I am creating this topic because I expect there will be a number of discussions unique to the concrete slab installation type. I may even use this first post as a summary of some of the previous discussions.
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So here’s my first two questions:
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I know that the recommended installation should include a 12"-18" (30 cm-45 cm) margin around the work piece. If I expect to mostly cut from 4’ x 8’ sheets but can’t guarantee I won’t want to cut bigger, is there any disadvantage to giving it a larger margin? Loss of accuracy?
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You would think with a concrete slab as your frame, all your problems with frame flex would go away, but there is a potential flex if the posts extending from the concrete are too small in diameter. What is a reasonable size?
- I know that the recommended installation should include a 12"-18" (30 cm-45
cm) margin around the work piece. If I expect to mostly cut from 4’ x 8’
sheets but can’t guarantee I won’t want to cut bigger, is there any
disadvantage to giving it a larger margin? Loss of accuracy?
as long as you have long enough belts, probably not
- You would think with a concrete slab as your frame, all your problems with
frame flex would go away, but there is a potential flex if the posts extending
from the concrete are too small in diameter. What is a reasonable size?
until we find out what the forces are, we don’t know.
David Lang
The strength (i.e. resistance to flex) of the posts would be greater with larger diameter, so a good way to anchor the Maslow4 would be to use a bolt through a tube (a spacer). The spacer provides the stiffness, while the bolt provides the downward force holding everything to the slab. Here’s a sketch:
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also note that if they are far enough out from your workpiece, you don’t need a
significant amount of stickout, just run the belts all the way down to the
ground.
David Lang
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Good point, thanks!
- Reducing height of the anchor assembly reduces the potential flex of the anchor assembly.
- Increasing height of the anchor assembly assures clearance of the belt over the work piece over the entire range of sled positions.
To address my initial question, we might need to consider the sag of the belt and how that affects the clearance. The greater the distance to the anchor points, the greater the sag. Ah, memories of catenary calculations!
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- Increasing height of the anchor assembly assures clearance of the belt over the work piece over the entire range of sled positions.
fair point, but we are generally talking about 3/4 or thinner material, you
don’t need a lot of angle or height to clear that
To address my initial question, we might need to consider the sag of the belt and how that affects the clearance. The greater the distance to the anchor points, the greater the sag. Ah, memories of catenary calculations!
The belts are light and the tension is relatively high, which reduce the effect
of sag. worth checking though.
David Lang
Do we know the kN / lbft force that the motors can exert on the anchors as I think we are in danger of over engineering the solution
I’m thinking something that can flip out of the floor that stay level when closed (magnet perhaps) and then secure with some sort of rod in the back to keep roughly at 90° to the floor. As long as it doesn’t move it’s not an issue and will always be under tension
no, Bar said it’s closest to the 57 rpm version of
https://www.aliexpress.us/item/3256805241838296.html
we do know that they flex the unistrut noticably.
and it doesn’t take much stretch to affect the positioning (in the worst case,
0.1mm of stretch can result in 0.2mm of error, and when you have 10+ ft of belt,
it is not hard to stretch it noticably.
David Lang
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I just measured it and it’s about 14kg (30lbs). That’s not nearly full power so we have the option to increase it significantly if we want, but that’s a good ballpark number for what we should be planning for when thinking about how hard the belts are pulling on the frame.
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One doesn’t need a fancy sleeve. A chunk of 4x4 with an appropriate sized hole, preferably offset away from the centre, would be all you need and more than stiff enough.
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On that note, does anyone have a good source on easily hideable concrete anchors? Either anchors that fold or telescope flush to the floor, or are easily removed without having to torque wrench them out every time. I’m thinking of using the car space of my garage with either 4 or 2 anchors (if I lean to the wall), but then pack it all up fairly efficiently to park after finishing up. Bonus points if the design has a dirt plate cover…
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I am still in the thinking stage about my setup, but I was envisioning a drop-in anchor, maybe epoxied into the hole, with a flat head cap for the hole secured with a flat head screw. I would probably use a hex drive screw and insert a plug into the drive hole. Even describing it, it seems rather complicated. Probably could be simplified.
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a embedded nut and have the hex head above the ground
or if you don’t like the hex head, a button-head bolt (the tradeoff is that the
button head is less likely to catch things, but harder to pull out)
you could even grind down the hex head to make it thinner, it’s not needing to
hold anything, just fill the hole to keep it clear when not in use.
David Lang
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Or just use a set screw. No head to drive over.
Pool cover anchors could fit the bill as they’re designed for constant force and hide flush to the ground when not in use.
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My other thought was drill a hole deep enough for a bolt to fit flush with the floor, and then make a space below the Bolt’s head for a hex nut to permanently sit. So when you want to use the Bolt’s, unscrew them and they will raise up, when done just screw them down flush. No caps, rings, joints to deal with since the bolt head is its own cover.
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the problem with that is tht the space around the bolt (or if a socket head, the
socket of the bolt) will fill up with gunk that will need to get picked out
before you can get a tool in to manipulate it.
David Lang
What retraction current was this measured with?
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I’ve been thinking about mounting some Maslow anchors in my (concrete) driveway. Since this is outside, I need to worry about corrosion and crud (e.g., dirt), as well as tripping hazards, etc.
By using stainless steel bolts and anchors, I should be able to avoid corrosion issues. Countersinking the anchors a bit and keeping short bolts in them when they’re not in use could keep out the crud and eliminate tripping hazards. Comments?