Luthier CNC

[bluesking]

Following on from my plate design work, yesterday I drilled the various holes:



Everything seemed to go well. I have a mixture of M5, M6 and M12 bolts to accommodate. Many of the holes don't need tapping so I created mostly 5.5, 6.5 & 13mm holes to accommodate the screws.

I also have a bunch of M5 holes that needed tapping. Here I made a bit of a mistake. Really didn't think it would make a difference so I just used a 4mm drill bit I had laying around. Drilling went well, but as soon as I started to tap this happened:



My lack of metalworking experience takes it's first casualty! Today I re-drilled all the offending holes and I think I got away with it overall.

Here's what I have learned:
1) If you want to use a M5 tap, you need a 4.2mm hole. No getting around this. Maybe 4.5mm would also work, but I didn't even try this as 4.2 worked perfectly.
2) If you break a tap in a hole there really isn't any way to recover. You try to drill it out, but because aluminium is so much softer than hardened steel, the drill bit wanders around the tap creating a bit of a disaster.
3) Cutting oil is a mixed blessing. Makes all the work super messy but I think improves results. If doing again I'd drill all the holes dry and use cutting oil just to do the tapping work.

The one hole I messed up was meant to attach to one of my z-axis HIWIN carriages. I'm hoping I can get away with using only 3 out of the 4 holes to mount this. I think this will be fine so I'm not going to bother buying/making a whole new plate for now.

[Doddy]

Here's what I have learned:

Been there. I think most of us have.

[mekanik]

Hi Bluesking
Broken drills/taps in non ferous materials can be dissolved using Alum solution.
https://www.google.com/url?sa=t&rct=...9aX0rtzdbxJhZP
Also when tapping go with the recommended size from your Zeus book ie 4.2mm but use a solid(no flute's) spiral tap.
Regards
Mike

[Clive S]

I also used Alum see this post #11 http://www.mycncuk.com/threads/8903-...highlight=alum I am not sure if there are different strengths of Alum

[Voicecoil]

The best taps I've found for through holes in ali are the blue ring spiral point types like this:

They're made specially for aluninium, the interrupted thread higher up seems to help with binding. Typically I've found you can tap a hole in 12mm plate in 3...4 seconds with one of these in a cordless drill with some cutting fluid. For blind holes as Mekanik said you need to use a spiral flute tap (to get the chips out), again I've found the blue ring types are marginally the best.

[mekanik]

This might be of interest
http://www.europatool.co.uk/threading/application-taps
Regards
Mike

[m_c]

In a nut shell for taps-
Spiral Point - Only good for through holes, as they push the swarf forward through the hole.
Spiral Flute - good for blind holes, as the swarf comes up around the shank, but are more prone to binding if the swarf doesn't clear.

My preference is always spiral point, as they're the stronger option.
If you're only going through relatively thin section, I'd avoid the interrupted flute ones Voicecoil posted a pic off, as a bit inadvertent sideways pressure with only two or three support flutes engaged can quickly jam the tap and snap the tap/rip the threads out the hole.
As always, lubrication is key. For tapping aluminium, a drop of good quality cutting oil is essential to stop things sticking.

There are also threadforming / roll forming taps which can give a stronger thread in aluminium, but drill hole size is very critical, so I would advise against them unless you can control the hole size well. Undersize and the tap will quickly jam, oversize and you won't have much of a thread.

[MikeyC38]

Hi Bluesking

Just seen your post here. Well done for joining the forum, doing a design and taking advice!. I wish I had seen this earlier because I would have told you to get the parts water-jet cut which also would have done some of the bigger holes for you and spotted the rest. This is the approach I took on my build and saved me a lot of grief (see build here http://www.mycncuk.com/threads/8206-...Axis-CNC-Build post #10 ) All I had to do was counterbore and thread the extrusions using a spiral tap. I then used a finishing tap to clean the threads or a plug tap where the hole is stopped.

Regards
Michael

[bluesking]

Hi guys,
Have not been on for a while as have been busy sorting out my new workspace and stocking up on parts. Thanks for everyone's various messages - don't have time at the mo to answer everyone individually.

Its all go on the CNC front now my space is sorted and I have all my parts, so I wanted to post an update.

Today I built a table for the CNC. 18mm MDF & C16 CLS timber from B&Q. About £30 worth of materials:

1 Cutting to size


2 Table top is 900mm x 1220mm; Supported by a timber frame:


3 700mm legs screwed on to table top:


4 timber base frame for support and shelf:


5 MDF shelf for base frame, and a few corner supports for increased rigidity (may need more of these in the short-dimension in future)
Table finished, ready to start assembling the machine itself:

[Doddy]

Looking good. Just a thought - is MDF the best, most appropriate worktop for something that could end up working with water (if only a failure-case for the spindle coolant, for example). My point - I have a sacrificial MDF layer on my "electronics" bench - and that's suffered some small water damage from an ultrasonic bath - and the surface is now perpetually feathering and impossible to flatten. My personal preference atm is ply, but most of the shops are stocking cheap eastern boards with large filled voids. The bench I've used for the mill is steel framed with ply top, sheathed in 1.2mm steel sheet (I had a bit lying around) - that's tough as boots.

I'm assuming that the bench is entirely separate from the machine? (it doesn't form part of the machine build?)