expansion of Aluminium is quite high.

I think the general mismatch in fit is about 2 thou per inch diameter (0.02mm/cm diameter). Has to be a quick fit, I expect.

Machined finish, degrease, and loctite. Bore hole to size, no reamer ! Noel

If you go with an engineering adhesive (Loctite-type product), rather than listening to numbers people pluck out of the air or that emerge from other parts of their body, download and read the manufacturer's data sheet for the particular adhesive you propose to use.

Problem is they don't seem to give you what gap to use, at least Loctite don't seem to in there technical data sheets. nor Truloc could you point the OP to where he should be looking.

Edited By JasonB on 16/08/2021 07:50:11

Hi, if you go to this page of Loctite Products and click on the product name in the list, not the download, and scroll down to technical information or use the headers, the maximum gap is shown, but it is also wise to look at the how to use information. With Truloc, look at max. gap filling ability.

Regards Nick.

Edited By Nicholas Farr on 16/08/2021 08:26:25

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There is a hint on that Loctite data-sheet … Products are often engineered to perform optimally under standard test conditions … so, in this case ISO 10123 is probably relevant.

MichaelG.

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As an aside, this is copied from one of my February 2014 posts

here is a very useful Design Guide from PermaBond

Nick although they both show the max gap it can fill that does not really answer the OP's question of what gap to leave, I'm sure most of us here would not go for a 0.25mm or 10 thou gap.

Likewise the OP may not really know what parameters to design for so was asking for practical info on what sort of size to make his hole.

Several members gave a suitable allowance yet were said to be talking out of their ............... by someone who said to look at documents that don't give the info needed so more likely to be talking out of theirs.

For the benefit of those who may flinch at the cost of purchasing ISO 10123 … here is a good summary of the relevant test: **LINK**

http://www.adhesivestoolkit.com/Docs/test/MECHANICAL%20TEST%20METHOD%201%20-%20Continued.xtp

MichaelG.

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Yes, I’ve posted it before.

Hi JasonB, I understand what you are saying, I don't know the answer to the actual question. The solution depends a lot on the design and parameters of the application and the exact material used. I don't know about Truloc, but Loctite do a service for such things, but of course that is likely to have a cost to it, which is probably beyond our hobbies pockets, but in the commercial world would save a company many thousands of pounds in lost production due to downtime caused by failures, so the OP can only go on guidance he is given from those that may have experience and follow what he thinks best but keeping within that stated in the technical documentation or do some subjective testing of his own. Myself, using the info and making a judgement of the gap needed has always been successful. As the OP seems to be using Aluminium, from the product list in the link I posted, 6300 may be worth considering.

Regards Nick.

I would be interested to know where the runout comes from in the first place. Maybe the bore is accurately concentric with the OD but a loose fit on the motor shaft also depending on how the pulley held on the shaft may throw it off centre (grubscrew/keyway). What does the bolt on the centreline do?

If the pulley is a solid lump of ali I would be tempted to put a conical bore in it to accept an off the shelf 'taper-lock' bush.

Ian P

Not sure I explained clearly. The final motor shaft hole will be reamed, but the hole for the plug will be bored. So the running fit will be fine for me.

I'd like to stay away from loctite and try to do an interference fit. I think it'll be more challenging to get things to the right size, then if I mess it up I can do loctite, although I currently only have loctite 271 which is thread locking.

It is a solid aluminium wheel.

So, if the plug and hole will be around 38mm, then the plug must be about 0.08mm larger than the hole?

Alternative suggestion - if the wheel fits well on the motor shaft and doesn't vibrate too badly as-is, then clamp the motor to the bed of your lathe, align carefully, spin it up and true the face and edge of the wheel as an assembly. Speed may not be ideal but you are only after a skim so proceed with caution and should be OK?

(yes there are hazards to doing this but probably similar to building and using the thing!)

The wheel was initially machined by a technician at the college I went to. I'm not even sure the college had a boring bar at the time so I think he just drilled it. The outside was turned on a mandrel but I believe the mandrel was not a tight fit in the hole, so the outside wasn't turned correctly. It really is just the center hole being oversized that has caused issues.

The bolt on the centerline holds the wheel on the shaft. I'm sure a setscrew would be better, but I don't think I have the right set up to try and do that.

The taper lock bush is also a good idea. It looks like the standard ones for 24mm motor shafts have a dia of 57mm so that could be a third option if the other two fail. Once again I don't have access to a milling machine so drilling the half holes may be troublesome.

38mm is 3.8cm. 3.8*0.02 = 0.076mm, which is 0.08mm to two significant figures.

Coeff. of exp. for Aluminium is given as 21*10^-6/K (but likely 10% higher) so a 100K delta would provide a very mild interference fit. You should be able to attain a delta T of 250K by heating the wheel and deep freezing the bush.

It requires the parts to be machined parallel and fitted together quite quickly as the conductivity of Aluminium is nearly ten times that of steel.

Never tried Aluminium myself, but have often shink-fitted steel parts.