I have mentioned before that I would like to find a way to attach a mill table to my Altmill via fasteners from the top. I have started to think about this again and am wondering if anybody has tried tapping the existing (4.3mm) holes in the stringers of the Altmill.
If you have done this, please read on. If you have not done this, please do not guess at answers.
I am thinking that tapping the existing holes for M5 fasteners. The material is 3.5 mm thick so in theory there should be enough thickness for the fasteners to hang on to.
I have always had a tough time tapping holes straight so I am trying to think of ways that I could tap the 60 holes straight and with reasonable efficiency. The only thing I have been able to come up with is to 3D print a guide that clamps onto the flat portion of the stringer and has a 90 degree (ie perfectly vertical) hole in it to guide the tap straight. I am thinking of using a cordless drill to power the tap (on low speed). Thoughts? What did you use to guide the tap?
Did you feel there was enough thickness in the aluminum to hold the fasteners securely?
What is the likelihood of being able to CNC drill the 60 holes at the correct locations in the table? I suspect that the holes in an individual stringer are very consistent but I don’t know how consistent the distance between adjacent stringers is. I would have maybe oversize the holes for the fasteners by 0.5 mm but that still means that things have to be pretty well aligned.
@Jens you said no guessing but I’m gonna guess anyways…
By stringers, I assume you mean the cross beams with the lips? If I were to use the CNC to tap the holes, I’d have the same concern as you. There’s no guarantee that the cross rails are perfectly parallel with the tolerance required for tapping with the CNC.
If I were to do this, I would instead purchase 1/4" x 3/4" aluminium bar stock. I would then drill and tap holes for M4 screws using the CNC. Those M4 screws will fit in the existing holes in the cross beam. You can then mount that bar to the underside of the lip using double sided tape or screw it in place. Effectively, you would be making a +/- 50" long nut plate with +/- 12 threaded holes
Using 1/4" bar stock would give you more holding power than the lip on the cross beam.
I would suggest using a threaded insert riveting tool that is about $20 at Harbor Freight. Amazon should also carry them. It will give you more thread to secure to. I have used a 1/4-20 threaded inserts with great success.
@2DogwoodenConcepts, I have considered that (and I even have inserts at hand) but the inserts have a shoulder that would stick out on the top of the stringers and that would prevent the milling table from resting on the stringers. Instead, the table would only sit on the small shoulders of the insert and this could cause stresses of with unknown consequences.
I had considered inserts that end up flush (without the rim) but I have never worked with those. I suspect they require a hydraulic press to properly seat (the aluminum stringer would cold-flow a bit during installation to retain the insert). While I do have a hydraulic press on hand, the only way of doing the install would require removing the stringer.
@Chucky_ott, how would you use the CNC to tap the stringers ?
I had not considered putting bar stock on the underside of the stringers and retaining the stock with double sided tape while the mounting hardware is not in place yet. What I had thought of was to manufacture a wooden cross beam that sits under the stringer and has pockets for nuts. I eventually discarded that idea as being too complicated … but double sided tape on flat stock should work. The catch 22 is that the issue with the cross rails being parallel and perfectly spaced remains. I think I have no choice but to remove the current table and verify the spacing of the stringers.
@Jens you can easily verify the spacing of the holes from one cross beam to the next by making a storyboard with two pins. Look at Onshape to see what the spacing is supposed to be, then use your CNC to drill two holes at that spacing in your story board. Insert two 4mm pins in those holes and then check how closely the pins in your story board and the holes in the cross beams align. It will be more accurate from the top though, so you’d want to remove you existing table. You could also measure with a ruler but I find a story board can be more accurate for repeat measurements.
Alternatively, you could increase the holes in the cross beams from 4mm to 5mm, but still use M4 screws for the nut plate. That might give you enough wiggle room.
I don’t know how I’d tap holes with the CNC. I have not done it, but others (e.g. Francis Creations on the Facebook page) have. Of course, you can do it manually as well.
At least with the nut plate method, you can restart if you mess up. But it’s also removable. And you have the advantage of the extra thickness.
Insert nuts could work too but you’d need a recess in your mill plate to accommodate the nut shoulder that would protrude above the cross beam. And it’s more permanent.
I will use the storyboard method. It’s the only way I can be reasonably sure that things will line up. Measuring always ends up having errors and is impossible when the distance is wider than the calipers can reach. Also, when doing measurements on multiple points, the storyboard method is MUCH quicker.
I tried tapping with a guide block and that actually worked surprisingly well on the far back (exposed) stringer.
@Jens Come to think of it, assuming the holes are precisely drilled in every cross beam, you don’t really need a story board with pins to check the hole-to-hole distance. You would only need a storyboard to verify the spacing between the cross beams.
The mill table is off. Visually and with a tape measure, the stringers appear to be evenly spaced. I have decided to 3D print my story board for verifying each space between the holes in the stringers. It’s looking good.
I have now verified all spacing and it appears consistent within 0.5 mm or so. I have tapped 5 holes on each stringer. About 7" on each side is not screwed down since the CNC can not reach the position where the hole is. I do not anticipate that to be an issue. If it turns out that this is an issue after all, I will drill and tap new holes in positions that the mill can reach.
Hole tapping took only about 5 seconds per hole, 10 seconds if you include cleaning and lubricating the tap.
Next project will be to lay out the holes in Fusion, generate the tool path and give it a go. The holes will be 6mm for a 5mm fastener which will obviously be set below the surface of the base board. I will do a trial run on some scrap to make sure everything works out and the hole locations in the base board works. The extra mm in the hole size should take care of any issues with tolerances of hole positions (I hope).
I have run into a problem setting x/y zero.
I have a board that is 53" wide and 48" deep which is my mill base board. I have located all the holes I need to mill in a fusion drawing. Problem is that in order to mill the holes at the correct locations I need to set an x/y zero but the spindle can’t reach x/y zero of the board.
Any suggestions how to get past this issue?
One possible solution is to position the spindle over one of the stringer holes while the mill table is off and manually enter the coordinates for x and y. The front left outermost hole is 65 mm from the front edge of the mill table and the y distance from the left edge of the stringer to the center of the hole is 187.7 mm . Locating the spindle at the right location requires my Eyeball Mark 1 measuring instrument but I doubt that I will be within an acceptable margin of error of the correct location.
Anyway, I am just putting the question out there and maybe somebody else has run into this issue. I will sleep on the problem overnight and maybe something will pop into my mind.
I believe this is why I started zeroing centre project. I like my workflows to be as simular as I can possible get them and have none or nihil od ones out.
Could you reach all milling points when having a centre project xyzero?
This is an interesting idea and something that I will need some time to wrap my head around. At this point I am not sure how this can help but it is certainly something to explore further.
I was unable to wrap my mind around the procedure to use zeroing to the center of the mill table. It might work or it might not but my brain didn’t stretch that far. Luckily I came up with a perfect solution that was painfully obvious now that I know about it.
Find or produce a rectangular piece of stock.
Use calipers to accurately measure the width and depth of this helper stock piece. The edges need to be perfectly parallel to each other.
3)Place the helper piece on the mill, butted up against the left and front of the mill ie: the corner of the helper piece will be at the location where the mill table front left corner will be.
Use your auto touch probe (or whatever you use) to locate the point diagonally across from where the mill table corner would be. The auto probe routine will set the spindle right over that corner and set x/y to zero
Manually change the x/y coordinates to the measurements taken in step 2
At this point, the front left corner of the mill table is at x/y zero even though the spindle can’t reach that point.
This sounds a lot more complicated than it is in real life. It is a very quick and straight forward routine that is difficult/awkward to describe..
I can now put the blank sheet of material that will be the mill table onto the mill stringers, load the tool path, adjust my Z height as needed and proceed to mill all the pockets in the mill table.
Next step will be to do a sample run with some scrap material and see if all the holes are at the right locations so that the screws hit the threads previously cut into the stringer holes.
The last update for those that are curious:
I learned an interesting lesson about what I think is tolerance stacking. I ran several test pieces for the holes on the left side of the table (didn’t want to waste an entire big sheet) and towards the end, the screws slipped into the holes and into the stringers pretty much without touching the hole walls. I then decided to run the full sheet and things did not work out as nicely. Most of the holes were at the right locations. Some fasteners were a bit tight to get in when the holes in the MDF didn’t quite line up with the threaded hole in the stringer below. One hole had to be expanded a bit manually.
I tried loosening the stringers where they meet the right side beam but there appeared to be no play in the location of the stringer. It could be because of all the screws being installed … I don’t know.
Mounting the MDF required leaving all screws loose in order to adjust things a bit here and there. One nice thing about MDF - it is relatively soft so lifting the MDF up and down and rubbing the threads against the holes expanded the tight holes by just enough.
The holes in the stringers were tapped for 5 mm fasteners. The hole in the MDF was 6.8 mm (7 mm would have been better). The one hole that did not allow a fastener to meet the stringer hole required a 7,3 mm (it was an imperial drill bit that I measured) drill bit to chase the hole to make it fit.
I spent an enormous amount of time with this in my attempt to get the holes at the right locations and I was a bit disappointed that a lot of the fasteners were not a ‘drop-in’ fit. I don’t know if the issue was with my measurements, the hole locations in the stringers being not quite right or whatever. Maybe I was expecting too much. The dimensions between holes were taken from the CAD documents (these were MK2 drawings and I have a MK1 Altmill but that should not have been an issue) and verified as good as was possible but none of my trial fits involved multiple holes in multiple stringers at the same time.
As it turned out, because of the tight fit of all the screws, the mill table does not move even with the fasteners being loose. Tightening things down just pulls everything flat and it doesn’t need to be heavily torqued down. In other words, even though there are only a few threads engaged in the stringer, there is no issue with screws pulling out as long as one doesn’t over tighten things.
Yet another update:
I generated an auto-dimensioned drawing of the mill table and noticed that there were extra dimensions that I didn’t expect. After some investigation I determined that when I drew the base stringer and located all the holes, one hole had not been restrained correctly. It was off by 0.3 mm. This error carried over to all stringers and then to the actual base sheet. This added to the other positional errors and/or tolerance stacking that existed. The error was not caught because it was only visible under high magnification.
I will not produce another base sheet but the error has been corrected for the future.
I have a question…was your sole purpose of doing this was to be able to just bolt the top down without using fasteners underneath? Im going to start attaching my top tomorrow and was trying to figure out what I wanted to do.
