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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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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Tormach Costing Sheet Update

Tormach changed their prices last year so I have updated the costing spreadsheet I created to reflect these changes. Note that the sheet now has the new M Series prices for the 770 and 1100.

Note that I believe I have interpreted their prices correctly but you can check this once you have placed a request for quotation and compare. Let me know if you spot any errors.

 

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Tooling Day

Each of my CNC mills has a home designed and produced tooling plate.  Both have a 25mm pitch matrix of tapped mounting holes and a further submatrix of 3.7mm tooling pin holes.   Why 3.7mm ?  So I can turn down 4mm silver steel for the tooling pins to create a retaining shoulder.

My small CNC plate has M5 tapped holes and the Tormach 440 has M8 tapped holes.   What struct me was that I has starting to create dual sets of hold down tooling, some with M5 and some with M8 mounting holes.   Not a good idea. (Entertaining and therapeutic though it might be to have ‘tooling days’).

Clearly a mounting with M8 holes was not much use with tooling having M5 mounting holes but the other way round would work if I had M8 to M5 adaptors.

As a result I have spent the day creating adaptors which you could call male and female.   Both are made from M8 hex head tensile screws with the female ones retaining the M8 head and the male ones utilising the cut off portion of thread.  I had to undercut the thread ends behind the heads so the female adaptors would sit flush.  Having undercut I then skimmed all the tops of the hex heads to be same depth.

All the turning was done with the ER25 collet chuck instead of the 3 jaw which is normally fitted to the Myford.  The male adaptor versions were a pain to turn down to M5 diameter and had to be done incrementally as the M8 threaded end could not be heavily gripped in the collet.

The female versions are quite useful if a job is being run on the Tormach that needs suspending above the table so it can machined to full stock depth.

Nothing revolutionary or original but a day well spent.

A picture explains : –

Male and female M8 to M5 adaptors

M8 to M5 adaptors in place on the Tormach tooling plate

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Tormach Purchase Costing Sheet

I haven’t been completely idle while on holiday …

After I decided to buy a Tormach milling machine I had debate whether to go for the 440 or the 770.   This confusion was based on available workshop space and to a lesser extent on cost.  I also did not have a feel for the total cost of not just the items I needed to buy but also what the total package would cost when it landed on my driveway.  In the UK we pay VAT on not just the goods but also the delivery cost.

To help my thinking I put together a spreadsheet on Excel that split out the basic machine parts and then had a common section showing all the accessories I would need.  This totaled everything up in USD and I then did a conversion to GBP at spot rate and then added VAT and duty factors for UK import.

This sheet helped my enormously and once I had all the key prices loaded from the Tormach site I could do ‘what if’ calculations to fit my budget.

I was recently contacted by another potential buyer of a Tormach and I sent him the sheet to help his thinking process.  For anyone else thinking of buying either in the US or an overseas country I thought the sheet might help so I have spent some time cleaning it up and and I attach the new version below.

Simply put a quantity of each item in the column associated and see the impact of your shopping list at the bottom, either as a 440 or a 770.   Clearly the sheet could be extended to a 1100 if that takes your fancy.  (Don’t forget to check the current pricing from the Tormach site by searching on the product code shown on the sheet).

Excel – Tormach mill costing

Don’t forget you will always need one more TTS collet than you have ordered …..

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