Dowel Pin Advice

[NylonAdmiral]

I am making a fixture plate for my mill and as such I am intending to locate it on the mill's slot bed with dowel pins before bolting it in place.

I basically want a tight enough fit that I can ensure reasonable repeatability of placement, but I would like to be able to remove the plate by hand (possibly with a gentle hammer tap if necessary).

My question really is; from experience can anyone adivse the best way to achieve this fit?

I'm intending to use 8mm dowel pins.

My plan was to drill a small pilot hole, followed by a slightly undersized hole and then ream the hole to the correct size.

Any advice on choosing an appropriate reamer for the job?

Thanks in advance!

[beege]

First, I might use these:

http://www.carrlane.com/Catalog/inde...3C3B2853564659

instead of dowel pins. One regular locator and one diamond. Give some fudge factor and still locates pretty accurately. If you opt with dowels, get the kind with grooves in it for hydraulic release. What material are you going to use for your plate?

[mactec54]

Hi NylonAdmiral

I guess the 8mm dowels are a nice fit in your table T-slot, What you are planing to do will work just fine, You only need to have 2 dowels in the centre of your fixture plate out at the ends of your fixture, the fixture plate in the photo has stepped dowels made to fit the tee-slot

[NylonAdmiral]

Thanks for the quick replies!

@beege
I'm using 10mm thick aluminium tooling plate. I had considered using those locating pins but I think the small size of my slotbed will be prohibitive. I had intended to locate the fixture plate with four dowels. If you look at the picture I've attached you can see the dimensions of my slotbed that I have to work with. Although I can easily fit a locating pin in the material thats 40mm wide, I wont really have space to do so in the bit thats only 14mm wide.

I guess I could use bullet nose pins similar to these: http://coburg.co.uk/productPrint.php?prodID=421

The smallest of the pins would fit and I'd have to fit the sleeves to the fixture plate but I would prefer to do it the other way round if possible.

@mactec54
I may have misunderstood you but my plan was to use the mill to actually drill into its own slotbed and fit 4 dowel pins into these holes. As suggested by beege I would have prefered to fit bullet nose dowel pin liners into these holes, then fit locating dowels to my fixture plate so I could drop it into place. However because of space restrictions I'm don't think it will be possible.
At the outboard edges of the fixture plate I intend to put recessed through-holes so that I can put a bolt through the plate and into a t-slot nut to secure the fixture plate in place.

Aside from the issue of actually locating the plate I'd still like some help with my original reamer sizing question as I intend to machine holes in the plate for locating dowels as well as threaded holes.

Thanks both for the input!

[mactec54]

Hi NylonAdmiral

You should not drill into you table, You can put the dowels in far enough so you have room for your tee-slot to be used to mount your fixture, you only need 2 dowels

[NylonAdmiral]

Hi Mactec,

Perhaps you could explain further, again I could well be misinterpreting you but by using two dowels won't that only provide stability in the Y axis? Surely this still allows movement along the X axis if the dowels protrude from the fixture plate and sit in the T-slot? Please forgive my misunderstanding!

I actually got the idea for the method I previously described from this thread:

http://www.cnczone.com/forums/showth...108#post716108

[mactec54]

Hi NylonAdmiral

I looked at the link, There is no way this is the right thing to do, on a machine that has a ballscrew lead error of around .002 per 12", those bushings he has put in his table would have to have clearance/slop so the pins could fit,

The way I have showed in the photo, you just put your fixture on the table, indicate the X0 Y0 start point & you are ready to go

& yes you can slide the fixture anywere along the X axes, a lot better than having it fixed

With the fixture plate in the link you would still have to indicate the X0 Y0 start point

[NylonAdmiral]

I see what you mean about the ballscrew lead error but surely if you know where all the the dowel pins are and you have correctly spaced all the holes for the locating dowels then its very easy just to mathmatically calculate the X0 Y0 Start point for most tasks?

Or for a better example, if you do a job that you know you will have to do again in the future, if you know the XYZ offsets for that job you can set the machine up for all kinds of other tasks before coming back to it, but when you do all you have to do is re-enter your known offsets and position the stock on the same dowel holes and your machine setup time is reduced to pretty much nothing. However, if the fixture plate has been removed in between repeating the job and it isn't fixed in place with some kind of locating pin then when you come to do the same task again you would need to spend a lot longer working out your offsets.

Thanks again for your advice here, I'm keen to work out the best way of implementing my fixture plate so it's usefull to speak to people who have some experience in this area.

Seeing as this thread has become less about dowel pin tolerancing and more about methods of securing a fixture plate I've started a new thread for it here:

http://www.cnczone.com/forums/showthread.php?t=98612

Hopefully it might bring more people to the discussion too.

[mactec54]

NylonAdmiral

I understand what you are saying, So If you take the table off your machine & have the holes put in on a jig borer & then the bushings ground on a jig grinder in place, then you could rely on them being in the right place, doing in on your machine with your machine you will not have the accuracy you need, to make it work with 4 pins without having to have oversize holes/slop in the fixture plate to fit over your 4 pins,

Set up time is the same, as I said before you have a X0Y0 point on your fixture, that all you need to set up each time, & should take no more than a minute

[rowbare]

If you are going the pinning route, two pins are sufficient to locate your fixture. As beege noted one should be a diamond pin. Four pins are overkill and make it really difficult to match up the fixtures and your pins. You have to be able to work to very tight tolerances or do both on the same setup as in the link you provided. Once you get a mile or two on the ballscrew, it might be difficult to make a new fixture to fit.

For most of us, Mactec54's approach is the simplest since it keeps the fixture square to the table. I use a similar approach and where space permits, I will bore a hole in the fixture and use the probing routine described here http://www.cnczone.com/forums/showthread.php?t=56079 to find my offsets.

bob

[NylonAdmiral]

Thanks rowbare,

I had actually seen that same probe video on youtube a while ago!

Unfortunately I don't have the capacity to do that.

From reading what you all have suggested I'm thinking I might adopt the method that mactec has offered and see how I get on.

I'm thinking I should fit two dowels into the underside of my fixture plate that will fit into my T-slots. Once set up I can accurately drill a hole in the top surface at a known point, say for example X0 Y0.

If the fixture plate is removed I can refit it and reset X0 Y0 using a wiggler.

Does this sound like a sensible approach? I'm sure some of this is probably blindingly obvious to anyone with experience but I'm really a bit of a "newbie" here so I really appreciate people's help

[mactec54]

NylonAdmiral

For your set up, If you are using a edgefinder/wiggler just use 2 edges X&Y of your fixture plate as your 0.0 point, the edge is easer with a edgefinder/wiggler, for a hole you would use a dial indicator