Advice to machine stationary engine base plate

[Greg HParticipant @gregh]

Hi,

I thinking of making a Bensen vertical steam engine and this is the base plate for it. I haven’t yet worked out how to machine it. I’m hoping someone might be help me out with soem advice.

The base plate is about 100mm long by 75mm wide. I might make it out of aluminium. I have a mill and a rotary table.

So for section A, B and C I thought I would use the mill and with pivot it around the centre of the circular segment  using my rotary table. I’ll use a R3 end mill cutter to get that concave radius around it.

How can I machine section D and get a similar R3 profile?

Thanks, Greg

[JasonBModerator @jasonb]

There are two ways depending on what size material you start with.

If you have a large sheet you can mark the ctr position of the two “D” arcs on that. You then just need to locate those marks centrally on the rotary table, clamp the plate in place then offset the mill table and machine the arcs.

If you only just have enough for the part then it can be mounted on a sub plate that only need be MDF or Ply just something to mark those ctr positions on.

This shows the two ctr positions outside of the actual part. You would just substitute your aluminium plate. Then I would sugged milling almost right through with say a 6mm cutter then change to your R3, alter the offset of teh mill table and cut the profile.

Whole scratch build detailed here

[Greg HParticipant @gregh]

Thanks Jason. That’s very helpful. I thought for the concave radi to would have required the base plate to be too far offset from centre to mount on my rotary table (150mm), but I hadn’t thought about using a larger plate to start with to keep it all rigid.

I’ll draw the base plate up with the pivot points to work out how far I need offset it, and then work out if I go with a larger plate or mount it on something like it looks like you’ve done.

Your build thread will be very helpful too. Thanks for the link.

[noel shelleyParticipant @noelshelley55608]

Or make a wooden pattern and cast it, might prove just as quick and you could sell the extras to cover your costs. Noel.

[JasonBModerator @jasonb]

Though you would still be faced with the question of how to produce the two concave parts of the pattern😊😊😊

If we are getting into alternative methods then there are a few.

• Build up from 3 layers (still need to do the concave bits)
• Fire up the CNC if you have one
• Draw in CAD and send off the file. Then wait for a jiffy bag with teh 3D printed part to arrive
• Cold chissel and files for the traditionalist

But you are probably going the right way, as Mr Westbury said many years ago

[PeteParticipant @pete41194]

As usual it depends on what you have available. Cnc would be the obvious, fastest, most accurate and best choice. If not, your Rotary table for the convex, and I’d use a custom ground HSS fly cutter in the horizontal cross hole in my boring head for both the convex and then the same for the concave radii without the rotary table. Especially for those much tighter inside radii at your B/C points. No great accuracy is needed, just a visual blending of the tangent points as the radii goes from a convex to concave radii. So some light blending with fine files might also be needed.

But if it were me, I think I’d make a trial run in any solid wood first just to double check the set ups before committing to metal. Where accuracy is needed on your engines base plate is the center lines and exact position of the cylinder, main column and pump. The rest is just visual aesthetics. Manual machining it? I think I’d want that sub plate with your starting base plate blank mounted to it that was mentioned. And with at least one true X & Y datum face to edge find from to then locate each radii, the cylinder, column and pump position to known coordinates. A second and larger sub plate fastened to your rotary table, machined flat and with a grid of drilled and tapped holes for off set clamping the part would also be a big help.

Industrial level rotary tables with a second and integral X,Y table on top were produced at one time that combined both the rotary and table off sets to the rotary axis that would make this fairly easy. Troyke and Advance were just two of them. But most I know of would need a fairly large mill compared to what most M.E.’s might have. https://www.machinio.com/listings/33082174-dm7-15-troyke-12-square-cross-slide-rotary-table-in-minneapolis-mn

[DiogenesParticipant @diogenes]

Please do keep us posted about how you finally approach this and how it goes, if you have the time – it’s these kind of  practical problem / solution threads that really add something to this forum IMO..

[JasonBModerator @jasonb]

I did consider suggesting the boring head for the main concave parts of the base but that still leaves the question of machining the 3mm quarter round detail. Doing it as I suggested allows both to be done at the one setting on the rotary table so reducing the number of setups and possible inaccuracies that may result.

I’m not sure how a boring head would work for the 3mm convex radii as you would not be able to swing a small enough circle particularly at the B/C junction and would also have to pack the work up to allow the boring had to pass beyond the bottom of the work

If using a subplate I would not be too worried about true edges for a job like this. Sharpie mark in the middle your blank stock will allow you to position your mill spindle over it and then set out the holes from there – both the ones for the external pivot points as well as those to locate ctr of cylinder, column and pump as well as any screw hole sto fix the cylinder base and column base. You then just need a peg in the rotary table’s middle and can locate all the arc centres from the holes. No nee dto us ean edge.

Yes a square and true datum edge and end are useful on some occasins but no nee dto overthing a part like this.

[JasonBModerator @jasonb]

Mentioning the BC junction brings up one point that should be considered when using a round over cutter.

You need to cut the basic shape with a suitably sized cutter so that the roundover cutters ctr when it gets to the junction is in the same position otherwise you need additional setups due to the roundover cutter having to follow a radius.

So depending on the size of your 3mm roundover cutter you may actually need smaller than the 6mm I suggested for cutting the shape out.

I did cover it in one of my published designs, possibly the Muncaster I’ll have a look later and post a picture or two.

Greg – If you can measure the tip diameter of your roundover cutter I’ll work out what cutter is needed for main shape. Also what scale are you building to to get the 100mm length

[Greg HParticipant @gregh]

Jason, I’ll be doing mine just as you have. I haven’t bought the radisued cutter yet, just thought R3 looked about right. Thanks for the offer of assistance.

On the 100mm length, that was just a rough size so readers had an idea of the size of the part. I bought a copy of the plans from Polly and I’ll be building it as per the plans. Although the flywheel looks a bit big to me. I might go 125mm or 100mm.

I’ll draw the plan in cad with the pivot point detail and go from there.

Aluminum plate is cheap, so if my first one doesn’t work out I can have another go until I’m happy.

[JasonBModerator @jasonb]

If you have not got the cutter yet then it would be worth considering a cheap TCT router bit, although meant for wood they will be fine with aluminium.

Most are sold with a bearing guide so have a larger tip diameter so you could cut the overall shape with say an 8 or 10mm cutter, the bearing is not needed when using the mill. The HSS ones for metal cutting tend to be about a 6mm tip radius so you would need a 3 or 4mm cutter to do the overall shape depending on the width of the “step” you go for.

[Greg HParticipant @gregh]

I got a milling cutter for $25AUD, about 13GBP. Australia is close to China, so I can get tooling fairly cheap. So all sorted.  🙂

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