A Mini Vacuum Clamping Table for PCB Engraving

You know only too well how I keep on going on about FlatCam and milling printed circuit boards on the Tormach PCNC440.

You will also have read about my preoccupation with trying to hold the PCB material flat to avoid variations in milling depth.

I have got it to a reasonably repeatable process using mechanical clamping but you know when a perfectionist starts something it has to be as good as possible …. step forward the Vacuum Clamping Table.

The thinking for this followed on from the Rosebud Grate experiments on my live steam locomotive.   The grate consisted of a matrix of larger holes on the underside of the grate leading to a small bore hole on the top side of the grate.   The theory as I understand it was that the reduction in size creates a Venturi type effect and boosts the air stream into the fire.   I wondered therefore if I reversed the air flow i.e. sucked the air from the large hole into the small hole whether this would be beneficial in providing a boost of the suction.   It is a bit tenuous I must admit and I can’t point to lots of science to back this up, but certainly worth a play.

First stop was Fusion 360 and a two part plate was designed.   This consisted of a top and bottom part.   The bottom part is 15mm cast aluminium with a milled trough and the top plate is 10mm cast aluminium with 6.8mm holes (no science – this is tapping size for M8 that was already in a Tormach collet) on the top side that reduce down to 1.3mm holes (ditto also already in a collet) as breakthrough holes on the bottom surface.   Around the edges are M6 screw holes to clamp the two plates together and also M8 mounting holes to fasten the plate to the tooling plate on the Tormach. I didn’t quite think the suction connection fully.   After I had worked out the total area of the 1.3mm holes I realised that to accommodate this I needed a 16mm diameter hole for the air inlet.  This was not going to be possible to mount on the 25mm overall edge of the plate.   The solution was to 3D print a connecting pipe and mount this on the top surface.   This adapts to the vacuum cleaner pipe being used as the suction source.    The 3D printed adapter did not provide a good seal to the top plate so I had to fit a rubber gasket on it.  The parts were all put together as shown below.

Finished vacuum plate on test in the bench vice
Close up view of the 6.8mm blind holes leading to 1.3mm through holes

To my amazement it seems to work !

There does not seem to be leakage on the joint between the two plates and the vacuum pipe adapter with the rubber gasket seems to seal alright.   If I put a large piece of PCB material over all the holes it is very difficult to move it.  Single sided board is naturally bowed in the manufacturing lamination process and I can see it visibly jump flat when I turn on the vacuum.  If the PCB is smaller than the total area of suction holes it does not seem to matter about covering over the ‘non-used’ holes to maintain the grip.

Proof will be when I try to run a board.   

The milling process will not have major sideways pressure as the depth of milling is quite small so it should be fine. Clearly I can’t go drilling the component mounting holes in the PCB material with this holding technique but I can spot drill them to say 1mm depth and then finish them by hand having got a guide hole to start me off.

But all this will have to wait as the X axis limit switch has come apart on the Tormach and a spare has been ordered and is on its way.

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Stretching FlatCam and PCB milling on Tormach PCNC440

Tormach M Code Expansion Card

I have got a project underway to use the Tormach USB M Code expansion board in association with an A axis rotary table.   Details of this will follow in due course.   The expansion card when added to a Tormach mill allows the operator to embed M Codes in their CNC program which will operate up to 4 dry contact SPCO relays or accept 4 inputs as handshaking acks.

My Cabling Masterplan

As part of this project I need to have cables from the USB expansion board to various devices and in a rush of blood to the head decided to use standard readily available Micro USB cables for this. Now the cable connectors are pretty small and the PCB mating socket is even smaller with its 5 connections.   Glibly overlooking this I asked a colleague to produce a PCB layout for the connector to a breakout connector strip.   Dave duly produced a layout and a scratching of head resulted.   How was I going to produce the PCB and how was I going to solder to the connections assuming I could see them ….

FlatCam to the Rescue

Elsewhere in my blog there is mention of the use of FlatCam to create a CNC GCode listing from PCB Gerber and Excellon files.   This program works really well and many successful PCBs have been produced but I have never attempted to mill such fine PCB tracks. A number of problems needed to be addressed to make this successful.   The PCB sheet needed to be held very flat on the PCNC440 tooling table and the correct milling tool with its associated feeds and speeds needed to be chosen. In the past I have used strips of aluminium to fasten the PCB blank down on the tooling table.   This is never perfect and leads to variations in the pressure around the edges of the board.   With single sided PCB there is a natural curvature of the board material as a result of the surface tension of the laminating process.  A single sided blank has a concave surface on the copper side.  I needed to create something more repeatable.

Milling Window Restrictions

Before I bought the Tormach PCNC440 I had a discussion with John Saunders at NYC CNC and he recommended going for the biggest machine I could fit in my workshop.   I could have squeezed the 770 in at a push but I would have had to sell off my Myford VMB which I was reluctant to do.   My order therefore went through as a 440.   With hindsight this decision has been justified on two counts.   I rarely need a larger working area than the 440 offers and the VMB gets used very regularly for quick jobs that don’t justify CNC.   This project was an exception. I wanted to make a frame that would clamp the PCB blank down onto the tooling table.   In order to get the maximum working area for the PCB blank the clamping frame would have to sit outside the machining area.   How was I going to manufacture it ? Fortunately my tooling plate was designed to have a mix of M8 clamping holes and 3.7mm tooling holes and I was going to use this to advantage.   The clamping frame would be symmetrical.   By adding some matching tooling holes in the frame I could cut just over half of the frame and then flip it round 180 degrees and cut the second half. Here is a picture of the CAD showing half of the machining on what will be the underside of the plate when in use. The outer holes are for the M8 clamping to the table and the four smaller holes are the tooling holes.   Being tight with my materials I did not want to just mill out the centre of the plate and have a mountain of swarf (chips).  Instead I designed it with two slots as shown,  one for the clamping surface and one that almost cut through the stock.   The partial cut was to ensure the central piece did not flip out once cut free and damage my cutter. First one half was drilled and cut and then the plate was rotated 180 degrees and the second half cut.   This left the central island just held in place by less than 0.5mm of material.   This was easy to hand cut through to liberate the central area.   The plate was then turned over and the cut edge cleaned using the same tooling position and doing the same 180 degree rotation. To my surprise the rotation process on the tooling pins worked very well with only a minor step transition at the overlap point on all cuts.  This was probably more down to my 3.7mm tooling pins being not quite concentrically turned from 4mm silver steel. With this finished I now had a much more robust clamp for the PCB material.   I had made the clamping step 4mm deep so I could put sacrificial backing boards behind the PCB being run.   This would allow drilling through as needed.   Checking the flatness of a clamped PCB blank with my Haimer showed variation of a few thou in the top surface of the PCB Z position. The worst case variation in Z was at dead centre where the PCB’s natural bow was most dominant.

Tooling and Feeds and Speeds

The next problem was the milling tool and feeds and speeds.   I experimented with various V shaped routers but was not happy with the results.   The 5 thou tip on a 10 degree V tool was incredibly fragile.   Also because the tool was V shaped, any residual bow on the PCB surface lead to a variable width cut.   In the end I opted for Think & Tinkers 15 degree, 2 flute tapered stub (P/N EM2E8-0051-15VC).  This has a 5.1 thou cutting tip which is parallel for the first section so depth variations have no impact on the width of cut.  I ran the program at 10,000 RPM (PCNC 440 maximum) and at 150mm per minute feed rate. The PCB does not look particularly beautiful after milling as there are burrs and shavings present but a gentle rub over with a fine wet and dry removes this and leaves a remarkably clean cut tracking.   The images below show some of the results.   The fine tracking for the USB connector connections is shown on the microscope with a scale for reference.   This shows the five fingers occupying 120 thou with fairly similar track to gap widths of around 15 thou. So now I just have to solder the connectors in place …. I will let you know how it goes.

Overview shot of the clamping plate in position on the my tooling plate on the PCNC440
Finished clamping plate in position on the Tormach PCNC440 holding down a 6″ square piece of single sided PCB.
Tracking on the USB micro connector mounting
Zoom shot on the USB connector tracks with the graticule giving an idea of scale (small divisions are 0.5mm)

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Tormach PCNC440 Bellows Protection

The Problem

It has been one of those things that has been nagging for some time….

I have had a couple of frights while severely destroying metal which were brought about by the rear Y bellows on the Tormach having got filled with swarf (chips).   The machine had tried to do a severe Y movement to the rear of the machine and the bellows began to try to crush the swarf (chips) that had accumulated in its grooves.   While it may not do any fundamental damage it does sound awful and does give rise to a transient expectation of an underwear change.

Some time ago I had ordered in some 1mm Nitrile rubber sheet to solve this problem and it had been sat gathering dust waiting a “non busy” day.   Today was that day. Time to sort this out.

The Nitrile sheet I had ordered in from EBay was 500mm square.  When cut down the middle it would nicely span the bellows.   I also had some asymmetric profile plastic angle section measuring 30mm x 20mm x 1mm which had been brought back from the Brico in France.  (For me the French Bricos are a regular source of material as their range of aluminium, steel and plastic sections far outshines our UK DIY stores).

The Method

I cut two lengths of the plastic angle at 250mm long to match the Nitrile, one for the top of the Z axis bellows and one for the table end of the Y bellows.   On the shorter arm of the each piece of plastic angle I put 5 x 3mm clearance holes and marked these though onto the rubber sheet.   I cut the matching holes in the Nitrile sheet using a rotary punch.   I put a slight countersink on the back of the holes in the plastic to reduce protrusion down into the bellows.

The Z axis mounting consists simply of two M5 clearance holes 120mm apart on the wider arm of the plastic angle.   I made these holes elongated to make adjustment of the mounting easier.

The Y axis was not so straightforward in that the two M5 bellows mounting holes into the table are set below the level of the slideway.   These are spaced at 210mm. The plastic angle had to be hacked out to allow for this but this was simple to do on my Gabro notcher as the 1mm plastic is quite soft.   Once again I made the two M5 clearance mounting holes in the plastic angle elongated for ease of adjustment.

The two pieces of plastic angle were fastened to the Nitrile using M3 x 5 countersink screws with a washer and nut facing out from the bellows.   Rather than tighten these down hard and distort the rubber I soft tightened after adding some Loctite.

Fitting the angle to the machine is a bit fiddly as I was working blind.  With hindsight the mounting holes should have been slots rather than elongated holes. This would allow the plastic angle to be slid into place without taking the bellows screws out.

The 500mm length on the Nitrile seems just about right as it does not bulk up too much at extreme table positions.  A little longer perhaps but no shorter. The 250mm width gives plenty of overlap across the bellows to keep them clear of swarf/chips.

I am pleased with the result and I am sure long term my underwear will also benefit.  Below is a guidance sketch of the plastic angle details and a few finished shots of the Nitrile in place on the Tormach PCNC440.

Bellows protection on Tormach PCNC440 using Nitrile sheet
Plastic angle dimensions for the Z and Y axis bellows Nitrile cover mounting
Full image with Nitrile sheet in place
tormach, bellows protector
Z Axis bellows protector sheet top mounting on Tormach PCNC440
tormach, bellows protector
Y Axis bellows protector sheet table end mounting on Tormach PCNC440

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CNCEST 3040T CNC Router Update

Mach3, Limit Switches and First Cut

Progress has been good in getting the 3040T running.   Mach3 is not like PathPilot but then it is an all things to all machines software whereas PathPilot is dedicated to the Tormach family.   As a result of this Mach3 does take a bit more getting your head around and there is a lot more under the hood settings and adjustments that you have to address.

The first problem was the RnR USB interface card and trying to work out which port was which on the card connections.  Once this was sorted the motors responded to keyboard directional commands with the arrow keys and page up/down.  I had to make changes to the Mach3 Config for this.   If anyone needs screen shots of these setups send me a message.

Having been used to homing the Tormach I decided that adding limit switches to the 3040 would be a good thing.   I fitted 6 microswitches and wired them in series via their normally closed contacts to create a loop.   I connected one end of the loop to the Input 2 terminal on the card terminal strip (Port 3) and the other end to ground.   Any switch when activated will now break the loop and create an alarm condition.   The same switches also perform the home reference function.  (Mach3 just looks for a break in the circuit relevant to the function being asked of it – it knows when it is homing and it knows when it is running and looking for a switch break).   

I made 3D printed mountings for the switches and covers for these.  I had to add extra wiring to the cable forms both on the machine, through the connector cable to the control box and inside the control box.   Fiddly but done. I slipped up with the +Y back stop switch in that I mounted it on the cross plate without realising that the interface cabling to the umbilical connector fouls the movement.   There was a protruding M3 screw holding the interface connector in place which was acting as a crude carriage stop.  I turned an eccentric ‘top hot’ to fit on this screw to activate the microswitch.

Fitting the switches has now made setting up more repeatable and it consistently goes to 0,0,0 when doing a Reference All. Having referenced the spindle head, it can then be moved to the WCS zero ready to run a job. So far I have played with the demo Mach3 which has limited lines of GCode capacity but I have run some of my small PCB milling routines successfully. So all looking good but as ever I wasn’t satisfied and wanted to make life easier by using the Tormach ShuttleXpress controller.   This involved downloading a plug in file to add to Mach3. It sort of works but it isn’t like it is on the Tormach so I am still trying to get to grips with it.

I am waiting for a delivery of TackPack superglue to stick the cable wiring in place, hence the fluorescent green masking tape.   The flat cable is standard ribbon cable stripped down to be the right number of cores as needed. Note that I replaced the backing plate on the Z mounting (the bright aluminium as seen in the first picture) as this protruded too far down and would not allow the microswitch to be easily mounted. A few pictures of progress below and more updates to follow.

3040T limit switches
View showing Z switches and one X switch with its printed cover in place
Limit switch with cover removed
Underside view showing Y limit switches and also ‘Top Hat’ stop fitted on connector mounting screw (aluminium disc in this view)
RnR USB interface card. IN1 is the emergency stop switch and IN2 is the series wired normally closed loop via the limit switches with the return going to GND

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A CNCEST router / engraver joins the workshop

You have probably gathered by now that I like to experience new challenges in the workshop.  I spotted a CNCEST 3040T for sale on EBay with no bids offered and on the last day of the auction.   I did some checking and discovered that these devices are quite common and are a good way to get into CNC.   There is a wealth of YouTube postings about the machines and whole libraries that you can delve into to find models to make.   I must state that it is not a Tormach type machine but more of an engraver/router device with nothing like the grunt of the Tormach or indeed other milling centres.

The one I had spotted had a USB interface and was intended to be controlled by Mach3 which is a totally different animal to PathPilot.  PathPilot is a dedicated controller for the Tormach range of devices and because of this there are lots of facilities that you take for granted but which are hidden ‘under the hood’. Mach3 on the other hand is a generic controller that can be configured to control all manner of CNC devices but because of that it is quite complex to get to grips with, particularly for someone starting out on CNC.   Some of the user interfaces leave a bit to be desired which doesn’t help but I am getting there.   Mach3 has been around a long time and has a large user base.

Back to the story.

The purchase included the control box with the USB interface, a set of ER11 collets and a fourth axis stepper motor with a chuck already installed.   It is nicely built in that it is quite substantial but is basic and my purchase did not come with a great deal of documentation. The USB interface is by BitSensor RnR and apart from the stepper motor control lines has four auxiliary 4 input and 4 output lines that can be configured for external control.

The weekend was spent YouTubing trying to get up to speed and from this I gleaned the following : – Mach3 allows you to download a demo version which is fully functional but has a restricted maximum number of lines for running a program.   No problem with this and nice to be able to play before committing to a licence.

Having loaded Mach3 onto my desktop (Win 7, 64bit) I could not get the program to talk to the mill.   This was solved by the discovery that you need to add a plug-in for the RnR USB interface card.   This allowed the spindle to be moved in XYZ fashion from the PC keyboard direction keys and Page Up/Page Down. Next problem was that the spindle movements did not seem to reflect the Mach3 DRO readout distances.   This was solved on the Settings tab using the Set Steps per Unit button.   This was rather nice in that you tell the machine how far you want the spindle to move (in XY or Z) and then you are asked how far did it actually move as measured and it then crunches in the program what the scaling factor needs to be.  Rather neat facility. I now have the basics of accurate controlled movement. 

I can run the initial lines of one of my earlier discussion FlatCam PCB milling programs and it looks as it should.   I can see this machine becoming a dedicated PCB and general engraving device that is offline to the Tormach. So where to now ?   

I want to replace the tool table with a dedicated tooling plate similar to the one I made for the Tormach.   The table as fitted is aluminium extrusion with the equivalent of T slots.  I also want to add limit switches to make it more user friendly.   This means working out how to enable the above mentioned auxiliary lines.   Some sort of height zero probe will be needed also. So a few things to think about over winter.   More notes on progress to follow. Similar or related subjects : –