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I have a CNC3040 from eBay and it works great. It has got me on the learning curve and given the confidence to try to make my own machine now :)
So I have a few questions if I may:
1) What are the pros and cons of ball-screw vs. lead-screw?
2) How important is supported rail vs. unsupported? I plan to build a machine that will have a 120cm x 60cm cutting area.
3) What factors dictate the max. speed of a machine? Is it the motors or the drivers, and how can I estimate the speed I'll get?
Thanks!
1) Ballscrews have far greater efficiency - about 90% vs 50% and significantly lower, but not zero, backlash however unlike leadscrews due to the use of rolling not sliding friction they can be preloaded to obtain practically zero backlash.
2) Extremely important. An unsupported rail will bend significantly more when a force is applied, ultimately leading to tool deflection and thus poorer accuracy. Supported rails are supported along the entire length, hence the force is largely transmitted to the frame which is significantly stronger than the rail and therefore deflects less. Does depend on the orientation, but either way a 12mm supported rail is probably stronger than a 25mm supported... huge difference.
3) Motors, driver voltage, ballscrew pitch, ballscrew diameter, drive ratio, mass of moving parts, type of rails used, preload of rails ... etc. You can estimate it using this calculator:
http://www.google.com/url?q=http://w...zkjSyaojkuQQKA
Thanks for the info Jonathan.
Could somebody please explain to me what is meant by 'fixed side' and 'supported side' with reference to ballscrews?
I don't understand why the same simple bearing can't be used on each end.
The ballscrew is going to push axially (lengthwise). It needs something to push against so one bearing must be fixed axially.Quote:
Originally Posted by Tenson
If you fix both ends axially you have to wonder what will happen when it expands and contracts due to temperature change.
Fixing at both ends is good if you can hold it in tension with springs, otherwise you have to let one end free to slide.
Robin explains it very well but a picture paints a thousand words.!!Quote:
Originally Posted by Tenson
Pics below show ballscrew with fixed end on right and a floating end on left, you will see fixed end as threads on ballscrew and the block is wider than the floating end because it has 2 angular contact bearings working togehter to handle the axial loads. The fixed end has just a normal bearing which is allowed to move inside the mounting block if any thermal expansion happens, it is still attached to the ballscrew via a small circlip so it can't fall off.
Simplizzz really.!!
Thank you, nice picture! This machine is getting more expensive ;) Still, good tools earn their value.
Hi chaps,
Which of these illustrations is the correct way to mount an X axis with supported rails?
(Yes I know there is no Z axis ;) )
http://i608.photobucket.com/albums/t..._uk/X-Axis.jpg
Also a question I feel stupid asking, but what is commonly called the X and Y axis? On my CNC3040 the Y axis is the length of the table and the X axis goes across. I've seen people talk about it the other way though.
What you have pictured is generally called the Y/Z axis, although as you say it does vary. I always call X the axis the gantry runs on, so generally the longest axis.
The orientation of the rails in the image on the left is considered better since the deflection due to each bearing is more even. For forces in the direction pulling the bearing off the rail, not surprisingly, the bearings are weaker and will deflect more. If the open sides are facing, as you have drawn, this evens out so the deflection will be the same for all axis which is much better than having significantly different deflection which is what will happen if the rails are both the same way round.
Thanks for the feedback. You mean like this?
http://i608.photobucket.com/albums/t..._uk/Y-Axis.jpg
Hmm.. I think I'll go for unsupported rails on the Y axis to keep the gantry weight down, plus build simplicity.
BIG BIG BIG MISTAKE if you do this unsupported rails are a complete waste of money.Quote:
Originally Posted by Tenson
Don't be affraid of weight, Mass really helps when it comes to cutting, It absorbs resonance and gives far better finish.
If your trying to cut weight to gain speed then you are sizing components wrong. The feeds you require for the machines intended purpose should be got from choosing components to allow it.
Ballscrew pitch and motor selection along with drives and correct voltage all play a part combined with build quality and component quality. Get anyone wrong and you comprimise the machines abilty in some way.
In your case unsupported rails with a light weight gantry of any resonable width would be a disaster for cutting any material denser than softwood, and even then with lower DOC.
DONT DO IT. . . . FORGET UNSUPPORTED RAILS.!!!!!!
Ooookay, I will use supported :)
What is the best way to drive the 150cm long X axis? Should I use a central ball-screw or two ball-screws, one on each side? If the latter, what is the best way to power them, i.e. 2 motors connected to 1 controller?
2 ball screws means that both sides are controlled and this will stop the gantry squewing. I have a motor on each side. Mach 3 can link the two together. A single motor linked by belts to the two screws may have been a better choice as the screws are guaranteed to rotate equally.
you will find quite a few threads about this.
Bruce
OO look at me showing off and giving advice:redface:
Good advice :)Quote:
Originally Posted by motoxy
It's possible to put two motors on one driver if they're mechanically linked, but I wouldn't advise it. Either put one motor on each screw, or one motor and a long timing belts to link the two screws. The former should go a bit faster, since you have twice the input power with two motors, but in practice I doubt it will go that much faster as you have to guarantee neither motor stalls. Clearly if one does then the gantry skews, which depending on the magnitude could damage things. Once the machine is tuned properly either is fine as the motors will never stall.
I used two motors and two drivers since it would require a very long belt and several idlers to link the rotating ball-nuts.
Eh Eh sorry if I shouted a bit load but can't stress enough what a bad idea unsupported rails are.!!Quote:
Originally Posted by Tenson
RE Single or twin screws on X. If Y axis much wider than say 600mm then personally I'd go for twin screws.
Powering them you can either have 2 motors each with it's own drive, 2 motors on 1 drive will work but not correct way to do it and could give problems. . Another don't do it.!!
You could use 1 slightly larger motor and connect the screws together with a timing belt, this is my personal favorite and how i've built my machine. It works perfect so don't be put off by timing belts.
Both work ok but you have to be aware of the downsides to each and choose which you prefer to live with.!
Using 2 motors works ok but comes with the issue if one motor stalls the other keeps going which can do major damage to the machine if happens at high feeds.
It can also be a problem keeping the screws in sync if both motors are not tuned perfectly in sync with each other and one runs ahead or behind the other, this will result in slowly twisting the gantry thou can be corrected by regular homing and squaring, but still wrong and not wanted.
The way to avoid these issues is to not stress the motors so you effectively need to run them below there full potential because you can't afford any missed steps or threat of stalling from lack of torque at high feed rates or rapid moves. The quality of build regards ballscrew aliagnment as to be spot on, which it should be on any machine, but even more so with twin screws/motors as no sticking or binding can be tolerated for fear of stalling motors at high feeds where torque is lower.
Mostly this is not a problem if your aware of it before building because you can choose higher lead ballscrews or build in gearing again via timing belts to the motors to achieve the speeds & feeds you require and keep the motors operating well below the point they start running out of torque.
The sync problem is just a slow setup and tuning job which can either take 2mins or 2 weeks depending how fussy you are.!! Warn you thou it can lead to premature baldness.!!. . I'm down to my last few strands. . :naughty:
The 1 motor pritty much eliminates all this hassle with perfect sync and also means you can run the motors closer to there max. It's also very easy to change pulleys to gear the system up or down.
The only downside is it's not as easy to build because you have build in the belt system and it's tensioning/tracking system and also have the ballscrews machined different from standard machining to add extra bearing support for the pulleys which will put greater side ways force on the ballscrews effectively trying to bend them when the belts are tensioned.
In my experience this is all worth the effort because when done and settled down it never loses position or sync and ounce setup thats it forget about it and never needs doing again with absolutly no fear of racking the gantry unless the belt snaps. (which I've only snapped 1 in over 3 yrs and that was because I left the pulley lose)
If good quality belts are used then belt stretch is minimal and with correct tension and good tensioning system never an issue and very very accurate.
It's no cheaper(well slightly) than 2 motors because you have the belts and extra bearing supports to consider.
Like I say both work ok if your aware of there limits and tendency's it's just I personnaly favor belt system.
Oh and at 150cm your in a funny area regards pitch and screw dia.???? I'm pritty sure you'll want 10mm pitch which often (well from china anyway's) only comes in either 16mm or 25mm dia they don't do 20mm dia only in 5mm pitch.
At 150cm 16mm is on the very edge of being to thin and will give big potential for whip at high speeds. 25mm would be better but comes with the cost that it requires more torque and power to over come the extra mass and inertia of the screw/nut when accellerating and deaccellerating. This means bigger motors which require larger power supply's and drives that can handle the power so it basicly cost's more money.
You can over come this by either taking the route like jonathan did and use rotating ballnut on 16mm screws, but again this will come with expense and i'm sure jonathan will tell you exactly how much and all it's bennifits. . . OR . . bite the bullet and pay for the larger screws and use with nema 34's and correct drives, psu etc. . . . OR . . .use 20mm 5mm pitch and gear 1:2 to give an effective pitch of 10mm at the cost of slightly less torque. ( My machine use's this system thou with nema 34 motors) This gives a slight advantage as it's easy to gear back down and double the resolution of the machine without comprimising the area where the motor works good. It can be done the other way round and gear 10mm pitch to give 5mm resolution but the motors work in an area which is not ideal.!
It's horse's for course's really so take your pick.!!!!!
Hope this helps.!
Thanks for the detailed reply.
So the problem with a single central ballscrew is that the gantry can twist a little back and forth? While the problem with using dual ballscrews + motors is that it can twist should one motor stall? I don't think my budget with allow the custom machining of ballscrew to fit a timing belt. Where did you get yours from JazzCNC?
The X axis will be 80cm wide. How about running a single 2505 ballscrew on the Y axis with a bigger motor? There is a nema 34 12Nm motor on Zapp Automation. However it needs 6.2A per phase, so what is a suitable driver for it? Is it best to use 2x 6A drivers, one per phase?
What is the problem with 5mm pitch on a 150cm axis? I'd have thought it best to use the same screw pitch on both X and Y axis so they have the same speed, provided the motors have enough torque.
Oh, and finally how can I build an auto tool change? I'd really like to have that at least to choose between 2 bits. Hmm.. I just searched that and it looks like I'd need a special chuck with gearing to transfer power from the motor, plus a system to open and close the chuck. PITA.
Ok the screws come from Chai at linearmotionbearings which you'll find on ebay. He's a great guy and very trust worthy. It's best to let Chia do the machining as he doesn't charge very much and will machine to your dimensions if asked. I do this with him all the time.
Yes single screw can twist the gantry when cutting at the outer edges, the wider the gantry the worse it will be. At 80cm your on the edge IMO.
If you do proceed with single then make the distance between the bearings as wide as possible to help combat this and have a strong wide cross brace undernieth.
The 25mm screw would work but you don't need 12nm motor 6nm would be ok.
I think you miss understand the phase/drive relationship you only need a drive which can handle the motors rated amp's, so in the case you gave 6.5A. you don't need 1 drive per phase.
The thing you have to be aware of is the differnce between nema 23/4 and 34 motors. Larger 34 motors tend to spin at lower speeds than 23/4 motors. typicly a 34 will runout of puff around 1000-1100rpm where as 23/4's will spin 1500rpm and above. The difference comes from how they produce and carry the torque up the rev range, 34's tend to hold higher torque further up the rev range then drop off really quick as it peaks. 23/4's will carry torque further up the rev range all be it a lower rate and gradually drop off to nothing.
Also 34's need really high voltage to get best performance. Voltage is key to speed and torque in both 23/4's and 34's but 34's require much higher to get best performance.
Unless you have a very well equiped workshop or lots of money then forget autotool changer far too expensive.!! . . . . better alternative is to buy Gerry's Mach screen it's about as close to autotool changing as you'll get without the expense, find it here and it will be the best $20 you have ever spent. http://home.comcast.net/~cncwoodworker/2010.html
Hi Jazz,
On the datasheet for the motor is said 6.2A (phase) which I take to mean each winding will draw 6.2A. So if I wire in parallel it will draw 12.4A. In series will will need about 3A but twice the voltage. Won't it?
Each side column of the gantry rides directly on the centre point of the linear bearings, and I was going to use solid 12mm alu sheet as a cross-brace underneath. In that case I guess maybe a single central ball-screw with a powerful motor would be okay. However you point that a nema 34 spins slower is worrying as I want all axis to be capable of the same speed. What do you think, is it okay to use a nema 34 only on the Y axis?
Why is running two motors from one controller not a good idea? Provided it can output the required current and voltage it should be okay, won't it?
No... in series it's 6.2A and in parallel 12.4A. Until I finish my own drivers you're not going to find drivers that will deliver 12.4A, so have to use 6.2A. As you say, that means a higher voltage is required ... so the only driver worth buying for that completely excessive and uncalled for Nema 34 motor is mains voltage:Quote:
Originally Posted by Tenson
http://www.zappautomation.co.uk/2m22...?cPath=9_3_132
Not at all - how strong you make the gantry with a single screw is largely irrelevant with regards to racking (although in general you clearly want to make it as strong as you can afford) since you're stopping the gantry rotating about the central ballnut, not stopping it bending. To rotate (rack) the rails do not have to deflect much at all, so the only way to do it would be to have a very large spacing on the X-axis linear bearings, however doing that you're just compensating for bad design in the first place. Use two ballscrews - I guarantee you'll regret one.Quote:
Originally Posted by Tenson
This happens with almost every build log - starts off thinking one ballscrew will be fine, then by about page 3 we manage to persuade that it's not.
You won't need a Nema 34 motor.Quote:
Originally Posted by Tenson
The only way to get a Nema 34 to have decent torque at higher speeds is to use the mains voltage drivers, so lets add up the costs:
Driver, £176
8.6nm motor, £108
Two RM2510 ballscrews from eBay, roughly £190
4 * 30T HTD pulleys, £22
Long (to link screws) and short timing belts, £27
Ballscrew end bearings & mounts, £50 (bit of a guess but shouldn't be far out, maybe more due to the long belt?).
Total £563
The other option is, as Jazz suggested, to use rotating ballnuts. See here for my experience with this setup:
http://www.mycncuk.com/forums/showth...-design-ideas/
(On the last page you'll find pictures of what I'm suggesting below, but the rest is worth reading for a more in depth explanation of why they work so well.)
To get the same (feederate and acceleration) as the above, probably better and certainly more than you'll ever need from a rotating ballnut setup you'll need:
2* m752, £80 (eBay)
PSU for 2 motors is shared with Y/Z motors so about £15 extra.
2* 3nm motor, £50 (eBay)
Two RM1610 ballscrews from eBay, £150
4 * 30T HTD pulleys, £22
2 * short timing belts, £10
4 * 7206 angular contact bearings, £40
Ballscrew end bearings & mounts, DIY for £10.
Total £376
So if I wanted to I could charge £187 to machine two rotating ballnut mounts and you'd end up with a better system for the same price. Realistically I'd charge less so it would be cheaper overall.* When I've worked it out before the price difference has been greater since I worked it out with two nema34 motors, not one, but either way it's a significant saving.
Or for about the same price (£376) you could just use the RM1610 and spin the screws as normal. I'm confident that will be adequate and a better option than RM2005 since the critical speed (speed at which the screw starts whipping, so the limiting speed) for a RM2005 screw is well under twice the critical speed of a 1500mm RM1610 screw so it wont actually get a better feedrate. Plus it clearly requires much more torque to accelerate a 20mm ballscrew compared to 16mm, so you'd need a bigger motor anyway.
In your situation I'd go for RM1610 and either rotate the screw or the nut. I went for rotating nuts since my machine has 2000mm ballscrews so rotating a 16mm screw wasn't an option...and equally because I wanted to try something new(ish). I've left my machine on 10m/min with 1m/s^2 acceleration on X and 1:1 ratio. It will do about 15m/min, even more if I change the pulleys, but 10m/min is already plenty so no point stressing the components. That's with the same 3nm motors and drivers, so you can expect better.
To give you an idea of how much more stable a system rotating ballnuts are; I recently didn't notice that one of my ballscrews had worked loose. So one end wasn't fixed at all and the other wasn't tight either. I only noticed the problem when after a few hours cutting aluminium I leant over/on the ballscrew to remove the tool and it moved... i.e. it made no difference! Try that with a rotating screw and you'd probably have ended up with a bent ballscrew.
Think what happens if one motor has a slightly different load.. then stalls.Quote:
Originally Posted by Tenson
*Other vendors are available.
Hmm, surely the parallel connection will draw twice the current of a single winding, not twice that of the series connections. Similarly the series connection will draw half the current of a single winding, not half that of a parallel set-up.Quote:
Originally Posted by Jonathan
Lol, okay I get it :tup: I'll go for two ball-screws.Quote:
Not at all - how strong you make the gantry with a single screw is largely irrelevant with regards to racking....
To rotate (rack), the rails do not have to deflect much at all...
This happens with almost every build log - starts off thinking one ballscrew will be fine, then by about page 3 we manage to persuade that it's not.
I still wonder though; why isn't it better to use the same screw pitch for all axis, so that providing the motors don't run out of torque, each axis will have the same speed and resolution?
I really don't want to go the rotating ball-nut route, nor linking the ball-screws with a belt. Going with dual ball-screw I'd like to use two motors. I'll bear the tedious tuning procedure when I first get it up and running. At least I know to expect that now thanks to you guys!
Quote:
Think what happens if one motor has a slightly different load.. then stalls.
My question was really about why it is better to power each motor and ball-screw from a separate drive circuit, rather than just using both motors from one circuit. The motors can surely still individually stall, even with two driving circuits.
In parallel the current is twice the current for one winding. In series it is the same as the current for one winding as you have the same current flowing through both.Quote:
Originally Posted by Tenson
GCSE Physics...
Each axis has a different mass and length of ballscrew, both of which affect the feedrate. So the same screw pitch on both will result in different speeds since other variables are still different. The resolution will be the same.Quote:
Originally Posted by Tenson
Your Y-axis is 600mm (?), so I'd still go for 10mm pitch on Y.
Why?Quote:
Originally Posted by Tenson
Same answer, but to put it bluntly if one steppers stalls when you have two on one driver then you can say goodbye to the driver. If one stalls with two drivers it's a bit annoying as the gantry is bent, but not the end of the world.Quote:
Originally Posted by Tenson
Tenson Jonathan's right on all counts here. I gave you the options he's giving you specifics about the rotating nut. If your wise you'll go with the rotating nuts at this length for best speeds verse's cost's.
The rotating nut will give you best of both worlds and save you some money.
The other reason for not using 1 drive with 2 motors is that if the pulse timing doesn't get to each motor at exactly the same time then you will slowly drift either infront or behind on 1 axis so all in all it's a lose lose situation and at best you'll be inaccurate and worst be letting out the magic smoke. . . Your choice.!!
Hi guys,
Jonathan, if you are genuinely willing to machine two rotating ball-nut/motor mount things for me at a good price then I'll go for it :)
I'm sorry to be persistently stupid but I still don't understand the desire for 10mm pitch. This is my thought process: Two screws with the same pitch, turned at the same RPM, will give produce the same speed and distance of movement. In practice, one axis will have a greater load than the other, so it will need more torque for the screw to maintain the same RPM. However, with adequate torque (two screws, two motors) there should be enough torque to do that. In fact I'd expect a larger screw pitch to need even more torque. If the 10mm screw moves twice the distance with one turn, it will require twice the force to turn it, not to mention the greater mass of the load.
Where is my thinking going wrong?
A ball bearing screw is just that: a screw which runs on ball bearings. The screw and nut have matching helical grooves or races, and the ball bearings recirculate in these races. There is no physical contact between the screw and the nut. As the screw or nut rotates, and the rolling balls reach the trailing end of the nut, they are deflected or guided from this "pitch" contact by means of a return tube and returned to the leading end of the circuit.
It's all about the speed vs torque. Remember with steppers as the speed rise's the torque drops away.Quote:
Originally Posted by Tenson
So with a 5mm pitch screw and usable max torque for cutting feeds translated to rpm is around 1000rpm with nema23 motor will give about 5mtr/min if your lucky . Taking into account the safety margin for twin direct drive screws the safe area will probably be around 4-4.5mtr/min(Obviously this will depend on mass and friction of each axis). Now at these feeds you'll have low torque so any deep roughing or hard material will require even lower feeds.
With a 10mm pitch the same 1000rpm gives you 10mtr/min with a similiar amount of torque. So obviously at the same 5mtr/min the motors are spinning half the speed so far more torque available so deep roughing cuts are no problem if the spindle and machine can handle them.
The other reason as Jonathan pointed out is 10mm pitch means for the same speed the 10mm pitch screws are spinnig half the speed of 5mm so less chance of whip.
Thou to be honest I don't hold much stall by this and Jonathan and I have debated this before.!
While in principle he's right in physics terms but in practice it doesn't quite work that way and my machine is living proof as it use's all the things Jonathan disagree's with like: twin 20mm x 5mm pitch geared 1:2 with nema 34 and non AC drives on 80V.
It will rapid at 12mtr/min if need be thou I restrict it by tuning to around 7mtr/min to save on screw wear as I don't require high rapid feeds and this allows higher acceleration which is much more usefull.
It will happly cut at 7mtr/min and if you check the vertical post I put up you'll see it cut 10mm deep 1 pass in dense resin inpregnated MDF with no problem, it even cut full depth 13mm for half the length untill the very very worn 6mm cutter snapped and even then the machine didn't stall.
So the critical speed issue is a non starter to me and I'm sure by the very fact of human nature most won't have the will to resist using all the available speed the 16mm x 10mm will give them so whip will be an issue because there's no strength in the screw. With 20mm you have more meat so better able to handle the slightly higher spinning if geared like I do.
All that said if you don't require high resolution then I would still advise you to go 16mm x 10mm pitch as it's far more versatile and cost affective. Rotating nut, direct drive or connecting belts is an individual's decision.!! . . . All I'll say is you still have the sync issue whether rotating or spinning screw.!!. . Thou Jonathans obviously then man to tell you exactly how it differs to direct drive spinning screws.
I see, so the motors drop in torque is not proportional to the increase in RPM? If you get more than double the torque at half the RPM then I can understand the argument for 10mm pitch vs. 5mm.
Jazz, have you tried not using the 1:2 gearing and just spinning the motors faster? If you found the slower motor speed is better then I should just follow your advice and go for 10mm pitch ;)
I'd like to go for rotating ballnuts if Jonathan will be kind enough to make the parts for me, as it would be nice to keep the drive motors within the machine frame, rather than stuck on the end. Also I could use ballscrew without end-machining since it would just be held in a clamp.
I can get 2510 ballscrew from Zapp, so can you make a rotating ballnut assembly for that Jonathan?
A good approximation is that the torque is fairly constant upto a particular rpm (the 'corner speed') after which the torque does drop proportionately. We've come to the point where hand waving explanations are insufficient as there's too many variables. It will depend on the chosen motor and thresholds.Quote:
Originally Posted by Tenson
The graph in the 3nm motor datasheet is helpful:
http://www.zappautomation.co.uk/redi...H88-3008BF.pdf
(They've just gone up in price by 20%!!)
Also Irving's spreadsheet is very helpful to understand which factors affect the feedrate:
http://www.mycncuk.com/forums/showth...otor-do-I-need
Even when critical speed isn't a problem (as is the case with the rotating ballnut) you'll find the 10mm pitch screw wins over 5mm as the torque required to accelerate it is less, due to the kinetic energy of the rotating screw being 4 times less if the screw is only rotating at half the speed.
I'll work out a price and let you know asap. Though it's theoretically possible to get away without end-machining with rotating nuts I wouldn't advice is as the surface area of the screw in contact at the clamp will be very small. It adds very little to the price with linearmotionbearings2008, so I got my ballscrews cut with the standard end machining, except the same on both ends with the threaded portion. That way you can put a nut on either end to clamp it.Quote:
Originally Posted by Tenson
Yep I can do any size, that's what I'm using:Quote:
Originally Posted by Tenson
Attachment 5559Attachment 5560
If you want 2510, although I think you'll be fine with 1610, you'd be much better off getting it from linearmotionbearings2008 on eBay. He sells 1605,1610,2005,2505 and 2510.
This is the version I did which is suitable for 16mm or 20mm screws:
Attachment 5557Attachment 5558
I thought about that too, but it seems easy to overcome. Double-nut the end of it or pot the thread then clamp. I hate waiting for long deliveries so I'd probably get it from Zapp!Quote:
Originally Posted by Jonathan
The bracket on the right of the picture (and in the final picture) looks good for me as I'd want to mount it under the gantry. If you think 1610 will be enough then lets go for that, though the saving seems small (about £15 each screw).Quote:
Yep I can do any size, that's what I'm using:
Attachment 5559Attachment 5560
If you want 2510, although I think you'll be fine with 1610, you'd be much better off getting it from linearmotionbearings2008 on eBay. He sells 1605,1610,2005,2505 and 2510.
This is the version I did which is suitable for 16mm or 20mm screws:
Attachment 5557Attachment 5558
I've started ordering some parts. My plan is to make the Z axis first, then the X and then finally make the machine frame and Y axis. I ordered a 2.2Kw spindle, 250mm ballscrew and some rail and bearings. I'll then make some 10mm plates to hold it all together.
Okay you have less rotational inertia from the screw, but twice the pitch means half the driving force on the nut so your max accelerations go down.Quote:
Originally Posted by Jonathan
Sounds a bit swings and roundabouts to me :naughty:
Your joking my last order came special machined from china in seven days. I'll bet you £10 between Zapp and Jonathan you can't get these screws delivered and machined in less time.!!Quote:
Originally Posted by Tenson
Agreed.!!Quote:
Originally Posted by Robin Hewitt
Where you going to use 10mm plate.?? Don't use 10mm plate on the Z axis or the gantry sides else again you'll come to regret it. 15mm minimum 19mm better.!!Quote:
Originally Posted by Tenson
Got to agree with Jazzmin on that, 10mm thick is to be considered as sheet material, personally i would say 20mm minimum/maximum myself.Quote:
Originally Posted by Tenson
Okie-dokie!
Are you guys sure that gantry weight is not a problem? I was thinking I could use extruded aluminium profile, but it needs to be quite thick to be stiff, so I'd prefer to go with solid 20mm aluminium. Alternatively I could go with thin sheet and re-enforce it with carbon fibre struts.
Yes we're sure. The mass of almost every CNC mill is significantly greater than a CNC router with good reason. It helps significantly to get a good finish and cut with a decent material removal rate. It is far more important to make the gantry strong than to make it fast and it wont make much difference anyway.Quote:
Originally Posted by Tenson
Extrusion is good for the gantry, but quite expensive. Consider steel box section or a combination of aluminium flat bar, extrusion or steel. You'll find plenty of examples of ways to do it with these if you search in the build log section.
Thanks Jonathan. I saw you used steel box section for the frame of your machine. Trouble is I can't weld! I only have a gas bottle torch for doing a bit of plumbing, and I have little experience with it.
My plan at the moment is to use aluminium profile for the frame and 20mm plate for the gantry. Given the cost of aluminium profile I don't want to use it for the machine bed, although it seems ideal for easy clamping, it would come to about £300! I have some 13.7mm aluminium honeycomb panel for another project so I thought I'd use that as the bed, and then fix some MDF on top as a sacrificial layer I can screw the cutting material on to.
That honeycomb stuff looks interesting, ty for the link.
With regards using plate for the bed, is there any advantage in bolting say 20x80 or 40x120 profile to either the top or bottom of the plate? Full cover or just certain strategic locations?
Also this stuff as a bed plate surface? http://stores.ebay.co.uk/Vakuumtisch...=p4634.c0.m322 , certainly not cheap but very convenient.
The basic question is, just how much benefit does one gain when increasing thickness by bolting two plates etc together?
Learn.!! . . Buy a cheap £40 quid stick(Arc) welder and give it a go you'll be surprised just how easy it really is.!!Quote:
Originally Posted by Tenson
DONT buy a cheap Mig thou they are NO GOOD for boxsection, you need good quality high power for material over 3mm. . . . Stick is cheap, relatively easy and eats 3mm boxsection for breakfast. With a few hours practice on some scrap you'll be surprised what you can do and how much you improve.
That said you don't need to weld!!. A steel framed machine can be easily bolted together using plates. You will need an half decent drill press which you'll need regardless whether you use profile, steel or what ever and it's a must have tool for building a CNC machine, and a good grinder with a selection of grinding and cutting disc's.
When you buy the steel buy a strip of 80 x 6mm flat bar and chop it up for the required plates with grinder.
Doing it with plates means you have loads of adjustabilty which you need in a DIY machine to account for the lack of precision grinding and equipment you don't have.!!
It will also save you a shit load of money compared to profile and be stronger.
There are a couple of things with profile just about every body who hasn't used it before doesn't realise.
1: It's even more expensive than they thought because the extra's like T-nuts and brackets etc which you need to keep it easy simple bolt together cost nearly as much as the profile it's self and if you try to skip them to save Wonga your into a world of pain and akwardness that's just not worth the cost.
2: Often it's not just simply bolt together and often needs counter bores drilling so bolt heads fit between slots etc so again requires a good drill press and little jigs so every thing aligns together.
Personnaly I find it far simpler and quicker using steel and the slight plus profile as in that it makes mounting things like switch etc easier is far too costly IMO and with good design every thing can be done with steel and Alu plate for less than half the price and really not that much longer to build.
I would have thought the honeycomb over time would eventually crush under plunging forces which can be very high. With a bit of experience you learn to avoid plunge machining and try to avoid it as much as possible because it knocks the shit out the machine.
Personally I don't like 1 piece beds like the German stuff because one cock up and the whole thing is knackerd. Far better to use individual pieces which can then be replaced relatively cheap if you Foo bar. On my steel framed bed I used 16mm x 38mm strips of Ali to create a T-slot bed and it works very good and is about the cheapist way (at the time) get a good bed.
The other route I'd consider is to use profile side by side with the bennifit being you have 2 shot's at killing it.!!! So if you damage a piece you can turn it over and use the other side before it's knackerd. These guys also sell single sided 80x16 at £14mtr which aint bad. http://www.metallin.co.uk/shop/index...ory&path=3_125
Why don't you agree with me on owt else I say.!!. :whistling: (Would kick you in the balls if I could find it.!!)Quote:
Originally Posted by Lee Roberts
I have a drill press and also a Makita table-saw with a steel cutting blade. So I could drill and cut box steel.
How accurate is water-cutting? There is is place near my Mum's home that does it, would it be precise enough to do the end plates and bits for the gantry? If it cut the outside dimensions and made a few marks where I need to drill for the mounting of rails etc.. then that could work well for me.
Water cutting can be very accurate. With 60,000psi and a garnet additive you can accurately cut 2" steel.