Author Topic: Help with lapping blind holes  (Read 2292 times)

Offline Learnerplates

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Help with lapping blind holes
« on: May 29, 2022, 01:07:37 PM »
Hey guys,
My first post- Iím on my phone so excuse the crappy drawing.

I need to lap a blind, very flat bottom angled hole in the end of a 6mm pin.
It needs to be polished to about 1200 grit.

Iíve tried so many ways to get this right.
Iíve tried valve grinding past with an aluminium lap i machines first, and itís pretty aggressive.

Iíve ordered some brass too as lap material.

I had an end mill ground with a 14 degree angle but it needs cleaning up,
And even after the end mill thereís a small nub in the middle of the bottom of the hole.

Can anyone advise how they would attack this?
Thanks team.
Learnerplates.





Offline Jo

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Re: Help with lapping blind holes
« Reply #1 on: May 29, 2022, 03:23:02 PM »
What is the "pin" made of. And out of interest what is it for? I can't think of anywhere on a model engine that would need this.

Valve grinding paste you buy in a motor factors is very coarse. You need to find yourself grinding grits in a number of grades, if you want 1200 grit then that is your finest to buy. It sounds more like polishing towards the end and rubbing compound will do that.

The lap just needs to be softer than the material you are lapping and you need to use sufficient honing oil both to hold the grit and to act as a lubricant.

Jo
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Offline Jasonb

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Re: Help with lapping blind holes
« Reply #2 on: May 29, 2022, 06:45:52 PM »
As Jo says car valve grinding paste is much too aggressive, for lapping I use Silicon Carbide powders down to 1000g but they do also make 1200grit. Mix the powder with a drop of light oil to form a paste

Look at some of the woodworking tool suppliers or stone polishing suppliers who often do small sets of grits, work down from 320g to the 1200g

ideally have a short parallel section at the bottom of the hole so the lap can go all the way in making contact with the taper but not the end of the hole.

Offline Learnerplates

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Re: Help with lapping blind holes
« Reply #3 on: May 29, 2022, 11:23:02 PM »
Hi guys.
Thanks for the tips, off to the lapidary shop today to get some gear.

The steel will be O-1 or 1045 and hardened and tempered afterwards.
Itís actually not for a model engine, but I did make a small sterling once years ago, and from my adventure into that I just knew that small engine makers such as yourselves are not only a wealth of knowledge and advice, experts at work arounds, and genuinely very nice people. (Old timers like us I suppose.)

Itís actually for a Swaging press to swage down lead and copper parts.
I tried making a D bit, but alas without a Dbit grinder itís hard to get real nice results.

Thanks team ill
Continue to have at it.
Iíll try to make a brass or alloy spring lap today and see how we go.
There is some JOSCO metal buffing compound in various grades available locally. Wonder if that would cut the steel/lap in ok? Iíve attached a link.

https://www.josco.com.au/product/brumby-fastcut-cutting-compound-grey/

Offline Jo

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Re: Help with lapping blind holes
« Reply #4 on: May 30, 2022, 07:42:51 AM »
If you can turn up the D-bit on the lathe and cut it to an accurate 1/2 distance, provide the backing off etc, then a fine hand whetstone will enable you to get a suitable smooth finish on its cutting edge.

When lapping rather than using a "dry sticky lump" (iron oxide combined in tallow or the like) which is what buffing mop polishing compounds are, you need your abrasive suspended in a fluid. You also need to be able to clean it off regularly and put on some new abrasive and oil. I was thinking car paint cutting compound or possibly a household abrasive surface cleaner for the final polish.

Jo

P.S. If you try using a buffing mop compound it is worth knowing they clean off with hot soapy water and not the sort of things you would use to clean up oil based residue.
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Offline Jasonb

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Re: Help with lapping blind holes
« Reply #5 on: May 30, 2022, 10:45:39 AM »
You will be there for ever with paint buffing compound and it's far finer than needed, the OP wants 1200g which is still in wet sanding terretory eg. wet and dry paper, compounds start around 4-5000grit. So the you want a fine abrasive like you find on W&D paper.

If you did want a polish then a chrome polish like Autosol is easier to charge the lap with

Offline Learnerplates

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Re: Help with lapping blind holes
« Reply #6 on: May 30, 2022, 11:40:23 AM »
If you can turn up the D-bit on the lathe and cut it to an accurate 1/2 distance, provide the backing off etc, then a fine hand whetstone will enable you to get a suitable smooth finish on its cutting edge.

When lapping rather than using a "dry sticky lump" (iron oxide combined in tallow or the like) which is what buffing mop polishing compounds are, you need your abrasive suspended in a fluid. You also need to be able to clean it off regularly and put on some new abrasive and oil. I was thinking car paint cutting compound or possibly a household abrasive surface cleaner for the final polish.

Jo

P.S. If you try using a buffing mop compound it is worth knowing they clean off with hot soapy water and not the sort of things you would use to clean up oil based residue.

I did try and make a D-Bit, i'll try again with O-1 and harden it after I finish it.
Just wondering, would you cut the entire flat area as one flat angle, or would you relieve one side of it at 5 degrees or so. I'm just not sure on the geometry for the end of it... (The blind hole section) thinking the approach would be with the tail stock?

Offline Jo

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Re: Help with lapping blind holes
« Reply #7 on: May 30, 2022, 01:02:52 PM »
The purpose of the clearance or relieved angle behind the cutting face is to provide somewhere for the material that has been cut off to go rather than have it rub on your newly cut surface and scratch it. It is not needed as much nearer the centre so this is often not cut back. An angle of 5 to 10 degrees is used for cutting steel but no more as it is thinning the steel and this increases the chance of it getting warm/hot and then it will loose its hardness and quickly its cutting edge.

You can also look to change the angle of the front or cutting face - a negative rake will improve the cutting performance but you are again reducing the thickness of the cutting point and risking it getting hotter. A 90 degree face will work but cannot be used as quickly.


I just leave my D bits without any angles.

Jo
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Offline Jasonb

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Re: Help with lapping blind holes
« Reply #8 on: May 30, 2022, 01:15:11 PM »
If you drill a pilot hole first then the end of the cutter is not actually doing anything. just make the cutter to the taper and cut to half thickness, any angling of that flat face will alter the taper angle cut.

You are really making a taper reamer so only side cutting needed if drilled first, although this is a through hole you can see I pilot drilled first and also added a pilot to my reamer but it is the sides of the half round section that are doing the cutting.



lapping the spindle into the hole with 1000g silicon carbide powder mixed with oil


Offline Learnerplates

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Re: Help with lapping blind holes
« Reply #9 on: May 31, 2022, 01:55:45 PM »
So I got a 2 flute end mill ground to the nose angle and it worked fantastic.
Only issue is I should have used a n 8mm end mill because the 6 mm seems to push the edges out and burn them rather than cut.

My O-1 just arrived so Iíll have another go at a tapered reamer, and drill to depth first.
Seriously considering buying a D-bit grinderÖ 1500 bucks thoughÖ ouch.
As for lapping I used a wood dowel with a split and ever increasing fine sandpaper to good effect.
Going well so far!

Iíll have another geometry problem soon.