# Tapping small holes in cast iron



## Bob Korves (Dec 8, 2017)

I recently helped a friend install magnetic scales on his new to him lathe, and had to drill and tap holes in cast iron for the small mounting screws.  The job came out fine, but I was less than happy with the quality and consistency of the work I did with drilling and tapping holes while holding the correct sizes and getting a good thread fits and accurate hole locations.

So, now I need to install DRO scales on my Millrite mill, 3 axes, all holes will be in cast iron, and all holes will be blind.  The hardware all uses M4 x .0.7 screws (or I suppose I could change to another size, like 8-32.)  All the holes will be drilled using hand held power drill motors.  I have quite a few, and can hold whatever RPM range I want to.  I have drill bits in all sizes imaginable, with backups, but only half millimeter increments in ISO sizes.  I only have one M4 x 0.7 mm tap, a 40-50 year old unused Craftsman Kromedge carbon steel plug tap.  Those are good taps, but I probably should have at least one more on hand for the job, and a bottoming tap may also turn out useful.  The results do not need to be to high tolerance for this job, but I would really like to achieve good fits, consistently, and no breakage.  I will definitely be hand tapping and taking my time.

Suggested taps for the job?
Tap drill size choice beyond the usual charts?  3.3mm (.1299") is the usual global recommendation, but other choices might be #30 (.1285") or #29 (.1360") or maybe others.  I have lots of #29 and #30 on hand.  One of my charts shows the preferred drill as #36, but that is definitely not correct, probably a typo for #30.
Specific advice for getting hole placement where I want it, considering hand held drill, awkward locations, hard spots in the iron, etc.?

I know for sure I will be making a drill guide to match the chosen tap drill size for keeping the drill normal to the surface while drilling the holes after starting them.

Other suggestions?
Thanks!


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## Frank Ford (Dec 8, 2017)

I'd get rid of the carbon steel taps, and go with HSS, spiral point taps.   Likewise, get some split point HSS drills if you don't have 'em already.  The last thing you want is to break a tap, and the spiral point ones will make that unlikely!  Drill the holes extra deep and you won't have to worry about where the chips go as you tap.  Spiral point taps shoot the chips ahead, so on cast iron, I work dry and back the tap out to evacuate chips with a blast of compressed air if it's a blind hole.


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## markba633csi (Dec 8, 2017)

I think of cast iron like wood, and in wood you use lag bolts, so I would use a coarse thread.
Do you really want to drill holes in your nice machine? I don't think I could bear to do it if I had one like yours.
Why not just epoxy some brackets? The forces are so minimal, I doubt it would ever come off..
Mark


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## Tinkertoy1941 (Dec 8, 2017)

Make a drill fixture to hold the Brown Tool cross hairs see link below.
The fixture could be no more than a flat plate about 3/4" thick that would accept a hole to hold a bushing for the drill and one for the tap
and the Brown Tool and held in place with c clamps.

https://www.browntool.com/Listview/tabid/344/ProductID/2322/Default.aspx


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## markba633csi (Dec 8, 2017)

DON'T DO IT BOB! LOL 
put that drill away son


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## magu (Dec 8, 2017)

markba633csi said:


> Why not just epoxy some brackets? The forces are so minimal, I doubt it would ever come off..
> Mark



I would advocate this method. Whether one cares to preserve some percieved purity of a machine or not is a personal choice, but from a purely functional setup, if anything ever induced a force on my DRO scales beyond a light contact I would much rather they pop free than break. Epoxy won't allow them to shift about and reduce accuracy, but a correct joint will break like glass at a certain point.


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## David S (Dec 8, 2017)

How about making some intermediate mounting plates with the correct hole patterns drilled and tapped in the drill press and epoxy those to the machine.  Then you can screw the DRO's to the plate and they can be taken off for cleaning, service etc.

David


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## Bob Korves (Dec 8, 2017)

markba633csi said:


> DON'T DO IT BOB! LOL
> put that drill away son


Excellent advice, Mark.  I have been careful since I have had the mill to make no changes that could not be returned to original condition.  Your post is a breath of fresh air and I will consider it strongly.  I would have to try to get whatever oil has penetrated the cast iron out, and make sure to use enough surface area to make an adequate bond.


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## Bob Korves (Dec 8, 2017)

Some of the mounting will pretty likely have to be over painted areas.  How do I do that without drilling holes or ruining the paint?  The X is easy, the Y looks perhaps possible, but on the Z  there seems to be no alternative to mounting over pristine paint.


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## whitmore (Dec 8, 2017)

Bob Korves said:


> I recently helped a friend install magnetic scales...
> [tapping holes in cast iron]
> Other suggestions?


Pin-drive anchors, screw  nails, or aluminum pop rivets would be my preference; something
in plastic or aluminum that can be drilled out.   I've been known to carve a wooden block
to mate to the cast iron, attach it with construction adhesive, and screw into the wood.


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## EmilioG (Dec 8, 2017)

I would invest in top quality taps from Emuge and/or  Guhring.  Hss-e Cobalt with TiAin coating.  Msc.com
White Ring straight flute taps from Guhring.


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## David S (Dec 8, 2017)

Bob if you were to attach a nicely done mounting plate, I would think that this would be an asset to the machine rather than a detractor.  Is there some other reason you don't want to make any non-reversible changes?

David


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## richl (Dec 8, 2017)

Holes can be filled with Bondo and repainted if you change your mind down the road. It's not like you are chopping the top, or shortening the table... just a thought


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## 4GSR (Dec 8, 2017)

Get some straight 4-flute taps to cut threads with.  Start with a tapered tap and go to a plug if needed.  4-flute tap will allow you to align the tap up with the hole and get it straight without problem as you would get with a gun tap.  Back out tap frequently to clean out shavings.  Use a can of Brake Clean to clean out holes. (This stuff may damage your paint job if left on the surface!) I would suggest going to 8-32 button head screws and throw the Metric one's in the trash.  That's just me.  Millrite castings are not very thick, drill holes all the way thru unless in a solid section of iron.


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## dulltool17 (Dec 8, 2017)

What about drilling oversize and using epoxy to hold the screws?  ...kind of like anchor bolts in concrete.


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## markba633csi (Dec 8, 2017)

Ya know- silicone RTV makes a good adhesive if you have a good surface area, and wouldn't harm the paint. The newer formulas don't have acetic acid.
Mark


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## Bob Korves (Dec 8, 2017)

David S said:


> Bob if you were to attach a nicely done mounting plate, I would think that this would be an asset to the machine rather than a detractor.  Is there some other reason you don't want to make any non-reversible changes?
> 
> David


A machine (or other artifact) in original condition, and even more so one in like new original condition and still being carefully used, is really quite special.  It is a view of living history.  A hot rod machine is a glimpse of one person's whim of the moment.  Restorations attempt to re-create "original", but you are not really looking at history, you are looking at a re-creation.  Not that there is anything inherently wrong with restorations or hot rods, they are just not the original artifact, showing exactly how it was and how it has survived.  All the more so in our mainly throw away lifestyle...


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## Bob Korves (Dec 8, 2017)

dulltool17 said:


> What about drilling oversize and using epoxy to hold the screws?  ...kind of like anchor bolts in concrete.


The problem is not the threads, it is the holes themselves, which I do not really want to add to an original and nearly pristine machine from 1965.


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## samthedog (Dec 8, 2017)

There are a number of ways to tackle this. One way is to just epoxy the scales on. Another way is to drill and tap the actual machine. You could also epoxy on an aluminium block and drill and tap the aluminium block(s). What I would recommend though is to use decent taps and ditch the carbon steel antiques you are using - they'll bring you no joy. 
This was the whole reason why I tested the Chinese high cobalt taps from China. You guys could replace your old taps with high quality import taps at a fraction of the price of buying European like Güring. For those who missed the test:


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## woodchucker (Dec 8, 2017)

markba633csi said:


> Ya know- silicone RTV makes a good adhesive if you have a good surface area, and wouldn't harm the paint. The newer formulas don't have acetic acid.
> Mark


But silicone will reduce the accuracy as it moves quite a lot.
I'm for screws, tried and true, been used for a long time.


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## Bob Korves (Dec 8, 2017)

The taps that some of you are worried about have been in service for about 45 years.  They are USA made and hard chrome plated.  I really do not know what material they are made of, but they do not say "high speed steel" on the tap or the packaging.  I have two sets, one Imperial and one metric, from 1/4 to 1/2" and from 4mm to 12mm.  They have collectively tapped and cleaned up many hundreds of holes in all kinds of materials, some of them extremely tough.  Not one of them has ever broken or chipped, and all of them are still acceptably sharp and unworn for fussier work.  Both sets look like new and work like new, and the metric taps mostly ARE new.  I have hundreds of other taps, mostly top brands in great condition.  They are no better than my old Craftsman taps, which are about equal to NOS GTD vintage taps.  I have had much worse results overall with modern "quality" taps than I have had with vintage taps, even ones with some wear.


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## Bob Korves (Dec 8, 2017)

What about using Cyanoacrylate (CA, super glue) to mount the brackets to mount the scales to?


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## Mitch Alsup (Dec 8, 2017)

http://web27.streamhoster.com/hvp/dropros/MillInstall1.mp4


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## markba633csi (Dec 8, 2017)

Yep superglue is another option, but the cyanide fumes might mar the paint


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## Bob Korves (Dec 8, 2017)

markba633csi said:


> Yep superglue is another option, but the cyanide fumes might mar the paint


I was more thinking to use it on the front of the table for the scale mounting.  Yes, the front, I am not going to give up ANY Y axis travel for this installation (or Y or Z).  I will also not give up the X axis hard stops, or the table power feed stops, and will be adding rubber sheet chip protectors to the front and back of the saddle as well.  And yes, all of that will make the installation much more busy and will require a well thought out plan.  I am using a "slim" glass scale for the X axis, which is about 1/2" shorter vertically than the standard scale (measures 1.150" high, .825" thick.)  It might be interesting to install the X axis scale inside a fairly rigid channel extrusion, with the table side glued to the table and the scale screwed to the inside of the outboard flange of the channel, with the channel opening facing downward.  Maybe something like this:
https://www.onlinemetals.com/merchant.cfm?pid=7016&step=4&showunits=inches&id=297&top_cat=60  (or perhaps 1 1/4 x 1 1/4".  The channel would be mounted upside down from the drawings at the link.)  I think I could make something like that work.


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## jocat54 (Dec 8, 2017)

What great timing on this thread--I am waiting for my 2 axis DRO glass scales for my lathe to arrive from China and have really been thinking of the best way to mount the scales on the cross slide. I know that I will use aluminum spacers so I can still use the cross slide lock screw.
Would really like to hear from some one "first hand" that has used epoxy or super glue to mount their scales---I'm really liking that idea.


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## kd4gij (Dec 8, 2017)

I find the cast-iron used in machine tools very easy to drill and tap.


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## Bob Korves (Dec 9, 2017)

kd4gij said:


> I find the cast-iron used in machine tools very easy to drill and tap.


Agreed.  It is not a big deal to drill holes in cast iron.  I just do not want to drill holes in this machine...


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## petertha (Dec 9, 2017)

I drilled & tapped 1/4-20 bolts in my CI lathe bed & I think same size in cross slide. I recall being very apprehensive about the job due to lack of experience. I used a simple drill guide which I highly recommend, ideally if it can be clamped in place so your hands can stay on the drill. My guide looked more like a 1/2" thick plate with a hole in it vs. a cylindrical collar. My HSS drills were new but I recall having to be patient to make the hole. Hand drilling is different than pulling a lever on a drill press, especially when your body is stuck in an awkward gap between the lathe & wall

One thing to check is the surface is actually perpendicular to your hole location. My lathe had all sorts of odd angles & chamfers &that can mess things up.

The Newall mount system is brilliant if you can copy the idea. You just have to get the post landed in position somewhat close. Thereafter the hardware blocks allows movement up/down left/right & in/out. Make your indicator measurements & lock it down with set screws. If I had to get the holes spot on to ensure the scales were within 0.001" that would be challenging. It needs to be able to float & progressively secured in each orientation me thinks.

Tapping wise what the guys said. I led in with a plug tap & use a guide to get it started perpendicular. The swarf is basically powder which I vacuumed out before switching to bottoming tap. All dry. There are 'CI specific taps' as I'm sure you are aware, but I didn't have them. On my mill I had to tap 10-24 or maybe 1-32 & it was a bit more fussy. My Taiwan iron has hard spots for sure, some holes were easier & better than others.

I think that's everything I know or can recall. Good luck  show us your results!


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## Bob Korves (Dec 9, 2017)

kd4gij said:


> I find the cast-iron used in machine tools very easy to drill and tap.


Usually, Chinese imports not so much...


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## rzbill (Dec 9, 2017)

I guess I will probably be the dissenting voice in regards to keeping a machine 'pristine' vs modifying for current use.  I understand the feelings about leaving a machine original. I have them too but they are not overwhelming. Look at it selfishly for a sec.  It is a functional improvement that a new owner would be nuts to remove.  Does one plan to sell the machine as a restored or restorable antique or are the plans to use the machine to the best of its capability for the rest of ones adulthood?   If it is the latter, then the dispensation of the machine after one is dead is irrelevant.  Modify.  If that means holes, so be it.


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## Boswell (Dec 9, 2017)

I had no problem drilling and tapping my Chinese lathe to install the DROs.  As for drilling an older machine, I personally would not hesitate. If I am using it as a machine to make things, then modifying it with improvements seems natural. If on the other hand I was trying to start a museum...


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## Robert LaLonde (Dec 9, 2017)

Bob Korves said:


> Usually, Chinese imports not so much...



Maybe closer to pig iron?

I never understood the extremist aversion to drilling a machine or modifying a vise.  I know some guys seem to brandish it like a badge of pride, but ultimately its about getting the job done.  Sure, I cry if the table on a mill gets dinged, or a mill slams into a vise due to pour programming, but it happens.  I have 3 screwless vises semi permanently mounted on three of my machines, and they all have modified jaws so I can take advantage of the full working envelope of the machines. 

My issue with Chinese cast iron is that with small fine threads it almost seems to crumble rather than cut.  High quality machine taps seem to work better than hardware store plastic pouch hand taps or Chinese sets from the usual sources.  A little lead in helps a lot.  If you have enough depth (sometimes you don't) then you can clearance drill a short lead in to help guide the tap in straight.  On steel or aluminum if you don't start the tap straight it will cut anyway.  If its really bad you will break the tap, but it will cut right up until that point.  With cast iron it will tap up until the cast iron gives effective stripping the hole.  Get the tap started straight.  Use quality taps.  With the cast iron the chips don't clear very well even with a spiral flute tap. Take the time to clear the chips.  Machine taps on average are better than hand taps, and just about any machine tap is better than a cheap import hand tap.  Spiral point taps do seem to cut with less force than a spiral flute tap so if practical use them, but chip packing can cause the tap to stop and rip the threads from the wall of the hole.  If trying to use them blind, back the tap all the way out periodically and blow the chips out of the hole.  

~~  I see some guys were posting while I was off getting my coffee ~~


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## petertha (Dec 9, 2017)

Bob Korves said:


> What about using Cyanoacrylate (CA, super glue) to mount the brackets to mount the scales to?



I'd say first test a coupon on something sacrificial out of sight. Although there are opinions that CA is Loctite & vise-versa, I mess around with a lot of different CA's (different hobby). Some have similar properties, others are very different. With CA you generally want very good surfaces & even then, very little working time to position. With LT, especially like the ones engineered for larger gaps it might be a bit more forgiving & better end result. They have lots of spec sheets to refer to whereas CA is a bit more iffy depending on the supplier. LT also has accelerators (CA=kickers) and primers to promote adhesion

I guess on the future removable side, they can come off under shear like hammer whack. They also fail about their rated temperature. Clickspring uses this principle a lot to hold skinny awkward parts in fixtures to be turned on the lathe & then heated to release. But you are in a better position to assess, would this be a semi-sacrifical bracket or something where the scale itself might get abused in the process. The beauty of fasteners & shims using threaded posts is it can be knocked down & reconstructed again, which I have had to do on my machines chasing down another mechanical problem. 

As an adhesive, epoxy might be another & better option. Even an off the shelf filled epoxy like JB weld will give you lots of working time, similar or better strength especially under vibration & shock, more forgiving of uneven or rougher surface. It also has a pretty decent shear force for removal but I would opt for localized heat. And consider that might be beyond the temp your paint cries uncle if its close by. I've heard of these specialized levelling/shimming epoxies for big boy machines which are no doubt engineered for that specific purpose. But goes to show you how composites can be integrated into 'metal' machines in the appropriate application.


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## Silverbullet (Dec 9, 2017)

I guess really , if you want dro on it or not. No matter which way you mount them. There's always going to be some damage at minimum gluing will mar or crack the paint , drilling small holes and tapping to me is the lesser of the evils. Tapped holes can be plugged and dabbled with paint . But the choice is do you want the dro or not. Mounting shouldn't be a problem if you want the machine to be more accurate.


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## dulltool17 (Dec 9, 2017)

Bob- got it- you really hate to deface the good machine.  There has been a great deal of progress made in adhesives.  This might just be the way to go.  If you reverse course sometime in the future, a stiff rotary wire brush could be used for removal.


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## David S (Dec 9, 2017)

There has been great progress in powerful magnets...but alas not sure this would be an appropriate application.....or would it??

David


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## talvare (Dec 9, 2017)

Sorry Bob.... couldn't resist. I'll try to be more helpful in the future.

Ted


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## Wreck™Wreck (Dec 9, 2017)

It's just a machine not a priceless historic artifact, drill holes wherever required.

Say this to yourself 3 times very slowly.

It Is Just A Machine


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## Bob Korves (Dec 9, 2017)

Wreck™Wreck said:


> It's just a machine not a priceless historic artifact, drill holes wherever required.
> 
> Say this to yourself 3 times very slowly.
> 
> It Is Just A Machine


It is just MY machine
It is just MY machine
It is just MY machine


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## Wreck™Wreck (Dec 9, 2017)

Bob Korves said:


> It is just MY machine
> It is just MY machine
> It is just MY machine


I understand completely, I once owned my own machines yet had no qualms about drilling holes in them when needed.

On a more serious note you may want to place hard stops on both axes to prevent crashing the scales, many manual machines have shear pins on the lead screw drives, they will however damage a scale before the pin shears.

I had a Bridgeport manual knee mill with power feeds in X and Y, was doing a long X cut and standing at the end of the table watching closely when much to my chagrin the rotating X axis handle latched onto the belly pocket of the hooded sweatshirt that I was wearing, the off feed control was to far away to reach so I just held the handle until the shear pin gave way. Before you point it out I fully understand the whole hobbyist mantra of DO NOT WEAR LOOSE CLOTHING AROUND MACHINES (-:


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## Bob Korves (Dec 9, 2017)

Wreck™Wreck said:


> On a more serious note you may want to place hard stops on both axes to prevent crashing the scales,


My mill has hard stops, I use them regularly, and they will not be going away.  I ordered the X scale with 1"+ additional travel beyond the maximum lead screw travel, so the read head can never make it to the ends of the scale.  Same on the other axes.  I will be installing the DRO scale on the front of the table, to avoid losing Y travel, and there will also be adjustable hard stops, adjustable power feed stops, the DRO scale, and new front and rear way covers to keep chips off the top of the knee.  It will be busy on the front of the table, but I have found room and have a plan for it all...


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## FLguy (Dec 9, 2017)

Bob Korves said:


> What about using Cyanoacrylate (CA, super glue) to mount the brackets to mount the scales to?


NO


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## Wreck™Wreck (Dec 9, 2017)

We just got a new Trac 3 axis mill this Summer, 60" X 23" Y and 24" Z, moved the Y axis soft stop switch a quarter inch toward the back to accommodate one part so it gets down right chip crushingly close when it rapids all the way back. Fortunately it only rapids at 200 IPM which is far fast enough to scare nearly anyone however.


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## Robert LaLonde (Dec 10, 2017)

Wreck™Wreck said:


> Fortunately it only rapids at 200 IPM which is far fast enough to scare nearly anyone however.



LOL.  Yeah, no kidding.  I had my Hurco running at upto 450 IPM and accelerating (with a lot of following error) at 100iss when I was testing the limits of my current servo drivers.  Its pretty scary when acelerating at near hammer blow forces back and forth while torture testing.  Especially with a 1000 lbs of saddle and table.  It was to much for me.  I backed it off to 200 and 30 and called it a day.  I to pushed my limits, but I did it in programming to accomodate the max size work pieces I can.  When it rapids to the back (Y-) max the drain tubes on the back of the table are about 1/16 away from the control cabinets.  Fortunately I am usually in front of the machine when that happens and I don't see it.  LOL.  

I can just imagine how it would feel with your much heavier table and saddle.


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