I'm making very low cost injection molds, and I regularly have the problems that I don't get perfect alignment on my guide pins because the drill wanders a little bit so they start jamming. The holes for the guide pins are tightly fitted to the pins (slip fit) with no perceptible play. The problem is that once the guide pins are press fitted into the "A" halves of the mold, I don't get them to slide into the "B" halves anymore (please see pic to get a better understanding what I'm talking about).
One solution is to drill the holes for the pins with the two mold halves stacked on top of each other, but unfortunately that is not always possible and introduces other issues. Any ideas on how this is properly done?
When making holes in multiple parts that need to precisely align make sure that each hole is approached from the same direction. In other words, do NOT use any zigzag or serpentine paths to make the holes. By always approaching each hole from the same X and Y directions you remove potential backlash from each hole location.
It is also important to make the holes at the same points on the lead screws. In other words, if hole #1 in part #1 is at lead screw locations X,Y you will also need to ensure that the identical absolute X,Y locations are used on hole #1 in part #2. The lead screws in the Tormach have a precision linearity of 560 micro-inches per inch of travel, with a cumulative error of under 800 micro inches. If you drill corresponding holes in multiple parts at different locations on the lead screws you will potentially have misalignments due to the quite small lead screw linearity errors.
And of course pilot holes should be made with stub drills and the finished holes should be milled rather than drilled.
Hi, you could always make a drill jig that you use to only drill both halves of the die for dowel pins independently........relying on the co-ordinate drilling of a machine that has as little as .02mm backlash will still give you misalignment.
Ian.
Drills will almost always walk a little. You might try drilling the hole undersized and plunge an end mill to correct location. If you are working with odd diameter pins make the drill and end mill undersized and ream to size.
Another thing to remember is that one pin creates the alignment in the x/y axis. The second pins job is to locate the pattern radially. Because of this a lot of precision locating dowels are ground to a diamond shape only leaving the diameter on two thin sections directly across from each other. When installing this second pin you align those two diameters perpendicular to the axis it share with the first pin.
Good replies guys,but have nothing to do with mold making & their guide pins & bushings placement, you have to be a mold maker to have done this hundreds of times to understand how this is normally done
Drilling is only a way to remove metal, can not be used for any accurately positioned holes, even if you reamed a drill hole to the correct size, it to would be not aligned enough for 4 pins,to be a slide fit as is needed for a Mold
Reaming with the A & B plates clamped together, works only sometimes, so even doing it this way is not going to alway give you correct alignment, Plus you have to Bore one side out for Bushings anyway
I see you are not using bushes on the B side,using bushes is the standard practice for a aluminum & steel molds, trying to do it cheap is never going to be successful, with this part of a mold
If you only have a mill then you have to Bore the holes, a Jig Boring or Jig Grinder machine is used to do these plates normally if you want accurately positioned holes
This can be done even on a Bridgeport type mill also,( Depending on your skill level ) with anything that you want to be accurately positioned like this, always move your axes the same direction,from your Datum Hole, this will eliminate any backlash that you may have in your machine, even on a CNC machine
( 1 ) Hole will always be the datum Point, usually the upper left top hole, which is normally offset from the other ( 3 ) holes for obvious reasons, this Datum hole is always your reference for doing other work on your mold
Photos of a split B side with ( 8 ) Tube Dowels for alignment to it's support plate, this has perfect guide pin alignment,as well, these holes were interpolated & Bored on a mill
@mactec54 Thanks for all the info, this is really useful. I actually already ordered bushings, just didn't receive them yet.
You write that one hole should be offset from the others for obvious reasons. Could you please elaborate why that is? I noticed looking at different CAD files for molds that one hold is usually offset, but never understood why.
As you can tell, I'm very much a novice when it comes to machining and building molds event hough I have done a few with satisfactory results (for very low precision ABS parts).
Oh, another question. I usually set my origin on the top left corner. Is there an obvious advantage from using a hole for setting the origin vs a corner?
AS for ( 1 ) hole being offset, this is so the mold can not be assembled, put together the wrong way round, you could have up to 4 ways to put the mold together if the Pins & Bushes are symmetrical, unless it is a simple mold, it is not worth the risk to crash a mold that has been assembled the wrong way round, so is a safety feature
As for a hole for your Datum, it is more accurate than ( 2 ) edges, ( in most cases ) to edges can be used, it just depends on the setup you are doing, before you put any holes in a plate, you will be using the the edges, but once you have the holes in, then everything should relate to your Datum Hole, indicating a hole, you will get good at it, you will not even think twice about how to set up different jobs, you will just use an Indicator, it takes a little longer, but worth it in the end
You will find that most molds are made this way, by using the Datum hole for any setup,( being the offset hole )
Ah ok, that makes sense. My molds are super simple and I just put a mark into the pieces so I always know where the origin is, so it has never been a problem for me.
Sorry to bother you again, but one more question: When you mill the holes for the A and B plates, how to you put them on the mill? Specifically, let's say your offset hole (origin) on the A plate is on the top left side when cutting the holes, do you then cut the holes in the B plate with the offset hole (origin) also in the top left corner (and then flip it for milling counter bores), or do you just mill the holes in the B plate with the offset hole on the top right side from the get go. (see drawing with the two possible options)
I'm asking because this might influence accuracy in case that the mill's axis are not 100% perpendicular to each other (which is the case on any mill to a certain degee). By milling both plates with the offset hole in the top left corner, it appears that the error in perpendicularity would be the same on both plates and therefore is less likely to cause an alignment issue later.
Yes counter bores in the A plate will be offset, a Mirror image of the B plate, 2 programs needed, counterbores for the pins/bushes are not so important as for the diameter, this is clearance for the head diameter, the depth needs to be correct for the head, or you just grind everything flush, you also need to vent the the Bush or the Pin, your holes may need a chamfer, (standard practice ) so the radius under the head of the pin /bush don't hang up, some pins / bushes have a relief under the head, then chamfers are not so important, chamfers are a good practice, anywhere they need to be used, so as you do each setup,do everything that needs, to be done in that operation, or that side of the mold Plate, this will save you setup time, have good planning of what you need to do each setup
Align Guide Pins so they don't jam...?
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