Author Topic: Lapping Tool  (Read 658 times)

Offline petertha

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Lapping Tool
« on: March 13, 2018, 04:35:48 AM »
Here is my lapping tool adventure. I had to bring some 5mm (~0.197")O1 drill rod down slightly to an acceptable fit & finish. Its an idler gear axle for the radial engine I'm building. The OD of the lap is 5/16", basically to match a nominal drill size. The smaller hole is a stress relief & springyness guestimate. My plan was to kind of keep the same basic design & grow progressively larger series of tools for larger laps maintaining a similar dimension of the lap annulus thickness. Too thick & its hard to compress & get a feel. Too thin & its fussy to machine.

The lap is 6061 aluminum. I don't have copper & the idea was I would test this aluminum against brass & CI. I made simple slits in the lap ID with the scroll saw but forgot to put a fine metal jewler blade in there so its not the best. I tried to maintain a rougher OD finish to help with grip, but now I think that's completely unnecessary. The interesting thing about drill rod (at least the stuff I'm buying), is that it looks shiny & dimensionally quite accurate, but when you first touch the lap to it with least amount of pressure, you can actually feel some 'oblong-ness'. I pre-blued it with a felt pen to try & take a pic hopefully showing rubbed off hills, but the ink itself washed off. Anyway, after a few strokes which I think also conditions the lap, it feels constant. Just a matter of increasing pressure, clean, check, repeat. I bought a set of Aliexpress diamond lapping paste tubes & took a guess at the grit. It is quite creamy but a drop of oil makes it slide a bit better on finishing.

Offline petertha

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Re: Lapping Tool
« Reply #1 on: March 13, 2018, 04:46:16 AM »
The intent of the one-piece bar with a slit was to make machining setup easier. But the variable is how much resistance to closure results. Mine was actually too stiff & its only 3/8" wide mild steel. So the relief hole should be bigger. I wanted the lap to be centered because it feels balanced in the hand & on the part while lapping. I made another tool for my valve stems last year which is much more un-symmetrical, like a squeeze handle off to one side. It has lots of mechanical advantage so you can gently teak the pressure but it didn't feel balanced.

So this is my next iteration plan. 2 equal pieces of bar stock. Sandwich it up with some sacrificial filler say 1/16" thick. Bottom bar holes are threaded, upper bar holes are clearance hole. Then just put a bolt & locknut on one side & use the other side as the squeeze adjustment.

Offline steamer

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Re: Lapping Tool
« Reply #2 on: March 13, 2018, 12:01:53 PM »
Ther is a very good thread on lapping posted by Ramon.     An excellent treatise!

Dave
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Online Mcgyver

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Re: Lapping Tool
« Reply #3 on: March 13, 2018, 04:47:58 PM »
good thread.  AL will work fine as a lap, and no, one should not expect drill rod to be round! (or super close to the nominal dimension).  TGP and is the stuff for when you need round and straight and to dimension.

For lapping in the round, I most often do the same - loose abrasive applied between the lap and work.  As you've found, its quite sensitive in being able to feel differences in the roundness and dimension.  The problem is the lap itself wears away because of the rolling abrasive.  With this approach you needn't worry so much about the geometry of the lap, it is self correcting - this only works if the lap is 1/2 the length of the work or less.

The alternative is to "charge" lap then apply the charged lap to the work.  It just gives a level different of control and lap itself is never really touched (its a cutting tool and work touches the abrasive not the parent material).  Here the geometry of the lap is critical as the lap will not wear/change.  This is ideal for things like small holes (say 1mm) or a taper or other jobs where you can keep the lap moving over the work.  You can't lap a taper with loose abrasive because if the lap's geometry is changed by wear in one spot more than another, there goes the accuracy of the taper

« Last Edit: March 13, 2018, 05:34:30 PM by Mcgyver »

Offline petertha

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Re: Lapping Tool
« Reply #4 on: March 13, 2018, 06:13:18 PM »
Ther is a very good thread on lapping posted by Ramon.

Thanks & agree. I've already bookmarked his posts & absorbed!

Another interesting resource specific to aluminum as the lap tool medium is Rob Renz. He does some bigger sized ones in this vid, but the same methodology gets utilized in other projects.