How do I machine a Fixture Plate?

ProjectZero · Mar 9, 2017

Hi guys,

I’m an amateur CNC machinist who needs to create a fixture plate for my ’05 Fadal VMC 3016. I’ve done some simpler parts in a vice, but for what’s coming up for work, a fixture plate would definitely help me do clean part flips. I’m basing mine off one I used to use at school (pic below).

It will have the same 1” grid of holes, with the top half of each hole being 5/16” press fit hole (.3125-.001 underreamed) and the bottom half being tapped 1/4-20.
Aligning it with the CNC bed is a little tricky because, unlike the Haas I learned on at school, the tracks are not a standard width that lend themselves to pressing in chunky off the shelf dowel pins. At school the track width was .625 so you pressed .625 dowel pins into your fixture plate and were set. My fadal has a weird .709 track width. I will either mill out (I don’t have a lathe unfortunately) .708 pins to press in or just press in .625 dowel pins, then push my plate back against the track as I tighten it down. Fortunately my work doesn’t have to be as hyper precise as a pro machine shop.
It will be a bit longer than the picture below - I’ll lengthen it from 8” x 12 inches, to 8” x 14 or 16 inches.

But my question is this - just *how* do I machine it? I’m scratching my head here. My 8” vice isn’t big enough to hold it. I could get a BIG piece of aluminum (10x18 wish), strap clamp it down on top of some scrap polycarbonate, and then machine it out of the inside of this big piece of stock. Is there a better way? How do you guys usually machine your fixture plates?

Thanks!

Screen Shot 2017-03-09 at 1.43.12 PM.jpg

Hazzert · Mar 9, 2017

Put your jaws on the outside of the Kurt, face one side flat, you can flip it over and see if you can get it parallel with the other side too.(I would use double sided tape for this) You could also buy tooling plate that wouldn't need to be faced.

Clamp it to a sac sheet on the short edges and do everything you can internally and on those long edges without hitting the clamps.

Have an M00 where you shuffle the clamps to the long edges and then finish the short sides and any holes you couldn't get to.

LostFab · Mar 9, 2017

.79 Inch Diameter Plug & Pin Gage 88872 - MSC
Buy pins that are the size you need, or offset the holes for the pins you modeled it for.
Use Mic6 so it won't need to be faced, clamp it down so you can cut one half, slide it over, adjust clamps, cut the second half.

lowCountryCamo · Mar 9, 2017

When I made my plate I learned that I could make more accurate holes, diameter and location by drilling undersize and interpolating the hole with wear comp vs. drilling under, plunging down with endmill, and reaming. However my mill has tight rails and ballscrews so you might have to experiment a bit.

ewlsey · Mar 9, 2017

Get some steel plate flame cut and blanchard ground.

ProjectZero · Mar 9, 2017

Hey guys,

Man this thread has been super useful already.

The tooling plate (Mic 6) is a great idea. It's not even that much more expensive than the sheet I could get from my local metal supplier, and not having to face off the top and bottom would simply the process quite a bit.

And I had no idea I could get .709" pins. Though, since the spec sheet I (finally) found says the diameter is .709", I should get .708" pins. And they have those too! That will make my life way easier than putting in smaller holes, offsetting them, and pushing up against the track.

Overall, I like the idea of getting tooling plate, strap clamping down, and drilling out half the holes. Then pausing, carefully moving the strap clamps over to the other side (at least 2 on at all times, I was taught), and drilling out the other half of the holes. If I get the pins ahead of time, I should be able to push the plate up against them before strap-clamping it down.

scadvice · Mar 9, 2017

I've machined alot of fixtures over the years and I found DME has a number of steel plates at such a low cost that I almost never machine my own. Here is an example of one from them.

7-3/8 x 18 x 1 - 6-1118RGM-

At just under 120 bucks it's not worth my time to try to get a piece of raw stock flat. Just by checking McMaster you would spend 150 to 160 bucks just for the raw material. So even shelling out 40 or 50 bucks for shipping your still ahead of the game. Mic 6 Alum. would also be near twice as much.

ProjectZero · Mar 9, 2017

scadvice said:
I've machined alot of fixtures over the years and I found DME has a number of steel plates at such a low cost that I almost never machine my own. Here is an example of one from them.

7-3/8 x 18 x 1 - 6-1118RGM-

At just under 120 bucks it's not worth my time to try to get a piece of raw stock flat. Just by checking McMaster you would spend 150 to 160 bucks just for the raw material. So even shelling out 40 or 50 bucks for shipping your still ahead of the game. Mic 6 Alum. would also be near twice as much also.

You usually create your fixture plates out of steel over aluminum? They must be heavy.

Monarchist · Mar 9, 2017

ProjectZero said:
You usually create your fixture plates out of steel over aluminum? They must be heavy.

Do yours have to fly LAX <=> HKG every day with a Jet A fuel surcharge?

Steel is also lower cost, stronger, stiffer, more dimensionally stable, and generally cheaper to alter or repair.

Save the light-metal for where it adds genuine value. As it can do.

Just not 'always'.

LostFab · Mar 9, 2017

ProjectZero said:
Overall, I like the idea of getting tooling plate, strap clamping down, and drilling out half the holes. Then pausing, carefully moving the strap clamps over to the other side (at least 2 on at all times, I was taught), and drilling out the other half of the holes. If I get the pins ahead of time, I should be able to push the plate up against them before strap-clamping it down.

If your pin and clamping holes are inset enough that its possible you should clamp it down and bore those all on one operation, then you can slide the plate around and chase clamps to put in the rest of the holes. That should give you the best starting point to make sure everything remains as square as possible.

Garwood · Mar 9, 2017

.709 is 18MM.

That's a hell of a lot more common than .625 on machine tool slots.

Gauge pins aren't expensive at all.

I mostly use fixtures on my mills. I make most of mine from aluminum to start and if the product takes off I will remake from steel or aluminum + hard anodize. Under 1000 parts aluminum does fine. For over 1000 parts you need hard anodize and helicoils or steel.

My recommendation is to put all the features in to attach it to the table then bolt to the table and machine everything else in-situ.

One trick I have used is to mill slots in the backsides of fixtures connecting threaded holes to the outside of the fixture. Deep enough slots for chips to pass through. Then when you blow air in the hole to clear the chips to screw in a bolt the chips and coolant will blow out the side of the fixture instead of back into your face.

ProjectZero · Mar 9, 2017

Garwood said:
.709 is 18MM.

That's a hell of a lot more common than .625 on machine tool slots.

Gauge pins aren't expensive at all.

I mostly use fixtures on my mills. I make most of mine from aluminum to start and if the product takes off I will remake from steel or aluminum + hard anodize. Under 1000 parts aluminum does fine. For over 1000 parts you need hard anodize and helicoils or steel.

My recommendation is to put all the features in to attach it to the table then bolt to the table and machine everything else in-situ.

One trick I have used is to mill slots in the backsides of fixtures connecting threaded holes to the outside of the fixture. Deep enough slots for chips to pass through. Then when you blow air in the hole to clear the chips to screw in a bolt the chips and coolant will blow out the side of the fixture instead of back into your face.

Haha, probably should have put two and two together about the 18mm. That's great. Means I won't have a problem finding an 18mm reamer. The only problem is that my slot is a nominal 18mm. Meaning I need my dowel pin to be undersized, I'd think. Or do you think an 18mm dowel pin is going to fit in there? I run the risk of finding an 18mm under reamer, making nice press fit holes for an 18mm dowel pin, and then having it not fit in the tracks. I might just have to get some 18mm dowel pins and see if I'm lucky. Surprisingly I haven't found 18mm dowel pins. The best I can do is the .709 ones I was linked to on MSC. I'm surprised I can't find metric dowel pins in 18mm size.

Regardless of all that, I like the idea of milling the dowel pin and t-nut/clamp holes, mounting it to the table, and then milling the rest of the features.

@Monarchist, haha fair enough. But I'm doing very low volume prototyping, and AL's gonna be a lot easy for me to machine! I'll go with Garwood's approach. If production somehow spikes, maybe I'll remachine out of aluminum or hardened AL.

Garwood · Mar 9, 2017

I don't think anyone buys 18mm dowel pins. They buy gauge pins. Gauge pins are typically 2" long and they cut fine with an abrasive wheel in a die grinder if you need shorter.

A 709 minus gauge pin will be a couple tenths on the big side. It will be a light tap fit in the slots if the fadal slots are ground. If you don't have a gauge pin set to check your slots then buy several pins and check the fit to see which you want to use. Then buy 20 or so more for the rest of your fixtures you haven't built yet.

scadvice · Mar 10, 2017

Not any more so at that size than a Kurt vise. Aluminum plates are fine but you do need to put thread inserts in them or they will strip out over time. With steel it's not necessary.

After drilling and tapping your plate be sure to wet stone the surfaces as there be a slight material rise around each hole.

irontoys · Mar 10, 2017

If you value your time, consider writing a check to Stevens Engineering. Problem solved.
Stevens Engineering Inc. Home

pirko · Mar 11, 2017

Dear,
Wery easy is camping on MDF board by vacuum. (MDF board is back wall in furniture. Regards Libor.

gorrilla · Mar 13, 2017

"Mark it with chalk, cut it with a torch! If you can't dazzle em with brilliance, baffle em with bulls__t!" Saw this on the bumper of a quick little Chevy II at the drags one day. Had to steal it. Seems applicable here.

ProjectZero · Mar 13, 2017

Garwood said:
I don't think anyone buys 18mm dowel pins. They buy gauge pins. Gauge pins are typically 2" long and they cut fine with an abrasive wheel in a die grinder if you need shorter.

A 709 minus gauge pin will be a couple tenths on the big side. It will be a light tap fit in the slots if the fadal slots are ground. If you don't have a gauge pin set to check your slots then buy several pins and check the fit to see which you want to use. Then buy 20 or so more for the rest of your fixtures you haven't built yet.

Thanks Garwood (and everyone else!).

Follow up question. I've ordered the 709s and some 708s in case the 709s are too tight. Do you think an 18mm reamer will be a good tool for making the press fit holes in my fixture plate for the gauge pins? I would of course assume I'd need an 18mm underreamer until you mentioned the 709s were going to be a couple tenths on the big side.

plastikdreams · Mar 13, 2017

In aluminum I'd go .001 under for a tight press fit on a 709, if you need to take them out a .708 will not stay in there, you could peen the rims but it's still not gonna stay long.

ProjectZero · Mar 13, 2017

plastikdreams said:
In aluminum I'd go .001 under for a tight press fit on a 709, if you need to take them out a .708 will not stay in there, you could peen the rims but it's still not gonna stay long.

Ah yea, that's what concerns me. If I go with an 18mm reamer for a .709 pin it's going to only be about .00035 under, about a third of the recommend 1 thou. Will probably be pretty loose.

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