Question 4th Axis diameter limitation

According to spec the 4th axis is limited to operate up to 32mm radius. Logic mandates that this is caused by the simple fact that this is the actual height of the chuck centre-line over it's footing.

Now imagine I cut a suitably large hole on my whiteboard in front of the chuck this limitation has no reason to be.

However hanging overhead there is still a spinning dervish called a cutter, and considering this what I have left is a clearance of something between 40mm to 60mm, depending on the cutter itself.

Unless... unless I also manage to drop the entire 4th-axis assembly itself through the floor if I make that nominal hole even larger.

The reason for all this is I need to cut some airplane model wings, and at present I have to slice them in half - top & bottom - to glue them together later. I'd like to model them in one piece using the rotary table but to do that I need to overcome the 32mm limit. Of course I'd have to change software parameters where appropriate.

Opinions please, before I reach for the hacksaw?

Unless I have misunderstood your requirements, the only option is to place a shim under the rotary table and tailstock...

I bought some 20 mm thick aluminium flat bar off eBay for this very purpose when I was considering milling my guitar neck from a 76 mm wide, 40 mm thick stock but yes it would have been close with respect to available clearance when spindle and cutter stickout is accounted for.

I subsequently decided to just do a straightforward 3 axis, two sided flip over cut.

Doug

Hi MMorao,

I would just do it in two jobs and turn the stock after the first oneby hand. This is not possible for all geometries, but often times you find a direction where you can mill the pice from just two sides. You might need to machine an inverse to hold the part after you machined the top side, but then you can just turn it around by hand and to another job from the other side. Alternatively you could use, for example, ESTLCAM, which allows you to place a 3d model inside a stock and calculates two sides for you, leaving enough stock to hold the piece (a few millimeters, wherever you want to have it). You have to cut these tiny block that hold the piece at the end, but you don't have to create any fixture.

SiSt

Thank you Doug and SiSt, your suggestions are quite correct for the example I gave but I should have provided a different one. In the wing example one dimension is predominant in cross-section, or simply put it's rather flattish. Instead think of a fuselage in which height and width are closer to each other, more of a cigar shape. Or a car body for another example. I'd like to turn these in the rotary table, but my dimensions are likely to go over the 64mm diameter limit.

Perhaps I might pull the whiteboard out for some 15cm-20cm provided the machine stability is not affected, I might have to screw in some element for that. The rotating table could then be set further down on a stiff tray rigidly attached to the machine.

Opinions? Objections?

I see no problem with lowering the work piece into the bed as you suggest. It's the kind of lateral thinking that appeals to me.

I'd been considering adding an additional "high" spindle mount position on the reverse side of the gantry to gain an additional inch or so that way. (Or a spacer on the same side just to move spindle up, or just slicing the gantry sides in half and adding spacers, or making interchangeable ones.)

If you go ahead please post some pictures.

Owen S.

Thanks Owen, for the moment I'm toying with working with perspex - so far so VERY good, keep tool passes fast and shallow - but I'm very interested in this. My narrowest wing is 70mm in chord so I really need to extend this. All right, inserting a <10mm plate underneath to raise the axis would work in this application but I'm aiming at rather larger objects. Raising the spindle itself had not occurred to me, but I fear stability issues.

Please just be careful with terminology, this is not lateral thinking, it's vertical

SiSt wrote: Hi MMorao,

I would just do it in two jobs and turn the stock after the first oneby hand. This is not possible for all geometries, but often times you find a direction where you can mill the pice from just two sides. You might need to machine an inverse to hold the part after you machined the top side, but then you can just turn it around by hand and to another job from the other side. Alternatively you could use, for example, ESTLCAM, which allows you to place a 3d model inside a stock and calculates two sides for you, leaving enough stock to hold the piece (a few millimeters, wherever you want to have it). You have to cut these tiny block that hold the piece at the end, but you don't have to create any fixture.

SiSt

Hi SiSt,

I took your suggestion up and did a test job milling one side and turning it over upside down on a previously milled bed, and I'm happy to say it did work well indeed

I still have to work on how to hold the upturned stock on the bed and also how to accurately determine displacement along the z-axis as milling the bed and adding 2-sided tape for instance will generate variations on zero Z height.

However, on the whole the method works, it all depends on finding an effective method to control these differences.

Hi MMorao.

Any chance of some pictures to help visualise your job?

I need to cut a guitar pickguard from 1/8" or 3.00 mm gloss black perspex in the near future. I would be very interested to learn of the exact cutting parameters you eventually get to work.

Many thanks.

Doug

Hi Doug, once again I'm sorry but I've been awfully busy killing braincells.

This comes late, but the parameters I use for perspex are: fast as you can, and cut in thin slices

In practice this means use max translation speed (2600 mm/min), thin cuts means 0.2 mm layer depth or less, and at most two flutes. If you're drilling use 0.2mm layers using the 'woodpecker' feature: cut 0.2, then come up, then dive again for the next 0.2, come up again, etc, etc.

Perspex melting point is only 105ºC, so the thin slices are to avoid generating heat, and the fast movement is literally to get away from possible hot spots! Also perspex is a good heat insulator, which means hot spots will remain hot so you really want your cutter to get out of there pronto!

Generally I use a 6mm dia. flatnose 2-flute cutter for rough work, ending with a ballnose of the appropriate dia. for the task and the finish is generally more than satisfactory, but it may need a proper polish depending on your requirements. Most of the time what comes out is good and smooth enough.

No problem at all with perspex, just avoid heat. At present it is my favourite material for originals for moulding.

Thanks for the tips MMorao.

You might have already seen my post but I successfully cut 3 mm acrylic sheet:



Material: 3.0 mm (nominal) cast gloss black acrylic (Perspex)
Spindle speed: 7,500 rpm
XY feed rate: 1,000 mm/min
Z feed rate: 30 mm/min
Depth per pass: 0.5 mm

I found that cutting with 2,000 mm/min feed rate also worked well with the 2 mm 2 flute cutter.

Doug

Hi Doug,

You arrived at values completely different than mine to process the same material! I'll be damned if I understand this

Happy Easter!

Miguel

MMorao wrote: Hi Doug,

You arrived at values completely different than mine to process the same material! I'll be damned if I understand this

Happy Easter!

Miguel

I just figured that to avoid heat build up, I would turn the spindle speed down and start at a relatively low feed rate then work upwards. I also figured that I would try 0.5 mm depth per pass and reduce this if required.

However, I found these parameters worked first time. I inadvertently ran a toolpath with feed rate set at 2,000 mm/min and found that worked well too so I presumed that the key variable was a low spindle speed in avoiding frictional heat build up.

Doug

Yo Doug, some braincell woke up from deep slumber. A Brian May Red Special? Do you mean this Brian May?

Brian Harold May, CBE (born 19 July 1947) is an English musician, singer, songwriter and astrophysicist, best known as the lead guitarist of the rock band Queen. He uses a home-built electric guitar, called the Red Special. His compositions for the band include "We Will Rock You", "Tie Your Mother Down", "I Want It All", "Fat Bottomed Girls", "Flash", "Hammer to Fall", "Save Me", "Who Wants to Live Forever" and "The Show Must Go On".

That's bigger than the White Whale! I heard him recently speaking on some astrophysics event, and I tell you, the man KNOWS

MMorao wrote: Yo Doug, some braincell woke up from deep slumber. A Brian May Red Special? Do you mean this Brian May?

Brian Harold May, CBE (born 19 July 1947) is an English musician, singer, songwriter and astrophysicist, best known as the lead guitarist of the rock band Queen. He uses a home-built electric guitar, called the Red Special. His compositions for the band include "We Will Rock You", "Tie Your Mother Down", "I Want It All", "Fat Bottomed Girls", "Flash", "Hammer to Fall", "Save Me", "Who Wants to Live Forever" and "The Show Must Go On".

That's bigger than the White Whale! I heard him recently speaking on some astrophysics event, and I tell you, the man KNOWS

The very same. Very famous guitar he and his father Harold designed and built themselves. Both very clever men.

Replicating it is proving to be like doing a second PhD. I can't afford a high end official replica:

http://www.guytonguitars.com/the-guyton-red-special.html

I bought one of these American versions used but it was still GBP2,500:

http://www.rscustomguitars.com/guitar_store/64_supreme

Doug

I'm sincerely impressed, please do keep us posted on your work, it's always a pleasure