So ive now got the maslow going well in the middle and upper sections of the sheet. However in the bottom corners of the frame the results are very poor. So ive decided to go ahead and raise and lengthen the top beam. I have a 4m length of RHS available that should do the trick so here is my proposed changes, top frame is the new one with the bottom my current setup. Any input and thoughts much appreciated.! I also believechain length will need to be around 4300mm is this correct?
to center may not be quite as good as the tension there is a smidge higher.
as for chain length, once you get to about 12’ top beam, the chain length to the far bottom corner is less than the distance between the motors. (in your case, you see the long length to the bottom corner is quite a bit shorter than the distance between the motors) so your required chain length is closer to 3800mm for a 4000mm motor spacing (a bit more doesn’t hurt)
note that as a step in calibration, it measures the distance between the motors with the chain, so it’s good to have the chain length be the distance between the motors plus a couple inches on each side to be able to get a good grip on each side’s sprocket.
42.96 lbs might be a little too much in top middle. I’d raise the bar to at least 30" to reduce the strain on the motors.
Okay sounds good. I did have an idea… instead of getting 2 new piece of chain (expensive where i live) could I use 800mm of 4mm stainless rigging wire (i have heaps leftover from when i put my mast up on my boat) its got a breaking strain of near 1000kg and is super flexible and doesn’t stretch. So I was thinking i could crimp it to the end of the chain and then connect it to the sled? Since this bit will never go through the gear anyway? For the distance measurement ill just cable tie the 2 chains together over say 10 links so I get the length then cut the cable ties once done? Thanks
Ok thanks for the input, does this look better? Is a minimum static force of 9.68 going to be enough to allow cutting in the bottom corners? I do need to use a full sheet at times.
Thanks
looks better
Ok thanks. @Metalmaslow what do you think about using a short length of cable to extend my current chains? The stock chains are 3353mm and I need 3800/4000mm so its not much extra. I’ve checked and it’ll be at least that distance from the motor to the corner of the panel so it should work? I’ll get a joining link so I can use the 2 chains together to measure the top beam. It should work?
I would worry about stretch in the cable ties.
If you can make it so that the length of the cable doesn’t change beteen 10 and
40 pounds of tension, this would work. But loops of cables may elongate, wire
ties may flex.
any change in length cn translate into 2x that amount of position error.
David Lang
the limits in the bottom corners are:
-
the tension on the slack side of the chain (if there is more tension on the
slack side, the gear backlach translates into error) -
how much friction does your sled have against the workpiece (the force
provided by gravity is tension modified by chain and frame angle), the more
friction, the more force you need. -
how fast are you trying to move, the slower you move, the more likely the
available force will move the sled correctly. (but slower cutting will generate
more heat)
so unfortunantly there’s no ay of giving a difinitive answer to your question.
There are people producting acceptable cuts to them with the stock setup.
Also when cutting near the side and bottom edges, the Z balance of the sled
becomes a factor. you need support below the workpiece as the bricks unbalance
the sled, and you need at least a couple inches of support beyond the side of
the workpiece.
David Lang
measuring the top beam via chain was a good thought (I’m the one who suggested
it 
) but in practice we have enough error in the chain length (aka chain
stretch) that the current number produced isn’t useful, you are better off using
a class 1 or class 2 tape measure to get the distance.
Ideally we should get both measurements and use the tape measure version for the
motor disance, and then use the chain measurement to get an estimate of the
chain stretch by looking at how much shorter the chain measurement is than the
tape measurement.
David Lang
Thanks for that ive use copper crimps now and the loops are snug on the cotter pins and on the chain. See attached I think itll work? I’m about to cut the calibration cuts now
thanks for that information also! My sled has no bricks its a stainless steel base with a linear rail z axis so it has more weight in the bottom already. I’m definitely going to get some teflon for the bottom of the sled as well. I’m just cutting the calibration cuts now and even on the first run I can see that in the bottom corners the cuts are way more straight than the old ones were so i thin its working!
So it seems to have worked! Here is a test cut of a back wall for a glasshouse im going to make. Nice solid curve right to the edge!
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looks like your top bar is out too far? 25mm wont’ matter, but it looks liker maybe 100mm which will cause errors towards the bottom.
chains should be parallel to work piece
I could ship you a small piece of chain to lengthen your chains, it’s pretty cheap, however shipping to Europe is $25.
By out too far do you mean off centre to one side? Its probably the angle I’m standing at when taking the photo? It was cutting when I took the photo so thats why the sled is off centre. Its within 1mm of centre ill check it again today but I did measure it about 4 times and got it as close as I could to spot on by myself. Thanks for the offer regarding the chain, I see its fairly cheap on ebay and shipping from China to NZ is free, but I’m going to keep using the cables for now they seem to be doing the job perfectly, ill see just how accurate I can get it after today I’m going to put some PTFE on the sled and 4lbs on the bottom of the sled to get it to my 27lbs target weight.
So I have now put 8mm uhmw on the sled bottom (couldnt get any teflon locally) and added 4lbs of steel to base as well to bring it up to 27lbs total weight. Cut my test frame join for my glasshouse in the extreme bottom left corner( right on the edge of the sheet. First photo is from the original frame second is from my new frame. As you can see the screws are coming out the pilot holes on the other side so I’m very happy with the accuracy!
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So I thought I would post quickly how I calibrated the machine so anyone else in a similar situation can do the same thing as I did. The cable used is 2.5mm stainless steel rigging, but anything around that diameter would do. I measured the motor spacing first using a tape measure as @dlang suggested. DONT crimp the cable onto the chain yet! Then I let the motor measure out the correct chain length, grabbed a nail cut the head off and nailed the chain to the backing board with a small amount of tension to remove slack through the end link. This gives a place to hook the eye of the wire over. Crimp a tight eye in one end of the cable, remove the chain from the nail and put the eye in its place. Then remove the chain from the gear and move it back around the length required and put back on the gear, slide the cable through the second link in the chain and pull it up so it has the same amount of tension and mark it with a sharpie at this point. Take the cable off the nail and crimp it on the chain end and cut the excess. Put the free end back on the nail to re check the tension. Repeat on the other side and now both ends are lengthened by the amount required by you. From this point attach the ends of the cable to you sled and calibrate from this point as normal.
:As a note put a mark on the gear so you know where the chain is meant to sit as you can’t re measure out the chain using the motors easily with the first link blocked with the cable.
Very long winded explanation but I hope it helps someone!
As a side note I still had heaps of chain between the gear and the crimp when the sled is at the top corner, so a longer top bar than mine (4000mm) is possible with stock chains. I lengthened mine by 600mm roughly per side. Some photos of the crimps attached
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