Undersized holes in new MFSlab

[Monte Milanuk]

Picked up a new MultiFunction Slab (http://www.multifunctionslab.com/) from a local CNC shop (at the local lumber yard). Got it home, eased the edges and the slots and hand-holes with an 1/8" round-over bit... and then dug out my PrecisionDog.us bench dogs and rail dogs.

The bench dogs *barely* fit in the holes, pretty much have to tap them in, and then tap them back out from the other side using a rod or something similar.

The rail dogs... well, only the tips fit, so that makes them fairly non-useful at this point.

Measured the dogs at ~19.95 mm, according to my Mitutoyo 6" Digimatic calipers.

Measured the dog holes at ~19.74 mm. I realize calipers aren't the most exact or best way for checking the ID of holes, but in this instance, they're a) what I had available and b) probably close enough.

Given that the plans provided to the CNC shop called for 20mm holes... what are my options? I put a feeler out to them to see if they can/will fix it, but I'm not sure if it's simply within the +/- tolerance of the machine they used.

I'd be fairly unhappy if next time the holes come back *over* sized rather than under - I had plans for using the dog holes for locating fences, guide rails, etc. so they need to be pretty much spot on.

Not sure if a split dowel with some sand paper to act as a sort of 'flap' sander to get each hole to the desired size is a better path, or if there is something else?

[Timothy Orr]

I would think a reamer would be the way to go to take off the .25mm or so. You can get a 20mm reamer or an adjustable one. They make hand held ones or you could use a power drill, but would probably need a jig to keep perpendicular. McMaster Carr would be a source, but might find cheaper at other metalworking suppliers.

[Bill Dufour]

hand reamer. Get an adjustable one for better fit. I would also chamfer the top of each hole.
Bill D

20mm= 13/16= 0.79 inches so 3/4 inch adjustable reamer should do it. Use a piloted reamer if possible. A HSS reamer should work depending on material?

https://www.ebay.com/itm/4pc-Adjustable-Reamer-Blade-Type-19-32-21-32-11-16-3-4-15-16-1-1-16-Plus-Parts/283729282979?hash=item420f9547a3:g:Uh4AAOSwdUtdMPC n

[Jim Becker]

While this is fixable by the methods mentioned, if the specification was 20mm, there's no reason why the CNC shouldn't have cut the holes at precisely 20mm unless there was either a human error or a worn or improperly sharpened piece of tooling was used. Even if I was choosing to fix it myself, I'd have a conversation with the shop because "my time" is also valuable and you were essentially paying to not have the drudgery of boring a billion holes by hand, etc.

[Frederick Skelly]

While this is fixable by the methods mentioned, if the specification was 20mm, there's no reason why the CNC shouldn't have cut the holes at precisely 20mm unless there was either a human error or a worn or improperly sharpened piece of tooling was used. Even if I was choosing to fix it myself, I'd have a conversation with the shop because "my time" is also valuable and you were essentially paying to not have the drudgery of boring a billion holes by hand, etc.

Jim,
From what you know of CNC, is there any chance they could put this back on the machine and open those holes to 20.00mm? Does CNC use any registration pins or anyhing that could put it back exactly where it was so that such a thing would actually work? Or would this be a "throwaway and do-over" for the shop?

Just curious.
Fred

[Brad Shipton]

Did you discuss a tolerance or what you were using it for? If so, operator should make new. If it was not made clear i would ask what he is willing to do. Maybe share cost for new? He clearly used pocket tool path. He needs to use 20mm boring bit. They are not expensive. I have done this before on my machine.

Some have pop up pins, but I would doubt he does given what he gave you. Even with pop up pins they could end up sloppy. Sounds like a do over. I would not fiddle with boring them out myself.

[Jim Becker]

Jim,
From what you know of CNC, is there any chance they could put this back on the machine and open those holes to 20.00mm? Does CNC use any registration pins or anyhing that could put it back exactly where it was so that such a thing would actually work? Or would this be a "throwaway and do-over" for the shop?

Just curious.
Fred

Fred, the answer is "maybe"...without knowing how they mounted and registered the workpiece to cut it, nobody could say for sure. And sometimes the method for registration to the machine bed gets cut off as part of the process, such as when a workpiece is cut out of a larger piece of material. In that case, remounting to make the holes bigger would be a tough row to hoe. Assuming this is MDF or similar, the material cost is low, so were it me doing the work, I'll probably re-cut from scratch after making adjustments in the design/toolpathing to fix the grid of holes that were cut too small. The holes should be barely snug for their intended purposes. If you need to force dogs and fixtures into them, the holes will get deformed and the grid may become inaccurate...that in addition to it just being a pain in the backside.

I'd go back to the shop...and take a sample of your hardware with you in addition to the piece that's not cut to spec, so they can see what the issue is and make the necessary adjustments for a replacement. They can put their calipers on the hardware, for example...

[Ron Selzer]

"Not sure if a split dowel with some sand paper to act as a sort of 'flap' sander to get each hole to the desired size is a better path, or if there is something else?"
40 grit paper and a cordless drill to see if a couple of revolutions or a lot of revolutions got the job done
couple of revolutions would not be too bad due to the work you have already put in to this piece and would have to do over on a new piece
Not certain how many holes we are talking about here either
I probably would try this on one hole and then decide, 10 holes versus 100 holes x how long for each hole
would not go back for anymore work to this guy again, either if he didn't make it right this time.
good luck
Ron

[Bill Dufour]

Sounds like he was using a worn or resharpened bit and interpolating the holes. Since the bit was undersized the holes are undersized. I do not understand why they are not all the same undersize amount?
Bill D

[Brad Shipton]

Bill, all CNC's are not created equal and not all operators calibrate their entry level machines often. If this is an entry level machine they use belt drives instead of precision gearboxes. Those can loosen over time. We are talking quite small errors, and when you get into the weeds of accuracy you will find different x/y accuracy in different areas of the table. This is another reason I am a fan of using a 20mm boring bit. With those you only have on point that can be out instead of many around the perimeter of a pocket tool path.

[glenn bradley]

The basic problem severity for me would be based on what I intended to use the table for. If I planned to rely on the hole positions for squaring things or aligning rails for a track saw, they would have to be more correct. There is no way that I can correct what you describe and get the level of accuracy I might want. As much as I would like to think I would have checked the product before I signed off on it, I can totally see myself getting the thing in my shop before I noticed something like this; especially if the errors are varied and spread out.

Even though you have done a lot of work on the thing, nothing you have done caused those holes to be wrong. Only you can decide on the level of wrong you are comfortable with. I also don't know how much it cost. I can plunge route with a jig and make pretty darn accurate holes all by myself but, I may pay for someone to do a few hundred holes auto-magically as I would incur costs in time and tooling.

In the end, if it is usable and good enough for what you paid for it, great. If you had the job done by a pro because you wanted CNC accuracy for your purposes, you did not get that and they should make you one that is as you agreed on. Otherwise, if it were me, I would silently growl and wish ill will on the maker every time I walked up to the thing ;-(

[Monte Milanuk]

My intention from the start was to use the dog holes for alignment of a fence and the guide rail for square cuts. That's literally what I wanted an MFT style top for. So not being able to fit dogs in the holes is pretty much a show stopper for me; it's an expensive waste of MDF otherwise.

I haven't done a lot of work on it *yet*, other than about 5 minutes worth of buzzing the edges and the slots with a round-over bit. I did plan on taking a rabetting bit to the under side of the holes, since regular Festool style clamps won't fit thru 1" thick material. No idea why the original design spec'd that thickness, but didn't include counter-bores on the backside of the holes in the plans. Not a big deal, maybe 15-20 minutes with a hand-held router doing some work that isn't really dimensionally critical - unlike the actual dog-hole diameter.

[Monte Milanuk]

Well, got a response back from the shop. Said they tried their Festool dogs which fit fine. They also said their computer said the holes should be 20.1 mm.

They suggested doing it over, so that's good. I'll definitely take my dogs (and my calipers) along next time to check before I bring it home.

[Frederick Skelly]

Well, got a response back from the shop. Said they tried their Festool dogs which fit fine. They also said their computer said the holes should be 20.1 mm.

They suggested doing it over, so that's good. I'll definitely take my dogs (and my calipers) along next time to check before I bring it home.

Glad to hear it's going to work out for you!

[Ronald Blue]

That's great Monte. It says a lot about them that they are willing to remake it to meet your needs and make you happy. Businesses like that deserve all the praise you can heap on them assuming they get it right the 2nd time and I'm sure they will. I'd also keep them in mind in the future if you need more CNC work completed. Sounds like a top notch company to me.

[Wes Grass]

I've seen holes come out undersize from too high a feed.