PDA

View Full Version : Press fit dowell pin



Lew Hartswick
12-20-2016, 10:50 PM
Dowel pin press fit problem
Two parts of a mechanism to be guided by a pair of 0.125" dowel pins. One part to be press fit the other sliding fit.
With the two parts held in alignment the holes were drilled through with a #31 bit (0.120") , the sliding part reamed with a 0.127" reamer and the pins slide fine.
Question: what is the options for getting a "reasonable" size press fit hole with no reamer in the size range??
I'm thinking that the 0.005" interference is going to be way too much and the next size bit is 1/8" so it may not be
tight enough. (no metric bits and even then the only one would be, I think 0.122") So will one of the "Locktite"
materials be adequate to lock a steel dowel pin to an aluminum part and will the 1/8" bit be appropriate???
The pins are 1" and the sliding block is 1/2" thick. The press fit is to be 1/4" deep in the other. (Is that sufficient?)
That will allow the sliding block to move a quarter inch or a bit more safely.
Opinions or preferably "knowledge" appreciated. Thanks
...Lew...

A.K. Boomer
12-20-2016, 11:02 PM
Lew - Iv had to use dowels for allot of power transfer mechanisms in things iv built over the years, and recently so with .125",,,
it's a coercing of an endmill and would recommend practicing on some same but scrap material first , but - iv been able to achieve press fits in pieces with same size endmill by just drilling really close to size with a bit first - then using the endmill as a finisher,,,
the other piece - the engagement piece that is - I don't drill nowhere near as close and sometimes depending on the material not at all - then the dowel seems to be more of a slip fit,,, I think the spiral flutes in endmills "fan out" some when under more pressure and create this effect just like a drill bit does...


the one good thing is when you get the fit you want in the scrap material you know what you just did to achieve it and if you get one you can usually get a half dozen or whatever...

dalee100
12-20-2016, 11:07 PM
Hi,

Been there. My first reaction is to make a D-bit. Then size is what ever you choose.

If something like that isn't an option, then I have drilled a matching size hole - i.e.1/8" drill for 1/8" dowel, (do a test drill first), then I either simply LocTite the dowel in. If the hole is a touch to big, I have scarred up the inside of the hole and then LocTited the dowel in with good results. But, it can be a YMMV situation.

Dalee

David Powell
12-20-2016, 11:29 PM
Hi,

Been there. My first reaction is to make a D-bit. Then size is what ever you choose.

If something like that isn't an option, then I have drilled a matching size hole - i.e.1/8" drill for 1/8" dowel, (do a test drill first), then I either simply LocTite the dowel in. If the hole is a touch to big, I have scarred up the inside of the hole and then LocTited the dowel in with good results. But, it can be a YMMV situation.

Dalee

IF the part is a small one, and can be held on the mill in such a way that you can heat it with a propane torch without damage to the mill,or vice then you could heat it, not to red but to black before red, without causing permanent distortion, then drill with a good 1/8 in drill. On cooling you should find the dowel a light press fit. (The actual part where the hole is to be must NOT be stopped from expanding by vice jaws or clamps, sometimes not easy to arrange) I have done things like that, sometimes works, but really is only for those Sunday nights when you HAVE to get her done. Hope this hint helps someone. Regards David Powell.

BCRider
12-21-2016, 01:17 AM
You're right. pushing the pins into that .120 or thereabouts size hole would not be good. If you really don't want to spring for a .123 or .124 size reamer then one other option would be to follow down with the pilot hole with a .125. Then with a 3/16 pin punch swage the mouth in a little with some "love taps" until the pin won't fit. Or you could make a few marks around the hole with a center punch to swage in the mouth for a good fit. Then goober up the hole and pin with a high strength version of Loctite or use some medium viscosity CA glue. Tap the pins down until they center in the pilot. Between seating in the pilot to some degree and the peening at the mouth it should hold the pins in place decently. Then the Loctite or CA will kick off and hold things that much better for the long term.

LKeithR
12-21-2016, 01:28 AM
To get a "good" fit you need to use the proper reamer. Another option is to open the hole up with a needle eye lap...

wierdscience
12-21-2016, 01:50 AM
The D-bit reamer idea is a good one,use one of your dowel pins for the blank.Chuck the dowel in the lathe and hit it with a few passes from an oil stone,check the diameter with a mic.Maybe shoot for .1245" try a few test holes before jumping in with both feet.

SGW
12-21-2016, 06:13 AM
Since a rule of thumb for press fits is one thou per inch of diameter, for a 0.125" dowel you don't need much. Many dowel pins are made a couple tenths oversize, so if you drill a 3/32" hole and finish with a good 1/8" drill, and your dowel pin is actually 0.1252" it may be about right. I like the suggestion to warm the block a bit before drilling.

A.K. Boomer
12-21-2016, 07:25 AM
true not all dowels are created equal either and that could be key in getting a proper fit,

the heating before boring is a great idea esp. with aluminum

last week I just went through this with 17-4 H-900 SS

first holes I bored I simply plunge cut with the .125 endmill - no pilot hole, they were so loose they were not even acceptable as a slip fit, next holes I center drilled and then pre-drilled to within .010" and then used the endmill,,, had to use a ball peen hammer to get them in and was seriously worried I was going to break a dowel -- barely made it .250" in

the receptor part needed to be slip fit for ease of assembly and disassembly,,, so I center drilled and then used a small pilot hole like about .050 or so - ended up being perfect, no play and yet ease of mating and disconnect...

The endmill I had seemed new but might have been worn some --- usually using an endmill of same size the dowel always seems to go in at least without a fight so maybe combo of 2/10ths larger dowel and an endmill slightly undersize

dian
12-21-2016, 07:34 AM
we get dowel pins in h6 and m6. if you have a good hole (H7) the former will be a medium sliding fit, the latter a light press fit. alternatively i play with oil and alcohol when drilling or reaming. the diffence usually does exactly what you want. as a matter of fact i will be doing it this aftenoon with 3 mm pins. i have no reamer (would not live long i guess) so it will be a drill +oil/alcohol.

edit: i predrilled 2.5 mm and its done. one side of the pins is pressed in, the other slides.

rkepler
12-21-2016, 09:40 AM
I wouldn't trust a drill to give an on-size hole, particularly when looking for a press fit. I'd ream that hole with a .125 under reamer, .1245 is I had one, .124 if not. .001 press on a .125 dowel pin is a pretty good press. (Lew, since you're local feel free to borrow the reamer(s) from me.)

J Tiers
12-21-2016, 09:40 AM
This is why there are "over" and "under" reamers. The loose side at 0.126 or 0.127, and the tight side at 0.124.

Drills are totally unreliable for press fits. In a pinch, you can measure you drills, find ones of suitable sizes, round off the "outer corners" on the cutting lips, and use that for a reamer. The rounding helps them self-guide and not cut oversize.

But if you use drills, Locktite is your friend.

Fasttrack
12-21-2016, 10:25 AM
Yep - a bearing retainer variety of loctite (can't remember the number) would probably work if you drill the hole. But I suggest buying the right size reamer OR... another trick not yet mentioned... stuff a tiny bit of oil soaked rag down one flute of an undersized reamer. It will cut oversize. I've never tried it on such a small reamer, but it works great with larger sizes. It's hard to control the exact dimension this way but if you just need a snug press fit or a nice sliding fit (i.e. not doing anything by numbers), this method can work in a pinch.

Toolguy
12-21-2016, 10:31 AM
For dowel pins, .0005 under is a medium press and .001 under is a tight press. You don't want tighter than .001 under. A nice slip fit is .001 over. With a DRO you can put the holes in one part, then do matching holes in the other part and they will be a perfect fit. Make sure to use a spot drill to make the drills start on center though. If all you have is drill bits, you can drill to a close undersize, then use an on size. I like LocTite #648 retaining compound (green) for this type of work.

As J Tiers says, drill bits vary in size. Use a micrometer to find one that fits the situation. If you only have one of that size, you can still mike it to know what you have.

David Powell
12-21-2016, 10:32 AM
This is why there are "over" and "under" reamers. The loose side at 0.126 or 0.127, and the tight side at 0.124.

Drills are totally unreliable for press fits. In a pinch, you can measure you drills, find ones of suitable sizes, round off the "outer corners" on the cutting lips, and use that for a reamer. The rounding helps them self-guide and not cut oversize.

But if you use drills, Locktite is your friend.

Hand reamers are( usually) slightly tapered and so If you do not go in full depth you can get a slightly undersize albeit tapered hole, getting tighter the deeper you pound the dowel in. The taper pin reamers have a steeper tape but can be used to give a start into an undersize hole. If you spring for the proper reamers and drill only slightly undersize with good drills the reamers will work for hundreds of holes . IF you have a spare reamer of the nominal size you can take a couple of tenths off it by reaming a hole in a block of steel, putting a little fine grinding paste on the reamer and running the reamer in the hole while gently pulling it to one side. The exact size you get is unpredictable. Remember, only the ends of machine reamers cut ( in theory!) Hope these further bodges help get someone out of a problem. Regards David Powell.

MTNGUN
12-21-2016, 10:39 AM
Consider using a spring pin.

A.K. Boomer
12-21-2016, 11:04 AM
Lots of methods can be manipulated esp. with endmills or drills, as I stated the amount of pressure has a direct effect on diameter and I will tell you as to "why" - its because pressure is directly related to torsional loading and torsional loading has a direct effect on both the endmill or drill due to the leading spiral wind, if the wind leads - the more the torsion the more the "unraveling" or "fanning out" - the unit will not only just grow in diameter - it will also grow in length too,


it's why reamers are straight - they are "for the most part" immune to this effect...

I brought this up many years ago and took some flack by a few members (and im not saying that in a bad way - I welcome that because I actually do my best work when backed into a corner lol) who asked for references or where am I getting my information what book and the like,
Was pretty much my book - the one I wrote and never published lol It just seemed elementary structural engineering to me,
and really thought others would have experienced allot of this effect on their own and at least questioned some of the reasons as to "why"
and by far it's not the only contributor,,, but if your not compensating for it then I can guarantee you that your at least leaving one "value" out of the equation...

Long story short I built a simple way of testing the theory and here's the pics I posted

first built a receptor to hold the end of the drill bit, aluminum was a good choice as I did not want to static load the bit in anything harder for fear of chipping the corners,

http://i146.photobucket.com/albums/r249/AK_Boomer/DSC00434.jpg (http://s146.photobucket.com/user/AK_Boomer/media/DSC00434.jpg.html)

then just inserted the bit --- took multiple "no load readings" of the flutes diameters

http://i146.photobucket.com/albums/r249/AK_Boomer/DSC00436.jpg (http://s146.photobucket.com/user/AK_Boomer/media/DSC00436.jpg.html)

then static loaded the draw bar with a predetermined amount of torque calculated by a torque wrench seen and my adjustable shower curtain rod,,,

http://i146.photobucket.com/albums/r249/AK_Boomer/DSC00435.jpg (http://s146.photobucket.com/user/AK_Boomer/media/DSC00435.jpg.html)

Conclusion; is that drill bits DO indeed grow in size the more torsion that is applied, I not only documented a diameter change --- I also kept track of its length by means of quill travel - and it only stands to reason - that if you try to straighten a spiral flute it will also make an effort to not just get larger but longer too...


I cannot find the old post - found a post after the fact I think but no specs in that one and it was not by a whole lot - but there also was no doubt about the end results...

A.K. Boomer
12-21-2016, 11:14 AM
Summary --- when it comes to a drill or even a very ridged endmill (in comparison)

If you want to stay as close to your tools size as possible then do not load the tool itself, drill up to as close to the finish size as possible...

on the flip side, if you want a slightly larger diameter hole either drill small and load the tool that way, Or don't drill all the way down at all, leave some meat at the bottom - you already got a good pilot hole for the tooling - get to the resistance and peck/load the tool some - fan it out and catch a little extra off the sides...

BCRider
12-21-2016, 12:29 PM
Speaking of drill bits varying in size..... I've noticed when sizing drills that are too small to have a stamped number on them or if the number is scuffed off that when I measure the drill bits I need to measure them near the end to be sure of the size. With that in mind could we not cut off or snap off a drill so it's shorter and re-sharpen it and that would make it cut undersize?

The center web would be crazy thick. But for a light ream out of this sort we might just get that half to one thou under fit that is needed.

A quick check of a new 1/8 drill bit is showing what appears to be a half thou smaller measurement across the flutes about 1/4 inch from the end of the fluting. This may not be consistent from brand to brand but it's worth checking.

Frank Ford
12-21-2016, 03:33 PM
. . . . will one of the "Locktite"
materials be adequate to lock a steel dowel pin to an aluminum part and will the 1/8" bit be appropriate?. . .


I'd say, "Yes." I'd drill it to 1/8 and glue the pins right in place. If the holes felt just a bit sloppy, I'd probably use the sliding portion as a guide to align the pins while the 680 or 609 green Loctite sets up - really just a few minutes. With the clearance holes at .127" I'd say you have room for small error, so it would be very likely to go easily.

ahidley
12-21-2016, 03:42 PM
I didn't read all the replies but may be consider this.
They make reamers on size, over and under, and also a set made just for dowel pins I.e. one for a dowel pin pressed it and one a sliding fit.
I'm sure the size that you need is available for a few dollars on eBay.
Buy the right tool for the job and life will be easier.

JoeLee
12-21-2016, 06:04 PM
I would practice first on a piece of scrap. Some drills may drill larger or smaller than the size that they are. Any eccentricity in you mill or drill chuck will also have an impact on the hole size. Drills always tend to drill larger at the start of a hole. Best results are obtained by drilling up to the finished hole size in small increments. Less drill walk. I usually do what AK mentioned, drill and finish up with an end mill. But still I always do a few practice runs.
Even a reamer can ream an oversize hole if it doesn't run dead center.

JL................

enl
12-21-2016, 06:30 PM
as I stated the amount of pressure has a direct effect on diameter and I will tell you as to "why" - its because pressure is directly related to torsional loading and torsional loading has a direct effect on both the endmill or drill due to the leading spiral wind, if the wind leads - the more the torsion the more the "unraveling" or "fanning out" - the unit will not only just grow in diameter - it will also grow in length too

It seems that this is rare knowledge, and not something I keep in my mind normally, but I had a vague recollection of seeing this before. Colvin and Stanley jigs and fixtures addresses this, tangentially, noting in several places that a jig only controls hole location and size, and the depth is only correct, if the feed and speed are correct.

A.K. Boomer
12-22-2016, 12:09 PM
You mean I may not be insane? see I told them I did not need therapy :p might be related and even if not - good to know,

Dowels come in handle for so many drive uses --- most my inventions would never get off the ground without them,

this is an external drive flange/mount that engages into some 6061 t-6 - the mount is made of 17-4 H900, it's one of my favorite SS to work with esp. on lathe work just cuts beautifully, these pieces are all butchered up esp. after many miles so don't look for any aesthetics here... but - dowels out at the farthest parameters ensure this piece will never budge even in aluminum,

http://i146.photobucket.com/albums/r249/AK_Boomer/DSC04165_zps4rrte5kh.jpg (http://s146.photobucket.com/user/AK_Boomer/media/DSC04165_zps4rrte5kh.jpg.html)

under the titanium end cap sits the "poe pholk" splines,,, just because you don't own fancy broaches or just plane don't want to spend the money for pre-fab special drive mechanisms does not mean you have to shut operations down, just come up with a way of solidly holding both pieces vertically - you can free float the outer piece and not have to hold it if the inner is ridged enough - just lock tight the outer to the inner first because it will try to rotate and immediately snap your endmill,
I counter sunk chamfered the ends for ease of installing dowels - some might look "loose" because of this but all have to go in with some light taps of a very small ball peen,,,
To get the best of your drive strength shear rating do not just go dead nuts in the center between the inner and outer piece - compensate for the lost radius that the inner piece will inflict on the dowels radius and adjust, anotherwords "hug" the inside drive or driven piece more than the outer...

http://i146.photobucket.com/albums/r249/AK_Boomer/DSC04162_zps0ilcs2e6.jpg (http://s146.photobucket.com/user/AK_Boomer/media/DSC04162_zps0ilcs2e6.jpg.html)


And Walla,, poe pholk splines that are not only extremely strong - they cost you about 50 cents and will take extreme torsional loading in BOTH directions for hundreds of thousands of cycles without loosening up like a common drive spline sometimes can...

http://i146.photobucket.com/albums/r249/AK_Boomer/DSC04169_zpsva483aj2.jpg (http://s146.photobucket.com/user/AK_Boomer/media/DSC04169_zpsva483aj2.jpg.html)

projectnut
12-22-2016, 12:41 PM
As mentioned good quality dowel pins are within .0002 of the specified size. In your case the suggested size for a reamer would be .1245. They are available from any tool house. McMaster Carr sells a HSS one for $12.41, and a Cobalt one for $14.71. Shars sells a .1240 reamer for about $7.50. If you plan to do a lot of dowel pin work I would buy a set. Several discount houses sell 1/16" to 1/2" by 64ths for around $125.00

dian
12-22-2016, 12:52 PM
what joe said. besides i realized long time ago that the right tool very often just is not around. what i also do is drill a smaller hole like 2.95 mm for a 3 mm pin. no pin will go in here. then i press through a m6 pin. one time, several times, dry, lubed, acohol, maybe rouge although i never did that, see how it is. then a h6 pin will either slide in with a heavy sliding or light press fit. dont predrill the hole in this case you want to take advantage of the larger diameter the driil produces at the entrance. especially in aluminum its easy to size a hole like this. every 0.001 mm makes a difference you can feel in this case. also heating/freezing will make a considerable differece.

comment: we dont get under/oversize reamers over here. h7 is standart and anything else is either not available or $$$.