Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: NormB on January 26, 2010, 04:42:14 AM
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It seems I am beginning to run more steady now and am looking more critically at the actual cuts being made and I find my table is not parallel to the gantry by 3/64 of an inch. This is from side to side on the short axis. #hat I am doing is setting Zero at onecorner of the table raising the Z move to corner 2 set zero and checking it's the same.
Having determined this small error what is the best material to use to shim the table flat. I have a solid base with an MDG top with screws holding the mdf down in four places. This is 1" MDF with T bar slots cut in it and I have T bar in the slots.
I am not sure if tape or some other material is best to use. I am sure others have had this situation, I can see why you really want to protect that table top now it takes a bit of time to level things out for accuracy.
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Can you not just surface your top plate?
Hood
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Well, I guess I can but wow that would take some time. I think you mean set Zero on the low corner and begin passed backa nd forth to level the table. This is 24" x 48" that might take some time.
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a large Flycutter shouldnt take too long but even if it does it will be the best in the long run I think.
Hood
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Is that what most do install a table then begin to rout the surface to level it? I did this backwards then I cut T track inot the top and mounted it. I would have to be careful not to hit the Alum T track.
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that would save a oot of effort finding the low spots for sure. As I think about it that really makes a lot of sense. the you know the entire surface not just corners are perfecly level to the gantry. and full length too.
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Would you use like a 3/4 dia bit to do this? Or is there some sort of fly cutter to use?
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I see you already told me that................thanks Hood, I will look for a fly cutter maybe 2" dia or more. Have to see what I can find I am not removing a lot of material so can be rather large with a 2 1/4 HP router.
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Oh a router, only use a flycutter if you can reduce the spindle speed, dont want a flycutter going fast as it is unbalanced and a router spindle is not the strongest and also it will likely shake your gantry quite a bit.
Hood
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Sorry Hood but i am going to disagree with you about surfacing the table.
I believe that the machine should be adjusted to get it flat using a flat reference "surface plate springs to mind".
Only when you have it almost perfect should then surfacing be the next step.
The reason being that anything that is substantial in thickness may have that error on the surface. You will also find out what is flat using this method.
Now if you only work on very thin items this advice can be ignored and Hood's method will be sufficient.
Phil
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You may be right Phil, I have no experience of routers but my thoughts are MDF (I presume thats what MDG was?), even at 1" thick will not take too kindly to being fixed only at the corners and when shimmed will only be supported at these 4 points, I think it will start to sag in the middle through time.
Hood
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Hood i know what you are saying is correct.
But i have painful memories of when i worked in a toolroom, the surface grinder J&S 540 was worn on the cross slide & i was forever complaining about it. The management naturally thought it was me, they had the service guy done who was the biggest bodger i had ever seen said it was OK.
So to prove it first i ground the mag chuck then i ground a piece of steel that was 1" wide X 6" long i ground this at 90 deg to the table
Got the management down to explain.
I mounted a DTI in the optidress ran it over the chuck perfectly level, then i put my test piece "Ground in situ" on the chuck ran it across again perfectly level .
Then turned the piece 90 degrees put the DTI on it. It read Zero +.002 Zero over the 6"
I got a brand new 540 ;D ;D ;D
I believe this is a common fault with all old at least J&S 540's
So this is why the OP needs to know that his axis are not curved i know he doesn't need that precision but my argument is valid
Phil
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Here what you are saying Phil, even an ex fisherman can grasp that clear explanation ;D
Hood
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the whole right side full length is low compared to the right side. It's as though one rail is a bit lower than the other that is why I thought of shimming up the whole side but with what. Now machining is I think a more permenant and correct way to make it right. I do have a variable speed router if necessary I have a 3/4 plunge bit can do the samething smaller steps.
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NormB,
Is there no adjustment anywhere ?
Can you post a picture
Phil
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No the sides are made up from extruded aluminium from 20/20 bolted together then round rod supports mounted to each side holding 2 veearings supporting the gantry on each side linear bearings. The table top rests on the side rails and bolts to them.
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If you have a way to view Autocad files I have a drawing of the assembly
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Well I will just attach it anyway....
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Found a way to see it without cad
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NormB,
DFX would be good, i looked at the bmp file but i cant get up close as i loose resolution.
Phil
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Okay here is the DXF file
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Okay I am in the shop now checking things out. What I have found is the right side is .021 lower than the left. That is off the DRO in Mach. I also found that the center of the MDF is approximately .045 lower than the either end. The MDF is warped in the middle. I was told that can't happen. *L* oh well nice fairy tail. I know plywood is not better the only thing I know that is strong enough not to bow across 24" is metal.
I guess I can try to run wood supports across the width to help reinforce the MDF. Not sure that will work really. I have to think this one through.
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I m thinking now maybe putting an aluminium plate on top and cover that with MDF with the T tracks cut into the MDF I could bolt the MDF from the bottom and then level the aluminium plate. That plate 1/2 thick I would think would be pretty flat.
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I'd make a bunch of ribs, notched around your screw, running across the table. The more the better. And use 2 pieces of 1/2" MDF laminated together. 4 pcs of 1/4" would be even stiffer. No matter what you use, it'll sag without support.
You should get it as level as possible by shimming, then surface it with the router. I've got a 2-1/2" slot cutter that came from an auction, that surfaces my 30x45 table in about 5 minutes. Amana make a nice surfacing bit, but it's around $200 or more. Her-Saf has a 2" for about $150. Cheapest is to by a 1-1/2" carbide tipped straight bit. About $25
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I have given some thought about making a frame but had not considered laminating sheets together, my thinking is it might not be as strong, I think you are telling me it's stronger to laminate the pieces. I know for one thing finding 1/4 is much easier than 1" Even 1/2 is common but 1" is rather hard to come by. The bad thing is my screw is dead center of the table and would require a rather large notch. I think I have about 2" between the table bottom and the top of the plate that holds the screw nut. "With the he aluminium side rails it would be easy enough to mount this frame to the sides with nuts and bolts. it could become a part of the entire frame. I have some white oak that is hard as blazes would make a good solid frame but even as you say over time it will warp too.
I have the table level now I but shims between the bottom tube and the side rail it was only about .021 not much but then the bow down in the middle wow that is about 1/16 from a straight edge placed across the corners and measuring in the center of the MDF.
If I can get this surface mounted accurately enough might not need to surface the finished top. MDF is really flat but I am not sure about how flat it will be laminating sheets together. Will they stay fairly flat?
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NormB
I would keep with an ali plate or a welded frame. i would then mount everything to that.
You will then have piece of mind, novel idea laminating MDF as you say keeping them flat while laminating would be a problem.
Phil
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The Ali plate is by far the more expensive fix but it's permanent. No more fooling around flat is flat a 1/2 plate is not going to bow and it's easier to mount simply attach with bolts through the top and mount the MDF to that it will take a little space form the Z height but with 5" not going to use all that anyway.
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Laminating the sheets to make 1" will be far stronger than 1 sheet of 1" mdf.
To keep them flat while laminating them you just need a flat surface like a table to support it while the glue dries.
You'll be amazed at how much stronger lamination makes sheet materials, I sometimes glue 2 sheet of 5mm PVC to make 10mm as it's a LOT stiffer than the 10mm PVC.
If you then put 1 or 2 supports across the bottom of the sheet that will also add to the strength, all a lot cheaper than an Ali sheet.
Steve
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Many great ideas, now I have tgo figure out whatI will do from here. Thank you for all the help ....
Norm