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  1. #1
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    Default Need some advice on lapping OD

    The diameter is 4" the length is about 9" the material is CI.

    Questions:
    • Is aluminum the best lap material for this?
    • Any acceptable plastics or composites for the lap material?
    • How long should be the lap compared to the cylinder length?
    • How thick should the lap walls be? (assuming aluminum)
    • What is the best abrasive material for this? Carbide, or Diamond?


    Thanks

    dee
    ;-D

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    Hi Dee:
    Are you lapping for geometry or for finish or for both?
    How close are you to your goal before you begin lapping?
    How will you power the part or the lap? All by hand? Lathe and hand work? Lapping machine,?
    How many laps do you think you'll need to get to your goal for precision and finish?

    The rules of thumb as I understand them are:
    - Lap needs to be softer than workpiece; aluminum works for CI, so does copper and soft brass and lead and zinc and cerrobend, but I've never tried any kinds of plastics.
    - Harder laps are better for geometry but worse for finish.
    - OD laps for best cylindricity need to be at least one diameter wide...1 1/2 diameters is better, 2 diameters is better still.
    - if geometric fidelity is the goal, a long lap with independently adjustable ends will help you control hourglassing or barreling.
    - If you need a specific crosshatch pattern, a short lap can be helpful if you can't run the lap off both ends of the cylinder and you're handworking it.
    - Laps that compress using a cone ring are more accurate than laps with a single slit and clamp screw, unless you put relieving bores or slits into the circumference of the lap so it compresses into a smaller cylinder rather than an ellipse.
    - for cast iron; aluminum oxide is adequate, silicon carbide is the standard (so far as I know) and diamond is overkill unless you have a specific requirement that can't be met without it.

    That about drains what I know of lapping...hopefully there is something useful for you in all that babble.

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining

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    I knew I was going to generate questions



    Quote Originally Posted by implmex View Post
    Hi Dee:
    Are you lapping for geometry or for finish or for both?
    How close are you to your goal before you begin lapping?
    How will you power the part or the lap? All by hand? Lathe and hand work? Lapping machine,?
    How many laps do you think you'll need to get to your goal for precision and finish?

    The rules of thumb as I understand them are:
    - Lap needs to be softer than workpiece; aluminum works for CI, so does copper and soft brass and lead and zinc and cerrobend, but I've never tried any kinds of plastics.
    - Harder laps are better for geometry but worse for finish.
    - OD laps for best cylindricity need to be at least one diameter wide...1 1/2 diameters is better, 2 diameters is better still.
    - if geometric fidelity is the goal, a long lap with independently adjustable ends will help you control hourglassing or barreling.
    - If you need a specific crosshatch pattern, a short lap can be helpful if you can't run the lap off both ends of the cylinder and you're handworking it.
    - Laps that compress using a cone ring are more accurate than laps with a single slit and clamp screw, unless you put relieving bores or slits into the circumference of the lap so it compresses into a smaller cylinder rather than an ellipse.
    - for cast iron; aluminum oxide is adequate, silicon carbide is the standard (so far as I know) and diamond is overkill unless you have a specific requirement that can't be met without it.

    That about drains what I know of lapping...hopefully there is something useful for you in all that babble.

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining

    So Qs and As
    1. I am going for round and straight, so that is for geometry
    2. I am already withing 1 thou round and within 2 tenths straight
    3. Planning to power it on the lathe and handhold the lap
    4. How many laps do you think I need? I do not mirror finish, can I get away with one?


    on the practical side making an 8" long lap is harder than a 4" lap. I assume I will bore the lap on the lathe. I can drill relief holes along the perimeter, so it will conform more. On let's say 6" lap is having an adjustment screw on either end would be sufficient, right?

    Tanks for the helpful answers


    dee
    ;-D

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    Dee, Marcus mentioned a bunch of great tips, but here's my $.005:

    1) If making a full cylinder with an internal working diameter, then you should either cut the OD so it's eccentric to the slit/clamp area, or saw a number of longitudinal slots radiating from the inside out. Slots closer to the clamp can be a little deeper, opposite the clamp a little shallower (replicating the stiffness gradient of an eccentric OD to get uniform collapsing).

    Inside slots also allow debris relief, bandsaw cuts are fine. You can cut relief slots on the outside of the cylinder, but then you lose the debris clearance benefit.

    Cleanliness! New abrasive, thoroughly cleaned part, fixturing, and lap if you need to stage grits with the same tooling.

    Your adjustment screw Q - think of it as simple support of a beam for minimal span deflection (similar to a surface plate mount). So roughly 1/5th the way in from each side for clamping screws. Also, add jack screws to allow you to control the clamping, and at least three (outside the two clamp screws, one in the middle). By playing with these you can also adjust for over or under cutting along the span (a bit).

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    Hi again Dee:
    You wrote:
    "I knew I was going to generate questions"

    Sorry about that, but that's what I always do when I'm confronting a problem I don't immediately know the answer to...it helps me to frame the essence of the question so I can at least try to find a way forward in my head and answer in a way that will be useful.

    Of course, for a project like this, there's really no substitute for trial and error, so take all of what I've said with due caution.

    Having said that, my own analogous experience was lapping case hardened mild steel that had been cylindrical ground but wasn't as accurate as I wanted it.
    It was for a column on a home built cutter grinder back in the late 1980's and I learned what I know of lapping at that time, mostly from reading of the experiences of others but also by taking chances on it, building the laps and finding out what worked for me.
    I had three contact pads on the cylinder, and a big anti-rotation flat to make as parallel as I could so I could run the head on the column with as little clearance as possible.
    So my task was not quite the same as your task is, because my goals were different. and I built different laps to accommodate my needs but I learned something useful from every fuckup and I did make a few before the project was done.

    However I've lapped lots of cylinders and bores since then; the latest was posts for running a bronze bearing on that had to run with a couple of tenths clearance, and the starting stock was dowel pins which are neither round nor straight when you buy them from the store.
    So I don't have exact experience, but I've solved similar problems and my little doctoral dissertation in post #2 was kind of a synopsis of what I've discovered about the process over time and trials.

    So forgive my impertinence in asking loads of questions...it's just my way of framing the issue.

    With regard to your specific questions in post #3 here's my take:
    If your goal is better than 1 thou round and you are already very straight, the length of the lap will be less important so long as it's long enough that you can't twist it on the workpiece and make some areas smaller in diameter than others.
    Since your lap to workpiece clearance is relatively small, you can accept a shorter lap; partly because your straightness is already very good, so you won't be lapping on a banana and so the lap doesn't need to straighten the geometry; it only needs to turn a cone into a cylinder; and a shorter lap can do that without much risk.

    Even at one diameter long; your ability to twist the lap will be restricted by the clearance between lap and workpiece...you won't have the physical strength to fuck it up.

    With regard to how many laps; well that's purely a function of what finish you need to achieve, because you need a new lap for every grit size and you can't just clean a coarse grit lap and put in finer grit.
    One leftover coarser grain in the wrong spot and you'll scarf up that beautiful finish you're developing and all will look like a pig's breakfast in an instant.

    For my big lapping job on the column I bought 150 grit, 320 grit and 800 grit Clover Compound (silicon carbide in grease for lapping in automotive valve seats).
    I wasted my money on the 800 grit because I could lap for a bloody week with it and not change the part by a tenth over the surface area I was lapping (3" column 14" long).
    I probably got within a tenth or two with the coarse grit and lapped with the 320 only out of a sense of masochism and a desire for "perfection" (I was very young at the time!).
    The result was better than I could measure and it all ran so silky smooth when assembled, that I have to keep the head at temperature so it doesn't bind on the column.

    With regard to the adjustment screw, it matters more depending on how far off the end of the workpiece you intend to run the lap.
    The risk is barreling the workpiece, and the problem worsens as the lap is shortened.
    I was hand lapping with no power at all, and I couldn't reverse the lap (asymmetric geometry) so the adjustment screws helped me a lot.
    For your task they may or may not be worth it because you can just reverse the lap as you need to and you can also run off the end of the part a bit before you reverse the stroke.
    But they're simple to put in so put in 3 screws; one at each end and one in the middle...use any or all of them as you see fit.

    For such a big diameter lap you may also need to relieve the bore a bit, leaving only a few lands to actually contact the workpiece.
    There's so much surface area potentially in contact you may not have the strength to hold the lap when you turn on the lathe, especially once you introduce grit into the interface.
    Put handles or knurls on the lap but make sure they're still small enough that you can let go if the lap grabs and that you can't get pinched between the handles and the lathe bed or the carriage.
    Take all the tools out of the toolpost and tailstock so you can't gore yourself on a pointy bit.
    Wear a short sleeved shirt!

    Lapping is normally pretty safe but you can hurt yourself pretty good if you are not on top of it and big diameters are quite a bit more dangerous than small ones when you're lapping in the lathe.

    Last point; you want it all sloppy with lubricant...it's going to make a godawful mess and it's abrasive as hell so protect everything including the chuck jaw joints with masking tape before you splatter shit everywhere.

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining

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    Robin Renzetti on Youtube (ROBRENZ) has a video "making and using large diameter laps"

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    Quote Originally Posted by implmex View Post
    Hi again Dee:
    You wrote:
    "I knew I was going to generate questions"

    Sorry about that, but that's what I always do when I'm confronting a problem I don't immediately know the answer to...it helps me to frame the essence of the question so I can at least try to find a way forward in my head and answer in a way that will be useful.

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining
    Sorry for that bit of sarcasm . I meant no harm. Don't worry you have been more than helpful. Thanks for all the good advice.


    dee
    ;-D

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    Quote Originally Posted by jcoehlo View Post
    Robin Renzetti on Youtube (ROBRENZ) has a video "making and using large diameter laps"
    Yeah, I saw that. That was the piece of inspiration that got me started on this. Except this is OD, not ID.

    dee
    ;-D

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    Hi again Dee:
    No problem...I did catch the gently eye rolling sarcasm and understood it for what it was.
    I didn't see it as anything at all for me to get bent about; but I grabbed the opportunity anyway, to babble a bit about why I did the interrogation in the first place.

    Some (but certainly not you) seem to take personal offence when I or others try to clarify what they're asking help for by responding with a gazillion questions.
    We've all seen some impressively pissy pushbacks from some of the nut jobs who've come on here over the years demanding help, so I took the time to try to explain myself; not to poke back at you but to make the general case for why someone responding to a query might seek clarification or more details before offering advice.

    No harm no foul...I was not at all upset or snarked out.

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining

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    Quote Originally Posted by implmex View Post
    Hi again Dee:
    No problem...I did catch the gently eye rolling sarcasm and understood it for what it was.
    I didn't see it as anything at all for me to get bent about; but I grabbed the opportunity anyway, to babble a bit about why I did the interrogation in the first place.

    Some (but certainly not you) seem to take personal offence when I or others try to clarify what they're asking help for by responding with a gazillion questions.
    We've all seen some impressively pissy pushbacks from some of the nut jobs who've come on here over the years demanding help, so I took the time to try to explain myself; not to poke back at you but to make the general case for why someone responding to a query might seek clarification or more details before offering advice.

    No harm no foul...I was not at all upset or snarked out.

    Cheers

    Marcus
    Implant Mechanix • Design & Innovation > HOME
    Vancouver Wire EDM -- Wire EDM Machining
    stop being so ....Canadian?... meant polite We are not very used to that south of 49.



    But honestly, you have been...lemme quote Tony, "grrrrreat". You guys confirmed what I was thinking, Now I have a plan I think I can execute.


    dee
    ;-D

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    A naive suggestion from an ignorant.
    It looks like the lap itself will be costly in terms of material and labor (e.g. large aluminum tubing, cutting internal reliefs, etc.).

    Instead of building multiple laps, one for each grit, wouldn't make sense building a lap with removable shoes, so that only the shoes need to be replaces between grit changes (or, taking inspiration again from Robin Renzetti, covering the shoes with aluminum foil and charging the foil with the abrasive)?

    Dee: if you need, I might have some alu tubing of the right diameter.

    Paolo

    PS Am I guessing wrong that you're building a cylindrical square?

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    Hello,

    I have close to 40 years lapping experience. ID and OD. There have been some good suggestions here and also some that are going to make your job very difficult. I suggest you contact either Helical Lap Company or American Lap Company. Contact them or a the very least visit their websites there is a lot of information there.

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    Quote Originally Posted by athack View Post
    Hello,

    There have been some good suggestions here and also some that are going to make your job very difficult.
    If any of my suggestions fall into the "difficult" category, please feel free to correct them here - I'd rather lose a little "face"" then leave bad advice for others.

    And since you are more experienced - when lapping OD's, what do you think of using curved segments rather than full cylinders? A segment is not likely to "catch" as long as it's less than 180 degree or so of engagement.

    Similarly, do you want to try to match CTE's as best as possible between workpiece and lap? Not sure if that's a worry...

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    The length you want would be about 4 inches. Sunnen makes an external hone that would do the job give them a call. It will make the part round and straight to a tenth if that is what you need and spend the time. A home made lap with grit and paste will be slow.

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    Quote Originally Posted by Paolo_MD View Post
    A naive suggestion from an ignorant.


    PS Am I guessing wrong that you're building a cylindrical square?
    Uhm, wild guess? Not gonna confirm nor deny. I like your idea of using replaceable shoes .

    I may take you up on the small section of tubing, but if I make a segmented lap with replaceable strips the outer rim can be anything including MDF or plywood


    dee
    ;-D

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    Quote Originally Posted by athack View Post
    Hello,

    I have close to 40 years lapping experience. ID and OD. There have been some good suggestions here and also some that are going to make your job very difficult. I suggest you contact either Helical Lap Company or American Lap Company. Contact them or a the very least visit their websites there is a lot of information there.
    This is a learning exercise for me if I had commercial application for this I would definitely invest in a hone or lap, I guess you already figured out this is a one-off and I am trying to do this on the cheap.

    dee
    ;-D

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    Hey Dee, I made a Robin inspired lap tensioned with a large engineers clamp to finish up a tailstock quill I turned. Diamond worked best by a long shot and spared the lap. I think i used mineral spirits or light oil for lube.


    At 4" be careful! Even at 2" odd its enough to get your attention. Maybe you could have a handle on the lap long enough to rest on wood covered way should things get tight. Keep hands out of the pinch point and a foot over the E stop

    Cheers
    D

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    Quote Originally Posted by athack View Post
    Hello,

    I have close to 40 years lapping experience. ID and OD. There have been some good suggestions here and also some that are going to make your job very difficult. I suggest you contact either Helical Lap Company or American Lap Company. Contact them or a the very least visit their websites there is a lot of information there.
    Can you give the specifics? I'm sure many of us would like to learn and get the benefit of your experience....but without knowing which are good and which are the bad it doen't help much

    thanks

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    There was a company that made lapping machines for jobs just like this ... Lapmaster ? It was two parallel rolls. You'd place the workpiece in the vee between the rolls, kind of like tires on a chassis dyno or a long version of a grinding wheel balancer. Then you would affect the size and straightness by the amount and location of pressure you put on the workpiece. It was a benchtop kind of doohickey, I always wanted one, seemed like it would be great for crankpins but you know how that is ...

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    Quote Originally Posted by Mcgyver View Post
    Can you give the specifics? I'm sure many of us would like to learn and get the benefit of your experience....but without knowing which are good and which are the bad it doen't help much

    thanks
    The thing that concerns me the most is the segmented idea. Lapping works by wearing the lap and work piece therefore making each very round and straight. Cast Iron is the best material to make a lap out of even to lap soft materials like brass and so on. Aluminum Oxide would be the Lapping Compound of choice for your material, cast iron.

    To do this job on the cheap I would buy a piece of Dura Bar tubing. Bore the ID to .002-.003 bigger than your shaft. Add an adjusting screw that goes thru the wall and split the tube so you can contract the tubing using the screw.

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