[ssutton]

What are the practical lengths that you can use a screw? I know that this depends somewhat on the speed, so lets say that you want to be able to cut at 150 ipm, maybe rapid at 300 ipm. Ball screws are out of the budget so we are talking precision acme with a zero backlash nut.

Thanks,
Scott

[CNC Lurker]

What are the practical lengths that you can use a screw? I know that this depends somewhat on the speed, so lets say that you want to be able to cut at 150 ipm, maybe rapid at 300 ipm. Ball screws are out of the budget so we are talking precision acme with a zero backlash nut.

Thanks,
Scott

I've heard on here that anything past 4ft is pushing it for an ACME 1/2-10 5-start.

If you want high speeds at a lower cost than ball screws, maybe rack and pinion would be the way to go.

Rack and Pinion Drive, Nema 23 LINK

[ger21]

It depends on how you mount them. If you use two bearings on each end, you can spin them a lot faster than with a single bearing on each end.

My 40" 1/2-8 2 start has no whip at 190ipm, but the 60" ones whip at ~140ipm.

[louieatienza]

What are the practical lengths that you can use a screw? I know that this depends somewhat on the speed, so lets say that you want to be able to cut at 150 ipm, maybe rapid at 300 ipm. Ball screws are out of the budget so we are talking precision acme with a zero backlash nut.

Thanks,
Scott

It depends on the number of starts too you use on the screw. I easily get over 400ipm on my y axis; then again I use 8-start ACME which gives me 1tpi effective, and .005" rmechanical resolution; which is probably what you'd get with a rack and pinion drive. To do that with 5 start you'd have to spin the screw at 800rpm; not going to happen, at least practically... On my x I get 250ipm reliably. But because of the weight, pulley system, and one screw being slightly bent, any more faster and it starts to whip.

My y screw is 36" oal and x screws are 42" oal. A lot is dependant on how the screws are mounted, their size, and alignment; so considerations should be taken as to allow for slight adjustments.

You COULD get faster rapids with leadscrews with a spinning nut design; though thos would probably add to the cost, whether you use a non-captive hollow shaft stepper/servo, or make a gear reduction unit, but this would help immensily with screw whip and ovverall length.

[rowbare]

Engineeering Product Selection Calculators - Nook Industries, Inc. see the Critical Speed calculator.

bob

[Born2dive9702]

I am looking to get speeds from say 10Ipm to 600 IPM while cutting.I was thinking on using a 1/2" acme with a bearing block on each end, of the table, then use 2 bearing blocks on each end of the gantry where the screw passes.I was thinkin of using a 1/2" acme but have been told to stay away from them on this length and speeds regardless of how many round guide rails are used in combination of V rails.I have been told that rack an pinion are more accurate, but are lot slower...What are your thoughts.

It depends on how you mount them. If you use two bearings on each end, you can spin them a lot faster than with a single bearing on each end.My 40" 1/2-8 2 start has no whip at 190ipm, but the 60" ones whip at ~140ipm.

[Born2dive9702]

It should also be noted I plan on also cutting alumimum plate up to 1.5" thick on this unit to make CF molds .

I am looking to get speeds from say 10Ipm to 600 IPM while cutting.I was thinking on using a 1/2" acme with a bearing block on each end, of the table, then use 2 bearing blocks on each end of the gantry where the screw passes.I was thinkin of using a 1/2" acme but have been told to stay away from them on this length and speeds regardless of how many round guide rails are used in combination of V rails.I have been told that rack an pinion are more accurate, but are lot slower...What are your thoughts.

[wizard]

Engineeering Product Selection Calculators - Nook Industries, Inc. see the Critical Speed calculator.

bob

I might add that you can find such charts and calculators on a number of manufactures web sites. In the end you can ask many questions but a few look ups or calculations can do wonders for confidence in a design.

It should also be noted I plan on also cutting alumimum plate up to 1.5" thick on this unit to make CF molds .

I'd strongly suggest looking at some of the stickies and build threads on this forum. There is a very high possibility that your machine will never give you the results you want out of aluminum. I say this due to your reference to V rails and round rails. The assumption here is that you need high quality surfaces to produce usable molds for Carbon fiber. Chatter will be a problem.

To put it another way, to get results comparable to routing wood, on a machine doing aluminum you will need a machine that is 10 times stiffer. You may even need more stiffness if you want to hold position tightly or want exceptionally good surface finishes. One other thing here size is a huge factor in issues with leadscrew whip, machine stiffness and other machine tool properties. If you have specific uses in mind don't make the machine any larger than required.

If the budget is too tight to do a machine that can do aluminum nicely, consider designing a machine that will be easy to upgrade over time. As noted above zero in on a minimal machine size to keep the cost of the main structure and the linear components under control. Search for scrapped and discounted materials. Oh one more thing positioning accuracy, for mold making, might be important which works against high lead leadscrews. In the end you will have to balance a bunch of competing needs against one and another to come up with a machine that meets most of your needs

[skrubol]

What sort of travels and how big of a spindle are you thinking?
To reach 600 IPM on anything other than a pretty small machine the extra servo/stepper power needed to turn ACME screws over ball screws or rack and pinion will probably pay for basic ball screws.

I'm slowly building a router to cut aluminum that I plan on having about 26x37" travels and 750/1100 IPM cut/rapid speed. I'll using 35mm linear rails and 1.5kW servos for ball screws.